21-255721-2557
ORDINANCE NO.
AN ORDINANCE OF THE CITY OF DENTON, TEXAS AMENDING THE STORMWATER
CRITERIA MANUAL; AND PROVIDING FOR SEVERABILITY, A SAVINGS CLAUSE
AND AN EFFECTIVE DATE.
WHEREAS, pursuant to Ordinance No. DCA18-0009q, the City Council of the City of
Denton, a Texas home -rule municipal corporation (the "City") adopted the newly revised Denton
Development Code, superseding and expressly repealing Chapters 31, 34 and 35 of the 1991 Code
of the City of Denton, Texas, as amended, and leaving all other Chapters intact and superseding
and expressly repealing the 2002 Denton Development Code, as amended; and
WHEREAS, the Denton Development Code established a process whereby the policies,
regulations, and procedures relating to zoning and development within the City and its regulatory
extraterritorial jurisdiction are legislatively established by the Council after public hearing in
accordance with State law: and
WHEREAS, the creation of specific design standards and methodologies (the
"Development Criteria Manual") are delegated to City professional staff possessing the necessary
and appropriate licensure and expertise who may also consult with their peers in both the public
and private sectors, consistent with the policy direction of Council; and
WHEREAS, the Development Criteria Manual process was intended to benefit the public
and the development community by empowering City professional staff to more quickly
implement new and improved materials and methods as they are developed, in accordance with
generally accepted design standards of the industry, as appropriate to achieving an equal or greater
public benefit for costs expended, for issues not involving policymaking decisions; and
WHEREAS, the latest version of the Stormwater Criteria Manual was last prepared in
December 2018; and
WHEREAS, the Stormwater Criteria Manual update will be adopted by ordinance; and
WHEREAS, after providing notice and conducting a public hearing as required by State
law, the City Council finds that these changes to the Stormwater Criteria Manual are consistent
with the Comprehensive Plan and are in the public interest; NOW THEREFORE,
THE COUNCIL OF THE CITY OF DENTON HEREBY ORDAINS:
SECTION 1. The findings and recitations contained in the preamble of this ordinance are
incorporated herein by reference.
SECTION 2. The Stormwater Criteria Manual is hereby amended and shall read as
contained in Exhibit "A."
SECTION 3. It is hereby officially found and determined that the meeting at which this
Ordinance was passed was open to the public as required by law, and that public notice of the time,
place, and purpose of the meeting was given as required by State law.
SECTION 4. If any section, subsection, paragraph, sentence, clause, phrase, or word in
this Ordinance, or the application thereof to any person or under any circumstances is held invalid
by any court of competent jurisdiction, such holding shall not affect the validity of the remaining
portions of this Ordinance, and the City Council of the City of Denton, Texas, hereby declares it
would have enacted such renaming provisions despite any such invalidity.
SECTION 5. Save and except as amended hereby, all the provisions, sections, subsections,
paragraphs, sentences, clauses, and phrases of the Code of Ordinances shall remain in full force
and effect.
SECTION 6. This ordinance shall become effective January 1, 2022.
The motion to approve this ordinance was made by s5e and
seconded by VA-, SeX� M" c Q, , the ordinance was passed and approved by
the following vote - 1:
Aye Nay Abstain Absent
Gerard Hudspeth, Mayor:
Vicki Byrd, District 1:
Brian Beck, District 2:
Jesse Davis, District 3:
Alison Maguire, District 4: ✓
Deb Armintor, At Large Place 5:
Paul Meltzer, At Large Place 6: ✓
PASSED AND APPROVED this, the 14+= day of e Q nnbE5;-2021
- Z �f �11i- - - �
GERARD HUDSPETH, MAYOR
Page 2
ATTEST:
ROSA RIOS, CITY SECRETARY
BY:�_®�
APPROVED AS TO LEGAL FORM:
MACK REINWAND, CITY ATTORNEY
–t lw&e— Hilary Negron
2021.12.0910:58:29
BY: -06'00'
Page 3
1W14IIM0II_1
Page 4
City of Denton
Stormwater Design Criteria Manual
D NTON
December 2021
Stormwater Design Criteria Manual
Table of Contents
Section 1.0 — Introduction.........................................................................1
1.1 Purpose.........................................................................................................................................1
1.2 Design Criteria...............................................................................................................................2
Section 2.0 — Definitions...........................................................................2
Section 3.0 Design Criteria.....................................................................10
3.1 Hydrologic Methods....................................................................................................................10
3.1.1 Types of Hydrologic Methods..............................................................................................10
3.1.2 Rainfall Estimation...............................................................................................................12
3.2 Acceptable Downstream Conditions for Open Channels and Floodplains.................................13
3.2.1 Streambank Protection..............................................................................................................14
3.2.2 Flood Mitigation..........................................................................................................................14
3.2.2.1 Introduction..........................................................................................................................14
3.2.2.2 Flood Mitigation Design Options.........................................................................................14
3.3 Stormwater System Design...............................................................................................................15
3.3.1 Introduction..........................................................................................................................15
3.3.2 Hydraulic Design Criteria for Streets and Closed Conduits................................................16
3.3.3 Hydraulic Design Criteria for Structures.............................................................................32
3.3.4 Channel Drop Structures.....................................................................................................38
3.3.5 Maintenance Access...........................................................................................................38
3.4 Culverts.......................................................................................................................................
39
3.5 Bridges........................................................................................................................................40
3.6 Detention Facilities......................................................................................................................40
3.6.1 Outlet Structures for Detention and Retention Structures..................................................42
3.7 Energy Dissipation... ...................................................................................................................
45
3.8 Floodplain....................................................................................................................................46
3.8.1 Floodplain Development Criteria.........................................................................................46
3.8.2 Procedures for Floodplain Alteration...................................................................................47
3.8.3 Fully -Developed Water Surface Elevation Calculations......................................................48
3.8.4 Floodplain Fill Requirements...............................................................................................48
3.9 Easements and Fences..............................................................................................................49
3.10 Water Quality...............................................................................................................................
50
3.10.1 Water Quality Protection Volume........................................................................................50
3.10.2 Construction Erosion and Sediment Control Requirements ...............................................
50
Section 4.0 iSWM'm Technical Manuals .................................................
52
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Section 1.0 - Introduction
1.1 Purpose
These design criteria and the North Central Texas Council of Governments (NCTCOG) integrated
Stormwater Management (iSWMTM) Planning, Water Quality, Hydrology, Hydraulics, Site Design Controls,
Construction Controls, Construction Control Standard Details and Landscape Technical Manual
Documents together shall be known as the Stormwater Design Criteria Manual. The purpose of this
Stormwater Design Criteria Manuals and the iSWMTm Technical Manuals is to implement the policies set
forth in the Denton Development Code (DDC) and the City of Denton Code of Ordinances, Subpart B,
Chapter 30 (Denton Flood Prevention and Protection Ordinance) by establishing standard principles and
practices for the design and construction of storm drainage systems within the City of Denton, Texas and
within its extraterritorial jurisdiction. The design factors, formulae, graphs and procedures are intended for
use as minimum engineering criteria for the design of drainage systems involving determination of the
quantity, rate of flow, method of collection, storage, conveyance, detention and disposal of storm water.
Responsibility for actual design remains with the design engineer. Users of this manual should be
knowledgeable and experienced in the theory and application of drainage engineering.
City of Denton
DDC Subchapter 7.4 Code of
DDC Subchapter 7.5 Environmentally Ordinances
Drainage Standards Sensitive Areas
Chapter 30
Flood
Prevention &
Stormwater Protection
Ordinance
Design Criteria
Manual
iSWMTm Hydrology iSWMTm Hydraulics
Technical Manual Technical Manual
iSWM "M Planning iSWMTm Landscape
Technical Manual
iSWMTM Site Design
Controls Technical iSWMTM Construction
Control Standard Detail
Manual
Technical Manual
iSWMTM Water iSWMTM Construction
Quality Technical Controls Technical
Manual Manual
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1.2 Design Criteria
A. The stormwater design criteria is not intended to be an all-inclusive design document and the
Denton Development Code and City of Denton Code of Ordinances shall be consulted for
additional policy and criteria.
B. The stormwater design criteria contained in this manual do not supersede the criteria contained
in the Denton Development Code and the Denton Flood Prevention and Protection Ordinance
and any revisions to those documents shall supersede the criteria in this manual.
C. The stormwater criteria contained under Section 3.0 of this manual supersedes any design
criteria contained in the iSWMTM Planning, Water Quality, Hydraulics, and Hydrology Technical
Manual Documents.
D. Any deviation from the criteria and principles of this manual must be approved by the City
Engineer or his/her designee.
Section 2.0 - Definitions
The definitions listed below are specific to this Stormwater Design Criteria Manual and may not
apply to other City of Denton manuals.
Abutment - A wall supporting the end of a bridge or span and sustaining the pressure of the bordering
earth.
Apron - A floor or lining of concrete, timber, or other suitable material at the toe of a dam, entrance or
discharge side of a spillway, a chute, or other discharge structure, to protect the waterway from erosion
from falling water or turbulent flow.
Backwater - The rise of the water level upstream due to an obstruction or constriction in the channel.
Backwater Curve - The term applied to the longitudinal profile of the water surface in an open channel
when flow is steady but non-uniform.
Baffles - Deflector vanes, guides, grids, gratings, or similar devices constructed or placed in flowing water,
to: (1) check or effect a more uniform distribution of velocities; (2) absorb energy; (3) divert, guide, or agitate
the stormwater flow; and (4) check eddy currents.
Baffle Chute - A drop structure in a channel with baffles for energy dissipation to permit the lowering of the
hydraulic energy gradient in a short distance to accommodate topography.
Base Flood Elevation(BFE) - The elevation shown on the Flooa Insurance Rate Map (FIRM) and found in
the accompanying Flood Insurance Study (FIS) for Zones A, AE, AH, Al -30, AR, V1-30, or VE that indicates
the water surface elevation resulting from the flood that has a one percent (-I%) chance of equaling or
exceeding that level in any given year.
Calibration - Process of checking, adjusting, or standardizing operating characteristics of instruments and
model appurtenances on a physical model or coefficients in a mathematical model. The process of
evaluating the scale readings of an instrument in terms of the physical quantity to be measured.
Channel — a man-made drainageway or watercourse, generally constructed to straighten a stream or
increase its capacity.
Channel Roughness - Irregularities in channel configuration which attenuate the flow of water and
dissipate its energy.
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Chute - An inclined conduit or structure used for conveying water to a lower level.
Conduit - Any open or closed structure for conveying flowing water.
Critical Flow - The state of flow for a given discharge at which the specific energy is a minimum with
respect to the bottom of the conduit. The Froude Number is equal to 1.0 for critical flow conditions.
Crown - The highest point on a transverse section of conduit or the highest point of a roadway cross section.
Culvert - Large pipe or other conduit through which a small stream passes under a road or street.
Curb - A vertical or sloping structure located along the edge of a roadway, normally constructed integrally
with the gutter, which strengthens and protects the pavement edge and clearly defines the pavement edge
to vehicle operators.
Dam- A barrier constructed across a watercourse for the purpose of creating a reservoir or diverting water
from a conduit or channel.
Degradation - The progressive general lowering of a stream channel by erosion, other than that caused
by a constriction.
Depression Storage - Collection and storage of rainfall in natural depressions after exceeding infiltration
capacity of the soil.
Design Storm or Flood - The storm or flood which is used as the basis for design.
Detention - The storage of storm runoff for a controlled release during or immediately following the design
storm.
a) Off-site detention -A detention pond located outside the boundary of the area it serves.
b) On-site detention -A detention pond which is located within and serves only a specific site or
subdivision.
C) Regional detention -Detention facilities provided to control excess runoff based on a
watershed -wide hydrologic analysis.
Development -Any man-made change to improved or unimproved real estate, including but not limited to,
buildings or other structures, paving, drainage or utilities. Development activities include: subdivision of
land; construction or alteration of structures, roads, parking, fences, pools, signs, temporary uses, utilities,
and other facilities; installation of septic systems; grading; excavation, mining or drilling operations; deposit
of refuse, debris, or fill materials; and clearing of natural vegetative cover (with the exception of agricultural
activities as defined and as permitted). Routine repair and maintenance activities are exempted.
Drop Structures - A sloping or vertical section of a channel designed to reduce the elevation of flowing
water without increasing its velocity.
Entrance Head - The head required to cause flow into a conduit or other structure; it includes both entrance
loss and velocity head.
Entrance Loss - Head lost in eddies or friction at the inlet to a conduit, headwall or structure.
Flash Flood - A flood of short duration with a relatively high peak rate of flow, usually resulting from a high
intensity rainfall over a small area.
Flood Control - The elimination or reduction of flood losses by the construction of flood storage reservoirs,
channel improvements, dikes and levees, by-pass channels, or other engineering works.
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Flood Hazard Area - Area subject to flooding by one percent (1 %) chance floods.
Flood Management or Flood Hazard Mitigation - Any program or activity designed to reduce damages
from flooding, including stream erosion.
Floodplain - The area designated as subject to flooding from the base flood (one percent chance flood) on
the flood insurance rate map. The floodplain includes the regulatory floodway and floodway fringe.
Floodway Fringe - The area located within the floodplain and outside the floodway.
Freeboard - The distance between the normal operating level and the top of the side of an open conduit
left to allow for wave action, floating debris, or any other condition or emergency without overtopping the
structure.
Frequency (of storms, floods) - Average recurrence interval of events, over long periods of time.
Mathematically, frequency is the reciprocal of the exceedance probability.
Froude Number - A flow parameter, which is a measure of the extent to which gravitational action affects
the flow. A Froude number greater than 1 indicates supercritical flow and a value less than 1 subcritical
flow. The simplest form of the Froude number is given by the equation:
F = V/(gD)a.s
where: V = Velocity
g = the acceleration due to gravity
(32.2 ft/sec/sec)
D = depth
Fully Developed Conditions —A description of hydrologic conditions in a watershed, if the watershed has
been completely built out based on the zoning and future land use maps of the City. This term is
interchangeable with the term "Ultimate Developed Conditions". This is not to be confused with a
Developed Floodplain as defined in Subchapter 7.5 of the Denton Development code, which refers to the
character of the streambed itself.
Gabion - A wire container filled with rock and used in the construction of dams, retaining walls, and
protection against erosion.
Grade - (1) The inclination or slope of a channel, canal, conduit, etc., or natural ground surface, usually
expressed in terms of the percentage of number of units of vertical rise (or fall) per unit of horizontal
distance. (2) The elevation of the invert of the bottom of a conduit, canal, culvert, sewer, etc. (3) The finished
surface of a canal bed, road bed, top of an embankment, or bottom of excavation.
Gutter - A generally shallow waterway adjacent to a curb used to convey stormwater.
Headwater - (1) The upper reaches of a stream near its sources; (2) the region where ground waters
emerge to form a surface stream; (3) the water upstream from a structure.
Hydraulic Control - The hydraulic characteristic which determines the stage -discharge relationship in a
conduit. The control is usually critical depth, tailwater depth, or uniform depth.
Hydraulic Grade Line (HGL) - A line representing the pressure head available at any given point within
the system.
Hydraulic Gradient - A hydraulic profile of the piezometric level of the water, representing the sum of the
depth of flow and the pressure head. In open channel flow it is the water surface.
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Hydraulic Jump -The hydraulic jump is an abrupt rise in the water surface which occurs in an open channel
when water flowing at supercritical velocity is retarded by water flowing at subcritical velocity. The transition
through the jump results in a marked loss of energy, evidenced by turbulence of the flow within the area of
the jump. The hydraulic jump is sometimes used as a means of energy dissipation.
Hydraulics - A branch of science that deals with practical applications of the mechanics of water
movement.
Hydrograph -A graph showing stage, flow, velocity or other property of water versus time at a given point
on a stream or conduit. Examples include:
a) Dimensionless Unit hydrograph
b) Unit Hydrograph
Hydrology — The science dealing with the properties, distribution, and circulation of water on and below
the Earth's surface and in the atmosphere.
Hyetograph - A histogram or graph of rainfall intensity versus time of storm.
Impervious -A term applied to a material through which water cannot pass, or through which water passes
with great difficulty.
Infiltration - (1) The entering of water through the interstices or pores of a soil or other porous medium.
(2) The entrance of water from the ground into a sewer or drain through breaks, defective joints, or porous
walls. (3) The absorption of water by the soil, either as it falls as precipitation, or from a stream flowing over
the surface.
Inlet - Inlets are drainage structures used to collect surface water through grate or curb openings and
convey it to storm drains or direct outlet to culverts.
Inlets used for the drainage of roadway surfaces can be divided into four major classes:
a. Grate Inlets — These inlets include grate inlets consisting of an opening in the gutter covered by
one or more grates, and slotted inlets consisting of a pipe cut along the longitudinal axis with a grate or
spacer bars to form slot openings.
b. Curb -Opening Inlets — These inlets are vertical openings in the curb covered by a top slab.
C. Combination Inlets — These inlets usually consist of both a curb -opening inlet and a grate inlet
placed in a side-by-side configuration, but the curb opening may be located upstream of the grate.
d. Drop Inlet (Y -Inlet) -A storm drain intake structure typically located in unpaved areas. The inlet may
extend above the ground level with openings on one or more sides of the inlet or it may be flush with the
ground with a grated cover.
Intensity - As applied to rainfall, a rate usually expressed in inches per hour.
Interception - As applied to hydrology, refers to the process by which precipitation is caught and held by
foliage, twigs, and branches of trees, shrubs and buildings, never reaching the surface of the ground, and
then lost by evaporation.
Invert - The floor, Bottom, or lowest portion of the internal cross section of a conduit.
Lag Time - The time difference between two occurrences such as between rainfall and runoff or pumping
of a well and effect on the stream. See Time of Concentration.
Lining - Impervious material such as concrete, clay, grass, plastic, puddled earth, etc., placed on the sides
and bottom of a ditch, channel, and reservoir to prevent or reduce seepage of water through the sides and
bottom and/or to prevent erosion.
Lip - A small wall on the downstream end of an apron to break the flow from the apron.
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Manning's Coefficient - The coefficient of roughness used in Manning's Equation for flow in open
channels.
Manning's Equation - A uniform flow equation used to relate velocity, hydraulic radius and the energy
gradient slope.
Model - Mathematical systems analysis by computer, applied to evaluate rainfall -runoff relationships;
simulate watershed characteristics, predict flood and reservoir routings, or use other aspects of planning.
Nappe - The sheet or curtain of water overflowing a weir or dam. When freely overflowing any given
structure, it has a well-defined upper and lower surface.
100 -year Event - Event (rainfall or flood) that has a 1 % chance of being equaled or exceeded in any given
year.
Open Channel - A conduit in which water flows with a free surface.
Orifice - (1) An opening with closed perimeter, and of regular form in a plate, wall, or partition through which
water may flow. (2) The end of a small tube, such as a Pilot tube, piezometer, etc.
Peak Flow (Peak Rate of Runoff) - The maximum rate of runoff during a given runoff event.
Percolation - To pass through a permeable substance such as ground water flowing through an aquifer.
Permeability - The property of a material which permits movement of water through it when saturated and
actuated by hydrostatic pressure.
Pervious - Applied to a material through which water passes relatively freely.
Porosity - (1) An index of the void characteristics of a soil or stratum as pertaining to percolation; degree
of perviousness. (2) The ratio, usually expressed as a percentage, of (a) the volume of the interstices in a
given quantity of material, to (b) the total volume of the material.
Positive Overflow - When the inlets do not function properly, or when the design capacity of the conduit is
exceeded, the excess flow must be conveyed overland along a paved course. This could mean along a
street or alley but could require a concrete flume and the dedication of special drainage easements on
private property.
Post -development - The condition of the given site and drainage area after the anticipated development
has taken place.
Precipitation - Any moisture that falls from the atmosphere, including snow, sleet, rain and hail.
Pre -development - The condition of the given site and drainage area prior to development.
Probable Maximum Flood (PMF) - The flood that may be expected from the most severe combination of
critical meteorological and hydrologic conditions that are reasonably possible in the region.
Probable Maximum Precipitation (PMP) - The critical depth -duration -area rainfall relationship for a given
area during the seasons of the year which would result from a storm containing the most critical
meteorological conditions considered probable of occurring.
Rainfall Duration - The length of time over which a single rainfall event occurs.
Rainfall Frequency - The average recurrence interval of rainfall events.
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Rainfall Intensity - The rate of accumulation of rainfall, usually in inches of millimeters per hours.
Rational Formula - A traditional method of computing peak flow using intensity of the storm rainfall.
Reach - Any length of river or channel. Usually used to refer to sections which are uniform with respect to
discharge, depth, area or slope, or sections between gaging stations.
Recurrence Interval - The average interval of time within which a given event will be equaled or exceeded
once. For an annual series (as opposed to a partial duration series) the probability of occurrence in anyone
year is the inverse of the recurrence interval. Thus a flood having a recurrence interval of 100 years has a
1 percent probability of being equaled or exceeded in any one year.
Regulatory Floodway — The channel of a river or other watercourse and the adjacent land areas that must
be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation
more than a designated height.
Return Period - See Recurrence Interval
Reynold's Number - A flow parameter which is a measure of the viscous effects on the flow. Typically
defined as:
Re = VD/v
Where: V = Velocity
D = Depth
v = kinematic viscosity of the fluid
Riprap (Revetment) - Forms of bank protection, usually using rock or concrete.
Routing - Routing is a technique used to predict the temporal and spatial variations of a flood wave as it
traverses a river reach or reservoir. Generally, routing technique may be classified into two categories -
hydrologic routing and hydraulic routing.
ROW (Right -of -Way) - A strip of land dedicated for public streets and/or related facilities, including utilities
and other transportation uses.
ROW Width - The shortest horizontal distance between the lines which delineate the right-of-way of a
street.
Runoff - That part of the precipitation which reaches a stream, drain, sewer, etc., directly or indirectly.
a) Direct Runoff -The total amount of surface runoff and subsurface storm runoff which reaches
stream channels.
b) Overland Runoff -Water flowing over the land surface before it reaches a definite stream
channel or body of water.
Runoff Coefficient - A decimal number used in the Rational Formula which defines the runoff
characteristics of the drainage area under consideration. It may be applied to an entire drainage basin as
a composite representation or it may be applied to a small individual area such as one residential lot.
Runoff Total - The total volume of flow from a drainage area for a definite period of time such as a day,
month, or a year, or it may be for the duration of a particular storm.
Scour - The erosive action of running water in streams or channels in excavating and carrying away
material from the bed and banks.
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SGS Runoff Curve Number- Index number used by the National Resource Conservation Service, formerly
the Soil Conservation Service, as a measure of the tendency of rainfall to run off into streams rather than
evaporate or infiltrate.
Sediment - Material of soil and rock origin transported, carried, or deposited by water.
Sidewalk - A paved area within the street right-of-way or sidewalk easement specifically designed for
pedestrians and/or bicyclists.
Slope, Critical - The slope or grade of a channel that is exactly equal to the loss of head per foot resulting
from flow at a depth that will give uniform flow at critical depth; the minimum slope of a conduit which will
produce critical flow.
Slope, Friction - The friction head or loss per unit length of channel or conduit. For uniform flow the friction
slope coincides with the energy gradient, but where a distinction is made between energy losses due to
bends, expansions, impacts, etc., a distinction must also be made between the friction scope and the
energy gradient. The friction slope is equal to the bed or surface slope only for uniform flow in uniform open
channels.
Soffit - In a stormwater pipe, the uppermost point of the interior of the pipe wall. The crown is the uppermost
point on the outside of the pipe wall.
Spillway - A waterway in or about a dam or other hydraulic structure, for the overflow of excess water.
Steady Flow - Open channel flow is said to be steady if the depth of flow does not change or if it can be
assumed to be constant during the time interval of consideration.
Stream — a natural drainageway that conveys stormwater, may also be referred to as a creek. References
to a stream or creek in this manual refer to the entire stormwater carrying component of the stream to the
limits of the floodplain, not just to the streambed.
Stilling Basin - Pool of water conventionally used, as part of a drop structure or other structure, to dissipate
energy.
Storm Hydrology - The branch of hydrology that concentrates on the calculation of runoff from storm
rainfall.
Stormwater Management - The control of storm runoff on-site or on small streams, by means of land use
restrictions, detention storage, erosion control, and/or drainage measures.
Stormwater Model - Mathematical representation of a stormwater network.
Subcritical Flow - The Froude Number is less than 1.0 for subcritical flow conditions.
Supercritical Flow - The Froude Number is greater than 1.0 for supercritical flow conditions.
Tailwater - The depth of flow in the stream directly downstream of a drainage facility.
Time of Concentration - The estimated time in minutes required for runoff to flow from the most remote
section of the drainage area to the point at which the flow is to be determined.
Total Head Line (Energy Line) - A line representing the energy in flowing water. The elevation of the
energy line is equal to the elevation of the flow line plus the depth plus the velocity head plus the pressure
head.
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Trash Rack - Racks, gratings, or mesh designed so as to prevent leaves and rubbish from plugging the
outlets from a dam or detention basin.
Trunk Line - The main line of a storm drain system extending from manhole to manhole or from manhole
to outlet structure.
Uniform Channel -A channel with a constant cross section and roughness.
Uniform Flow - Open channel flow is said to be uniform if the depth of flow is the same at every section of
the channel.
Unit Hydrograph - The direct runoff hydrograph resulting from one inch of precipitation excess distributed
uniformly over a watershed for a specified duration.
Valley Storage — Refers to the water storage capacity of a stream and is a volume that is measured below
the base flood elevation. Restrictions on loss of valley storage refer to compensation for the loss of storage
caused by fill below the base flood elevation.
Velocity Head - The energy per unit weight of water due to its velocity (v). The velocity head also represents
the vertical distance water must fall freely under gravity to reach its velocity (v). The velocity head can be
computed from:
Vel. Head = VI/2g
V = Velocity
g = acceleration due to gravity
(g = 32.2 feet per second)
WaterYear-The water year commonly used in the United States is the period from October 1 to September
30 of the following calendar year.
Watershed - The area contributing storm runoff to a stream or drainage system. Other terms are drainage
area, drainage basin and catchment area.
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Section 3.0 Design Criteria
Section 3.0 of this manual includes the criteria for addressing the key adverse impacts of development on
stormwater runoff.
Design Focus Areas
The design criteria for stormwater management include the following Focus Areas:
• Streambank Protection: Regulate discharge from the site to minimize downstream bank and channel
erosion
• Flood Mitigation and Conveyance: Control runoff within and from the site to minimize flood risk to
people and properties for the conveyance storm as well as the 1 percent chance storm.
Design Storms
Design is based on the following four (4) storm events.
Table 3.0 Storm Events
Storm Event Name
Storm Event Description
"Water Quality"
Criteria based on a volume of 1.5 inches of
rainfall, not a storm frequency
"Streambank Protection"
1 -year, 24-hour storm event
"Conveyance"
25 -year, 24-hour storm event
"Flood Mitigation"
100 -year, 24-hour storm event
Throughout the manual the storms will be referred to by their storm event names.
• For design of closed pipe systems, culverts, open channels, the City of Denton requires use of the
Flood Mitigation storm event only, unless specified otherwise in the manual.
3.1 Hydrologic Methods
3.9.9 Types of Hydrologic Methods
There are several empirical hydrologic methods available to estimate runoff characteristics for a site or
drainage sub basin. However, the following methods have been selected to support hydrologic site analysis
for the design methods and procedures included in this manual:
• Rational Method
• SCS Unit Hydrograph Method
• Snyder's Unit Hydrograph Method
• USGS & TXDOT Regression Equations
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Table 3.1 lists the hydrologic methods and the circumstances for their use in various analysis and design
applications. Table 3.2 provides some limitations on the use of several methods.
In general:
• The Rational Method is acceptable for small, highly impervious drainage areas, such as parking lots
and roadways draining into inlets and gutters.
• The U.S. Geological Survey (USGS) and Texas Department of Transportation (TXDOT) regression
equations are acceptable for drainage areas with characteristics within the ranges given for the
equations shown in Table 3.2. These equations should not be used when there are significant storage
areas within the drainage basin or where other drainage characteristics indicate general regression
equations are not appropriate.
Table 3.1 Applications of the Recommended Hydrologic Methods
Method
Rational
Method
SCS
Method
✓
Modified
Rational
Snyder's
Unit
Hydrograph
✓
USGS /
TXDOT
Equations
Streambank
Protection Volume
SPv
Flood Mitigation
Discharge (Qf)
_
✓
✓
✓
Storage Facilities
✓
✓
✓
Outlet Structures
✓
✓
Gutter Flow and Inlets
✓
✓
Storm Drain Pipes
✓
✓
✓
Culverts
✓
✓
✓
✓
Bridges
✓
✓
Small Ditches
✓
✓
✓
Open Channels
✓
✓
✓
✓
Energy Dissipation
✓
✓
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Table 3.2 Constraints on Using Recommended Hydrologic Methods
Method
Size Limitations'
Comments
Method can be used for estimating peak flows and
Rational
0 —100 acres
the design of small site or subdivision storm sewer
systems.
Method can be used for estimating runoff volumes
Modified Rationale
0 — 200 acres
Any Size
for storage design.
Unit Hydrograph (SCS)3
Method can be used for estimating peak flows and
hydrographs for all design applications.
Unit Hydrograph
1 acre and larger
Method can be used for estimating peak flows and
(Snyder' S)4
hydrographs for all design applications.
TXDOT Regression
10 to 100 mit
Method can be used for estimating peak flows for
Equations
rural design applications.
USGS Regression
3-40 mit
Method can be used for estimating peak flows for
Equations
urban design applications.
' Size limitation refers to the drainage basin for the stormwater management facility (e.g., culvert, inlet).
2 Where the Modified Rational Method is used for conceptualizing, the engineer must use the iSWMTm Hydrology
Technical Manual Document when using this method.
3This refers to SCS routing methodology included in many readily available programs (such as HEC -HMS or HEC -
1) that utilize this methodology.
4This refers to the Snyder's methodology included in many readily available programs (such as HEC -HMS or HEC -
1) that utilize this methodology.
3.9.2 Rainfall Estimation
Rainfall intensities, provided from NOAA, are based on Atlas 14 and shall be used for all hydrologic analysis
within Denton County.
https://hdsc.nws.noaa.qov/hdsc/pfds/pfds map cont.html?bkmrk=tx
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3.2 Acceptable Downstream Conditions for Open Channels and
Floodplains
The downstream impacts of development must be carefully evaluated for the two focus areas of
Streambank Protection and Flood Mitigation. The purpose of the downstream assessment is to protect
downstream properties from increased flooding and downstream channels from increased erosion potential
due to upstream development. The importance of the downstream assessment is particularly evident for
larger sites or developments that have the potential to dramatically impact downstream areas. The
cumulative effect of smaller sites, however, can be just as dramatic and, as such, following the Focus Areas
is just as important for the smaller sites as it is for the larger sites.
The assessment shall extend from the outfall of a proposed development to a point downstream where the
discharge from a proposed development no longer has a significant impact, in terms of flooding increase
or velocity above allowable, on the receiving stream or storm drainage system. The City shall be consulted
to obtain records and maps related to the National Flood Insurance Program and the availability of Flood
Insurance Studies and Flood Insurance Rate Maps (FIRMS) which will be helpful in this assessment. The
assessment must include the following properties:
• Hydrologic analysis of the pre- and post -development on-site conditions
• Drainage path that defines extent of the analysis
• Capacity analysis of all existing constraint points along the drainage path, such as existing floodplain
developments, underground storm drainage systems culverts, bridges, tributary confluences, or
channels
• Offsite undeveloped areas are considered as "full build -out' for both the pre- and post -development
analyses
• Evaluation of peak discharges and velocities for three 24-hour storm events
• Streambank protection storm
• Conveyance storm
• Flood mitigation storm
• Separate analysis for each major outfall from the proposed development
Once the analysis is complete, the designer must answer the following questions at each determined
junction downstream:
• Are the post -development discharges greater than the pre -development discharges?
• Are the post -development velocities greater than the pre -development velocities?
• Are the post -development velocities greater than the velocities allowed for the receiving system?
• Are there any increases in post -development flood heights above the pre -development flood heights?
These questions shall be answered for each of the three storm events. The answers to these questions
will determine the necessity, type, and size of non-structural and structural controls to be placed on-site or
downstream of the proposed development.
Section 2.0 of the iSWMT1 Hydrology Technical Malwal Document gives additional guidance on calculating
the discharges and velocities, as well as determining the downstream extent of the assessment.
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3.2.1 Streambank Protection
The first focus area is in streambank protection. There are two options by which a developer can provide
adequate streambank protection downstream of a proposed development. The first step is to perform the
required downstream assessment as described in Section 3.2. If it is determined that the proposed project
does not exceed acceptable downstream velocities or the downstream conditions are improved to
adequately handle the increased velocity, then no additional streambank protection is required. If on-site or
downstream improvements are required for streambank protection, easements or right -of -entry agreements
will need to be obtained in accordance with Section 3.9. If the downstream assessment shows that the
velocities are within acceptable limits, then no streambank protection is required. Acceptable limits for
velocity control are contained in Tables 3.10 and 3.11. If existing stream velocities exceed the maximum
allowable velocities, then no increase in velocities will be permitted.
Option 1: Reinforce/Stabilize Downstream Conditions
If the increased velocities are greater than the allowable velocity of the downstream receiving system, then
the developer must reinforce/stabilize the downstream conveyance system. The proposed modifications
must be designed so that the downstream system is protected from the post -development velocities. The
developer must provide supporting calculations and/or documentation that the downstream velocities do
not exceed the allowable range once the downstream modifications are installed.
Allowable bank protection methods include stone riprap, gabions, and bio -engineered methods. Sections
3.2 and 4.0 of the iSWMTm Hydraulics Technical Manual Document give design guidance for designing
stone riprap for open channels, culvert outfall protection, riprap aprons for erosion protection at outfalls,
and riprap basins for energy dissipation.
Option 2: Install Stormwater Controls to Maintain Existing Downstream Conditions
The developer must use on-site controls to keep downstream post -development discharges at or below
allowable velocity limits. The developer must provide supporting calculations and/or documentation that the
on-site controls will be designed such that downstream velocities for the three storm events (Streambank
Protection, Conveyance, and Flood Mitigation) are within an allowable range once the controls are installed.
3.2.2 Flood Mitigation
3.2.2.1 Introduction
Flood analysis is based on the flood mitigation storm event.
The intent of the flood mitigation criteria is to provide for public safety; minimize on-site and downstream
flood impacts from the flood mitigation storm event; maintain the boundaries of the mapped 100 -year
floodplain; and protect the physical integrity of the on-site stormwater controls and the downstream
stormwater and flood mitigation facilities.
Flood mitigation must be provided for on-site conveyance systems, as well as downstream outfalls as
described in the following sections.
3.2.2.2 Flood Mitigation Design Options
There are three options by which a developer may address downstream flood mitigation as discussed
below. When on-site or downstream modifications are required for downstream flood mitigation, easements
or right -of -entry agreements will need to be obtained.
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The developer will provide all supporting calculations and/or documentation to show that the existing
downstream conveyance system has capacity (Qf) to safely pass the full build -out flood mitigation storm
discharge.
Option 1: Provide Adequate Downstream Conveyance Systems
When the downstream receiving system does not have adequate capacity, then the developer shall provide
modifications to the off-site, downstream conveyance system. If this option is chosen, the proposed
modifications must be designed to adequately convey the full build -out stormwater peak discharges for the
flood mitigation storm event. The modifications must also extend to the point at which the discharge from
the proposed development no longer has an impact on the receiving stream or storm drainage system. The
developer must provide supporting calculations and/or documentation that the downstream peak discharges
are safely conveyed by the proposed system, without endangering downstream properties, structures,
bridges, roadways, or other facilities, and no increase in water surface elevation.
Option 2: Install Stormwater Controls to Maintain Existing Downstream Conditions
When the downstream receiving system does not have adequate capacity, then the developer shall provide
stormwater controls to reduce downstream flood impacts. These controls include on-site controls such as
detention, regional controls, and, as a last resort, local flood protection such as levees, floodwalls,
floodproofing, etc.
The developer must provide supporting calculations and/or documentation that the controls will be designed
and constructed so that there is no increase in downstream peak discharges or water surface elevations due
to development.
Option 3: In lieu of a Downstream Assessment, Maintain Existing On -Site Runoff
Conditions
Lastly with Option 3, on-site controls shall be used to maintain the pre -development peak discharges from
the site. The developer must provide supporting calculations and/or documentation that the on-site controls
will be designed and constructed to maintain on-site existing conditions.
It is important to note that Option 3 may not require a downstream assessment. It is a detention -based
approach to addressing downstream flood mitigation after the application of the integrated site design
practices. However, a downstream assessment may be required for sites adjacent to or near streams in
which delayed release of flows from detention facilities could potentially increase the peak flow in the stream
due to coincident peaks. This assessment of the impact of coincident peaks is required for all sites with a
contributing drainage area greater than or equal to ten percent (10%) of the stream drainage area at the
subject discharge point.
3.3 Stormwater System Design
3.3.1 Introduction
Stormwater system design is an integral component of both site and overall stormwater management
design. Good drainage design must strive to maintain compatibility and minimize interference with existing
drainage patterns; control flooding of property, structures, and roadways for design flood events; and
minimize potential environmental impacts on stormwater runoff.
Stormwater collection systems must be designed to provide adequate surface drainage while at the same
time meeting other stormwater management goals such as water quality, streambank protection, habitat
protection, and flood mitigation.
Design
Fully developed watershed conditions shall be used for determining runoff for the flood mitigation storm.
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3.3.2 Hydraulic Design Criteria for Streets and Closed Conduits
A. Introduction
This section is intended to provide criteria and guidance for the design of on-site flood mitigation
system components including:
• Street and roadway gutters
• Stormwater inlets
• Storm drainpipe systems
• Parking lot sheet flow
B. Streets and ROW
1. Design Criteria
a) Flow spread limits for curbed streets are shown in Table 3.4.
b) Inverted crown sections are permitted only in alleys.
c) Street crowns shall be reduced for approximately one hundred (100) feet on each side of valley
gutters. No valley gutters will be permitted across collectors or arterials.
d) For non -curbed streets the flood mitigation storm event shall be contained within paralleling
roadside ditches, within the public right-of-way (Figure 5-2).
e) Roadside ditches shall be designed to carry the flood -mitigation runoff below the roadway
elevation.
f) Streets or alleys adjacent to an open channel shall have the edge of the pavement designed
with an elevation of minimum of one (1) foot above Flood Mitigation elevation or as directed by
the City Engineer or designee.
g) Where additional hydraulic capacity is required on the street, the proposed street gradient must
be increased, or curb inlets and storm sewers installed to remove a portion of the flow.
h) The maximum concentrated flow directed into the street (from a driveway or flume, etc.) is 3
cfs.
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I- CURE 5-2
3.0.11,11.
FLOOD WITIGATICN SURFACE1l
f-VATIIIO'jN'
P !�
x.0."1".
FLUUU M111�3.AIIUN SU-�h ACE aLEVAII(A
WATER SPREAD LIMITS FOR
NON -CURBED BOND;"/ISYS
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2. Flow Spread Limits
Inlets shall be spaced so that the spread of flow in the street for the flood mitigation storm shall not
exceed the guidelines listed below, as measured from the gutter or face of the curb:
Table 3.4 Flow Spread Limits
Street Classification
Allowable Encroachment
Residential and Non -Residential Collectors
• one travel lane on Collectors, one traffic
and Arterials
lane must remain open each direction on
Arterials
Residential Streets
. curb depth
The allowable drainage flow across street intersections for the flood mitigation storm event shall be as
follows:
Table 3.5 Permissible Flow Across Street
Intersections
Street Classification
Cross Flow
Arterial Street (divided and undivided))
None
Non -Residential Collector Street
None
Residential Street and Residential
Gutter Flow of 2
Collector
inches or less
3. Minimum Street or Alley Elevations
No lowering of the standard height of street crown shall be allowed for the purposes of obtaining
additional hydraulic capacity. Street crowns shall be in accordance with the City of Denton Standard
Details.
C. Drainage Related Minimum Elevations
1. Lots shall have a minimum elevation for the buildable area (including parking areas) of the lot of
one (1) foot above the one percent chance (100 -year) base flood elevation (BFE), or as directed
by the City Engineer or designee.
2. Where lots are positioned on a downhill side of a steep lead-in road to a "T" or "L" intersection, or
a sharp turn in a steep alley, the portion of the lot facing toward the high-water flooding danger
area will be 6" above the top of the curb.
3. For lots in the influence of a sag area and a positive overflow, the lot elevation will be at least one
(1) foot above the sag area top of the curb, or one (1) foot above the possible maximum pool
elevation when the positive overflow is functioning, whichever elevation is higher.
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4. Where lots do not abut a natural or excavated channel, minimum floor elevations shall be a
minimum of one (1) foot above the street curb, edge of alley, or rear property line (at the midpoint
of the lot), whichever is lower, unless otherwise approved by the City Engineer or designee.
D. Stormwater Inlet Design
1. Permissible Types of Inlets
Drop Inlets (Y -inlet). Drop inlets are sump inlets which are not located along the curb line of a
roadway.
Grate Inlets. The use of grate inlets is not allowed on public drainage systems without prior consent
from the City Engineer or designee. If allowed, the inlet opening shall be designed twice as large
as the calculated opening to compensate for clogging. Grate inlets may be used on private
systems.
Curb Inlets. Curb inlets may be located at roadway low points (sumps) or on grade at such points
as to meet the water spread limitations and cross flow depth requirements. Curb inlets may be one
of the following:
Recessed curb inlet. Recessed curb inlets are curb inlets constructed such that the front of
the inlet is 2.0 feet behind the not mal face of curb and the depression does not extend into the
traffic lanes.
Standard curb inlet. Standard curb inlets are curb inlets that are in line with the roadway curb.
2. Design Criteria
a) Public curb inlet size shall be 10, 15, or 20 feet. Maximum length of inlet at any one curb
location shall be 20 feet on each side of the street. Inlets will be placed only in straight sections
of curb, and at least 10 feet from the curb return. Curb inlets are not allowed in intersection or
curb returns.
b) Recessed inlets will be required on arterial and non-residential collector streets.
c) The maximum inlet opening shall be six (6") inches. Openings larger than six inches shall
require approval by the City Engineer or designee and shall contain a bar, or other form of
restraint.
d) Inlets shall be located in the following locations:
i. At low points,
ii. Upstream of pavement crown transitions at intersections (or identify flow patterns and
depths to show these inlets are not needed),
iii. Where street flow spread limits or permissible intersection depths are exceeded.
e) Where possible, inlets at intersections shall be located on the street with the lesser
classification, or on alleys.
f) A by-pass of no more than 5% of the inlet capacity will be allowed for the flood mitigation storm
event.
g) Water flowing in gutters of arterials should be picked up prior to super -elevated sections to
prevent water flowing across the street for the flood mitigation storm event.
h) In super -elevated sections of divided arterials, inlets placed against the center medians shall
have no gutter depressions. Interior gutter flow (flow along the median) shall be intercepted at
the point of superelevation transition to prevent street cross flow.
i) At bridges with curbed approaches, water should be intercepted before flowing onto the bridge
to prevent icing during cold weather.
j) Recessed inlets shall not decrease the width of the sidewalk or interfere with utilities.
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k) Design and location of inlets shall take into consideration pedestrian and bicycle traffic.
1) The use of slotted drains is discouraged except in instances where there is no alternative and
requires approval by the City Engineer or designee. If used, the manufacturer's design
guidelines should be followed.
m) Depressed inlets are recommended on continuous grades that exceed one percent; although
the use in traffic lanes should be avoided whenever possible.
n) Positive Overflow. Positive overflow will be required for all sag or low point inlets. The excess
flow will be required to be conveyed overland along a paved course. This could mean along a
street or alley but could require the construction of a concrete flume within a drainage
easement on private property. The flume should be sized to convey a 2 -year storm, and the
easement should contain the 25 -year storm. Reasonable judgment should be used to limit the
easements on private property to a minimum. In specific cases where the chances of
substantial flood damages could occur, the City may require special investigations and
designs. The overflow elevation shall not be higher than two -inches (2") above the top of the
curb at the low point. Artificial sags created by "seesaw" of street or alley grades will not be
permitted.
3. Inlet Computations
a) Sump Inlets and Drop Inlets
Curb inlets and drop inlets in a sump or low point can be considered to function as a rectangular broad -
crested weir with a coefficient of discharge of 3.06. The capacity shall be based on the following
weir equation:
Q/L or Q/P = 3.06 HI
Q = Capacity in cfs. of curb opening inlet or capacity in cfs. of drop
inlet
H = Head at the inlet in feet
L = Length of curb opening inlet in feet; or
P = Length of portion of perimeter of inlet opening which water
enters the drop inlet in feet
Inlets should be located such that the inlet openings do not become submerged. In some cases where
this is not possible, and the inlet operates under completely submerged conditions, the orifice
equation should be used to compute the inlet capacity rather than the weir formula. The capacity
of a completely submerged inlet shall be based on the following orifice equation:
Q=4.84AH1/2
A =Area of inlet opening
The curves shown in Figures 6-2 and 6-3 provide for direct solution of the above equations.
In order to facilitate the computations required in determining the various hydraulic properties for curb
inlets and drop inlets in sump conditions, Computation Sheet 6-1 has been prepared.
Column 1 Inlet number and designation.
Column 2 Total flow in cfs to inlet. For inlets other than the first inlet in a system, flow is
the sum of runoff from contributing area plus carry-over flow from inlet or
inlets upstream.
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Column 3
Assumed length of inlet opening or perimeter in feet.
Column 4
Total area of inlet opening based on assumed inlet
opening length and opening height.
Column 5
Discharge per unit foot of inlet opening.
Column 2 divided by Column 3.
Column 6
Computed head at inlet for weir flow conditions based
on Figures 6-2 or 6-3 or the following equation:
H = (q/3)1
Column 7
Computed head at inlet for orifice flow conditions
(submerged inlet) based on Figures 6-2 or 6-3 or the
following equation:
H = [(Q/A)/4.82]2
Column 8 Maximum allowable head at sump inlet. This value is determined from
topographic conditions at the sump
inlet site.
Column 9 Width of spread of water for curb inlets in sump. Use Figure 5-3 or 5-4, or
Appendix B to determine Sp for roadways.
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10.0
8.0
6.0
5.0
4.0
3.0
2.0
w
w
w
z_
1.0
0 0.8
a
w
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0.5
0.4
0.3
0.2
0.1
0
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O b Q
D Q-
VJ
a
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A = NET AREA
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� O
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0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 2 3 4 5 6 8 10 20
DISCHARGE IN CFS PER FT OF PERIMETER (q L) DISCHARGE IN CFS PER SF OF AREA (Q/A)
HEADS UP TO 0.4 USE CURVE (a)
HEADS ABOVE 1.4 USE CURVE (b)
AT HEADS BETWEEN 0.4 AND 1.4. TRANSITION SECTOR
AND OPERATION ARE INDEFINITE
CAPACITY OF GRATE INLET IN SUMP
FIGURE 6-2
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10.0
8.0
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6.0
5.0
4.0
3.0
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HEAD IN FEET (H)
CAPACITY OF DROP INLETS AND
CURB INLETS IN SUMPS FIGURE 6-3
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Y =Depth of Flow
a = Gutter Drop
h =Th
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roat Opening
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SHEET 6-1
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Stormwater Design Criteria Manual
b) Inlets on Grade
The capacity of a curb inlet on grade will be based on the following equation'.
Q/Lo = 0.7[1/yo] [(H) 5/2_(a)512]
Where:
Q = Discharge into inlet in cfs.
Lo = Length of inlet opening in feet
H=a+yo
a = Gutter depression in feet
y, = Depth of flow in approach gutter in feet
The curve shown in Figure 6-4 provides for the direct solution of the above equation when the
value of yo is known. The curve shown in Figure 6-5 provides for the determination of the ratio of
the intercepted flow by the inlet to the total flow in the gutter.
In order to facilitate the computations required in determining the various hydraulic properties for curb
inlets on grade, Computation Sheet 6-2 has been prepared.
Column 1
Inlet Type and number.
Column 2
Location of inlet by station number.
Column 3
Drainage Area designation of area entering between the
previous pick up point and the inlet being designed.
Column 4
Peak Discharge (QP) from area of Column 3.
Column 5
Carry-over flow (q) which has been passed by the last
preceding inlet to the inlet under consideration.
Column 6
Total gutter flow (Qo) in cfs. For inlets other than the
first inlet in the system, total gutter flow is the sum of the
runoff from the contributing area plus carry-over flow
from the inlet or inlets upstream. Column 4 plus
Column 5.
Column 7
Reciprocal of the pavement cross slope for pavements
with straight crown slopes.
Column 8 Reciprocal of the pavement cross slope (Z) divided by
the pavement roughness coefficient (n)
Column 9 Slope of approach gutter (So) in ft. per ft.
Column 10 Depth of gutter flow " yo " in approach gutter from
Figure 5-3. Figure 5-4. or Appendix B solution or direct from Manning's
equation for triangular gutters:
yo = 1.245 (Q 318)[n 318/S3116] [1/Z]3/8
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Column 11 Spread of water (SP) or width of ponding in the gutter measured from the
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face of curb. Column 7 times Column 10 (Figure 5-3 or Appendix B).
Column 12
Width of street and height of parabolic crown.
Column 13
Slope of approach gutter (So) in ft. per ft.
Column 14
Depth of gutter flow " yo " in approach gutter from Figure 5-4 or Appendix B.
Column 15 Spread of water (Sp) or width of ponding in the gutter
measured from face of curb from Figure 5-4 or Appendix B.
Column 16
Discharge in cubic feet per second (Q) which will be intercepted by an inlet
one foot in length for a given depth of flow in the approach gutter (yo).
Determined from Figure 6-4 or from the solution of the following equation:
Q/Lo = 0.7[1/yo] [(H)s'2-(a)si2]
Column 17 Length of inlet (Lo) in feet which is necessary to intercept a
given discharge Qo. Column 6 divided by Column 16.
Column 18
Actual length (L) in feet of inlet which is to be provided.
Column 19
Ratio of the length of inlet provided (L), to the length of the inlet required for
100% interception (Lo). Column 18 divided by Column 17.
Column 20
Percentage of discharge intercepted by the inlet in question determined from
Figure 6-5 using the values determined in Column 19 and Column 10 or
Column 14.
Column 21
Discharge (Q) in cubic feet per second which the inlet in question actually
intercepts. Column 6 times Column 20.
Column 22
Carry-over flow (q) is the amount of water which passes any inlet, and is the
difference between the total flow (Qo) of Column 6 and the intercepted flow
(Q) of Column 21.
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J
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E. Storm Drain Pipe Design
1. Design Frequency
Pipe Design: flood mitigation storm event
2. Design Criteria
a) Storm drain systems capable of conveying the flood mitigation storm event are required when
water spread, intersection cross flow, and lot to lot drainage flow limits are exceeded, or when
the minimum time of concentration shown in Table 1.5 of the iSWMTm Hydrology Technical
Manual Document are reached. Closed pipe systems are required for discharges up to and
including 300 cfs in public systems.
b) Pipe material in a public storm drain system or in public right-of-way shall be reinforced
concrete for all pipe sizes with appropriate bedding and class type depending on cover.
c) Proposed storm drains may discharge into existing watercourses. See Section 1.2.10 of the
iSWMTM. Hydraulics Technical Manual Document for guidance related to the Tailwater
elevation to be used for hydraulic grade line calculations.
d) The maximum hydraulic gradient shall not produce a velocity that exceeds 15 feet per second
(fps). Table 3.8 shows the desirable velocities for most storm drainage design. Storm drains
shall be designed to have a minimum mean velocity flowing full at 2.5 fps.
Table 3.8 Desirable Velocity in Storm Drains
Description
Maximum Desirable Velocity
Culverts (All types)
15 fps
Inlet laterals
No Limit
Collectors (up to 24")
15 fps
Mains (Larger than 24")
12 fps
e) The minimum desirable physical slope shall be 0.5% or the slope that will produce a velocity
of 2.5 feet per second, as required, for the Streambank Protection Storm when the culvert is
flowing partially full, whichever is greater.
f) The potential hydraulic grade line elevation shall not exceed ground elevation or the gutter flow
line, whichever is lowest.
g) Access junction boxes are required at intermediate points along straight runs of closed
conduits. Table 3.9 gives maximum spacing criteria.
Table 3.9 Access Junction Sox Spacing Criteria
Pipe Size (inches) Maximum Spacing (feet)
18-36 600
42" and Larger 1000
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h) Junction Boxes shall also be located at:
• pickup points having two (2) or more curb inlets or laterals at the same relative point of a
street or alley;
• trunk line size changes for pipes greater than twenty-four (24) inches; pipejunctions having
any pipe 36" and larger;
• grade changes;
• the upstream end of the storm drain system;
• Bends greater than 30 degrees and pipe junctions greater than 45 degrees;
• At the connection point of public and private storm sewer pipes or boxes. If this connection
point is in the public right-of-way, pipe materials on the private system must meet public
storm drain system materials requirements. If the connection point is on private property,
then the public portion of the system must be contained in an easement. The connection
on private property may be at an inlet rather than a manhole.
Generally, inlets will not be allowed to serve as a junction box. Under special circumstances
the City Engineer or designee may allow an inlet to serve as a junction box. Where inlets serve
as a junction box, the width of a new standard inlet, at a minimum, shall be doubled in size.
Storm drain systems parallel to the street will not be permitted to run directly through inlets.
j) No 90 degree bends will be allowed in storm sewer pipes or box culverts. Bends shall be 30
degrees or less. Pipe junctions shall not be greater than 45 degrees.
k) The minimum storm drain pipe diameter shall be eighteen (18) inches.
1) Pipe diameters shall not normally decrease downstream.
m) Manufactured Pipe inverts at change in sizes should be set at the same elevation.
n) A prefabricated eccentric reducer may be used when pipe size changes are required on trunk
lines between 18 -inch and 24 -inch diameter pipes. Cast -in-place pipe collars are not allowed.
o) Laterals shall be connected to collector or main lines using manholes or manufactured wye
connections. Special situations may require laterals to be connected to the trunk lines by a cut-
in (punch -in), and such cut -ins must be approved by the City Engineer or designee.
p) Vertical curves in the conduit will not be permitted and horizontal curves will be permitted only
in specific cases with the approval of the City Engineer or designee. If permitted, radius pipe
shall be installed for curves with radii between 100 -ft and 500 -ft.
F. Parking Lot Design
Parking lots shall be designed for the flood mitigation storm not to exceed top of curb with a maximum
depth at low points of one (1) foot. The flood mitigation storm shall be contained on-site or within
dedicated easements.
G. Driveway Culverts.
Driveway culverts are only permitted in non -curbed roadway sections. All driveway culvert construction
shall be inspected by the City during construction. Public Works Inspection Department will not
approve the driveway approach formwork for concrete pours until the driveway culvert, ditch survey,
and material invoice (Aluminized Type II only) have been reviewed and accepted by the Drainage
Department. All driveway culverts shall meet the following requirements:
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1. All driveway culverts will be designed to convey the flood mitigation storm.
2. Culverts shall have a minimum pipe diameter of 18 inches
3. Acceptable Culvert Material shall be Reinforced Concrete Pipe (RCP) (Class III minimum) or
Aluminized Type II Corrugated Metal Pipe (CMP) (18" to 42" diameter)
4. Pipes shall be jointed together properly by materials approved by the City. Box culverts shall have
a minimum height of 24". Culverts under City streets used for entrances to a subdivision shall be
made of approved classes of reinforced concrete pipe or box.
5. The top of pipe elevation must be at or below the adjacent roadway edge of pavement elevation.
6. Pipe culverts shall utilize a safety end treatment conforming to the most current version of Texas
Department of Transportation (TxDOT) standard detail SETP-PD with the slope of the rip -rap being
6:1.
7. Low Water Crossings may be allowed in lieu of culverts in low flow areas and high points when a
18' pipe does not meet the design intent. (as approved by the City Engineer).
1. Must have the lowest elevation at least 8 inches below the edge of pavement elevation.
2. Minimum cross slope of 1.5%
3. Toe walls at both ends shall be extended at least 15" below grade to prevent undercutting.
8. Culverts Slope requirements:
i. Must provide positive drainage
ii. Culvert slope shall be set as shown on the approved subdivision construction plans
iii. Minimum slope shall be 0.3%
9. Ditch Slope requirements:
i. The ditch shall be graded upstream and downstream as far as necessary to provide
positive drainage with no areas of standing water.
ii. Minimum earthen slope is 0.5%
iii. Minimum concrete slope is 0.3% for 2 -foot concrete pilot channels if 0.5% earthen is
unobtainable.
3.3.3 Hydraulic Design Criteria for Structures
A. Introduction
This section is intended to provide design criteria and guidance on several on-site flood mitigation
system components, including culverts, bridges, vegetated and lined open channels, storage design,
outlet structures, and energy dissipation devices for outlet protection.
B. Open Channels
1. Design Frequency
a) Open channels, including all natural or improved channels, swales, and ditches shall be
designed for the flood mitigation storm event
2. Design Criteria
a) Depending on velocities (See Table 3.11), constructed or improved channels shall be designed
with either an earthen channel or a 10 -ft minimum concrete pilot channel section and
appropriate side slope protection up to the streambank protection elevation, as described in
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Section 3.2.1.
b) All channels with contributing drainage basins larger than one square mile shall remain in their
natural condition.
c) Channels with a contributing drainage area of less than one square mile shall remain in their
natural condition if they are identified as being within an Environmentally Sensitive Area (ESA)
by the current City of Denton ESA map. Channels not identified as being within an ESA may
be channelized, with the channelization method being determined by analysis of the erosive
velocities.
d) All improved channels shall be designed to carry the flood mitigation flow and shall have one
foot of freeboard as illustrated in Figure 8-1. Freeboard requirements at bends in all improved
channels shall be the greater of the following:
1. One (1) foot or
2. Ten (10) percent of the flow depth
e) At a minimum, channels that require concrete lining shall be lined up to an elevation of the
water surface resulting from the flood mitigation storm.
f) Unlined improved channels that contain bends may be required to be armored if maximum
permissible velocities are exceeded.
g) Unlined improved channels shall have side slopes no steeper than 4:1 and concrete lined
channels shall have side slopes no steeper than 2:1.
h) The minimum grade allowed on any channel, outfall channel or ditch shall be three -tenths foot
per one hundred (100) feet for concrete lined channels and five -tenths foot per one hundred
(100) feet for grass lined channels.
i) Geotechnical investigations will be required for open channel designs, except in cases where
the City Engineer or designee deems it not necessary.
j) For vegetative channels, flow velocities within the channel shall not exceed the maximum
permissible velocities given in Tables 3.10 and 3.11.
k) If relocation of a stream channel is unavoidable, the cross-sectional shape, meander, pattern,
roughness, sediment transport, and slope shall conform to the existing conditions insofar as
practicable. Energy dissipation will be necessary when existing conditions cannot be
duplicated.
1) Streambank stabilization shall be provided, when appropriate, as a result of any stream
disturbance such as encroachment and shall include both upstream and downstream banks
as well as the local site.
m) HEC -RAS, or similarly capable software approved by the entity with jurisdiction, shall be used
to confirm the water surface profiles in open channels.
n) The final design of artificial open channels shall be consistent with the velocity limitations for
the selected channel lining. Maximum velocity values for selected lining categories are
presented in Table 3.10.
o) Seeding and mulch shall only be applied with use of erosion control blanket when the design
velocity exceeds the allowable velocity for bare soil. Velocity limitations for vegetative linings
are reported in Table 3.10. Vegetative lining calculations and stone riprap procedures are
presented in Section 3.2 of the iSWMTM Hydraulics Technical Manual Document.
p) If relocation of a stream channel is unavoidable, fill material into Waters of the United States
must comply with Section 404 of the Clean Water Act. Provide either proof of mitigation or
letter of permission from the United States Army Corps of Engineers.
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The design of stable rock riprap lining depends on the intersection of the velocity (local boundary shear)
and the size and gradation of the riprap material. More information on calculating acceptable riprap velocity
limits is available in Section 3.2.7 of the iSWMTm Hydraulics Technical Manual Document.
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Table 3.10 Roughness Coefficients (Manning's n) and Allowable Velocities for Natural
Channels
Max. Permissible
Channel Description
Manning's n
Channel Velocity
(ft/s)
MINOR NATURAL STREAMS
Fairly regular section
1. Some grass and weeds, little or no brush
0.0:30
3 to 6
2. Dense growth of weeds, depth of flow materially
greater than weed height
0.035
3 to 6
3. Some weeds, light brush on banks
0.035
3 to 6
4. Some weeds, heavy brush on banks
0.050
3 to 6
5. Some weeds, dense willows on banks
0.060
3 to 6
For trees within channels with branches submerged at high
stage, increase above values by
0.010
Irregular section with pools, slight channel meander,
increase above values by
0.010
Floodplain — Pasture
1. Short grass
0.030
3 to 6
2. Tall grass
0.035
3 to 6
Floodplain — Cultivated Areas
1. No crop
0.030
3 to 6
2. Mature row crops
0.035
3 to 6
3. Mature field crops
0.040
3 to 6
Floodplain — Uncleared
1. Heavy weeds scattered brush
0.050
3 to 6
2. Wooded
0.120
3 to 6
MAJOR NATURAL STREAMS
Roughness coefficient is usually less than for minor streams
of similar description on account of less effective resistance
Range from
offered by irregular banks or vegetation on banks. Values of
0.028 to
3 to 6
"n" for larger streams of mostly regular sections, with no
0.060
boulders or brush
UNLINED VEGETATED CHANNELS
Clays (Bermuda Grass)
0-035
5 to 6
Sandy and Silty Soils (Bermuda Grass)
0.035
3 to 5
UNLINED NON -VEGETATED CHANNELS
Sandy Soils
0.030
1.5 to 2.5
Silts
0.030
0.7 to 1.5
Sandy Silts
0.030
2.5 to 3.0
Clays
0.030
3.0 to 5.0
Coarse Gravels
0.030
5.0 to 6.0
Shale
0.030
6.0 to 10.0
Rock
0.025
15
For natural channels with specific vegetation type, refer to Table 3.11 for more detailed velocity control.
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Table 3.11 Maximum Velocities for Vegetative Channel Linings
Vegetation Type
Slope Range
Maximum Velocity' (ft/s)
Bermuda grass
0-5
6
Bahia
4
Tall fescue grass mixtures3
0-10
4
Kentucky bluegrass
0-5
6
Buffalo grass
5-10
>10
5
4
Grass mixture
0-51
5-10
4
3
Sericea lespedeza, Weeping
lovegrass, Alfalfa
0-54
3
Annuals'
0-5
3
Sod
_
4
Lapped sod
5
' Do not use on slopes steeper than 10% except for side -slope in combination channel.
2 Use velocities exceeding 5 ft/s only where good stands can be maintained.
3 Mixtures of Tall Fescue, Bahia, and/or Bermuda
4 Do not use on slopes steeper than 5% except for side -slope in combination channel.
'Annuals - used on mild slopes or as temporary protection until permanent covers are established.
Source: Manual for Erosion and Sediment Control in Georgia, 1996.
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DRAINAGE EASEMENT
FLOOD MITIGATION FLOW
1,0 FOOT FREEBOARD
FLAT BCJTTOM
1
KI
2 MIN I - FEET I
{ MIN
2 {MIN}
TYPICAL IMPROVED CHANNEL WITH CONCRETE LINING
DRAINAGE EASE MI N
F 1.0 FOOT FREEBOARD
1
\—FLOOD MITIGATION FLOW
1 4 (MIN)
10 FEET
4 (MIN) MIN � $TREAMBANK
PROTECTION FLOW
STREAM1'�3IANK E U
�&%&S COSNOIRED IF ERGSIVL.
TYPICAL IMPROVED CHANNEL WITH PILOT CHANNEL, LINING C R
FREEBOARD REOUIREMLNlS AND
CHANNEL SECTION ILLUSTRATIONS
mcipla2=.7=
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3.3.4 Channel Drop Structures
1. Sloping channel drops are permitted and are required to have a maximum slope of 4:1. Vertical channel
drops are not permitted.
2. The flow velocities in the channel upstream and downstream of the drop structure need to satisfy the
permissible velocities allowed for channels (Table 3.10). The velocities shall be checked for flows
produced by the streambank protection and flood mitigation frequency events.
3. An apron shall be constructed immediately upstream of the chute or stilling basin to protect against the
increasing velocities and turbulence which result as the water approaches the drop structure. The apron
shall extend at least five (5) feet upstream of the point where flow becomes supercritical. In no case
shall the length of the upstream apron be less than ten (10) feet.
a. An apron shall be constructed immediately downstream of the chute or stilling basin to protect against
erosion due to the occurrence of the hydraulic jump. The apron shall extend a minimum of ten (10) feet
beyond the anticipated location of the jump.
5. The design of drop structures is based on the height of the drop, the normal depths upstream and
downstream of the drop structure and discharge.
6. When used, channel drop structures shall be located near bridges or culverts, as directed by the City
Engineer or designee.
7. The location of the hydraulic jump should be determined based on the upstream and downstream flow
depths and channel slopes.
8. The length of the hydraulic jump should be calculated to determine the length of the downstream apron
required to prevent erosion.
3.3.5 Maintenance Access
1. Access areas and ramps shall be provided for all improved channels to allow for maintenance of the
channels. Access areas shall have a width of at least fifteen (15) feet and a cross slope no greater than
2%.
2. Access easements shall be provided from the Public Right of Way to the Access area if it is not directly
connected.
3. Access to all improved channels shall be provided by one (1) of the following methods, as approved
by the City Engineer or designee.
a) By providing a combination of the bottom access and clear access on one (1) side of the channel,
if the depth of the channel will allow maintenance from the top of the channel.
b) For channels exceeding a depth of four (4) feet or four (4) to one (1) side slopes, clear access
in an easement shall be provided on both sides of the channel where none of the other
methods would be sufficient to provide for maintenance of the channel access area.
4. All lined channels, and earthen channels with concrete pilot channels shall have a minimum bottom
width often (10) feet and shall be provided with concrete access ramps located as directed by
the City Engineer or designee. Concrete access ramps shall not be less than twelve (12) feet wide,
with a maximum slope of six (6) to one (1) and maximum cross slope of 5%. All access roads
shall be located within a dedicated easement.
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3.4 Cu Iverts
Culverts are cross drainage facilities that transport runoff under roadways or other improved areas.
A. Design Frequency
1. Culverts shall be designed for the flood mitigation storm. Consideration when designing culverts
includes: roadway type, tailwater or depth of flow, structures, and property subject to flooding,
emergency access, and road replacement costs
2. The flood mitigation storm shall be routed through all culverts to be sure building structures (e.g.,
houses, commercial buildings) are not flooded or increased damage does not occur to the highway
or adjacent property for this design event.
B. Design Criteria
1. Velocity Limitations
a) The maximum velocity shall be consistent with channel stability requirements at the culvert
outlet.
b) The maximum allowable velocity is 15 feet per second, but outlet protection shall be provided
where discharge velocities will cause erosion conditions.
c) To ensure self-cleaning during partial depth flow, a minimum velocity of 2.5 feet per second is
required for the streambank protection storm when the culvert is flowing partially full.
C. Headwater Limitations
1. The allowable headwater is the depth of water that can be ponded at the upstream end of the
culvert during the flood mitigation storm event, which will be limited by one or more of the following
constraints or conditions:
a) Headwater will be non -damaging to upstream property.
b) Culvert headwater plus 12 inches of freeboard shall not exceed top of curb or pavement for
low point of road over culvert, whichever is lower.
D. Tailwater Considerations
1. If the culvert outlet is operating with a free outfall, the critical depth and equivalent hydraulic grade
line shall be determined.
2. For culverts that discharge to an open channel, the stage -discharge curve for the channel must be
determined. See Section 2.1.4 of the iSWMTm Hydraulics Technical Manual Document on methods
to determine a stage -discharge curve.
3. If an upstream culvert outlet is located near a downstream culvert inlet, the headwater elevation of
the downstream culvert will establish the design tailwater depth for the upstream culvert.
4. If the culvert discharges to a lake, pond, or other major water body, the expected flood mitigation
storm event of the water body will establish the culvert tailwater.
E. Other Criteria
1. Culvert skews shall not exceed 30 degrees as measured from a line perpendicular to the roadway
centerline without approval.
2. Reinforced concrete pipe shall be required for culverts.
3. Erosion, sediment control, and velocity dissipation shall be designed in accordance with Section
4.0 of the iSWMTm Hydraulics Technical Manual Document.
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3.5 Bridges
A. Design Frequency
1. Flood mitigation storm for all bridges
B. Design Criteria
1. A freeboard of two -foot shall be maintained between the computed design water surface and the
low chord of all bridges.
2. Design guidance is in Section 3.4 of the iSWMTm Hydraulics Technical Manual Document.
3.6 Detention Facilities
A. Design Frequency
1. Detention facilities shall be designed for the four storms (water quality, streambank protection,
conveyance, and flood mitigation storms) for the critical storm duration that results in the maximum
(or near maximum) peak flow.
B. Design Criteria
1. Dry detention basins are sized to temporarily store the volume of runoff required to provide flood
protection up to the flood mitigation storm.
2. Routing calculations must be used to demonstrate that the storage volume and outlet structure
configuration complies with Section 3.2.2.2 of the Stormwater Design Criteria Manual. Outlet
structures shall be designed per Section 2.2 of the Hydraulics Technical Manual. Refer to Section
2.2.3 of the Hydraulics Technical Manual for design of extended detention (for Water Quality)
outlets.
3. Private Detention Basins shall be designed with a 10 -foot wide unobstructed maintenance access
around the entire perimeter of the pond.
4. Public Detention Basins shall be designed with a 20 -foot wide unobstructed maintenance access
around the entire perimeter of the pond.
5. No earthen (grassed) embankment slopes shall exceed 4:1. Concrete lined embankment slopes
shall not exceed 2:1 slope. Vertical walls may be allowed but must be structurally designed to
account for inundation of the base and drawdown upon pond draining and must have a six-foot
security fence at the top.
6. The side slope for any excavated detention basin, which is not in rock shall not exceed a 4:1 slope.
7. A freeboard of 1 -foot will be required between the flood mitigation storm water surface elevation
and top of bank.
8. A calculation summary shall be provided on construction plans as found on computation sheet 10-
1 or equivalent. Stage -storage -discharge values shall be tabulated and flow calculations for
discharge structures shall be shown on the construction plans. Detention design shall follow
iSWMTM guidelines. It is the responsibility of the Engineer of Record to use appropriate
methodologies presented in iSWMTm based on specific basin characteristics. Detailed calculations
and a design narrative shall be provided for review in a supplemental report that is referenced on
the construction plans. In general, the narrative shall provide basic design information, such as
hydrologic method applied, design assumptions, pre- and post -development site conditions,
downstream constraints, environmental considerations and design software version used, if
applicable.
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9. An emergency spillway shall be provided at the flood mitigation maximum storage elevation with
sufficient capacity to convey the flood mitigation storm inflow rates with six inches of freeboard.
Spillway requirements must also meet all appropriate state and Federal criteria.
10. The emergency spillway shall be constructed of concrete, unless the City Engineer or designee
approves alternative materials.
11. All detention basins shall be stabilized against significant erosion. Design guidance is in Section
1.45 of the iSWMTm Landscape Technical Manual Document.
12. Design calculations will be provided for all spillways and outlet structures.
13. Storage and dam safety design may be subject to the requirements of the Texas Dam Safety
Program based on the volume, dam height, and level of hazard. Earthen embankments 6 feet in
height or greater shall be designed per Texas Commission on Environmental Quality guidelines for
dam safety (see the Texas Administrative code, Title 30, Part 1, Chapter 299 Dams and Reservoirs
for current dam safety criteria).
14. Riprap-protected slopes shall be no steeper than 2:1.
15. The embankment crown width shall be at least 12 -feet wide for public detention ponds and 8 -ft
wide for private detention ponds and shall be determined based on a geotechnical investigation of
the detention facility site.
16. Earthen embankments used to impound detention water must have a non -permeable core and
shall be based on a geotechnical investigation of the site. The geotechnical investigation shall be
performed by a licensed engineer and shall include at a minimum the type of material on-site (or
other material to be used in the embankment), moisture content, liquid limit, plasticity index, and
required compaction.
17. Where deemed necessary by the City Engineer or designee, security fencing with a minimum
height of 6 feet shall encompass the detention storage area if the velocity, depth, or slopes create
a potentially dangerous condition. The fence shall be designed to allow access for maintenance
and so as not to restrict stormwater flow into or out of the detention basin. Unless approved by the
City Engineer or designee, a maintenance equipment access ramp shall be provided for all
detention facilities. The slope of the ramp shall not exceed 6:1 and the minimum width shall be 12
feet.
18. Bottom slopes should not be less than one (1 %) percent.
19. Concrete pilot channel at least ten (10) feet wide, and a minimum slope of 0 5 percent shall be
constructed in the bottom of the detention pond. Privately maintained ponds shall have a concrete
pilot channel with a minimum width of six (6) feet.
20. Limited recreational equipment (such as picnic tables or playground equipment) and trees may be
permitted in private detention facilities with the following restrictions:
a) Provide user access at a maximum 10% slope in at least two locations (or one location that
comprises not less than 20% of the perimeter of the facility);
b) No recreational equipment is permitted in any portion of the facility that lies more than 18"
below the flood mitigation level of the facility;
c) Recreational structures such as picnic tables, playground equipment, etc. must be rust
resistant, and anchored to the ground;
d) Mulch, wood chips, gravel or rubberized pellets will not be permitted within the detention facility
due to the likelihood of floating into the outlet structure;
e) If recreational equipment is installed in the detention pond, the area designated for recreational
use must be delineated with a post & cable fence (or other means as approved by the City
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Engineer), with signs warning users of the danger of proceeding beyond the allowable limits.
No chain link fence will be allowed in the detention pond;
f) Trees, shrubs and other woody vegetation will not be permitted in the embankment of any
detention facility, nor within the maintenance access area around the facility;
g) Isolated trees, a maximum of one per 5,600 square feet, may be permitted in the recreational
area of the pond as described above. A trash rack will be necessary to prevent clogging of
the outlet structure. No bark mulch will be permitted around the trees.
21. All private detention basins shall require an Operation and Maintenance Manual submitted at time
of building permit submittal. Private detention basins shall be inspected by Public Works Inspection
to ensure conformity to the design standards required by Section 7.5.3.F of the Denton
Development Code (DDC). Current inspection fees will apply.
22. Use of parking lot surface area as detention is not permitted.
3.6.1 Outlet Structures for Detention Facilities
A. Design Frequency
1. Water Quality
2. Streambank protection storm
3. Conveyance storm
4. Flood mitigation storm
B. Design Criteria
1. Estimate the required storage volumes for water quality, streambank protection, conveyance storm,
and flood mitigation.
2. Outlet velocities shall be within the maximum allowable range based on channel material as shown
in Tables 3.10 and 3.11.
3. Design necessary outlet protection and energy dissipation facilities to avoid erosion problems
downstream from outlet devices and emergency spillway(s).
4. Perform buoyancy calculations for the outlet structure and footing. Flotation will occur when the
weight of the structure is less than or equal to the buoyant force exerted by the water.
5. Any outflow structure, which conveys water through the embankment in a conduit shall be
reinforced concrete, designed to support the external loads. The conduit shall withstand the
internal hydraulic pressure without leakage under full external load or settlement and must convey
water at the design velocity without damage to the interior surface of the conduit.
6. The minimum pipe size and box size shall meet the following requirements (these minimum sizes
apply even when used in conjunction with weirs or other flow control devices, and must be
accessible for maintenance):
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7. Minimum opening of inlet shall be 6 inches in diameter or 6" x 6" square. Smaller inlet openings
may be used with junction box and properly sized outlet structure.
8. Design guidance is in Section 2.2 of the iSWMTm Hydraulics Technical Manual Document.
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Outlet .- and Box Size
Discharge Pipe Length
Minimum Pipe
Size
Minimum Box
Size
100 feet or less
18 inches
2 feet x 2 feet
Greater than 100 feet
Winches
3 feet x 3 feet
7. Minimum opening of inlet shall be 6 inches in diameter or 6" x 6" square. Smaller inlet openings
may be used with junction box and properly sized outlet structure.
8. Design guidance is in Section 2.2 of the iSWMTm Hydraulics Technical Manual Document.
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3.7 Energy Dissipation
All drainage system outlets, whether for closed conduits, culverts, bridges, open channels, or storage facilities,
shall provide energy dissipation to protect the receiving drainage element from erosion.
A. Design Frequency
1. Flood mitigation storm
B. Design Criteria
1. Energy dissipaters are engineered devices such as rip -rap aprons or concrete baffles placed at the
outlet of storm water conveyance systems for the purpose of reducing the velocity, energy and
turbulence of the discharged flow.
2. Erosion problems at culvert, pipe and engineered channel outlets are common. Determination of
the flow conditions, scour potential, and channel erosion resistance shall be standard procedure
for all designs.
3. Energy dissipaters shall be employed whenever the velocity of flows leaving a stormwater
management facility exceeds the erosion velocity of the downstream area channel system.
4. Energy dissipater designs will vary based on discharge specifics and tailwater conditions.
5. Outlet structures shall provide uniform redistribution or spreading of the flow without excessive
separation and turbulence.
C. Recommended Energy Dissipaters for outlet protection include the following:
1. Concrete or grouted rock rip -rap apron
2. Riprap outlet basins
3. Baffled outlets
4. Grade Control Structures
Design guidance is in Section 4.0 of the iSWMTm Hydraulics Technical Manual Document.
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3.8 Floodplain
3.8.1 Floodplain Development Criteria
The following decision chart is intended to consolidate the floodplain development criteria in the City
of Denton. It references information found in the Denton Development Code (DDC) Subchapters
7.4 and 7.5, and in the Floodplain Development Ordinance, and this Criteria Manual. It is not to be
considered an exhaustive list of criteria but is only to be used as guidance to the information
provided in the above -referenced documents. Criteria in those documents supersedes this decision
chart.
Greater Than or Equal To
One Square Mile
F - -
Stream
Developed Habitat
Condition
(DDC 7.4)
Hardship Variance
Required for
Floodplain Alteration
• No Rise in Fully
Developed WSEL
• No Floodway
Alteration
• No Valley Storage
Loss
• No Adverse Impact
• Comply with ESA
Riparian and/or
Water Related
Regulations
• FEMA
CLOMR/LOMR
Proposed
Project
What is the
Size of the
Drainage
Area?
Less Than One
Square Mile
�-- Stream
Undeveloped Developed Habitat Undeveloped
Condition
(DDC 7.4)
Alternate ESA and
Floodplain Alteration
Alternate ESA
Hardship Variance
Allowed
Required for
Required for
Floodplain Alteration
Floodplain Alteration
• Set Fully Developed
BFE (No Rise)
Set Fully Developed
• No Rise in Fully
• Establish Floodplain
BFE (No Rise)
Developed WSEL
• Establish Floodway
• Establish Floodplain
• Minor Fill (50 CY)
• Maximum 15%
• Establish Floodway
• No Floodway
Valley Storage Loss
• Maximum 15%
Alteration
• No Adverse Impact
Valley Storage Loss
• No Valley Storage
• Comply with ESA
• No Adverse Impact
Loss
Riparian and/or
• Comply with ESA
• No Adverse Impact
Water Related
Riparian and/or
• Comply with ESA
Regulations
Water Related
Riparian and/or
• CLOMR/LOMR or
Regulations
Water Related
Flood Studv
• CLOMR/LOMB OR
Regulations
Flood Study
• FEMA
CLO M R/LOM R
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Stormwater Design Criteria Manual
3.8.2 Procedures for Floodplain Alteration.
Fill and alteration of floodplains, containing drainage areas one (1) square mile or less, when it
is not unreasonably damaging to the environment, is permitted where it will not create other flood
problems. The following are the engineering criteria for such requests.
A. FEMA Submittal.
Developments which impact designated Federal Emergency Management Agency (FEMA) flood
plains in the City (Zones A E , A, X shaded) will be required to submit the minimum data
which shall be sent to FEMA for conditional approval of the proposed project. The Conditional
Letter of Map Revision (CLOMR) shall be submitted to the City prior to approval of any preliminary
plat. Approval of (CLOMR) from FEMA will be required prior to acceptance of a final plat.
1. A written description of the scope of the proposed project and the methodology used to analyze
the project's effects.
2. Hydraulic backwater models of the 10, 50, 100, and 500 -year flood for the following:
a) Duplicate of the effective Flood Insurance Study (FIS) model;
Existing conditions (effective FIS model including cross-sections through the project
site. All cross-sections should reflect conditions prior to construction of the project);
Proposed conditions (existing conditions model reflecting the proposed project); and
3. Floodway hydraulic backwater models of the following:
a) Duplicate effective;
a) Existing condition; and
b) Proposed conditions.
4. A copy of the Flood Insurance Rate Map with the project area indicated.
5. Topographic mapping of the entire area covered by the proposed condition model, indicating the
locations of all cross-sections used in the hydraulic model and delineating the proposed 100 -
year flood plain boundary.
6. Topographic mapping of the entire area covered by the proposed conditions model, indicating
the locations of all cross-sections used in the hydraulic model and delineating:
a) The proposed 100 -year and 500 -year floodplain boundaries; and
b) The proposed floodway boundary.
7. Certification that the project meets the requirements of the 44 Code of Federal Regulations 60.3
(d) (2)_
8. Upon completion of the proposed project, "as -built" and final LOMR plans certified by a
registered professional engineer shall be submitted to the City for review and subsequent
transmittal to FEMA. FEMA requires that individual legal notices be sent to all affected property
owners when developments (cut or fill) occurs in the regulatory floodway that would cause any
rise in the 100 -year FIS water surface elevation. Public notice in the official community
newspaper is required for proposed modifications to the regulatory floodway. In all the above
hydraulic models, the following rules will apply:
a) The hydraulic parameters, such as bridge loss coefficients, "n" values, etc., used in the
effective FIS models will only be changed where obvious errors or changes have taken
place and must be documented.
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b) The computed water surface elevation profiles must converge with the existing profiles
c) The information should be shown on a map of suitable scale and topographic definition to
provide reasonable accuracy.
d) All items should be labeled for easy cross-referencing to the hydraulic model and
summary data.
9. FEMA may have questions regarding the project. The engineer must address all of FEMA's
comments. It is not anticipated, but if revisions to the development are required by FEMA, the
developer will be responsible to do so.
3.8.3 Fully Developed Water Surface Elevation Calculations.
A. The following hydraulic data should be submitted to the City, preferably using the Corps HEC -RAS
program to compute the channel's water surface elevation. The data should be submitted
electronically and in,a bound report.
1. Duplicate of the effective City fully developed backwater model or as developed by developer
or property owner and approved by the City.
2. Modified existing condition backwater model — this model should include pre -development
cross-sections through the project side obtained from field surveys or updated topographic
information.
3. Proposed condition reflecting the development's impact on the flood plain area.
4. Water surface elevation and velocity summary tables tabulating the results of the above analysis.
5. Topographic map at a suitable scale with cross-sections, existing and proposed one percent
chance (100 -year) fully developed flood plain delineated, and the area being developed shown.
6. Analysis of the existing and proposed valley storage conditions of the area.
7. Documentation from the Corps of Engineers determining if a 404 permit is required for the project.
3.8.4 Floodplain Fill Requirements
A. Side Slopes.
1. To ensure maximum accessibility to the floodplain for maintenance and other purposes, and to
lessen the probability of slope erosion during periods of high water, maximum slopes of filled
area shall usually not exceed 4 feet horizontal to 1 foot vertical. Grass cover is required for all
cut and fill slopes unless other armoring is required. Concrete rip -rap or an approved equal
erosion protection measure is required on slopes steeper than 3:1. Vertical walls, terracing and
other slope treatments will be considered only as:
a) Part of a landscaping plan submission, and
b) If no unbalancing of stream flow results
2. Vegetation/Landscaping. - Engineering plan submission shall include plans for:
a) Erosion control of cut and fill slopes;
b) Restoration of excavated areas; and
c) Tree protection in and below fill areas.
d) Landscaping should incorporate natural materials (earth, stone, and wood) on cut or
fill slopes wherever possible.
e) Applicant shall show in the plan the general nature and extent of existing vegetation on the
tract, the location of trees in accordance with the requirements of the tree survey required
by Section 7.7.4.E of the Denton Development Code (DDC), the areas which will be
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Stormwater Design Criteria Manual
preserved, altered, or removed as a result of the proposed alterations.
f) Locations and construction details should be provided, showing how trees will be preserved
in areas which will be altered by filling or paving within the drip line of those trees.
g) Applicant shall also submit plans showing location, type, and size of new plant materials
and other landscape features planned for altered flood plain areas.
3.9 Easements and Fences
A. Drainage and floodplain easements shall be provided for all open natural streams or manmade
channels. Easements shall encompass all areas lower than a ground elevation defined as being
the highest of the following:
1. Fifteen (15) feet outside the calculated water surface elevation and associated flood boundary
based on a design storm whose frequency is 100 years. All contributing watersheds are to be
treated as fully developed for purposes of calculating the water surface elevation.
2. The top of the high bank plus a minimum of 20 feet, if higher than (1) above.
3. Existing natural banks with a slope steeper than 4:1 shall have the easement line no closer
than the intersection of a 4:1 line extending from the toe of the slope to the proposed grade at
the top of the bank plus an additional fifteen (15) feet.
3. Additional access area may be required according to the section below.
B. Storm Drain Easements
1. Above Ground Systems.
1. Where an access road is required adjacent to a channel, an additional easement area of
a minimum width of fifteen (15) feet shall be provided. The maximum cross slope
shall be 5 percent. All access roads adjacent to improved channels shall be located
within the drainage easement.
2. No driveways, sidewalks, patios, etc. shall be placed in a drainage easement except
where the easement is a public or private open space or park, pedestrian and vehicle
access may be provided as determined by the City Engineer or designee.
2. Closed Systems.
a. Easements for closed drainage systems shall meet the following minimum standards,
unless special circumstances warrant additional or reduced easements; as determined
by City Engineer or designee:
Easement Requirements forClosed Drainage
Pipe or box Size Minimum Easement
Width
36 inches and under
16 feet
42 to 54 inches
20 feet
60 to 66 inches
25 feet
r772 inches and above
30 feet
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b. Utilities such as water and sanitary sewer lines may share a portion of a drainage
easement, containing an underground enclosed drainage system where an additional
easement width for a minimum of ten (10) feet is added to create a public drainage
and utility easement. No utilities shall be located in any lined channel, pipe, or box in
such a way as to interfere with flow capacity or maintenance of or access to the channel,
pipe or box.
c. A drainage easement shall be provided for the area within a required outfall channel or
ditch to the point where the flowline "day lights" on natural grade or matches existing
topography.
d. To provide for maintenance, a drainage easement shall be provided at least twenty-five
(25) feet beyond any outfall headwall.
e. No driveways, sidewalks, patios, etc. shall be placed in a drainage easement except
where storm sewer system is enclosed and is designed for 100 -year storm, unless
this easement serves a positive emergency overflow route.
C. Detention Facilities Easements
1. Detention facility easements must encompass the entirety of the detention facility plus any
required maintenance access areas adjacent or leading to the facility.
D. Fences
1. Fences in drainage easements are prohibited by the Denton Development Code, except as
specifically provided for below.
a. Fences in drainage easements that contain an underground stormwater system
may contain any type of non -masonry fence if the fence is constructed with
knock -out panels to facilitate maintenance.
b. Fences in drainage easements that contain overland flow may cross easement if
the fence is constructed with wrought iron (pickets and rails), pipe, or pipe and
cable. Vertical picket spacing must be a minimum of 4 inches clear space with
rails, pipes and cables maintaining a minimum of 4" ground clearance.
c. No fences are allowed in drainage easements that serve as a sump with a
concrete overflow or in combination with water or sewer facilities.
2. Fences in floodplain are prohibited.
3.10 Water Quality
3.10.1 Water Quality Protection Volume
All new detention ponds must include provisions for extended detention of the Water Quality Protection
Volume (WQ„). Seethe iSWMTM Water Quality Protection Technical Manual for information regarding the
design of this WQ„ and appropriate discharge design.
3.10.2 Construction Erosion and Sediment Control Requirements
All land disturbing activities must include provisions for erosion and sediment control in accordance with
the Denton Development Code, Subchapter 7.3 Land Disturbing Activities, the iSWMTM Water Quality
Technical Manual Document, the iSWMTII Construction Controls Technical Manual Document and iSWMTM
Site Development Technical Manual Document.
A. Phased erosion control plans are required. Plans must include existing and proposed contours.
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Stormwater Design Criteria Manual
1. Initial grading phase.
2. Individual lot phase with behind curb controls.
a. Behind the curb controls shall be installs upon completion of street segments.
b. Identify areas of permanent inactivity and provide timeline for vegetative stabilization.
B. Drainage area maps and calculations are required for three phases of development. Calculations
shall utilize a 2 year 24-hour storm for the design of any hydraulic component of the erosion control
plan including sediment basins, swales, channels, berm height, weir length or any other outlet or
conveyance structure required by the plan.
1. Existing pre -development conditions.
2. Mass graded conditions prior to public/private infrastructure installation stage.
3. Proposed post construction conditions.
C. Sediment basins are required when disturbed areas of 10 acres or larger disturbed area are part
of a common drainage area at any phase of development.
1. Sediment basin must provide at least 3,600 cubic feet of storage per acre drained until final
stabilization of the contributing drainage area.
2. Decentralized treatment such as sediment traps may be used instead of sediment basins where
the calculations are provided demonstrating equivalent volume treatment.
D. Vegetative stabilization is required for all permanent and temporary channels and basins. Plant
selection guidance is in Section 1.4 of the iSWMTm Landscape Technical Manual Document.
E. Erosion and sediment control Best Management Practices (BMPs) design criteria shall adhere to
the most current version of the iSWMTM Construction Controls Technical Manual Document.
1. Linear projects may instead follow TxDOT standards.
2. Proprietary erosion or sediment control devices may be utilized when:
a. Independent performance data is provided to prove a demonstrated capability of
meeting stormwater management efficiency equivalent to iSWMTM methods.
b. System or device must be appropriate for use in North Central Texas conditions.
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Stormwater Design Criteria Manual
Section 4LD iSWMTm Technical Manuals
Section 4.0 of this manual identifies the technical manuals documents that govern the design and
construction of stormwater drainage systems. The Stormwater Design Criteria Manual utilizes the most
current version of the following eight technical manuals documents produced by the North Central Texas
Council of Governments (NCTCOG):
• iSWMTM Planning Technical Manual Document
• iSWMTM Water Quality Technical Manual Document
• iSWMTI Hydrology Technical Manual Document
• iSWMTm Hydraulics Technical Manual Document
• iSWMTM Site Development Controls Technical Manual Document
• iSWMTM Construction Controls Technical Manual Document
• iSWMTM Construction Control Standard Details Technical Manual Document
• iSWMTm Landscape Technical Manual Document
These manuals are not reproduced here but can be downloaded free of charge at the following website
location: http://iswm.nctcog.org/technical-manual.htmi
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