The purpose of this division is to establish policies governing
storm drainage facilities within the city limits, and its extraterritorial
jurisdiction in order to protect the general health, safety and welfare
of the public by reducing flooding potentialities, controlling excessive
runoff, minimizing erosion and siltation problems, and eliminating
damage to public facilities resulting from uncontrolled stormwater
runoff.
(Ordinance 2018-20 adopted 5/8/18)
Whenever the words, forms, phrases defined herein, or pronouns
used in their place, occur in this division, the intent and meaning
shall be interpreted as follows:
Closed system.
A system that carries storm drainage by way of conduit, inlets,
and other types of closed structures.
Detention basins.
Artificial or man-made depressions that have the capability
of storing storm drainage runoff.
Flume.
A lined channel section used to carry stormwater between
structures.
Headwalls.
A structure either upstream or downstream near a pipe or
culvert used to transition the flow of water and to resist erosion.
Inlet.
Type of opening designed to allow stormwater to enter a system.
There are low point inlets or on grade inlets.
Manning equation.
An equation that was developed by Manning to determine the
flow of water in a system.
Offsite drainage.
Drainage that initially or collectively crosses subject property
and onto adjacent property.
Roughness coefficient.
Also referred to as an “n” factor. A number developed
to account for the degree of roughness that affects the flow of water.
Runoff coefficients.
Also, referred to as a “C” factor. The factor
that reflects the development conditions of a tract of land.
Time of concentration.
The longest time that will be required for a drop of water
to flow from the upper limit of the drainage area to the point of
concentration.
(Ordinance 2018-20 adopted 5/8/18)
(a) The purpose of this section is to establish standard criteria, principles,
procedures and practices for design of storm drainage facilities.
(1) The design factors, formulas, graphs and procedures presented or
referred to herein are intended for use as engineering guides in the
design of drainage facilities and in the solution of drainage problems
involving the quantity, method of collection, transportation and disposal
of stormwater.
(2) Methods of design other than those indicated or referred to herein
may be considered where experience clearly indicates they are preferable;
however, these deviations shall not be attempted until approval has
been obtained from the city engineer.
(3) The methods outlined or referred to herein include accepted principles
of surface drainage engineering and should be a working supplement
to basic design information obtainable from textbooks and publications
on drainage.
(b) Determination of design discharge.
The two methods acceptable
for use in the city are:
(1) Rational method to be used on small watersheds of 1,000 acres or
less.
(A)
Q = CiA, in which:
(i)
Q is the peak runoff rate in cubic feet per second (cfs). Actually,
Q is in units of inches per hour per acre; since this rate is in/hr/ac
differs from cfs by less than one percent, the more common cfs is
used.
(ii)
C is a runoff coefficient depending on characteristics of the
drainage area.
(iii) i is the average rainfall intensity in inches
per hour (in/hr).
(B)
The rational method is based on the following assumptions:
(i)
The peak rate of runoff at any point is a direct function of
the average rainfall intensity during the time of concentration to
that point.
(ii)
The frequency of the peak discharge is the same as the frequency
of the average rainfall intensity.
(iii) The time of concentration is the time required
for runoff to become established and flow from the most remote (remote
in time, not necessarily distance) part of the drainage area to the
point under design.
(2) Unit hydrograph techniques may be used for areas greater than 1,000
acres. The city engineer shall approve the technique and the data
to be used for the determination of the design discharge prior to
the calculations being completed. A complete set of all detail calculations
must be submitted to the city engineer for approval prior to completion
of the plans for the drainage system.
(c) Drainage area.
(1) Area of the watershed shall be determined using approved engineering
methods.
(2) Outline of drainage areas must follow natural drainage features in
nonurbanized areas. Consideration shall be given to man-made features
in urbanized areas.
(d) Runoff coefficients.
(1) Storm drainage shall be designed for ultimate development of the
watershed and, therefore, runoff coefficients used shall consider
these fully developed conditions. Master plans, zoning maps and land
use plans shall be used to determine the ultimate development.
(2) Table I gives values for runoff coefficients that shall be used in
the determination of stormwater runoff.
Table I. Runoff Coefficient “C”
|
---|
Retail or Commercial Areas
|
0.90
|
Industrial Areas
|
0.70
|
Residential Areas
|
0.50
|
Park or Open Space Areas
|
0.30
|
(e) Rainfall intensity - duration - frequency curves for this area have
been developed using the most current data available. These curves
are as published in the drainage criteria manual for the city, which
data is approved and incorporated herein by reference. A copy of these
curves is on file with the director of public works and is available
for inspection.
(f) Roughness coefficients “n” have been determined for the
city. table II shall be used when assigning values for “n.”
Table II. Computation of Composite Roughness Coefficient For
Excavated and Natural Channels
N = (n0 + n1 +
n2 + n3 + n4) m
|
---|
|
Channel Conditions
|
Value
|
---|
Material involved - n0
|
Earth
|
0.020
|
|
Rockcut
|
0.025
|
|
Fine Gravel
|
0.024
|
|
Coarse Gravel
|
0.028
|
Degree of irregularity - n1
|
Smooth
|
0.000
|
|
Minor
|
0.005
|
|
Moderate
|
0.010
|
|
Severe
|
0.020
|
Variation of channel cross section - n2
|
Gradual
|
0.000
|
|
Alternating Occasionally
|
0.005
|
|
Alternating Frequently
|
0.010–0.015
|
Relative effect of obstructions - n3
|
Negligible
|
0.000
|
|
Minor
|
0.010–0.015
|
|
Appreciable
|
0.020–0.030
|
|
Severe
|
0.040–0.060
|
Vegetation - n4
|
Low
|
0.005–0.010
|
|
Medium
|
0.010–0.025
|
|
High
|
0.025–0.050
|
|
Very High
|
0.050–0.100
|
Degree of meandering - m
|
Minor
|
1.000–1.200
|
|
Appreciable
|
1.200–1.500
|
|
Severe
|
1.500
|
Roughness Coefficient for Line Channels
|
Concrete - n = 0.017
|
Rubble Rip Rap - n = 0.022
|
Open Channel Hydraulics Line - Ven Te Chow, Ph.D.
|
(g) Time of concentration.
(1) Time of concentration shall be calculated for the watershed under
consideration.
(2) No time of concentration less than 15 minutes shall be used.
(Ordinance 2018-20 adopted 5/8/18)
Drainage and street facilities shall be designed to accommodate
at a minimum the following minimum design frequency:
Type of Facility
|
Minimum Design Frequency
|
---|
Channels, Creeks
|
50 year
|
Culverts
|
25 year
|
Streets at right-of-way line (with 12 inches over top of curb)
|
100 year
|
Flumes between buildings
|
100 year
|
Closed Systems Storm sewers (with inlet on grade)
|
5 year
|
Storm sewers draining at low points
|
25 year
|
(Ordinance 2018-20 adopted 5/8/18)
Headwalls shall be constructed at the inlet and outlet of all
storm sewer systems.
(Ordinance 2018-20 adopted 5/8/18)
The five different types of basic drainage features or systems
are as follows:
(1) Closed systems (storm sewers).
(3) Natural or earthen channels.
(4) Playa lakes and/or detention basins.
(Ordinance 2018-20 adopted 5/8/18)
(a) To be reinforced concrete or other alternatives approved by the city
engineer.
(b) Access to be provided for maintenance purposes.
(Ordinance 2018-20 adopted 5/8/18)
(a) Access shall be provided for maintenance purposes.
(b) The city engineer will approve all detention basins on an individual
basis.
(c) Natural playa lakes shall not be filled unless such loss in drainage
capacity is equally compensated for in some other manner.
(d) Finished floor of buildings in or adjacent to playa lakes, or drainage
way shall be a minimum of 1 foot above the 100-year storm.
(Ordinance 2018-20 adopted 5/8/18)
(a) Street crowns may vary from standard street crowns for drainage purposes
if approved by the city engineer.
(b) Inverted streets may be used on local streets if approved by the
city engineer. Inverted streets may not be used on collector or thoroughfare
streets as shown on the master thoroughfare plan except as approved
by the city engineer.
(c) Overwidth streets may be used for drainage purposes if approved by
the director of public works.
(d) Buildings adjacent to flumes shall set at least one foot above the
top of the flume.
(e) Buildings shall have finished floor elevations set one foot (1')
above top of curb or greater if required to maintain 100 year flood
within right-of-way or as approved by the city engineer.
(Ordinance 2018-20 adopted 5/8/18)
(a) All channel improvements such as filling shall be protected with
sodding, backsloping, cribbing or other bank protection, as approved
by city engineer.
(b) All channel sides shall have a slope at least equal to 4 to 1 or
flatter.
(c) Such systems shall not have velocities in excess of ten feet (10')
per second.
(d) Access to such channels shall be provided for maintenance purposes.
(Ordinance 2018-20 adopted 5/8/18)
(a) Bridges and culverts shall be designed to pass runoff from a 25-year frequency storm unless a large structure is required to comply with subsections
(b) and
(c) below.
(b) For drainage structures designed for residential streets, water overtopping
the structure from the 100-year frequency storms shall not flood the
roadway to a depth greater than fifteen inches (15") above the roadway
gutter and all water shall be redirected into the immediate downstream
system.
(c) For drainage structures designed to carry any street other than a
residential street, water overtopping the structure from a 100-year
frequency storm shall not flood the roadway to a depth greater than
nine inches (9") above the roadway gutter and all water shall be redirected
into the immediate downstream system.
(Ordinance 2018-20 adopted 5/8/18)
The requirements contained in all city ordinances with regard
to surface drainage or water sources shall be considered as minimum
requirements only, and the owner, his agent and independent contractor,
shall be responsible for complying with all state and federal laws
as well as all duties imposed on such owner, agent or independent
contractor by common law. Such owner, agent or independent contractor
shall also be required to obtain all permits required by the state
or the federal government.
(Ordinance 2018-20 adopted 5/8/18)
All construction shall be in accordance with the city specifications,
which are approved and incorporated herein by reference, a copy of
which is on file with the director of public works and is available
for inspection.
(Ordinance 2018-20 adopted 5/8/18)
Where the city engineer is given the authority by this division
to make a determination or finding, any person disagreeing with such
determination or finding, may appeal to the city council for a final
determination. In case of such an appeal, the city council can consider
and by motion approve any drainage requirements that meet a standard
of good engineering practice.
(Ordinance 2018-20 adopted 5/8/18)