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:
“C” factor determination.
The same as “runoff coefficient.”
Closed system.
A system that carries storm drainage by way of conduit, inlets, and other types of closed structures.
Design “storm frequency.”
The frequency in years at which the specific intensity of rainfall will probably occur.
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.
Fully developed watershed.
A tract of land defined by topography that has been or can be fully developed.
Head losses.
Loss of energy in a system due to friction.
Headwalls.
A structure either upstream or downstream near a pipe or culvert used to transition the flow of water and to resist erosion.
Hydraulic capacity.
Ability of system to carry the designed storm.
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.
Natural channels and/or earthen channels.
A system, either natural or manmade, that carries storm drainage by way of an open system that is not lined.
Offsite drainage.
Drainage that initially or collectively crosses subject property and onto adjacent property.
Outfall of storm sewer system.
The point at which a closed system empties into an open system.
Playa or playa lake.
A natural depression that has the capability of storing storm drainage runoff.
Reinforced concrete lined open channels.
A system that carries storm drainage by way of open channels with reinforced concrete used as a means to prevent erosion or to increase capacity.
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).
(2) 
Lined open channels.
(3) 
Natural or earthen channels.
(4) 
Playa lakes and/or detention basins.
(5) 
Streets and/or flumes.
(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)