[Ord. No. 777 §26-146, 3-23-1994]
Stormwater runoff and the velocity of discharge are considerably
increased through development and growth of the City. Prior to the
development of the land, surface conditions provide a higher percentage
of permeability and longer time of concentration. With the construction
of buildings, parking lots, etc., permeability and the time of concentration
are significantly decreased resulting in an increase in both the rate
and volume of runoff. These modifications may create harmful effects
on properties downstream. Therefore, to minimize these effects, the
following minimum stormwater detention requirements have been established.
[Ord. No. 777 §26-147, 3-23-1994]
A complete set of stormwater detention plans and calculations
shall be provided for all construction projects which increase stormwater
runoff.
[Ord. No. 777 §26-148, 3-23-1994]
Differential runoff evaluation consists of determination of
the rates of runoff, before and after development, determination of
required volume of detention and verification of adequacy of discharge
and control structures. The 100-year (frequency) runoff coefficients
shall be used. Differential runoff rates shall be evaluated by equation:
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R
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=
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(Cd X I100) - (Cu X I100)
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Where
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R
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=
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Differential Runoff Rate
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Cd
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=
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Runoff Coefficient for developed conditions
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Cu
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=
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Runoff Coefficient for undeveloped conditions
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I100
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=
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Intensity for 100-year storm
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"C" values shall be determined from the following table:
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Suggested Runoff Coefficients
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|---|---|---|
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"C" Value
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Surface Conditions
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.10 — .15
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Tall grass, brush
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.15 — .20
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Parks, golf courses, farms and 1 acre single-family residences
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.35
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Single-family residences on lots of not less than 15,000 square
feet
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.45
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Single-family residences on lots of not less than 10,000 square
feet
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.47
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Single-family residences on lots of not less than 7,500 square
feet
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.51
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Single-family residences on lots of not less than 6,000 square
feet
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.90
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Gravel surfaces
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.95
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Asphalt concrete surfaces
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1.00
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Buildings and other structures
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[Ord. No. 777 §26-149, 3-23-1994]
Volume of detention can be evaluated according to the "Simplified
Volume Formula" or other method with approval of the City Engineer.
Total volume of detention shall be computed by the equation:
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V
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=
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R X A X tc (min.) x 60 (sec./min.)
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V
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=
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Total volume of detention (cu. ft.)
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R
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=
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Differential Runoff Rate
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A
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=
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Area of project in acres
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tc
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=
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Time of concentration as determined for use with differential
runoff rates
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The design volume of detention shall be determined from the
following table:
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Calculate Volume
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Design Volume
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|---|---|---|
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1 cubic foot thru 500 cubic feet
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500 cubic feet
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501 cubic feet thru 4,999 cubic feet
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Round up to nearest 500 cubic feet
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5,000 cubic feet thru 9,999 cubic feet
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Round up to nearest 1,000 cubic feet
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10,000 cubic feet thru 49,999 cubic feet
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Round up to nearest 5,000 cubic feet
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50,000 cubic feet thru 99,999 cubic feet
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Round up to nearest 10,000 cubic feet
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100,000 cubic feet and above
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Round up to nearest 25,000 cubic feet
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[Ord. No. 777 §26-150, 3-23-1994]
A.
The
following conditions and limitations shall be observed in the selection
and use of method of detention.
1.
General location. Detention facilities shall be
located within the parcel limits of the project under consideration
with the following exceptions:
a.
No detention or ponding will be permitted within public road rights-of-way
without specific written approval of the City.
b.
Location of detention facilities immediately downstream of the project
will be considered by special request if proper documentation is submitted
with reference to practicality, feasibility, proof of ownership of
right-of-use of the area proposed and provisions are made for perpetual
maintenance.
2.
Dry reservoirs. Wet weather ponds or dry reservoirs
shall be designed with proper safety, stability and ease of maintenance
features. Maximum side slopes for grassed reservoirs shall not exceed
one (1) foot vertical for three (3) horizontal (3:1). In no case shall
the limits of maximum ponding elevation be less than two (2) feet
vertically below the lowest sill elevation, nor should the maximum
limits of ponding be designed closer than ten (10) feet from a building
unless waterproofing of the building and pedestrian accessibility
are properly mulched, sodded or paved. A minimum of one (1) foot of
freeboard is required above the spillway. The outlet structure shall
be concrete or other equivalent material. Spillway areas shall be
saved with a minimum of six (6) inches of concrete.
3.
Open channels. Normally permitted open channels
may be used as detention areas, provided that the limits of the maximum
ponding elevation are not closer than thirty (30) feet horizontally
from any buildings with habitable areas below ground level and less
than two (2) feet below the lowest sill elevation of any building.
In no case should the maximum limits of ponding be designed closer
than ten (10) feet from a building unless waterproofing of the building
and pedestrian accessibility are properly documented. No ponding will
be permitted within public rights-of-way without specific written
approval of the City. Maximum depth of detention in open channels
shall be four (4) feet. Maximum flow line grade shall be one-half
percent (0.5%).
For trapezoidal sections, the maximum side slopes of the detention
area of the channel shall not exceed one (1) foot vertical for three
(3) horizontal (3:1). For design of other typical channel sections,
the features of safety, stability and ease of maintenance shall be
observed.
The entire reservoir area of the open channel shall be seeded,
fertilized and mulched, sodded or paved.
The hydraulic elevations resulting from channel detention shall
not effect adversely adjoining properties.
4.
Permanent lakes. Permanent lakes with fluctuating
volume controls may be used as detention areas, provided that the
limits of maximum ponding elevations are no closer than thirty (30)
feet horizontally from any building and less than two (2) feet below
the lowest sill elevation of any building.
Maximum side slopes for the fluctuating area of permanent lakes
shall be one (1) foot vertical to three (3) feet horizontal (3:1)
unless proper provisions are included for safety, stability and ease
of maintenance.
Maximum fluctuation from permanent pool elevation to maximum
ponding elevation shall be three (3) feet.
Special consideration is suggested to safety and limiting accessibility
of small children in design of permanent lakes in residential areas.
The entire fluctuating area of the permanent reservoir shall
be seeded and fertilized and mulched or sodded or concrete paved.
Any area susceptible to or designed as overflow shall be paved with
concrete.
5.
Parking lots. Detention will not be permitted in
primary parking lots. A primary lot will be considered to be the most
accessible eighty percent (80%) of total parking for a facility.
In no case should the maximum limits of ponding be designed
closer than ten (10) feet from a building unless waterproofing of
the building and pedestrian accessibility are properly documented.
When detention is being effected on parking lots by means of
retaining walls or curbs, these retaining walls and curbs must be
constructed of reinforced concrete.
The minimum freeboard from the maximum ponding elevation to
the lowest sill elevation shall be two (2) feet.
[Ord. No. 777 §26-151, 3-23-1994]
A.
Analysis
of all elements of design is always performed by the engineer. The
following outline is provided to ascertain that certain critical elements
of design are in workable compliance to the aims of design.
1.
Volume of detention for the total project.
2.
Tributary (Q) peak runoff to basin.
3.
Sizing of the overflow facilities.
4.
Stability of detention dikes.
5.
Safety features.
6.
Maintenance features.
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Routing calculations shall be submitted in legible tabulated
form. Proof of adequacy of the volume of detention and sizing computations
for low-flow structure shall also be submitted. Features of stability
and safety will also need to be documented if the scope of the project
requires special attention in this area of design.
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Spot elevations shall be included in sufficient detail on the
site plan so that the final direction of water flow can be determined
and so that the volume of detention can be ascertained. Projects over
two hundred (200) acres in area shall provide documented verification
of adequacy according to scope and complexity of design.
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[Ord. No. 777 §26-152, 3-23-1994]
Detention facilities shall be provided with obvious and effective
outlet control structures. These outlet structures may include v-notch
weirs or rectangular weirs, as well as pipe. Plan view and sections
of the structure with adequate detail shall be included in plans.
The design discharge (Q) for the low-flow outlet shall not exceed
the existing runoff for the 1-year storm. The maximum discharge shall
be designed to take place under total anticipated design-head conditions.
The design-head storage volume is not to be considered as part of
the volume of detention required.
Low-flow pipes shall not be smaller than four (4) inches in
diameter to minimize maintenance and operating problems, except in
parking lot and roof detention where minimum size and configuration
of opening shall be designed specifically for each condition. The
low-flow pipe shall be provided with a bar-screen on a minimum 2:1
slope to reduce blockage by debris.
Overflow spillways will be required on all detention facilities
which have storage volumes of one thousand (1,000) or more cubic feet.
The overflow opening or spillway shall be designed so that the
combination flow of the low-flow outlet and the flow over the spillway
will not exceed the total peak runoff for the improved area. The total
peak runoff is to be determined from a 25-year frequency rain for
drainage areas less than one (1) square mile and from a 100-year frequency
rain for drainage area one (1) square mile or greater.
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Street Classification
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Allowable Depth and Inundate Areas 100-Year Storm
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|---|---|
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Local and collector
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Residential dwellings, public, commercial and industrial buildings
shall not be inundated at the ground line. The depth of water over
the gutter flow line shall not exceed eighteen (18) inches.
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Arterial
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Residential dwellings, public, commercial and industrial buildings
shall not be inundated at the ground line. Depth of water at the street
crown shall not exceed six (6) inches to allow operation of emergency
vehicles. The depth of water over the gutter flow line shall not exceed
eighteen (18) inches.
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