[Res. No. 662 §26, 8-19-1997]
Storm water 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 development engineer shall consider the use of storm water detention.
[Res. No. 662 §27, 8-19-1997]
A complete set of storm water detention plans and calculations shall be provided for all construction projects which increase storm water runoff and where storm water detention is a recommended storm water management option.
[Res. No. 662 §28, 8-19-1997]
Storm water detention facility evaluation consists of the development of pre- and post-development runoff hydrographs from both the site and the entire watershed at the point(s) of discharge, and consideration of the impacts of post-development runoff downstream of the proposed development. Allowable basin discharge flow rates should be based on the downstream conveyance system capacity requirements. Post-development discharge rates may be less than pre-development rate.
[Res. No. 662 §29, 8-19-1997]
A. 
Volume of detention required can be estimated using the procedures outlined in the SCS TR55 runoff hydrograph procedures, or other method with approval of the City Engineer.
B. 
Actual volume of detention shall be verified in design through the use of Muskingham-Puls routing procedures. As-constructed surveys of the constructed detention basin shall verify the stage-storage and stage-discharge relationships identified in the basin design.
[Res. No. 662 §30, 8-19-1997]
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 off-site of the project development site will be considered by special request if proper documentation is submitted with reference to practicality, feasibility, proof of ownership or 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 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 paved with a minimum of six (6) inches of concrete.
3. 
Open channels.
a. 
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%).
b. 
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.
c. 
The entire reservoir area of the open channel shall be seeded, fertilized and mulched, sodded or paved.
d. 
The hydraulic elevations resulting from channel detention shall not effect adversely adjoining properties.
4. 
Permanent lakes.
a. 
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.
b. 
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.
c. 
Maximum fluctuation from permanent pool elevation to maximum ponding elevation shall be three (3) feet.
d. 
Special consideration is suggested to safety and limiting accessibility of small children in design of permanent lakes in residential areas.
e. 
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.
a. 
Detention will not be permitted in primary parking lots. A primary parking lot will be considered to be the most accessible eighty percent (80%) of total parking for a facility.
b. 
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.
c. 
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.
d. 
The minimum freeboard from the maximum ponding elevation to the lowest sill elevation shall be two (2) feet.
[Res. No. 662 §31, 8-19-1997]
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.
B. 
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.
C. 
Spot elevations and finish contours 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.
[Res. No. 662 §32, 8-19-1997]
A. 
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. Use of channel chokes is not acceptable.
B. 
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.
C. 
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.
D. 
Overflow spillways will be required on all detention facilities which have storage volumes of one thousand (1,000) or more cubic feet.
E. 
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 100-year frequency rain.
Street Classification
Allowable Depth and Inundate Areas 100-year Storm
Local and Collector
Residential dwellings, public, commercial and industrial buildings shall not be inundated at the ground line. The depth of water over the gutter flowline shall not exceed eighteen (18) inches.
Arterial
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 flowline shall not exceed eighteen (18) inches.