[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:
[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:
[Ord. No. 777 §26-150, 3-23-1994]
The following conditions and limitations shall be observed in the selection and use of method of detention.
General location. Detention facilities shall be located within the parcel limits of the project under consideration with the following exceptions:
No detention or ponding will be permitted within public road rights-of-way without specific written approval of the City.
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.
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.
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.
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.
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]
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.
[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.