The following shall govern the design of any improvement with respect to the detention of storm water runoff.
(A) Acceptable detention methods.
(1) (a) The increased storm water runoff resulting from a proposed development shall bedetained on-site by the provisions of appropriate detention or retention basins, underground storage, parking lots, streets, lawns or other acceptable techniques.
(b) Measures, which further retard the rate of overland flow and the velocity in runoff channels may also be required to partially control the runoff rate.
(2) (a) Detention basins or underground storage shall be sized to store excess flows from storms with a 100-year return period.
(b) Control devices shall limit the maximum release to a rate no greater than that prescribed by this chapter.
(B) Design storm.
(1) Design of storm water detention facilities shall be based on a return period of once in 100 years, or a 1% chance of occurrence in any given year.
(2) The storage volume and outflow rate shall be sufficient to handle storm water runoff from the most critical of a one-hour or six-hour storm.
(3) Rainfall depth-duration-frequency relationships and intensity-duration-frequency relationships used shall be those given in Tables 5 and 5A.
Table 5: Point Precipitation Rainfall Depth (inches) for a Given Recurrence Interval | ||||||
Duration | 2-year | 5-year | 10-year | 25-year | 50-year | 100-year |
6-hr | 2.27 | 2.82 | 3.28 | 3.92 | 4.45 | 5.01 |
1-hr | 1.48 | 1.84 | 2.12 | 2.49 | 2.79 | 3.09 |
NOTES TO TABLE: Reference: NOAA Atlas 14, Volume 2, Version 3 | ||||||
Table 5A: Rainfall Intensity (inches per hour) for a Given Recurrence Interval | ||||||
T (hrs) | 2-year | 5-year | 10-year | 25-year | 50-year | 100-year |
Table 5A: Rainfall Intensity (inches per hour) for a Given Recurrence Interval | ||||||
T (hrs) | 2-year | 5-year | 10-year | 25-year | 50-year | 100-year |
0.08 | 5.07 | 5.95 | 6.72 | 7.88 | 8.90 | 10.04 |
0.17 | 4.03 | 4.73 | 5.34 | 6.27 | 7.08 | 7.99 |
0.25 | 3.37 | 3.96 | 4.47 | 5.24 | 5.92 | 6.68 |
0.33 | 2.87 | 3.37 | 3.80 | 4.46 | 5.03 | 5.68 |
0.5 | 2.13 | 2.50 | 2.82 | 3.31 | 3.73 | 4.21 |
0.67 | 1.65 | 1.94 | 2.19 | 2.57 | 2.90 | 3.27 |
0.83 | 1.34 | 1.58 | 1.78 | 2.09 | 2.36 | 2.66 |
1.5 | 0.69 | 0.81 | 0.92 | 1.08 | 1.22 | 1.38 |
Derived using IDF Equation 2.2.13 and Evansville data from Table 2.2.2 LTAP Stormwater Drainage Manual, Revised February 2008 | ||||||
Table 6: Rainfall Distribution Ordinates for Hydrologic Modeling (Temporal Distribution: 6-Hour Duration) (1st Quartile Distribution - 50% Exceedance Probability) | |
% Storm Time | % Storm Rainfall Depth Total |
Table 6: Rainfall Distribution Ordinates for Hydrologic Modeling (Temporal Distribution: 6-Hour Duration) (1st Quartile Distribution - 50% Exceedance Probability) | |
% Storm Time | % Storm Rainfall Depth Total |
0 | 0 |
10 | 25 |
20 | 44 |
30 | 63 |
40 | 77 |
50 | 84 |
60 | 90 |
70 | 94 |
80 | 97 |
90 | 99 |
100 | 100 |
Note: Use for both 1-hour and 6-hour duration storm events.
| |
Reference: NOAA Atlas 14, Volume 2, Version 3
(C) Allowable release rate.
(1) The allowable release rate of storm water from development, re-development, and new construction shall not exceed the storm water runoff rate from the land area in its present condition as described in § 151.21.
(2) If more than one detention basin is involved in the development of the area upstream of the limiting restriction, the allowable release rate from any one detention basin shall be in direct proportion to the ratio of its drainage area to the drainage area of the entire watershed upstream of the restriction.
(D) Drainage system overflow design. The drainage system shall have adequate capacity to safely and adequately convey the storm water runoff from all upstream tributary areas through the development under consideration for a storm of 100-year design return period calculated on the basis of the upstream land in its present state of development. This conveyance shall be accomplished without damage to structures and improvements. An allowance, equivalent to the reduction in flow rate provided, shall be made for existing detention basins in upstream tributary areas; provided, evidence of its construction can be shown.
(E) Determination of storage volume.
(1) For areas up to and including 20 acres, the Rational Method may be used to determine the required volume of storm water storage. The procedure is described in the HERPICC Storm Water Design Manual. Other design methods as described below in division (E)(2) below may also be used.
(2) For areas larger than 20 acres, methods other than the Rational Method for determining runoff and routing of storm water shall be used to determine the storage volume required to control storm water runoff. The procedures or methods used shall be proven methods in the practice of hydrology, subject to approval of the Board. The Advanced Interconnect Pond and Channel Routing, SWMM, HEC-HMS, NRCS TR-20 (Project Formulation, Hydrology), and NRCS TR-55 (Urban Hydrology for Small Watersheds) models are approved by the Board for appropriate use in analysis of the runoff and routing of storm water.
(F) General detention/retention basin design requirements.
(1) Basins shall be constructed to temporarily detain the storm water runoff which exceeds the maximum peak flow rate authorized by this chapter. The volume of storage provided in these basins, together with the storage as may be contained in other on-site facilities, shall be sufficient to control excess runoff from the 100-year storm.
(2) The following design principles shall be observed.
(a) The maximum volume of water stored and subsequently released at the design release rate shall not result in storage duration in excess of 48 hours unless additional storms occur within the period.
(b) The maximum planned depth of storm water stored should generally not exceed five feet.
(c) All earthen storm water detention facilities shall be separated by not less than 50 feet from any building or structure to be occupied.
(d) Debris and safety screens having a maximum opening of six inches shall be provided for any pipe or opening to prevent children or large animals from crawling into the structures.
(e) Earthen dams that are a component of storm water facilities shall be designed and constructed according to sound engineering and construction principals. Dams shall not be designed to overtop in a 100-year storm, unless approved by the Board.
(f) Danger signs shall be mounted at appropriate locations to warn of deep water and possible flooding conditions during storm periods or other dangers that exist. Fencing shall be provided if deemed necessary by the Board.
(g) Outlet control structures shall be designed to operate as simply as possible and shall require little or no maintenance and/or attention for proper operation. They shall limit discharges into existing or planned downstream channels or conduits so as not to exceed the predetermined maximum authorized peak flow rate at maximum storage levels.
(h) Emergency overflow facilities such as a weir or emergency spillway shall be provided for the release of exceptional storm runoffs or in emergency conditions should the normal discharge devices become inoperative. The overflow facility shall be of a design that its operation is automatic and does not require manual attention. It shall be designed to be stable and resist erosion in exceptional storm runoffs. It shall direct all flows to the receiving outlet without passing through any off-site property.
(i) Grass or other suitable vegetative cover shall be provided throughout the entire earthen basin area. Grass shall be maintained in healthy condition and should be cut regularly at approximately monthly intervals during the growing season or as required.
(j) Debris and trash removal and other necessary maintenance shall be performed on a regular basis to assure continued operation in conformance to design.
(k) A report shall be submitted to the Board describing:
1. The proposed development;
2. The current land use conditions;
3. The method of hydraulic and hydrologic analysis used, including input and output files; and
4. A maintenance plan clearly outlining the party responsible for the permanent ownership and maintenance of the drainage system and all periodic maintenance requirements.
(G) Detention basin design requirements. Provisions shall be incorporated to facilitate complete interior drainage of detention basins, to include the provisions of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facilities, paved gutters or the installation of subsurface drains.
(H) Retention basin design requirements. The following additional design principals shall be observed.
(1) Basins designed with permanent pools or containing permanent ponds shall have a water area of at least one-half acre. If fish are to be maintained in the pond, a minimum depth of approximately ten feet shall be maintained over at least 25% of the pond area. The remaining pond area shall have no extensive shallow areas, except as required by division (H)(3) below.
(2) In excavated ponds, the underwater side slopes in the pond shall be stable with a maximum slope of two and one-half to one. In the case of valley storage, natural slopes may be considered to be stable.
(3) A safety ledge four to six feet in width is required and must be installed in all ponds approximately 18 to 24 inches below the permanent water level. The slope above the safety ledge shall be stable, with a maximum slope of three to one, and of a material such as stone or riprap which will prevent erosion due to wave action.
(4) Periodic maintenance is required in ponds to control weed growth. The pond shall also be designed to provide for the easy removal of sediment, which will accumulate during periods of pond operation.
(5) For emergency use, basin cleaning or shoreline maintenance, facilities shall be provided or consideration given for auxiliary equipment to permit emptying and drainage.
(6) Dry fire hydrants and drafting basins for fire protection may be requested for installation in areas where fire protection water supplies are not available.
(I) Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of storm water on all or a portion of their surfaces. Depth of storage must be limited to a maximum depth of six inches so as to prevent damage to parked vehicles and so that access to parked vehicles is not impaired. Ponding should, in general, be confined to those positions of the parking lots farthest from the area served.
(J) Facility financial responsibilities. The construction and maintenance cost of storm water control systems and facilities as required by this chapter shall be accepted as part of the cost of land development. If general public use of the facility can be demonstrated, negotiations for public participation in the cost of the developments may be considered.
(K) Facility maintenance and ownership responsibilities.
(1) Facility maintenance responsibility and permanent ownership of detention/retention facilities during construction and thereafter shall be the responsibility of the land developer/owner, unless the responsibility and ownership is formally dedicated to and accepted by a public body.
(2) Assignment of responsibility for maintaining facilities serving more than one lot or holding shall be documented by appropriate covenants to property deeds, and shall be determined before the drainage plans are approved.
(3) Storm water detention and retention basins may be dedicated to the county or other unit of local government for permanent ownership and maintenance providing:
(a) The county or governmental unit is willing to accept responsibility;
(b) The facility has been designed and constructed according to all applicable provisions of this chapter;
(c) All improvements have been constructed, approved and accepted by the county for the land area served by the drainage basin;
(d) Dry detention basins shall have all slopes, bottom of the basin and area above the high water line seeded and stabilized; and shall have the high water line not closer than 50 feet to any development boundary; and
(e) The Board has accepted as-built documentation and all conditions of any permits have been met.
(L) Inspections. All public and privately owned detention and retention storage facilities must be made available for inspection by representatives of the Board not less often than once every two years and as necessary following large storm events. If the Board determines that maintenance is needed or that the original objectives are not being realized, a certified inspection report covering the physical conditions, available storage capacity and operational condition of key facility elements will be provided to the owner.
(M) Corrective measures. If the Board finds deficiencies, the person or entity responsible for the storm water control facilities will be required to take the necessary measures to correct the deficiencies. If the person or entity responsible fails to do so, the Board will undertake the work and collect from the responsible party, using lien rights if necessary.
(N) Joint development of control systems. Storm water control facilities may be planned and constructed jointly by two or more developers as long as compliance with this chapter is adhered to and future operation, maintenance and ownership are assured.
(O) Installation of control systems. Runoff and erosion control facilities shall be installed as soon as possible during the course of site development. Detention/retention basins shall be designed with an additional 6% of available capacity to allow for sediment accumulation resulting from development and to permit the basin to function for reasonable periods between cleanings. Basins should be designed to collect sediment and debris in specific locations so that removal costs are kept to a minimum.
(P) Detention facilities in floodplains. If detention storage is provided within a floodplain, only the net increase in storage volume above that which naturally existed on the floodplain shall be credited to the development. No credit will be granted for volumes below the elevation of the regulatory flood at the location unless compensatory storage is also provided.
(Q) Off-site drainage provisions. When the allowable runoff is released into an area that is susceptible to flooding, the developer may be required to construct appropriate storm drains through the area to avert increased flood hazard. The requirement of off-site drains shall be at the discretion of the Board.
(Ord. 8-2014, passed 7-21-2014)