1. The subdivider shall furnish projections of the increase of stormwater runoff created by the proposed development from the 2-year, 10-year, and 100-year frequency, 24-hour duration type III distribution storms, as computed in accordance with Technical Release #55, Urban Hydrology,
Engineering Division, Soils Conservation Service, USDA, January, 1975, as amended or by use of other methods conforming to sound engineering practice. Rational method shall not be used in computing drainage flows in drainage basin areas in excess of one thousand (1,000) acres.
2. Information to Be Submitted By Developer
A. All storm drainage calculation must be designed and certified by a registered Professional Engineer. The following date shall be submitted for review by the Public Works Department or its designated agent:
1. Topographic Contour Maps(s) showing drainage area(s).
2. Written description and computation including at least the following:
a. Method used to calculate storm water runoff.
b. Runoff characteristics of the property before and after development.
c. Drainage calculations.
d. Maximum velocity and quantity at point or points of discharge from the system.
e. Design calculations for all drainage piping and structures.
f. For detention structures:
(1) Inflow and outflow hydrographs for detention area.
(2) Maximum storage volume.
(3) Design of spillway or other measures for the release of excess flows beyond that of the design capacity of the structure.
(4) Flood routing of all runoff greater than the design capacity of the detention facility.
(5) Time which is required for the facility to drain completely.
(6) Materials used in construction of the facility.
(7) Methods employed to avoid clogging the discharge mechanism.
(8) Safety measures.
(9) Proposed landscaping and vegetative measures used to stabilize slopes and bottom surfaces.
3. No increase in peak flow from those storms referred to in 1. above shall be allowed unless downstream increases are compatible with an overall flood plain management system. The following items should be considered in determining whether increased peak flows are compatible with an overall flood plain management system:
A. The timing of peak flows from sub-watersheds
B. The increased duration of high flow rates
C. The stability of the downstream channels
D. The distance downstream that the peak discharges are increased.
4. When stormwater detention structures are required, they shall be designed so that the peak runoff after development shall not exceed nor be substantially less than the peak runoff prior to development for each of the above flood events. The subdivider shall be responsible for maintenance of the structure. The structures shall be designed and constructed in accordance with good engineering practice. Basin(s) shall be designed for easy access for maintenance purposes and be provided with safety measures as needed.
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