Runoff quantities shall be computed for the area of the parcel under development plus the area of the watershed flowing into the parcel under development. The quantity of runoff which is generated as the result of a given rainfall intensity may be calculated as follows:
(A) Area less than or equal to 200 acres.
(1) For areas up to and including 200 acres, the rational method may be used. Typical runoff coefficients are listed herein. In the rational method, the peak rate of runoff, Q, in cubic feet per second is computed as:
Q=CIA
where C equals runoff coefficient, representing the characteristics of the drainage area and defined as the ratio of runoff to rainfall; I equals average intensity of rainfall in inches per hour for a duration equal to the time of concentration (tc) for a selected rainfall frequency; A equals tributary drainage area in acres. Guidance to selection of the runoff coefficient "C" is provided in Appendix B, Tables 1 and 2, which show values for different types of surface and local soil characteristics. The composite "C" value used for a given drainage area with various surface types shall be the weighted average value for the total area calculated from a breakdown of individual areas having different surface types.
(2) Table 3 of Appendix B provides runoff coefficients and inlet times for different land use classifications. In the instance of undeveloped land situated in an upstream area, a coefficient or coefficients shall be used for this area in its present or existing state of development.
(3) Rainfall intensity shall be determined from the data shown in Appendix B, Table 6. The time of concentration (tc) to be used shall be the sum of the inlet time and flow time in the drainage facility from the most remote part of the drainage area to the point under consideration. The flow time in the storm sewers may be estimated by the distance in feet divided by velocity of flow in feet per second. The velocity shall be determined by the Manning Formula.
(4) Time of concentration (tc) is the combined time required for the runoff to reach the inlet of the storm sewer. It includes overland flow time and flow time through established surface drainage channels such as swales, ditches and sheet flow across such areas as lawns, fields, and other graded surfaces, it may be computed by using the Time of Concentration Worksheet, in Appendix A.
(B) For areas larger than 200 acres, hydrograph techniques and/or computer modeling methods may be used. Hydrograph techniques and computer modeling methods used to determine storm water runoff shall be proven methods, subject to approval of the Board.(see also § 157.27(F)
(Ord. 00-22, passed 12-28-00; Am. Ord. 2001-17, passed 1-14-02)