(A) Once the developer has made a decision on how the site is to be developed and on the drainage system that is going to be used, the runoff calculations can be made, control criteria that has to be met can be determined and a check can be made to determine if the proposed drainage system meets that criteria. If the runoff criteria have not been met, then it may be necessary for the developer to go through an iterative process of revising development plans and making runoff calculations until the runoff criteria can be met.
(B) Following are the recommended steps in making runoff calculations for a proposed development plan.
(1) Determine critical storm by calculating the volume of runoff from the one-year storm after development and comparing it to the volume of runoff from the one-year storm before development (§ 153.037). The percentage increase will determine the critical storm (§ 153.032). The allowable peak rate of discharge from the site for all more frequent storms up to the critical storm is then limited to the peak rate of discharge from the one-year storm before development.
(2) Calculate volume of runoff and peak rate of discharge from the critical storm after development and compare with the allowable peak rate of runoff and storage capacity of any proposed structures.
(3) Calculate volume of runoff and peak rate of discharge from all storms more intense than the critical storm for both before and after development conditions. After-development peak rates of runoff must be less than or equal to before-development peak rates of runoff. Check capacity of control structures. For the purpose of these regulations, the following storms shall be considered one-year, two-year, five-year, ten years, 25 years, 50 years, and 100 years.
(Ord. 16-96, passed 4-15-1996)