15.25.020: RAINFALL HYDROLOGY:

   A.   Policy: All storm drain systems shall be designed to carry the 100-year storm.

   B.   Storm Specifications: Local storm drain piping shall be designed for the 10-year storm where the road or other aboveground conveyance will carry the difference to the 100-year storm. All basins regardless of local or regional, or retention or detention, shall be designed to accommodate a 100-year storm event. The storm duration used for the sizing of basins shall be based upon the worst case scenario and not the time of concentration.

   C.   Intensity-Duration-Frequency (IDF): For the use of the "rational method", an IDF curve shall be obtained from the city engineer for the project location.

   D.   Calculation Basis: For single site plans and small subdivisions, the rational equation may be used. For larger sites a city engineer approved computer model shall be used.

   E.   Rainfall Pattern: For the use of computer models, the following rainfall pattern shall be used. This pattern is based upon the "Farmer-Flecher distribution". This pattern is for a one inch (1") unit storm and must be multiplied by rainfall depth for storms of other magnitudes.

   F.   Rainfall Total: For the use of the following rainfall pattern, a rainfall total from the "NOAA atlas" must be obtained. Such total shall be based upon a 1-hour storm as follows:

1	0	11	0.004	21	0.033	31	0.052	41	0.012	51	0.005
2	0	12	0.005	22	0.034	32	0.045	42	0.011	52	0.005
3	0.002	13	0.008	23	0.035	33	0.04	43	0.01	53	0.004
4	0.002	14	0.009	24	0.038	34	0.035	44	0.009	54	0.004
5	0.002	15	0.009	25	0.039	35	0.03	45	0.009	55	0.004
6	0.002	16	0.013	26	0.045	36	0.022	46	0.008	56	0.003
7	0.002	17	0.017	27	0.052	37	0.02	47	0.006	57	0.003
8	0.002	18	0.02	28	0.054	38	0.018	48	0.006	58	0.002
9	0.003	19	0.024	29	0.054	39	0.016	49	0.005	59	0.002
10	0.003	20	0.029	30	0.054	40	0.014	50	0.005	60	0.001

 

(Ord. 2012-05)