§ 152.203 DESIGN METHODOLOGIES.

   (A)   An applicable hydrologic design method may be selected from the IDOT Drainage Manual with the following modifications and clarifications. Minor conveyance systems for areas up to 100 acres, and major conveyance systems up to ten acres may be designed using the rational formula. Design runoff rates may be calculated using a continuous simulation model or by event hydrographic methods. If event hydrographic methods are used they must be HEC-HMS, HEC-1, SCS TR20, or SCS TR55. Event methods must incorporate the following assumptions:

      (1)   Antecedent moisture condition 2 (normal moisture); and

      (2)   Huff or SCS type 2 distribution.

   (B)   For design events the Illinois State Water Survey Bulletin 70 (northwest) rainfall data must be used. Storage volumes for detention must be 24-hour events. Stormwater conveyance capacities must be designed for the critical duration creating the highest peak. The following table lists data from Bulletin 70:

FREQUENCY DISTRIBUTIONS (IN INCHES) FOR ZONE 1 ILLINOIS STATE WATER SURVEY, BULLETIN 70
Duration	Frequency
	2-month	6-month	1-year	2-year	5-year	10-year	25-year	50-year	100-year
5 minutes	0.17	0.25	0.31	0.37	0.47	0.56	0.067	0.78	0.89
15 minutes	0.38	0.57	0.70	0.84	1.07	1.25	1.51	1.76	1.99
30 minutes	0.52	0.77	0.95	1.15	1.46	1.71	2.07	2.42	2.77
1 hour	0.67	0.98	1.21	1.46	1.86	2.18	2.63	3.07	3.51
2 hours	0.84	1.23	1.52	1.83	2.33	2.74	3.31	3.86	4.47
6 hours	1.06	1.56	1.93	2.33	2.96	3.48	4.20	4.90	5.69
12 hours	1.23	1.81	2.24	2.71	3.43	4.03	4.88	5.66	6.51
18 hours	1.30	1.92	2.37	2.86	3.63	4.26	5.15	6.01	6.92
24 hours	1.40	2.08	2.57	3.11	3.95	4.63	5.60	6.53	7.36
48 hours	1.47	2.24	2.80	3.42	4.28	4.96	6.07	7.02	8.07

 

(Ord. passed - -)