3.10 Design Calculations and Capacity

a. Design Calculations

1) Design calculations shall utilize ODOT Hydraulics Manual.

2) Peak design discharges shall be computed using the rational formula, Q=CiA, where Q = flow in cfs, C = runoff coefficient, i = rainfall intensity, and A = area m acres.

b. Design Storm

1) Rainfall Intensity-Duration Curve - The rainfall intensity-duration-frequency (IDF) curve for use in the City of Silverton is the ODOT Zone 8 IDF curve (enclosed herein).

2) Design Frequency - The intensity-duration design frequency is based on the time of concentration for the area and the size of the drainage facility. The adopted criteria are listed in the following table.

Design Storm Frequency
AREA	FREQUENCY
Residential areas	25-year storm
Commercial and high value districts	25-year storm
Trunk lines (18" pipe and larger)	25-year storm
Minor creeks and drainage ways (not shown as a flood plain on the Flood Insurance Rate Map (FIRM))	50-year storm
Major creeks (shown as a flood plain on the FIRM)	100-year storm

ODOT Zone 8 IDF Curve Tabular Data (Silverton)
Rainfall Duration (Min)	Rainfall Intensity, inches/hour
Rainfall Duration (Min)	5 year Storm	10 year Storm	25 year Storm	50 year Storm	100 year Storm
5	2.01	2.25	2.63	3.00	3.35
6	1.90	2.12	2.50	2.81	3.12
7	1.81	2.01	2.35	2.68	2.95
8	1.71	1.91	2.24	2.55	2.80
9	1.65	1.83	2.14	2.43	2.69
10	1.60	1.78	2.07	2.33	2.58
11	1.51	1.70	1.98	2.25	2.48
12	1.48	1.65	1.90	2.18	2.40
13	1.41	1.60	1.85	2.10	2.31
14	1.38	1.55	1.79	2.01	2.24
15	1.32	1.50	1.72	1.95	2.19
20	1.13	1.30	1.50	1.69	1.90
25	1.00	1.14	1.35	1.50	1.69
30	0.91	1.02	1.21	1.36	1.51
35	0.82	0.92	1.10	1.21	1.38
40	0.75	0.84	0.98	1.11	1.24
45	0.69	0.78	0,92	1.02	1.15
50	0.64	0.73	0.85	0.95	1.08
55	0,60	0.68	0.80	0,89	1.00
60	0,57	0,64	0.75	0,84	0.94
70	0.53	0.59	0.68	0.76	0.85
80	0.49	0,54	0.63	0.70	0.78
90	0.46	0.52	0.59	0.66	0.74
100	0.44	0.49	0.56	0.62	0.69
110	0.42	0.47	0.53	0.60	0.66
120	0.40	0.45	0.51	0.57	0.63
130	0.385	0.44	0.49	0.55	0.60
140	0.37	0.420	0.48	0.53	0.58
150	0.36	0.410	0.46	0.520	0.56
160	0.35	0.400	0.45	0,50	0.540
170	0.340	0,390	0.44	0.49	0,53
180	0.33	0.38	0.43	0.48	0.52

Chart 3.10b2

3) The coefficients of runoff "C" are listed below. Use of coefficients other than those listed must be based on field investigations which demonstrate conclusively that the proposed coefficients are justified.

Runoff Coefficients
SOIL COVER	FLAT TERRAIN S<2%	ROLLING TERRAIN 2%<S:s;l0%	STEEP TERRAIN S>l0%
Cultivated Land	0.30	0.35	0.40
Parks & Cemeteries	0.15	0.20	0.30
Woodlands & Forests	0.10	0.15	0.20
Meadows & Pasture Land	0.25	0.30	0.35
1)Single-family residential in urban areas, except comer lots with duplex potential	0.40	0.45	0.50
2)Gravel parking lots	0.50	0.55	0.60
3)Mobile home parks	0.60	0.65	0.70
4)Multi-family residential, zero-lot-line single- family residential and potential duplex lots in single-family residential	0.70	0.75	0.80
Highly impermeable (roofs and paved areas)	0.90	0.90	0.90

1) For land in a pre-development condition, the minimum time of concentration from the most remote point in the basin to the first defined channel (e.g. gutter, ditch or pipe) shall be 10 minutes. Pre-development shall be defined as a site with natural vegetation on native soil.

2) For developed residential and commercial/industrial property, the maximum time of concentration from the most remote point in the development to the closest inlet shall be 10 minutes, unless calculations by an acceptable method show the time to be longer.