City of Evanston, WY
Uinta County
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Table of Contents
Table of Contents
A. 
Hydrologic design shall be predicated upon full urban development of the tributary watershed. Storm water runoff of these urban development areas shall be in accordance with the type of development stipulated by the zoning ordinance and by the adopted land use plan in the City's impact area.
B. 
The drainage areas shall be divided into major and minor waterways as defined below.
A. 
Major waterways shall be defined as those waterways with a drainage area larger than four square miles.
B. 
Modifications to any major waterway shall be designed for a design storm of one hundred years. Analysis and design shall be correlated with existing City models of these waterways and shall utilize stream gauging and computerized analysis techniques such as the HEC II model. Corps of Engineers, Federal Emergency Management Agency (FEMA), and other governing agency approval shall also be obtained and submitted as a part of the final drainage plan.
A. 
Minor waterways shall be those waterways with contributing drainage areas less than four square miles. These storm drainage facilities shall be designed for a design storm of ten years for all transport facilities and for a one-hundred-year design storm for all detention facilities.
B. 
For all minor watersheds and waterways, the design discharge shall be determined by the use of the rational method as per section 7-104A or the simplified method as per section 7-104B.
(1) 
Rational method. The rational formula below shall be applied and used as follows:
Q = C I A
Where:
Q
=
design discharge in cubic feet per second
C
=
runoff coefficient (see Table I below)
I
=
intensity of rainfall, inches per hour (see Tables II of III below)
A
=
tributary watershed area in acres
Table I. Runoff Coefficients.
Type of Area
Runoff Coefficients
Rural, park, forested, agricultural
.10—.30
Low residential (single-family dwellings)
.30—.50
High residential (multiple family dwellings)
.60
Business and commercial
.80
Industrial
.80
Impervious
.90
Schools
.50
Table II.
10-Year Design Rainfall
Intensity—Duration Data
Duration in Minutes
Intensity (in/hours)
5.0
2.46
6.0
2.31
7.5
2.04
10.0
1.73
12.5
1.53
15.0
1.46
20.0
1.16
25.0
1.02
30.0
0.94
45.0
0.69
60.0
0.59
90.0
0.45
120.0
0.35
180.0
0.27
240.0
0.20
300.0
0.14
360.0
0.12
Table III.
100-Year Design Rainfall
Intensity—Duration Data
Duration in Minutes
Intensity (in/hours)
5.0
4.28
6.0
4.03
7.0
3.69
8.0
3.41
9.0
3.19
10.0
3.00
12.5
2.64
15.0
2.55
20.0
2.02
25.0
1.77
30.0
1.63
45.0
1.26
60.0
1.02
90.0
0.94
120.0
0.72
180.0
0.52
240.0
0.36
300.0
0.18
360.0
0.15
420.0
0.14
480.0
0.12
The time of concentration shall be based on an initial lot to street time of ten minutes for lots smaller than one-half acre, and fifteen minutes for lots equal to or greater than one-half acre.
Where the size of the watershed is too large for the application of the rational method in one step, the waterways shall be subdivided into reaches of reasonable length, and the rational formula applied to each reach step-by-step, properly accumulating parameters and the time of concentration.
For all transport facilities the engineer shall determine, using a design storm of one hundred years, the impact and/or surcharging that would occur in all proposed storm drainage facilities.
(2) 
Simplified method. For small drainage basins where the developer does not want to apply the rational formula and the step-by-step approach, 1.46 inches of water shall be used for a ten-year design storm, and 2.55 inches of water shall be used for a one-hundred-year storm. These values shall then be used for the intensity of rainfall or "I" in the rational formula in order to calculate the peak runoff flows for the appropriate contributing area, "A." The total volume of water discharged shall be calculated assuming the entire volume of water will be discharged from the site.