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.
