§ 35-29.5. Hydrologic methods.

(a)

The following methods may be used to support hydrologic site analysis for the design methods and procedures included in NCTCOG-iSWM:

(1)

Rational Method.

(2)

SCS Unit Hydrograph Method.

(3)

Snyder's Unit Hydrograph Method.

(4)

TxDOT Regression Equations.

(5)

USGS Regression Equations.

(6)

iSWM Water Quality Protection Volume Calculation.

(7)

Water Balance Calculations.

(b)

Table 35-29H lists the hydraulic methods and circumstances for their use in various analysis and design applications. Table 35-29H provides some limitations on the use of several methods. The rational method is acceptable for small, highly impervious drainage areas, such as parking lots and roadways draining into inlets and gutters.

(c)

The U.S. Geological Survey (USGS) and Texas Department of Transportation (TxDOT) regression equations are acceptable for drainage areas with characteristics within the ranges given for the equations shown in Table 35-29H. These equations should not be used when there are significant storage areas within the drainage basin or where other drainage characteristics indicate general regression equations are not appropriate.

Table 35-29H Constraints on Using Recommended Hydrologic Methods
Method	Size Limitations	Comments
Rational	0-100 acres	Method can be used for estimating peak flows and the design of small site or subdivision storm sewer systems
Modified Rational	0-200 acres	Method can be used for estimating runoff volumes for storage design
Unit Hydrograph (SCS)	Any Size	Method can be used for estimating peak flows and hydrographs for all design applications
Unit Hydrograph (Snyder's)	1 acre and larger	Method can be used for estimating peak flows and hydrographs for all design applications
TxDOT Regression Equations	10 to 100 mi	Method can be used for estimating peak flows and hydrographs for rural design applications
USGS Regression Equations	3-40 mi	Method can be used for estimating peak flows and hydrographs for urban design applications
ISWM Water Quality Protection Volume Calculation	Limits set for each Structural Control	Method can be used for calculating the Water Quality Protection Volume (WQv)

 

(Ord. No. 2017-10008, § II, 10-26-17)