FLOOD00425

• County: Statewide
• Title: HYDPAR- users manual
• Date: 11/1/1978
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

SAMPLE PROBLEM
PERCENT IMPERVIOUSNESS AND SNYDER'S METHOD

Problem Statement

Calculate the hydrologic subbasin parameters, percent impervious. surface and

basin lag, for the Trail Creek Watershed under existing conditions using the

weighted average technique for computing imperviousness and the regression

equation form, equation 3-3. Use Table 1 to assign values of percent imper-

vieusness to each of the ten land use categories and Table 2 to define the

necessary physiographic characteristics utilized in the regression equation.

(Table 3 is a listing of the data base file used in the Trail Creek Watershed).

Discription of Input Requirements

Land surface slope values (different from stream slope values) are not necessary
input for this computation, so they are not included on the coding sheets shown
in this sample problem. The regression coefficients are input on the EQ
card, providing the necessary equation values. (The EQ card format is found on
the first of the two coding sheets given). Percent imperviousness values are
provided for each land use category on the LU cards, one value for each cate-
gory indicated in NLUSE (J2.4). The SB cards contain particular subbasin
information for each subbasin to be analyzed, NSUB (J2.2).

Discussion of Results

Output for subbasin 1 and the summarized data for the entire watershed has

been provided on the following pages. Since no land slope values (SCS slope

class averages) were input, the specific subbasin data and the final summary

printout include a 'not available' (N/A) indicator sunder SURFACE and SUB-

BASIN SLOPE. (If this summary is desired and land slope values are in the

data bank, input the necessary data as done .n the previous sample problem

Exhibit 3-4
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