Watershed Delineation Using HEC-HMS GIS Tools (Part 3 of 3)

Watershed Delineation Using HEC-HMS GIS Tools (Part 3 of 3)

The newest HEC-HMS Version 4.7 (as of January 8, 2021) has the capability to estimate hydrology parameters for watersheds delineated. This post is Part 3 of 3 to demonstrate such functions of HEC-HMS V4.7. Bear Creek Watershed has been delineated in Post Part 1 of 3 and the delineated subbasins and reaches were further refined by merge/split in Post Part 2 of 3.

The files used for this demonstration can be downloaded by clicking this link which includes the completed HEC-HMS model of Post Part 2 of 3. The hydrology parameter estimate capability of HEC-HMS V4.7 is still not fully functional for all loss and transform methods. This instruction is to show its application for ModClark transform method.

  1. For this demo, ensure ModClark transform method is chosen for all the subbasins. The equations of ModClark transform method used for this demo is shown on Figure 1. The Time of Concentration (TOC) equation establishes a relationship between TOC and Longest Flowpath Length (L) in miles, Centroidal Flowpath Length (LC) in miles, and 10-85 Slope in feet per mile. It is important to note that these types of equations are usually developed for use with specific watersheds and are not always applicable to other regions. Furthermore, for simplicity, it is assumed an R/(TC+R) relationship of 0.5 which means that the Storage Coefficient (R) will be equal to the Time of Concentration (TC). 
Figure 1
  1. Click top menu of Parameters, then Transform —> ModClark to open the parameter estimate window for ModClark transform method. Click “Calculator…” at the bottom of the window to open the Expression Calculator.
  2. In the Expression Calculator, choose Time of Concentration for Field and click “Stats” Tab to show all the variables to be used for Time of Concentration calculation. Note the variables shown here are calculated previously by clicking top menu of Parameters, then Characteristics —> Subbasin. In the bottom of the Expression Calculator, type in the TOC equation as shown on Figure 2. Click the button of Calculate to fill in Time of Concentration value for each subbasin. Since we assume R/(TOC+R)=0.5 for this demo, Storage Coefficient (R) is equal to TOC. Simply copy TOC values and paste to Storage Coefficient (R) column (Figure 2).
Figure 2

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