Evaluating Surface Water Transformation in HEC-HMS

Dr. Nelson


Overview

You have learned the basic modeling techniques and performed various sensitivity studies with the precipitation and infiltration methods in HMS and GSSHA. In this exercise we will examine how HMS develops the runoff hydrograph, which is often termed Transformation

Our sensitivity studies will again center around the Judy's Branch Watershed and you can use the same data as you have previously. We will use different transformation methods and see how they affect output. You will perform similar analyses with GSSHA (in the next assignment) and at the end you will compare the response


Model Set up

Use your basic Judy's branch WMS project with the SSURGO soil information and the same land use data you have been using.

Create land use and soil type coverages in WMS and determine the curve number for the watershed as usual when necessary.


Processes

1. Transformation method comparison

In this part we will the see variation in output hydrograph as we change the transformation method. Create a HMS model with:

    a.   SCS Transformation Method

    b.    Clark Transformation Method

Compare the results for the two different methods.


2.    Sensitivity Analysis of R in Clark Method

The Parameters for the Clark Transformation method are Time of Concentration tc and Storage Coefficient R. In HMS, open the project file that you used for 1b where you used Clark Method (you might want to save it as with new name so that you will have previous project unchanged). Now under Transform tab, leave the time of concentration the same but change the value of storage coefficient:

Compare the outflow hydrographs with normal value (Determined by WMS in part 1b) and these two cases.


3.    Sensitivity of Lag Time

The parameter for SCS Transformation method is lag time TLag.  In WMS, open project the file you used for creating HMS file for 1a where you used SCS method. In the Edit Parameter Window, when you compute basin data for SCS Transformation method, change the Lag time equation as:

Compare the outflow hydrographs from all the different cases.


4.    MODClark Method

The performance of MODClark method is affected by the parameters like initial abstraction, potential retention factor and grid size resolution. Along with these, the time of concentration and storage coefficient R also play a significant role. But as we already analyzed the sensitivity of time of concentration and R, we will not change these values. Here we will be analyzing the sensitivity of Initial abstraction, potential retention factor and grid size resolution.

  1. From the WMS base project create a 100 by 100 MODClark grid cells. Enter all values to create a MODClark HMS project. Use following:

Run HMS for all three cases and compare the results

       b.   Now In WMS, delete the grid you had and create grids of 5 by 5, 25 by 25 and 200 by 200. Then, in each of those models use the following:

Run MODClark in HMS for all the different grid cells sizes. you already have results for 100 by 100. compare the results for four different grid resolutions.


To Turn in Now:

To Turn in with Assignment 23 (think about these now):