INTRODUCTION TO HEC-HMS

CE 394K.2 SURFACE WATER HYDROLOGY

UNIVERSITY OF TEXAS AT AUSTIN - SPRING 99

#1) Map of Waller Creek HMS Model

#2) Subbasin drainage areas:

SUBBASIN	AREA (sq. km.)
12	3.602
13	6.912
14	0.966
15	0.563
16	1.633
17	0.437
18	0.252
TOTAL	14.365

#3) a) Description of Subwatershed #13 - The drainage area of subwatershed 13 is the largest of all the other basins (6.912 sq. km.). Since it is the largest basin, it is understandable that this watershed also has the longest SCS lag time (321 minutes), which is the time for a drop of water to travel from the edge of the basin to its outlet. The initial/constant loss rate method is used for calculating rainfall losses absorbed by the ground. Initially, the basin loses 5 mm of rainfall to the ground. The rest of the water is then lost at a rate of 2mm/hr. Also, the model assumes that no area is impervious throughout the basin.

Description of Reach #7 - The method used to route the water through the reach is the Muskingham Routing Method. The first parameter involved with this method is K, which is the travel time of a flood wave passing through the reach. For this reach, the flood wave takes 0.9579 hrs to move from one end of the reach to the other. The second parameter used in the Muskingham method is X, which is a measure of the degree of storage in the reach. For this reach, X = 0.15. This number indicates a relatively high amount of storage in the reach (range in X is from 0 to 0.5). Finally, the reach has been divided into two subreaches to keep the computations stable.

#4) Design Precipitation Input File:

#5) Time Intervals of Computation - For a 3-day storm at 10 minute intervals, there will be 432 intervals of computation (3*24*60/10=432)

#6) Drainage and Discharge Figures for Waller Creek:

OUTLETS	PEAK DISCHARGE (CMS)	DRAINAGE AREA ABOVE POINT (SQ. KM.)	DISCHARGE/UNIT AREA (CMS/SQ.KM)
Hemphill	46.149	10.514	4.389
MLK	45.836	11.48	3.993
15^th St.	45.631	12.043	3.789
7^th St.	45.611	13.676	3.335
1^st. St.	45.192	14.113	3.202
Sink	44.870	14.365	3.124

#7) Design Study of Flood Diversion Tunnel for Waller Creek:

For my design discharge study, I used the web link given in the exercise to revise the initial/constant loss factors of the basin model. Looking at the maps, I noticed that a very large area of the watershed is now covered by impervious material (such as sidewalks, buildings, roads, etc.). Therefore, I chose high values of imperviousness and lowered the initial loss factors accordingly. In the case of the precipitation model, I used the values given in the chart on the exercise to change from a 10 year design storm to a 100 year design storm. Both models are shown below:

Once the basin and precipitation models were specified, I ran the configuration through HMS using the same control specifications as the previous model. With this new run, I found the design discharge at 15^th St. to be 79.188 cms, which was well above the discharge of the 10 year design storm. The discharge hydrograph at 15^th St. is shown below. If there is a known flow gage in the area, this discharge can be checked against historical data in order to judge the accuracy of the model.