Application of HEC-HMS in Watershed Modeling

1. Introduction The purpose of this project is to show how the combination of GIS and HMS can be used to predict the impacts of future urban development on a watershed. Development does not effect the soil parameters of the watershed but it does increase the percent imperviousness of the area. I wanted to find the significance of this change by observing the watershed’s runoff behavior before and after development. I chose to use a 100 year storm on the Salado Creek watershed for this analysis. Sub-basin Size Sensitivity Analysis One of the first inputs required by CRWR-PrePro is a stream definition threshold. This threshold determines the number of sub-basins that will exist in the model. A small threshold will result in a large number of small sub-basins where as large thresholds will result in a small number of large sub-basins. Figure 6 shows how this relationship varied. A threshold of 2,000 resulted in 146 sub-basins with an average size of 811 acres and a threshold of 50,000 resulted in 8 sub-basins with an average size of 14,800 acres. I determined the six thresholds shown in Figure 6 would be adequate to test the model’s sensitivity to this characteristic. Study Area The Salado Creek basin is located in Bexar County where it flows southeast for 30 miles through the urbanized city of San Antonio and then drains into the San Antonio River. This 185 square mile watershed can be described as having rolling terrain with elevations as high as 622 feet and as low as 90 feet. The basins location is shown below in Figure 1. Figure 2 shows the roads and buildings in this watershed. The city covers all of the southern half (below the red line) of the watershed and only a small percentage of the northern half (above the red line). Texas A&M UniversityDepartment of Civil EngineeringCven689 – CE Applications of GISInstructor: Dr. Francisco OliveraLogan BurtonApril 29, 2003 Threshold of 2000 Threshold of 4000 Threshold of 8000 Figure 1: Bexar County and the Salado Creek Watershed Figure 2: Urbanized Watershed Methodology 1. Gather all data – The data required for this analysis is: 30 m DEM, NHD reach files, urban files, and gauging stations. 2. Load Extensions – The extensions required are: CRWR-PrePro, CRWR-Raster, CRWR-Vector, and HEC-GeoHMS. 3. Manage and Organize Data – All data must be projected into a common projection. Use ArcView 8.x series for raster projections and CRWR-Vector for vector projections. 4. Run CRWR-PrePro on the study area -- Follow the documented lab exercises titled “Delineating the Stream Network and Watersheds of the Guadalupe Basin” and “Developing a Hydrologic Model of the Guadalupe Basin” written by Dr. Francisco Olivera and Dr. David Maidment. 5. Import schematic into HMS – This will be the basin model used for the analysis. 6. Construct the Meteorological model and the Control Specifications in HMS – A 100 year event was chosen with 15 minute time intervals. 7. Calibrate the model – USGS gauging stations were used to calibrate the peak flow from a 100 year storm (Figure 3). 8. Edit model to simulate future development – The soil parameters do not change. The only parameter that was changed was the percent imperviousness (HMS Applications Guide 2002). 9. Run HMS again to see the significance of development to watershed runoff. Compare both hydrographs. Threshold of 12000 Threshold of 20000 Threshold of 50000 Application of HEC-HMS in Watershed Modeling Figure 6: Sub-basin Size Variation Methodology Each of the six basins was built in ArcView and imported into HEC-HMS. The default soil parameters were used so the only difference in the trials would be the basin size. The same meteorological model and control specifications were used for each trial. The peak flows could then be compared to find how sensitive the model is to varying basin sizes. Results It was found that a reasonable threshold would yield resulting flows within a range of 0.5%. Reasonable thresholds are defined as those that produced an average sub-basin size of less than 5.6% of the total watershed area. The only trials significantly different than the rest were the two with thresholds of 20,000 and 50,000. They resulted in flows that were 2.18% and 5.44% off target, respectively. Figure 7 shows the complete results. Figure 3: Calibration Results The two hydrographs in Figures 4 and 5 show the effects urbanization can have on a watershed. The flow increased from 65,000 cfs to 75,000 (about 16%) with the development of 80 square miles in the northern half of the basin. This is a significant increase that could easily cause a rise in the back water surface elevations if the existing drainage system was at or nearly at full capacity to begin with. This should be given careful consideration before the development is allowed to presume. Conclusions Engineers can use the information from this analysis to see if the existing drainage systems need improvements or upgrades to handle the increase in flow caused by the future development. A 16% increase in runoff could cause backwater elevations to rise several feet, causing additional damage to surrounding property. Figure 7: Results of Sensitivity Analysis Conclusions Overall, it was better to use a conservatively low threshold than one that was too large. The lower thresholds allow the model to better represent the actual basin by preserving its characteristics like slope and lag time more accurately. However, a threshold that is too small increases the size of the data set which can significantly increase the run time. Figure 4: Hydrograph for Undeveloped Basin Figure 5: Hydrograph for Fully Developed Basin