Upper Reaches of Leon Creek Watershed Frequency Analysis for Storm of June 30-July 2, 2002 Phase I

Author Pape-Dawson Engineers
Year 2003
Description Analysis of runoff and peak flow from a major rainfall event June 30-July 5, 2002 in the Leon Creek Watershed, Bexar County, TX
Publisher Pape-Dawson Engineers
Location Bexar County, Leon Creek Watershed

The storm of June 30-July 5, 2002 in the San Antonio and Bexar County area produced major flooding on various local watersheds. This report addresses the hydrology of the rainfall and runoff analysis for this event on one specific location on the Leon Creek watershed at state highway FM 1604, approx. 39.371 Square Miles or 25,197 Acres.

Data gathering disclosed that the NWS (NOAA) did not have continuous recording rain gages on the Leon Creek watershed, but did have daily rainfall totals both in digital image form and printed sheets. It was found that the Edwards Aquifer Authority operates several continuous recording rain gages in the NW area of Bexar County, one of which is station BE 10 located on Leon Creek west of I-10 at approximately the center of the watershed (20 sq. mi.). This station is a continuous 6 Min. recording station, and was used to develop the accumulative mass “S”-Curves in 15 Min. increments for use in an existing HEC-1 model of Leon Creek at FM 1604. From this data peak discharges were calculated for the June 30, July l , & July 2, 2002 rainfall events. Those peak discharges on Leon Creek at FM 1604 were 9,437, 12,838, and 11,941 cfs respectively.

Although the total rainfall for the six-clay period was approx. 20 inches at this rain gage, the rainfall was spread out over a longer time frame then the October 17-18, 1998 storm event. As can be seen from the summary sheet, the daily rainfall amounts of 3.72, 5.63, 4.62, & 1.88 inches for Jun30-July 3 were low values for 24-hour periods. In addition, the hyetograph values of rainfall within the 24-hour daily events were spread with hours of no rain. This can be seen from the plotted storm hydrographs included in the HEC-1 runs.

Since no isohyetal maps of this storm were available to obtain weighted average rainfall over specific areas, the rain gage values of the NWS Scenic Loop station, located at the western end of the watershed was also used for analysis. This gage showed a 24-hour value for July 2, 2002 of 7.51 inches vs. the EAA gage BE l0 for the same date of 4.62 inches. Therefore, using the cumulative rainfall coefficients for July I and July 2 rains, a rainfall value of 7.51 inches was applied in the HEC-1 analysis. The results of the peak flow analysis showed an interesting result as the flow peaked at 18.320 cfs for the July 1 rain pattern, while the July 2 pattern caused a peak flow of 24. L 72 cfs for the same total amount of rainfall. This shows how the actual pattern of rainfall events greatly affects peak flows for the same amount of 24-hour rainfall.

In conclusion I would say the max. frequency of this event for runoff discharge would be approximately 23 years, and without any other sustaining data, the realistic frequency could be in the 2 to 7 year range at the specific point in question.