FIGURE 2.2 Flood flow hydrograph for Leon Creek at Interstate Highway 35, San Antonio, Texas, for the July 2002 flood. SOURCE: USGS National Water Information System.

tributary, and the right-hand side of this figure shows a HEC-RAS (River Analysis System) flood hydraulic model of Rosillo Creek. The Hydrologic Engineering Center of the U.S. Army Corps of Engineers (USACE) in Davis, California, is the principal source for flood simulation models used in FEMA flood map studies.

The Hydrologic Modeling System is a computer program that transforms storm rainfall input to streamflow discharge output using watershed characteristics such as drainage area, slope, length of the longest flow path, and land cover and soil type, to modulate the conversion of storm rainfall to streamflow. For a FEMA flood map study, the storm input is a rainfall event, determined by statistical analysis to have a 1 percent chance of being equaled or exceeded in any year. The HEC-HMS program takes this storm and transforms it into a flood hydrograph such as that shown in Figure 2.2, computed at each of the points indicated by a red dot in the center of Figure 2.3. This calculation determines the maximum flow or discharge of water that the creek will experience during this storm event at each computed location.

The HEC River Analysis System is a computer program that takes the maximum flood discharge at each point along a river reach and transforms it into a water surface elevation

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