FLOOD00835 - Laserfiche WebLink

Home Browse Search

e , . precipitation excess is characterized by a unit hydrograph which is directly determined from basin "lag" that is computed as a function of the curve number, mean land surface slope within the subbasin and subbasin hydraulic length. Conventional calibration studies are performed to develop the appropriate relationships between iand use and the hydro. logic parameters (CN and lag). The respective subbasin mean curve numbers for an alternative land use pattern are determined automatically by assigning the calibrated curve number to each grid cell within each subbasin depending upon the grid cell hydrologic soil group and grid cell land use. and computing the mean value for each subbasin. The average subbasin land surface slopes are computed in a similar manner. The lag, and thus unit hydrograph is developed from an empirical lag equation containing relationships of CUIVe number, land surface slope. and subbasin hydraulic length. The technique adopted is a modest extension and automation of the method developed for San Diego County (Franzini,et al.. 1971). e Trail Creek Test Application The Trail Creek Watershed was subdivided into the 21 subbasins shown on Figure 3 and HEC-I model parameters were determined for each. The ioss rate functions (CN's) and unit hydrograph lags were determined automatically from the grid cell data file using the calibration data developed and tabuiated in Table 2. Table 2 also summarizes the hydro- logic parameters for each subbasin that were automatically developed from the grid cell data file by the data processing interface computer program HYDP AR. Table 3 summarizes the results of evaluating the alternative conditions indicated. Note that the flow rate increases for each of the specified probabilities but at a less propor- tionate rate for rarer events. Note also that the flow rate change for say the IOO-year event is different between control points and that the change in flood elevation is not directly proportional to the change in flow. Study of the table indicates that the hydro- logic consequences of land use and engineering works are complex and require careful analysis. FLOOD DAMAGE ANALYSIS ; The objective of the analysis is to evaluate the damage potential of alternative land use pallerns and/or specific development proposals. The existing conditions (1975) land use pattern is evaluated with no assumptions as to land development controls (e.g., the development exists). An alternative future land use pattern evaluation requires examina- tion of policy assumptions regarding land use development controls. A method was developed to automatically extract information from the data file and format it for ex. pected annual damage assessments for the alternative land use patterns for alternative land use development policies that consisted of generating damage potential functional relationships from the grid celi data bank. The method constructs a unique elevation- damage relation for each grid cell within the flood plain (based on ground elevation. land use, and damage potential) and aggregates the individual cell damage functions to an index location for each designated damage reach. The technique for aggregating the damage functions is similar to the conventional method of using a reference flood to properly re- ference each cell to the designated index location. The damage functions are merged with hydrologic (flood frequency) and hydraulic (rating curve) data within the HEC.l program e 9 ...