FLOOD01360 - Laserfiche WebLink

Home Browse Search

. . ~ ~ .. . ~ ~ . ~ ~ . .. Table 2-Peak determination methods. Ga2ed Sites I) Normal Distribution 2) Log-Normal Distribution 3) Gumbel Extreme Value Distribution 4) Log-Pearson Type III Distribution t . ~ I t ~ l . 3. Check Flows. Culvert operation should be evaluated for flows other than the peak design flow because: (1) It is good design practice to check culvert performance through a range of discharg- es to determine acceptable operating conditions, (2) flood plain regulations may require the delineation of the 100-year flood plain, and (3) in performing flood risk analyses, estimates of the damages caused by headwater levels due to floods of various frequencies are required. t .. .. t t . t t .. . , . ~ l- t ~ ~ ) Check flows are determined in the same manner as the peak design flow. The hydrologic procedures used should be consistent unless unusual circumstances dictate otherwise. For example, a stream gage record may be long enough to estimate a 10-year peak design flow but too short to accurately generate a 100-year check flow. Under these circumstances the check flow should be evaluated by another method. 4. Hydrographs. The entire flood hydrograph at the culvert site must be defined if upstream storage is to be considered in culvert design. Passing the peak design flow through a culvert neglects the attenuating effects of up- stream storage, If this storage is taken into account, the required culvert size may be substantially reduced. Since volume considerations are now involved, the flood hydrograph becomes an integral part of the design process. A flood hydrograph is a plot of dis- charge versus time. Figure II-I depicts a typical flood hydrograph showing the rise and fall of stream flow over time as the flood passes, Actual flood hydro- graphs can be obtained using stream gage Unlla2ed Sites I) USGS Regression Equations 2) FHWA Regression Equations 3) Regional Peak Flow Methods 4) SCS Peak Discharge Method S) Rational Method records. These measured storm events can then be used to develop design flood hydrographs. In the absence of stream gage data, empirical Or mathematical methods, such as the Snyder and SCS synthetic hydro- graph methods, are used to generate a design flood hydrograph. The unit hydrograph technique is a popular procedure for determining the response of a watershed to a specified design rainfall. A unit hydrograph repre- sents the runoff response of a watershed to a uniform I-inch rainfall of a given duration. A unit hydrograph may be gener- ated from data for a gaged watershed or synthesized from rainfall and watershed parameters for an un gaged watershed, Both methods are briefly described below. a. Unit Hvdrollraoh Formulation - Ga2ed Watershed. To develop a unit hydro- graph for a gaged watershed, the designer should have streamflow and rainfall re- cords for a number of storm events. The rainfall data must be representative of the rainfall over the watershed during each storm event. In addition, the rain- fall events should have relatively con- stant intensities over the duration of the storm. Unit hydrograph generation involves four steps which are illustrated in figure 1I-2. (I) The groundwater or low flow contribution of the gaged flood hydrograph is estimated and removed from volume consideration. This groundwater or low flow contribution is generally regarded as constant and estimated to be the amount of stream flow prior to the storm event, (2) The volume of the remaining runoff hydrograph is calculated. This is termed the direct runoff volume. (3) The direct IS