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<br />Each of the bridge, geomorphic, and flow factors is related to the bridge <br /> <br />design process in several ways, and changes in one factor may subsequently <br /> <br />affect others. There are instances where it is desirable to modify the <br /> <br />channel, flood plain, amount of flow, or flow alignment at a bridge site. <br /> <br />Knowledge of existing geomor~hic and flow factors can be used as guidelines to <br /> <br />estimate the effects of a prcposed site modification. <br /> <br />The determination of the design flood at a bridge is usually based on historic <br /> <br />or observed flow data. Sometimes these data are available at the site, but <br /> <br />most often they are estimated from rainfall records, or transferred by <br /> <br />statistical methods from a streamflow gaging station nearby. The water level <br /> <br />(stage) associated with the design flood is estimated from historical data or <br /> <br />computed from channel conditions as measured at the bridge site. Estimates of <br /> <br />these flow factors are then ~.sed in the design of the bridge or <br /> <br />countermeasure. A lack of we.ter level or discharge data at a bridge may give <br /> <br />misleading design criteria if unexpected or temporary flow conditions, such as <br /> <br />backwater occurs. Also, flo~' factors determined at a bridge site are affected <br /> <br />by geomorphic changes. For Example, several geomorphic and flow factors will <br /> <br />change if a meander cutoff occurs in the vicinity of a bridge site. The <br /> <br />occurrence of man-induced or natural channel changes presents a problem to the <br /> <br />bridge designer because hydn.ulic conditions used in the design of the bridge <br /> <br />may change with time. <br /> <br />At sites protected with rock riprap, there is evidence that the success or <br /> <br />failure of bank protection measures are related not only to proper selection <br /> <br />of the flow stress/stone SiZE~ (DSO) in the design procedure, but also to the <br /> <br />evaluation of the hydraulic Hnd channel factors that are typical or <br /> <br />potentially applicable to th" site. <br />