Laserfiche WebLink
<br />Flood Plain and Channel Geometry <br /> <br />Cross sections of the valley are surveyed for use in computing the <br />water-surface profiles where a one-dimensional step-backwater technique <br />is used. When two-dimensional flow models are used, more detailed data <br />may be needed. The computational methods chosen govern the amount of <br />data necessary. <br /> <br />Land Use <br /> <br />Land use is an important variable in computing backwater damages. <br />A survey will quantify the types and values of the various structures <br />(houses, warehouses, factories, and so forth) and other uses (agriculture, <br />recreation, and so forth) of the land which may be inundated by flood <br />waters. <br /> <br />Hydraulic and Hydrologic Data <br /> <br />Summarize and discuss the source and availability of hydraulic <br />information pertinent to the site. These data may include stage-discharge <br />relations, measured water-surface elevations f~om previous floods, <br />velocity distributions measured at an old bridge and discharge measurements. <br />Earlier site reports may be available which summarize this information. <br />Manning's n value should be selected for each section or subsection to <br />be used in the hydraulic analysis. Measured water-surface elevations <br />should be used to calibrate and verify the flow model selected to compute <br />the water-surface profiles. <br /> <br />Hydrologic data may be developed from gaged records at the site or <br />from nearby sites. A discharge frequency curve will be constructed and <br />a discharge hydrograph constructed for each discharge used in the analysis. <br />The discharge frequency relation is used in computing the risk while the <br />hydrograph is used to obtain the length of time that an embankment might <br />be overtopped (and traffic interrupted) during a flood. The overtopping <br />time may also be useful in evaluating embankment erosion potential. The <br />degree of local scour may depend on the duration of the hydrograph. <br /> <br />Geologic Data <br /> <br />Several types of geologic data may be useful in the bridge site <br />investigation. These include surface geology, channel morphology, soils <br />information, and scour history. <br /> <br />Surface Geology.--Surface geology supplies information on the <br />regional geological conditions. The surface formation can be determined <br />and characteristics of the formation related to bridge> designs could be <br />specified. > <br /> <br />Channel MOrphology.--Channel morphology is a complex problem <br />(Richardson and others, 1975) which can affect the stability of bridges <br /> <br />15 <br />