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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />can contain the maximum amount of water without inundating of the noodplain. A channel's <br />bankfull can usually accommodate the flow that occurs every 1.5 years. <br /> <br />Specific to field measurement are surveying the river cross sections. Cross section profiles are a <br />necessary input to the completion of the hydraulic analysis and. therefore. must be obtained. <br />There are a number of different techniques for surveying that can be used. Those techniques <br />include the use of a level. a total station, GPS equipment. or aerial surveying. The technique that <br />is recommended for floodplain delineation is using a survey grade GPS unit that has the <br />capability of acquiring coordinates within centimeter accuracy. GPS surveying is less expensive <br />than aerial surveying and less time consuming than both using both a level and a total station. <br />GPS is also the most accurate of the four techniques and the data can easily be imponed into a <br />GIS. If. however. there is enough funding to obtain an aerial survey of the channel and drainage <br />basin it should be considered. especially if no previously GIS data exists. <br /> <br />The operation of individual GPS units may vary depending on the manufacturer. but all units will <br />acquire point data in the acquisition process. Points are taken wherever there is a significant <br />break in the slope or where there is a significant elevation change within a cross section. In <br />addition. points are taken at left and right bankfull and at the thalwag of each cross section. Cross <br />sections are taken immediately downstream and immediately upstream of each bridge. culvert. or <br />other obstructive crossing. A single point is taken on the deck surface of each crossing. An effort <br />is made to take cross section profiles no more than J(X)() feet apart. If the GPS data acquisition <br />program allows the input of descriptions. all separate cross section should have an identifying <br />name and the banks and center of the channel should be noted. The GPS output file is In a <br />comma delimited text file reponing the point number. latitude. longitude. elevation. and <br />description. This text file can then be handled in the procedures described below. The surveying <br />of the cross sections concludes the data collection process. <br /> <br />4.2 Data Analysis <br /> <br />The GPS points obtained from surveying are directly downloaded into the computer program <br />thai should accompany the GPS data collector. The points are opened in the fonn of a <br /> <br />16 <br />