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<br /> <br />".. ..... "'''''<....9...'!?i;\j!.;.:.........'.i....,<: <br /> <br />..:;..::;.... -:~::. .\:.:.:.::~ . :':":.:'. .':': ..' -::::.;:::: <br />.::.;. ;:.....:::...",",. . '. ;:...:, ::. .:.: -.: ;.'; ",' .:. :...: ;.i...:.~..:..::...:..:.....;..,..,....::.. '. .. ::::;: .'...:...:.:".",",.;.",.. <br />. -.' ":~x':':::""'"' ." "." -. ..:..~::~!.::...i;.~:...~.!:i~:.:it::. <br />.......:...::...::...:.'.....'...:~......;..:....:..~:.\..:~.:.:;...:...:...:.............. <br />": '~:':'.:' _;.",.. ~{?:......~....~..~.:.;..:.::.:.,.:.\.:~.~~..~;..:..;.;.'.~:;:'.):... .....;.: :::/::.i>.;?X:.:;;.::~.;:.\\..<... ,': .... .",. ';:. <br />:~:?iY/)!.\.: ....;.:. ::.:.::::-:x;,:,,:~:::: :.:.:.:.:.:..:,':";::: ." ..::.::/:..::....:...:::::"... .' ::::.:::}\.\(:/):\.~.:::.~:::::::.::.:.::::::. <br /> <br />NOT TO SCALE <br /> <br />8-----8' <br />C....--C' <br />0-'-'-'-'-0' <br /> <br />BFW <br />ACW <br /> <br />EXPLANATION <br /> <br />ACTIVE-CHANNEL REFERENCE LEVEL <br />BANKFULL REFERENCE LEVEL <br />LOW-FLOW WATER LEVEL <br />BANKFULL WIDTH <br />ACTIVE-CHANNEL WIDTH <br /> <br />Figure 6. Block diagram of a typical stream channel. <br /> <br />Tbe active-channel and bankfull reference <br />levels for a typical stream channel are <br />illustrated in figure 6. Photographs of <br />active-channel and bankfull reference levels at <br />six gaging stations in Iowa are shown in figure <br />7. <br /> <br />A standard particle-size analysis (dry sieve, <br />visual accumulation tube, and wet sieve) was <br />performed for each of the composite sediment <br />sampl"s collected from the channel bed and the <br />left and right channel banks (Guy, 1969). The <br />local gradient (ORA) was measured from <br />1:24,OOO-scale topographic maps and was <br />calculated as the slope of the channel between <br /> <br />the nearest contour lines crossing the channel <br />upstream and downstream of the gaging station. <br /> <br />Of the 157 gaging stations selected for study <br />using the channel-geometry flood-estimation <br />method, 46 were on stream channels that were <br />or were suspected of being channelized. <br />Bankfull width (BFW) and bankfull depth (BFD) <br />measurements could not be made for these sites <br />because channelization affects the long-term, <br />stabilizing conditions of stream channels. <br />Active-channel width (ACW) and active-channel <br />depth (ACD) measurements were made at these <br />46 sites because channel conditions indicated <br />that the active-channel portions of these <br /> <br />l <br /> <br />20 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA <br /> <br />, <br />I <br />