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<br />I <br />I <br />I <br />I <br />I <br />1 <br />I <br />I <br />I <br />I <br />I <br />1 <br />I <br />1 <br />I <br />I <br />I <br />I <br />I <br /> <br />CHANNEL CHARACTERISTICS SUMMARY <br /> <br />CHAPTER IV <br />HYDRAULIC ANALYSIS <br /> <br />Channe 1 <br />Reach <br /> <br />Section <br /> <br />Degree of <br />Irregularity <br /> <br />Effect of <br />Obstruction <br /> <br />Roughness <br />Coefficient <br /> <br />A. General <br /> <br />Confl uence Main chan. smooth to minor minor .032 <br />to Carr St. Overbank minor appreciable .051 <br />Carr St. to Main chan. mod. to severe mi nor .045 <br />Garri son St. Overbank minor moderate .051 <br />Garrison St. Main chan. minor negl . to apprec. .030 <br />to West DFC Overbank minor moderate .046 <br />West DFC to Main chan. smooth to minor negligible .028 <br />study 1 imits Overbank mi nor negligible .030 <br />South of DFC Main chan. mi nor negl. to mod. .032 <br />along Alameda Overbank minor negl. to mod. .032 <br />C. Flood Plain Determination <br /> The flood plain for a majority of the channel in the study area was <br /> <br />A hydraulic analysis was performed for the major drainageways of <br /> <br /> <br />McIntyre Gulch and consisted of determining the flood plain, the water <br /> <br /> <br />surface profile, and the floodway for the 100 year flood. The water <br /> <br /> <br />surface profile was obtained by performing backwater calculations using <br /> <br /> <br />a computer program. The data were then plotted on the 1" = 100' scale <br /> <br /> <br />mapPing for the study and the flood plain outline defined. The basic <br /> <br /> <br />input data and methodology used in the analysis is presented in this <br /> <br />chapter. <br /> <br /> <br />B. Channel Characteristics <br /> <br />To estimate the channel and overbank roughness coefficients, <br />several field investigations were made to establish the basic channel <br />characteristics. The coefficients were then estimated taking into aCCOunt <br />the degree of irregularity, cross-section variation, obstructions, <br />vegetation, and meandering. The entire drainageway was subdivided into <br />four reaches with similar channel characteristics. A summary of the <br />channel characteristics for the various reaches is presented below. All <br />the channel reaches were considered to have minor meandering, low degree <br />of vegetal effects and cross-sections which alternate overbank flow zones <br />occasionally. The roughness coefficients were used as baseline values and <br />were adjusted accordingly for local cOnditons. <br /> <br />determined using a computerized backwater program. The program is called <br />"Multiple Backwater Profiles with Provisions for Bridge and Weir Losses", <br />Program No. 22G1M405 and written by William A. Thomas of the Little Rock <br />District, Army CQrps of Engineers. The methodology used for the backwater <br />calculations is "Method II, EM 1110-2-1409" and the program is commonly <br />referred to as "NEC-II". The required input to the program is the channel <br />cross-section coordinates, the hydraulic properties of each sub-section <br />within the cross-section, and the discharge value. The backwater calculations <br />are initiated by inputting a known water surface elevation and discharge <br />value. The longitudinal spacing of the cross-sections varies with channel <br />condition, but generally the spacing was less than 300 feet. The starting <br /> <br />7 <br />