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<br />'. <br /> <br />t- <br />rP <br />c.J <br />C'1 <br /> <br />.' <br /> <br />Normal maintenance of the channel will begin vii th the comple- <br />tion of construction and will require removing sediment deposits <br />periodically. It also will require the removal or control of weeds <br />and plants along the channel. The levee will require maintenance <br />following all substantial flows in the floadway which are expected at <br />about five-year intervals. Groins and other structural work probably <br />will be required where the current tends to erode the levee. It also <br />may be necessary to construct pilot channels to encourage the flows <br />into desired courses and to improve sediment transport characteristics. <br />The levee must ue I~ised from 0 to 2 feet, for about one-third of its <br />length, during the first 30 ~'€ars in order to compensate for increased <br />heights of water surface in tile flcodway resulting from sediment <br />accumuJati.on in McMillan Reservoir. <br /> <br />Controlling growth of salt cedars is expected to be the chief <br />maintenance proulem in tile floodway. The plan provides for airp]~ne <br />spraying of the floodway with chemicals, The spraying schedule .lill <br />be coordinated with nearby farmin~ operutions so that crops will not <br />be damaged, Ground spra;,'ing or control by mechanical means can be resor- <br />ted to if necessary. <br /> <br />Maintenance of the heading structure is expected to consist <br />chiefly of replacing the temporary dike section across the head of the <br />floodway after it has been breached by floods. The gates must be main- <br />tained ill good operating condition, <br /> <br />Major repairs must be made in tho event of floods greater <br />than the design capacity of the project, In view of the probability <br />of such floods, the plan fnr maintenance provides for replacing one- <br />third of the E vee and con'le~-ance channel once every 25 years. <br /> <br />Floodway Desi.gn <br /> <br />The floodway capacity, originally set at 50~000 cubic feet <br />per Becond by consultatinn with the iOngineering Advisory Committee to <br />the P"cos River Commission, was later reduced to 40,000 cubic fe8t per <br />second as a combined result of design cnnsiderations and the .rlshes of <br />the Carlsbad people. A Hazen-type frequency analysis of the 50 years <br />of recorded flow at Artesia indicates a flood of this magnitude can be <br />expectGd about once in 17 years, The floodway Jevee was designed so <br />that it will permit the passage of a maximum discharge of 40,000 cubic <br />feet per secnnd with a minimum freeboard of 3 feet, The required height <br />of the levee was determined by backirater studies ca.rried upstream <br />from McMillan Dam, beginning with the wo:ter surface elevation at the dam <br />required to pass a discharge of 40~000 cubic feet per second through <br />the existi~! spillways. <br /> <br />Channel Design <br /> <br />From comparative studies of the water salva.ge that could be <br />accomplished with conveyance channel capacities of 1,200, 1,500, and <br /> <br />5 <br />