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<br />i <br /> <br />United States <br />Department of <br />Agriculture <br /> <br />Natural <br />Resources <br />Conservation <br />Service <br /> <br />1475 Pine Grove Road. Suite 201 A <br />Steamboat Springs <br />Colorado 80487-8803 <br /> <br />To: Bi 11 Green <br />Colorado Water Conservation Board <br /> <br />RECEIVED <br />AUG 1 1 1997 <br /> <br />Colorado W <br />Consef\lati ater <br />on Board <br />Date: 8-8-97 <br /> <br />The following is short history of the design sequence that led to the planned dam diversion <br />design for the Smith Ditch. <br /> <br />The capacity of the exisfingold concrete dfversion structure is 855 c.f.s. with the water <br />to the top of the right sidewall. A new channel has eroded around the right side of the <br />existing old concrete diversion. Most of the flow is carried by the new channel with only <br />the highest flows going through the diversion. The Elkhead Creek stream channel invert is 4 <br />feet below the Smith Ditch irrigation channel invert. <br /> <br />The first design concept for the repair of the dam diversion was to increase the capacity of <br />the existing old concrete diversion structure with the addition of an emergency spillway. <br />The design flow rate that I used for this concept was 1430 c.f.s.. the 25 year snowmelt flow <br />that I took from the design reports for Elkhead Reservoir. The emergency spillway was <br />planned using a gabion lined drop structure with the 132 foot wide crest of the structure <br />above the irrigation water level. The existing old eoncrete diversion structure would have <br />remained an integral part of the system and would have been required to pass a considerable <br />portion of the design flow rate. Stop logs would have been installed in the existing old <br />concrete diversion to control the water level for irrigation. <br /> <br />Consultation with Kent Holt of the Division of Water Resources placed the design flow rate <br />in doubt. The stream gage on Elkhead Creek near Elkhead has recorded and instantaneous peak <br />exceeding the 855 c.f.s. capacity of the existing old concrete diversion 26 of the 43 years <br />of record from 1953 to 1995 and exceeded the 1430 c.f.s. design flow rate 7 of the 43 years. <br />The drainage area at the stream gage is 64.2 square miles. The drainage area at the Smith <br />Ditch Diversion is 209 square miles. My NRCS state office analyzed the hydrology for the <br />watershed and recommended a 25 year design flow rate of 2415 c.f.s. <br /> <br />The Smith Ditch Dam Diversion as planned will restore the ability to divert water from <br />Elkhead Creek into the Smith Ditch for irrigation without reliance on the existing old <br />concrete diversion. The planned dam diversion is located in the channel downstream from the <br />existing old concrete headgate in an area that has not been disturbed by the stream eutting <br />a new ehannel around the existing old concrete diversion. The upstream and downstream <br />invert of the proposed dam diversion are planned at the present invert of the stream channel <br />of Elkhead Creek. <br /> <br />The planned diversion structure has an adjustable height weir that ean raise the water level <br />in Elkhead Creek to allow diversion of water into the Smith Ditch. The irrigation water can <br /> <br />The Natural Resources Conservation Service. <br />formerly the Soil Conservation Service. <br />is an agency of the <br />UnHed States Deoartment of Aori culture <br /> <br />File Name: <br /> <br />AN EOUAL OPPORTUNITY EMPLOYER <br />