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bridge: <br />Numerous positive factors favored a site about 1000 ft. downstream from the Atwood <br />A well-defined low water channel about 3 ft. deep and 60 ft. wide has been carved within <br />the main channel. This channel runs right against the main north bank. Elsewhere in <br />District 64 the river wanders about in a main channel bed several hundred feet wide, and <br />often distant side channels exist. The narrow focus of the flow at this site seems to be <br />related to the presence of a historic bridge, the wood pilings of which are still visible. <br />2. The narrowing of the channel at the historic bridge site acts as a control. The channel is <br />wider above and below this location. This indicates that a more stable stage discharge <br />relationship will exist than in a wider, wandering channel. The discharge rating should <br />be about as stable as possible for a sand channel gage. <br />The site was far enough downstream from the bridge that it is not adversely affected by <br />the hydraulics of flow around bridge piers and piled debris. <br />4. The site was on land owned by the State of Colorado, making it possible to have an <br />easement for access and long-term use. <br />GAGE TYPE <br />Having awell-confined low water channel right up against the main flood bank allowed <br />for the installation of a stilling well gage, where stage is measured directly with float <br />instruments. This is the most accurate and reliable way to measure stage. It is also an expensive <br />gage to install, since a permanent stilling well must be excavated into the bank and inlet pipes <br />run out into the stream. Our Kersey and Julesburg gages use stage sensor lines rather than <br />stilling wells. Stage sensors are cheaper but less accurate and dependable. <br />The extra expense of this installation is expected to pay back in terms of fewer <br />maintenance and measurement trips, and more reliable flow data. <br />An important design factor for installation of the gage was the elevation of the shelter, <br />since being on the bank it would be in the flood plain. Survey levels were run back to the bridge, <br />so that the instrument shelf could be set at a level that was higher than the bottom of the bridge. <br />Logan County or CDOT did not have mean sea level elevations available for the bridge, so the <br />actual elevation of the gage will need to be determined at a later date. <br />COST & MAN-HOURS <br />The South Platte River at Atwood gage was put into operation on July 19, 2005. <br />The cost of the materials and installation was $12, 391. The instrumentation and Satellite <br />Monitoring equipment cost approximately $5000. Additionally, $1100 was paid to the Colorado <br />State Land Board for Right of Way consideration and R.O.W. document preparation. This <br />brought the total cost of the gage to approximately $18,491. <br />