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<br />Hvdraulics
<br />Hydraulic analyses conducted in this study were done using u,s.
<br />Army Corp of Engineers computer model HEC2. The general land
<br />slope in the study reach is less than 1 percent, therefore
<br />backwater effects from roads, bridges, ete are very significant.
<br />In some oases HEel was used to develop discharge-elevation rating
<br />tables for use in TR20. Water surface profiles were computed for
<br />three general reaches.
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<br />Reach number one inclUdes a profile of Pawnee Creek from the
<br />South Platte River upstream through the Burlington Northern
<br />Railroad and U.S. Highway 6 bridges, then continuing on through
<br />County Road 33 and 31.
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<br />A considerable out of bank division of flow occurs between County
<br />Road 31 and the Pawnee Ditch crossing. The Pawnee Creek channel
<br />at this location has a very limited carrying capacity. A portion
<br />of the out-of-bank flow moves toward the southeast to the
<br />vicinity of Atwood. Water surface profiles were not computed for
<br />this out-of-bank segment of flow, except along U.S. Highway 6
<br />between Atwood and the main channel, because of the lack of
<br />topographic data.
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<br />A significant out-of-bank division of flow also occurs just below
<br />County Road 33. The out-of-bank flow at this location moves
<br />overland away from the main channel towards the northeast and
<br />eventually to the vicinity of the Riverside Cemetery. Water
<br />surface profiles were not computed for this segment of flow
<br />because of the unpredictable meandering flow pattern through
<br />farmland; however, a probable flow path was Sketched on the flood
<br />plain maps.
<br />Alluvial fan type topography exists along Pawnee Creek
<br />immediately below U.S. Highway 6 and the railroad. The flow
<br />here divides three ways: the main channel, left overbank, and
<br />right overbank. Only a profile along the main channel is shown
<br />in the included flood profiles. B<lckwater computations were
<br />made, however, for the overbank flow segments for purposes of
<br />defining the flood boundaries on included flood plain maps.
<br />A second reach includes a short segment of flow along the
<br />railroad and U.S. Highway 6 between the main Pawnee Creek channel
<br />and the town of Atwood. The source of this flow is from out-of-
<br />bank conditions at the upper end of reach one, discussed
<br />previously. A major part of this segment of flow will overflow
<br />the highway and railroad near Atwood with the residual moving on
<br />to the main channel at U.S. Highway 6.
<br />The third reach is a profile along U.S. Highway 6 and the
<br />railroad from the Riverside Cemetery upstream to the bridges on
<br />Pawnee Creek.
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