South Platte River Basin District 64 Diversion Structure Invent

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August 4, 2004 South Platte River Basin District 64 Diversion Structure Inventory 2 Pawnee Narrative of Structure Components and Repair Plan The Pawnee structure consists of a dam/spillway with three 5-foot-wide timber stoplog regulator gates, and a diversion consisting of provisions for six timber stoplog headgates (none currently present). Similar to the "South Platte" structure, a 4-foot-high by one- foot-thick wall was added just upstream of the original structure, and is functioning as a low-level dam. The tops of the original six piers appear to have been purposely broken off, to match the configuration of the improvements. The piers are now connected to the low-level dam, and act as counterforts. The intermediate piers from the original structure have been abandoned-in-place. There is a 1-foot-high by 4-foot-wide sill at the downstream end of the spillway apron, with 1.5-foot-wide by 1-foot-high notches every 30 feet. The regulator gates are located along the north half of the dam/spillway. Downstream of the dam, it appears that low flow channels have been excavated for several hundred feet. One is toward the center of the river bottom and one is along the south bank. The overall condition of the structure appears to be fair. The downstream edge of the spillway apron and abutments are being supported by driven steel sheetpile. Approximately 2 to 3 feet of vertical scour has occurred downstream of the apron. Numerous hairline cracks were found in the south endwall (abutment). The north endwall also has numerous hairline cracks, as well as significant cracks at its angle points. The flow concentrated along the south bank downstream of the structure is eroding the bank for several hundred feet, and undermining the northeast corner of the south endwall. Upstream flow is seeping through the dam (east), then south along the structure toward the south bank, then continues downstream. Upstream dredging is needed for several hundred feet, after which south bank stabilization should be installed. Stabilization of the downstream river bottom and banks is needed. Placement of proper stabilization downstream of the spillway apron is critical, to help reduce or eliminate additional scour. Refer to the "Stabilization" portion of Section 4.0, Conceptual Repair Plan. Additional gates should be constructed toward the south end of the dam, to help flush excess sediment downstream. Provisions should be made for a walkway with handrail, for maintenance of, or access to, the structure. Currently, almost the entire river is flowing north through the diversion structure into the Pawnee Ditch. There is a small amount seeping through the dam to the east, as described above. None of the gates are present for the diversion structure. There are provisions for five stoplog gates, which should be installed. The condition of the apron could not be seen due to silt deposition. Numerous hairline cracks were found throughout the concrete of the diversion, with more significant cracks found at the angle points. The last 15 feet of the west downstream wingwall has completely broken off, with only reinforcing steel holding it in place. This section of wall should be removed and replaced in accordance with the "Crack Repair" Subsection of Section 4.0, Conceptual Repair Plan. The new section of wall should be keyed into the west bank of the ditch. Downstream bank stabilization is needed, and new decking and handrail should be installed. Farnsworth Group