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COMMENTARY - SHEET 8 OF 12 COST ESTIMATE - SHEET 8 OF 12 The recommended plan for this reach is to construct a 40 -acre-foot detention basin designed to control peak flows for the more frequent flood events. The basin will have sufficient volume to store the runoff from a 10-year flood event. The principle outlet is sized to release flows at a rate equivalent to a 2-year peak runoff rate. Larger flows will be released through an emergency spillway sized to pass approximately twice the 100-year flood without over- topping the dam embankment. Emergency spillway flows will discharge into the Welch Ditch and be outletted at a controlled location by the use of flow restrictors and grading of the downstream bank of the ditch. Downstream of the ditch, fl ows will be confi ned to the proposed emergency spill way outlet channel, which will tie in with proposed berming and an improved channel through the Golden High School property (see Commentary for Sheet 7). ITEM Channel Improvements Street Crossings Utility Relocation Detention Facility (Excavation, Embankment, Emergency Spillway, and Welch Ditch Flow Restrictors) Subtotal Contingencies (15%) Engineering, Legal & Administrative (15%) TOTAL CONSTRUCTION COST The maj ority of the 1 and requi red from the detention fac i 1 ity is located on State-owned property of the Lookout Mountain School for Boys. The State-owned land is referred to locally as the Baseball Gulch area. A 4.5-acre portion of the detention facility is located on private property being planned for condominium development. Options may be available to relocate the embankment along the private property boundary and excavate additional storage as needed. Trade-offs may be possible in working with the developer since the privately owned land is currently in the floodway and a major drainage channel and 100-year flood protection would have to be considered as part of any development plan. The right-of-way cost estimate following this commentary considers the public and private properties separately. Right-of-Way Acquisition TOTAL CAPITOL COST OF IMPROVEMENTS TOTALS IN EACH JURISDICTION City of Golden Jefferson County Additional cost data pertaining to the detention facility is provided following the commentaries for Sheets 7 and g. The costs reported for this sheet refl ect only the ri ght-of-way costs and const ruct i on costs for the embankment, excavation, emergency spillway and Welch Ditch improvements. Annual Operation and Maintenance Costs Until improvements have been constructed, floodplain regulation is the recommended plan. COST I I I I I I I I I I I I I I I I I I I $ -0- -0- -0- 624,000 624,000 93,600 93,600 811 ,200 832,400 1,643,600 1,643,600 -0- (7,850)