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CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 42 February 27, 2015 DATE DESCRIPTION 1967 SEO plan approval, C-1195 CARRIAGE HILLS RESERVOIRS. CH 2 dam shows 2H:1V upstream and downstream slopes, 10-ft crest width. The embankment is zoned with an upstream impervious zone, followed by a 10 MIL Polyethylene membrane, and a clay & disintegrated granite downstream zone. The plans show a 24” dia. 12 ga. Corrugated metal pipe (CMP) outlet conduit with an upstream slide gate and trash rack (outlet capacity = ~35 cfs). The spillway was to be 10-ft wide with vertical side slopes through rock and then laid back at some unspecified slope, with 5-ft of freeboard. Spillway capacity was listed as 377 cfs. [Plans are not marked as-constructed]. 1983 1st record of SEO inspection. Water Commissioner (WC) inspection. Capacity estimate: 5AF, Surface Area: 1 Ac, Height: 18 ft. Notes trees & brush on the embankment. 1984 WC inspection. Reservoir full, 1 cfs of spillway flow 1985 WC inspection. Flow obstructed by willows in spillway. “No outlet found” 1986 Engineer’s Inspection Report (EIR). Willows obscure embankment. Willows obstructing spillway flow & there is flow along downstream toe of dam 1988 WC inspection. Seepage flow “adjacent to outlet”, “outlet obstructed by overgrowth and dirt”. 7/24/91 Town of Estes Park annexed land and took ownership of the CH dams. 7/31/91 EIR (Mark Haynes): Rated Conditionally Satisfactory due to spillway and outlet condition. Required Actions: Remove obstructions from spillway and provide minimum 3-ft of freeboard, locate outlet and make operable or Replace. Notes no change in downstream hazard, and hydrology not rated pending evaluation [No record that hydrology study was every performed] 8/21/91 Town of EP requested that they be allowed to use pumps for emergency drawdown in lieu of new outlet works. 9/3/91 Response to SEO allowing use of pumps with conditions: reliable power source, access to dam maintained, accessible during emergency, 4 cfs min. capacity 11/12/91 Dam Breach Analysis by SEO using NWS Breach 87. Four cases were investigated: (1) piping failure of CH1 (u/s) only, (2) piping failure of CH2 only, (3) simultaneous piping failure of both dams, and (4) overtopping failure of CH2 dam caused by a piping failure of CH1 dam. Peak breach flows for cases (2) and (4)[considered most relevant] were 1162 cfs and 5409 cfs. No formal review of hazard classification was performed, but the Hazard is denoted as 3 in the analysis, where 3 in the previous 1-3 hazard rating system corresponds to Low Hazard. 6/14/96 EIR (Jim Dubler). Spillway return flows along the toe of the dam and obscures seepage inspection. Outlet was not located and suggests that outlet may not exist. Cites confusion on the Town’s part about their pump procedure for emergencies. Required Actions: 3-ft min. freeboard. 10/22/02 EIR (Jim Dubler). Crest: Right end appears low. Outlet could not be located, but describes (with a photo) a 4-ft dia. pool at the downstream toe that presumably was the outlet outfall. Required Actions: Restore the spillway to the C-1195 design conditions (10-ft wide, 5-ft freeboard) or hire an engineer to do an alternative study/design. 11/7/02 Letter from Town of Estes Park to SEO documenting available pumps for emergency drawdown requirement. 10/1/08 Outlet can’t be found but presumed to be at wet area at downstream toe. No record that outlet was properly abandoned and C-1195 plans show conduit is CMP. States recent work was done on the spillway, reportedly restored to the design width of 10ft, as well as riprap protection added in the approach and downstream channel. Spillway flow routed along the downstream toe, seepage noted in “vicinity of outlet”. Required Actions: Plans & specs for outlet rehabilitation, Monitor seepage around the downstream end of the conduit. Conditional Satisfactory Rating & Conditional Full Storage based on the requirement for outlet rehab.