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the dam were surveyed. The soils and bedrock in the spillway were visually classified during <br />the visit. Photographic evidence of the post- breach configuration was collected and sketches <br />of the site were prepared for later use in the breach investigation. <br />The breach of the spillway was modeled with two methodologies. First the surveyed sections <br />were input into a HEC -RAS v4.1 (US Army Corps of Engineers, 2010A) model with an in -line <br />structure modeling the spillway control section and a storage area simulating the reservoir. A <br />rainfall runoff model was prepared using HEC -HMS v3.5 (US Army Corps of Engineers, 20106) <br />to generate an inflow hydrograph to the reservoir. The HMS model utilized rainfall provided <br />by Applied Weather Associates (AWA) for the Irondale Gulch drainage basin for the actual <br />event. The RAS model was primarily used to verify that the potential affect of the <br />downstream flow hydraulics on the breach flow with its more advanced hydraulic <br />calculations. Once it was verified that the breach was not impacted by tailwater affects <br />through the breach channel, the breach simulation was incorporated into the HMS model <br />which verified the results of the RAS model and allowed the simulation of the entire Irondale <br />Gulch basin within one model. <br />The portion of the RMA that received rain contains a somewhat complex system of ponds and <br />channels including Havana Street Dam, Lake Ladora Dam and Upper and Lower Derby Lake <br />Dams. The whole system collects runoff from northeast Denver which includes areas of <br />intense urban development including residential and industrial neighborhoods and also <br />significant portions of undeveloped land on the Refuge. The system of drainage channels and <br />reservoirs all end up draining to Irondale Gulch which coalesces and drains under a railroad <br />embankment and CO Highway 2 at a location approximately 1/4-mile northeast of 88th Ave. as <br />illustrated in Figure 1. In order to determine the impacts of the dam breach, this entire basin <br />had to be modeled to determine the relative magnitude of flows coming from each part of <br />the system and the incremental damages which may be attributable to the dam failure. <br />3.0 Dam Description <br />Havana Street Dam is located within the RMA northeast of Denver. More specifically, the dam <br />is located in Section 11, T3S, R67W 6th P.M. in Adams County, Colorado. The dam, originally <br />constructed and owned by the US Army Corps of Engineers (USACE) as part of the Rocky <br />Mountain Arsenal, has transferred ownership over the past few decades from the USACE to <br />USFWS. More recently the Urban Drainage Flood Control District (UDFCD) agreed to take on <br />maintenance responsibility for the dam in exchange for securing its use as a flood control <br />structure within the Irondale Gulch drainage. A brief description of the dam is provided <br />below. A vicinity map of northeast Denver showing the dam site is provided in Figure 1. <br />The dam is constructed as a homogeneous rolled earth embankment. Its primary purpose is <br />flood control. In its pre- breach configuration, the reservoir impounded by the dam included a <br />small pool of dead storage of approx 67 acre feet at a depth of approximately 5 - 8 feet. At <br />the dead pool elevation of 5248 ft MSL, a service spillway consisting of a 30 in RCP pipe <br />penetrates the embankment near the maximum section. An emergency spillway consisting of