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I I I I I I I I I I I I I I I I I I I Mountain Modelinq Comparison of statistical analysis of the Eldorado Gage stream flow data with the UDSWMM deterministic model reveals an apparent disparity as the statistical methods give lower discharges than UDSWMM. We believe this is explained by the flood storage which is taking place in Gross Reservoir and the numerous storages upstream of the Moffatt rail line. To a lesser degree, refining the South Boulder Creek channel routing parameters based on representative field survey sections might reduce (or increase) the UDSWMM model result. Storage exists upstream of the rail line on South Draw, the major tributary above the Eldorado gage. Other storages upstream of the rail line can be observed on the topographic maps, but we have not documented their characteristics or effects. Further, the Denver Water operational data for Gross Reservoir illustrate significant storage of flood type events, such as the flood of May 1969. The regulations only allow recognition of storage above fixed spillways of water supply storage operated by governmental entities. Specifically prohibited is recognition of storages such as those formed by rail lines unless there are institutional agreements and the facilities are documented as adequate. Therefore, there are only two basic choices regarding selection of analytical model techniques: 1.a) Make institutional arrangements to either preserve the operational practices of Denver Water which result in flood storage, or explore other institutional arrangements and improvements which would enhance storage at Gross. b) Investigate characteristics, needed physical improvement, and make institutional arrangement with the Railway to preserve or enhance this storage. c) Refine the UDSWMM modeling accordingly to reflect the physical condition and changed facilities, comparing again with statistical methods if the physical and operational conditions are the same. Or 2. Don't rely on the statistical data at the Eldorado gage, since it represents the result of inadvertent storage. The sponsors have decided to proceed with the latter choice and issue this report on an Interim Basis, since making arrangements with Denver Water and the Railway could take too long. The sponsors are pursuing possible cooperative arrangements with Denver Water. They may pursue storage with the Railway, both to evaluate washout hazards and for flood control, but at a later date after the Master Planning effort has made significant progress toward an agreed solution. 1-17