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I I I I I I I I I I I I I I I I I I 5. 2. their historic operating practices, or other arrangements made for providing flood storage, then the hydrology downstream would be lower, cfV'b,,^'<'-fkZ^<\ The raillin~to the Moffatt Tunnel provides, we believe, significant flood storage on the south tributaries from the foothills to above Gross Reservoir. We are familiar in detail with one site on South Draw where about 80 to 100 acye:teet of, sto.~age olccurs and reduces the 100-year flood pe~k by morEj . than hal1 0:.\ ~ <,(,,,\'1,,,. c~ /' / ^ J-6."",Jd'l'l ,0. "^" ~ ~'IW / r"'^3-'l v There may be as many/a's 20 to 30 storage~ that exisy{ that ~ from very small to sizes such asilsouth Draw. Since agreemeQts are not in placlt to sustain these storages, they have not been includecf These storage~xist, benefitting the downstream conditions for most events, but also maypresent hazard as the storages are not engineered for flood control and may be susceptible to breeching. 3. When flood control facilities, floodplain mapping, or refined surveys are available the hydrology model can be further refined to better depict flow splits and hydraulic controls. The 500-year flood model prediction requires further review to confirm flow splits and patterns. Thus, we can envision further revisions to the hydrology model in the future. 4. The UDSWMM program has very limited stream flow routing algorithms. We feel it is inadequate to simulate the dynamic floodplain storage routing of the South Boulder Creek below Eldorado Springs and the West Valley Overflow. Thus the flows depicted in these areas shown in the present draft modeling are probably high. Effectively the peaks of the hydrographs are "trimmed" and sent to the West Valley Overflow without the natural attenuation that exists. Our limited sensitivity tests indicate that the model is highly responsive to the geometry given for the South Boulder Creek main stream. While our work for the Colorado State Park has afforded the opportunity to incorporate field surye~ of representative sections at Eldorado State Park, it may be worthw~~to conduct field surveys of representative se~' ns at other locations in order to better represent the mountain strea s. ) ,.,,/ RECOMMENDATIONS 1 . The results are reasonable to begin a review and refinement by independent experts and the sponsors, 2. The results are reasonable to begin the alternative evaluation process. Alternatives developed might consider options to "firm up" existing flood 1- 15