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,;'" .-. STREAM LOCATION D.A, (mi2) 0100 (cfs) Wonderland Ck. UIS limit 0.38 110 Broadway 0.85 480 N. Goose Ck. confl. 1.91 1410 The above figures for Wonderland Creek include an unspecified diversion of flow from Fourmile Canyon Creek into Wonderland Creek (we think it may be 400 cfs), We were told that current estimates of the diversion from Fourmile Canyon Creek range from 1800 to 2600 cfs. Those estimates are coming from the Love and Boyle studies and have not been finalized yet. There are clearly hydraulic implications to the above hydrological information. When compared to the magnitude of flows in each stream, the size of the diversion may be very significant. It appears that Fourmile Canyon Creek cannot carry its own 100-year flow, even with its own floodplain to supplement the channel. Wonderland Creek is apt to be overwhelmed by such large flows from another watershed. There are potential implications for the floodplain maps and for the damages that might be predicted in both watersheds. There are implications for mitigation options to address the apparent problems. There are questions about the appropriate regulatory responses to ongoing proposals for new development near the two streams, At our meeting there was also discussion of the topographic and geologic nature of the two floodplains. The city staff said that Fourmile Canyon Creek exhibits some alluvial characteristics, That would probably be related to flow spills from Fourmile Canyon Creek into the Wonderland Creek watershed. It could also be related to the need for previous engineers to place "invisible walls" in the hydraulic models. The purpose of these "invisible walls" was to keep water in the HEC-2 or HEC-RAS models (to prevent it from escaping) so that the models would work. (That is true. I didn't make it up.) It could also be related to "inconsistencies" observed by city staff and by citizens. After the meeting I took a brief tour of the mouth of the canyon on Fourmile Canyon Creek. I would initially agree with the notion that it is an alluvial fan, even though it does not look like Telluride, Ouray or Glenwood Springs. It could be alluvial without having much (or any) debris associated with it. I also looked at both streams between Broadway and US 36. I would suggest that serious consideration should be given to using a 2-dimensional hydraulic model like FLO- 2D (the model used by the COE in Ouray and Glenwood) to follow flow from the mouth of the canyon to a point between Broadway and US 36 (28th). The sense I got when I looked at both streams from above is that the streams are trying to go downhill in a sideways fashion. I would also strongly recommend that floodplains like these need a geomorphological analysis, because slope, geology and soils all playa part, along with hydraulics, in determining where the floodplain really is. In these cases, engineers need all the help they can get. We need to know when we are in or out of the 100-year floodplain. At this particular location knowing where the floodplain is located is very important, because there is already a lot of existing development near or along the two streams. I will keep you informed about the work of this committee and our findings.