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more storage volume due to the routing nature of the hydro graph and due to the fact the pond started filling at the start of the storm thus making the storage less efficient. The detention alternate was rejected for all of the study reaches due to the enormity of the project and the negative impacts it would have on park or open space lands. Acquisition of flood prone properties was rejected as a final alternate for all of the study reaches. There are approximately 200 properties, with structures on them, within the lOa-year floodplain of Fourmile Canyon Creek. Acquisition of all 200 structures does not make economic sense. Please note that the acquisition of individual structures may be required for implementation of the selected flood control improvements This alternate would include flash flood forecasting and warning systems and evacuation plans. Flood insurance and floodproofing of structures would also be recommended (at the property owner's expense). Floodplain regulations would be strictly enforced. Post flood relief would be provided. Public information and flood hazard education programs would be undertaken. Flood insurance is recommended for all flood prone properties. Non-Structural Methods: 6.2.6 I I I I I Non-structural methods should be considered as an interim solution (until the selected master plan is implemented) and as a sub-alternate of every alternate. However, this alternate does not make a good "stand alone" alternate since, flood waters will still inundate large areas of land and additional flood waters would be added to the Wonderland Creek floodplain from extreme events. Other - Trans-Basin Diversion of Excess Flood Waters: meetings held on this project, it was suggested that trans-basin diversions the basin to the north be looked at as a potential alternate. At one of the public of excess floodwaters to 6.2.7 I Trans-basin diversion of excess floodwaters was examined in a cursory manner. A channel route was laid out on U.S.G.S. mapping. The channel would originate upstream of the North Briar Subdivision and would be capable of conveying lOa-year flood discharges in excess of 1,000 cfs to the next drainageway north of Fourmile Canyon Creek. The components of this alternate would include: a diversion for Fourmile Canyon Creek, a flow limiter, approximately 4 miles of channel construction to Six Mile Reservoir, numerous road crossing structures, modifications to Six Mile Reservoir to handle the increased discharges without adversely impacting downstream properties, and drainage easement acquisition from numerous property owners. This alternate was rejected due to the cost; the legal liabilities involved with a trans-basin diversion of flow and due to the fact that larger floods than the lOa-year would still cause flood spills to Wonderland Creek. other alternates should be AL TERNA TE EVALUATION Maintaining the existing floodplain configuration is the alternate that all compared with and as such is one of the final alternates for consideration. 6.3 I I A natural type of waterway following the general historic channel alignment, will also be a final alternate. The construction of a channel to convey a flood of a certain design frequency will decrease or contain the regulatory floodplain, will enhance life safety (by removing structures from the floodplain) and will decrease flood damage potential. I I Following is a discussion on a reach by reach basis of the variety of alternates that were considered as a part of this report for possible implementation Maintain existing floodplain configuration. Select structural improvements and Channelization (replacement of 47th Street culvert and debris removal, bank stabilization, and drop structure construction). Non-structural methods. analyzed were 119 to Palo Park Channelization) the following alternates (State Highway In Reach 1 in detail: Selected structural improvements were evaluated for this project. The majority of the floodplain is controlled by floodplain geometry and configuration and not by structures. For example, if the existing 1 a-year capacity Broadway box culvert is replaced with a lOa-year structure, the floodplain would remain essentially the same. The spatially varied flow distribution in the floodplain would change locally; however, the footprint of the floodplain would not change appreciably and the flow in the main channel floodplain and the spills toward Wonderland Creek south of Violet would approximately be the same. Select structural improvements at roadway crossings would reduce the frequency of roadway overtopping, reduce life and safety hazards due to roadway overtopping and would aid in emergency response. I I I . . . Constructing channel improvements for only selected reaches of Fourmile Canyon Creek is problematic due to the wide floodplain footprint. The transitions from the wide floodplain to the narrower improved channel and back again would be difficult and costly I I not of Palo Park channelization to 30th Street) the following alternates was alignment 1, a natural type waterway following the general historic channel since a natural type waterway already generally exists in this reach. In Reach 2A (downstream limits were analyzed in detail In Reach considered, Off-channel and in-channel detention facilities were investigated. An off channel pond was sized upstream of Broadway on City park or open space lands. The pond would have a side channel spillway crest from the Fourmile Canyon Creek floodplain and would have the capacity to store the lOa-year flood volume with the peak discharge downstream being the 1,000 cfs. This pond required approximately 30 acres of land, a ten-foot high embankment and excavation of 1.3 million cubic yards of earth from the pond below the embankment. An on-channel pond required significantly I I 6-3 I I