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I I I I I I I I I I I I I I I I I I I Mr. Ben Urbonas, P.E. Page Four May 8, 1978 Mr. Ben Urbonas, P.E. Page Five May 8, 1978 The engineer, in its report, suggests two approaches in this reach and the affected entity must choose in this case between a more economic approach or one that is legally safer. suggest that since this area is relatively undeveloped, that a design frequency of lOO-years be selected and then when the land is developed, that the developer be required to make the suggested improvements. This suggestion is based upon the assumption that if the suggested improvements upstream are built with no construction in this reach, that damages in this reach would not be increased by such improvements upstream. If this is not the case, then an alternative to alleviate that increase in damages would have to be implemented with any improvements upstream. e) Reach AlA. The regulation of the residual 100- year floodplain and a flood warning system are not mentioned as recommendations in this Reach or in any of the other A 1 series reaches. I assume that these recommdations will be part of the engineer's Phase "B" report, if not already an assumed recommendation in the Phase "A" report. In any event, the recommendation in this reach is not consistent throughout the reach, since it leaves a portion of the reach unimproved from its present state. The benefit/cost analysis for a continuous channel for certain design frequencies appears to be positive and, therefore, I would suggest that this recommendation be re-examined. In regard to the area north of 76th Avenue, where a hiker/ biker/maintenance trial is recommended, the engineer comments that "no further channel improvements are needed." I assume from that statement that this portion of the reach is able to handle any increased flows from upstream without increased damage in this reach. Finally, I assume that the new culvert recommended in this reach will be sized according to the improvements upstream and downstream of the new culvert. The engineer recommends a 10-year system for the channel and culverts on the main stem of this reach.. Is a 10-year sy- stem adequate here with improvements upstream capable of hand- ling the 100-year event? The engineer should be requested to confirm the fact, if true, that no increased damages downstream will be realized from going from a 100-year system to a 10-year system. If you have any further questions which are left unanswered by this opinion, or are raised by this opinion, please let me know. f) Reach 3. With the proposed improvements upstream, can the West 75th Avenue culvert be left unchanged? Is it ade- quate to handle without any additional damage the increased flows, if any, from upstream improvements? Very truly yours, ~Cr/-. The engineer might be asked what improvements he would suggest for the area between Winona Court and West 75th Avenue when it is eventually developed. g) Reaches 2A and 2B. Recommendations appear to be legally sufficient. EJK:sc h) Reach 1. It is unclear whether the channel improve- ments and selected structural improvements are to be sized to a 10-year or 25-year design. The benefit/cost data all refer to a design frequency of either a 25-year or 100-y~event. I assume the engineer is recommending a 25-year design frequency. I might