Laserfiche WebLink
<br />~-, <br /> <br />~- <br /> <br />Mr. Stan Szabelak, City Engineer <br />April 27, 1998 <br />Page two <br /> <br />the subdivision) were modeled. The physical changes proposed by the applicant led to a <br />reduction in floodplain width, but they did not result in significant (i.e. greater than 1 <br />foot) increases in flood elevations. <br /> <br />Comments <br />The proposed floodplain delineation for Tributary L (both for "existing conditions" flows <br />and for "future conditions" flows) would match the current F1RM approximate <br />delineation at the downstream and upstream limits of the CLOMR. There are no current <br />flood elevations to tie to because there is no current detailed floodplain information and, <br />therefore, no water surface profile. The CLOMR analysis is more detailed than the <br />current FEMA analysis and the proposed delineation poses no conflict with the current <br />FEMA approximate delineation at the upstream and downstream CLOMR limits. <br /> <br />There are no significant changes to the floodplain boundary on the north side of Tributary <br />L. The significant changes to the floodplain boundary are all along the south side. The <br />most significant changes of all, in terms of land area in the floodplain, are in the vicinity <br />of the existing 66" culvert around Station 13+63, and continuing upstream to Station <br />16+ 16. Despite the substantial proposed reduction in floodplain area there, the water <br />surface profile is unchanged when compared to existing conditions. Our understanding is <br />that the area is a "pond" and that the filling is simply filling along the edge of the "pond" <br />in a region of ineffective flow. For that reason there no perceptible effects on the water <br />surface profile. <br /> <br />The south side of the floodplain is also changed farther upstream, from Station 16+ 16 <br />upstream to Station 23+50. Through much of that reach there are only minor changes in <br />the water surface elevations due to the proposed filling. We presume there may be some <br />increases in flow velocities to compensate for the reduced cross-sectional area. Weare <br />not sure whether erosion will become a substantial concern. The report stated that, <br />"There will be some channel stabilization improvements proposed." That is a good idea - <br />if, indeed, existing channel grades are steep in some locations and if velocities might be - <br />increased by proposed filling along the channel. We did not see any specific plans for <br />channel stabilization improvements, but we expect that those improvements will be <br />included in the LOMR phase of this project. <br /> <br />After reviewing the information we have received, we recommend that work continue on <br />channel stabilization needs and that any stabilization improvements be included in the <br />LOMR application that would follow implementation of this project. We also <br />recommend that coordination continue with the City of Thornton and the Urban <br />Drainage and Flood Control District regarding the proposed detention facility upstream of <br />