1996-10-04_REVISION - M1981302 - Laserfiche WebLink

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A cursory review of old and recent topography mapping clearly demonstrates that the South Boulder Creek had many alignments over recent history enroute to its confluence with Boulder Creek. Interviews with knowledgeable individuals stated that today's channel alignment was manmade in the late 1800's. It is difficult to say "where is the historical flow path" due to the manmade changes. COMPILATION OF FLOOD-RELATED INFORMATION Under item no. 5 in the RESEARCH OF THE FLOODPLAIN ISSUES, one can see that much study has been undertaken in the determination of the 100-year flood hazard/ risk within the South Boulder Creek Floodplain. The most recently completed analysis was performed by Love& Associates, Inc. Love evaluated the assumptions and recommendations of the previously completed study investigations. It appears to us that the Love study included a complete inventory of all the published floodplain information. EVALUATION OF THE TECHNICAL INFORMATION/DATA Surveys and Mapping - The Love investigation appears to use available data with some field verification. The Flatiron mining operation has greatly modified the lands in and near the gravel pit area. However, new topographic data does exist which shows more recent topography. There appears to be some question regarding the accuracy of the centerline profile for U. S. Highway 36. The Love base data and modifications through field verification are reasonable for the level of detail which is expected for a flood hazard analysis. Hydrology - The Love investigation used the Corps of Engineers' 1979 hydrological values which are consistent with the State's designated ones. Therefore, their values are acceptable. Hydraulic Routing - The Love investigation used the FEMA and CWCB base hydraulic model in the preparation of their HEC-2 model for the study reach. The investigation did use state-of- art assumptions and input coefficients. Two floodplain conditions were analyzed: 1) existing channel with no FEMA certified levee in place, and 2) existing channel with a FEMA certified levee in place. These two assumptions may be may be too simplified, considering the large amount of floodplain lands involved and materials excavated during the gravel mining operation. A third condition is possible and should be considered. This condition is the relocating of the channel back to a historic flow path which may place floodwaters through the presently developed Frasier Meadows and Keewaydin Meadows subdivisions. In addition, there may be other viable alternatives that could be considered for the site. The HEC-2 model which was developed by Love appears to be reasonable and is an acceptable hydraulic routing procedure to us. Determination of the 100- year Floodplain Areas - For the assumed hydraulic conditions, the Love 100-year delineations appear to be reasonable and appear to represent the 1996 field conditions. However, the U.S. Highway 36 roadway embankment elevations may require further investigation should a project design be considered adjacent to the roadway. We will