2009-02-17_REVISION - M2001023 (12)

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Dave Zehner Milner Pit No. 2 Expansion Floodplain Evaluation Page 3 of 7 February 3, 2009 4. The dashed lines represent the channel of the Yampa River and Trout Creek as of October, 2003. Note that in places the channel is actually outside of FEMA's mapped floodway. This reveals a discrepancy in FEMA's maps, because by definition the floodway must be at least as wide as the channel, and the channel cannot be outside of the mapped floodway. This type of discrepancy occurs when FEMA mapping is based only on cross-section data, without the benefit of simultaneous aerial photography, or when, subsequent channel erosion occurs. In reality, what this type of discrepancy shows is that floodplain mapping is no better than the input data, and secondly, is only a snapshot in time which can be out of date in short order simply because streams are dynamic systems that change over time. From a pragmatic perspective, the floodplain maps are useful for general planning purposes, but have! to be adjusted to meet current reality, and have a limited useful life. Map No. 2 provides greater detail of the proposed expansion area. It includes site specific topography. Several observations are likewise in order. 1. Vertical datums are critical issues for all mapping, and perhaps more so for floodplain mapping. 2. The site datum for the existing pit and proposed expansion are based on the 1929 NGVD vertical datum. However, variances in survey methodology lead to various degrees of error. 3. The site elevations for FEMA's work were originally based on the 1929 NGVD, but were later adjusted to supposedly be on the 1988 NAVD datum. Unfortunately, in many conversations and correspondence with FEMA staff over the course of the past half dozen years, there is incomplete documentation as to exactly how the current maps were generated. FEMA's best conclusion is that WATM REBOURCS CONSULTANTS, LLC