Laserfiche WebLink
Dave Zehner <br />Milner Pit No. 2 Expansion <br />Floodplain Evaluation <br />Page 5 of 7 <br />February 3, 2009 <br />1. Frequency and spacing of cross-section data. FEMA's cross-sections are <br />spaced widely apart, sometimes % to Y2 mile apart. Thus, a lot of detail is missed <br />when compared to site specific cross-sections that are only 300 to 500 feet apart. <br />2. Base maps. FEMA usually uses USGS maps to provide a base map to overlay <br />field surveyed cross-section data. In the Milner area, the USGS contour lines are <br />at 40 foot intervals. The accuracy of USGS contour maps is typically one-half of <br />the contour interval, or in this case, plus or minus 20 feet. Interpolating between <br />surveyed cross-sections, to an accuracy of less than one foot, is simply not <br />possible. This margin of error is very obvious towards the southeast portion of <br />the proposed expansion (Map 2). Note that site specific topographic data shows <br />a steep bank that is well above the floodplain. However, FEMA's mapping shows <br />this same area within the limits of the mapped floodplain. <br />3. Flood flows. At the time of the study in 2000, FEMA had not determined flood <br />flows for the Yampa River. Our analysis in 2000 showed flood flows to be 14,010 <br />cfs. FEMA's model was based on flood flows of 14,924 cfs, or about seven (7) <br />percent higher flow rates. <br />4. Choices of friction factors. Friction factors directly affect the calculated base <br />flood elevation. The choice of factors is usually based on engineering judgment, <br />simply because calibration is not possible. A variance of 10% to 20% in choices <br />of factors is common. <br />5. In a riverine system, there is always the question of whether or not the main <br />channel of the river has actually decreased, or increased, in hydraulic capacity <br />over time. For this reach of the Yampa River, the answer over the past nine <br />years is: not significantly. Even though there have been some erosional and <br />depositional changes in the main Yampa River channel since the year 2000 <br />when the initial floodplain study was performed for the existing pit, observation of <br />the river channel at low flows and at peak runoff in 2008 revealed there has not <br />been significant changes that would increase or decrease the base flood <br />elevation by more than a inches. <br />WATER RESOURCE CONSMTAWM, LLC