Laserfiche WebLink
Western Sugar Reclamation Land Development Project <br />Flood Analysis <br />Such analysis - as with any computer modeling of natural processes - is not an exact science but <br />will provide a "sanity check" and relative comparisons of potential for headcutting and erosion. <br />Such analysis is valuable for determining extent of certain erosion scenarios such as berm width, <br />tailwater elevation in the pit, time to headcut back to the river, etc. <br />The dam- breach model being considered is the National Resource Conservation Service (MRCS) <br />WinDam B (Windows Dam Analysis Modules) (National Resource Conservation Service, 2011.) <br />WinDam B is a modular software application for the analysis of overtopped earth embankments <br />(berms in this case) and internal erosion. The model addresses routing of a flood over the dam <br />and evaluation of the potential for vegetation or riprap to delay or prevent failure of the <br />embankment (berm in this case). In addition, the model includes erosional failure of an <br />embankment (berm) through overtopping. <br />The three -phase erosion model was jointly developed by the Agricultural Research Service <br />(ARS) and the NRCS. Those three phases are: <br />1) Phase one of the erosion or failure process is failure of the vegetal cover and development of <br />concentrated flow. <br />2) Phase two is downward erosion in the area of concentrated flow, resulting in headcut <br />formation. <br />3) Phase three is downward and upstream movement of the headcut, potentially breaching the <br />dam (or, berm with regard to gravel mines). <br />Each phase is described by a set of threshold -rate relationships based on the process mechanics. <br />A headcut erodibility index describes the resistance of the exposed geologic materials to erosive <br />attack during the third phase of the process. <br />For a complete erosion estimate, the geotechnical composition must be characterized. Typical <br />parameters are representative particle size, percent clay fraction, plasticity index, total unit <br />weight, undrained shear strength, and critical shear stress. Specific to WinDAM B software, the <br />headcut erodibility index and detachment coefficient also need to be estimated. <br />4.2 WinDAM B Parameters <br />4.2.1 Key Parameters <br />Model parameters are listed in the Appendix. Berm width (dam crest) of 100 ft was applied for <br />three different scenarios. The scenarios, Weakest, Best, and Strongest involved changing a key <br />parameter which would result in the model to predict weakest (most) erosion potential, strongest <br />(least) erosion potential, and best which is in- between weakest and strongest. All parameters <br />remained constant for the three scenarios except for the erodibility factor, Kd which is the most <br />sensitive parameter. For this study, Kd had a range of two orders of magnitude from 10 for <br />Weakest, to 0.1 for Strongest. Such a range of magnitude was chosen to cover a very large range <br />pg. 6 <br />