2012-11-09_REVISION - M2010049 (3)

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Western Sugar Reclamation Land Development Project Flood Analysis of erosion possibilities due to the many uncertainties of such modeling with the intent that the actual values would fall somewhere within the range of model results. 4.2.2 Hydrograph Development The model is driven by an inflow flood hydrograph based on the right bank overtopping 100 -yr peak discharge of 900 cfs as previously mentioned. The hydrograph was developed by selecting the largest historic thunderstorm flood, and proportioning the amount of time that it flowed overbank. A thunderstorm (vs, snowmelt) was the flood of choice because it would have less warning time to make for a conservative argument that Tract C pit would fill if ample flood warning (due to shutting pumping and allowing groundwater inflow) is received by Varra Companies, Inc. before flood flows enter the pit. Discharge records exist for the Cache LaPoudre River near Greeley, Colorado stream gage (Cache LaPoudre, 2011) from 1903 to 2011 (see Appendix). Review of those records indicates that the largest thunderstorm occurred during June, 1947 (see Appendix) . That flood hydrograph was adjusted by linearly proportioning the ordinates to a ratio of the FEMA Effective 100 -yr discharge to the June 1947 peak discharge (see Appendix ). The purpose of such was to estimate a flood hydrograph for use in WinDAM B that would have the same peak discharge as the FEMA Effective 100 -yr flood at the site (7,080 cfs). It is important to note that the FEMA Base Flood (100 -yr flood) just upstream from the site is 10,600 cfs which splits to the north leaving 7,080 cfs at the Site. The largest historial peak discharge based on the 107 years of the Cache LaPoudre River near Greeley streamflow gage is only 6,090 cfs (WY 1983) — far short of 10,600 cfs. Thus, the discharge hydrograph used in this analysis — based on the 100 -3r flood - may be conservative and the FEMA Base Flood is questionable. After developing a daily peak 100 -year hydrograph, another hydrograph was developed that just considered overbank flow onto Tract C (see Appendix). That was done by subtracting the difference between the 900 cfs overbank flow (discussed above) and the adjusted 100 -yr hydrograph. Peak discharge is enough to flow overbank for 3 days only - from June 23 to June 25. Because the Greeley gage data are daily peak discharges, the peak was assumed to occur at 12 hours from midnight June 23 and decay through June 25 with all ordinates linearly proportioned until zero flow occurs (see Appendix ). This is an approximation of an actual hydrograph (hydrographs are not available), but sufficient for the purpose of this study and the WinDAM B model. Different hydrograph ordinates would make little difference in the final model results. The most important factors are the discharge peaks and amount of time that overtopping is occurring. 4.3. WinDAM B Modeling Results A summary of headcutting and erosion results is presented below with two different scenarios. 4.3.1. Conservative Scenario - No Pit Tailwater The first scenario is the most conservative (worst -case) because it is based on the ultimate pit not being evacuated (i.e., pumps not shut down). That is, it assumes that Tract C has no water and p9. 7