2014-01-22_PERMIT FILE - M2013064 (10)

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Hydraulics Hydrology Water Resources P.O. Box 6100 Breckenridge, CO 80424 970 - 547 -3823 www.flowtechnologies.biz Plasticity Index. This parameter is mentioned in Section 3.2.1 for the purpose of giving a basic description of WinDAM, but was not needed in this analysis. The plasticity index is required only when a vegetative surface is being modeled. Because the erosion modeling of Coulson Pit did not consider a vegetative surface — rather bare soil for conservativeness — input of plasticity index was not necessary. Total Unit Weight. Total unit weight is the total density of the in -place soil in the berm. A value of 115 lb /ft3 was input for this analysis. Total unit weight was not available from soil sample data for Coulson Pit. Due to such, the value obtained from soil boring data of a nearby gravel mine (Western Sugar) was assumed applicable to Coulson Pit. This is a reasonable assumption because total unit weight is not a sensitive parameter with respect to final results. Undrained Shear Strength. WinDAM B uses the undrained shear strength approximated by taking one -half of the unconfined compressive strength of the soil material. ASTM D2166 provides a description of the testing methodology to determine the unconfined compressive strength of soil — which is a complex, expensive procedure and beyond the scope of practicality of this study. Due to such, guidance contained in a paper authored by Hanson et el. ( 1 ) was applied. Hanson's paper includes a table provided by Moore (2) for estimating unconfined compressive strength of soil which was applied for estimating undrained shear strength for the purpose of this analysis. In addition to Moore's procedure, engineering judgment was applied. Because the WinDAM methodology is not an exact science (as with any computer model of natural processes), undrained shear strength values used on other WinDAM analyses (provided as references with the model) were viewed and considered also. Most importantly, due to the uncertainties of estimating undrained shear strength, sensitivity analysis was performed and results indicate that variations within the bounds of reasonable values have minimal effects on the study conclusions for the scenarios evaluated. Based on the above, a value of 200 lb /ft2 was input to WinDAM. Critical Shear Stress. The critical shear stress can be determined from the jet erosion test or some other erosion test (e.g., flume test, hole erosion test, or erosion function apparatus). Those tests would be impractical for the purpose of this study. However per the WinDAM manual, the critical shear stress is usually enough smaller than the applied hydraulic shear stress that it may be taken as zero with minimum error. However, engineering judgment was applied to arrive at a value slightly greater than zero, or 0.1 lb /ft2. It was reasoned that for the event being modeled, the berm would have a high moisture content due to heavy rains preceding the flood event. Also, the berm (upper soil profile) would 1 Development and Characterization of Soil Material Parameters for Embankment Breach, G.J. Hanson, D.M. Temple, S.L. Hunt, R.D. Tejral, Applied Engineering in Agriculture, Vol. 27(4):587 -595, 2011. z Moore, J.S., Field procedures for the headcut erodibility index. Trans. ASAE (40(2): 425 -436), 1997. Page 2