2012-02-13_REVISION - M2005050 - Laserfiche WebLink

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Hydrology Comments: 1. There appears to be an error in the Win TR -55 runoff depth. DRMS has duplicated the error using the Win TR- 55 software. The runoff depth for basin U -1 assuming a CN = 86 & Pio = 1.97 inches) should be 0.826 inches, not 0.506 inches. Correcting this software output error, yields a 10 -year storm runoff volume of 2.1 S7 cu -ft, thereby making the 1,396 cu -fl design volume too small. There is a similar error for the runoff depth from basin L -1. It should be 0.773 inches, not 0.748 inches. However, the increase in runoff volume from 8,146 cu- ft to 8,415 feet is capable of being stored in the 8,528 cu -ft design volume. Please increase the storage capacity of the Upper Pond to store the estimated runoff volume of 2,187 cu -ft. Response: Runoff volumes have been recomputed using the SCN Formula duplicating the DRMS volumes. Spillway elevations have been raised to provide the required design capacities. 2. Based on our understanding of the design presented on the 10/25/11 "Upper Pond Details" drawing and referencing the 8/27/11 "Revised Drainage" map, DRMS concludes the discharge from the Upper Pond spillway will make its way to the Lower Pond if no other way than flowing parallel to the sill fence identified on the "Revised Drainage" map. The spillway in the Lower Pond should include the 100 -year design peak discharge (routed appropriately} from the Upper Pond. Please revise the design flow for the Lower Pond spillway as appropriate. Response: The Lower Pond spillway was sized for the combined 100 year peak flow to allow for routing of the Upper Pond discharge through the Lower Pond as an option for the future - the operator may wish at some point in time to eliminate a discharge monitoring point. At this time, however the intent is to keep the Upper Pond discharge in the location specified in the 2009 plan. At the time that the decision is made to reroute the Upper Pond Discharge a revision will be made to the plans with the appropriate notification to DRMS. The Drainage Plan and Upper Pond Detail have been modified to more accurately depict this. Spillway Comments: 3. The spillway and riprap sizing methodology used are appropriate. However, there are some significant errors in obtaining values for "D50^Cu ^1/4 from Figure 4 in the Frizell paper. Most notably, the fact that a riprap with a specific gravity of 2.24 is proposed, whereas the selected methodology assumes a specific gravity of 2.65. DRMS notes that an arbitrary size increase is used to account for the significantly lighter rock. Fortunately, DRMS has obtained the equation used to develop Figure 4 from the Bureau of Reclamation, which they obtained from the Ph. D. dissertation used in the Frizell paper (ref. Mishra Subhendru K., 1998. RIPRAP DESIGN FOR OVERTOPPED EMBANKMENTS. Ph. D. dissertation, Civil Engineering Department, Colorado State University, For! Collins.. CO, July 7, 1998). The equation is as follows: D50 Cu "1/4= 0.55(gf"0.52 /S "3 /4)(sin a /(Gs cos a 1)(cos a tan phi - sin a)) "1.11 a. Where: D50 = median stone size (meters); Cu = coefficient of uniformity: of = unit discharge (m/s /m); S = embankment slope (dimensionless); a = embankment slope (.in degrees): Gs = specific gravity of the riprap; an phi = angle of repose of the riprap. Note that this equation should be used with metric units as it is not certain what physical parameters are represented by the 0.55 coefficient b. Using this equation, please provide revised riprap sizes to account for differences in i) specific gravity: and ii) angle of repose for the proposed riprap. Please contact me if you have any questions. Response: Equation noted and used - thanks again for obtaining it. Riprap D50 recalculated using this equation. Per the note in the PAP -790 Design Guide with respect to safety factors the design D50 was adjusted to add an additional safety factor.