2013-01-14_PERMIT FILE - M2012032 (6)

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half pipe channels were estimated by the Division's engineering staff to have a half - velocity -head of greater than 4 feet. As such these channels are not properly designed because insufficient freeboard is available. a. Please provide a rationale for the selected roughness coefficients, and evaluate each designated channel /ditch design slope for both capacity and stability. NOTE — the `flow capacity" and "velocity capacity" presented on the drawing is not specifically related to the design flows. b. Please design all engineered ditches with the appropriate freeboard and provide channel design depths for construction. SMO Response Table 6 -1 contains the hydraulic calculations for all of the channels in question. These calculations include a set for an anticipated minimum Manning's n (stability) and a set for the maximum Manning's n (capacity). The two sets of calculations show the maximum flow that the channel can handle given the design inputs. These are then compared to the Channel Requirements to determine if the design is sufficient in both cases to contain the flow with the minimum required freeboard (either 0.5 ft. or greater than one -half the velocity head of the anticipated flow). Map G -5 shows the design detail for each channel segment, with dimensions (freeboard + depth of flow = design depth for construction) and basic geometry. DRMS Comment Map G -7: 23. As discussed in Comment #22, the velocity in the half pipe channels can be expected, in some cases, to well exceed 20 fps. Please provide some hydraulic calculations to demonstrate the 10 -foot long rock -lined energy dissipation zones are sufficient to dissipate the hydraulic energy; and demonstrate the proposed 24 -inch riprap will be adequate. SMO Response As shown in Table 6 -2, the highest flow velocity from a half pipe culvert on a steep slope into a "earth" channel is 13.0 fps (Div #1B Final Segment 2 into Segment 3). Therefore the energy dissipation zone has been designed around this interface. Using a procedure from Applied Hydrology and Sedimentology for Disturbed Areas (Barfield, Warner, Haan. 1983. Pg. 534 -6) the length of the energy dissipation basin needs to be either 3W (W half pipe diameter) or 10h (h = depth of scour). The overall basin length is 4W or 15h whichever is greater. For the design shown on Map G -7, h = 0.96 ft and W = 4.0 ft., requiring a length of dissipation pool of 12 feet and a total length of 16 ft. A riprap lining of D50 = 0.5 ft meets the requirement of 2 < h , 4, from the Barfield procedure.