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model input is appropriate. It should also be noted that the first five "Land Flow <br /> Conditions" in Sedcad are for overland/sheet flow(see below). <br /> 2. Minimum tillage cultivatior <br /> 3. Short grass pasture <br /> 4. Cultivated,straight row <br /> 5. Nearly bare and untilled,, <br /> 6. Grassed waterway <br /> 7. Paved area and small upl <br /> 8. Large gullies,diversions, <br /> Screen capture from Sedcad, Subwatershed,routing ca/culatians, Land Flaw Conditions <br /> Chapter 3 Time or Concentration and Travel Time Technical Release 55 <br /> Urban Hydrology for Small Watersheds <br /> Sheet flow For sheet flow of less than 300 feet.use Manning's <br /> kinematic solution(overtop and Meadows 1976)to <br /> Sheet flow is flow over plane surfaces.It usually compute T, <br /> occurs in the headwater of streams.With sheet flow, <br /> the friction value(Manning's n)is an effective rough- _ 0 0.0076 " <br /> ness coefficient that includes the effect of raindrop Tt (py 10%0-4 l� 3 ] <br /> impact;drag over the plane surface;obstacles such as <br /> litter,crop ridges,and rocks;and erosion and trans- where: <br /> portation of sediment.These n values are for very <br /> shallow flow depths of about 0.1 foot or so.Table 3-1 Tt - travel time(hr), <br /> gives Manning's n values for sheet flow for various n - Manning's roughness coefficient(table 3-1) <br /> surface conditions. L a flow length(ft) <br /> P� -2-year,24-hour rainfall(in) <br /> s = slope of hydraulic grade line <br /> Table 3-1 Roughness coefficients(Manning's n)for (land slope,fttft) <br /> sheet flow <br /> Screen capture from TNT 55,note 300-faot limit fGP sheet AN(underlined) <br /> RESPONSE: The SEDCAD model has been reconfigured to eliminate those <br /> inconsistencies. Copies of the Syr, 10 yr, and 100 yr output are attached to this <br /> response (See Attachment 13). In addition to modifying the flow segment descriptions so <br /> that SEDCAD and the Civil3D are consistent,former basin "Lower South"has been <br /> split into two basins for the two channels at the actual tunnel site. In addition, those two <br /> basins are color coded on Drawing CCVSA11-7 and an inset on that drawing shows the <br /> flow segments (See Attachment 13). <br /> 63. Results of Storm Routing, p. 7 <br /> • Based on Comment No. 62 channel/culvert sizes and riprap should be re-evaluated. <br /> Please keep in mind that Sedcad's PADER method for riprap sizing typically undersizes <br /> riprap. On the other hand,the Simons/OSM method(the other Sedcad option)results a <br /> very conservative (large)riprap size. [Note: The Division will periodically inspect <br /> stormwater channels and if frequent maintenance is required for riprap-lined channels, <br /> the Division may require a technical revision to increase the riprap size.] <br /> RESPONSE: In my professional opinion and through experience, the Simons/OSM <br /> method is significantly conservative and the Pader method is more appropriate, even if it <br /> provides slightly undersized armoring. A look at the hydrograph of Photo 2 illustrates <br /> that the peak flows on which the riprap design is based last only for a few minutes, much <br /> Page 29 of 30 <br />