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.r~ <br /> <br />Memo to Dan Hernandez - 2 - October 27, 1989 <br />1. The 25-year event and HW/D=2 are the appropriate design level and are <br />consistent with previous Division guidelines. <br />2. I did not recalculate all of the watershed measurements and h~~drologic <br />calculations presented in the Appendix to derive peak flows, however, <br />based upon the parameter values for curve number, time-of-concentration <br />and hydrologic soil group used, the runoff values do not appe~~r to be <br />underestimated and are suitable. <br />3. The "typical culvert installation and roadway section" detail shows an <br />18" minimum thickness of fill over a culvert. This 18" depth would be <br />adequate only for Culvert No. HR-5 based on an actual HW/D value of <br />1.75. All of the other culverts will require more than 18 inches of <br />cover to provide the required freeboard. These amounts of freeboard <br />should be specified when site-specific designs are submitted. <br />4. Entrance and exit conditions are described on page 9 as needing to be <br />addressed on a "site by site" basis and that grouted rock headwalls or <br />special end sections may be necessary in some locations. The provision <br />for stable, non-erosive channels below the culverts is of primary <br />concern. Splashblocks of riprap are shown on the plates but riot <br />described specifically in the text. These specifications and supporting <br />calculations should be addressed in the site-specific designs. <br />5. The omission of anti-seep collars where culvert length is less: than 100 <br />feet is acceptable. If lengths greater than 100 feet are necessary, <br />collars which provide an equivalent flowpath length of 120 percent of <br />culvert length should be provided. <br />6. The sizing specified for Culvert Nos. HR-1 through HR-5 are acceptable. <br />7. The design proposed for Culvert No. HR-6 is somewhat difficult.. To size <br />this culvert consistent with the design method chosen and applied to the <br />other culverts, a 36-inch culvert is recommended. However, this implies <br />the downstream 24-inch culvert on Cedar Heights Drive East presently <br />installed is undersized and should be replaced. <br />The plan proposed is to use a 24-inch culvert at the HR-6 location and <br />allow it to overtop and run down the roadside ditch to the existing <br />54-inch culvert. The rationale for sizing the two culverts the same size <br />is that the upstream 24-inch culvert will pass the same flow to the <br />downstream culvert and thus not overwhelm it. The difficulty is that a <br />24-inch culvert, with HW/D=2, would pass 26 cfs safely but the design <br />flow is 56 cfs, or routing about 30 cfs of flow down the roadside ditch <br />to the 54-inch culvert. Approval of this proposal would require a <br />demonstration that: <br />a. The ditch (or road/ditch combination) is designed to pass 30 cfs in <br />a stable, non-erosive manner; and, <br />