2014-04-22_REVISION - M1990057 - Laserfiche WebLink

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Mr. Michael Cunningham CDRMS April 22, 2014 riprap typically range between 0.035 (stability) and 0.045 (capacity). The Division's erodibility and revetment considerations are as follows: a. In general, earth -lined channels with a flow velocity greater than 5.0 feet per second (fps) will require revetment such as riprap to reduce the potential for scouring. b. Revetment, when required, should be sized for each reach /segment using industry accepted methods. Sizing methodology and calculations should be included in the submittal. The Division can suggest various methods appropriate for different conditions. Response: The Manning's n values have been revised for riprap -lined channels to the values indicated by the CDRMS. 0.035 will be used for stability checks on riprap -lined channels, and 0.045 will be used for capacity checks on riprap -lined channels. Riprap will be installed on all channels with flow velocities greater than 5 fps. Riprap was sized using the FHA's Design of Roadside Channels with Flexible Linings Hydraulic Engineering Circular Number 15, Third Edition, 2005. 3. Appendix B, West Diversion, max slope. The maximum slope is identified as 62.5 percent. Even though the peak flow is very low, riprap will not stand at this slope. A 40 percent slope is typically the highest recommended slope from available research and publications. As the topography may make it impractical to flatten the slope to 40 percent, the Division recommends diverting this small flow to the east at the outfall /design point for Basin OS1 towards the east where Basins OS2 and OS3 are diverted. Please provide a revised design for the West Diversion that is on a slope less than or equal to 40 percent or revise the direction of the diversion. Response: Per the Division's recommendation, the flows from Basin OS1 above the property have been directed to Basin OS2. The peak flows in Basin OS2 have been revised to include the additional runoff from OS1. Basin OS1 has been eliminated from the drainage plan. 4. Page 6, Second Paragraph: The response indicates the upland diversion "will handle a 200 year, 24 -hr rainfall event ". The precipitation depth (2.47) and statements earlier in the response suggest this should be the 100 -year, 24 -hour event. Please clarify which design storm is intended. Response: The hydraulic analyses have been revised to reflect the proposed linings of the individual channels depending on the proposed linings. All channels with flows greater than 5 feet per second are riprap - lined, and channels with flows less than 5 feet per second are earth - lined. 5. Appendix B, South TSF Contact. The hydraulic analyses for this channel indicate the bottom width is five feet. There are no details on Drawing DR -2 with a trapezoidal section. Please revise the analyses to match a proposed channel section or add a trapezoidal section to drawing DR 2.