2013-10-29_REVISION - C1992080 (2)

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R Harries, Goff Engineering C- 1992 -080 TR -17 PARNo. 3 29- Oct -2013 Page 2 of 3 It would be appropriate to provide text for Appendix 5 -12 that will be inserted ahead of the subparts A -B -C. This text (not in the form of a letter to the DRMS) will describe what is provided in the appendix, and include references to the subparts and maps. DRMS 10/29: The information was revised, as requested. A few formatting modifications and clarifications are requested, as we discussed earlier today. 2. This item was resolved with the October 15 response. 3. This item was resolved with the October 15 response. 4. DRMS 09/20: The HEC -RAS information provided for the upper segment of the Carbon Junction Channel indicates that the existing channel capacity is adequate to convey the design event. Predicted velocities (5.17 -8.08 fps), however, are in excess of the acceptable range (3 -5 fps, depending on soils) for a mixed grass vegetated waterway. Please revise the proposed channel designs to include lining as necessary (in areas where the channel will be disturbed) to ensure that the channels will be stable and not contribute suspended solids to streamflow and runoff outside the permit area (Rule 4.05.3(4)). Also, the design information submitted indicates that the channel is designed to convey the 100 yr — 24 hr event. The watershed contributing to the channel is less than one square mile, so the requirements of Rule 4.05.3 would apply to this section of Carbon Junction. Rule 4.05.3(3) requires the channel to be sized to adequately convey the runoff from a 10 yr — 24 hr event. The diversion portion of the channel, however, must be sized for the 100 yr — 24 hr event. Please see comment 7, below, for further clarification. DRMS 10/21: The design has been revised based on a 10 yr — 24 hr event, as suggested by the Division. The runoff value used, however (2.19 "), is inconsistent with the value in the currently approved permit application (2.25 ") and that shown on the NOAA distribution maps located at http: / /hdsc.nws.noaa.gov /hdsc /pfds /pfds map cont.html ?bkmrk =co (also 2.25" for latitude 37.24722 and longitude - 107.8556). When the designs have been revised using the appropriate rainfall amount, please determine whether the predicted velocities will be erosive, and provide designs for channel lining if necessary. DRMS 10/29: The revised design is based on a 2.25" value for the 10 yr — 24 hr event, as required. Goff determined that predicted velocities would not be high enough to require channel lining, and therefore is not proposing to install rip rap or other lining in any of the Upper Carbon Junction Channel. There is one segment (43 +00) that indicates that the expected velocity will be 5.37 feet per second (fps), and several segments do approach 5 fps. The Division will approve Goff s proposed plan to leave the Upper Carbon Junction Channel in its current (vegetated) configuration, but advises Oakridge that Rule 4.05.3(1)(e) requires all channels to be designed to minimize adverse impacts to the hydrologic balance and to be stable. If it becomes apparent that the channel is not stable, the Division will require Oakridge to submit designs for the channel that incorporate channel lining (such as rip rap or turf reinforcement mat) to ensure long term stability. Also, in the event that vegetation in the existing bottom of the Channel is disturbed during reclamation of the berm, channel lining may be necessary until vegetation within the channel is reestablished.