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R Harries, Goff Engineering <br />C -1992 -080 TR -17 PAR No. 2 <br />21- Oct -2013 <br />Page 3 of 5 <br />Division was able to ascertain that the water surface elevation and freeboard would be <br />at or below the elevation of the riprap lining within all segments of the channel. <br />3. DRMS 09/20: It appears that the runoff volume used for the Carbon Junction Permanent <br />Diversion is based on the previously approved design in the permit application, which <br />may have resulted from modeling of the site at maximum disturbance. Modeling for <br />current conditions, using data from recent vegetation sampling may yield a lower runoff <br />volume and subsequently indicate a smaller required ditch capacity. <br />DRMS 10/11: Review of Appendices 5 -9 and 5 -10 confirms that the previously <br />approved model was based on maximum disturbance. Given the current conditions on <br />site, it would be appropriate to revisit the runoff calculations and likely arrive at a lower <br />discharge volume. This new volume might show that the existing configuration of the <br />channels is adequate to convey the design events. (Essentially, the model will need to <br />either be revised to show that what's on the ground will work, or else construct the design <br />we approved with TR -11, including the 500 ft. of riprap in the Upper Carbon Junction <br />Channel.) <br />DRMS 10/21: This item is resolved; Goff elected to retain the original calculations in <br />the permit application package <br />4. DRMS 09120: The HEC -RAS information provided for the upper segment of the Carbon <br />Junction Channel indicates that the existing channel capacity is adequate to convey the <br />design event. Predicted velocities (5.17 -8.08 fps), however, are in excess of the <br />acceptable range (3 -5 fps, depending on soils) for a mixed grass vegetated waterway. <br />Please revise the proposed channel designs to include lining as necessary (in areas where <br />the channel will be disturbed) to ensure that the channels will be stable and not contribute <br />suspended solids to strearnflow and runoff outside the permit area (Rule 4.05.3(4)). Also, <br />the design information submitted indicates that the channel is designed to convey the 100 <br />yr — 24 hr event. The watershed contributing to the channel is less than one square mile, <br />so the requirements of Rule 4.05.3 would apply to this section of Carbon Junction. Rule <br />4.05.3(3) requires the channel to be sized to adequately convey the runoff from a 10 yr — <br />24 hr event. The diversion portion of the channel, however, must be sized for the 100 yr <br />— 24 hr event. Please see comment 7, below, for further clarification. <br />DBMS 10/21: The design has been revised based on a 10 yr — 24 hr event, as <br />suggested by the Division. The runoff value used, however (2.191), is inconsistent with <br />the value in the currently approved permit application (2.25') and that shown on the <br />NDAA distribution maps located at http://hdsc.nws.noaa.gov/hdsc/pfds/ <br />pfds map cont.html ?bkmrk =co (also 2.25" for latitude 3724722 and longitude -- <br />107.8556). When the designs have been revised using the appropriate rainfall amount, <br />please determine whether the predicted velocities will be erosive, and provide designs <br />for channel lining if necessary. <br />5. DRMS 09/20: The sixth page of Appendix 5 -2 includes information on the Permanent <br />Channel Realignment of Carbon Junction Channel to run through the reclaimed pit area. <br />Since the Carbon Junction Diversion will be a permanent structure and Carbon Junction <br />