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Mr. Rob Zuber <br />August 25, 2015 <br />Page 4 <br />(for the 25 year event) when runoff volume increased? Is the decrease in <br />the annual sediment loading related to the increase in undisturbed area? <br />Response: Since the actual undisturbed area did not increase from the 2013 TR -95 value, and in <br />fact has reduced slightly, this comment pointed out an error in the most recent SEDCAD <br />modeling. Upon close examination it was found that 98.1 acres tributary to Str. No 7, SW <br />Tributary, had been incorrectly input as "Undisturbed" with CN 57 instead of "3+ year <br />reclamation" at CN=62. The SEDCAD models for the 10 year and 25 year storms have both <br />been re -run with the correct CN and are resubmitted herewith. This correction brings the <br />"undisturbed" total back nearer to the value presented in the 2013 TR -95 Table 6. It should be <br />noted that overall the watershed area increases in this modeling effort by about 70 acres from the <br />2013 value. Because runoff from areas with either of these low curve numbers is typically very <br />low, correcting the 98.1 acres to CN=62 had a very small impact on the results. For example, it <br />raised runoff volume from 13.25 ac -ft. to 13.50 ac -ft., and raised the peak pool 0.09 feet in the 10 <br />year, 24 hour storm. (The value of runoff volume of 4.75 in a previous version of this table <br />appears to have been a transposition error. It is important to note that all entries in Table 6 have <br />to be manually transposed from the SEDCAD output and other derivative spreadsheets.) Table 6 <br />has been revised to account for the corrected SEDCAD modeling for this 98.1 acres as "3+ years <br />reclamation" instead of "undisturbed." <br />The base flow rate change from 0.15 cfs to 0.00 cfs was a carryover from an earlier <br />model performed in 2006. It has been corrected to 0.00. <br />With respect to the 25 year storm spillway issues, that difference may be due to an error <br />in the manual interpolation of the spillway flows from the SEDCAD output. SEDCAD does not <br />provide splits on the separate flows in the primary and emergency spillways when both are <br />flowing, but only provides the total combined outflow. If both spillways are flowing, these split <br />values can only be obtained by going into the actual separate stage/discharge curves and <br />interpolating which portion was attributable to each spillway. This split is a meaningful data <br />point in the 10 year storm because it demonstrates zero flow in the emergency spillway while <br />routing the 10 year, 24 hour storm, as required in the regulation. Further, for the 10 year flow it <br />is directly observable in the SEDCAD storm output because all discharge is through the primary <br />spillway if the pond is properly designed. Accordingly, Table 6 will continue to provide the <br />individual spillway flow splits in the 10 year storm simply to demonstrate that the emergency <br />spillway is not being engaged. However, for the emergency spillway design flood (typically a 25 <br />year or 100 year flow), this distinction adds nothing of value and requires a manual interpolation <br />of SEDCAD input data. Accordingly, we are modifying Table 6 to present only the combined <br />flow from all active spillways flowing in the spillway design storm. This data is directly <br />provided in the SEDCAD program output, and will allow a simple conformance check of Table 6 <br />and the submitted SEDCAD output. The Table 6 attached herewith incorporates this change. <br />A T ,itch,t,me Euu' C ; hrrau c <br />