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0 Appendix Exh7-140 — Section 16 Pond Ditch Extension <br />• <br />The Section 16 Pond Ditch Extension is an approximately 1,939 foot extension of the <br />Section 16 Pond Ditch. Its headwaters are located in the NE1 /4 of Sec. 20, and it passes <br />through the extreme northwest corner of Section 21 enroute to its connection with the <br />upper end of Section Sixteen Pond Ditch in the NW1 /4 of Sec. 21. The combined <br />Section 16 Pond Ditch Extension/Section 16 Pond Ditch is a temporary ditch that serves <br />to convey runoff from disturbed lands in Sections 20, 21 an 16 to the Section 16 <br />Sedimentation Pond (009 -A) prior to release to undisturbed lands in Section 21. It will <br />remain in service until all contributing areas are ready for bond release, at which time it <br />will be removed and blended into the surrounding topography. Its alignment is shown on <br />Map 41A. Watershed details and reclamation status for SEDCAD modeling are <br />presented on Exh. 7 -140, Figure 1 and Exh. 7 -140, Figure 2. The ditch profile is <br />presented on Exh. 7 -140, Figure 3. For reference, the profile of the original Section 16 <br />Pond Ditch is presented on Map 33A. <br />Pursuant to post- mining topography revisions and a pull back to the northwest of the <br />limits of mine disturbance submitted in TR -81, the watershed of the Section 16 Pond <br />Ditch Extension was slightly modified. Its watershed remains quite small at 32.8 acres, <br />of which 24.7 will be disturbed and 8.1 will remain undisturbed. The worst case <br />hydrologic condition, which forms the basis of the SEDCAD modeling, assumes the <br />disturbed portion of the watershed has been regraded and topsoiled but does not yet have <br />a vegetative cover, corresponding to a runoff curve number (CN) of 83. The ditch is <br />designed for the 10 year /24 hour storm in accordance with Rule 4.05.3 for watersheds <br />smaller than one square mile. Peak flow is 9.7 cfs. The ditch is designed as a trianglar <br />ditch at a 2.1 % grade, with side slopes at 3H:1 V, and a minimum excavated depth of 4 <br />feet. It will be cut into the natural shale /sandstone bedrock. Calculated flow velocity is <br />approximately 3.6 feet/sec, which is acceptable for this material. As a construction <br />expedient, it may be built with a bottom width greater than zero, which would only result <br />in shallower and slower flow when delivering the design flow. <br />The SEDCAD modeling results for this ditch is presented on the following pages. The <br />geometry is the same as that used as input for the revised SEDCAD modeling of the <br />Section 16 Sedimentation Pond, presented in Vol. 2E. The ditch flow results are also <br />summarized in Table 1 in Exh. 7, Item 14 found in Vol. 2C. <br />Exh.7 -140 -2 Revision Date: 12/31/10 <br />Revision No.: TR -81 <br />