2020-07-14_PERMIT FILE - C1981008

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Other Inflow Sources Currently, spoil spring flows and backfill drainage (ground water) is directed in an 18-inch HDPE pipe around the north side of Pond 013, as shown on Map 2.05.3(3)-21. Flows from the trench drains that gravity feed to this pipe are typically on the order of the 250 to 450 gpm, with seasonal variation mainly due to due to irrigation. These flows may be routed into Pond 013. Also, inflow from other sources such as Tuttle Draw and CCC irrigation ditch may be routed into Pond 013. These inflows will also vary seasonally and the total inflow from the trench drain and other sources will not exceed 1,000 gpm. Modeling a constant inflow using SEDCAD is somewhat complex. The methodology suggested by SEDCAD's primary author, Pam Schwab, was used to model the impact of a constant inflow. In the current version of SEDCAD, a fixed flow can only be input by inserting a "dummy" upstream reservoir with a watershed large enough to produce a "tank" flow, and then setting the output of the dummy reservoir as a constant "User Defined" outflow curve, independent of pool elevation in the dummy reservoir. A flow of 2.228 cfs (1,000 gpm) was conservatively utilized, well in excess of the normal inflows. The synthesized 1,000 gpm flow was then dropped into directly into Pond 013. SEDCAD allows the user to override the customary starting pool that is set by the lowest drain hole. In the case of the 10 year storm, where the starting pool is normally set by the twin 6-inch valves with invert at elevation 5552.0, the starting pool at the start of the design storm needs to be 1.43 feet higher, or elevation 5553.43 to account for the 1,000 gpm already flowing through the two orifices at the start of the storm. For the 25 year emergency spillway design storm, where it has been conservatively assumed that the twin 6-inch orifice holes are closed, the normal starting pool is the principal spillway elevation of 5555.0. This must be raised 0.23 feet to account for the 1,000 gpm over the riser lip, treated as a sharp-crested weir. The starting pool for the 25 year spillway design storm was thus set to elevation 5555.23. Other System Components An existing ditch, C9 Ditch, routes the easternmost part of the affected area runoff to the pond, as shown on Map 2.05.3(3)-32. It is modeled as vegetative lined trapezoidal ditches with a 3-foot bottom and 2H:1 V sideslopes, and with gradient of 3.2% as determined from field surveys, as depicted on Map 2.05.3(3)-32. Several cross sections are also shown on Map 2.05.3(3)-32. The actual side slopes and bottom width may vary in the field, and the ditches may have a flatter side slope or a wider bottom in some locations. Some reaches may be cut in rock. As such, the actual ditch sections will be at least as hydraulically capable and erosion resistant as the modeled template described above for the modest runoff flows described below. As can be observed on Map 2.05.3(3)-32, the central and western portions of the watershed flow directly into the pond. Culvert C177 is located near the inlet of Pond 013 in Ditch C9 as shown on Map 2.05.3(3)-l. The design for this 18" HDPE pipe is included in the SEDCAD model for the 10 year, 24 hour precipitation event. Up to three surface baffles will be installed in Pond 013 to route water through the pond, and ensure the entire flow path of the pond is utilized to treat water moving through to the outlet of the pond. The surface baffles do not change the capacity of the pond; therefore, they are not included in the modeling of the pond. Attachment 2.05.3(3)-32 Revision Date: 3/30/20 Page 2 Revision No.: TR-99