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Other Inflow Sources <br />Currently, spoil spring flows and backfill drainage (ground water) is directed in an 18 -inch <br />HDPE pipe around the north side of Pond 013, as shown on Map 2.05.3(3)-21, Trench Drain <br />Design. Flows from the trench drains that gravity feed to this pipe are typically on the order of <br />the 250 to 450 gpm, with seasonal variation mainly due to due to irrigation. These flows may be <br />routed into Pond 013. Also, inflow from other sources such as Tuttle Draw and CCC irrigation <br />ditch may be routed into Pond 013. These inflows will also vary seasonally and the total inflow <br />from the trench drain and other sources will not exceed 1,000 gpm. <br />Modeling a constant inflow using SEDCAD is somewhat complex. The methodology suggested <br />by SEDCAD's primary author, Pam Schwab, was used to model the impact of a constant inflow. <br />In the current version of SEDCAD, a fixed flow can only be input by inserting a "dummy" <br />upstream reservoir with a watershed large enough to produce a "tank" flow, and then setting the <br />output of the dummy reservoir as a constant "User Defined" outflow curve, independent of pool <br />elevation in the dummy reservoir. A flow of 2.228 cfs (1,000 gpm) was conservatively utilized, <br />well in excess of the normal inflows. The synthesized 1,000 gpm flow was then dropped into <br />directly into Pond 013. <br />SEDCAD allows the user to override the customary starting pool that is set by the lowest drain <br />hole. In the case of the 10 year storm, where the starting pool is normally set by the twin 6 -inch <br />valves with invert at elevation 5552.0, the starting pool at the start of the design storm needs to <br />be 1.43 feet higher, or elevation 5553.43 to account for the 1,000 gpm already flowing through <br />the two orifices at the start of the storm. For the 25 year emergency spillway design storm, <br />where it has been conservatively assumed that the twin 6 -inch orifice holes are closed, the <br />normal starting pool is the principal spillway elevation of 5555.0. This must be raised 0.23 feet <br />to account for the 1,000 gpm over the riser lip, treated as a sharp -crested weir. The starting pool <br />for the 25 year spillway design storm was thus set to elevation 5555.23. <br />Other System Components <br />An existing ditch, C9 Ditch, routes the easternmost part of the affected area runoff to the pond, <br />as shown on Map 1. It is modeled as vegetative lined trapezoidal ditches with a 3 -foot bottom <br />and 2H:1 V sideslopes, and with gradient of 3.2% as determined from field surveys, as depicted <br />on Map 1. Several cross sections are also shown on Map 1. The actual side slopes and bottom <br />width may vary in the field, and the ditches may have a flatter side slope or a wider bottom in <br />some locations. Some reaches may be cut in rock. As such, the actual ditch sections will be at <br />least as hydraulically capable and erosion resistant as the modeled template described above for <br />the modest runoff flows described below. As can be observed on Map 1, the central and western <br />portions of the watershed flow directly into the pond. Culvert C177 is located near the inlet of <br />Pond 013 in Ditch C9 as shown on Map 2.05.3(3)-1. The design for this 18" HDPE pipe is <br />included in the SEDCAD model for the 10 year, 24 hour precipitation event. <br />Pond Construction <br />Pond 013 was designed and constructed as a largely subgrade impoundment with a low <br />(approximately 5 feet high) embankment along the northwestern end across a natural swale. <br />The details can be observed on Map 1. The primary, or "service" spillway consists of an 24 -inch <br />perforated riser connecting to an 24 -inch CMP under the above described low embankment. The <br />Attachment 2.05.3(3)-32 Revision Date: 3/09/2016 <br />Page 3 Revision No.: TR -70 <br />