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~ ~ III IIIIIIIIIIIIIIII <br />subia°t CC&V Mada by TLB ~0b 003-2142 Task 6 <br />Wla~c^ Bateman Pond Hydrology Checked by Date 7/I I/2001 <br />®~`.'.~~.'~`~~.~' APProved by Sheet <br />Incl. Contributing Ponds 1 of 2 <br />OBJECTIVE: <br />The objective is to design sediment control and flow attenuation structures for the Arequa Gulch <br />overburden storage azea, Crusher azea, and portions of the Bateman Creek watershed. The structures <br />-Crusher Sedimentation Pond, Arequa Gulch Sedimentation Pond, Bateman Creek Detention Pond, <br />and BMP Sump -are integrated in a hydrologic model. The hydrologic model was run for the 100- <br />yeaz, 24-hour storm event. The Bateman Creek Detention Pond is sized to attenuate the peak flow <br />resulting from the 100-yeaz, 24-hour storm. The Crusher Sedimentation Pond, Arequa Sedimentation <br />Pond, and BMP Sump are designed to capture two times the runoff volume of the 10-yeaz, 24-hour <br />storm event. The ponds' spillways aze sized to route the 100-yeaz, 24-hour storm dischazge. <br />METHOD: <br />Drawing 1 shows the three ponds and BMP Sump as well as the complete subbasin map of the <br />contributing areas. The Bateman Creek Detention Pond, BMP Sump, and Arequa Gulch <br />Sedimentation Pond broad crested weir spillways aze designed to pass the 100-yeaz, 24-hour storm <br />discharge. The Crusher Sedimentation Pond has a 42-inch CSP as the spillway. The capacity of this <br />culvert is designed for the 100-yeaz, 24-hour storm dischazge. <br />An azea weighted composite SCS curve number (CN) was calculated for each subbasin (Ref. 1) shown <br />on Drawing 1. The USACE HEC-1 model (Ref. 2) and the SCS method was used to estimate <br />dischazges from the three ponds resulting from the 100-yeaz, 24-hour storm event. The weir equation <br />(Ref. 5) was used to size the spillways for the Bateman Creek Detention Pond, Arequa Sedimentation <br />Pond, and BMP Sump. HY8 software (Ref. 6) was used to determine a rating curve of head versus <br />dischazge for the Crusher Sedimentation Pond outlet culvert. <br />ASSiJMPTIONS: <br />• Precipitation depth of 3.5 inches for the 100-year, 24-hour storm event; 2.4 inch depth for the <br />10-year, 24-hour storm (Ref. 3). <br />• The 100-year, 24-hour storm was assumed to have a SCS Type II distribution. <br />• Times of concentration (t°) were determined using SCS methodology (Ref. 1). <br />• Volume required for the ponds was determined using the SCS runoff equation and composite <br />curve number for contributing areas. <br />• A SCS curve number of 71 was used for reclaimed slopes and original ground terrain that is <br />grass covered; a SCS curve number of 66 was used for reclaimed slopes and original ground <br />terrain that is tree covered; a SCS curve number of 90 was used for roads, a SCS curve number <br />of 100 for full ponds (Ref. 4). <br />• All three ponds and the BMP Sump are assumed to be empty at the onset of the 100-year storm <br />event for the HEC-1 model (Ref. 2). For spillway weir sizing, ponds were assumed full when <br />the 100-year, 24-hour storm occurs as a worst case scenario. <br />CALCULATTONS: <br />File Name: C:\DOCHME" I\jlupo\LOCALS"1\Temp\aa¢man PoM Hydrology ircl aonir ponds.doc <br />