REV93826

• Permit No: M1982036
• IBM Index Class Name: Revision
• Doc Name: EXHIBIT G WATER RESOURCES
• Type & Sequence: TR2
• Media Type: D
• Archive: No

Design rationale and parameters for the existing enlarged sedimentation pond are
• presented in Appendix G-1. Exhibits G-2 and G-3 illustrate the original and revised pond
design. Based upon the site-specific hydrology, this sedimentation pond will contain the
10-year, 24-hour storm flood volume (1.07 acre-feet) with adequate freeboard. The
emergency spillway will convey the 25-year, 24-hour storm flow (7.5 cfs) at a velocity of
3.66 feet per second.
Runoff from approximately 47.7 acres of land in dryland wheat production, disturbed, and
reclaimed land will be routed through the new west sedimentation pond, which will be
located in the northwest corner of the Permit Area on the ephemeral tributary to
Hutchinson Gulch. Outflow from the new pond will be monitored at CPDES Discharge
Point 002, for which SCC has submitted a permit revision request. Design calculations
for the new sedimentation pond are provided in Appendix G-4 and the pond design
configuration and specifications are illustrated by Exhibit G-4. The new sedimentation
pond is designed to temporarily contain and treat approximately 2.5 acre-feet of
stormwater runoff, corresponding to the runoff volume resulting from the 10-year, 24-hour
storm event over the contributing watershed. The pond is designed with a CMP riser and
• discharge pipe with a dewatering orifice above the base of the pond. The location of the
dewatering orifice provides approximately 0.9 acre-feet of dead storage for sediment
accumulations.
Initial drilling and gravel extraction operations encountered very limited ground water,
however as operations have progressed to the west, ground water inflows to the active
pits have increased. Pit progression to the west has advanced toward the minor drainage
divide, both the unconsolidated overburden and the gravel material are relatively shallow
in depth and have high permeabilities, and the gravel is underlain by a relatively low
permeability clays and shales. These factors seem to indicate that the ground water
encountered in the pits is shallow ground water resulting from direct infiltration of surface
precipitation and downgradient flow within the permeable gravel layer and above the
confining shales. The limited ground water encountered in the drillholes and initial
excavation area is probably an indication of rapid flow through the gravel unit and
resulting rapid loss of head, flow depth, and flow volume with distance from the point of
. recharge. Given these considerations, maximum pit inflows are anticipated in the vicinity
of the drainage divide, declining as operations progress away from the divide.
G-11 Revised May 1998