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3.0 Data Summary and Pond Assessment Results 3.1 Stormwater Pond Survey and Visual Inspection Table 2 summarizes the information collected as part of the stormwater pond survey including water surface elevation. pond bottorn elevation, and the primary outlet control elevation. Table 2 also summarizes the calculated information such as permanent pool surface area and permanent pool storage volume. Maintenance needs based on the visual inspection were noted for the pond site. Appendix A contains information on the specific observations and accompanying site photos for Silver View Pond. Estimates of the soft sediments were taken at 4 locations in the pond. Figure 2 shows the location of the survey shots on the "rnuck elevation" of the pond. Location I is at about the mid -point of the southern arm of the pond, near the bank. The muck layer thickness at Location l is 0 feet, This reveals that minimal sedimentation has occurred in the upper levels of the pond. Location 2 is in the southern arm of the pond, near the center of the pond. The muck layer thickness at Location 2 is 3.8 feet. As such, significant sedimentation has occurred in this location. Location 3 is in the eastern arm of the pond, near the center of the pond. The muck layer thickness at Location 3 is 2.8 feet. This indicates that significant sedimentation has also occurred in the southern arm of the pond, where there are several inlet pipes. Location 4 is in the eastenl arm of the pond, at approximately the mid- point of the arm. The muck layer thickness at Location 4 is 2.7 feet, revealing that sedimentation is extensive and relatively uniform across the eastern arm. The average muck layer thickness across the entire pond is 2.3 feet. Given that the pond is about 11.5 acres in size and assuming 2.3 feet of sediment, there is approximately 26.7 acre-feet of sediment (43,076 cubic yards) accumulated in Silver View Pond. This value is similar to the volume of sediment calculated by comparing current pond bathymetry to estimated pond original construction conditions (28 acre-feet). Therefore the soft sediments likely represent sedimentation that occurred after the pond was constructed, and they are occupying a significant portion of the pond's design volume. it is possible that the bulk of this sedimentation occurred as the watershed was developed due to poor erosion control during construction. An analysis of past and current watershed activities would be needed to confirm this idea and identify major sources of sediment. But it is clear that approximately 38% of the pond's originally excavated volume has been lost due to sediment accumulation. P_ Mpis 23 MN li_' '3621OV-2 libel Vie% Pond Asaesalncnl wnrkrik;s Rtpt,rl Siher View Pond Assrssmcni-rinal.docx I I