Agenda - Council - 10/27/2015 - Laserfiche WebLink

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Yellow — The peak pond elevation is above the emergency overflow spillway Red — The peak pond elevation may threaten structures All red and yellow areas will be "ground truthed" to verify the condition depicted. These maps can be used to evaluate regional pond opportunities. In reviewing these maps, one can easily see where the current ponds or depressions are overtaxed for the 100-year event. Odd numbered figures from Figure 11 through Figure 23 are maps of critical SSA pipe data superimposed on the most recent aerial photo of Ramsey. They are color coded to highlight the 10-year storm conditions as follows: Orange — The pipe is flowing normally Red — The pipe is flowing full The following is a brief discussion of the opportunities and recommendations associated with each watershed: Figures 10 & 11 — D43 Watershed This is the Ditch 43 watershed. From Figure 10, it can be seen that this area is fully developed around a significant amount of existing wetlands. This leaves little opportunity to construct regional ponds. The red areas on Figure 10 indicate ponds that are subject to flooding during the 100-year event. The yellow areas indicate that the emergency overflow is reached during a 100- year event. These ponds should be reviewed relative to the following: • Should the outfall pipe be replaced with a larger one? • Can the pond be enlarged? • Can the overflow spillway be lowered? • Can rain gardens be incorporated into the upstream watershed? • Is the upstream watershed larger than allowed by the original design? • Can some of the upstream watershed be diverted? The two ponds in the southwestern part of the watershed that are highlighted in red, P26308 and P26310, appear to be infiltration ponds with no outlet pipe. Because our model conservatively assumes no infiltration, the threat of flooding from these ponds may be exaggerated. In comparing this area with the historic flooding map of Figure 8, there have been no reports of flooding in this area. Pond P25454 appears to need a raised emergency overflow to the west in order to relieve potential flooding. Our model suggests that a 12-inch outfall culvert under Sunwood Drive with an inlet elevation of 866 will eliminate this threat from a 100-yr rainfall event. Since the D43 watershed is comprised of a significant amount of wetland which acts as satisfactory runoff storage, no other significant designs are necessary with the exception of the localized flooding associated with the red highlighted ponds. Otherwise, upstream watershed review for infiltration/rain garden opportunities is recommended. Figures 12 & 13 — D66 Watershed Section X October 21, 2015 Page 66