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Table 1: Summary of Critical Elevations at the Low Point Along Morton Ave. <br />Description <br />Elevation <br />(ft) <br />Depth (ft) <br />Morton Low Point <br />895.52 <br />0.00 <br />Morton Top of Curb <br />896.02 <br />0.50 <br />Morton EOF <br />896.30 <br />0.78 <br />Table 2: Summary of Runoff Depth at the Low Point on Morton Ave. <br />Rain Event <br />Rain <br />Depth <br />Depth at Low Point <br />(in) <br />Ex (ft) <br />Option 1 (ft) <br />Option 2 (ft) <br />Option 3 (ft) <br />2-Year <br />2.85 <br />0.11 <br />0.11 <br />0.08 <br />0.10 <br />10-Year <br />4.24 <br />0.35 <br />0.35 <br />0.18 <br />0.24 <br />25-Year <br />5.24 <br />0.79 <br />0.79 <br />0.26 <br />0.35 <br />50-Year <br />6.07 <br />1.02 <br />1.02 <br />0.38 <br />0.55 <br />100-Year <br />1 6.97 <br />1 1.23 <br />1.23 <br />0.71 <br />0.98 <br />Discharge Rates <br />Since the storm sewer capacity is increased with some of the proposed options, an associated <br />increase in discharge rate is observed at the storm sewer outlet. Also, since the land use <br />upstream does not change, there is no noticeable change in runoff volume to the ultimate <br />discharge point. Currently, the City maintains a stormwater management pond near the <br />Mississippi River. Figure 4 in Appendix B shows the discharge location and approximate extents of <br />the stormwater management facility. Table 3 summarizes the anticipated decrease in discharge <br />rate for each proposed option. It is proposed to eliminate any increase in flow at the discharge <br />point. We have identified three primary options to mitigate the increase in discharge rates. <br />1) Expand the current facility. The current facility is in a narrow easement on private property <br />and surrounded by trees. While the City maintains this area, it is known for relatively high <br />rates of sedimentation, inundation from flooding from the Mississippi river and existing <br />washout issues have already been a concern, so expansion in this location is not preferred. <br />2) Expand a dedicated regional stormwater facility at Bailey Point Nature Preserve. By routing <br />stormwater into the park and away from private property, the City may increase stormwater <br />retention and improve water quality to a higher extent than the existing basin. In general, A <br />portion of the Morton Ave discharge could be diverted through a control structure into a new <br />basin in Bailey Point Nature Preserve and the overflow continue on its existing discharge <br />route. <br />a. Preliminary sizing of a stormwater diversion and retention facility within Bailey Point <br />Park Preserve was performed. Similar design criteria could be applied to the existing <br />ponding area east of Morton Ave. The available depth of a feature is approximately 3 <br />feet and has a footprint of approximately 5,000 square feet. The volume of the <br />feature would be approximately 11,000 cubic feet to mitigate the peak discharge rate. <br />A 24" diversion pipe from the catch basins on Morton Ave would allow back flow of <br />water out of the system for temporary retention and infiltration. <br />Prepared by: Bolton & Menk, Inc. Proposed Flooding Solutions <br />Morton Avenue Drainage Study I OR11.127809 Page 5 <br />