4.4 SR 10-17-2022 - Laserfiche WebLink

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Hydraulic Modeling and Assessment of Flooding The source of the flooding at Morton was confirmed through hydraulic modeling. Autodesk Storm and Sanitary Analysis (SSA) was chosen for its comprehensive and full hydrodynamic ability. The low area was modeled as a storage area to best understand the depth of water in the intersection, how the pipes may surcharge and add to flooding, the bypass flow from upstream catch basins during large events, and the capacity of the pipe system through the rain fall event. From this assessment, it is evident that multiple factors contribute to flooding. 1) The total watershed area upstream of Main St is 11.3 acres. Morton Ave has a directly contributing drainage area of 2.33 acres. The drainage areas outside of directly contributing areas do not provide substantial overland flow contribution to the Morton Ave low point. 2) The low point on Morton Ave is currently served by 2 catch basins, one on either side of the road. This offers limited inlet capacity for the contributing direct overland discharge. 3) There are 2 catch basins on the north side of Main St at Morton Ave. Overland flow draining to these catch basins exceeds the inlet capacity, and bypass flow makes its way to the Morton Ave low point. This flow contribution is small, but a contribution, nonetheless. 4) The storm sewer pipe in Morton Ave is 18" from 4t" St to the Morton catch basins and 21" form the catch basins to Main St. Then, the pipe is reduced to a very flat 12" diameter across Main St, then a steeper (-4%) 12" pipe before turning back into an 18" pipe down Morton towards the river. The 12" bottleneck at Main St causes stormwater to surcharge out of the Morton catch basins. Figure 2 in Appendix B shows the approximate water inundation area for a variety of rainfall events. IV. Proposed Flooding Solutions Several solutions to flooding were assessed. These generally include storm pipe size increases and adding catch basins and manhole structures along with discharge rate mitigation. All options were assessed in SSA to measure the flood reduction outcomes. The following phenomena were observed in the modeling. 1) Increasing pipe size downstream of the low point in Morton Ave (i.e. upsizing the 12" pipe across Main St) lowers the hydraulic grade line in the pipe. That is, the water surface is lower such that stormwater is not surcharging from the pipe to roadway. a. The upsize in pipe has a minor impact on the depth of water in Morton Ave because the flow into the pipe is inlet controlled. That is, the catch basin inlet capacity is limited relative to the overland runoff rates. 2) Increasing the inlet capacity by adding more catch basin structures raises the hydraulic grade line in the pipe. So, there is a point of diminishing returns when it comes to adding catch basins and speed at which we can remove water from Morton Ave. 3) Increasing the pipe size downstream of Main Street relieves the remaining bottle neck in the system to address the flooding at the low point in Morton Avenue. The following options have been identified as the primary alternative solutions to this issue. Refer to Figures 3-5 in Appendix B for maps representing the proposed alternatives. Prepared by: Bolton & Menk, Inc. Proposed Flooding Solutions Morton Avenue Drainage Study I OR11.127809 Page 3