5E, Drainage Report for Townhouse Villages North and Townhouse Villages at Arden Hills

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When the model was run with a 100-year, 24-hour storm event, assuming no restrictions on the inlets or pipes, the high water levels (HWL) for each low point was: Northern Low Point (on Arden View Drive) = 973.35 Southern Low Point (rectangular grate east of Arden View Ct) = 965.15 There is no emergency overflow to provide relief if the catch basin grates or storm sewers are plugged, or if water in the pipe surcharges and comes back out of the catch basins. When either one of these conditions occur there is potential that the low points will flood. Water will not escape overland from the north low point until it reaches an elevation of approximately 980. At this point, from reviewing the survey, if water reached an elevation of 976.7, it would' enter the window sills of the adjacent townhomes. Water will not escape overland from the south low point until it reaches an elevation of appro�cimately 968.5. At this point, the survey shows that if water reached an elevation of 966.6, it would enter the garages/doors of the adjacent townhomes adjacent to this low point. Outions An on-site field visit and a review of eacisting topography maps and construction drawings were used to develop several drainage options. The storm sewer map for this project area shows that appro�mately 75% of the northern portion of the site flows into the southwest corner pond through a single run of storm sewer pipe. The improvement options considered were based upon the location of where the flooding occurred and what appeared to be potential solutions based upon a visual review of the existing drainage system. The following options were modeled and their associated costs calculated: Option 1 (Figure 2) This option involves building a berm azound the southwest corner of Pond 2 and reconstructing the.pond outlet. The existing overflow for Pond 2 is in the southwest corner where an existing pathway is located: Building a berm in this area and replacing the existing outlet with a new structure would allow additional storage (pond bounce) to occur, while at the same time limit the pond discharge to the downstream storm sewer system. Modeling Option 1 showed that this action would have an insigni�cant impact on either the nortt� or south low points. The north low point HWL was 973.26 (-0.09') and the south low point HWL was 965.08 (-0.07') with this altemative. The estimated cost for Option 1 is shown in Table 1. Option 2 (Figure 3) This option would provide an emergency overflow pipe for the low point area located to the southeast of the Arden View Court cul-de-sac. A pipe or a swale could be constructed that would extend from the driveways between units 4364 Arden View Court and 4324 Arden View Court. For the purposes of this analysis a pipe was considered, since this would likely be less