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ARDEN HILLS CITY COUNCIL WORK SESSION—OCTOBER 10, 2011 2 <br /> Marcus Thomas, Bolton and Menk, stated that the storm drainage system on this site was <br /> designed to accommodate a ten year storm. The system relies on overland conveyance to carry the <br /> water away when a storm is greater than a ten year event. He explained that they modeled the <br /> existing storm sewer system then routed a ten year storm through the model to determine what <br /> would happen. They found that the water was contained in the system for a ten year event. He <br /> stated they then routed a 100 year storm through the model and found that the catch basins could <br /> not keep up with the water which causes the storm sewers to back up causing extensive ponding <br /> in the streets. He then pointed out a natural low area on the north end of Arden View Court and <br /> Mr. Thomas stated that their <br /> another low lying area at the south end of the development. <br /> Y g <br /> modelin showed that with the 100 year storm the water does not reach the lower garage <br /> g Y <br /> elevations in the area. He explained that what they found is, assuming the storm sewer catch basis <br /> are open, unobstructed, and operating normally, there should not be a problem, other than isolated <br /> street/driveway flooding with the current drainage system during a 100 year event. <br /> Councilmember Holden asked if the modeling showed ponding at the north entrance road that <br /> would make accessing the site difficult. <br /> Mr. Thomas responded that the modeling for a 100 year storm did show a half foot to a foot of <br /> ponding in the roadway which would be up and over the curbs, but it dissipates quickly. <br /> Councilmember Holmes asked if the July 16th event was considered a 100 year storm. <br /> Public Works Director Maurer responded that the July 16th storm was probably a little greater <br /> than a 100 year event. <br /> Mr. Thomas explained that what they were looking for was easy, low cost modifications to the <br /> existing system that would bring some benefit to the two low areas. He stated they have identified <br /> the following alternatives: <br /> Option 1. . Make modifications to the existing pond and the outlet structure to modify the <br /> amount of water and length of time it is held in the pond. In addition, they propose doing <br /> some berming around the pond to allow it to store more water. They plugged these <br /> modifications into their model and found that it did not amount to much of a benefit and only <br /> reduced water elevations about an inch for a 100 year storm. <br /> Option 2. Reconfiguration of the existing storm sewer by eliminating head-to-head flow by <br /> rerouting the storm sewer pipe, but that did not accomplish any significantly lower water <br /> elevation at the low point. <br /> Option 3. Reduce the volume of flow going into the system by routing water out of the pond <br /> to a different location; to the wetland area to the west. The modeling showed this option does <br /> not make a big difference in the high water elevations in the low areas. <br /> Mr. Thomas stated that they considered making all the changes referred to in the first three <br /> options. What they found was that even with all the proposed changes, there was no benefit to the <br /> north ponding area and at the south ponding area the benefit was about a three inch reduction <br />