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10062 Flanders Court NE, Blaine, MN 55449 – Ph: (763) 780-0450 – Fax: (763) 780-0452 MEMORANDUM To: Mr. John Anderson, Interim Public Works Director From: Kristie Elfering, Elfering & Associates Subject: 2015 PMP Improvements Project 1776 Venus Avenue; Drainage Date: August 22, 2016 As you are aware the resident at 1776 Venus Avenue is concerned with the storm water runoff that their property experiences during large rain events. They have stated that the construction has changed the drainage characteristics of the neighborhood resulting in more runoff being directed to their property. We have extensively reviewed the drainage area and their property and have the following findings. The overall drainage area that is directed to the low point that existed both before and after construction in front of 1776 Venus Avenue remains unchanged. The storm sewer inlet configuration did change as portions of the existing storm sewer system were modified as part of the project. However, the storm sewer system functions adequately for a 10 -year storm design event, which is the City standard. The street itself was modified in front of the property to accommodate an urban section into the existing topography. The property directly across from 1776 Venus Avenue has a very steep driveway and the road was tipped up towards this property. While the standard crown was eliminated the concrete gutter maintains the initial drainage runoff on the north side of the street into the catch basins on that side. In addition, the overall street elevation was lowered from the original conditions, which provides more of a buffer between the street water that ponds in the low point in front of the property at 1776 Venus Avenue. A cross section is attached that details the old and new profile in this location. As part of the project, a screen was installed in the catch basin to the east of driveway at 1776 Venus Avenue to aid in collecting sediment prior to discharging to the wetland behind the property. During laboratory tests the screen had shown to aid in minimizing the sediment that was suspended and transported downstream. However, in practice the screen became clogged with leaves, sticks, and other debris from the neighborhood. This reduced the ability of the pipe to flow freely. The screen was in place during the large rain event that the resident e-mailed the City with their concerns on July 18th, 2016. After this rain event the screen was removed from this location and all other critical low point locations where they had been installed. Since that time we have monitored the property after most large rain events, including the following: Date Approximate Rain Received July 23 2.14-inches July 27 2.25-inches August 4 2.04-inches August 10 2.85-inches