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

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Introduchon The Townhouse Villages North and Townhouse Villages at Arden Hills experienced a significant flood event in July of 20l l. This incident caused street, yard and home flooding, along with damage to some vehicles. Bolton & Menk, Inc. was retained to investigate the storm sewer drainage system to determine if feasible solutions could be made to the system that would provide relief from future heavy rainfall events. BackSround The watershed area that drains into the storm sewer system analyzed consists of approximately 46 acres (see Figure 1). Land use in the watershed consists of single family residential dwellings (townhomes), open space and some natural woodland. The storm sewer system contains reinforced concrete pipe (RCP) ranging in size between 12" and 27" in diameter. The drainage outlet for the main storm sewer system empties into in a pond located near the southwest corner of the property. On July 18, 2011 the Minnesota Climatology Working Group website reports that the general area near the Townhouse Villages received 5.45 inches of rainfall. Localized flooding was reported at two road low points and adjacent townhouses. These two areas are identified as the "northern low point" and the "southern low point" on Figure 1. To complicate matters, the 2011 Pavement Management Program was being constructed in this area when the storm event occurred. As a result, the Contractor had inlet protection installed in the street catch basins to catch sediment from discharging into the downstream water bodies, but also restrict the flow of water into the storm sewer castings. Methodology Using Autodesk� Storm and Sanitary Analysis (SSA), the drainage area was modeled to evaluate impacts associated with various storm events. A theoretical 100-year, 24-hour storm event was used to model the options that were looked at. The 100-year event is defined as that event likely to occur on the average once every 100-years. This is not to say that a 100-year rainfall cannot occur in consecutive years or even twice in the same year. In reality, the 100-year storm has a 1% chance of occurring in any given year. The 100-year, 24-hour storm is defined as a 5.9-inch event for Ramsey County. Existing Condition Analysis In July of 2011, the two land-locked low points in the development flooded due to what appeared to be an approximately 100-year storm event. The model results show that the existing drainage system has the capacity to handle a 10-year storm event without surcharging. The results also showed that the e�sting drainage system has the capacity to handle a 100-year storm event with minimal, localized flooding at the low points, which would not cause flooding into adjacent homes.