07-28-1999 Council Agenda - Laserfiche WebLink

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County Road D — North Inlet There is an existing inlet on the north side of County Road D that collects storm water from the areas to the north and discharges it through a 24 -inch CMP culvert under County Road D and into the swale along the east property lines of the lots along Maple Leaf Court. The areas that contribute water to this inlet include a portion of County Road D and the areas surrounding the apartment complex on the north side of County Road 0, including the parking lot. The total peak discharge from this area from a 5 -year storm event, as stated in the memo dated September 30, 1998, is approximately 16.2 cfs. This inlet is undersized as the stormwater from larger storm events often bypasses the inlet and overflows County Road D to the south, adding to the erosion problems along Maple Leaf Court. The inlet is only about 0.6 feet below the north edge of the travel lane of County Road D. This indicates that the inlet can only have 0.6 feet of head on it before encroaching onto the travel lane of County Road D. Assuming that the maximum head that can be developed on the inlet is 0.6 feet, the capacity of the inlet is approximately 7.5 cfs, for the type of grate installed. We have determined the elevations of the catchbasin and the 24 -inch CMP culvert under County Road D and have calculated the slope on the culvert to be about 3.5 %. The full flow capacity of the 24 -inch culvert would then be about 23.0 cfs, which is well above the existing estimated 5 -year peak flow of 16.2 cfs as mentioned in the September 30, 1998, memo. As shown in the above information, the catchbasin with an inflow capacity of 7.5 cfs cannot accommodate the flow from the 5 -year storm event of 16.2 cfs and results in the excess stormwater overtopping County Road D to the south. The culvert under County Road D does have the capacity to handle the stormwater from the 5 -year event, if the stormwater was detained by the inlet. The inlet structure is the limiting factor of this system. As part of the solution to the erosion problem along Maple Leaf Court, the inlet on the north side of County Road D should be replaced with an inlet having greater capacity to take advantage of the flow capacity of the 24 -inch culvert under County Road D and lessen the chance of excess stormwater overtopping County Road D. The replacement of this inlet should be completed regardless of what solution is chosen south of County Road D. The estimated cost of the inlet replacement is $5000.00. Riprap /Enkamat Swale This option would include the construction of a combination Enkamat and Riprap swale along the drainage route on the east property lines of the lots along the east side of Maple Leaf Court. The north portion of the swale, where the erosion is not as prominent, would be constructed as a shallow swale excavated and lined with Enkamat erosion control blanket. The swale would be 1 foot deep with a 4 -foot bottom and 4:1 side slopes. This Enkamat swale would be at the existing slope of approximately 3.0% with a velocity of about 3.6 fps. This velocity would be below the 4 fps maximum recommended for an earthen channel and also well below the maximum allowable flow velocity of about 8 fps for an Enkamat type erosion control blanket. At the location where the severe erosion and gully starts, there would be a transition to riprap as a channel liner. A riprap surge basin or a stepped riprap terrace would be utilized to transition down the 4 foot drop off at the start of the gully. The deep gully would be re- graded and lined 0: 1PROJ\804490j10 32 5149 0 -1 304. bct varc CO PAGE 128 any