Re: drainage problem - Laserfiche WebLink

Home Browse Search

Ms.Kathy Pollock May 21,2015 Page 4 Observations and Conclusions After observing your property and the 990 property(looking south from your property line) and reviewing available topographic and aerial photographic information,we have come to the following conclusions: 1. Prior to the construction of the new home and driveway on the 990 property in 2013, much of the runoff from that property drained onto your property; however,those flows were likely dispersed sheet-flows, not the more concentrated, channelized flows that now occur. 2. Construction of the house and driveway on the 990 property in 2013 resulted in approximately 27,900 square feet (0.64 acres) of impervious surface (Figure 1).That amount of impervious cover, combined with the lot grading and the conversion of forested woodland and tall-grass-meadow vegetation areas (Figure 2) to a mowed lawn has: (a) significantly increased the volume, rate, and flow velocity of runoff entering your yard from the south, and (b) converted what were once largely sheet-flows running from the 990 property to concentrated, channelized flows at three distinct locations on your property(the east side of your house, near the bump-out parking/turn- around area on your driveway, and coming down your driveway from North Shore Drive). 3. You have removed much of the rock that was deposited in your front yard and you have attempted to control the erosion in your backyard. 4. In our opinion, the damage to your yard and the deposition of rock and debris will be chronic and worsen until the increased, concentrated flows from the 990 property are prevented from entering your property. 5. In our opinion, the rock on the road ditch slope at the curb drain cutout channel along North Shore Drive was not properly placed and is not functioning as a channel armoring/erosion-control feature. Flows are not contained within the rock as they pass from the road, down the steep road shoulder slope, and onto the roadway ditch near your north property line.The rock was mounded and not dished to contain flows. Water is forced to flow along the outside edges of the rock, carving an eroded channel on both sides. This causes the rock to drop into the scoured zones and renders it useless as an erosion-control feature. Further,there appears to be no fabric or granular filter beneath the rock. As a result, the rock is sinking into underlying soils. Recommendations To stop or minimize the problems described above, we recommend the following actions: 1. Have the owner of the 990 property place a bituminous, concrete or wood-timber curb along the entire north side of their bituminous-paved and concrete-paved driveway.This will prevent water from the roof, driveway and much of the yard from rapidly flowing onto your property. 2. Ask the County to place features on North Shore Drive to prevent flows from the 990 property and North Shore Drive from running down your driveway.It is my understanding that the County may be planning to construct a stormsewer with a catch basin in the road gutter between your driveway and the 990 property driveway.Typically catch basins are designed to capture flows from a 5-year to 10-year storm event. But in this case,I recommend designing in much more catchbasin capacity for the following reasons: • The velocity of flows and the resulting flow vector of flows from the 990 property may create a condition where they could bi-pass the catchbasin and come down your driveway. P:\Mpls\23 MN\27\23271432 Drainage Issues-950 N Shore Dr\WorkFiles\Report