01-11-1988 - Agenda Packet City Council - regular meeting

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Mr. Waldron's proposal includes the construction cif terraces around a cut for a walk -out basement to minimize the sloping land on the property. It is important that this area not be paved and increase the hard cover. However, The improvements in the quality of surface runoff may be offset by degradation of the wat. r that infiltrates and reaches the lake as ground water. Perhaps the best solution, given the state of knowledge, is to choose the most aesthetically pleasing design that avoids an increase in impervious surfaces. Mr. Waldron's proposal also includes a deck that would be installed at the level of the first floor. This would improve the surface runoff situation by intercepting and retaining rainfall much as trees do. I lowever, it is important that the area under the deck not be paved or made impervious. 3. Prevention of Recurrence The erosion damage done to Mr. Waldron's property in July probably did more damage to Lake Minnetonka than twenty or more years of normal conditions. We believe that the best thing that can be done to maintain water quality is to avoid a recurrence. This can be done linstalling subsurface drains, controlling surface runoff, and avoiding retaining wawhere possible. This problem occurred because rains had saturated the ground prior to a very large rain. During the latter event, excessive ground water pressure caused the slope to fail just above the level of the lake. With this support removed, the rest of the slope quickly failed. The most important step to prevent recurrence i , to regrade the slope to a grade no steeper than 50%, revegetate, and install subsurface drains at the bottom of the slope. The drains will prevent excessive ground water pressure. Next, surface water runoff must be controlled to minimize the velocity of flow at any point. One way to do this is to install gutters on rooftops and pipe the water to an area where it no longer has the energy to erode or the opportunity to entrain other materials. Other landscaping can be done to avoid the channeling of flow. Finally, we cannot over -emphasize that retainin& walls do not solve slope problems that are caused by ground water conditions similar to those encountered here. Retaining walls are a solution to landscaping problems, not (iiainage problems. Subsurface drains must be placed behind all retaining walls and the walls must be properly designed. 4. Summary In conclusion, we believe that Mr. Waldron's proposal ,kill remedy his erosion problem and will prevent a recurrence. The keys to this are regrading to slope. less than 50%, installing subsurface drains, avoiding retaining walls, and controlling surface runoff. We do not believe that the proposal wii', create any new water quality problems.