PRECA 01-23-1980 - Laserfiche WebLink

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r There are three possible solutions to the problem: 1. Ponding in the proposed park to hold the runoff The Watershed District requires that the excess runoff from a 100 year storm to held on site with a two foot freeboard to the lowest building entrance. A pond to hold all of the runoff from the 18.8 acre area would cover. most of the proposed park since the existing storm sewer outlet. is too shallow to permit a deep pond. The area where the pond would be constructed i;s now quite high and we estimate that about 50,000 yards of earth would have to be removed from the site at a cost of about $100,000. If a lesser .storm event could be used for design, this pond could be reduced in size. 2. Anew storm sewer pipe from the rear of the center north to Mississippi Street thence east to the existing sewer in Silver Lake Road A new storm sewer pipe would have to be a 42 inch RCP with an estimated cost of $70,000.- This would-provide capacity-for- the--ten year-event:-~-~-~ 3. Flood proofing the shopping center building In order to adequately floodproof the building, the retaining wall at the rear of the center would have to be raised about four feet. This wa17 would have to be waterproof and structurally adequate to hold back the water. We have not estimated this costa Flood proofing the building only will still result in flooding of the parking lot and a portion of the park. A combination of any two or all of the above solutions is also possible. Discussions with the City engineer and the Watershed engineer, Bill Wiedenbacher, indicate that a.compromise plan can be proposed. It would require a pond of about 1.15 acres, (125 ft. by-380 ft.) on the northerly portion of the park and raising the retaining wall by two .feet. The existing. outlet would-be modified to provide--more positive drainage out of the park and to provide treatment of a one year_ form as-per the Watershed District's policy. __ . My computations indicate that this pond would drain within three hours .after a one year storm, and within 4 1/2 hours after a 25 year storm. In my opinion this represents a.very minor impact on the use of the proposed park. This is so because .any park would remain too wet to use for at least a few hours-and possibly a day after a rainfall. Summary - The drainage problems of Park Place can be economically resolved with a limited pond and some minor masonary work on the Rice Creek Center. We have not yet prepared cost estimates for this work, but it would b'e much less than the other alternatives considered and it appears to offer a means to satisfy both the City, and the Watershed District. Copies of all calculations .and plans are on file in my office. Sincerely, cc Les Proper James P. De6enedet, P.E.