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Home My WebLink AboutA009 - report from Iowa State University on urban stormwater management and water supply impact Iowa State Water Resources Research Institute IOWA STATE 355 Town Engineering Building Ames, Iowa 50011 UNIVERSITY Telephone 515-294-8921 January 3, 1980 Mr. Neil A. Carpenter Ames Engineering and Testing Co. 1212 McCormick Ames, IA 50010 Dear Mr. Carpenter: Re: Stone Brooke Subdivision, North Ames Area This is a brief letter report concerning urban stormwater management and water supply impact of this subdivision project as it relates to the water supply research studies at the Hallett's Quarry area.. Because of various statements attributed to our office and me, this clarification should assist in land use planning studies in this area. Mr. Larry Antosch, Graduate Research Assistant, has conducted a preliminary analysis of the runoff potential of the proposed urbanized subdivision, Stone Brooke. This has indicated that it would be possible to detain all runoff from the area in the northeast marsh area, by con- structing a 3 to 4 foot high berm around the north and east sides of the property (top elevation of 79 ft. , City Datum, proposed) , This would detain all runoff from all storms up to and including the 10-yr, 25-yr, 50-yr and 100-yr rainfall frequencies, for 24-hour durations, etc. This is severe criteria, but does make maximum use of the available marsh area's detention potential. A compacted earth berm, with a 6 ft. to 8 ft. top width, 2: 1 or 3: 1 sideslopes with a shallow core trench at center line, should be adequate. Release of the detained runoff would be possible under one of two .options: 1. Underground seepage release of detained water Convert marsh area to grassed area. Install agriculture tile in drainage trenches, backfill trenches with sand and gravel to serve as "french drains". Entire area seeded to grass, kept mowed and used for open-space and recreation uses. Convert to ice-skating in winter, if desired by se- quential flooding to freeze soil surface, etc. This probably would require a tile outlet to the southwest corner of Hallett's Quarry water area. Need stream channel profile from Top-of- Hollow to Hallett's Quarry to work out final details. This alternative would eliminate mosquito nuisance situation which continued marsh area might present. It would also permit Neil A. Carpenter 2 January 3, 1980 periodic clean-up of flooding residue, etc. following storm- water runoff periods, and additional dissipation of phenols, oil residues etc. , from urban runoff. This is our first preference. An emergency overflow section, about 100-200 f t. in length, at an elevation one-ft. below the berm elevation, would be needed at the northeast corner of the tract. 2. Surface water pond detention Construct a permanent pond, with a normal water depth of three to four feet. A temporary storage volume of about 23 ac-f t of detention storage is needed, from the 71 acre site. This alternative would measurably eliminate mosquito nuisance situation a shallow marsh would present--the stormwater deten- tion, storage volume would be above the normal water surface. Elevations for normal water surface, temporary stormwater storage and berm top elevation, would have to be worked out in a detailed design. A small concrete spillway would have to be designed also, to discharge eastward to the creek channel. It might be necessary to line such a pond with bentonite or other sealant materials to retain the permanent pond. Either option would satisfy urban stormwater detention requirements, and control urban runoff quality inputs to the Hallett Quarry area. Additional details can be provided in consultation with our research group, the city and the developers, and other interested groups. It is recommended that the high-density housing units be placed to the north and the northwest parts of the subdivision, so that the in- creased urban runoff will drain to the detention area and not flow directly to the nearby creek channel (outside of the detention area watershed) . This will also move such high density uses to a more re- mote location, and reduce the impact on the -individual residential housing units to the south, along Top-of-Hollow Road. Otherwise, we can see no conflict with this subdivision project and the proposed use of Hallett's Quarry as a supplemental water source for the City of Ames. Construction control over sediment production can be provided using silt barriers, bales, etc. on each localized site. Later if it is needed (following the two-year research study) , the detention area could be modified or included in a larger pond area for the entire "south" watershed. It would assist our studies and your detailed engineering studies if a comprehensive profile survey was made of the stream draining this "south watershed" to Hallett's Quarry. This would involve commencing at Top-of-Hollow Road near the electric substation, proceeding north, then west, to the Stone Brooke subdivision, then north and on to Hallett's Neil A. Carpenter 3 January 3, 1980 Quarry. Obtaining elevations of the existing water level in the quarry, and outlet and culvert dimensions at the Dawes Drive outlet, should be included in such a survey. We will not be able to do this until next summer; your assistance would be helpful and would be a valuable con- tribution to the total studies. A copy of this information will be forwarded to the City Planning and Zoning Commission, and appropriate city officials. I will let you forward copies to your clients. Yours very truly, Merwin D. Dougal Director, ISWRRI MDD/ee cc: City Planning and Zoning (2 copies) City Public Works Director City Manager's Office Water Plant - Harris Seidel _ 4 1 Oki CNJ Q r ul c}�..'^ •-mraC`�..-• - J .A.'.yti.�.� �� O �. .'�,Ild 14 i� •.fir^' -13 Orw it CH/CA8O -- i PQ Cl) Tzl! 1 017- 0.0 - ��, ��•`•'-- -+aye—�:J� r �. - � �i� w� -. .._- .— _ - ___..- 1:; la / '� i. v i r C�/�j ~Jai' �i�� 1 _/•t\�� `'�1 i`