Although the State of Wisconsin soon will be requiring stormwater infiltration on certain types of new developments, few developments in southeastern Wisconsin successfully have used infiltration as part of a stormwater management plan. This is largely because of the soil composition in the area, which is generally clayey and not conducive to infiltration. At a large new development just west of Milwaukee, however, a number of factors – including favorable soils – allowed stormwater infiltration to play a large part in the stormwater plan.
Infiltration basin at Pabst Farms with one season of growthLocated halfway between Milwaukee and Madison in the center of America’s Dairyland is a tract of farmland previously owned by Pabst Brewery. The property still is referred to as Pabst Farms. The farm consists of 1,500 ac. of mostly cropland, in recent years particularly corn and mint. In the earlier part of the century, the Pabst family used the farmland to become a world-renowned breeder of Holstein cows. Situated in Waukesha County on Interstate 94 connecting Milwaukee and Madison, the farm is visible to tens of thousands of motorists each day. Given the exposure and freeway access, the land makes an ideal retail, commercial, and residential site. When the project first was proposed, however, there were concerns from local residents regarding the effect of stormwater runoff on surrounding lakes.“Lake Country”Waukesha County locally is referred to as “lake country” because of the abundance of inland lakes in the immediate area. The majority of the lakes are groundwater lakes, meaning that the water elevation of the lakes is the elevation of the groundwater. Surrounding the project to the west, south, and east are lakes with year-round and summer homes.Due to the flat topography and pervious soil makeup of the site, only a small portion of stormwater ever flowed off the site while it was farmed, further heightening the concerns of local residents. The topography is so flat that anyone standing on one end of the site is able to see almost all of the 1,500 ac. The soil consists of approximately 1 ft. of topsoil over glacial till. Groundwater is fairly uniform at about 17-20 ft. below surface grade. Because the subsoils are so transmissive, the topsoil dries out fairly quickly. The farming operations required that high-production irrigation wells operate during most of the dry periods of the year.Technical Advisory GroupTo address the concerns of local residents and as part of the initial stormwater planning, the developer, Pabst Farms Development LLC, created a technical advisory group (TAG) to assist its consultant in formulating an overall strategy for regulating stormwater runoff before development began. TAG included representatives from the developer, the Wisconsin Department of Natural Resources (WDNR), the Waukesha County Land Conservation, the City of Oconomowoc, and the town of Summit, along with consultants for the developer, the City of Oconomowoc, and the town of Summit. Two-thirds of the Pabst Farm development is located in Oconomowoc, with the other third in Summit. RecommendationsThe primary recommendation from TAG was that peak discharge and volume of stormwater runoff from the developed property should not exceed that of the site in its existing, undeveloped condition. To accomplish this, runoff from the development would need to be treated to meet WDNR stormwater-quality requirements and then be infiltrated.The consultant identified the areas where stormwater discharges off-site and quantified the associated peak flows and volumes that could be matched with the proposed flow. The consultant also identified areas where stormwater facilities (complexes of stormwater-quality ponds and infiltration basins) would be located on the site, along with their conceptual sizes and deposits.The site was broken into regions with different requirements. Development in some regions will be allowed to drain all stormwater to developer-provided, community-owned facilities. These are primarily residential and dense commercial areas. In other regions of the site – primarily larger commercial and industrial lots – the individual sites will be required to treat and infiltrate 65% of the stormwater runoff, with the balance allowed to flow to community facilities. It was envisioned that some private onsite facilities would fit on the larger lots in areas already allocated to green space requirements. ImplementationOther factors besides soils hamper the use of infiltration in southeastern Wisconsin. Infiltration requires a commitment by the developer to this innovative type of solution because of the higher initial and ongoing costs. Infiltration basins cost more to construct than simple stormwater detention ponds do, and they use more land. More care has to be taken not to compact or clog the infiltration area with sediment during construction. The infiltration basins need to be tested and maintained throughout their lives.Pabst Farms Development charged National Survey & Engineering with the responsibility to design and implement the plan and begin the design effort. The Peter Bell family and the Pabst family own the property. Peter Bell has a history of developing large parcels into high-end facilities and therefore represents the development side of the partnership. It became a focus to create stormwater facilities that not only are functional but also enhance the development. In some cases, the stormwater facilities became an integral focal point of the Pabst Farms development. It was only by the commitment of the developer that infiltration successfully could be implemented on this site. The result of this collaborative effort was the Pabst Farms conceptual stormwater management plan, which was adopted by all of the participants as the overall guiding document. Two areas totaling 300 ac. were identified by Pabst Farms Development as the first phase to be designed. These areas were commercial and retail parcels. The retail area of about 200 ac. will be served solely by community facilities, and the 100-ac. commercial area will share responsibility of the stormwater requirements between private onsite and public community facilities. Basin Complexes
Infiltration basin under construction
Water-quality pond discharging from Oconomowoc to SummitAlthough the general location of basin complexes was defined at an early stage by TAG, during detailed design the exact location of the basins had to be determined based on grades and preliminary road layouts. Stormwater facilities were sized and located either to maintain existing discharges off-site or contain all flows on-site, based on matching existing conditions. Drainage areas for the facilities were identified mostly by land use and road layout of the major roadways planned for the site. Under a tax incremental finance district (TID), the City of Oconomowoc’s engineer designed two major internal roads, including sanitary sewer, storm sewer, and water mains within the right of way. Utility locations were added to the layout of the basins to ensure that the sanitary sewer could serve future areas with adequate depth in regard to frost depth and potential basements in areas designated residential.Because the site was so flat, large amounts of stormwater could be directed from the site to a basin of choice without extraordinary amounts of earthwork. This also limited the amount of fall in the ditches that could be provided to get stormwater to drain to the basins. Because the groundwater was 17-20 ft. below the surface and because of the need to keep infiltration above the groundwater, basin depth below existing grade also was limited. Although the facilities were sized for future conditions based on predicted land use, there were no firm site layouts for most of the development. Tracts of land were identified, and major roadways were shown conceptually. Assumptions were made as to travel paths for storm flows under developed conditions. Overland flow and ditches were used to enable the developer to be as flexible as possible to create parcels sized for yet-undefined potential users, as ditches are easier to change than storm sewer lines are. Conservative runoff factors were assumed using local green-space requirements as a guide. Water-Quality Pond Design
Partially filled 6-ac. water-quality/feature pondWater quality was essential on this project: sediment must be removed from the stormwater before it has a chance to enter the infiltration basins, thus reducing their capacity. One accepted method of improving water quality is to allow the suspended solids to settle in water-quality basins. The latest WDNR regulations require removal of 80% of total suspended solids of a 5-micron particle size during the 10-year event on an average annual basis.To meet this requirement, ponds were designed using the WDNR conservation practice standard Wet Detention Basin, Code 1001. This conservation practice ensures 80% removal by requiring a minimum surface area based on land use, a 3:1 length-to-width ratio, a minimum 3-ft. depth in the wet pond, sediment storage room in the wet pond, and a release of the one-year storm over 24 hours. These requirements combine to give the 5-micron particle adequate time to drop out of incoming stormwater. Based on this standard, the size of the water-quality ponds ranged from 1 to 6 ac. The 6-ac. pond was sized larger than required for stormwater quality to serve as an aesthetic feature in the retail portion of the site. The basins were designed with irregularly shaped sides to try to make them look more natural than typical oval stormwater-quality ponds appear. Water-quality ponds were designed to be adjacent to infiltration basins and were connected to the infiltration basins by earthen weirs. The water-quality ponds were lined to ensure that they held water and provided a water-quality benefit. Infiltration Basin DesignInfiltration basins were designed based on the requirements in the Wisconsin Stormwater Manual (a WDNR publication) and the requirements of Wisconsin Administrative Code Chapter NR151. They were designed to infiltrate stormwater from the 100-year storm within three to four days, so as not to drown plantings in the basin. To accomplish this, at least two infiltration tests were taken in each proposed basin location at the proposed basin elevation. The design values chosen were one-half of the tested values to better represent the average infiltration rate over the life of the basins. Computer models were run using PondPack software to help size the basins, and the maximum water surface was generally about 3 ft. deep. Postconstruction infiltration tests verified that the design infiltration rate was achieved. When TAG originally contemplated the infiltration basin in the first phase of design, it envisioned two sorts of infiltration basins. The first type infiltrated all storms up to and including the 100-year rain; the second type infiltrated only storms in excess of the 10-year storm, with smaller storms draining to an existing onsite pond. During detailed design, it was determined that infiltrating smaller storms and only releasing storms in excess of the 10-year storm would be preferred where possible to infiltrate more overall stormwater. Of the three basin complexes designed for this phase of development, all three types of basins are present: One basin infiltrates all rain events up to and including the 100-year rain; a second basin infiltrates stormwater up to and including the 10-year event (with the remainder draining to the existing onsite basin); and a third basin infiltrates events in excess of the 10-year event (with the remainder draining to the existing onsite basin). Basin geometry consisted of relatively flat bottoms and 3:1 sideslopes. Ditches were placed around the infiltration basins to intercept any runoff during and after construction, diverting it to the water-quality ponds. Just like the ponds, the basins were created with irregularly shaped sides to make them look more natural. Infiltration Basin Plantings
An infiltration basin after wetland plant seedingThe developer desired a combination of wetland and prairie plantings in the water-quality ponds and infiltration facilities. The original plan was to topsoil the bottom of the infiltration basins and plant deep-rooted wetland plants. Because onsite topsoil was being used to line the water-quality ponds, however, there were concerns about placing it in the infiltration fields over the more pervious material. Although deep-rooted wetland plants allow stormwater to pass though topsoil and infiltrate underlying soils, the plants cannot be expected to do this until they mature after several years. For these reasons, only the sides and no more than 40% of the bottom of the infiltration basins were topsoiled and planted with the deep-rooted wetland plantings, with native granular material covering the rest of the basin. As the deep-rooted plants become more established, infiltration tests will be performed. Once the plants are proven to provide the design infiltration rate, additional topsoil and plantings might be added, covering the native granular material. Different plants were used for different portions of the infiltration basins, depending on expected exposure to water. Impervious Liner, DewateringBecause of the geology of the site, the ponds were required to be lined with an impervious liner. Synthetic liners and clay liners were investigated. It turned out the topsoil on-site had many characteristics of clayey soils. The geotechnical engineers determined the topsoil would be suitable for a lining material if applied at proper moisture and compaction. In areas where the bottom of the pond was below groundwater, groundwater had to be pumped in to install the liner. A plan of operation was required for the WDNR permit that indicated a discharge location for the water. Discharging into one of the surrounding lakes was not an option, as the town’s engineer was concerned with the adverse effects of the increase of lake elevation. Located on the southeastern portion of the site is a 16-ac. lake created by the Wisconsin Department of Transportation to construct one of the overpasses spanning the interstate highway. This area was identified as having the potential to receive the pumped groundwater. Because the pond was approximately 1,500 ft. away, the zone of influence was not a concern. A 15-in. discharge line was constructed across the state highway and into the pond.
Construction of the water-quality pond along I-94The dewatering effort continued throughout the installation of the liner. After the liner was installed, the discharge from the dewatering pumps was directed into the pond. This was necessary to create ballast for the pond to counteract the hydrostatic uplift as the groundwater elevation was restored to normal elevation. During periods of dry weather, it will be necessary to regularly monitor the water elevation of ponds whose bottoms are lower than groundwater to ensure that the elevation does not get too low, thereby creating enough hydrostatic pressure to damage the pond liner.EarthworkGiven the flat topography of Pabst Farms, the grading design of the 100- and 200-ac. parcels was centered on shaping the flat topography into drainage areas for the identified stormwater facilities. Initially the drainage basins were identified solely on land use. As the stormwater design progressed, the site design became more defined, and additional input was incorporated into the design. In particular, such items as pond sizing changed as the planner and developer determined which features to enhance and how they would best fit into the overall theme of the development.As the pond sizes were redefined, the drainage basins were adjusted based on the ability of the ponds to meet the needs of the drainage area. Each time the drainage areas changed, the site had to be rebalanced. Changes to the drainage basins required moving high points hundreds of feet. Because of the enormous size of the development, changes in drainage basins could result in the import or export of tens of thousands of cubic yards of materials. Each time the basin configuration changed, grades and slopes had to be adjusted to balance the earthwork. Based on representative soil borings, the topsoil depth was assumed to be approximately 1 ft. In reality, the topsoil depth varied between 8 in. and 3 ft. The deeper topsoil zones generally were isolated and concentrated to localized 1-ac. depressions that were filled by windblown topsoil. To shape the identified areas, topsoil was stripped and stockpiled for use as a dressing to establish ground cover and stabilize the site.
A 6-ac. water-quality/feature pond, with the infiltration field in the distanceInitial topsoil stripping and stockpiling resulted in numerous piles being placed in areas that would not interfere with short-term development but in close enough proximity to minimize haul distances for scraping and respreading. On the initial 200 ac. of the development, more than 300,000 yd.3 of topsoil was stripped and stockpiled. Shaping of the stripped area then began. Ponds and infiltration areas were dug as the beginning point. Materials cut from these areas were used to fill areas at the edges of the basins to create grade. The grading design attempted to use a minimum grade of 1% over the site; however, in ditches with concentrated flows, a minimum of 0.5% was used. Overall shaping of the drainage basins resulted in moving more than 700,000 yd.3 of material.The geotechnical engineer monitored the topsoil stripping and directed the contractor to segregate topsoil suitable for pond lining. The segregated topsoil was stored separately. The stripping, moving, and hauling of the material was observed to ensure that no large foreign matter or stones were mixed into it. After the ponds were shaped, a topographic survey was taken of each basin. Topsoil was placed and compacted in layers to the finished grade. A second topographic survey was done to check the depth of the liner. The ponds were filled, and the water elevation was observed to ensure that no leakage occurred. Erosion ControlErosion control measures were installed at the outfalls of the pipes, outfalls of the ditches, and overflows to the infiltration basins. The geology that made this development ideal for infiltration also made it extremely susceptible to erosion. Protection of the topsoil layer over the granular material was absolutely essential as the plantings were being established. The topsoil eroded very easily, and once the granular subgrade was exposed it also eroded quickly, as it had little soil cohesion. It was very important to establish the annual grasses and protect the areas until they were established. Correct installation of the erosion matting – temporary and permanent – was also very important. In any small areas where topsoil could erode, it did, and it created defined areas requiring immediate repair. Once established, the ground cover consists of native blends of annual and perennial grasses, such as Kentucky blue grass, red fescue, and rye. The ground cover has held up well, enduring a 6-in., three-day rain with minimal erosion problems.Stormwater DistrictBecause of its location within two municipalities, the Pabst Farms development required a separate public entity to regulate the stormwater facilities. The creation of a stormwater district was required to represent each community and the developer regarding the operation and maintenance of the public stormwater facilities. In addition, the district monitors, regulates, and approves the design, construction, and maintenance of all private stormwater facilities in conformance with the overall stormwater requirements regardless of municipal boundaries. The stormwater district includes two members from the City of Oconomowoc, two members from the town of Summit, and one member representing Pabst Farms Development.
Summary
