PROJECT AREA: WARWICK/GREENWOOD LAKE

HUDSON VALLEY REGIONAL COUNCIL

3 Washington Center, Newburgh NY 12550 http://www.hudsonvalleyregionalcouncil.com

GREEN INFRASTRUCTURE CONCEPT PLAN -- FRONT OF WARWICK TOWN HALL (SITE #1) WG-7 Project type: Municipal parking lot retrofits DECEMBER 2011

Proposed practices: Two bioretention gardens

Google 2011

2

The following report describes a schematic landscape design proposal using green infrastructure practices for stormwater management. This plan is intended to give practical guidance for the owner, design professionals, contractors, and other interested parties to use in developing a final design. It is not intended to be used as a final design or for construction documents. The following report was prepared by Kevin Sumner, District Manager of the Orange County Soil and Water Conservation District. Thanks to Supervisor Michael Sweeton and Town Engineer Edward S. Butler for their support and contributions.

3

Figure 1 View northeast of proposed rain garden location

Figure 2 View southwest showing existing gully

BOARD OF DIRECTORS

Gary Keeton David Hulle Michael R. Pillmeier John Wright Tom Pahucki 394 Guymard Turnpike 28 Hulle Lane Orange County Gov’t Center 329 Kings Hwy. 111 Celery Ave Middletown, NY 10940 Middletown, NY 10941 255 Main Street Warwick, NY 10990 New Hampton, NY 10958 Goshen, NY 10924

ORANGE COUNTY SOIL & WATER CONSERVATION DISTRICT 225 Dolson Avenue, Suite 103, Middletown, NY 10940

PHONE: (845) 343-1873 FAX: (845) 344-1341 kevin.sumner@ocsoil.org

Green Infrastructure Planning for Warwick Town Hall 12/2011 KMS Site #1 On 11/30/2011, a field trip was conducted to identify potential sites for the installation of Green Infrastructure/Runoff Reduction practices at the Town of Warwick, NY municipal complex on Kings Highway. Participants on this field trip were Kevin Sumner (Orange County SWCD), Ed Butler (Tectonic Engineering), and Simon Gruber and Marcy Denker (Hudson Valley Regional Council representatives). Town Highway Supervisor Jeff Feagles spoke with the group briefly regarding potential projects around the DPW complex. Several potential projects at the DPW complex were briefly discussed. A site on the access road to the Police Complex was reviewed as well, but consensus was that projects visible to visitors to Town Hall would better suit the overall goals of the GI planning project. Therefore, attention was focused on the Town Hall complex. Ed Butler assisted Kevin Sumner with collecting survey data and evaluating the sites. Two primary sites were given attention at the Town Hall Complex. One was in front of Town Hall, between Kings Highway and the ‘drop off’ area at Town Hall’s main entrance. This long, narrow lawn area slopes from the edge of the Town Hall asphalt driveway down to the road ditch next to Kings Highway. This area receives runoff from ~9,600 SF asphalt driveway, building roof surfaces, concrete sidewalks and lawn areas. Most of the contributing area, some 6,200 SF, is asphalt driveway. Runoff from the drop off area currently concentrates and leaves the asphalt surface directly across from Town Hall entrance – causing gully erosion through the receiving lawn area. Erosion is also evident at the northeast end of the site where driveway runoff concentrates along the driveway/lawn area edge before draining to the Kings Highway road ditch. It appeared that much of the runoff concentrating at this area could be directed into the proposed GI practice. Additional driveway runoff could potentially be directed into the treatment practice if a runoff collection device, such as a slotted drain, were installed across the Town Hall driveway entrance. Such a device could collect an additional 7770 SF of driveway surfaces. The practice that is proposed for this area is a rain garden or bioretention basin (BRB). These two GI practices are similar, with rain gardens generally being simpler in design – typically relying on native soil for treatment, and BRB’s typically employing a select soil mix and often an under-drain. Some installations could be considered a hybrid of the two practices. For simplicity’s sake, the term rain garden (RG) will be used in the remainder of this report. For preliminary planning purposes, a reasonable formula for determining if adequate area is available for a RG is ten percent of the contributing area. Assuming the full drainage area including that which would be collected by a runoff collection device across the TH driveway entrance, the available area should be at least (9598+7770=) 17368*0.1=1737 SF. The available area is narrow, as noted above, and sloping, so the entire area from the asphalt driveway edge to the road ditch cannot be included in the treatment area. Allowing for slope issues, a 12 foot wide treatment area surface should be feasible. Therefore, a length of about 150 feet will be needed to meet the general sizing recommendation. This is feasible within the existing lawn area between Town Hall driveway and Kings Highway, even without disturbing the existing row of landscape trees southwest of the drop off area. Therefore, the

Orange County

Soil & Water Conservation District

page 2

proposed RG practice appears feasible for this site. If the additional drainage area is not included, the RG could be reduced in length to roughly 80 feet ([9600*0.1]/12), or oversized beyond 80 feet in length. Other Design Considerations SOIL - The Orange County Soil Survey indicates the area to be Erie Silt Loam, a somewhat poorly drained soil, and one which could be expected to have an infiltration rate too slow to function effectively for a RG. A cursory examination of the soil in the field using a hand-held soil auger yielded no reason to question the published soil mapping. Therefore, preliminarily, it should be expected that to make this area function for a RG, removal of native soil and replacement with a select soil mix will be necessary. Or at a minimum, heavy amendment of the native soil might be an option. WATER TABLE SEPERATION – Erie soils can be expected to have a seasonal water table perched above a fragipan layer that is 24 inches or less from the surface. It appears, and would be expected, that this area was disturbed/altered in the past during road and building construction activities. Based on the observed water table immediately on the other side of Kings Highway, it is estimated that a 4 foot or greater separation from the regional water table could be maintained assuming perched water table/fragipan will not be an issue as a result of both past disturbance and expected improvements/amendments to soil as part of any RG design requirements. CROSS SLOPE – As noted above, the site slopes in its narrow dimension, typically as much as 8%. This will require considerable grading, and some type of embankment or retaining wall on the down-slope side of the RG. Although this will increase costs, creative design measures can be employed to allow the embankment or wall to appear as an intentional and pleasing feature of the garden. VISIBILITY – The location of this area, with its visibility both from passing traffic on Kings Highway and from driving and walking visitors to Town Hall, offers advantages in terms of the educational value of a RG in this location. Its easy access from Town Hall and associated parking lot, along with its high visibility, would hopefully enhance the possibility that a locally based group would ‘adopt’ the RG and attend to modest, though necessary long term maintenance requirements. UNDER-DRAIN/INFILTRATION CONSIDERATIONS – As noted, there are some concerns with the texture of the native soils at this site. These concerns would often argue for the inclusion of an under-drain in the RG design to better ensure that ponding does not exceed the recommended duration. Some designers routinely call for under-drains in their RG designs. Others have raised the concern that under-drains to some extent defeat the purpose of RG’s in terms of promoting infiltration of storm water. Various approaches have been suggested to address this concern. One is raising the under-drain above the bottom of the improved soil/aggregate layers of the garden. Some believe that this will allow the native sub-grade under the garden to accept storm water to the extent of its infiltrative capacity, with the under-drain functioning as a ‘relief outlet’. Others suggest that the under-drain will still ‘short-circuit’ infiltrative ability and possibly compromise pollutant removal functioning if positioned too high in the improved soil layers. Another possible approach to this concern is to specifically design the under-drain (number and size of perforations) to control the removal rate – maximizing retention time while avoiding unacceptably long ponding. The under-drain could then be placed at the bottom of the improved soil layers – avoiding any compromise of pollutant removal functions and, assumedly, promoting more infiltration into the sub-grade as the native soils allow. A third possibility would be to install a gate valve in the under-drain downstream of the RG. With this feature, an operator could observe the performance of the garden and adjust the valve accordingly. For example, if the RG drains within the recommended time frame with the valve closed, infiltration is maximized. If not, the operator could experiment with the degree to which the valve is opened in order to influence the ponding time. This approach adds an additional maintenance/monitoring component to the practice, which may not be acceptable in some settings. However, the gate valve approach, it would seem, may provide the most options and flexibility of the above approaches. In the event that an attentive operator is not available, the designer could set the valve initially to achieve acceptable ponding duration. At this point, the RG would function like any other RG design without a valve

page 3

control, but would have the added advantage that future adjustment would be simple and essentially without cost. There would be an additional up-front cost for the valve, but this would be a modest cost in relation to the overall cost of the practice installation. UNDERGROUND UTILITY ISSUES – A utility mark-out was not conducted as part of this conceptual design. The issue was discussed in the field with the parties mentioned at the beginning of this document. It appeared that there could be buried electric issues to contend with, but no other buried utilities were observed or noted. Should this conceptual design be pursued, a formal utility mark -out will be necessary. ADVANTAGES TO THIS SITE – A pre-treatment/inflow control feature could be designed that would eliminate the current gully erosion occurring in this lawn area. The existing road ditch provides a potential outlet for an under-drain. DISADVANTAGES TO THIS SITE – Increasing the area treated as described with a slotted drain would add considerable expense, perhaps as much as the construction of the entire RG. However, even with the smaller contributing area, the garden would provide treatment and serve the educational function. COST ESTIMATE – Not enough study was done to calculate a reliable cost estimate for this project. It can be noted, for general planning purposes, that RG’s of similar size and design have been built locally at a cost of between $10,000 and $20,000 (design and construction).