Located comfortably on Cape Cod Bay in southeastern Massachusetts, Plymouth owns top bragging rights to American heritage: Thanksgiving, Pilgrims, and of course, Plymouth Rock. Like many Massachusetts towns, the road networks in Plymouth — which carry tourists to the custom portico on the shore of Plymouth Harbor where the 10-ton rock rests — pre-date modern drainage and stormwater management. As a result, the Samoset Street Outfall watershed, referred to in historic records as First Brook, empties into the harbor with little or no treatment. With the town’s population growing noticeably over the last three decades, and with all that history to protect, Plymouth had to be proactive.

Jay Thomson, Engineering Division, Plymouth Rain garden after completion

To address the problem, the town applied for and received a Coastal Pollution Remediation Grant from the Commonwealth of Massachusetts Office of Coastal Zone Management (CZM) in 2002, and then hired the consulting firm of Horsley & Witten, Inc., to start a comprehensive assessment. The watershed, which is located in an urbanized area, is more than 100 acres in size and extends west from the harbor toward Route 3 (a limited access highway constructed in the 1950s from Boston to Cape Cod); Samoset Street (Route 44) is the primary access road from Route 3 to downtown Plymouth. Typical of many streams in urban areas, the watershed was diverted to pipes. Many of these pipes are located under the street and portions of the old stream bed and flood plain have been filled and developed. The watershed empties into Plymouth Harbor through a 36-inch pipe.

The comprehensive assessment resulted in six proposed locations to consider for best management practices (BMP) treatment:

1) Memorial Drive parking lot — A “proprietary water quality treatment device,” e.g., Stormcepter, to Cultec infiltration chambers

2) Office building complex — Diversion of stormwater and conversion of an existing detention pond to a pocket wetland system

3) Standish Avenue wetland — Alteration of an existing wetland to create two micro-pools to increase detention

4) Depot Square — Rain garden or bio-retention system to capture and infiltrate surface drainage

5) Bates Park — Rain garden

6) Water Street — Rain garden

All six locations presented challenges for development. One of the locations considered (Memorial Drive parking lot) would treat the runoff from a large portion of the drainage area; however, it was the site of a potential parking garage.

Of the two proposed sites in private ownership (office building and Standish Avenue wetland), one of the sites was an existing wetland, and the only portion of First Brook not diverted to pipes. The wetland location was subject to flooding “due primarily to clogging of the outlet pipe and its flow capacity.” It was also “highly disturbed…with large areas of observed fill, trash, debris, and invasive/nuisance species.” The proposed work at this site would involve reconstructing the wetland area to create “micro-pools” that improve the detention capacity of the wetland. It was clear that Plymouth would need to work with the various owners of the site to garner support for the project, which would have required easements over the properties involved. Permitting required for work in wetlands would also need to be determined and obtained.

The remaining three sites (Depot Square, Bates Park, and Water Street) were explored further for implementation under the time constraints of the grant process. Depot Square was ruled out because of impermeable soil conditions; and Bates Park would have treated a very small portion of the drainage area and was put on hold due to opposition from abutters. This left only one site remaining to construct a BMP within the time frame of the grant.

The location of the remaining site, the Water Street Rain Garden, made up for the small watershed area that would be treated. One major advantage to the Water Street site is that it is highly visible. Rain gardens come close to approximating the natural or predevelopment conditions. Pollutants addressed were pathogens (fecal coliform), nitrogen, phosphorus and total suspended solids.

Interestingly, the public is largely unaware of the history of this waterfront park. The site is actually an earthen embankment with stone armoring built in 1948 to shore up Water Street. This portion of Water Street was laid out in 1881 and was a product of “land-making” — the practice of developing by filling in the harbor — which was common at the time. In the 1940s, Water Street had subsided as much as 5 feet at the foot of Chilton Street. The springs that attracted the Pilgrims are still present along the waterfront and were being used for drinking water as late as the 1980s. Historically, the springs complicated the construction along the waterfront. During this project the construction of the diversion weir required diverting the flow of spring water until the concrete set.

The Water Street Rain Garden
The Water Street Rain Garden project involved installing a weir in the existing drainage system and diverting the initial storm flow (a ½-inch storm event) to a bio-retention area for water quality treatment before discharging it to Plymouth Harbor. The rain garden basin area is approximately 850 square feet. The drainage area treated is about 4 ½ acres. A weir diverts the initial storm flow to the rain garden. Excess water flow to the rain garden goes to an overflow basin to the ocean.

Jay Thomson, Engineering Division, Plymouth Under-drains being installed with overflow manhole on the left.

The project required blocking four to six metered parking spaces for a period of six weeks. A compromise was struck blocking four spaces only, requiring the contractor to feed the parking meters while they worked. The original planting scheme was changed due to concerns from local residents that the few small trees proposed would block their views of the harbor.

The next step was ensuring that no debris from the site would reach the waters of the adjacent harbor. The site is located on the waterfront at Plymouth Harbor, just a few hundred feet north of State Pier where the replica of the Mayflower is moored. The site is also just south of the town pier where the fishing and whale watching boats are moored.

When the embankment was placed 50 years ago, the material used was clean gravel with a great deal of stone. This time around, the contractor placed a silt sock rather than the more traditional silt fence/hay bales; removal of loam from the designated area was soon followed by excavation. Although the finish grade of the detention area was about 3 feet below the existing surface, the initial excavation was about 5 feet.

Once excavated, the construction of the bio-retention area began. An under-drain of crushed stone and four inch perforated pipe, which is connected to the overflow basin which drains to the ocean, was covered with permeable filter fabric and a specially processed loam. At this point a stone retaining-wall was placed on the uphill side. Rather than a grassed slope, which would be difficult to maintain, slope plantings were specified. After the area was brought to sub-grade, the next step in the process was placing a layer of triple ground hardwood mulch. The contractor took pleasure in transforming the “hole-in-the-ground” to a vibrant garden of plantings designed to improve the water quality as it percolates through before discharging to the ocean.

Weir construction
An interesting aspect of the project was the weir construction. The existing brick structure — the point where the stormwater was diverted to the rain garden as part of the 4½-acre drainage system serving the watershed, with an outfall of approximately 500 feet to the north — to be remodeled was approximately 2½ feet by 5 feet. The plan was to build a brick dam or weir to divert the initial storm flow. Brick was specified, as it would be easy to tweak the height to meet actual flow requirements. Upon closer inspection the brick structure was found to have no bottom. Either it was removed when cleaning the basin or the structure was built without one. The gravel bottom in the structure allowed the water to pass under the weir, preventing it from functioning as designed. A reinforced concrete weir, anchored to the brick sidewalls with reinforcing rods, was placed. Once the concrete weir had hardened, the forms were removed and the contractor was able to install concrete floor in the existing brick structure and place hydraulic cement around the existing pipes. The structure was now able to fill and divert runoff to the rain garden.

A fence was determined to be warranted behind the stone retaining wall; coming up with an appropriate fence for this historic site was a challenge. Fortunately the town had some rough cut granite posts left over from another job, and black vinyl-coated chain was used between the posts.

Within 24 hours of the 90-degree elbow being installed, and putting the system on-line, a storm event with a one-inch rainfall occurred. Everyone involved was pleased with the performance of the rain garden.

Jay Thomson, Engineering Division, Plymouth Rain garden just after a one-inch storm event

The final step in the process was placing a sign, again appropriate for a historic district, explaining the function of the rain garden, and Plymouth’s commitment to improving the water quality of the harbor.

Per maintenance requirements, an inspection of the rain garden must be performed annually, as well as after heavy storm events of more than 2 inches. When first installed, a rain garden requires frequent attention, including pruning or replacement of plantings as required, and if need be adjusting the weir to control the diversion flow. Every few years the mulch may need to be tilled or in severe cases removed and replaced. As with any garden, those that are cared for look the best.

Sid Kashi, P.E., is the town engineer for Plymouth, and Jay Thomson, E.I.T., is the civil engineer Plymouth. Kashi can be reached at skashi@townhall.plymouth.ma.us.