Watson Park Proposed Flood Protection Berm Existing Site The approximately 18-acre Watson Park site is located on Gordon Rd. (Assessors Map 3001 0 1B). The site includes eight ballfields, a splash pad, a playground, a tennis court, a basketball court and a walking trail. The site is located on the Weymouth Fore River and much of the park is within the floodplain. The Base Flood Elevation is 10 feet (NAVD 88) while most of the field area of the park is at elevation 8 feet. Figure 1: FEMA SFHAs for the Watson Park area (Effective Date: June 9, 2014). Background The impetus for the project was the eroding shoreline of the park as well as the flooding of the park which occurred in January and again in March 2018. The Town received grant funding from Coastal Zone Management to develop a solution for the eroding shoreline and to plan for the impacts of sea level rise and storm surge. Staff from Planning and Community Development and the Public Works Department (including the Engineering, Recreation and Stormwater Divisions) have been working with consultants from Woods Hole Group. Alternatives for the park were developed and scored based on multiple criteria by Town Staff. The analysis and discussions with the Town and CZM identified Figure 2 as the preferred alternative. The overall project includes salt marsh restoration, stabilization of the eroding coastal bank and construction of the flood protection berm and rain gardens. Page 1 of 7 Figure 2: Overall Shoreline Stabilization Plan The Conservation Commission issued an Ecological Restoration Order of Conditions for the salt marsh restoration and shoreline stabilization components as well as a general Order of Conditions for the flood protection berm in fall, 2020. Project Need The park is a valuable recreational area that is heavily used by many Town residents. The park has been flooding more frequently in recent years due to coastal storms. Based on the modeling of sea level rise and storm surge done by Woods Hole Group, the park is likely to flood with increasing frequency in the future. While most residents will not be out during a storm, the park will remain flooded for longer periods of time, which renders the park unusable and potentially dangerous for longer periods of time. Additionally, the flooding of the park with sea water kills the grass within the park and on the heavily used baseball fields. This requires the Town to perform increased maintenance to bring the park and the baseball fields into a usable condition. The FY19 Coastal Zone Management Resilience Grant-funded study recommended the installation of a temporary earthen berm that can provide added protection to the park from flooding associated with most storms in the near-term. Based on modeling done by Woods Hole Group, daily tide levels are expected reach the elevation of the field in approximately 2070. At that time, the berm will be removed to allow for salt marsh migration landward into the park. Project Description The berm will be located in the location of the current walking path and the walking path will be recreated on along the crest of the berm. This berm would tie into the existing concrete seawall and would extend northeast to join higher elevations near the splash pad to provide a cohesive flood barrier. The crest of the berm will be built to an elevation of 10 feet (NAVD88) and will be 8 feet wide (consistent with the width of the existing foot path). The berm construction will require 173 cubic yards of material which will be laid in 6-inch layers and compacted to a minimum 95% density. The slopes of the berm will be covered with 6 inches of a Filtrexx Compost Erosion Control Blanket media for added slope stability and seeded with a New Page 2 of 7 England Native Warm Season Grass Seed Mix. The walking path will be relocated along the crest of the berm in order to ensure that scenic views are not obstructed and to minimize the amount of space needed for both the earthen berm and the path. The footpath along the crest of the berm will consist of 6 inches of stone dust. The proposed project will provide a stormwater benefits in the form of rain garden infiltration basins. These will be constructed along the field-side of the berm to reduce the amount of surface runoff that flows directly from the playing fields into the river. No impervious surfaces will be added as part of this project. After finished grades are established in the rain garden, the 2:1 side slopes of the rain garden will be seeded with New England Wetland Plants Native New England Warm Season Grass Mix at a rate of 23lb./acre, blended into a Filtrexx Compost Erosion Control Blanket media for added slope stability. Once it has been seeded and stabilized, the rain garden will be planted with switchgrass (Panicum virgatum), Indian grass (Sorghastrum nutans), little bluestem (Schizachyrium scoparium), and big bluestem (Andropogon gerardi) plug plantings at 3’ on- center. After consultation with the Stormwater Division, grass and herbaceous plants were deemed easier to maintain by the grounds maintenance crew (i.e., the whole area could be mowed or weed whacked once annually). Therefore, grass plug plantings will be supplemented with black eyed Susan (Rudbeckia hirta) and purple coneflower (Echinacea purpurea) in 1-2- gallon pots, planted at 6’ on-center. Coastal Analysis This Section provides a summary of an estimation of sea level rise impacts from extracting water level information from the Boston Harbor Flood Risk Model (BH-FRM) for existing and future 5-, 10-, 20- and 50-year return period storms. This coastal processes analysis provides a more in depth understanding of the potential historic processes contributing to the ongoing flooding of Watson Park, as well as insight into future conditions that may influence the flood protection project design and the long-term management of Watson Park. As with any coastal flood mitigation project, an accurate understanding of water level elevations is crucial for developing a successful design. Accurate present-day water levels can be derived from a detailed analysis of long-term historical records and detailed site specific topography. Historical water level data from the National Oceanic and Atmospheric Agency (NOAA) tide gage in Boston, MA (station ID: 8443970) were collected as hourly observations between May 3, 1921 and July 31, 2014: a 92-year period of record that was used as an input dataset for the BH- FRM. Water levels for this station have been rising continuously for each epoch since the beginning of data collection in 1921. To calculate accurate present-day water levels, historic values were adjusted based on the observed annual sea-level rise rate for the station. In addition to an accurate calculation of present-day water levels, BH-FRM also provides data on water levels during storms of various intensities. The BH-FRM results provide high resolution water level elevations for locations throughout the greater Boston area, including Braintree. The present day mean high water (MHW) and mean low water (MLW) elevations are presented in Table 1, and are 4.82 feet and -4.89 feet (NAVD88), respectively (Table 1). In addition, Table 1 also presents the water level elevations expected today under 5-, 10-, 20-, and 50-year storm conditions. In addition to the present-day water level elevations, corresponding data from two Page 3 of 7 future years, 2030 and 2070, are also presented in Table 1, assuming a high sea-level rise (SLR) scenario (i.e., assuming a high emissions trajectory). SLR by itself and SLR combined with storm events have most commonly been evaluated by simply increasing the water surface elevation by the projected SLR and comparing the new water elevation with the topographic elevations of the land. While this simplified “bathtub” approach can provide a first order assessment of potentially vulnerable areas, it does not accurately represent the dynamic nature of coastal storm events. To develop more refined flood vulnerability projections, the BH-FRM utilized high-resolution hydrodynamic modeling, which included multiple key processes that affect coastal water levels, such as riverine flows, tides, waves, winds, storm surge, sea level rise, and wave set-up. SLR scenarios were modeled for four distinct time periods (present-day, 2030, 2070, and 2100) to bracket the potential future sea level rise outcomes for the Boston Harbor area. SLR estimates were taken from the United States National Climate Assessment (NOAA 2012). Final sea level heights were adjusted for local subsidence rates. Ground elevations throughout much of Watson Park range between 7.5 and 9.0 feet (NAVD88). The results in Table 1 indicate that even today, high return frequency storms (i.e., 5- and 10-yr events) will impact portions of the park, while larger, less frequent storm events (i.e., 20- and 50- yr events) will flood almost the entire park area. Although everyday water levels will not exceed 8 feet (NAVD88) and be able to reach the field of the park until approximately 2070, by as early as 2030 even a 5-yr return period event could produce water level elevations of 9.1 feet (NAVD88), which is capable of flooding almost all of Watson Park. Flood Protection Function During multiple coastal storms in 2018, the entirety of Watson Park experienced flooding. An initial feasibility assessment conducted under a CZM Coastal Resilience Grant (FY19) looked at the probability of two types of flooding: First, the probability of the park experiencing storm surge flooding under present day conditions, as well as into the future due to sea level rise.