Coastal Resiliency Planning for the Surf Drive Area
A phased management approach for reducing vulnerability to natural hazards and enhancing coastal resiliency
Introduction
With almost 70 miles of shoreline along Buzzards Bay and Vineyard Sound, the Town of Falmouth is particularly vulnerable to coastal flooding due to storm surge and future sea-level rise. Many areas vulnerable to flooding contain public infrastructure and facilities, commercial development, and residential communities that can be adversely impacted by flooding. The Town of Falmouth has therefore taken proactive steps to fully evaluate its risk and establish an integrated climate change planning and preparedness strategy.
The Town of Falmouth commissioned the Woods Hole Group to conduct a detailed town wide climate change flood vulnerability assessment, which was completed in January 2020. This study identified the Surf Drive area as one of the most vulnerable areas in Town. Surf Drive and the adjacent shoreline already experience significant stressors (e.g., regular inundation, erosion, overtopping, storm damage, ecosystem evolution, etc.) under today’s climate conditions. Storm damage to the roadway and the bike path can cause disruption to the collocated utilities, including water, sewer, and electricity. Climate change and sea-level rise will increase the frequency and severity of these stressors, resulting in increased vulnerability in the future. To further evaluate these risks and begin long-range planning for the Surf Drive area, the Town of Falmouth sought funding through the MVP Action Grant Program to complete a detailed coastal resiliency planning study.
This study provides a conceptual phased management approach to improve the coastal resiliency of the Surf Drive Study Area over time – identifying key time frames and sea level thresholds for action. This process utilized a dynamic adaptation pathways approach, which utilizes the key steps outlined below.
Step 1: Identification of the Problem
Although this study focused primarily on the risks from flooding, when considering term adaptation plans for coastal areas, other coastal hazards, such as erosion, cannot be ignored. The following coastal hazards were considered:
Daily High Tide Flooding
Daily High Tide Flooding: Sea level rise will result in higher high tides each day, creating shallow “nuisance flooding”. However, nuisance flooding can have significant impacts, as inundated roads cannot provide reliable transportation corridors or emergency access, and buildings surrounded by water cannot be readily accessed. The map on the right shows areas that are likely to experience daily high tide flooding in 2030, 2050 and 2070. Click on the buttons below to move through the layers showing the daily high tide flooding.
The extents of flooding are based on the projected mean high water (MHW) elevations shown here.
Storm Surge Flooding
Storm Surge Inundation: Storm surge is the rise in water level caused by a storm. The probability of inundation during storm events was evaluated using the Massachusetts Coast Flood Risk Model (MC-FRM), a high-resolution hydrodynamic model that simulated tides, waves, winds, storm surge, and sea level rise to identify locations at risk of flooding. The model was used to evaluate present-day risks (2008), as well as climate change influences for 2030, 2050, and 2070 time-horizons. The resulting flood risk maps on the right can then be used to identify the estimated annual probability, or likelihood, that any location will experience flooding. Much of the Surf Drive Study Area already has a high probability of inundation during a storm event today; this risk will only increase in the future given sea level rise.
Click on the buttons below to see the impacts of storm surge flooding now and into the future.
Coastal Erosion
Although the focus of this study was flood risk, when evaluating long term adaptation options for a coastline, it is valuable to consider erosion as well. The Massachusetts Office of Coastal Zone Management has completed a statewide shoreline change assessment. Using data compiled from the mid-1800s to 2009, rates of erosion or accretion have been determined at transects every 50-meters along the shore. The average long-term (mid-1800s to 2009) shoreline change rate within the Surf Drive Study Area is -0.4 feet/year, indicating a slow, but steady erosional trend. The paved roadway of Surf Drive and coastal engineering structures have likely influenced this rate over the last century. Surf Drive, the Shining Sea Bikeway, and the Trunk River sewer main have been repeatedly damaged in recent years by undermining caused largely by coastal erosion.
Step 2: Identification of Key Assets
The next step in an adaptation pathways approach was to identify important at-risk assets. This study identified the flood risk to each asset and gathered information about the current use and importance of each. The following assets were considered:
MHW Risk to Roads and the Shining Sea Bikeway
Sea level rise will result in daily tidal flooding of 0.2 miles of Surf Drive and Mill Road, in addition to sections of Bywater Court by 2050 (red sections). By 2070, an additional 0.8 miles of Surf Drive and roads in the Bywater Court area will experience daily tidal flooding (yellow sections).
In addition, a 0.4-mile section of the bikeway adjacent to Oyster Pond and Salt Pond will be at risk of daily tidal flooding by 2070 (yellow dashed sections).
Storm Surge Risk to to Roads and the Shining Sea Bikeway
Under present-day conditions, 1.6 miles of Surf Drive and adjacent connecting roads have a 50-100% annual chance of inundation during a storm (red sections), meaning these sections of roadway are likely to flood at least once every other year.
Under present-day conditions, approximately 0.3 miles of the bikeway has a 50-100% annual chance of inundation as a result of storm surge flooding (red dashed sections).
MHW Risk to Sewer Infrastructure
By 2050 and 2070, daily high tide flooding will regularly inundate sections of the Woods Hole sewer main along the bikeway, 0.2-miles of sewer infrastructure along Surf Drive and Mill Road, and 0.3 miles of sewer infrastructure in the Bywater Court neighborhood, which could result in infiltration.
In addition, the area around the Surf Drive Pump Station will begin to experience daily high tide flooding by 2070.
Storm Surge Risk to Sewer Infrastructure
Under present-day conditions, 0.4 miles of the Woods Hole sewer main along the bikeway has a 50-100% annual chance of inundation during a storm.
Under present-day conditions, the Surf Drive Pump Station is vulnerable to flooding during a 10% chance storm event. This likelihood increases in the future.
MHW Risk to Other Utilities (Water/ Electrical)
Small sections of the water distribution infrastructure along Surf Drive and the Bywater Court area will be vulnerable to daily high tide flooding by 2050 (red sections), with an additional 0.8-miles of water main subject to daily high tide flooding by 2070 (yellow sections).
The Mill Road power substation will become vulnerable to flooding as a result of daily high tides by 2050 (red dot) and the Elm Street substation by 2070 (yellow dot).
Storm Surge Risk to Other Utilities (Water/ Electrical)
Under present-day conditions, 0.5 miles of the Surf Drive water main along and water infrastructure within the Bywater Court residential area have a 50-100% annual chance of inundation during a storm (red sections).
The electrical substations within the Mill Road parking lot and off Elm Street both have a 50-100% annual chance of inundation during a storm under present day conditions (red dots).
MHW Risk to Town Buildings and Parking Lots
Both Town Hall and the Mitchell Bathhouse could become vulnerable to daily tidal flooding by 2070.
The Surf Drive Beach parking lot will become vulnerable to regular high tide flooding by 2070. The four additional parking lots along Surf Drive are expected to be impacted by regular high tide flooding between 2030 (e.g., Mill Road parking lot - purple) and 2070 (e.g., Surf Drive Beach parking lot - yellow).
Storm Surge Risk to Town Buildings and Parking Lots
The lower level of Town Hall is vulnerable to inundation during a 5% chance storm today and a 20% chance storm in 2030, with flooding coming from Siders Pond. The Mitchell Bathhouse is also vulnerable to flooding during a storm, with a 20% annual chance of inundation today and a 50% chance in 2030. The Surf Drive Beach parking lot also has a 50-100% annual chance of inundation due to storm surge in present day.
The four additional parking lots along Surf Drive already have a 50-100% annual chance of inundation during a storm event under present-day conditions.
MHW Risk to Low-Lying Residential Neighborhoods
A significant portion of the Old Stone Dock and Surf Drive neighborhoods is extremely low-lying. By 2050, regular daily high tides will flood 30 structures (red) and by 2070 the number of homes and structures impacted by daily high tide flooding will increase to a total of 169 (yellow).
Storm Surge Risk to Low-Lying Residential Neighborhoods
Within this same area, under present-day conditions, 122 structures already have a 50-100% annual chance of inundation during a storm event and an additional 123 structures have a 20-50% annual chance of flooding.
MHW Risk to Natural Resources
There is a coastal beach that runs the entire length of the project area and a series of disconnected low dunes. As sea level rises and erosion continues to act on the beach, the width of the dry high tide beach will decrease without intervention. By 2030, the dry high tide beach will be narrowed to half its current width and by 2070 high tides will regularly flood almost all of the existing coastal beach - in other words, there could be little to no dry beach at high tide. In addition, the elevation of the roadway at each pond inlet is low enough that without action by 2070 water will flow over the road and the barrier beach into these coastal ponds at high tide, rather than through the existing culverts.
Storm Surge Risk to Natural Resources
Given their proximity to the waterline, much of these natural resources have a 50-100% annual chance of inundation during a storm event even under present-day conditions.
Step 3: Development of Potential Actions
Once the risks and vulnerabilities to key assets have been evaluated, the next step in a dynamic adaptation pathways approach is to develop and consider a full range of potential adaptation actions, which can help reduce or eliminate those vulnerabilities. These actions were developed within four main themes.
Below are four slideshows for the short-, mid-, and long-term potential actions proposed within each of the four themes. Use the arrows on the left of each image to flip through each action plan. Clicking on one of the action plan maps will enlarge the graphic.
Step 4: Action Assessment
Once a full list of potential actions was developed, each action was evaluated based on estimated costs, potential benefits (i.e., target effects) and potential adverse impacts. Examples for four assets are provided below. Note that all cost estimates provided should be considered planning level estimates.
Surf Drive
A major consideration for the barrier beach portion of Surf Drive is whether to maintain the roadway in place, and if so, how to protect it, or to abandon the roadway through a phased retreat plan. Key potential actions, along with their estimated costs, target effects and potential adverse impacts, are listed to the left.
Shining Sea Bikeway
Similar to the barrier beach portion of Surf Drive, a major consideration for the bikeway is whether to maintain the bikeway in place, to elevate it, to relocate it or to abandon a portion through a phased retreat plan. Key potential actions, along with their estimated costs, target effects and potential adverse impacts, are listed to the left.
Mitchel Bathhouse and Surf Drive Parking Lot
Major considerations for the public beach facilities include whether to transition to portable facilities or to protect the existing bathhouse, and if so how. Key potential actions, along with their estimated costs, target effects and potential adverse impacts, are listed to the left.
Low-Lying Neighborhoods
A major consideration for the low-lying neighborhood in the vicinity of Bywater Court is whether to plan for a phased retreat from the area or to protect the neighborhood, and if so how. Key potential actions, along with their estimated costs, target effects and potential adverse impacts, are listed to the left.
Step 5: Development of Potential Actions
Dynamic adaptation pathways (DAPs) can provide a unique visualization of the potential adaptation actions and how they can fit together into an overall implementation strategy over time.A key to interpreting these figures is presented below.
As you move left to right along a colored/themed pathway for a particular action, at key time steps you will encounter “transfer stations”, representing decision points and opportunities to shift to a different action. In many cases, the most beneficial and cost-effective approach is to phase in different actions over time, as rising water levels prompt an alteration in the way an asset may be used. Some actions will be effective throughout all time frames considered (solid colored line will continue to the right edge of the figure). For others, an adaptation tipping point (the point where that action can no longer function as intended) may be reached at a certain water level, indicated by a black vertical bar. When this occurs, the solid colored line representing an action will either end at that tipping point terminal (i.e., that action is no longer effective), or the line will continue past the tipping point terminal as a dashed line (i.e., that action will have a change in function).
Another feature to consider when interpreting the dynamic adaptation pathways figures is whether each path is light or bold. Pathways that are depicted as lightly shaded lines are technically possible but either not yet required or not recommended. These paths could still be selected at an earlier time than required; however, if sea level rise does not occur at the rate projected, then this adaptation may not ever be required.
Finally, at the bottom of each figure are three threshold lines (see below): the amount of sea level rise in feet, and at what outyear that amount of sea level rise is expected to occur given a high (RCP 8.5) or intermediate (RCP 4.5) sea level rise scenario. The bottom two lines provide a range of actionable timelines for when each action will be needed and effective. The top line, indicating the total amount of sea level rise in feet, can be compared to actual measured water levels over time to track whether climate change impacts are proceeding closer to the high or intermediate scenario. For the purposes of the individual asset discussion that follows, all timelines will be discussed related to the high sea level rise projections.
Below is a map tour of four example assets within the Surf Drive study area. There is a slide show of DAP and scorecard graphics for each asset. To view all graphics associated with a particular asset, click the arrow on the right of the figures to scroll through the graphics. Clicking on one of the graphics will zoom in.
01 / 04
1
Surf Drive
The status quo (i.e., current actions) will only be effective for maintaining this asset until approximately 2030 (or approximately 1.4 feet of SLR from 2008 conditions), at which point a tipping point will be reached, as indicated by the vertical black bar in the DAP figure to the right. At that point, a different pathway should be chosen. Under the Managed Retreat themed actions (blue), public outreach and policy planning can start today, leading up to ending maintenance on the barrier beach. Under the Connection actions (purple), bridge construction is not recommended until approximately 2050, but improved maintenance could keep the roadway functional until a bridge is warranted. With the Natural Resources themed actions (green) beach and dune nourishment are unlikely to be effective after 2050, at which point the roadway should be abandoned and the pavement removed to restore a natural beach. Finally, with the Protection actions (orange) storm protection measures could be constructed in the short-term (2030) but if the goal of elevation is to protect from daily high tide flooding, then action is not warranted until 2050. The preferred alternative includes both Managed Retreat and Natural Resources actions. As shown in the score card (second image) the preferred alternative is one of the most cost-effective options, eliminating ongoing maintenance and avoiding major infrastructure development.
2
Shining Sea Bikeway
The status quo (i.e., current actions) will be effective for maintaining this asset until approximately 2050 (or approximately 2.7 feet of SLR from 2008 conditions), at which point a tipping point will be reached, as indicated by the vertical black bar in the DAP figure to the right. At that point, a different pathway should be chosen. Under the Managed Retreat themed actions (blue), sections of the bikeway would be abandoned by 2050. Under the Connection actions (purple), portions of the bikeway could be converted to an elevated boardwalk by 2050. With the Natural Resources themed actions (green), portions of the bikeway and associated parking areas could be relocated to less vulnerable locations. Finally, with the Protection actions (orange), the bikeway could be protected with a revetment or seawall by 2050. The preferred alternative includes both Connection and Natural Resources actions. As shown in the score card (second image), although the preferred alternative is not one of the least expensive alternatives, it does maintain access along the bikeway, reduces the amount of maintenance required overtime, and avoids major infrastructure development along the coastal beach.
3
Low-Lying Neighborhoods
The status quo (i.e., current actions) will be effective for maintaining this asset until approximately 2050 (or approximately 2.7 feet of SLR from 2008 conditions), at which point a tipping point will be reached, as indicated by the vertical black bar in the DAP figure to the right. At that point, a different pathway should be chosen. Under the Managed Retreat themed actions (blue), areas of the neighborhood that are vulnerable to regular high tide flooding would be converted to a waterfront park starting in 2050. With the Natural Resources themed actions (green), living shorelines would be utilized to provide flood protection for the neighborhood starting in 2050. Finally, with the Connection and Protection actions (purple and orange), neighborhood could be protected by elevating portions of Mill Road and Surf Drive. The preferred alternative includes both Connection and Protection actions. As shown in the score card (second image), the preferred alternative is one of the most cost-effective alternatives, providing flood protection to more than 100 residences while also maintaining reliable transportation access along the roadways being elevated.
4
Mitchell Bathhouse
The status quo (i.e., current actions) will only be effective until approximately 2030 (or approximately 1.4 feet of SLR from 2008 conditions), at which point a tipping point will be reached, as indicated by the vertical black bar in the figure below. At that point, a different pathway should be chosen. Under the Managed Retreat actions (blue) removal of the bathhouse (2030) and parking lot (2050) would remove these assets from a high-risk area, avoiding expensive repairs in the future. Connection actions (purple) suggest transitioning to portable facilities by 2030, while using beach nourishment and access walkways to provide additional protection and ensure access to the public beach. Under the Natural Resources theme (green), beach and dune nourishment could begin as soon as 2030, with continued maintenance through 2070 and beyond. Finally, with the Protection actions (orange), the bathhouse could be floodproofed in the short-term (2030) and protective seawalls could be constructed by 2050 to protect the parking lot, bathhouse, Surf Drive and nearby residences. The preferred alternative includes both Connection and Natural Resources actions. As shown on the score card (second image) the preferred alternative is one of the most cost-effective options, enhancing the coastal ecosystem, while preserving public beach access.
Step 6: Develop Phased Implementation Plan
Based on the feasibility and effectiveness of the individual actions presented for the four themes, the water level and timing thresholds presented in the dynamic adaption pathways, and the cost and effectiveness of each action, a recommended phased implementation plan was developed. The short-, mid-, and long-term components of this plan are displayed in the Recommended Alternatives figures below. Use the arrows on the sides to flip through each of the recommended alternative figures.
For More Information
For more information, including the full Coastal Resiliency Planning for the Surf Drive Area report and an Executive Summary document, please visit the Falmouth Coastal Resiliency Action Committee’s Surf Drive project webpage:
