Managing Flooding in Nantucket

Sponsor: Natural Resources Department
Sponsor Liaison: Peter Morrison, Vincent Murphy
Student Team: Robert Blythe, James Casella, Kaija Gisolfi-McCready, Maura Walsh
Abstract: Nantucket is particularly susceptible to flooding and is increasingly concerned about the exacerbating effects of climate change as sea levels rise and storms become more frequent and intense. Unfortunately, little of the local data on flooding has been systematically analyzed. To address this problem, we built a database comprising 703 flooding events and 856 storm and non-storm flood instances. We found that 88% of the events were minor, nuisance flood events from high tide flooding and winter storms account for the major of major flood events. We identified the causes and effects of flooding at three location (Polpis Road near Sesachacha Pond, Polpis Road near Fulling Mill Creek, and Children’s Beach) and recommended several potential mitigation strategies.

Report: Flooding Final Report

Presentation: Floods Final Presentation

Database: Nantucket Flood Database

GIS Layers: Nantucket Floods GIS Layers

Executive Summary

As an island 30 miles south of Cape Cod, Nantucket is particularly susceptible to tropical
storms, hurricanes, and Nor’easters, and the flooding that ensues across the island. Looking
toward the future, climate change could increase flood damage and coastal erosion as sea levels
rise and storms become more intense and frequent.

Distinct parts of the island flood in different ways. Nantucket Harbor, the largest harbor
on the island, undergoes ‘tidal stacking’ when northerly winds push water into the harbor and
inundate downtown. Affected areas overlooking the harbor are home to many historic buildings,
commercial establishments, as well as critical infrastructure. These assets are frequently
inundated by flood waters; some may be severely damaged by wave and wind action during
storms. Other areas, such as Brant Point, Long Pond, Polpis, and Madaket, also frequently
experience flooding.

Mitigating local flooding requires understanding the local hydrodynamics that produce
periodic flooding and assembling robust strategies to mitigate what nature delivers. The
objectives of our study are to (1) assemble and analyze a database of local flooding events,
gathered from varied historical and other sources; (2) evaluate three locales that experience
frequent and unique flooding; (3) build a knowledge base of research on urban flooding whose
mitigation strategies are applicable to selected locales; and (4) recommend specific mitigation
strategies for each of our selected case study areas. Our focus is on public infrastructure.
To satisfy objective one we formulated a robust database containing 856 flooding
instances that occurred across the island. Each event’s general location, latitude and longitude,
type of flooding, date, verified highest mean higher high water and causes are included. Analysis
of our database indicates areas that flood most frequently, causes of both major and minor
flooding on the island and maximum and minimum heights for flood water per area. Data from
the database also serves in understanding the complexities of flooding in our three case study
We recommended three locales for expansive research: Polpis Road near Sesachacha
Pond, Polpis road by Fulling Mill Creek and Children’s Beach.

Polpis Road near Sesachacha Pond suffers from wave action from the pond during high winds and overflow from surrounding
wetlands. Flowing water inundates and erodes the roadway making it inaccessible to residents.
This section of Polpis road is part of an evacuation route for residents and needs to remain
passable during storms. We recommended two major mitigation strategies; building a causeway
underneath Polpis Road by Sesachacha Pond to protect against erosion and reduce the buildup of
water on the roadway and building a pipe from the pond to the Atlantic Ocean to drain the pond
more effectively twice a year. Additionally, our recommendations include planting vegetation
along the barrier between the pond and ocean and building a Rip Rap wall along the barrier
between the pond and road.

Polpis Road near Fulling Mill creek floods due to coastal flooding, run off from Fulling
Mill Creek and when the wetland becomes overwhelmed. This section of Polpis Road is part of a
separate evacuation route so its access must be maintained during storms. To address the three
types of flooding in this area of road we developed two mitigation strategies; replacing the
existing culvert with a larger, higher culvert and building a bridge in the central part of the road
to connect the wetlands on either side and to raise the roadway to protect against sea level rise.
Part of the culvert replacement strategy includes raising the height of the roadway which protects
against future sea level rise.

Our final location, Children’s Beach, floods from coastal flooding.
As our lowest lying area closest to the water our mitigation strategies include raising the
beachfront to protect against sea level rise, installing a padlock and bollards at the boat ramp,
implementing a municipal fine for opening the ramp’s tidal gate and repairing the pump station
on Children’s Beach. We suggest part of the pump station repairs also includes increasing the
size and capacity of its associated tank. Elevating the waterfront could increase the burden on the
pump station and a larger tank would increase the quantity of water the pump could manage at
once reducing chances of backflow and upstream flooding.