2.5 Floodplain Resources
Because of Dennis’s location on the ocean between Cape Cod Bay, and Nantucket Sound, it is at high risk for hurricanes and other types of severe storms which carry high winds and heavy precipitation; especially coastal storms. This, combined with the fact that over 25% of Dennis’s land area is low-lying makes it also highly vulnerable to flooding. Beyond flooding, Dennis is also potentially impact by erosion and sea level rise issues, both of which could exacerbate flooding issues beyond the limits of current floodplain mapping
Dennis participates in the National Flood Insurance Program, which requires that new shorefront development meet engineering standards for floodproofing, but does not prohibit development. Flood velocity zones, or V-zones, are land areas where storm surge or direct wave action occurs. A 1988 analysis by the Massachusetts Coastal Zone Management Office found that in the 1970s, Dennis had more structures built in the velocity zone than any other Cape Cod town except Bourne and Falmouth. About 124 buildings are clearly located in the V-zone (looking at the 2005 aerial photograph and the latest FEMA flood zone mapping), primarily between West Dennis Beach and the Swan River, portions of Dennisport and Chapin Beach. In addition to these homes, several hotel structures also appear to be located within the Velocity Zones. Hurricane Bob and the 1991 Halloween Storm damaged some houses along the Southside. State and local wetlands protection legislation should help prevent future development in this high hazard area.
Landward of the velocity zones are other flood-prone areas (A-Zones) in which standing waters can be expected during 100-year storm events. These areas consist mostly of salt marshes and shorefront uplands up to about the 13-foot contour. Both commercial and residential developed areas, including portions of Route 28, occur in the A-Zone.
According to the 2010 U.S. Census, the Town of Dennis has 567 people living within the V Zone. This represents a drop from 815 people in these areas in 2000. However, housing units in the V zone in 2010 was 1,236 units, up 80 units from 2000 (1,156 units). The biggest changes in housing occurs in the construction of new seasonal housing and the conversion of some year round housing to seasonal showing an increase of 200 seasonal housing units (1,108 in 2010 up from 908 in 2000).
The Census data also reveals that there were 3,690 people living within the A Zones in Dennis in 2010 a drop from 4,153 people in 2000. However, total housing increased from 4,629 units in 2000 to 5,134 in 2010. Seasonal housing units represent 3,069 units in 2010 or 59.7% of all housing in the flood zone in 2010. In 2000, there were 2,476 seasonal housing units in Zone A, or 53.5% of all housing in this area.
The result is 4,257 year-round residents are living within a flood zone and are at risk of flood impacts in the event of a one-hundred year storm. As these flood areas are also predominantly representative of the portions of Dennis with seasonal populations (Dennisport and West Dennis) the impacts of a summer-time storm will be even more severe. As noted above, nearly 60% of the homes in the flood zone are classified as seasonal under by the Census Bureau. Looking at normal occupancy rates (year-round and seasonal rates) the result is between 10,600 and 22,600 people are living in the flood zone on any given summer day.
From a land mass perspective, there are about 1,076 acres of land located within the V Zones. Significant portions of these areas are currently under various forms of protection as conservation areas (Crowe’s Pasture Area for instance) or public beaches (West Dennis Beach, Chapin’s Beach, etc). Slightly over 1,900 acres of land are found within Flood Zone A, despite the population numbers noted above, much of this area is also protected, with the largest areas being along Chase Garden Creek, Sesuit Creek and Swan Pond River. Other, large expanses of land, in these areas are considered to be wetland resource areas and have been provided extra protections under the Dennis Wetland By-law. These two figures suggest that there is about 3,000 acres of land in Dennis that could be considered at risk of flooding in a 1% storm event.
The Federal Emergency Management Administration is in the process of updating the Flood Insurance Rate Maps for Dennis. New maps will go into effect in the summer of 2012. During the development process of these maps, large land areas were initially identified as being potentially at-risk of flood damage. These areas have been targeted for additional study. It is quite likely that much of the area identified in the May 12, 2009 Preliminary Flood Insurance Rate Maps will be added to the flood zones within the next 3-5 years. These changes identify nearly 600 additional parcels of land at-risk of flooding, and as many as 2,049 residential units, a 40% increase in residential structures in harms way. Particularly, the cottage colony areas of Dennisport are illustrated in the May 12, 2009 maps as being in harm’s way. These structures are among the most poorly constructed facilities in Dennis, and they are most fully occupied during the summer, prime hurricane season. The potential for these properties to be flooded in a storm is significant. The area clearly needs to be identified as a high risk area for property damage and personal casualty in a flood event.
These flood zone issues are not the only “flooding” problems faced by Dennis. The town has its share of poor drainage areas where much smaller, typical summer thunder-storm events lead to flooding problems. It is not uncommon for the area to experience a quick inch or two of rain falling in a one hour time frame. These storm events lead to flooding concerns of a more localized street and urban type flooding problems. Low lying areas with poor drainage are of particular concern. These events are often exacerbated by the amount of impervious surface in a particular location.
In 2010 and 2011 two significant east coast hurricanes threatened Cape Cod. In 2010 Hurricane Earl approached the Cape at times threatening to strike the area with a Category 2 strength storm. This storm passed to our east. In 2011 Hurricane Irene approached with similar ferocity, causing significant damage to inland areas of the state.
In 2004 the Cape had several near misses during Hurricane Season with Bonnie, Charley, Gaston and Hermine all passing near or over the Cape. In 2001 Tropical Storm Allison passed to the south of the Cape. In 2000 Hurricane Gordon passed over the Cape, followed shortly thereafter by Tropical Storm Helene passing south of the area. In 1997 Hurricane Danny passed south of the Cape. In 1996 Tropical Storm Josephine passed over Cape Cod.
Hurricane and coastal storms bring with it the one-two punch of heavy rains and high winds, creating flooding, storm surge and wind based damage issues. With 11 tropical events over fifteen years, the town clearly recognizes the problems associated with hurricanes and coastal storms.
In August 19, 1991 Hurricane Bob brought a storm surge of about 10 to 15 feet to portions of Cape Cod, with wind gusts of up to 100 mph being registered in North Truro. This storm was followed shortly thereafter by the “Unnamed Storm” on October 30, 1991 which passed south of Cape Cod causing extensive shoreline and property damage.
Dennis residents memories of these two storm events vary widely. Some talk about boats lifted out of the Bass River and deposited on adjacent properties, others recollect very little coastal damage. However, even moderate hurricane events can trigger significant damage. Hurricane Charley in 2004 took down tree limbs and caused urban type flooding in many locations. Even Dennis Town Hall was not immune to water problems as the basement area flooded due to the sheer volume of rain that occurred in a short period of time.
Beyond the “tropical” events are the far more frequent Northeaster events. Northeasters pose significant risks especially to the town’s shoreline. Over the past several winters there have been several winter storms that left nearly 3 feet of snow. The winter of 2004/2005 had two such events. Schools in many Cape communities were closed for up to a week due to the amount of snow left in the streets. In June 2009 a late season Northeaster pounded the Dennis coast for almost a week. When it was over, the town had experienced its worst barrier beach damage in over three decades. While the Dennis Barrier Beach system is not as exposed as that in Chatham, this storm event brings a strong reminder that we are part of a large sandbar that is always at risk.
Sea, Lake and Overland Surge from Hurricane (SLOSH) analysis provided to Dennis by the Cape Cod Commission and developed by the Army Corps. Of Engineers, provides additional information about at-risk properties and people due to potential SLOSH impacts. A Category 1 Hurricane will impact approximately 2,026 acres of land. Note this land area is less than the 1% storm event. This is an important consideration for the residents of Dennis in that many people have come in to Town Hall with questions about their flood zone status and state that they were here for Hurricane Bob (a Category 1 storm) and did not see any flooding impacts.
A Category 2 Hurricane will add an additional almost 1,000 acres to the storm impact area. A Category 3 Hurricane will increase this figure by an additional 950 acres. Finally, a Category 4 Hurricane will add yet an additional 970 acres, increasing the total impacted area to just over 4,900 acres of land.
In population numbers, a Category 1 Hurricane could displace 3,129 year-round residents and 4,799 housing units. Of these housing units 66% are seasonal in nature, resulting in an estimated actual summer population in excess of 9,150 people that could be displaced due to storm impacts.
A Category 2 Hurricane would increase the number or year-round residents impacted to 4,076 and summer population displaced to 12,460. A Category 2 Hurricane Storm Surge will reach 6,671 homes.
A Category 3 Hurricane would trigger the displacement of 5,485 year-round residents and a summer population of 15,400 people. A Category 3 Hurricane Storm Surge would also impact 8,394 housing units.
Finally, a Category 4 storm would increase the number of people displaced to 6,373 year-round residents and about 17,000 summer population. A Category 4 hurricane will impact almost 9,300 housing units.
Dennis has three barrier beaches, Chapin’s Beach, West Dennis Beach and Crowe’s Pasture Beach. These barrier beaches are subject to regular erosion action, as are many other portions of the Dennis Coastline. The following coastline change illustrations were drawn from the Massachusetts Coastal Zone Management website using the Shoreline Change Browser.
Chapin’s Beach illustrates significant coastline loss over the past century, the green line illustrates the location of the coast between 1892 and 1938, the black line illustrates the coastline as measured between 1948 and 1975, the blue line illustrates 1978 to 1982 and the red line illustrates the coastline in 1994. The coast in this area illustrates consistent shoreline loss, with annual average losses as high as 11 feet per year.
Crowe’s Pasture coastline change is also indicative of steady erosion. There were some shifting of the shoreline in and out prior to 1938, however, since 1938 the trend has been towards shoreline loss. However, the data since 1978 seems to suggest that the erosion has slowed, and that areas that were lost (possibly during the Blizzard of 78) have recovered.
West Dennis Beach illustrates relative stability in comparison to the other barrier beaches. The beach has had significant sand deposition since the 1938 measurements, but has been relatively stable since the 1948-1975 measurements were made.
Beyond the three barrier beaches, the remainder of the Dennis coastline also experiences changes.
As the above charts illustrate, the town coastline has been withdrawing on average 0.65 feet per year and more beaches are losing sand than are gaining. This means that Dennis is losing almost an acre of shoreline annually. The following discusses examples of this change in various locations along the north and south coasts
The Nobscusset Point section of the northern coastline illustrates some deterioration of the coastline. The majority of the deterioration occurred prior to 1938. Since 1938, while there has been some erosion, the amount of erosion has been limited.
The area east of Sesuit Harbor suggests that while the area has been subject to erosion in the past, the groin adjacent to the harbor has managed to capture sand, and maintains the coastline in this area.
The mouth of the Swan Pond River is another area that illustrates significant coastline change over the years. Presently the town is implementing a town-wide dredging plan designed to maintain access to Swan Pond River, Chase Garden Creek, Bass River and Sesuit Harbor. The sands collected during these dredging projects is being used to restore coastal beaches and dunes.
Sea Level Rise
In coming decades, flooding and erosion will be increasingly exacerbated due to relative sea level rise. This phenomenon, the result of land subsidence and ocean expansion from global warming, could result in the loss of between 113 and 394 acres of upland in Dennis between the years 1980 and 2025. These areas will basically coincide with the 100 year floodplain. Sea level rise will also mean an increase in the severity of storm damage. Owing to its low-lying coastline that intrudes far inland, Dennis can expect to experience a shoreline retreat (as a percentage of its land mass) worse than any other Cape Cod town.
Revised November 18, 2011