DFM News’ five part series on the potential impact of sea level rise in Delaware continues with the start of a county-by-county look at the issue. Today: New Castle County.
- Aerial view of Broad Marsh Dike in New Castle.
(photo courtesy: DNREC)
As global climate change scientists and state officials warn of rising sea levels, the City of New Castle is getting serious about shoring up its defenses against the waves.
The historic city is preparing to repair and reinforce four earthen dikes that protect it from the tidal Delaware River whose waters have overtopped the dikes 18 times in the last five years, according to state records.
City managers are considering an engineer’s report recommending improvements to the dikes that were built by Dutch and Swedish settlers more than 300 years ago, and whose condition was assessed as “fair to poor.”
New Castle has $3 million in state funds to spend on the repairs this year, and expects to decide this fall which specific works to begin with, said City Administrator Cathryn Thomas.
When they get the go-ahead from officials, contractors will begin removing trees and other vegetation that have grown on the dikes, and which compromise their ability to withstand rising sea levels. Workers will also repair cracks, fill in eroded sections, and evict animals that have burrowed into the dikes.
The report by Green Stone Engineering attached “unacceptable” ratings to many aspects of the dikes and said that infestation by animals was a particularly acute threat to the dikes’ integrity.
At the Gambacorta Marsh Dike, for example, “Significant maintenance is required to fill existing burrows, and the levee will not provide reliable flood protection until this maintenance is complete,” it said.
Over an unspecified time, the dikes will be overtopped or will fail if they are not properly maintained, the report said. It recommended that all four dikes be an estimated cost of close to $ 4.5 million.
Overtopping would affect the city more severely than natural flooding because it happens with little or no warning and can be “extremely damaging,” the report said. “The risk of overtopping is increased with the anticipated impacts of sea-level rise.”
New Castle got a taste of the risks it faces on Mother’s Day in 2008 when northeast winds combined with tides to drive water over the Gambacorta Dike, one of those slated for renovation in coming months. Property damage was minimal because flooding mostly affected a marsh behind the dike but the waters left behind a mess of debris, forcing the City to mount an expensive cleanup operation, Thomas said.
The dikes are essential to the city’s ability to withstand storms, Green Stone said.
“Without these dikes, or in the event of dike failure, approximately 25 percent of the land in the City of New Castle boundaries would flood during high-intensity rainfall events, affecting homes, other commercial and industrial structures and properties.”
The Green Stone report added that not doing the proposed work would mean inundation of more than 70 buildings that are currently protected by the dikes, even while assuming the minimum water elevation in the projections.
But if waters rose to the level classified by the Federal Emergency Management Agency (FEMA) as a 100-year flood event (1-percent-annual-chance), the number of flooded buildings would jump to 238, according to an October 2010 City of New Castle Dike Maintenance and Emergency Planning report. Under that scenario, more than a fifth of the land now protected by Broad Dike would be flooded, the study projected.
“If we don’t do something now to address these problems then we could have much greater problems in future,” said New Castle City Administrator Cathryn Thomas.
Rising Threat: City of New Castle Plans Response to Sea Level Rise
Excerpts of interview with New Castle city administrator Cathryn Thomas
While city managers are very concerned about the prospect of sea-level rise, it’s less clear that the public knows or cares much about the issue, Thomas said.
“I don’t think it’s on too many people’s radar screens,” she said. “We want it to be.”
New Castle residents like most Delawareans have little concern about sea-level rise, according to a survey that found the issue at the bottom of the list of most people’s worries.
Only 32 percent of 1,505 respondents in the 2010 survey said they were very concerned about the environmental issue of sea-level rise, although Sussex County residents said they worry about it more than people in Kent or New Castle Counties.
New Castle County residents were the most likely to strongly agree that human effort can reduce the impacts of climate change and sea-level rise when compared to residents of the other two counties.
Across the state, a third of respondents said they were not so convinced or not at all convinced that sea-level rise is happening, and only 22 percent said they have personally experienced its effects.
- Some examples of issues along the City of New Castle's dikes are visible. Click here to view a slideshow
In the City of New Castle, proposed works include installing new flood gates at existing road or rail culverts between the dikes and most buildings in the areas that would be flooded, effectively treating the culverts as additional dikes.
It also estimated costs for building up dikes to withstand higher water levels. Raising Broad Dike, for example, to the 100-year flood elevation, would cost about $0.8 million but this would be heavily outweighed by the economic benefits of all improvements to that dike, which the report estimated at $72.6 million.
Improving all four New Castle dikes to the 100-year standard would be classed as economically feasible by FEMA which issues the designation if a ratio of damage to improvement cost is greater than zero.
The city has also attracted state support in the form of a “coastal resiliency project” being drawn up by the Delaware Department of Natural Resources and Environmental Control (DNREC).
The project is designed to reduce the city’s vulnerability to natural hazards such as flooding and road closures that currently exist and which may increase in future because of climate change and sea-level rise.
A current assessment of the city’s vulnerabilities will look at current hazards “as well as future risks associated with climate change, including the potential impacts of sea-level rise, increased storm frequencies and intensities, tide gate and dike problems or failures, wetland loss, as well as other impacts,” DNREC said.
For Cathryn Thomas, the Mother’s Day storm of 2008 — which saw wind gusts of 50 to 60 mph, two to four inches of rain, and wave heights up to 21 feet – alerted her to the threat facing New Castle.
“When I saw that storm, I freaked out,” she said. “The water was coming up all over the land.”
Next in the Rising Threat series: A look at the how the issue of rising sea levels is viewed in Kent County.
Previously: Part I – Rising seas threaten Delaware coast, test policymakers.
