Objectives

What are the worst case effects of hurricanes, riverine flooding, high tides, and sea level rise inundation on DelDOT’s assets? The University of Delaware Water Resources Center (UDWRC) and Center for Applied Demography and Survey Research (CADSR) utilized climatic, hydrologic, hydraulic, and geographic information systems (GIS) data to document and map the worst case effects of hurricane, fluvial flooding, high tide, and sea level rise inundation on the highway, railroad, transit, and pedestrian/bicycle transit assets of the Delaware Department of Transportation.

Conclusions

  1. Delaware, a Low Lying State: Situated at a mean elevation of 60 feet above sea level, Delaware is the lowest lying state in the U.S. and therefore is especially vulnerable to coastal and riverine flooding accentuated by changes in the climate and sea level rise. With sea level rise, Delaware is likely to see record-breaking coastal floods within the next 20 years, and near certain to see floods more than 5 ft above the high tide line by 2100. Over 62,000 acres of land lie less than 5 feet above the high tide line in Delaware and $1.1 billion in property value and 20,000 homes sit on this area.The Governor has directed that state agencies such as DelDOT plan to address future impacts of flooding and coastal storms on infrastructure. In September 2013, Governor Markell signed Executive Order 41 creating the Cabinet Committee on Climate and Resiliency to address climate change at the state level. Delaware’s Climate Framework is based on the 2012 Sea Level Rise Vulnerability Assessment and 2014 Climate Change Impact Assessment. In June 2017, Governor Carney announced Delaware was among 10 states to join the U.S. Climate Alliance to adhere to the Paris Climate Accord.
  2. Bridge Hydraulic Analysis: The DelDOT roadway design manual requires design of pipe culverts to pass the 50-year flood. The DelDOT bridge design manual requires that interstate, principal, and major arterial bridges pass the 50-yr flood. Local roads and streets are designed to pass the 25-y flood.Of 547 bridges along streams with FEMA Flood Insurance Study (FIS) delineated floodplains in Delaware, 230 bridges (42%) have inadequate hydraulic capacity to convey the 10-yr flood, 353 (65%) inadequately to pass the 50-yr flood, and 405 (74%) inadequately convey the 100-yr flood. Of 547 bridges statewide, 78 bridges (14%) have bridge decks overtopped by the 10-yr flood, 175 (32%) are overtopped by the 50-yr flood, and 245 (45%) are overtopped by the 100-yr flood.
  3. Highway Flood Inundation: We assessed over 7,000 DelDOT total road miles and over 1,700 major route (Federal interstate/highway/state principal/major collector) miles and mapped the road miles flooded (inundated) within the riverine floodplain (100- and 500-yr flood) and/or the Category 1, 2, and 3 coastal storm surge zone for the following scenarios

Existing Conditions (Mean High Water)

  • 100-yr Riverine Flood with Coastal Flooding from Category 1 Hurricane
  • 500-yr Riverine Flood with Coastal Flooding from Category 1 Hurricane
  • 100-yr Riverine Flood with Coastal Flooding from Category 3 Hurricane
  • 500-yr Riverine Flood with Coastal Flooding from Category 3 Hurricane

Future Conditions (w/0.5 m sea level rise)

  • 100-yr Riverine Flood with Coastal Flooding from Category 1 Hurricane
  • 500-yr Riverine Flood with Coastal Flooding from Category 1 Hurricane
  • 100-yr Riverine Flood with Coastal Flooding from Category 3 Hurricane
  • 500-yr Riverine Flood with Coastal Flooding from Category 3 Hurricane

Along all DelDOT roads, hurricanes and severe storms may inundate 437 miles (6% of roads) in the 100-yr floodplain, 533 miles (8%) in the 500-yr floodplain and 212 miles (3%) during a Category 1 storm and 794 miles (11%) during a Category 3 storm. Along major Federal/state highways, flooding would inundate 119 miles (7%) in the 100-yr floodplain, 143 miles (8%) in the 500-yr floodplain and 71 miles (4%) during a Category 1 coastal storm and 229 miles (13%) during a Category 3 storm.

Table 1. Road miles inundated by flooding in Delaware

Storm Category

Total Roads Inundated (mi)

Major Roads Inundated (mi)
100-yr Storm 437 (6%) 119 (7%)
500-yr Storm 533 (8%) 143 (8%)
Cat 1 Storm 212 (3%) 71 (4%)
Cat 2 Storm 450 (6%) 138 (8%)
Cat 3 Storm 794 (11%) 229 (13%)
Road Miles 7,000 (100%) 1,700 (100%)

Historic Storm Analysis: Hurricane Irene (August 28, 2011) and Sandy (October 29, 2012) were two of the most severe Category 1 storms to ever pass near Delaware causing significant flooding. Two of the more severe storms to cross over Delaware were Tropical Storm Bertha in 1996 with 58 mph winds and Tropical Storm Floyd on September 16, 1999 with winds up to 64 mph and 24-hour rainfall of 10.58 inches at Greenwood, Delaware at state record and destroyed 33 homes in New Castle County. According to the EPA storm surge inundation map, there is a 10- to 30-year probability that a hurricane will impact Sussex County, Delaware and a 30- to 100-year probability that a hurricane will impact New Castle County and Kent County, Delaware.

Originally forecast to hit near Lewes, Delaware, in October 2012 the eye of Superstorm Sandy hit Atlantic City, New Jersey coast just 60 miles north of the Delaware beaches (and then passed through Wilmington) and caused the highest flood tide on record damaging the Route 1 bridge over the Indian River inlet. The peak stage ranged from 6.51 ft at Indian River Bay at 9:00 hours on October 29, 2012 to 7.20 ft at 12:00 hours on October 30 at Delaware River at New Castle. Peak stages ranged from 4.82 ft at Little Assawoman Bay to 9.37 ft at Ship John Shoal, Delaware. The inundation analysis indicates that if the storm crossed Delaware to the south as originally forecast, flood peaks during Sandy would have increased from 6.1 ft to 15.8 ft at Delaware City and from 6.1 ft to 13.8 ft at Lewes. The flood inundation area would have spread inland from Indian River and Rehoboth Bay, miles west along Delaware Bay to Route 1 in Sussex County and past Route 9 in Kent and New Castle counties.

Recommendations

  • Review and revise DelDOT road design and bridge design manuals to strengthen hydraulic design criteria for bridges and culverts to pass the 100-yr flood (instead of the current 50-yr flood specification).
  • Conduct a systematic review of the DelDOT system to enlarge and/or replace bridges and culverts to adequately pass the 100-year flood and raise bridge deck elevations above the 100-year flood elevation with at least 2 feet of freeboard.
  • Conduct a strategic review of the DelDOT highway system to determine road segments at high risk to flood inundation and allocate capital funding to raise or flood proof these vulnerable roadway sections.

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