New Castle, Delaware
Community History and Archaeology Program 

Areas at Risk of Flooding in Historic New Castle

Recent Storms

LIDAR Elevation

DNREC Inundation

FEMA Floodplain

Dike Profiles

Dikes north and south of New Castle were damaged during Hurricane Sandy in October 2012 and an unnamed storm on December 21. The latter had just the combination of wind, rain and tide to cause almost as much flooding as the hurricane. With two storms in close succession, residents wondered:

What areas are at risk of flooding if current dikes fail?
What areas would flood with water 1 or 2 feet higher than in the recent storms?
Where are new dikes needed to protect homes in the both the historic district and other parts of town?

A wide variety of information is available on the web, but the differing vertical reference points cause confusion. There are DNREC inundation maps with elevations referenced to MHHW (Mean Higher High Water), proposed and actual dike elevations referenced to MSL (Mean Sea Level) using two zero points which differ by 0.88 feet (NADV88 and NGVD29, respectively) and FEMA flood maps with two different zero points: 2011 vs NGVD29 and 2013 vs NADV88. It's confusing! To make it clearer, this page shows elevations flooded in the recent storms, and what areas would be flooded at one or two or four feet greater height of water.

Recent Storms

Hurricane Sandy	Pre-Christmas storm
Bull Hill the morning after Hurricane Sandy. Note the debris including large logs. Picture courtesy Andrew Hayes

Water levels at the Delaware City tide station (shown above) rose to about 6.5 to 7 ft above MSL. Thomas Moran (Del. Div. Fish & Wildlife) reports that at the Bull Hill tide station, maximum elevations vs NGVD29 (0.88 ft above NADV88, the reference for the LIDAR map) were 8.06 ft, 10/30/12 2:04:59 AM (Hurricane Sandy), and 7.57 ft, 12/21/12 6:12:26 AM.

While interesting, the high tide information doesn't directly help us in understanding inundation without information about what areas become flooded by a given rise in water level. What does help is comparison of elevation maps with areas known to be flooded by observing debris lines or in pictures such as that above.

LIDAR Elevation Maps

LIDAR is a technique that allows gathering elevation information. Unlike radar which uses radio waves, LIDAR involves flying over an area, sending out laser beams and collecting backscatter information at 3 foot intervals. Elevation data are calculated and referred to mean sea level. I downloaded the data from NOAA's digitalcoast website. The resulting maps are of interest to us because they allow us to look at the elevations of areas that were flooded recently, and see what would be flooded in storms with water 1 or 2 or 3 foot higher than the recent ones. Water level is not the only factor relevant to what gets flooded in a storm (storm duration, wind driven wave action and surface topography also play roles), but it's the major one.

The map above and to the right show LIDAR elevations of 0-5 (blue), 5-6 (pink), 6-7 (red), 7-8 (dark red), and 8-10 feet above mean sea level in downtown New Castle. A pdf version that includes closeups of Bull Hill, the Strand, St. Peter Church area and W. 3rd st and Buttonwood is available. After the storms, debris remained in both the 5-6 feet and 6-7 foot elevations on the Broad (Narrow) dike, Bull Hill, Battery Park and Gambacorta dike near Dobbinsville (shown on the map as "reference points"). The storms therefore with their wind and wave additions reached about 6 feet above MSL.

Looking at the maps reveals:

FEMA floodplain

FEMA makes a variety of maps available including maps with 100 and 500 year flood zones. These are areas they state have a 1 or 0.2 percent chance of flooding in a given year. They are now being updated to reflect rising sea level and increasing chance of storms. The one shown here is an "old" one (<2013). It shows that the 500 year flood plain would cover the Read House and Immanuel Church parish house. An interactive version is available online.

Draft versions of Panel 166 and 162 of the "new" (2013) maps of New Castle are now available. The areas marked "AE## are 100 year flood zones of the indicated height above MSL (NADV88) and appear to be more extensive than in the previous flood zone maps. (See a list of FEMA abbreviations acronyms). Hundred year zones do not mean that floods only occur every 100 years, but it is of interest that in the great October storm of 1878, water "rose far as the residence of John M. Clayton" (the Read House). Since that time the sea level has risen 9 inches. Hundred year floods actually means that the chance of a flooding of an area is 1 percent/year. It is apparently based on historical information not projections that include sea level rise and changes in storm intensity or frequency. Projections for sea level rise vary 10 fold for the rise by the end of the century, (1 to 6 feet). The frequency and intensity of North Atlantic tropical storms have risen markedly in the last 30 years.

DNREC Sea Level Rise Inundation Maps

These interactive maps cover the entire state, and are referred to MHHW, which is 0.93 feet above MSL at Delaware City according to the NOAA. MHHW is also given as about 3 ft above MSL in the engineering report for the dike. Confusing!?! tide station data (8.657-7.727) Shown here are snippets including New Castle at the four available elevations. The highest one shown is 0.93 + 4.92 = 5.85 above MSL, which is a little above the height of recent storms and 2 feet below the planned dike level. Note that the area north of town which is protected by the exiting dike is shown flooded at 4.92 feet. The maps have a google-like interface and are easy to use at the four fixed elevations. This map is constructed from LIDAR data. Similar but more extensive maps are available for New Jersey, which also include economic impact, vulnerability, facilities and FEMA maps and explanatory material. These maps are also referenced to MHHW, perhaps since Sea Level Rise (SLR) ties in more directly to it.

Existing and planned dike profiles

In 2011, a dike profile was prepared by Greenstone Engineering (Appendix A) based on a survey by DNREC's Coastal Conservation District using NGVD29 as a datum. According to NOAA's online datum converter at longitude -75.56, latitude=39.658 (New Castle), NGVD29 is about 0.88 ft. above NAVD88, so one can mentally subtract 1 foot from the elevations in the profile for comparison with the LIDAR data.

The dike's elevation at that time ranged from about 6 to 11 feet; the FEMA 100 year flood elevation was 9 ft (vs NGVD29). The map to the right shows the 100 and 500 year flood zones based on 9 and 11 feet elevations.


In August 2012, (before the storms) a New Castle Dike Design Presentation made recommendations including:
* Raise Dikes to Elevation 8.5 Feet NAVD 88 (current low= 5.2 feet)
* Extend Dikes to Tie-Out as Required.

The profile of the proposed improvements vs exisiting conditions shows similar width (17ft at the top) with a flood-side layer of rip rap down to MSL. Rip rap is a sloping layer of loosely packed rock used to reduce the scouring and undermining that occurs at vertical structures like bridge piers and seawalls. The area between MSL and MLW would allow some vegetation to occur.