Floodplain/Stormwater Management

Introduction

Situated on the Delmarva Peninsula between the Delaware Bay and Chesapeake Bay, Delaware is the lowest state in the United States, with a mean elevation of just 60 feet above sea level.  Vulnerable to flooding from rising sea levels and ocean-fueled tropical storm systems, coastal Delaware is one of only three states located on a peninsula.  Delaware floods originate along the hilly, rocky Piedmont streams in northern New Castle County and from the tidal bay and Atlantic Ocean.

The 100-year Floodplain

More than 331 square miles, or 17 percent of Delaware’s landmass, lie within a mapped 100-year floodplain.  The distribution of floodplains in the three Delaware counties is similar, ranging from 16 percent to 18 percent of the land.

Area of the 100-year Floodplain in Delaware

Road Miles in Floodplain

Approximately 621 road miles are in the 100-year floodplain in Delaware.  New Castle, Kent, and Sussex Counties contain 128, 75, and 418 road miles in the 100-year floodplain, respectively.  Watersheds with the largest mileage of floodplain roads include the Christina River in New Castle County (44 miles), Murderkill in Kent County (16 miles), and Indian River Bay (106 miles).

Structures in Floodplain

More than 18,000 structures exist in the 100-year floodplain in Delaware—2,431 in New Castle County, 1,853 in Kent County, and 13,760 in Sussex County.  Watersheds with the most structures in the 100-year floodplain include the Christina River in New Castle County (1,007 structures), St. Jones River in Kent County (567 structures), and Indian River Bay in Sussex County (3,856 structures).

Flood Discharge

Watersheds with the largest FEMA 100-year-flood flow per drainage area include Shellpot Creek in New Castle County (1,161 cfs/sq mi), Duck Creek in Kent County (327 cfs/sq mi), and Indian River in Sussex County (52 cfs/sq mi).  Some of the largest recorded floods occurred on July 8, 1989 (July 4th storm), September 16, 1999 (Hurricane Floyd), Sept. 15, 2003 (Tropical Storm Henri), and Hurricane Irene (August 28-29, 2011).

Floodplain Mapping

• Delaware citizens and governments can access the mapping to determine flood insurance, flood warning, and flood response needs by visiting FEMA’s Flood Map Service Center.​

Tropical Storm Ida Remnants HECRAS Hydraulic Analysis Brandywine River at Wilmington, Del.

UDWRC, Oct 19, 2021

The UDWRC utilized the USACOE HECRAS hydraulic model developed by FEMA to reconstruct and examine the effects of Tropical Storm Ida remnants on neighborhoods along the Brandywine River in Wilmington, Del. on Sep 1-2, 2021. The Ida peak flood of 33,700 cfs (>100-yr) occurred on Sep 2, 2021 the highest on record along Brandywine Creek at Wilmington dating to 1946 surpassing Hurricane Agnes of 29,000 cfs on Jun 23, 1972. The peak discharge recorded at Brandywine Creek at Chadds Ford, PA was the highest in 110 years since the USGS gage was installed in 1911. Ida peak flood of 8,500 cfs (>10-yr) on Sep 2, 2021 highest on record along White Clay Creek near Newark dating to 1943 just behind Hurricane Agnes of 9,080 cfs on Jun 22, 1972. Ida peak discharge Brandywine Creek at Chadds Ford, PA > 33,000 cfs recorded at 2 am on Sep 2, 2021. The Ida peak discharge Brandywine Creek at Wilmington 33,700 cfs occurred 9 hours later at 11 am Sep 2, 2021. The normal high tide along Christina River at Wilmington is 3 to 4 ft msl. Fortunately the Ida peak high tide of 5.4 ft occurred at 10 pm Sep 1, 2021 and at 4.8 ft 11 pm Sep 2, 2021 out of phase with the peak riverine flood discharge. There are 4 bridges over the tidal lower Brandywine moving upstream: AMTRAK RR, Northeast Blvd., E. 16th Street, and Bus. Route 13/Market St.. Each is sized to safely convey the 100-yr design flood without overtopping the bridge deck. Ida peak discharge of 33,700 cfs (>100 yr flood) overflowed into Wilmington neighborhoods at El. 9 to 10 ft msl upstream from the Northeast Blvd bridge. Ida peak discharge (33,700 cfs > 100-yr flood) caused about 1 ft of backwater at the AMTRAK RR (El. 9 ft) and NE Blvd (El. 10 ft) bridges and along with likely debris jams at the bridges contributed somewhat to flooding in the neighborhoods sitting at interior drained ground elevations below 10 ft msl. The spillover point was just upstream of the NE Blvd bridge and backwater through storm sewers. The 100-yr floodplain is 9 to 12 ft above msl at and upstream the Northeast Blvd. bridge. Ida peaked at 9 to 10 ft msl there and spilled over into the neighborhood. Given that Ida just about overtopped the banks of the Brandywine at El. 10 ft msl, a mildly sloped 4 to 5 ft high grass Dutch style dike (1000 to 1,500 LF) at TOB 15 ft msl with storm sewer backflow valves along with extension of Brandywine Park downstream to the AMTRAK RR bridge could flood proof this neighborhood employing a modest budget tucked into the 2019 EPA brownfields plan.

Ida HECRAS Hydraulic Analysis Brandywine River Wilmington, Delaware, September 1-2, 2021

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Tropical Storm Ida Remnants along Brandywine River at Chadds Ford, Pennsylvania

The UDWRC utilized the HECRAS hydraulic model developed by FEMA and LIDAR mapping to examine the effects of Tropical Storm Ida remnants along the Brandywine River in Chadds Ford, Pennsylvania on Sep 1-2, 2021.

Conclusions

1. Based on instrumental record to 1911 Ida was a truly historic event only two floods > 25,000 cfs and not until 1999 then 20 yrs later in 2021.
2. There looks like there could be a fix here involving earthwork with the RR embankment to the east. We have a USGS map showing the RR in 1896 and looking at the history realize the mill buildings and RR are circa Civil War.
3. This began a century and a half ago and if we had computer hydraulic models in the 19th century the code to protect the riparian rights of the mill would have specified the RR build a trestle to east out to Rte 100 to span the floodplain ala the short trestle to west that does the job job.
4. At high 50-yr floods and over like Ida, the RR trestle can’t adequately pass flood without backwater and serves as dam with debris jams, this insufficient hydraulic capacity increased Ida flood elevations at museum and Rte 1 by 4 ft which is difference between water damage and not.
5. The museum is also at the confluence of the Brandywine itself and 2 incoming tributaries Ring Run and Harvey Run.
6. Ida at 33,000 cfs far exceeded Floyd (1999) at 27,000 cfs. Ida at 33,000 cfs > USGS 100 yr flood discharge 31,380 cfs.
7. From USACOE 1963 report, Jan 26, 1839 flood on Brandywine rose to 22 ft all bridges except for highway and RR bridge swept away. Estimate Jan 1839 Brandywine flood = 30,100 cfs second only to Ida and above Sep 1999 Floyd of 27,000 cfs. Ida was highest flood in two centuries.
8. Ida peak flood wave travel time from Modena (8 pm Sep 1, 2021) to Chadds Ford (2 am Sep 2, 2021) = 6 hours.
9. PADCNR & CCWRA reservoirs attenuated flood. Marsh Cr. reservoir release 100 cfs < than inflow. Just 300 cfs release Chambers Lake reservoir.
10. Chadds Ford USGS gage recorded Ida peak flood elevation 172.2 ft msl upstream from the RR trestle.
11. FEMA HECRAS flood profile: RR trestle and embankment causes 3.6 ft rise in 100-yr flood elevation (170.6 ft US – 167.0 ft DS).
12. Removing part of RR embankment to east to grade could reduce 100-yr flood and historic Ida elevations by 4 ft at museum and campus buildings which could flood proof these facilities and leaves Rte 1 unflooded so businesses at Rte 100 intersection/PENNDOT benefit too.
13. The elevated RR trestle to west spans the floodplain providing effective flood storage there.

As The River Runs, Philadelphia Inquirer, November 08, 2021 (PDF)

Ida Hydraulic Analysis Brandywine River at Chadds Ford, PA, September 1-2, 2021 (PDF)