Flooding in Gloucester County

As a whole, Gloucester County is not especially flood prone, although the low-lying tidal tributaries of the Delaware River Estuary, along the western portions of the county are vulnerable to local and regional flooding that could result from the tragic coalescence of extraordinary hydrologic conditions (i.e., Gloucester County's own version of the Perfect Storm). These low lying tidally influenced areas are vulnerable to flooding from somewhat unpredictable combinations of tides in the estuary, river flow in the expansive upstream portions of the Delaware River watershed and regional (flash flood) storm events in Gloucester County.

Dams that were built along many of the county's streams to from ponds and lakes for aesthetic, recreational, irrigation and other purposes are also vulnerable to flooding from regional/local flash flood storm events. Gloucester County has been growing and the new land uses have transformed some portions of the once primarily agricultural landscape into suburban impervious cover, potentially altering the hydrologic character of the County's watersheds and increasing runoff volumes and rates and stream flow. These ponds and lakes provide storage buffers that may help prevent flooding, provided that the storage volumes are available and the dam spillways adequate and do not fail at critical times.

Other manmade and natural features that obstruct runoff and stream flow may also inadvertently increase vulnerability to flooding. These include roadway bridges and culverts, tide gates, and debris jams in stream channels and locally, sediment, debris, leaves, trash and snow and ice that plug storm sewer inlets, pipe systems and outfalls. Maintenance of these features is required and upgrades may be needed as time and conditions change.

Fortunately, many of the county's watersheds retain natural features and some manmade features that help prevent flooding and these features should be protected. Important natural and manmade features that help prevent flooding in Gloucester County are described below.

1. Abundant Hydrologic Soil Groups (HSGs) and geology that support significant average annual recharge rates (i.e., soils in HSG Groups A and B and/or within the four surficial aquifer systems in the County, including the Potomac-Raritan�Magothy, Englishtown, Wenonah-Mount Laurel, and the Kirkwood-Cohansey aquifer systems) that provide runoff storage and maintenance of stream base flow. The importance of these soils in the hydrology and hydrogeology of Gloucester County cannot be overstated. Impervious cover should be minimized and the potential loss of recharge and storage mitigated in these areas.
2. Drainage patterns and natural topography that function efficiently, involving many dendrites. These efficient natural patterns should not be disrupted.
3. Numerous sub-watersheds that may still remain in optimal condition, preventing flooding. Regional assessment of these sub-watershed conditions should continue in order to identify sub-watesheds where optimal conditions remain and where sub-optimal conditions can be remediated.
4. Stream channels that are typically shallow, flat-bottomed and gently sloped and stream banks that are moderately sloped. Resulting stream velocities are low and the energy available to cause flooding is thus low. These stream channel conditions should be preserved.
5. Ponds, lakes and their dams that further reduce stream slopes and provide sediment and runoff storage buffers that may help prevent flooding.
6. Flood plains adjacent to stream banks that are typically wide, containing forested wetlands and marshes that are very important in preventing flooding. These floodplains, their vegetation and valuable habitats should be preserved.
7. Stormwater management facilities, primarily detention and retention ponds associated with land development over the last 15 to 20 years, that mitigate increases in runoff volumes and rates. These facilities must be maintained in order for them to meet their flood control purposes.
8. Agricultural land uses that are adjacent to floodplains and stream corridors and significantly benefit a watershed's runoff rates, volumes and quality.
9. Significant natural forested areas that work in conjunction with topography, soils and geology and fully infiltrate rainfall and eliminate excess overland runoff. To the extent possible these forests should be buffered and retained.

Although land development could adversely alter the natural hydrologic characteristics of the county's watersheds, the new stormwater regulations require the use of Low Impact Development techniques (LIDs) that when properly employed will maintain some or all of the natural pre-developed conditions in watersheds. LID techniques control peak stormwater runoff rates, reduce runoff volumes, maintain groundwater recharge and thus minimize the potential for flooding. More detailed information concerning these techniques can be found on the New Techniques for Stormwater Management webpage and in the NJDEP's BMP Manual.

Flooding and Hydrologic Information

DRBC
The DRBC website provides links to recent flood events in the Delaware River watershed.

USGS
The United States Geological Survey (USGS) provides links to Real-Time stream flow information for various streams in New Jersey. This site also includes Real Time New Jersey Tide Gages, some of which are located in the Delaware River Estuary.

There are two locations (Station Numbers) with Real-Time gage height (flow level) and flow rate information in Gloucester County. These sites also provide the estimated flood stage gage height:

01475000 - Mantua Creek at Pitman
01477120 - Raccoon Creek near Swedesboro

There are two closely related stations on the Maurice River at Norma in Cumberland County and on the Great Egg Harbor River at Folsom in Atlantic County.

01411500 - Maurice River at Norma
01411000 - Great Egg Harbor River at Folsom

The US Geological Survey provides the closest Real-Time stream flow information for the Delaware River at Trenton (just above the beginning of the tidal estuary).

The site includes a web-based GIS map depicting all USGS gage locations in the Delaware Basin.

NOAA-National Weather Service (NWS)
The National Weather Service (NWS), a division of NOAA, is the agency responsible for providing flood warnings and forecasts for the United States.

The NWS is continually working to improve forecasting methods. The Advanced Hydrologic Prediction Service (AHPS), under development since 1997, is an example of the National Weather Service's continuing efforts in working toward more accurate and timely hydrologic forecasting.

Existing NWS river stage forecasts generally provide only short-term (1-to 3-days in advance), text and tabular formatted forecast products for specific locations along a river. These forecasts are the product of applied historical weather data, stream gage data, channel cross-section data, water supply operations information and hydrologic model characteristics representing large regions.

AHPS is a step above the current forecasting methods. Applying the same hydrologic data used in current methods, AHPS also uses advanced hydrologic models with characteristics specific to local watersheds and tributaries. These advanced, localized hydrologic models increase forecast accuracy by 20% over existing models. AHPS depicts the magnitude and probability of hydrologic events, and gives users an idea of worst-case scenario situations. AHPS provides forecasts farther in advance of current methods, allowing people to gain additional time to protect themselves, their families, and their property from floods.

AHPS has recently been implemented in parts of the Delaware River Basin and implementation in other portions is ongoing. Information specific to the basin can be obtained from the Middle Atlantic River Forecast Center. The Delaware River Basin Commission (DRBC), with technical input from the DRBC Flood Advisory Committee, has developed a report recommending upgrading and modernization of the basin's flood warning system. The recommendations calls for development of improved flood forecast modeling, graphical forecast products, and integration of Doppler radar and Geographic Information System (GIS) technology to improve headwater flood warning. Implementation of AHPS in the Middle Atlantic region is scheduled to continue through 2008.

The Lower Delaware Basin AHPS site provides current information about flooding in the Delaware River Basin. No streams in Gloucester County are depicted by the AHPS.

The Lower Delaware River Basin link on the NWS Middle Atlantic River Forecast Center site provides information about flooding only as far south as Big Timber Creek, which is located at the north end of Gloucester County.

The NWS Northeast River Forecast Center can be found at the following link: http://www.erh.noaa.gov/er/nerfc/.

Flood Hazards, Maps and Insurance
The Federal Emergency Management Agency (FEMA) and its National Flood Insurance Program (NFIP) provide information on flood zones, hazards and flood insurance. "Flood zones are land areas identified by the Federal Emergency Management Agency (FEMA). Each flood zone describes that land area in terms of its risk of flooding. Everyone lives in a flood zone - it's just a question of whether you live in a low, moderate or high risk area." Homeowners insurance does not cover flood damage. However, mortgage companies require flood insurance for homes in Special Flood Hazard Areas and relatively inexpensive flood insurance can be obtained through the NFIP Insurance Center and its agents.

A Flood Insurance Rate Map (FIRM) is a map created by the NFIP for floodplain management and insurance purposes. Digital versions of these maps are called DFIRMs. A FIRM will generally show a community's base flood elevations, flood zones, and floodplain boundaries. One can use this map to get a reliable indication of what flood zone you're in. However, maps are constantly being updated due to changes in geography, construction and mitigation activities, and meteorological events. Therefore, for a truly accurate determination, contact your insurance agent or company or your community floodplain manager." A link to the FEMA NFIP website that allows one to find out your relative flood risk by entering the property address.

"Flood zones are geographic areas that the Federal Emergency Management Agency (FEMA) has defined according to varying levels of flood risk. These zones are depicted on a community's Flood Hazard Boundary Map or a Flood Insurance Rate Map (FIRM). Each zone reflects the severity or type of flooding in the area."

"Land areas that are at high risk for flooding are called Special Flood Hazard Areas (SFHAs), or floodplains and these areas are indicated on Flood Insurance Rate Maps (FIRMs). A home located within an SFHA has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage. A Non-Special Flood Hazard Area (NSFHA) is an area that is in a low- to moderate-risk flood zone (Zones B, C, X Pre- and Post-FIRM). An NSFHA is not in any immediate danger from flooding caused by overflowing rivers or hard rains. However, it's important to note that structures within an NSFHA are still at risk [from flash floods]. In fact, one out of four floods occurs in a NSFHA."

Below are brief definitions of the FEMA flood zones.

Moderate to Low Risk Areas

Zones B, C, and X
Areas with less than a 1% chance of flooding each year; areas that have less than a 1% chance of sheet flow flooding with an average depth of less than 1 foot; areas that have less than a 1% chance of stream flooding where the contributing drainage area is less than 1 square mile; or areas protected from floods by levees. No base flood elevations or depths are shown within these zones.

High Risk Areas
In communities that participate in the NFIP, mandatory flood insurance purchase requirements apply to all A zones.

Zone A
Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. Because detailed analyses are not performed for such areas; no depths or base flood elevations are shown within these zones.

Zone AE and A1-A30
Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. In most instances, base flood elevations derived from detailed analyses are shown at selected intervals within these zones.

Zone AH
Areas with a 1% annual chance of shallow flooding, usually in the form of a pond, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Base flood elevations derived from detailed analyses are shown at selected intervals within these zones.

Zone AO
River or stream flood hazard areas, and areas with a 1% or greater chance of shallow flooding each year, usually in the form of sheet flow, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Average flood depths derived from detailed analyses are shown within these zones.

Zone AR
Areas with a temporarily increased flood risk due to the building or restoration of a flood control system (such as a levee or a dam). Mandatory flood insurance purchase requirements will apply, but rates will not exceed the rates for unnumbered A zones if the structure is built or restored in compliance with Zone AR floodplain management regulations.

Zone A99
Areas with a 1% annual chance of flooding that will be protected by a Federal flood control system where construction has reached specified legal requirements. No depths or base flood elevations are shown within these zones.

High Risk - Coastal Areas
In communities that participate in the NFIP, mandatory flood insurance purchase requirements apply to all V zones.

Zone V
Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves. These areas have a 26% chance of flooding over the life of a 30-year mortgage. No base flood elevations are shown within these zones.

Zone VE and V1 - 30
Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Base flood elevations derived from detailed analyses are shown at selected intervals within these zones.

Undetermined Risk Areas
Zone D
Areas with possible but undetermined flood hazards. No flood hazard analysis has been conducted. Flood insurance rates are commensurate with the uncertainty of the flood risk.

Source: information was obtained from the FEMA-NFIP website.