TIDAL IMPACT TABLES

The tidal impact tables were developed by the National Weather Service in Mount Holly, New Jersey in cooperation with local and county emergency managers and the Delaware Geological Survey.

The tables attempt to correlate tide levels at select National Ocean Service gages with their expected impact on local communities.  Some historical tide data are also included.

Click on a location of interest.

DELAWARE MARYLAND
NEW JERSEY PENNSYLVANIA

ABOUT THE TIDAL IMPACT TABLES

By examining coastal flooding events of the past a fairly reliable correlation has been developed between the tide levels at select National Ocean Service gages and their expected impact on local communities. In order to establish those relationships each county in the region was assigned a reference tide gage.

* The Sandy Hook, New Jersey gage is used for the counties of Middlesex and Monmouth.

* The Atlantic City, New Jersey gage is used for the counties of Ocean and Atlantic.

* The Cape May (Ferry Terminal), New Jersey gage is used for the counties of Cape May and Cumberland.

* The Philadelphia gage is used for the counties along the tidal part of the Delaware River north of Marcus Hook, Pennsylvania.

* The Reedy Point, Delaware gage is used for the counties of New Castle in Delaware and Salem in New Jersey.

* The Lewes (Breakwater Harbor), Delaware gage is used for the counties of Kent and Sussex.

* The Tolchester Beach, Maryland gage is used for the counties of Cecil and Kent.

* The Cambridge, Maryland gage is used for the counties of Queen Anne's, Talbot and Caroline.

The values listed along the left side of each tidal impact table refer to a water level at the specified reference gage.  The heights are not to be interpreted as an indication of the water level at the actual locations mentioned in the corresponding impact statements.  Those locations are often a number of miles away from the reference gage.

It is important to remember that the impact statements are based on estimates.  A number of factors can cause the flooding associated with a particular tide level to vary somewhat from event to event.  Those factors may include rainfall, wave action and the number of tide cycles during which there is an onshore flow.

Being that the impact statements are based on estimates, it may be best to place more emphasis on the descriptive categories of flooding rather than focusing on the specific height values associated with flooding at a given location.  The three categories (minor, moderate, major) provide a much better overall picture of the type of tidal flooding that can be expected throughout a county or region.

In the  MINOR category there is flooding of the most vulnerable roadways due to high water and/or wave splash-over. The majority of roads remain passable. There is no significant threat to life. Any impact on property is minimal. This type of event is covered by a Coastal Flood Advisory..

In the MODERATE category there is widespread flooding of roadways due to high water and/or wave action with many roads becoming impassable. Lives may be at risk when people put themselves in harms way. Some damage to vulnerable structures may occur. This type of event is covered by a Coastal Flood Warning.

In the MAJOR category there is flooding that is severe enough to cause structural damage along with widespread flooding of roadways. Some evacuations become necessary. Vulnerable homes and businesses may be severely damaged or destroyed. Numerous roads become impassable and some neighborhoods may be isolated. The flood waters become a danger to anyone who attempts to cross on foot or in a vehicle. This type of event is covered by a Coastal Flood Warning which may contain enhanced wording to emphasize the significant threat to life and property.

The National Weather Service issues Coastal Flood Watches and Warnings for events that are forecast to be in the moderate or major range.