Over the past year we have been working with the Federal Emergency Management Administration on the Flood Insurance Rate Maps for Dennis. These maps identify properties considered to be at risk for flooding should the town be impacted by a 1% (100 year) storm event. As this type of storm event is quite rare many do not understand the level of flooding in contemporary terms.
Many have asked me about this flooding, and in particular have noted the lack of significant flooding with Hurricane Bob or the October No-Name Storm. I hope to explain, in layman’s terms, the 1% storm and the confluence of events that may be needed to actually experience such a storm event.
First, the graphic below illustrates the reach of the 1% storm event along the Swan River.

As the picture illustrates, the 1% storm has quite a reach. Much of the low lying areas in South Village, along Swan Pond Road and out almost to Trotting Park Road would be impacted. These areas have been arrived at from a variety of information. This includes basic storm data (such as the potential for increased tidal elevation); historic storm data (hurricanes in 1938 and 1954 provide much of the benchmark data for this area); ground elevation; and sea level. The current sea level information, high and low tides, moon tides, etc. are combined with information from the historical storms in the region and the expected increases in tidal and wave elevations with a storm surge to determine how high water levels might be if a storm hit and everything were aligned perfectly for maximum impact. This information is then looked at against land elevations, above sea level, to determine the inward reach of a storm. Ultimately, that leads to the mapped area that is considered to be at risk from a 1% storm event.

The map above illustrates National Weather Service mapping of hurricanes that have impacted Dennis. As you can see, many have passed over or near our area. The graphic below, illustrates how a storm surge works.

Essentially, the tide rises and falls daily, when a storm is out to sea, pushing the water in with strong winds, the tides build to higher and higher levels. The longer a storm churns off the coast, the higher potential storm surge. Everything within the ground elevation affected by the higher than normal tides are considered at risk of flooding. The 1% storm, generally means the storm surge would reach a flood elevation of about 11 feet above sea level for most of Dennis.
Now, let’s look at Hurricane Bob as our reference point. Hurricane Bob was a Category 1 Hurricane. The map below illustrates the anticipated greatest land side impacts of a Category 1 Hurricane. As the picture illustrates, a Category 1 Hurricane could inundate much of the area considered at risk of flooding in a 1% storm event.
To see this impact, however, everything must be aligned. High tide, or higher than normal tides. Being on the wind side of the storm. A slower moving storm rather than a fast moving one. Hurricane Bob was moving to quickly to build a tremendous surge when it hit. Hurricane Bob also arrived off-tide, its biggest surge being during a low tide. It also did not hit during a full or new moon cycle which traditionally have the larger tidal impacts. Even then, Hurricane Bob caused some flooding damage. Properties in close proximity to the water reported damage. Some boats along the Bass River were beached. The damage was nothing, though in comparison to what could have happened.
A Category 2 Hurricane, Hurricane Bill this past summer before it veered off to sea, could have far more drastic impacts.

Here you see potential storm effects that extend beyond the presently designated 1% storm event flooding area. However, the originally proposed updated flood maps in May 2009 looked eerily like this area.

This similarity suggests that a 1% storm event could be the equivalent of a Category 2 Hurricane. If Hurricane Bill had approached Dennis as originally thought, with proper conditions. We might have seen that 1% storm event, or one very near to that.
But, while many wonder why they are in a flood zone. It is probably better to feel protected and be glad that we have not seen storms the size of those that would lead to this flooding in quite some time. Or we might be looking at:

