A swamp might seem like the worst place to be during a flood, but I endured Hurricane Floyd from the middle of the Great Swamp, and it was a serene and beautiful experience.

Floyd's arrival came as no surprise. Most of us were beginning to suffer from what might be called prediction fatigue as the storm's path was plotted in more detail than we wanted to know. All of the predictions could not prepare us for the intensity of the storm itself. At its peak, great torrents of driving rain fell, and fell, and fell. When it was over, the gages in and around the Great Swamp were showing rainfalls ranging up to 14 inches! We will round it off to an even 12 inches.

When the sun came up on Friday morning, everything was quiet in the Great Swamp. There were no raging streams, no cars being swept away, nobody being rescued by officials in boats. Much of the swamp had turned into a quiet lake that crept up over the roadways in places, blocking the normal commuter traffic that streams through. Local residents of the Great Swamp gathered at bridges and intersections for impromptu block parties, comparing the water levels to previous floods, exchanging stories about family members, and shaking their heads in disbelief about the reports of devastating floods in the towns of Bound Brook, Manville, and Lodi.

How is it possible that the impact of Floyd could be felt so differently in communities that are only a few miles apart?

The charts tell us that a rainfall of 7.5 inches can be expected only once in every hundred years, so Floyd had delivered a monumental rain-- some say a 200- or even a 500-year storm event. As an aficionado of stormwater, I ventured out several times during the storm to record the water level from the staff gage where Black Brook crosses New Vernon Road about a mile north of Meyersville. I would come back in and log onto the USGS web site ((http://wwwnj.er.usgs.gov/station # 01379000) that presents the data from the automated stream gage in the Passaic River at the Millington Gorge. I continued this ritual for several days after the storm.

The Black Brook gage is located right in the heart of the Great Swamp basin and the Millington Gorge station monitors all of the water that leaves the Great Swamp. Together, they provide an excellent picture of how the swamp handles a large storm. The USGS data from the Millington Gorge station is especially valuable, because this automated station has been in continuous operation since 1920, and routine manual measurements go back to 1900, giving us a hundred years of Great Swamp stormwater data.

One way of measuring a storm is by the amount of water flowing in a river. By this measure, the storm of the century happened three years ago in October '96. Most of us remember that storm. We had a 9.5-inch rainfall, confirming one of the favorite jokes of hydraulic engineers who nervously describe a 100-year storm event as one of those huge rainstorms that we get every couple of years. The Millington Gorge station recorded a peak of 2200 cubic feet per second (cfs) making it bigger than Hurricane Doria in 1971, bigger than the great flood of 1905, and yes, even bigger than Floyd, who comes out in sixth place for the century, barely breaking the 1500 cfs mark.

There are all kinds of technical terms for what happens in the Great Swamp during a major storm event. Hydrologists speak of evapotranspiration, runoff curves, infiltration and the like. In lay terms, this can be translated into stormwater being collected and held back so much of it can seep into the ground and be absorbed into the grasses and leaves of the trees.

Floyd arrived when all the plants were still in their growing season and the soil was still parched from a summer's drought. He didn't have a chance. The Great Swamp took the foot of rain and tamed it into sixth place. Here's how:

The total amount of rain water that fell on the 55.1 square miles of the Great Swamp watershed can be calculated (It is more than 11 billion gallons!). The continuous data from the USGS station in the Millington Gorge allows one to calculate how much water leaves the Great Swamp. Within the first 48 hours of the storm, the rain stopped and although Floyd had dumped 12 inches of rain, the Great Swamp had allowed less than 2 inches to leave. Over the course of the next two weeks, the Great Swamp would allow another 2 inches worth to slowly make its way through the gorge. But the remarkable conclusion is this: a full 8 inches of the stormwater was retained in the swamp. For good. That's roughly 8 billion gallons of stormwater that did not contribute to the flooding of our neighboring communities along the lower Passaic River.

The answer to the flooding problem is not the construction of concrete levees and huge detention systems. These manmade monuments will surely fail, as they did along the Mississippi River and the Red River a few years back. Instead, we need to insist that our local officials preserve our natural floodplains and take advantage of modern building practices, leaving most of the water from our rooftops and roadways where it belongs--in the ground.

In addition to its aesthetic beauty, the Great Swamp is a flood tamer, making it a natural resource of inestimable value. As officials and flood victims around the state begin to sift through the damage and make plans for the future, they might visit the Great Swamp and reflect on the wisdom of allowing natural resources to fulfill their role in the planning process.

Floyd occurred on 16 SEP 99 and 17 SEP 99.

Copyright 1996-2004. Great Swamp Watershed Association