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Wednesday, February 29, 2012

Ninigret Pond dredging - EXPLAINED

Dredging overview - click to enlarge
by Tom Ferrio with assistance from Steven McCandless, Town GIS Specialist

Permanent breachways like that connecting Ninigret Pond to the ocean are not natural features of our shallow ponds along the coast.

Keeping the breachway open and protecting the health of the pond requires special design and maintenance.

It's an obvious statement to say that the ocean has waves and Ninigret Pond is much calmer. The turbulence in the Block Island Sound suspends sand and other fine particles from the beach and ocean bottom into the water. As the tide flows into the Ninigret Pond the water slows down and much of the suspended material drops to the bottom.

Without special areas to catch this sand it would spread across large portions of the pond bottom, doing great damage to vegetation and shellfish in the pond. To prevent that, the breachway widens out just past the narrow channel so the water will slow down and deposit its load in a sedimentation basin (see the map above).

Over time that sedimentation basin fills up. If it's not cleaned out periodically it will stop doing its job of catching sand before it reaches the pond. Especially after tropical storm Irene last year, the need was critical to get rid of the accumulated sand.

During 2006 through 2008 there was a major habitat restoration project which included the dredging of approximately 122,000 cubic yards of sediment from the flood tidal delta of the Charlestown Breachway and approximately 75,000 cubic yards of sediment from the sedimentation basin and relief channel, restoring approximately 40-acres of prime eelgrass habitat in Ninigret Pond. Eelgrass beds are a primary source of food and shelter to an abundance of marine life, including economically important finfish and shellfish, such as the bay scallop. It is widely recognized that the vitality of an estuary’s eelgrass beds is an indicator of its health. The sedimentation basin within the Breachway retains migrating sediment before deposition into the flood tidal delta and migrating over the newly restored eelgrass beds.

Sedimentation Basin

In this year's maintenance dredging, sediment within the basin is being removed to an approximate average depth of 6 feet.  It's estimated that approximately 70,000 cubic yards will be removed from the sedimentation basin.

Relief /Navigation Channel
Further into the pond, accumulated sand is also being removed from the relief (or navigation) channel. No other areas are being dredged and no new navigation channels will be created. Sediment within the relief channel is being removed to restore a water depth of 4.5 feet below mean low water and achieve a width of 60 feet, resulting in an estimated dredge depth of one yard on average throughout the relief channel with approximately 12,000 cubic yards to be removed there.

The sand removed is being pumped to the town land on Charlestown Beach where it is replenishing beach sand and is far enough away from the breachway that it will not rapidly re-enter. One or two storms will spread and mix the sand and the predominant current along the coast will tend to move the sand to the east. The small amount of organic material in the sand will rapidly be bleached by the sun and our gorgeous beach will be even more beautiful this summer.

In the meantime, take a trip in this fine weather to visit the dredging. If you walk out on Charlestown Beach you can see the sandy water flowing out of the pipe in a huge volume.

There isn't much to see at the business end in the Breachway Park unless you like to see and hear a huge pump in action. But while you're there be sure to walk out on the beach and see the Mystery Ship exposed on the beach from Irene, romantically thought of as a rum-runner by some but most evidence says it is much older than that.
The Mystery Ship - click to enlarge