Coastal Resource Guide Pleasant Bay and ... - Town to Chatham

Coastal Resource Guide for Pleasant Bay and Chatham Harborincreased the overall tidal prism entering Chatham Harbor andPleasant Bay. This larger volume of water, in turn, increased tidalranges approximately 1.6 ft as well as increased current velocitiesthroughout the system. These increased velocities caused themigration of some shoals and channel systems that had beenrelatively stable for many years prior to the 1987 inlet. Within afew years after the inlet break, the channels and shoal systemswithin the mid and upper regions of Pleasant Bay became moreequilibrated to the new hydraulic regime. Chatham Harbor,however, was still undergoing considerable change twenty yearsafter the inlet breach due to close proximity to the inlet.Dredging with an excavator at the Fish Pier. Photo: Ted KeonThe total volume of material, which would be generated if thechannels were dredged to the design dimensions, is shown in Table6. The volume is based on cubic yards and the over-depth assumesone foot of additional dredging beyond the design depth. Notethat the dredge volume assumes establishing the project designdepth along the full channel width.8.7 Shoal Migration PatternsAs stated above, much of the Pleasant Bay system is characterizedby shallow sandy shoals and meandering channels. Unlike finegrained silts and clays which are easily disturbed and transportedwithin the water column, sand needs a relatively high energyenvironment to become mobile. Therefore, the areas of greatestshoal movement are generally in the vicinity of tidal inlets andchannels with high tidal currents. 3Areas adjacent to tidal inlets are particularly prone to migratingshoal patterns given the large potential for sediment introductionfrom the ocean beaches, strong tidal currents and high waveenergy. Chatham Harbor quickly became recognized as havingsome of the most dynamic channel and shoal systems in theCommonwealth following the formation of the new inlet near theChatham Lighthouse in 1987. The inlet caused the developmentof a highly variable flood and ebb shoal complex that werechallenging to navigate. These changes required the developmentof an extensive dredging program to maintain navigation accessfor the Chatham’s commercial fishing fleet. The 1987 inlet alsoenabled higher energy waves to enter Chatham Harbor, whichresulted in considerable erosion of the adjoining, previouslysheltered, internal beaches and upland property along themainland. This erosion contributed littoral sediments to PleasantBay, which modified shoals in Chatham Harbor.The more recent inlet breach in 2007 has already caused manychanges to the adjacent interior shoal systems and it is anticipatedthat considerable modifications will continue to occur as the inletfurther develops (see Figure 6 found in section 2). Given thenorthern location of the 2007 inlet, the upper portions of ChathamHarbor and southern waters of Pleasant Bay near Ministers Pointand Strong Island are likely to be regions of greatest change. Thisdynamic potential is related to the influx of large, new sedimentsupplies and further increases in tidal flow. Many of the channelsystems between the Chatham mainland and Strong Island alreadyhave become more dynamic due to the higher current velocities.A new flood shoal complex is forming inside the new inlet andat this time it is unclear how, where, or in what manner thisflood shoal will ultimately migrate. It is equally unclear how thetraditional navigation channels will respond to the intrusion of newsediments, shoal movement and increased current velocities. Map18 identifies the area where the greatest amount of shoal andchannel migration is expected to occur over the next several years.The 2007 inlet has altered sand and channel stability within theportion of Chatham Harbor between the two inlets. The 2007inlet has captured a significant volume of the overall tidal prismfor Pleasant Bay. Therefore, current velocities have moderatedsomewhat between the two inlets since the southern inlet iscontributing less water to the estuary. This has slowed the rateand extent of shoal migration in Chatham Harbor and it is likelythat the previously highly mobile flood shoal complex will becomeincreasingly more stable in the short term. However, the longerterm changes within this portion of Chatham Harbor may still besignificant. Over the next few decades, it is assumed that theremnant island between the two inlets will break-up and ultimatelymigrate in an anticipated west to southwesterly direction. Thishas the potential to significantly alter the network of shoals andchannels and shoreline morphology in Chatham Harbor in thecoming years.In addition to causing the growth and migration of large floodshoal systems within the adjacent harbor, the 1987 inlet also353 Barrier beach overwash is another mechanism whereby sediments are added to the interior estuary which can then be redistributed by waves and currents. This is amore episodic event generally only occurring during large storm events and effecting areas immediately landward of the barrier beach.Pleasant Bay Alliance 2011