Chatham Harbor first began formally marking the north channel in 1998 for the
fishing fleet.1 Controlling depths have been approximately 4.0 to 6.0 ft (1.2 to
1.8 m) mllw in spot areas at the terminal lobe. With the northward movement of
the north channel, the north swash platform has reduced its area and the north spit
has changed from a stable feature to one that has grown south and has become
larger, with the formation of several recurved spits and bars since 1997. As of the
summer of 2000, the south channel has been essentially abandoned for navigation.
Sediment discharge through the north ebb channel has caused the northern
third of the ebb shoal to expand seaward 500 ft (153 m) in a crescent shape, remi-
niscent of the initial ebb shoal formation in 1987/88. The southern ebb channel tip
has retreated landward slightly (300 ft or 92 m) over the same period. It appears
that the northern ebb channel will become the dominant channel in the future.
Southward transport of the ebb shoal is still evident in the growth of the shoal
2,500 ft (762 m) further to the south along the South Beach shoreline. This south-
ern portion of the ebb shoal has complex channel margin shoal features. The
shoreline along the land bridge and northern 3,000 ft (914 m) of South Beach has
remained relatively stable, somewhat protected with this nearshore shoal complex.
Some of these shoal features are beginning to migrate onto the South Beach as of
the summer of 2000.1 South of this area the shoreline is still retreating.
Figure 53 illustrates the sediment transport pathways based on morphology
from July 1999. There are numerous ebb and flood dominated channels inter-
spersed with swash bars on the ebb shoal. The main ebb flow is shared with the
north and south ebb channels. A complex flow pattern is also evident around the
expanding flood shoal, with the east flood channel being more dominant at this
time. With the expansion of the ebb shield around the north end of the flood
shoal, the west flood channel has been blocked in since 1997. The west flood
channel has tried to break through the ebb shield in several spillover lobes in the
past 3 years, and it now appears that it is trying to re-establish a channel with a
large spillover lobe through the ebb shield, just about in the same position and
with a similar orientation to the original west flood channel of the 1987/89 time
period. Deposition on the west ebb spit of the flood shoal has further restricted the
southern portion of the west flood channel, which has flowed into the center of the
flood ramp area since 1995. It is hypothesized that the west flood channel will re-
establish itself through the existing flood shoal with the breakthrough of the
spillover lobe. The western flood shoal will become detached and form a new
Tern Island south shoal.
Personal Communication, 2000, T. Keon, Dept. of Coastal Resources, Town of Chatham, MA.
Chapter 5 Sediment Pathways