are Mortar and Coastal 6 gullies,
where wide plunge pools below their
respective headwalls are expanding
laterally through bank collapse and
rotational slumping (Fig. 23d and 30).
Figure 23h shows one section of
the monitoring network at the River-
North site, where slope processes are
consuming mudflat sediments and
introducing material directly into the
Eagle River (Fig. 24). Rapid lateral
erosion on the order of 3.2 m was
recorded at this site between July and
November of 1994, and there was up
to 7.4 m of recession by late October
1995. Erosion occurs along large ro-
tational slumps that fail because of
river undercutting and removal of
sediment supporting the toe of the
slope (Fig. 24b). Prior to failure, large
extension cracks develop parallel to
the scarp, gradually leading to
catastrophic failure (Fig. 24a). The
location of the crack becomes the new
scarp of the riverbank.
Recession rates are variable be-
cause the erosional processes are
rapid, short-duration events (Table
2). Recession appears to be caused
mostly by currents scouring the
lower, unvegetated portion of the
gully walls during ebb tide when wa-
ter velocities are highest. Because the
uppermost 2030 cm of material is
consolidated and root-bound (Fig.
30), this soil and root mat are under-
mined and only fail after an erosion-
Figure 28. Aerial view of Bread Truck Gully in October 1995.
al niche of approximately 0.5 m or
deeper is cut below it. In the gullies
between two tributaries to the main gully, about
themselves, current scour during ebb tide removes
75 m downchannel of the gully headwall. Gully
material from the toe of the slope, thereby remov-
erosion is characterized by the formation of cus-
ing the base of the slope and causing slumping
pate embayments that have gradually enlarged
and sediment flow (Fig. 31).
since 1992, with a maximum of 5.3 m change (Table
Eroded sediments form deposits in the gullies
8, Fig. 29). This site is significant because we have
that are eventually transported into the Eagle
repeatedly detected WP in transport within ebb
River. Along steep scarps, blocks of consolidated,
waters (Lawson et al. 1995), as well as in fauna
root-bound sediments fall and roll into the gully
sampled within the gully (Bouwkamp 1995). The
bottoms and remain intact until currents eventu-
source of this WP is unknown at this time; how-
ally break them apart. The lateral walls of gullies,
ever, it is likely to have either eroded out of the
which tend to develop low-angle slopes as head-
gully headwall or been flushed from C-Pond.
walls recede inland, fail mainly by rotational slump
Data (Tables 8) indicate that, in general, lateral
flow, or by creeping slowly as a mudflow into the
recession is lower (often by an order of magni-
gully channel. On gentler slopes, blocks of root-
tude) than headward recession. The exceptions
bound material remain intact as they are trans-
38
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