ported downslope by slow-moving mudflows that
2.9 m/year (Table 9). The other gullies that
are active in the latter stages of the ebb cycle.
have shown a recent acceleration of reces-
sion are those of Bread Truck, which has been
Historical rates
the site of considerable monitoring (see be-
Historical rates of gully extension by headward
low), and Coastal 6. Recession at Bread Truck
erosion have been reconstructed for B, Parachute,
Gully has been accelerating in an unpredict-
In-Between, Mortar, Bread Truck and Coastal 5
able manner as the gully headwalls have en-
gullies (Fig. 32; Table 9). Long-term average re-
croached on Bread Truck Pond and entered
cession rates over the past 45 years range from 3.6
into unvegetated mudflats that are intermit-
m/year at Mortar Gully to 13.7 m/year at the
tently ponded.
Coastal 5 Gully. Recession ranged from 1.1 to 3.5
m/year between 1950 and 1960 and increased to
Recent vs. long-term
between 2.1 and 14.7 m/year between 1960 and
rates of recession
1972. Recession continued to increase between
The modern rates of net headward and lat-
1972 and 1986, when rates ranged from 5.1 to 23.6
eral recession measured using the hub and
m/year. In the decade of 1986 to 1995, recession
line stake technique are depicted in Figure 33
rates have remained high but have in general
to show the spatial patterns in gully erosion
slowed slightly, ranging from 3.9 to 21.6 m/year.
along a transect from the mouth of the Eagle
The aerial photographic record suggests that
River inland to the head of the Flats. Nearest
there was little physical change before the 1964
the coast, sites Coastal 1 and Coastal 7 have
earthquake. At four of the six locations studied,
experienced more headward than lateral re-
recession rates reached their peak between 1972
cession. In the mid-coastal regions, Mortar
and 1986. The recent high rate of recession at In-
and Coastal 6 have experienced similar rates
Between Gully reflects a rapid, shallow distribu-
of lateral and headward recession, with lat-
tary that advanced into the mudflat by about 40 m
eral erosion dominating slightly as these gul-
during the 199293 winter (Lawson et al. 1996),
lies have been widening by bank collapse and
which increases the 9-year average by 4.4 m/year.
slumping. Slightly inland from the mouth of
Therefore, the last decade of recession at In-Be-
the river, Bread Truck and Coastal 5 gullies
tween Gully may be best represented by a rate of
have been experiencing drastic, yet rather
Figure 31. Sediment flow at Bread Truck Gully. Saturated sediment fails when erosion during ebb tide
removes material from the slope toe.
40
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