measured at 11 sites established in 1992 and at 3
to 30 cm of material is consolidated and root-
additional sites established in 1993 (Fig. 8). Net
bound, this soil and root mat are undermined by
recession rates were measured between 28 May
current erosion and only fail after an erosional
and 22 September 1992, 22 September 1992 and 2
niche of approximately 0.5 m or deeper is cut be-
June 1993, and 2 June and 22 September 1993,
low it.
while measurements on 26 August and on 22
Eroded sediments form deposits within the
September 1993 permit comparison of erosion
gullies and are eventually transported down-gul-
and recession resulting from a single month's
ly into the Eagle River. Along steep scarps, blocks
cycle of flooding tides (Appendix A).
of consolidated sediments bound by roots fall
The amount of recession of the scarp crests
and roll into the gully bottoms where currents
was highly variable within a gully, as well as
eventually may break them down by scour or by
among the sites (Table 6). Maximum recession
rolling them along the gully bed. The lateral
rates ranged from 0.14.9 m during May to Sep-
walls of gullies, which tend to develop low angle
tember 1992 , 0.46.3 m during September 1992
slopes as headwalls recede inland, fail mainly by
to June 1993, and 0.09.8 m during June to Sep-
retrogressive slump flow, the lower half or more
tember 1993 (Table 6).
of the slope creeping slowly as a mudflow into
At each site, however, some parts of the scarp
the gully channel. On gentler slopes, blocks of
crests did not retreat at all. This spatial variabil-
rootbound material remain intact as they are
ity in recessional rates is evident in plots of the
transported downslope by slow-moving mud-
scarp crest location with time (Fig. 30). Reces-
flows that are active in the latter stages of the ebb
sion rates for the September 1993 flooding cycle
cycle. Because this sequence of events happens at
were a maximum of 1.4 m. The extreme rate of
different times and rates, recession varies within
change of 45 m was generated by the develop-
as well as among the study sites (Table 6).
ment of a new network of shallow (0.5 m) gullies
If our measurements of recession rate are char-
in the mudflats between C and Bread Truck
acteristic, erosion will cause increased drainage of
ponds during the 199293 winter (Fig. 30c).
ponds within the next 10 to 15 years. Two ponds
The variability in recession rate reflects the
in particular will likely be affected: one is Bread
nature of the erosional processes in causing rapid
Truck, on its northern side, and the other is the
short-term changes (Table 2). Recession appears
pond complex between the Bread Truck and C
to be caused mostly by current erosion of the
ponds (Fig. 5). Further analyses of recession rates
lower, unvegetated part of the near-vertical gul-
are needed to determine if the 1992 to 1993 rates
ly walls during ebb. Because the uppermost 20
are representative of more than this 1-year
period.
Table 6. Headwall recession rates in 1992 and 1993.
Recession range (m)
Erosion
Location
MaySept 92 Sept 92June 93 June-Sept 93
site
Position* Min Max
Min Max
Min Max
B†
L
0.0
2.1
0.3
3.2
0.1
3.9
1
2
B
L
0.0
4.9
0.0
2.0
0.0
1.2
3
C
L
0.0
0.6
0.0
0.9
0.0
0.0
4
C
H
0.1
3.1
0.5
1.5
0.0
3.8
5
C
L
0.0
1.3
0.0
0.8
0.0
0.2
6
D--S. Fork
L
0.1
2.4
0.0
0.6
0.0
1.1
7
D--S. Fork
H
0.0
1.1
0.0
0.4
0.0
0.8
8
D--Cent. Fork
L
0.0
0.1
0.0
1.0
0.0
0.4
9
D--Cent. Fork
H
0.0
0.5
0.1
6.3**
0.0
9.8
10
D--Cent. Fork
L
0.0
0.2
0.0
0.5
0.0
0.0
11
D--N. Fork
L
0.0
0.5
0.0
0.4
0.0
1.0
12
F
L
--
--
--
--
0.0
3.1
13
F
L
--
--
--
--
0.1
1.8
14
F
L
--
--
--
--
0.0
1.2
* L = lateral wall, H = headwall.
† See Figure 8.
** 45-m shallow erosion extending into mudflats (Fig. 30).
31
Previous Page
