search is still required to resolve the absolute mag-
INTRINSIC REMEDIATION:
nitude and nature of the tectonic response to the
POND DRAINAGE BY GULLY
1964 earthquake.
EROSION AND EXTENSION
The complex way in which the physical system
Erosion and the inland extension of tidal gul-
of ERF responded to this disturbance is difficult
lies are among the most visible mechanisms cur-
to reconstruct. In a previous report, we specu-
lated that the recent erosion and stepped profile
rently modifying the ERF physical system. This
in the gully gradients reflected a dynamic equi-
activity is critical because our initial assessments
librium related to changes in base level vs. mean
(Lawson et al. 1996) suggested that gully exten-
sea level (Lawson et al. 1996). However, it is ques-
sion could drain contaminated ponds, create dry-
tionable as to whether or not the physical system
ing conditions conducive to in-situ WP degrada-
would respond in such a dynamic manner to a
tion (Walsh et al. 1995), and result in a natural
0.6- to 0.7-m change in base level given the
attenuation in a relatively short time. The length
macrotidal range of 911 m for Knik Arm. We
of time for drainage to occur will be a function of
now feel that erosion and sedimentation rates are
several factors and may vary across the Flats. The
a response to increased tidal flooding, coupled to
1964 Alaskan Earthquake appears to have had
sedimentary subsidence. Both the increase in fre-
the single greatest effect on ERF, compared to all
quency of flooding and larger volumes of water
other external and internal factors. It significantly
inundating the Flats are important factors that
altered the hydrological system, which in turn
would increase the amount of sediment entering
has initiated major changes in drainage, gully ero-
ponds while also increasing the volume and in-
sion and extension, and pond and mudflat sedi-
tensity of drainage during ebb.
mentation.
External forcing
Gully erosion and discharge
The effects of major tectonic events in the
The volume of water that flows through each
greater Anchorage area are reasonably well known
gully is controlled by the height and duration of
and best documented following the 1964 earth-
tidal flooding, as supplemented by river dis-
quake (Hansen 1965, Plafker et al. 1971). Strati-
charge and the additive effects of wind, ice and
other factors. The range and magnitude of ebb
graphic analyses of coastal marshes indicate that
velocity is critical to determining both the flux of
large earthquakes (>Mw = 9.0) have a recurrence
sediment and water into and out of the ponds
interval of 600800 years (Combellick 1993, 1994),
and mudflats, and the ability of those currents to
while magnitude 8.0 or greater earthquakes are
scour and resuspend pond, mudflat and gully
more frequent (230460 years; Nishenko and Jacob
sediments.
1990). Tectonic disturbance associated with such
The velocity of tidal flood and ebb currents
events causes co-seismic subsidence or uplift (de-
at Mortar, Bread Truck, B, Parachute and In-
pending on site position relative to local displace-
Between gullies ranges between 0.75 and 2.07 m/s
ment), which is followed by a post-seismic recov-
(Table 7). The peak velocity at each site changes
ery in the opposite direction (e.g., Savage and
with the height of flooding (Fig. 22), the lower
Plafker 1991). These crustal movements cause ad-
peak and range in velocities occurring during the
justments in surficial processes, which are mani-
lower elevation flooding tides. Flow velocities are
fested at ERF by the interactions of the internal
greatest at B-Gully, with moderate values recorded
factors listed in Table 1.
at Bread Truck, In-Between, Mortar and Parachute
Leveling data collected over the decade fol-
gullies.
lowing the earthquake of 1964 show that Anchor-
Peak ebb velocity is greater than peak flood
age subsided 6070 cm during it (Brown et al.
velocity (Table 7, Fig. 22). This asymmetry in flow
1977). Brown et al. (1977) estimated that there had
velocities, and therefore discharge, determines
been about 20 cm of post-seismic recovery by
when erosion and sediment transport will take
1975, although this is not apparent in the tide
place and to what magnitude. During tidal flood-
gauge records (Savage and Plafker 1991). Cohen
ing, gully water levels rise passively before spread-
et al. (1995) and Cohen (1996) suggest that uplift
ing out onto the mudflats and suspended sedi-
in the first decade following the earthquake was
ment loads increase as Knik Arm waters flood the
rapid and has subsequently decreased; Savage
gullies, mudflats and ponds. In contrast, higher
and Plafker (1991) estimate the current uplift rate
to be around 1.0 2.2 mm/yr. Considerable re-
velocities during ebb dictate an increase in turbu-
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