160
160
Tidal Duration
Tidal Duration
120
120
80
80
Tidal Delay
Tidal Delay
40
40
0
2000
4000
6000
1000
0
2000
3000
Gully Length (m)
Distance to Knik Arm (m)
b. Distance to Knik Arm vs. tidal duration and de-
a. Gully length vs. tidal duration and delay. Re-
lay. Regression line for tidal duration is expressed by
gression line for tidal duration is expressed by y =
y = 1E-06x2 0.004x + 112.72 (R2 = 0.1283); and
8E-06x2 + 0.0437x + 79.818 (R2 = 0.6964); and
for tidal delay y = 5E-07x2 + 0.0005x + 22.94 (R2 =
for tidal delay, y = 7E-06x2 0.0187x + 35.887
(R2 = 0.9655).
0.8915).
Figure 29. Relationships among the duration of tidal flooding and tidal delay with gully length and distance from
gully headwall to Knik Arm.
flooding cycle, allowing even longer sedimenta-
the Arm is the primary source of water flowing
into the gullies; however, secondary controls on
tion times. This may be significant because ponded
water dynamics in the gullies modulate the indi-
water has low turbulence and reduced particu-
vidual site responses.
late transport capacity. Increased runoff time may
Water level measurements at the river and gully
therefore allow for increased sedimentation. In
sites (Fig. 16) verified our observations that the
addition, the mixing of flood waters with the pond
flood cycle normally starts with a progressive rise
and marsh waters increases the amount of sedi-
in the river channel and adjacent gullies before
ment suspended in the ponds and marshes, and
the mudflats gradually flooded. Flooding begins
is the primary source of newly deposited mate-
first in the coastal mudflats on Knik Arm while
rials.
water is moving progressively inland up the Eagle
The timing of flooding is a function of the el-
River channel. The delay in flooding at the
evation of the levees and mudflats surrounding
hydrostation sites from that predicted for An-
the gullies, the distance of the gully headwall
chorage is illustrated by data from June through
from the coast, and gully length. Water moving
November (Fig. 30).
down the river can also alter the timing of flood-
ing inland from the coast, depending upon its
volume relative to the volume of tidal waters.
Analyses indicate that tidal flooding is best de-
60
June
August
October
scribed by a polynomial regression (R2 = 0.97) for
November
July
September
the relationship between gully length and tidal
delay (Fig. 29a). A good correlation (R2 = 0.89)
40
also describes the relationship between tidal de-
lay and gully headwall distance from the coast
(Fig. 29b). A similar comparison using tidal dura-
20
tion indicates little relationship with either gully
length or distance to Knik Arm.
The characteristic tidal response observed at
0
our hydrostation sites is governed by gully char-
acteristics (i.e., sinuosity, gradient, cross-sectional
area, perimeter and channel roughness) that in-
fluence the tidal inundation rate in the gullies.
Thinking intuitively, we feel that the distance to
Figure 30. Delay in peak high tide from Anchorage to
Knik Arm also must be an important factor, since
ERF sites, June through November.
26
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