150
160
120
100
80
50
40
0
0
4.8
5.0
5.2
3
4
5
6
Peak Water Elevation (m)
Peak Water Elevation (m)
Figure 27. Peak water elevation vs. flooding time at
Figure 25. Relationship of water rise time to peak
Bread Truck Pond, September 1994.
water elevation at Bread Truck Gully.
6
Jun
Jun
Cover Levees (5.21 m)
600
Jull
Ju
Aug
Cover Mudflats (4.87 m)
Aug
Sep
Sep
Inundate Ponds (4.6 m)
Oct
Oct
400
Nov
Nov
4
200
1994
0
3
4
5
6
2
8 Oct
15 Oct
22 Oct
29 Oct
Peak Water Elevation (m)
Figure 26. Water elevation record at Parachute Gully
Figure 28. Peak water elevation vs. runoff time at
relative to elevations required to flood selected land-
Bread Truck Gully, JuneNovember 1994.
forms. Open circles represent the time and elevation
of predicted high tide at Anchorage.
height slightly below that predicted, reducing the
the peak elevation and each landform's threshold
inundation. Higher tides attributable to wind
for flooding.
surge increase the height and volume of flood wa-
During flooding, ponds have relatively high
ter and prolong the period of runoff during ebb.
water levels, increasing the area available for sedi-
The time between the start of water rise and
mentation. Successive flooding tides move new
peak high tide was calculated for each hydro-
material into the ponds and higher deposition
station site. We found a linear relationship be-
rates can be maintained for extended periods.
tween water rise time and peak elevation (Fig.
Pond drainage is restricted by the gullies' cross-
25). Then, average elevations for tidal flooding
sectional area, channel roughness and drainage
across the Flats were estimated from the surveyed
density, which limit the volume of water that can
transects (Fig. 7). These data reveal that ponds
escape from the pond. This produces a bottleneck
will begin to flood at a tidal height of 4.6 m, that
where water in the pond remains dammed de-
mudflats are inundated at 4.9 m and that levees
spite turbulent, fast flowing conditions in the gully
are covered at 5.2 m (Fig. 26). These elevations
heads. The response is a nonlinear relationship
also yielded a linear relationship between the pe-
between pond elevation and runoff time (Fig. 28).
riod of flooding (i.e., time when water elevation
Pond sedimentation results mainly from set-
was greater than 4.6 m) and peak water elevation
tling-out of sediment suspended in the water col-
(Fig. 27). How often each landform is flooded in a
umn (Table 5); sediments are presumably depos-
given period can be estimated using these eleva-
ited during and immediately following slack high
tions. The length of time available for sedimenta-
tide (Fig. 22). Ponds commonly did not completely
tion during tidal inundation is directly related to
drain between consecutive tides during a high
25
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