4000
A Pond
In-Between Gully
B Gully
Bread Truck Pond
C Pond
3000
Mortar
Parachute
Racine Pond
+
Racine Gully
+
River
+
Coast
TSS
(mg/L)
2000
+
+
+
+
1000
Figure 44. Maximum monthly TSS
comparison at all sites. Dashed lines
0
connect data from pond sites.
4 May
3 Jun
3 Jul
2 Aug
1 Sep
1 Oct
31 Oct
6
1600
1200
5
TSS
(mg/L)
800
4
400
Figure 45. Typical TSS variation
3
0
through a flooding tide.
6 Oct `94
7 Oct
ment concentration during ebb reflects the loss of
pond storage capacity and the capacity of the gully
material by deposition on mudflats and in ponds,
to drain water from the ponds and mudflats. These
as well as the mixing of the tidal waters with
differences in sediment flux into and out of the
sediment-poor pond and river waters.
ponds and mudflats result in a spatial variability
in deposition, which is compounded by the effects
of localized differences in elevation and vegetation.
Landform sedimentation rates
Sedimentation rates vary with morphology and,
A typical pattern in TSS response, as measured
in a general sense, elevation (Table 10). The over-
at the heads of gullies through a flooding cycle, is
all trend is an increase in rate from levees to
a sharp increase because of the influx of highly
mudflats, to ponds, to marshes and to gully bot-
turbid flood water from Knik Arm and resus-
toms, with the more heavily vegetated areas of
pended gully bed sediment (Fig. 45). TSS values
mudflats having higher rates of accumulation than
remain high through slack high tide and the start
those that are not vegetated. Typical annual sedi-
of ebb. TSS then abruptly decreases during ebb
mentation ranges from about 10 to 30 mm among
runoff. The duration of ebb runoff depends on
the various landform types.
the ability of the gully or network of gullies to
Net accumulation is generally higher during
transfer water stored in the ponds and mudflats.
the "winter" 8-month period from September to
The greater the volume of water stored in an area,
May than during the "summer" 4-month period
the longer it will take for gullies to effectively
from May to September. This difference probably
drain the ponds. The decrease in suspended sedi-
53
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