Pond Beyond
MG
BT
Ea
N
g
35-50 years
Bread Truck
Pond
17-20 years
PC
25-30 years
C-Pond
Figure 56. Drainages within C/D Area
near permanent ponds with arrows show-
BG
ing locations of anticipated progressive ero-
sion. Where rates have been measured, es-
timated time to natural draining of ponds
is indicated (MGMortar Gully, BT
Bread Truck Gully, PCParachute Gully
0
500 m
and BGB Gully).
undercut the capping material and damage or
The two ponds most likely to be naturally
destroy its effectiveness.
drained by headward gully extension are Bread
Gullies are also routes for pond outflow and
Truck in an estimated 3550 years and C-Pond
therefore are potential pathways for migration of
in an estimated 1725 years (Fig. 56). Further
WP off-site. As the gullies continue headward
analyses of both modern and historical reces-
expansion into contaminated ponds and mudflats,
sion rates are required to determine if the 1992
WP-bearing sediment will be introduced into the
1994 data are representative of longer-term
water draining the ponds. As gullies encroach on
rates.
pond margins, erosion will probably increase run-
off, stimulating increased scour, and once reach-
Implications for
ing unvegetated pond bottoms, increase the rate
remediation methods
of gully expansion within them.
An understanding of gully recession enables
Dredging has also been proposed as a poten-
us to better assess the effectiveness of the pro-
tial method to remove WP from pond sediments.
posed remedial measures for WP contamina-
This method will, however, affect the physical
tion. In a positive way, headward recession will
properties of the dredged areas and will affect
eventually drain C-Pond, Bread Truck Pond and
hydrological conditions, including drainage. Gul-
Pond Beyond, thereby producing drying condi-
lies intercepting a dredged area may deepen and
tions that will favor a natural degradation of
extend rapidly through those areas because of the
WP (see Walsh et al. 1995). Gully erosion, how-
loss of vegetation and compaction of sediments.
ever, could threaten the integrity of capping
Extension could be rapid, and move large
measures (i.e., geotextiles and AquaBlock) if
amounts of material while destroying the ponds.
these are used in the wrong places. Erosion will
45
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