The results of this analysis should be taken as indicative of what is possible. It should
be noted that the results presented in this chapter are “typical” and are not expected to
reflect the best possible fit with observed values. There are a number of deficiencies
with the approach which are currently under active review. At the moment
comparisons between the model and long term observations appear to indicate that the
model over-predicts at low elevation and under-predicts at high elevations.
Discussion
The Effect of Saltmarshes on Sediment Deposition
241
Sediment trapping by vegetation
Sediment accumulation by different species is based on a series of short term (single
tide) observations. Filter papers are fixed horizontally within the different types of
vegetation before the tide approaches. After the tide recedes the filter papers are
collected, dried and reweighed to estimate the amount of sediment trapped.
Environmental data around each filter paper, such as the elevation and distance to the
nearest creek, is measured as well as the sediment concentration on the incoming tide.
Results
Fig. 5 shows the predicted accumulation of sediment for 1995 using a sediment
concentration in the water of
and a sediment density of
Users of the
model can select and alter these Fig.s according to their own knowledge and
experience. The average accumulation rates are relatively low, averaging only about
three millimetres per year over the vegetated part of the marsh. This still represents a
significant volume of sediment (roughly 48 thousand cubic meters). Note that the Fig.s
for accumulation are rather lower than the predicted rate of sea-level rise, which by
some estimates (Warick and Oerlemans, 1990) may exceed 10 mm/year over the next
hundred years.
Rigorous validation of the model is still being carried out, but, initial results
can be compared to rates reported in Steers (1960) along three transects at Scolt Head
Island. Accretion was measured three times over a twenty two year period.
Unfortunately, there is no information about the vegetation and only the only elevation
data is for the first station on each transect. Rates of accretion varied from zero to 12
mm/year. This was measured in relation to patches of sand spread on the marsh
surface. Current practice is to use fine white clay that may more safely be assumed to
be neutrally buoyant in the surrounding sediment. Little systematic variation can be
seen along individual transects or over time. Fig. 6 shows the data from the lowest
transect, Missel Marsh, for the 17 points (out of the original 37) which could be found
in all years. Whether the missing sites represent erosion or other changes is unknown,
so that conclusions about the net accretion over the whole marsh are difficult to draw.
Fig. 7 shows the data from all three transects plotted against the elevation of the first
station on each transect it demonstrates how relatively minor differences in elevation
can have a major influence on the rate of accretion.
be noted that the results presented in this chapter are “typical” and are not expected to
reflect the best possible fit with observed values. There are a number of deficiencies
with the approach which are currently under active review. At the moment
comparisons between the model and long term observations appear to indicate that the
model over-predicts at low elevation and under-predicts at high elevations.
Discussion
The Effect of Saltmarshes on Sediment Deposition
241
Sediment trapping by vegetation
Sediment accumulation by different species is based on a series of short term (single
tide) observations. Filter papers are fixed horizontally within the different types of
vegetation before the tide approaches. After the tide recedes the filter papers are
collected, dried and reweighed to estimate the amount of sediment trapped.
Environmental data around each filter paper, such as the elevation and distance to the
nearest creek, is measured as well as the sediment concentration on the incoming tide.
Results
Fig. 5 shows the predicted accumulation of sediment for 1995 using a sediment
concentration in the water of
and a sediment density of
Users of the
model can select and alter these Fig.s according to their own knowledge and
experience. The average accumulation rates are relatively low, averaging only about
three millimetres per year over the vegetated part of the marsh. This still represents a
significant volume of sediment (roughly 48 thousand cubic meters). Note that the Fig.s
for accumulation are rather lower than the predicted rate of sea-level rise, which by
some estimates (Warick and Oerlemans, 1990) may exceed 10 mm/year over the next
hundred years.
Rigorous validation of the model is still being carried out, but, initial results
can be compared to rates reported in Steers (1960) along three transects at Scolt Head
Island. Accretion was measured three times over a twenty two year period.
Unfortunately, there is no information about the vegetation and only the only elevation
data is for the first station on each transect. Rates of accretion varied from zero to 12
mm/year. This was measured in relation to patches of sand spread on the marsh
surface. Current practice is to use fine white clay that may more safely be assumed to
be neutrally buoyant in the surrounding sediment. Little systematic variation can be
seen along individual transects or over time. Fig. 6 shows the data from the lowest
transect, Missel Marsh, for the 17 points (out of the original 37) which could be found
in all years. Whether the missing sites represent erosion or other changes is unknown,
so that conclusions about the net accretion over the whole marsh are difficult to draw.
Fig. 7 shows the data from all three transects plotted against the elevation of the first
station on each transect it demonstrates how relatively minor differences in elevation
can have a major influence on the rate of accretion.