Environmental Engineering Reference
In-Depth Information
representative transect of the marsh. Sediment cores (15 cm in diameter,
50 cm in
length) are taken along the transect with a thin wall cylinder, with care taken to
prevent compaction of the cores. The retrieved core is sectioned, dried, and 137Cs
activity is counted. Sediment accretion is measured by counting the 137Cs activity
relative to distance down into the core. Bulk density, percent C, and percent organic
matter should also be determined by depth in each core to allow for calculations
relative to total mass, C, or organic matter.
7.10.4 SETS
Sediment erosion tables (SETS) measure net changes in sediment surface elevation
relative to a benchmark (Cahoon et al.
2002a
). A SET can be used to determine
either the impact of a single meteorological event on sediment surface elevation or
a long-term trend in elevation change. SETS consist of a supporting base pipe
permanently placed at each site and a portable portion with four components:
horizontal arm, vertical arm, flat table or plate, and pins. The base pipe, an
aluminum pipe that is driven into the soil to the point of refusal, serves as a
benchmark. The table provides a horizontal reference plane. The pins pass through
the table at right angles until they touch the marsh surface. Marsh surface elevation
relative to the benchmark is determined by measuring the length of pin above the
table. Changes in the elevation of the marsh surface are indicated by changes in
the distance between the marsh surface and the table. An integrated measure of
elevation (accretion minus subsidence) can be calculated by combining data
obtained from SETs with that provided by feldspar marker horizons.
SET stations should be located where the sediment surface is plainly visible and
not obscured by vegetation or deep water. Data should be collected at times of
lowest water level. Efforts should be made to minimize disturbance from foot traffic
during SET installation and data collection. One option is to construct platforms with
removable planks. The planks should be removed each time you exit the site. Data
collection frequency should be based on expected sedimentation rates at the site. In
areas with high sediment fluxes, a frequent collection schedule will produce a higher
resolution of elevation changes over time. In areas with low sediment fluxes, a
similar collection schedule may produce results within the range of methodological
error so that actual changes in elevation will not be detected. The conservative
approach is for each practitioner to construct a data collection schedule based on
preliminary work onsite. For further details on SETS, including design specifics, the
reader is referred to Cahoon et al. (
2002a
,
b
) and Callaway et al. (
2001
).
7.10.5 Sediment Collection Tiles
Sediment collection tiles provide a simple and inexpensive approach to assessing
deposition in both tidal and non-tidal wetlands. Sediment is deposited on the upper
