Environmental Engineering Reference
In-Depth Information
Table 2.2 Various products that might result from a bathymetric analysis
Mapping
dimensions
Products
Two dimensions
(X and Y)
Wetland perimeter and area (derived from ground surveys, aerial imagery,
topographic maps, vegetation, soils)
Total area
Reference elevation and elevation datum (benchmark/ground control)
Three dimensions
(X, Y, and Z)
Wetland bottom elevation (Z) at various X, Y locations
Wetland water level (stage) and water depth
Outflow elevation and potential surface connections with other wetlands
(outflow/inflow)
Wetland drainage basin boundary
wetlands map need to be placed in a larger geographical context with real world
coordinates (e.g., longitude and latitude)? If this is the case, then GPS technology
will play a role in the field, and you will use GIS tools and map projections to ensure
the data are accurately georeferenced to real world coordinates. If the questions
about the wetlands include biophysical features (e.g., vegetation patterns, geologic
features, or evidence of animal activity) associated with the bathymetry, the survey
work might need to map those features too. Successful characterization of the
bathymetry will be guided by clearly defining the products that will be needed for
conducting the bathymetric analysis (Table
2.2
). The products desired will dictate
which mapping dimensions will be needed and what type of analyses will be
conducted. Finally, the available resources and budget will ultimately constrain
what can be accomplished. In general, a well-defined question will result in efficient
use of field and analysis time.
The goals of the wetland survey will dictate the boundaries, number of survey
points, and needed resolution. The boundaries of interest may be defined legally
(i.e., a jurisdictional wetland) or may include a larger context (i.e., the watershed
contribution). In either case, the mapped wetland should include enough area
outside the wetted area to avoid interpolation inaccuracies near the defined bound-
ary of interest. “Resolution” is a broad term, generally indicating the smallest
physical feature visible in the data set. Resolution is a function of survey point
spacing, with higher resolution achieved by closer spacing of elevation data. Your
choice of resolution will depend in part on the sources of variation in the wetland
itself and how much of that variation needs to be captured, the number of sampling
points you can afford to survey, and the precision of individual survey shots. For
example, Haag et al. (
2005
) found that bathymetric data containing a high density
of data points provided the most useful stage-area and stage volume-relations
characterizing isolated marsh and cypress wetlands in Florida. Moreover, bathy-
metric maps generated from a low density of data points underestimated by
50-100 % the wetland area and volume over certain ranges of stages compared to
maps generated by a high density of data points. This emphasizes the importance of
collecting data from an appropriate number of data points when determining
wetland bathymetry. From a pragmatic perspective, the size of the wetland feature,
