Environmental Engineering Reference
In-Depth Information
Fig. 2.24 A representation of the relationship between a wetland's water stage and a wetland's
volume. (a) Wetland ponds can be geometrically complex, so more than one equation is needed to
adequately model the volume to stage relation. The simplest wetlands will have a minimum of
three volume zones (bottom filling or point of zero volume [pzv], pond filling, and flooding or
point of incipient flooding [pif]) as they are analogous to a bathtub with a wide, relatively flat
bottom, steep sides, and overflow. (b) The bottom of Molera Wetland is relatively flat, and the
contour lines are far apart (refer to Fig.
2.23a,b
for a representation of its bathymetry), so an
increase in stage which inundates the lowest contours results in a very small increase in wetland
volume. Nearer to the perimeter, the wetland has relatively steep sides, and the contours are closer
together, so that an increase in stage results in a much larger increase in wetland volume
Table 2.3 The relationship
of water stage to wetland
area and volume in our
Molera Wetland example
Area (m
2
)
Volume (m
3
)
Water stage (m)
1.05
3.54
0.007
1.07
61.5
0.58
1.09
120
2.3
1.11
183
5.3
1.13
256
9.7
1.15
360
15.8
1.17
536
24.7
1.19
748
37.7
Wetland water volume and surface area can be determined at a range of water
levels. To calculate water volume and surface area, you will select 3D Analyst
Tools
to input your trimmed DEM
filename, indicate the water elevation for which you want the calculations, and then
indicate that the analysis is “below” the plane. You should be sure to create an
output file. This output text file stores the analysis results. We placed our file in a
folder called “stage_vol directory.” If wetland volume or surface area is calculated
for a range of stages, a visual graph and mathematical “rating” equation can be
created that relates stage to volume or area (Fig.
2.24
). Rating equations can be
linear, power, or polynomials, and different parts of the data set might require
different equations (Fig.
2.24
). It is important that you use good judgment in
limiting extrapolation beyond the data, given that wetland geomorphology can be
very complex as demonstrated by our example of how changes in stage can
dramatically change wetland area and volume (Table
2.3
). For our calculation of
Functional Surface
Surface Volume
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