Hydrogeomorphic approach to assessing wetland functions developed
under Corps' research program
by R. Daniel Smith, U.S. Army Engineer
Waterways Experiment Station
Scientists at the U.S. Army Engineer Waterways Experiment
Station are developing a procedure for assessing the functions of
wetlands under the Wetlands Research Program. The procedure is
intended primarily for U.S. Army Corps of Engineer use and
measures the ability of wetlands to perform specific functions.
The procedure supports the Corps' Regulatory Program mandated by
Section 404 of the Clean Water Act and can be used
- to compare project alternatives
- to compare pre and post project conditions for determining
impacts or mitigation success
- to provide guidance for avoiding and minimizing project
impacts, and
- to determine mitigation requirements.
The modular and hierarchical format of the procedure will make
it easily adaptable to a variety of planning, management,
educational, and other regulatory situations that involve the
assessment of wetland function.
Wetland functions introduced
Wetland functions are the actions that are naturally
performed by wetland ecosystems, or simply, the things that
wetlands do. Wetland functions are a result of the interaction
between the structural components of wetlands--such as soil,
detritus, plants, and animals--and the physical, chemical, and
biological processes that occur in wetlands. A process
is a sequence of steps leading to a specific end. For
example, the microbially mediated process of denitrification
occurs in many wetlands, and leads to the relatively simple
wetland function of nitrogen removal (Fig. 1
(25K)). More complex functions resulting from the interaction of structural
components and multiple processes can also be identified. For example, the
physical processes of overbank flooding, reduction of water
velocity, and the settling of suspended particulates interact
with physical structures and result in the wetland function of
particulate retention.
Hydrogeomorphic classification introduced
Wetland ecosystems in the United States occur under a wide
range of climatic, geologic, geomorphic, and hydrologic
conditions. This diversity of conditions makes the task of
assessing wetland functions difficult, because not all wetlands
perform functions in the same manner, or to the same degree, if
at all. Therefore, to simplify the assessment process, it is
useful to classify wetlands into groups that function similarly.
Classification narrows the focus of attention (1) to the
functions a particular type of wetland is most likely to perform,
and (2) the characteristics of the ecosystem and landscape that
control these functions. The benefits of classification are a
faster and more accurate assessment procedure.
The assessment procedure being developed uses a
hydrogeomorphic classification to group
wetlands on the basis of three fundamental characteristics: geomorphic
setting, water source, and hydrodynamics. At the highest level of the
classification, wetlands fall into one of five basic
hydrogeomorphic classes including: depression, slope-flat,
riverine, fringe, and extensive peatland.
The classification can applied at a regional level to narrow
the focus even further. The regions identified by Omernik
(1987), Bailey (1994), or Bailey et al. (1994) and based on
climatic, geologic, physiographic, and other criteria provide a
convenient starting point for applying the classification Within
a region, any number of regional hydrogeomorphic wetland
subclasses can be identified based on landscape scale factors
such as geomorphic setting, water source, soil type, and
vegetation. The number of regional subclasses identified depends
on the diversity of condition in a region, and assessment
objectives. Experience shows that regional subclasses provide a
scale at which assessment efficiency and accuracy can be
maximized in the context of the Corps' 404 Regulatory Program.
Assessment procedure introduced
The assessment procedure is unique in that it utilizes the
concepts of hydrogeomorphic classification, functional capacity,
reference domain, and reference wetlands. These will be
discussed in the context of the three phases of the procedure
which include: characterization, assessment, and application.
The characterization phase includes the following steps:
- Definition of assessment objectives
- Characterization of the proposed project, the wetland
ecosystem, and landscape context
- Screening for "red flag" features, and
- Identifying wetland assessment area(s) within the project
area on the basis of hydrogeomorphic classification,
physical separation, and potential project impacts.
The assessment phase of the procedure provides a measure the
ability of a wetland to perform functions. Wetland functions are
measured in terms of functional capacity. The concept of
functional capacity is based on two assumptions. First, the
inherent capacity of a wetland to perform a function is dictated
by the structural components, and the physical, chemical, and
biological processes of the wetland. Second, the functional
capacity of a wetland (the level at which a function is actually
performed), is determined, to a greater or lesser degree
depending on the function under consideration, by interactions
between the wetland and the surrounding environment.
An analogy to explain functional capacity
An analogy is useful to explain the concept of functional
capacity. Consider a water pump and its function of moving
water. Assume the water pump is designed to move 100 gpm, and
its inherent capacity is 100 gpm. However, the functional
capacity of the water pump--or the rate at which it actually
moves water--depends not only on its inherent capacity to move
water, but also the context in which the water pump occurs. If
the water pump is attached to a hose that delivers 100 gpm, the
functional capacity of the water pump is 100 gpm, the same as its
inherent capacity. However, if the water pump is attached to a
hose that is capable of delivering only 50 gpm, the functional
capacity of the water pump is 50 gpm, though its inherent
capacity is still 100 gpm. Like most analogies, this one is
oversimplified and imperfect. While the inherent capacity of a
water pump is static, the inherent capacity of an ecosystems is
dynamic, and can change over time. For example, the inherent
capacity of a wetland to provide habitat may change as plant
succession takes place.
The concepts of inherent and functional capacity can be
applied to wetland ecosystems and the functions they perform.
For example, consider the floodwater storage function performed
by some wetlands (Fig 2. (127K)). The
inherent capacity of a riverine wetland to store overbank floodwater
depends on characteristics of the wetland's storage capacity by volume
to store floodwater (how big is the bucket?). However, the functional
capacity, or actual amount of floodwater stored in the wetland depends
on the ability of the watershed to generate overbank floods. This
ability is dictated by watershed characteristics such as the size
of the watershed, the intensity and duration of precipitation in
the region, runoff coefficients of the watershed, and the
location of control points in the stream above and below the
wetland. A wetland with a potential inherent capacity to store
100,000 gallons, based on peak flood elevation during the average
annual peak flood, could have an actual functional capacity
ranging from 100,000 to 0 gallons depending on the conditions in
the watershed of that wetland.
Functional capacity index introduced
The functional capacity of a wetland is determined using a
functional capacity index (FCI) An FCI is a ratio of the
functional capacity of a wetland under an existing, or predicted,
condition, and the functional capacity of a wetland under
attainable conditions. Attainable conditions are by
definition the conditions under which the highest, sustainable
level of functional capacity is attained across the suite
of functions that wetlands in a reference domain naturally
perform. The reference domain is simply the group
of wetlands for which a functional capacity index is developed.
The reference domain will normally be a regional hydrogeomorphic
subclass. However, depending on assessment objectives, it could
be composed of a larger or smaller number of subclasses and
geographic extent. For example, if the assessment objective is
to compare a subclass of wetlands in the watershed, the reference
domain would include all wetlands in the subclass in the
watershed.
Attainable condition, or the highest, sustainable level of
functional capacity is presumed to occur in wetland
ecosystems and landscapes that have not been subject to
anthropogenic disturbance with long term effects. When
undisturbed wetlands and landscapes do not exist or cannot be
reconstructed from historical data, attainable condition
is presumed to exist in the wetland ecosystems and
environments that have been subject to the least amount of
anthropogenic disturbance.
Functional capacity indices are based on an assessment
model that defines the relationship between the ecosystem and
landscape scale variables and functional capacity. The condition
of a variable is measured directly, or indirectly using
indicators (observable characteristics that correspond to
specific variable conditions). Variables are assigned an index,
ranging from 0.0 - 1.0, based on the relationship between
variable condition and functional capacity in the reference
domain. This relationship is established using reference
wetlands. A reference wetland set is a group of
wetlands that represent the range of conditions that exist in
wetland ecosystems and their landscapes in the reference domain.
The range of conditions represented include those resulting from
natural processes (succession, channel migration, erosion and
sedimentation) and anthropogenic disturbance.
Reference wetlands and their environments serve as the basis
for scaling and calibrating variables in assessment models. The
relationship between variable condition and functional capacity
in the reference domain is established using either empirical
data, expert opinion, best professional judgment, or combination
of these options. The relationship is formalized by using
logical rules or equations to derive an FCI ranging from 0.0 -
1.0. An FCI of 1.0 corresponds to the level of functional
capacity that exists under attainable conditions for the
reference domain. An FCI of 0.0 reflects the absence of
functional capacity.
Functional capacity units introduced
The functional capacity index provides a measure of the
ability of a wetland to perform a function relative to similar
wetlands in the region. Theoretically, the functional capacity
index represents an estimate of an absolute quantification of
function on a per unit area and time basis. For example, a FCI
of 1.0 for the flood storage function represents the absolute
number of cubic feet of water that are stored, in a specified
wetland area over a specified period of time, under attainable
conditions in the reference domain. The actual number cubic feet
of water, with estimates of uncertainty, could be determined
empirically for the same specified area of wetland and the same
specified period of time.
In the 404 Regulatory Program, the primary application a FCI
is to compare different wetland areas such as project
alternatives, or pre/post condition. However, comparing two
wetland areas on the basis of a functional capacity index alone
can lead to erroneous conclusions. For example, consider the
following scenario. A new highway is being planned, and there
are two alternative routes under consideration. The first route
will impact 5 acres of wetland with a FCI of 0.8 for a particular
wetland function. The second route will impact 25 acres of
wetland, also with a FCI of 0.8 for the same function. In
comparing the two alternatives based on functional capacity it
would be correct to say that on a per unit area basis there was
no difference between the alternatives. However, when
incorporating the size of each wetland area into the comparison a
conclusion of no difference would be erroneous. The comparison
of the two alternatives based on the functional capacity index
and size of wetland would lead to a more appropriate conclusion
that the first alternative is the least damaging to the selected
wetland function.
The functional capacity indices resulting from the assessment
phase can be applied in a variety of ways during the application
phase using functional capacity units (FCUs). Functional
capacity units provide a measure of the ability of a wetland area
to perform a function, and are calculated by multiplying a
functional capacity index by the area of wetland the FCI
represents. For example:
FCU = FCI x size of wetland area
where:
FCU = Functional capacity units for wetland area
FCI = Functional capacity index for wetland area
Once the functional capacity of a wetland area is expressed in
terms of FCUs, a number of the comparison necessary in the 404
permit review process can be made. For example:
- Comparing the same wetland area at different points in time
(e.g., pre/post project conditions).
- Comparing WAAs in the same hydrogeomorphic wetland class at
the same point in time.
- Comparing WAAs in different hydrogeomorphic wetland classes at
the same point in time.
Implementation of the assessment procedure
The assessment procedure described in this article is being
published as a WRP technical report. A variety of assessment
models are being developed for use with the assessment procedure
and will be published as WRP technical reports. These will be
published in the form of guidebooks for each hydrogeomorphic
class, and case studies of regional hydrogeomorphic subclasses.
The guidebooks will:
- provide a general description of the hydrogeomorphic class
- identify and provide rationale for the functions wetlands in
that class are likely to perform
- provide generic assessment models and functional capacity
indices; discussion of variables, variable conditions, their
relationship to functional capacity, and the scaling of variable
and functional capacity indices, using reference wetlands. Case
studies represent the application of the hydrogeomorphic approach
to a regional subclass. Case studies scale assessment models and
FCIs to the regional hydrogeomorphic subclass, using reference
wetlands. Case studies are being conducted for a variety of
regional hydrogeomorphic subclasses, and will provide a template
for applying the hydrogeomorphic approach to additional regional
subclasses.
More information is available from Dan Smith at (601) 634-2718.
References:
Bailey, R. G. 1994. Ecoregions of the United States (map),
revised edition. US Forest Service, Washington, D. C. Scale
1:7,500,000, colored.
Bailey, R. G., P.E. Avers, T. King, and W. H. McNab
(compilers/editors). 1994. Ecoregions and Subregions of the
United States. US Forest Service
Omernik, J. 1987. Ecoregions of the Coterminous United
States.Annals of the Association of American Geographers 77:
118-125.
back to contents