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North Dakota Water Resources Research Institute
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A
Review of Models for Investigating the Influence of Wetlands on Flooding November
1999 Melanie
L. Bengtson, Lecturer and G. Padmanabhan, Professor Department
of Civil Engineering and Construction North
Dakota State University The
aftermath of the 1997 Red River of the North flood led to questions regarding
the value of wetlands for flood control. In
order to assess the impact of wetlands on flooding, the International Joint
Commission, the North Dakota Water Commission and the Minnesota Department of
Natural Resources funded a study to model the influence of wetlands more
accurately. The purpose of this
study was to assess existing hydrologic models for their usefulness in
simulating wetland hydrology on a river basin scale, review previous studies of
simulation of wetland hydrology, and recommend a modeling approach applicable to
assessing the effectiveness of wetlands in flood control. It
was determined that the desirable model would be capable of modeling large
watersheds (greater than 1000 km2), and that modeling individual
flood events rather than continuous hydrologic modeling would be adequate for
studying floods. The model should
be able to incorporate the spatial distribution of wetlands and their storage
capacities. The model must also be
capable of modeling rainfall or snowmelt, overland runoff, and streamflow
routing. Reviewing
the literature for studies of hydrologic modeling of wetlands revealed that
generally most models used for this purpose were continuous water-balance
models, applied to single wetlands or a single wetland complex.
Most modeling efforts were not applied to flood events.
The few studies concerning flood control generally showed that wetlands
provided some mitigation for high frequency flood events, but little to no
mitigation for low frequency flood events. Based
on the literature review and desirable model characteristics, the HEC-1, PRMS,
HSPF, and AGNPS hydrologic models were evaluated.
It was decided that HEC-1 was the best choice due to its ability to model
flood events, incorporate wetland storage (either as reservoirs or diversions),
reflect spatial variation in wetland location by subdividing the watershed into
subwatersheds, and the variety of overland and streamflow routing methods
available in the model. It was determined continuous models such as PRMS and HSPF
would be too difficult to calibrate with the existing data, and these models
lack overland and streamflow routing methods that would be most suitable for the
relatively flat slopes in the Red River Valley. A
Hydrologic Model for Assessing the Influence of Wetlands on Flood Hydrographs in
the Red River Basin November
1999 Melanie
L. Bengtson, Lecturer and G. Padmanabhan, Professor Department
of Civil Engineering and Construction North
Dakota State University This
report describes the development and application of a hydrologic model for
investigating the integrated effect of wetlands on flooding over a watershed
scale. The HEC-1 model was
chosen for its capability to: 1) simulate major hydrologic processes during a
flood event, 2) subdivide the watershed to reflect spatial variations in
parameters, 3) simulate the attenuation of flood flows due to wetlands, 4)
explicitly simulate flood storage available in wetlands with a variety of
initial water levels, 5) simulate watersheds with drainage areas exceeding 1000
mi2, and 6) interface with GISs and DEMs for preparation of input
data and post-processing of the results. The
Maple River watershed was modeled. After
the watershed was subdivided in ArcView using GIS coverages including the rivers
and watershed boundaries, parameters such as hydrologic soil types, land uses,
stream lengths, and areas were determined for each subwatershed.
Various categories and surface areas of wetlands were identified in each
subwatershed. Based on the surface
areas, storage volumes were estimated for the drained wetlands in each
subwatershed. The estimated storage
provided by restoring these drained wetlands was modeled using diversions in the
HEC-1 model. As flow arrives at the
outlet of each subwatershed, a percentage is diverted from the system,
representing the runoff that might be held within wetlands.
This method has the advantage of not assuming that all runoff will be
intercepted by the restored wetlands. The
rate of diversion can be varied so that all storage is sure to be utilized.
The WMS (Watershed Management System) software was then utilized to build
the HEC-1 model and enter the necessary parameters.
Available DEMs were not of satisfactory quality to use WMS to
automatically generate subwatersheds and streams, but WMS was a useful tool to
quickly and easily assign parameters to the subwatersheds in the model.
The HEC-1 model was run within WMS, and output hydrographs were generated
by the WMS model. The
results indicated that restoring all drained wetlands (representing about 0.24%
of surface area in the Maple River watershed) would lower the flood stage for
the 100 year flood event by less than 0.4%) even when the wetlands are modeled
as empty before the flood event begins. Increasing
the percent of restored wetlands to 1% by watershed area (4 times the present
area) reduced the 100-year flood stage by 0.9%.
Therefore it does not appear that the volume of storage gained by
restoring the drained wetlands in the Maple River watershed would significantly
affect a low-frequency event such as the flood of 1997. |
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