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

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.