FLOODING and its effect on TREES

Forestry Strategies To Protect Floodplain Agricultural Systems

Frank Hershey, Douglas Wallace and John Dwyer

Watershed Specialist, USDA Forest Service, Columbia, Missouri; Forester, USDA Soil Conservation Service, Columbia, Missouri; Associate Professor, The School of Natural Resources, University of Missouri, Columbia, Missouri.

Presented at the Restoration of Aquatic Ecosystems symposium, The Association of State Wetland Managers, St. Paul, Minnesota, June 20-23, 1994.

Introduction

Damages to agricultural lands and associated infrastructure from the record floods of 1993 in Missouri were staggering. Floodplain croplands, some of the most valuable and productive in the state, were impacted by debris accumulation, sediment deposition and scour erosion. Cropland damaged by sand and scour for just the Missouri portion of the Missouri River was estimated at 455,000 acres, with an estimated $500 million to reclaim sand damaged land (Soil Conservation Service, 1993). Additional millions of dollars will be required to rebuild levee breaks and restore fertility to flood-damaged cropland. In some instances costs for recovering the land exceeded its market value as cropland.

Questions were raised about the future crop production capabilities of the damaged land. Programs activated to assist with the clean-up and repair of damaged farmlands in the floodplains and the associated agricultural infrastructure were designed to make repairs to restore pre-flood conditions. Mixed with the damages were some excellent examples of floodplain use and management that could serve to guide changes in the way floodplain agriculture is conducted to avoid or minimize future severe flood damages. The strategic use of trees in floodplain agriculture is the key to accomplishing the desired changes and realizing the benefits.

Role of Trees In Floodplains

Historically, trees performed some important functions and their presence in the river bottomlands significantly influenced the floodplain landscapes farmed today. Woody vegetation stabilized the soil and controlled scour erosion. Stands of trees absorbed the energy from floodwaters and caused the deposition of water borne sediments. Floodplain forests stored the overflow waters and drove many of the processes to support aquatic life systems and improve water quality.

Following the 1993 floods it was apparent that there were not enough trees in the floodplain. Extensive damages to floodplain cropland and the associated agricultural infrastructure were preventable with the strategic placement of trees and with more effective management of opportunities offered by natural stands. Major forms of damage that can be addressed by manipulation of tree resources are debris accumulation, scour erosion, and sand deposition. Also, woody vegetation appears to be under-utilized and discriminated against as a biological system of levee protection.

Brumfield (1993) described the influence of strategic cottonwood plantings on debris trapping and sand deposition in the Thompson Bend high flow channel of the Mississippi River in Mississippi County Missouri. Satterlund (1972) discussed the use of Manning's equation in hydraulics to express velocity. There is a ten-fold increase in the roughness coefficient factor of Manning's equation between flow over a packed clay surface and a surface composed of dense shrubs and forest litter. Woody vegetation on floodplains causes significant reductions in flow velocity causing the deposition of suspended particles and trapping debris.

Scour erosion is controlled by the dense mat of intertwined, fibrous roots that reinforce the top foot of soil in the forest floor. Perry (1989) reported that trees develop root systems that extend horizontal distances of up to 2 times tree height. Any one square foot of soil on the forest floor will contain the intermingled roots of 7 or 8 different trees.

During flood events, levee systems frequently function as stream banks. Shields and Gray (1993) reported positive effects of woody vegetation on levees in California supporting observations in Missouri during and after the 1993 floods that trees could be excellent levee protection.

Levee Maintenance

Scheifele (1928) installed and documented the effectiveness of numerous levee and streambank protection plantings utilizing woody vegetation, many in the Midwest. He also discussed the concerns of engineers about woody vegetation on constructed embankments. Three major concerns are the basis for levee maintenance standards that specify that no woody vegetation be allowed on levee embankments.

  1. Large trees will windthrow from saturated levees, removing a large soil mass and creating a breach point in the levee.

  2. Large tree roots will extend through the levee and cause piping during floods. This is primarily a concern when large old trees die.

  3. Woody vegetation attracts burrowing wild animals to the levee embankment and their activities create breach points in the levee.
These concerns must be addressed and their validity documented or disproved before recommendations to armor levees with woody vegetation can be successfully advanced. Bottomland hardwood ecosystems in the floodplains of Midwestern streams have been converted to agri~ecosystems. These converted systems lack stability and are more susceptible to environmental damage (Bratton, 1993). Restoring trees to these systems can add stability, increase diversity and supplement the agriculture systems that will continue to occur on floodplains.

Agroforestry Systems

Trees that work for agriculture are called agroforestry systems (U.S. Forest Service, undated). Some agroforestry systems with specific application to floodplains include windbreaks to stabilize sandy soils, filter strips and riparian areas for bank stabilization and water quality, alley cropping for enhanced crop production and protection, wildlife habitat, woodlots and fuelwood plantations. Agroforestry systems are installed for several objectives including profit, productivity enhancement, energy conservation, natural resource conservation, environmental diversification and modification and to enhance the environment for people.

Hershey and Wallace, (1993) found that waterbreaks of trees planted perpendicular to the flow of high energy flood waters were economical based solely on the reduction of damages to crops, assuming floods of 10 year frequency. Tree species adapted to the fioodplains include species valued for their lumber, and pecan, valued for the nut crop. Agroforestry systems designed specifically for the floodplain are needed to develop and analyze all of the possible alternatives of maintaining productive agriculture while increasing environmental stability and protecting the agricultural infrastructure of the floodplains.

Floodplain Agroforestry Systems

The system designed for Thompson Bend is an example ofthe potential benefits of this system. The Thompson Bend plantings trapped debris so efficiently that mountains of debris rode the trees down, submerging their tops in the 1993 floodwaters and killing sections of the plantings. However, the tree rows still functioned to hold the debris and deposit sand within the confines of the dead tree rows and leaving the crop land clear and unscoured for 1994 planting.

Conclusions

Agriculture will always have a role in the fertile floodplains of Midwest streams. Agroforestry systems are one means of creating a stable balance between human needs and natural forces.

References

Bratton, G. 1993. Agroforestry practices for riparian areas. Plenary paper presented at Management of Riparian Forests Workshop. Missouri Society of American Foresters, Kansas City, MO. June 21-25 1993.

Brumfield, W.H. 1993. The trees at Thompson Bend. Presentation at Floodplain Forests Program, Fall Meeting, Missouri Society of American Foresters, Hannibal, MO. October 1, 1993.

Hershey, F.A. and D. Wallace. 1993. Tree planting on flood damaged farmland. Unpublished paper. U.S. Department of Agriculture, Soil Conservation Service, Columbia, MO: 19p.

Perry, T.O. 1989. Tree roots: facts and fallacies. Arnoldia 49(4):3-24.

Scheifele, 0.5. 1928. Protection of river banks and levees. Reprinted from the Canadian Engineer, January 10, 1928.

Shields, F.D. and D.H. Gray. 1993. Effects of woody vegetation on sandy levee integrity. Water Resources Bulletin 28:45 917-931.

Soil Conservation Service. 1993. Impacts of the 1993 Flood on Missouri 5 Agricultural Land. United States Department of Agriculture, Soil Conservation Service, Columbia, MO.

United States Forest Service. undated. Agroforestry: working trees for agriculture. United States Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Center for Semiarid Agroforestry, Lincoln, NE: 6p.

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