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CAUSES AND EFFECTS OF VALLEY
PLUGS IN WEST TENNESSEE

Timothy H. Diehl

Cite this report as: Diehl, T.H., 1994, Causes and effects of valley plugs in West Tennessee, in Sale, M.J., and Wadlington, R.O., eds., Symposium on Responses to Changing Multiple-Use Demands; New Directions for Water Resources Planning and Management, Nashville, Tenn., April 17-20, 1994, Proceedings of extended abstracts: American Water Resources Association, p. 97-100.

Introduction

Valley plugs, also called channel blockages, are common along low-gradient alluvial streams in West Tennessee. This paper presents observations on the causes and effects of selected valley plugs in West Tennessee, and discusses some human responses to valley plugs. These observations and discussions are based on a field reconnaissance of valley plugs in several West Tennessee streams during 1992 and 1993.

Valley plugs selected for the reconnaissance included four in the Obion-Forked Deer basin that previously had been mapped ( U.S. Army Corps of Engineers, 1982 ; Obion-Forked Deer Basin Authority, 1983 ). These sites are on the South Fork Obion River near Jarrell; on Stokes Creek where it enters the flood plain of the North Fork Forked Deer River near Tigrett; and on the Middle Fork Forked Deer River near Spring Valley and near the village of Law. Other valley plugs selected for reconnaissance are on the Tuscumbia River near Chewalla; on the Wolf River near LaGrange; on Nash Creek where it enters the flood plain of the North Fork Forked Deer River near Tigrett; on Bear Creek near Brownsville; and on Crooked Creek near Huntington. These sites were selected on the basis of verbal reports from local residents and an examination of topographic maps. Several other sites that seemed to have significant potential to develop valley plugs were also included in the reconnaissance. These included reaches of the Hatchie River near the mouths of Cub Creek, Porters Creek, and Piney Creek.

The sites selected for reconnaissance included sites with drainage areas as large as 1,400 square miles and channels as wide as 100 feet. The sites included nine sites where the main channels were blocked and three where drainage canals had been reopened through valley plugs.

The physical features of valley plugs and the principles of their formation were defined by Stafford Happ ( Happ and others, 1940 ; Vanoni, 1975 ). The basins studied in greatest detail by Happ are geologically and geomorphically similar to many in West Tennessee.

Causes of valley plugs

Valley plugs typically form at sites where the sediment transport capacity of the stream decreases in the downstream direction to the point that the stream cannot carry the sediment load. In reaches of uniform slope, plugs typically form where a channelized or high-gradient tributary delivers more sediment than the larger stream can transport, or where driftwood accumulates and causes a local reduction in flow velocity. Plugs also tend to form at points where channel slope decreases, such as at the downstream end of a channelized reach where there is an abrupt decrease in channel slope, or where a tributary enters the nearly level flood plain of a larger stream.

Regardless of the location of the plug, channel aggradation and driftwood accumulation typically combine to block the original channel completely. When this occurs, flow shifts abruptly into a single alternative channel or multiple distributary channels, or is dispersed through backswamps. As a result of these changes in flow pattern, sediment transport through the valley plug is greatly reduced, and sedimentation is accelerated throughout the plug. The channel blockage tends to grow in the upstream direction by trapping additional driftwood and sediment.

Effects of valley plugs

Valley plugs produce hydrologic and vegetational changes over extensive areas of valley bottom. They typically increase the depth and area of seasonal flooding and the area of semipermanent inundation. Decreased flow velocity and increased filtering by vegetation promote accelerated sedimentation. These physical changes tend to promote the development of open-water communities, marshes, shrub communities, in place of bottomland-hardwood swamps and crop species. Eventually, marshes and shrub communities may be replaced by bottomland-hardwood swamp through a combination of accelerated sedimentation and vegetational succession.

The reach of a stream downstream from a valley plug can be affected in several ways. Downstream flood peaks are typically delayed and attenuated by storage in the inundated area associated with the plug. The accelerated sedimentation in the plug results in reduced sediment concentrations downstream from the plug. The trapping of sand in the plug can protect downstream locations from channel aggradation and the formation of additional valley plugs. However, complete removal of sand can induce degradation and widening downstream, leading in turn to increased production of driftwood.

Human responses to valley plugs

Human responses to valley plugs commonly include one or more of the following elements:

I. Channel excavation

A channel can be excavated through the valley plug. The channel can be straight or sinuous; it can follow a new alignment, or represent the restoration or enlargement of a previously existing channel. This response provides the best likelihood for locally increasing the areas available for profitable agriculture and forestry. Where a valley plug has formed in a drainage canal, an excavated channel might need to be dredged periodically to maintain channel capacity unless the human response also contains elements that reduce the sediment influx.

If the channel is greatly modified, erosion, transport, and deposition of sediment will determine the future evolution of the valley, upstream and downstream as well as in the modified reach. Channel excavation can lead to upstream degradation and widening and increase the amount of sediment entering the excavated reach. An enlarged or straightened channel also can promote formation of a valley plug downstream.

Creation or restoration of a small or sinuous channel, as opposed to a straight channel of large capacity, is less likely to promote upstream erosion and downstream plug formation, but such channels are more vulnerable to repeated blockage. Where a sinuous channel is excavated through a valley plug, and the upstream channel has a higher gradient than the sinuous excavated channel, a plug is likely to form in the upstream end of the sinuous channel.

II. Removal of woody debris

Driftwood can be removed from the channel bed, and unstable trees can be removed from the banks. Wood removed from the main channel can be placed to block the entrances to alternative channels, concentrating the flow in a single main channel. Because of the strong tendency of driftwood accumulation to promote local aggradation and channel blockage, removal of woody debris can delay or prevent formation of some valley plugs. Wood removal typically involves less disturbance to the environment and lower cost than excavation, but also removes fish and invertebrate habitat.

III. Forced deposition of sediment in selected areas

The sediment load of a stream available for formation of valley plugs can be reduced by promoting sedimentation in areas where it will not cause a problem. Sediment deposition in upstream impoundments, sloughs, or on alluvial fans can reduce the potential for valley plug formation in downstream reaches.

IV. Erosion reduction

The sediment load of streams can also be reduced by reducing erosion in the basin. Excessive erosion from gullies and entrenched intermittent streams is the major source of the sand that forms most valley plugs in West Tennessee. Some basins that no longer have numerous active gullies still produce enough sand from entrenched channels to promote formation of valley plugs downstream. Nearly any downstream channel configuration can be protected from valley plug formation if upstream erosion is sufficiently reduced.

V. Adaptation to existing conditions

An alternative to responses involving intentionally altering the drainage system is to adjust to the hydraulic, geomorphic, and vegetational changes brought about by the valley plug. Institutional methods can be used to capture the benefits and ameliorate the damages associated with valley plugs. Public land and easement purchases, subsidies, and regulations can be used to reduce the need for construction projects in some instances.

The choice of the most effective response to a given valley plug will depend largely on the channel characteristics, hydrologic setting, sediment transport rates, rates of degradation and aggradation, and vegetation and land use in the basin. Where quantitative information is available on these factors, the likelihood of selecting an effective response is greatly enhanced.

References

U.S. Army Corps of Engineers, 1982, Draft Environmental Impact Statement for Stream Renovation Activities in the Obion and Forked Deer Rivers Basin of West Tennessee. Memphis District, U.S. Army Corps of Engineers, Memphis, Tennessee, 187 pp.

Happ, S.C., Rittenhouse, Gordon, and Dobson, G.C., 1940, Some Principles of Accelerated Stream and Valley Sedimentation. U.S. Department of Agriculture, Technical Bulletin No. 695, Washington, D.C., 133 pp.

Obion-Forked Deer Basin Authority, 1983, Comprehensive Development Plan, Obion-Forked Deer Basin Summary Report. Jackson, Tennessee, 41 pp.

Vanoni, V.A., editor, 1975, Sedimentation Engineering: American Society of Civil Engineers, New York, New York, 745 pp.


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