The Use of Geomorphic Features in Low-Gradient, Lowered, and Artificial Drainage Networks

Leslie Zucker, Larry C. Brown, Brent Sohngen and Andy Ward,
The Ohio State University,
Dept. of Food, Agri. and Bio. Engineering, Columbus, OH;

Norman R. Fausey, Jay Dorsey, Eric Zwierschke, and Barry Allred,
USDA-ARS, Soil Drainage Research Unit,
Columbus, OH;

Dan Mecklenburg
Ohio Department of Natural Resources,
Division of Surface Water, Columbus, OH.

In the last several decades, dramatic advancements have been made in our knowledge of river mechanics, geomorpohology, hydraulics, and sedimentation. Despite the new knowledge, traditional engineering approaches are typically applied to such problems as flood control, navigation, irrigation, hydroelectric development, and municipal and industrial water needs. Alternative designs have been demonstrated that enhance natural tendencies for the channel to seek quasi-equilibrium between sediment and water, both at low flow and in flood (Leopold, 1996). One application of alternative engineering techniques that remains relatively unexplored is the management of low gradient, artificially drained streams on agricultural landscapes.

A large portion of the Great Lakes basin and extensive portions of the Midwest have received drainage improvement for agricultural production. Agricultural drainage is the removal of excess water from the soil surface and/or soil profile of cropland, by either gravity or artificial means. Agricultural drainage allows timely field operations, helps plant growth to begin early, and provides a well-aerated root environment that enhances plant uptake of nutrients. Subsurface drainage pipes are typically installed at a depth of 30 to 40 inches and often outlet to an open ditch or stream. To expedite stormwater removal, either as runoff or subsurface drainage, natural channels have been straightened and widened to increase the flow capacity of the channel and prevent overbank flooding. The channels are also deepened to allow water to flow freely from drainage pipe outlets and to promote runoff near the channel.

This traditional drainage channel design usually has a trapezoidal cross section (Figure 1a). Although it achieves the purpose of timely soil drainage, the trapezoidal channel design has a tendency to aggrade (Schwab 1993). As a result, drainage channel capacity and shape must be continually maintained at considerable cost to counties, soil and water conservation districts and landowners. The most common reasons for maintenance activities include brush growth in the channel, and sedimentation and submersion of subsurface drain outlets (Nolte 1972). The annual cost of drainage channel maintenance in western Ohio counties averaged $470 per mile from 1994-1996. Total maintenance costs exceeded $1.7 million annually over more than 3,000 miles of open ditches under county maintenance programs (Atherton 1999).

The artificial, modified state of traditional agricultural drainage channels contrasts sharply with natural channel morphology. Studies of fluvial geomorphology show that a river system will evolve toward a state of dynamic equilibrium following disturbance (Leopold 1994). Once this state of dynamic equilibrium is achieved, the river exhibits a stable dimension, pattern, and profile such that, over time, channel features are maintained and the stream system neither aggrades nor degrades (Rosgen 1996). This state of equilibrium is called "dynamic" because it might involve meandering of the channel, although stable channel dimensions are maintained. A modified channel that is not continually maintained will undergo a process of adjustment toward a stable state. As an example, this process might follow a pattern of aggradation, downcutting or incising, undercutting and then sloughing of the banks, and finally floodplain deposition resulting in a stable channel cross section. The readjustment process can involve significant erosion. Aggradation follows channel widening because flow velocities decrease near the channel bed. Eventually, the velocity gradient at the bank might increase as the stream tries to regain its meander geometry and shape. Channels that are over-deepened encourage bank erosion, as cross-channel movements tend to be reinforced (Brookes and Sear 1996). Modification and adjustment of channel variables often extends spatially upstream and downstream of a disturbed site, where impacts might not be intended. It is the evolutionary process of channel adjustment following modification that provokes channel maintenance activities. An alternative approach is to work with natural channel processes as much as possible, allowing the river to efficiently manage sediment, flow and morphology.

A team of scientists in Ohio proposes to research and demonstrate an alternative drainage channel design that incorporates naturalized fluvial features to enhance stream integrity and maintain or improve drainage capacity. Implementation and monitoring are planned under low-gradient conditions in an agricultural watershed. The naturalized drainage channel will feature width to depth ratios that accommodate bankfull stage comparable to that of a stable channel in the region. The bankfull stage is when the water fills the channel to the level of the floodplain and begins to overflow onto the floodplain, dissipating energy and depositing sediment. The bankfull stage corresponds to the discharge at which channel maintenance is most effective (Dunne and Leopold 1978). A "two-stage" design will contain bankfull flows within the first-stage channel, and floodwaters within a larger second-stage channel. The resulting design might be visualized as "a stream within a ditch" (Figure 1b). Naturalization of an existing incised channel would involve creation of a new low-level floodplain corridor and construction of a meandering channel within this corridor (Hey 1995).

Traditional drainage channel designs typically alter channel hydraulics and promote loss of instream and riparian habitats. Habitat modification, largely related to drainage improvement, is now the leading cause of aquatic life use impairment in Ohio (Ohio EPA 1998). Brush and tree removal deprive instream biota of important spawning habitats, food supply and cover and shelter (Schoof 1980). Reductions of habitat diversity, and in particular the elimination of pools adversely affects fish populations (Swales 1982). In comparison, a two-stage channel design might enhance instream habitat and the potential for biological integrity through the following mechanisms: 1) reduced flood peaks and critical low-flow periods that alter aquatic community composition and lessen chances of survival; 2) decreased suspended solids and fluctuations in dissolved oxygen and ion concentrations that impact species survival and community composition; 3) restored pool and riffle habitats; 4) creation of a number of microhabitats, niches and substrate diversities; and 5) reduced substrate embeddedness. We also hypothesize that the restored, low-gradient stream, when properly designed and constructed, will provide the required drainage capacity for agricultural production. One potential obstacle is the necessity for a wider drainage channel, and the subsequent loss of agricultural land to accommodate channel migration within the second-stage channel. The channel corridor would be at least 1 to 3 times bankfull width (U.S. Army Corp of Engineers 1994).

If the design proves effective, significant educational challenges exist at many decisionmaking levels. Because many maintained streams remain in relatively constant configurations for decades following modification, they are widely viewed as "successful" management practices for stormwater management (Rhoads and Herricks 1996). However, drainage networks established and maintained over the last 150 years are increasingly failing. Increased runoff from urban development exacerbates flooding and channel erosion downstream. Downstream communities and landowners must solve their problems and evaluate channel management options. It is our hope that naturalized drainage channel designs will eventually contribute to solutions that balance multiple objectives including agricultural soil drainage, water quality, flood relief and water retention, habitat enhancement leading to restored fisheries, and decreased bank erosion and sedimentation. Implemented at larger scales, naturalized agricultural drainage channels might prove a potent best management practice for water quality concerns such as Hypoxia and Pfeisteria in the Midwestern and Eastern U.S.

 

(a)                                      (b)

Figure 1.

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