Constructed Floodplain: The Two-Stage Ditch

Summary

Nearly 3,000 linear feet of channel at Waterman Farm were protected and enhanced with traditional and innovative best management practices that reconnected the stream to it's floodplain, reduced bank erosion, and improved water quality. 

Overview

Highly modified channels drain extensive portions of the United States particularly in the Midwest where soils tend to be highly cohesive and gradients tend to be extremely flat. These channels are important because they serve as an outlet for surface runoff and subsurface drain systems where natural drainage is insufficient for the proposed land use, which typically is agricultural production or urban development.

Usually are located at the upper end of small watersheds, drainage ditches often are straightened and deepened, taking on a characteristic trapezoidal shape, to enable more water to flow faster downstream. These modified ditches have become detached from their floodplains and, often having steep side slopes, can be come highly unstable leading to excessive bed and bank erosion. This can result the need for frequent maintenance to prevent problems arising from a shape and function that is not what nature intended on the landscape.

In addition, streams and ditches often provide the only source of water for livestock. When livestock are given free access to streams and ditches they can contribute to: bacteria such as E. coli entering the water via feces, disturbance to instream habitat features that affects local aquatic biota, and destruction of stream banks via grazing and trampling that contributes sediment to the system.

Water quality issues such as excessive siltation, transporting nutrients like nitrate-nitrogen and phosphorus downstream, and loss of in-stream and riparian habitat can follow this instability resulting in chronic problems in the watershed including flooding, algal blooms, and burying tile outlets necessary for drainage. Additionally, commodities (i.e., crops) or infrastructure vital to our economy have built up around these modified channels and some have thrived as a result of them.

The Problem

In the pasture of the Waterman Farm Dairy, the herd had free access to the small, ephemeral stream running through the pasture. The stream channel was characteristic of a Rosgen Type E channel. The bed and banks were heavily degraded in the pasture due to many years of livestock access and no riparian buffer. Upstream of the pasture is a small pond that discharges to the stream. The pond has a high Canada geese population. In this section of stream, sediment and E. coli were likely contributed to the Olentangy River mainstem.

Downstream of the pasture, the stream was straightened and deepened at some point and had a characteristic trapezoidal ditch shape. The channel served as an outlet for subsurface drainage tiles from the Waterman Complex and was criss-crossed by several gas and utility lines. The channel had not been maintained by the Farm and was showing signs of recovery including small benches forming and meandering of the inset channel. A small riparian buffer had been allowed to establish along both banks, but it mostly consisted of honeysuckle, brambles, and some large but poorly developed trees. Frogs were seen in this section of ditch, but no fish were found when sampled in 2010. Macroinvertebrates were not sampled.

The Goal

With a need to both improve water quality and preserve as much of the existing land use as possible, demonstrate multiple options for returning ecological function to modified channels that employ cost-effective traditional and innovative BMPs.

The Objectives

1. Exclude livestock from the upstream section stream to reduce the source of sediment and nutrients.

2. Enhance the downstream section of stream to re-establish nutrient cycling and sediment transport function to act more as a sink for sediment and nutrients.

3. Demonstrate a traditional BMP, livestock fencing, and passive recovery of a stream channel.

4. Demonstrate an innovative in-stream BMP, the two-stage ditch design, to create a connected floodplain.

The Result

The Waterman Farm Agricultural Best Management Practices Demonstration and Education Project, funded by the Ohio EPA 319 program, endeavors to demonstrate that a healthy functioning stream system can coincide with agricultural production and urban development.  The project also hopes to broaden the public’s understanding of the vital role streams play in improving water quality.

Upper Section of Stream

Cattle were excluded from nearly 2,000 linear feet of stream channel in the Waterman Farm Dairy pasture. The livestock fencing was installed by the Dairy Manager, staff, and students. The project resulted in approximately 2 acres of riparian buffer along the stream channel. Originally intended to be planted with native trees, shrubs and grasses, regrowth from the existing seed bank was so vigorous that additional plant materials were not installed resulting in a project cost savings of $XXX.

Lower Section of Stream

A two-stage ditch design was selected for the lower section of channel at Waterman Farm.  The design strategy consists of: (1) a channel that is sized to convey the effective discharge, (2) a bench to serve as a floodplain for the smaller channel, and (3) a stage of adequate width to prevent flow overtopping the ditch banks and flooding surrounding land.

In order to demonstrate typical conditions in a rowcrop agricultural setting, trees and woody vegetation were removed from the lower half of the ditch during construction. The lower half of the project, constructed in June 2010, consisted of 325 feet of two-sided two-stage ditch construction and 126 feet of one-sided two-stage ditch construction to preserve a treatment wetland on the left bank of the ditch. The upper half of the project, constructed in June 2011, consisted of 285 feet of one-sided two-stage ditch construction. The small wooded riparian area along the ditch was preserved in the upper half of the project.

The Ohio Department of Natural Resources provided the engineering designs for the two-stage ditch with technical assistance from OSU Extension. The average inset channel width was 6 feet and the average depth was 0.8 feet. Benches were constructed at 3x the channel width for the two-sided section and 4x the channel width for the one-sided sections. The channel bed slope ranged from 1.0% to 2.8% along the project length. The watershed area at the project location is 0.35 square miles.

Ohio Land Improvement Contractors (OLICA) performed the construction as a field demonstration. Primary costs of two-stage ditches are associated with earthwork necessary to increase the ditch width. Costs for construction increase with both watershed size and ditch depth and generally range from $5 to $20/linear foot. Costs may be higher than this range given certain site conditions such as removal of trees and stumps or if excavated material cannot be spoiled on-site. All material was spoiled on-site, leveling out an area used for corn silage adjacent to the ditch and improving this size of area that could be planted. Trees and shrubs were chipped on site and used around the Waterman Complex for mulch and livestock bedding.

The ditch was seeded with annual rye and mulched with straw immediately after construction. No additional seeding or planting was performed and the ditch was allowed to regenerate from the existing seed bank.

During construction of the lower half of the two-stage ditch, an irrigation line was disturbed. OLICA members performed an on-the-fly repair to continue construction. Franklin Soil and Water Conservation District and a local engineering firm returned to the site to do a full repair of the irrigation line and installed an on-off valve.

Impacts

  • 13,764 square feet of constructed floodplain.
  • 2 acres of riparian buffer.
  • 2,736 linear feet of stream channel protected/enhanced.
  • Demonstration of innovative ditch construction to 40 land improvement contractors.
  • Improvement to adjacent corn silage field.
  • Multi-institutional collaboration.