Two-stage ditches

Description

In order to keep fields in good cultivation status, the excess water must be removed from the fields by drainage. Balanced water economy is also needed for sufficient bearing capacity of the fields so that they can bear the machinery without excessive compression of the soil which, in turn, would be harmful for both cultivation and environment. Large-scale basic drainage of the fields in Finland was made after the World War II in order to enable better cultivation conditions. Presently, in over 70% of the drained arable area in Finland the conveyance capacity and drainage depth are no longer sufficient due to the deterioration of this aged channel network. Moreover, there is increasing need for mitigation of the harmful environmental impacts of conventional drainage (Kinnunen et al. 2020). 

This “debt of maintenance” related to this invaluable part of Finnish rural infrastructure must – with one way or another – be dealt with in the next years to come. As an alternative to the conventional way of dredging the existing main ditches, the two-stage channel design is a promising new nature-based solution (NBS) for managing the excess (and shortage) of water in agriculture. Two-stage channels consist of constructed floodplain(s) on the side(s) of the existing main channel to provide extra flood capacity (Figure 1).

Localisation and implementation

Drainage channels are traditionally designed using a trapezoidal cross-section to i) serve as an outlet for subsurface tiles draining adjacent lands and ii) move that water downstream as efficiently as possible. Trapezoidal drainage channels lack floodplains and are thus prone to bank erosion and excessive buildup of sediments as the channel attempts to regain a balance between sediment transport and supply.

Two-stage channels are designed take advantage of the benefits of active floodplains. At the lower, first stage, a relatively narrow channel allows for enough velocity to minimize sediment deposition during normal or low flows. The second, horizontal higher stage provides bank stability, can act as a recipient of subsurface drainage waters, and has extra water storage capacity for the flow during larger storm events. This approach can be considered one type of in-stream best management practice (BMP) and, if properly designed, should require little or no maintenance.

Effects, duration and maintenance

Pilot studies in Midwest United States and Finland show that two-stage channels may provide water quality benefits by trapping and processing of suspended sediment (SS), phosphorus (P) and nitrogen (N) on the floodplain (e.g. Mahl et al. 2015; Västilä et al. 2016). In Finland, the effectiveness of the two-stage channel design has been evaluated thoroughly by computing the percentage of SS retention and sedimentary accumulation of P, N and carbon (C) with respect to the total transported loads over a 9-year period between summers 2010 and 2019 (Västilä & Jilbert 2021). The net retention and erosion of SS on the floodplain, main channel banks and the low-flow channel were obtained from repeated high-resolution surveys of the cross-sectional geometry. The accumulation rates of P, N and C in the deposited sediment were quantified from the vertical sedimentary mass distributions. Based on this 9-year monitoring, the sedimentary retention in the 830 m long floodplain was 6450 kg/a for SS, 4.7 kg/a for P, 23.2 kg/a for N and 228 kg/a for C. Comparing the retention to the total transported loads revealed that the retention efficiency of the floodplain was 6.8% for SS, 3.5% for P, 0.42% for N and 0.62% for C.

It is hypothesized that two-stage channels reduce sediment deposition on the channel bed, which decreases the need for maintenance and increase the channels’ life cycle. The new floodplain habitat accompanied by the more natural-like low-flow channel may also contribute to enhanced biodiversity, but scientific evidence on the impact of two-stage channels on biodiversity is so far limited (DeZiel et al. 2019). However, in a recent Finnish study (Västilä et al. 2021), two-stage channel construction appeared to be beneficial for plant biodiversity; highest number of species was recorded in the two-stage channel area, while all three conventionally dredged reference areas had lower number of plant species. The two-stage channel appeared to be beneficial to herbs favoring moist or wet conditions. This was probably due to the proximity of the wider and more uniform flooded section, i.e., in the man-made floodplain some species ‘creep’ up the bank. In terms of pollinating insects, the counts showed that various kinds of wild pollinators as well as the honeybee were relatively abundant along both kinds of studied reaches and any systematic differences between the two-stage and conventional channels with respect to pollinator species’ richness and abundance was not detected (Västilä et al. 2021).

Maintenance of a conventional, trapezoidal channel can be an ongoing problem particularly in areas that contain highly erodible soils, which are easily transported and fill in the ditch bottoms over time. Removal of excessive sediment to maintain drainage requires the use of heavy machinery, which disrupts the channel system and is costly for landowners. When coupled with appropriate upland BMPs, the two-stage design can be self-sustaining, significantly reducing the need for expensive and destructive clean-outs.

It has been estimated that the periods between the required maintenance operations of two-stage channels may be up to several decades (Paradis & Biron 2017), which increases the economically viability of two-stage channels in the longer term. The lesser need for maintenance is believed to be due to the fact that two-stage channels mimic natural conditions with natural sedimentation and flooding processes (D’Ambrosio et al. 2015). However, maintenance may still be required to remove woody vegetation, such as willows, which may obstruct water flow or shade the grasses needed to stabilize the banks.

Nevetheless, it should be kept in mind that even the oldest two-stage channels have only existed for less than 20 years and thus little is known and peer-reviewed papers about their actual life cycles are not yet available.

Pamphlets, reports, videos and websites

Kinnunen, O., Hjerppe, T. & Väisänen S. 2020. ELY-keskusten näkökulma luonnonmukaisten periaatteiden toteutumiseen maatalouden peruskuivatushankkeissa. Shared Waters-project report. ELY centers' perspective on the implementation of nature-based principles in agricultural dredging projects. 8 p.

Wikipedia - Two stage drainage ditch 

Scientific papers

Västilä, K., Karttunen, K., Koskiaho, J., Kuussaari, M., Lehtoranta, V. & Väisänen, S. 2021. Nature-based solutions for climate change adaptation and water pollution in agricultural regions. Sustainability (submitted).

Västilä, K. & Jilbert, T. 2021. (manuscript in preparation).

DeZiel, B., Krider, L., Hansen, B., Magner, J., Wilson, B., Kramer, G. & Nieber, J. 2019. Habitat Improvements and Fish Community Response Associated with an Agricultural Two-Stage Ditch in Mower County, Minnesota. Journal of the American Water Resources Association 55(1): 154–188. DOI 

Paradis, A. & Biron, P.M. 2016. Integrating hydrogeomorphological concepts in management approaches of lowland agricultural streams: Perspectives, problems and prospects based on case studies in Quebec. Canadian Water Resources Journal / Revue canadienne des ressources hydriques, 42(1): 54–69. DOI 

Västilä, K., Järvelä, J. & Koivusalo, H. 2016. Flow–vegetation–sediment interaction in a cohesive compound channel. Journal of Hydraulic Engineering 142(1): DOI 

D'Ambrosio, J.L., Ward, A.D. & Witter, J.D. 2015. Evaluating Geomorphic Change in Constructed Two-Stage Ditches. Journal of the American Water Resources Association 51(4): 910–922. DOI 

Mahl, U.H., Tank, J.L., Roley, S.S. & Davis, R.T. 2015. Two‐Stage Ditch Floodplains Enhance N‐Removal Capacity and Reduce Turbidity and Dissolved P in Agricultural Streams. Journal of the American Water Resources Association 51(4): 923–940. DOI