Abstract
Conventional dredging of ditches and streams to ensure agricultural drainage and flood mitigation can have severe environmental impacts. The aim of this paper is to investigate the potential benefits of an alternative, nature-based two-stage channel (TSC) design with floodplains excavated along the main channel. Through a literature survey, investigations at Finnish field sites and expert interviews, we assessed the performance, costs, and monetary environmental benefits of TSCs in comparison to conventional dredging, as well as the bottlenecks in their financing and governance. We found evidence supporting the expected longer-term functioning of drainage as well as larger plant and fish biodiversity in TSCs compared to conventional dredging. The TSC design likely improves water quality since the floodplains retain suspended sediment and phosphorus and remove nitrogen. In the investigated case, the additional value of phosphorus retention and conservation of protected species through the TSC design was 2.4 times higher than the total costs. We demonstrate how TSCs can be made eligible for the obligatory vegetated riparian buffer of the European Union agri-environmental subsidy scheme (CAP-AES) by optimising their spatial application with respect to other buffer measures, and recommend to publicly finance their additional costs compared to conventional dredging at priority sites. Further studies on biodiversity impacts and long-term performance of two-stage channels are required.
Highlights
In Boreal and Continental climatic conditions, an important aim for agricultural water management is the removal of excess water from the fields by drainage
The two-stage channel (TSC) design is an alternative to the conventional dredging of channels, and we focused on the additional benefits that can be achieved by the TSC design in comparison to the conventional dredging
These benefits are mainly generated through the nature-based features of TSCs, which mimick the geometry of natural lowland streams, whereas the conventional dredging produces straight, trapezoidalshaped channels with overwide beds (Figure 1)
Summary
In Boreal and Continental climatic conditions, an important aim for agricultural water management is the removal of excess water from the fields by drainage. Drainage ensures the good cultivation status of the soil and a sufficient load-bearing capacity of the fields to carry agricultural machinery, while preventing harmful compression of the soil. 90% of the field area requires drainage [2]. The field drainage consists of sub-surface drains beneath the soil surface or, more seldomly, of open field ditches laid in parallel across the field surface at a spacing of 10–50 m. The field drainage network discharges to larger ditches or to natural streams and rivers that have typically been dredged, straightened, and channelised to ensure adequate flow conveyance and sufficient drainage depth (Figure 1a)
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