Two-stage drainage ditch
[3] In acknowledgement of such environmental issues, the primary purpose of the two stage drainage ditch is to more effectively transport sediment and other contaminants using natural fluvial processes of existing channels with minimal maintenance whilst allowing existent activities on the land proximate to the water body to continue such as farming, irrigation or roadworks.
Furthermore, much of the vegetation, except for the land needed to implement the adjacent grass benches (see disadvantage analysis below) and the natural shape and meander of the water channel remains largely unaltered during the formation of the two stage drainage ditch, thus, protecting the system's ecology and substantially reduces ongoing maintenance costs, contrary to other drainage management systems.
[4] The two stage drainage ditch, with the introduction of the benches, creates a larger area on a cross-sectional basis than the existing, natural, trapezoidal channel.
Construction costs for such earthwork are contingent upon the watershed size and depth of the ditch and approximately range from $5–20 USD/linear foot.
Evidently, referring to the diagram on the left, the existent channel needs to be extended by an additional 13 ft of land to create the floodplains (benches) "Over a mile long reach [of the water channel] adds up to over 1.5 acre of land needed for the two stage drainage ditch design".
[6] The optimal region to establish a two stage drainage ditch is where there are benches already naturally forming for binary reasons.
In fact, more than 25,000 miles of these traditional drainage ditch lines exist along agricultural fields in Minnesota, particularly in the South East in parts of the Upper Mississippi River.
[7] The Nature Conservancy's investigation into the Minnesota region affirmed the above limitations of traditional drainage management above, finding that these deep and wide conventional ditches are highly susceptible to erosion and sedimentation, hindering the stability of the ditch, finding excessive levels of nitrogen and phosphorus in streams which culminate in eutrophication, in addition to increased flooding.
Therefore, to restore and alleviate issues of water quality, given agriculture is the largest income generator of Minnesota, the Nature Conservancy have begun to test a value analysis associated with the formation of two stage drainage systems in the region in the past decade.
A longer time horizon of data would further demonstrate the positive impact the two stage drainage ditch has on nitrate levels in the channel during periods of precipitation.
