Using a Two-Stage Ditch
The development of silt bars or benches within the channel bottom reflects natural adjustments to maintenance or channel disturbance. The bench acts as a floodplain within the over-wide and over-deep channel to dissipate energy. Benches also help to confine low-flow in the large drainage channel within a well-defined small channel in the center of the large channel, thereby protecting banks of the large channel from erosion. Observations of naturally-formed benches have led to a procedure to construct these features as an approach to enhancing ecosystem function in a channel. This approach can be considered a floodplain creation approach and often is referred to as a Two-Stage Channel (watch a video about Two-Stage channels).
Construction of a two-stage channel.
The same channel a few years after construction.
Goals and Implementation
It is important to keep in mind that the goal a two-stage approach is not to return a channel system to its pre-development condition. The primary goal is to allow existing channels to efficiently transport sediment using natural fluvial processes with little maintenance while allowing adjacent activities on the landscape to continue. The overall benefit to the system will be stability requiring little or no maintenance, increased capacity, reduced flow rates during high flows, and reduced sediment lost from the system. A key ecological benefit of this approach is that vegetation is left along the fringe of the existing channel and no work is done to reshape or narrow the current channel. This helps preserve any existing in-stream ecology and greatly reduces construction costs. This approach can improve water quality particularly for nutrient assimilation by improving soil-water interactions on the benches, which function more like long, linear wetlands. The best locations for the two-stage channel approach may be where benches already are naturally forming because: (1) there is an available supply of very fine sediment at the bottom of the channel; and (2) a major part of the flow entering the channel is subsurface drainage, which contains very little sediment so this flow picks up sediment once inside the ditch.
The constraints of land loss and economics imply that there is a maximum width to which benches can be constructed, and it often is not cost-effective or practical to form a floodplain as wide as fluvial processes would like. In order to maximize channel system stability and ecological function, we recommend that the total width of the benches plus the existing inset channel be 3 to 5 times the inset channel width that will form over time. In some cases bench construction is only performed on one side of the channel.
Generally, additional land area is required for this approach has been found to be approximately 2 acres per linear mile of channel for a bench width ratio of three times the inset channel width. Large, deep drainage channels often already have been constructed to facilitate discharges from subsurface drainage systems. Making these systems even larger would result in extensive earth moving, higher cost, and substantial losses in valuable land. Average costs of this approach might range from $5 to $50 per linear foot, excluding engineering service and the value of the land. These costs depend on the length of channel to be constructed, the depth of the channel, infrastructure constraints (i.e., municipal utility lines), and any bank stabilization measures that are taken. Ditches draining small drainage areas that are relatively shallow are the most cost effective. Not only is construction less expensive but also the water treatment potential is highest.
Click here for a factsheet that describes why you might consider utilizing the two-stage ditch design.
Click here for a factsheet on the critical steps in sizing a two-stage channel design.
Click here to learn more about important economic and design issues related to two-stage ditches.