A two-stage ditch is a design conversion that modiﬁes the geometry of a ditch to establish benches within the ditch. The ditch provides a low-ﬂow channel and then a vegetated bench that is ﬂooded during higher ﬂows. The vegetation provides some slowing of water ﬂow where sediments and other heavier material in the ﬂow might settle. A two-stage ditch is an in-channel practice.
Figures 16a and 16b. A conventional ditch (16a) compared to a two-stage ditch (16b).
- It is best suited to ditches with a grade of less than 2 percent, and ﬁelds that are fairly ﬂat (less than 0.5 percent slope) with subsurface drainage installed.
- It is appropriate for ditches that experience bank erosion or for undersized ditches.
- It can be implemented on ﬁelds with greater slope, but grade-control structures might be required.
- It is appropriate for sites where good aquatic habitat is already present or desired.
- It reduces wetness in ﬁelds with subsurface drainage, where outlets are frequently under water.
- It makes the banks more stable and reduces the maintenance cost.
- It reduces downstream exports of nutrients and sediment.
- It improves plant-soil-water interactions in ditches, increasing nutrient cycling
Studies in Ohio, Indiana, and Michigan indicate that two-stage ditches:
- accumulate little sediment (1–13 mm) over their lifetime and do not need further clean out or maintenance (D’Ambrosio et al., 2015).
- reduce soluble reactive phosphorus concentrations by 3–53 percent and reduce turbidity by 15–82 percent, suggesting reduced suspended sediment export. Reach-scale nitrogen removal also increased 3- to 24-fold when benches were ﬂooded (Davis et al., 2015; Mahl et al., 2015).
- showed decreased nutrient export at sites with lower nutrient loads, and that more benches were wet for longer portions of the year (Mahl et al., 2015).
Once constructed, a two-stage ditch is relatively self-sustaining. Periodic mowing or removal of woody vegetation might be required.
- Management costs vary based on the existing ditch’s size, topography, engineering design, existing infrastructure, and spoil material.
- Costs are site-speciﬁc, but they typically range from $10–$50 per linear foot.
- Planning to use spoil material on-site or nearby reduces costs signiﬁcantly.
- Sandier soils might not be well-suited for this practice unless vegetation can establish quickly.
- Two-stage ditches might impact existing grass buﬀer contract. Consult your local NRCS technician prior to construction.
- Channel width will vary by site. Land taken out of production should be considered in the cost analysis.
Figure 17. A two-stage ditch during construction. Credit: Food, Agricultural and Biological Engineering, The Ohio State University.
Often called a ditch-within-a-ditch, a two-stage ditch is designed so that low ﬂows stay in the ﬁrst-stage main channel and ﬂood ﬂows stay conﬁned within the ditch banks in the second stage. A wider ditch with less steep banks slows down the speed of ﬂood waters and reduces the height of the water in the ditch. This makes the banks more stable. Tile outlets will be submerged less often and will not be clogged by sediment deposits. Benches are below tile outlets, so ﬂow from ﬁeld tiles falls on to a vegetated bench rather than into the ditch’s main ﬂow of water. Vegetation is the key to bench and bank stability by slowing water ﬂow so that sediment can drop out of suspension. The vegetation also utilizes nutrients deposited with sediment.
Figure 18. A two-stage ditch three months after construction. Credit: Food, Agricultural and Biological Engineering, The Ohio State University.
Extensive design details are provided in NRCS Standard 582: Open Channel