Research Topics > Geomorphology: Self-Maintaining Ditches > Results > findings on ditches with stable versus unstable benches

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Using Knowledge of Fluvial Processes to Design Self-Maintaining Agricultural Drainage Ditches in the Midwestern Region of the USA

Results: findings on ditches with stable versus unstable benches

Stable low benches are covered by dense grass and the channel meandered slightly within the confines of the ditch. (See Figure 9 below).

Figure 9

Stable benches form in wider ditches that provide less confinement. (See Figure 10 below.)

Figure 10

The width of the benches and the ratio of the bench width to channel width were significantly different. The mean widths for the stable and unstable bench groups were 8.3 m and 2.9 m respectively. There were also significant differences between the two groups for the width of the flood prone area, the width at twice the maximum channel full depth and the ditch width at three times the average channel mean depth for all the sites. These measurements are indicators of the confinement of the flow and the ability of the flow to dissipate energy as the flow exceeds the channel full stage. (See Figures 11 and 12 below.)

Figure 11: Wide benches	Figure 12: Narrow benches

For the stable benches a ratio of the bench width to channel width at the bench full stage was highly correlated (R² = 0.84) with a linear combination of drainage area and d₈₄ of the bed material. As the drainage area and d₅₀ increased the ratio decreased. (See the table below.)

For the stable benches, the particle size at incipient motion is similar to the measured d₅₀ or between the d₅₀ and d_{84. (See Figures 13 and 14 below.)}

Figure 13

Figure 14

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