Major Findings > Geomorphology

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

Results

All reaches exhibited one to four grade breaks up the side slopes (banks) of the ditches. At most sites the dominant grade break was between a small channel and a low depositional bench.

All the benches have coarse material at the elevation of the bed of the fluvial channel. Above the coarse layer there are fine materials interspersed with one to three poorly defined thin sandier layers that might also contain some gravel.

There was good agreement between precipitation, stream gauge and pressure transducer data, ADAPT daily simulation model discharge estimates, empirical discharge predictions, calculations based on Manning's equation and channel full discharge being associated with events that occur 10-20 times annually.

The probable dimensions of the fluvial channel can be empirically determined based on regional studies similar to those that are conducted on natural streams.

The channels exhibit some riffle and pool features. The pattern and profile characteristics of the channels were not as well defined as in natural streams and did not consistently fit expected relationships with bankfull.

Stable low benches are covered by dense grass and the channel meandered slightly within the confines of the ditch.

Stable benches form in wider ditches that provide less confinement.

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.

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.

For the stable benches, the particle size at incipient motion is similar to the measured d₅₀ or between the d₅₀ and d_84.

Conclusion:

The probable dimensions of the low-flow channel can be empirically determined based on regional studies similar to those that are conducted for natural streams. Similarly, measurement of the study sites begins to suggest that a broad ditch with a total bench width approaching or exceeding the channel width will result in stable benches.

The fluvial channel within a ditch has the potential to provide better habitat for biota in the same way that a bankfull channel is superior to an over-widened trapezoidal channel. It might also be useful in improving water quality particularly for nutrient assimilation in headwater streams. Further research is needed to ascertain what combination of factors will establish and maintain fluvial features.