DETERMINING NON-EROSIVE VELOCITIES FOR DESIGN OF CONTOUR BANKS
Mar 10, 2017

Agronomy & physiology WCRC Agro-physio-australia WCRC1
Abstract                                                                         Back to Table of contents

One farming system found to give good control of erosion on sloping agricultural land is the use of agronomic measures in conjunction with contour banks.  Contour banks constructed with a constant gradient result in layouts with interbank areas of uneven width.  This leads to inefficient operation of machinery.  For more parallel workbays, gradients along contour banks must vary.  Limits to the gradients, both upper and lower, need to be defined to prevent damage.  Water was run down five contour banks, of 0.1, 0.3, 0.6, 1 and 3% gradient respectively, at five rates, increasing to 450 l/s, on a cracking clay soil when in a fine tilth state and again after a crop of forage sorghum had been grown and grazed.  Unacceptable soil loss occurred from all channels except that on 0.1% gradient, where soil was redistributed over the channel floor.  No net deposition of sediment occurred in any of the contour bank channels studied here.  Soil shear strength increased with depth and time since wetting.  This influenced sediment concentrations which, for a given flow rate, decreased with time.  It the stream power of flows exceeded about 0.003 g m3 /cc s3 then excessive soil movement occurred.  The stream power of design flows should be kept below that when designing contour banks for cracking clay soils.

Conclusions

Unacceptable scouring of contour bank channels occurred for all flows where the channel gradient was 1% or greater.  No net loss of soil occurred for a contour bank with a channel gradient of 0.1%.  For channels with gradients between 0.1% and 1%, increasing discharges resulted in increasing soil loss.

When designing contour banks on dark cracking clay soils, the stream power of design discharges should be kept below 0.003 g m3 / cc s3.  This design criteria is limited to this soil type.  Similar criteria should be developed for other soil types.

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