Zambia Pivot Contour Banks
Designing Contour Bank Spacing with the Runoff Analysis Feature
Field Area: 47 ha or 116 acres
Irrigation Method: Pivot Irrigation
The client was having erosion issues and was looking to improve the existing contour banks. They wanted the field smoothed out and they wanted the contour banks parallel as possible.
Below is the Existing Topography along with the results from the Runoff Analysis. The Runoff Analysis is simulated with a 1 in 10 year rainfall event of duration up to 3 hours. The arrows on the Runoff Analysis show the water velocity building up as the water runs down the field. Without the contour banks, the water velocity becomes erosive over most of the pivot, thus requiring the need for the contour banks.
Figure 1. Existing Topography (left) and Runoff Analysis (right). Drag the slider left or right to view each.
The first step was to smooth the field with OptiSurface 4Way design to eliminate ponding areas and make sure the surface of the field is smooth and the contour banks will be as straight as possible.
Figure 2. Existing Topography (left) and Proposed Topography (right). Drag the slider left or right to view each.
Figure 3. Cut/Fill Map.
The next step was to determine where the first contour bank will be positioned. A more scientific way of determining the spacing was to check where erosion starts to occur. The Runoff Analysis feature is ideal for this. We need to know the rainfall depths for a 1 in 10 year storm event for 1 hour duration (60mm) and for 24 hours duration (160mm) as shown in Figure 4 below.
Figure 4. Runoff Analysis inputs.
The results of the Runoff Analysis is shown below. Where the water velocity gets above 0.5m/s, the erosion risk increases significantly.
Figure 5. Proposed Runoff Analysis.
The first contour bank will be placed where the runoff velocities start to go beyond 0.5 m/s or where the velocity arrows start to go to yellow. The contour bank is designed to fall from the middle of the field then slope down to the edges at 0.15% slope.
An example of a contour bank profile and the position is shown in the figures below along with the inputs for the Contour Bank Design calculations.
Figure 6. Contour Bank 1A Profile falling to the west.
The figure below shows the eastern side of the same contour bank.
Figure 7. Contour Bank 1B Profile falling to the east.
Then, to determine where the next contour bank will be placed, the process is repeated but the runoff analysis will now be carried out on a new boundary where the first contour bank started as shown in the yellow boundary in Figure 8. The second contour bank will start where the runoff velocities start to go to yellow as discussed previously.
The updated Runoff Analysis is shown below with the yellow polygon indicating the extents of the Runoff Analysis for the second contour bank.
Figure 8. Runoff Analysis after the first contour bank, to allow positioning of the second contour bank.
This process was repeated again for below the second contour bank to define where the third contour bank should be.
Figure 9. Runoff Analysis after the second (left) and third (right) contour banks.
The final contour bank design only required three contour banks saving significantly on construction and maintenance cost while minimizing the impact on farm machinery operations.
Figure 10. Proposed Contour Banks.
OptiSurface Designer gives the unique capability of determining the best contour bank spacing and design with the Runoff Analysis and Contour Bank Design features.
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