Strategic drainage planning for Nakamabala Estate, Zambia.

Nakambala, located south of the Kafue River and spanning approximately 15,197 ha, is the largest sugar operation in Zambia. It is operated by Zambia Sugar, which is committed to promoting sustainable agriculture and the upliftment of the local stakeholders and the communities within which they operate. In alignment with that purpose, it is vital that Zambia Sugar control all impacts from and on their estates and surrounding environment.

One of the biggest risks to the estate and surrounds is flooding, especially associated with the Kafue River. To mitigate flood risks, Nakambala required an assessment of their drainage plan and associated water management infrastructure. To that end, GroundTruth were appointed to carry out a strategic drainage planning exercise for Nakamabala and surrounding areas.
Given the extensive area to be surveyed and modelled, GroundTruth adopted a phased approach.

Phase 1 involved desktop analyses to identify areas known to be prone to flooding or likely to flood. GroundTruth analysed data sent by Zambia Sugar and developed a Digital Terrain Model of the entire assessment area. Using the Digital Terrain Model, flow accumulation and ponding models were generated using Geographic Information System (GIS) tools. These models informed water flow direction and accumulation during a flood, as well as where localised flooding may occur. The results were used to delineate high-risk zones. All drains in those zones formed part of the drainage assessment.

Phase 2 involved a detailed flood risk assessment of high-risk zones and drainage networks to determine the causes of flooding and provide recommendations on alleviating flood risk. To inform the detailed assessment, GroundTruth conducted in-field surveys of Nakambala and the surrounds. The field surveys helped the GroundTruth team

1. Verify drainage network placement and condition;
2. Understand the flow inputs and outputs to the primary and secondary drainage lines;
3. Understand the external influences on the drainage plan such as irrigation canals, debris, and solid waste;
4. Verify the catchment delineations and land use within the catchment;
5. Note any alterations to the drainage network since previous data were collected;
6. Note of any impacts to the existing drainage network, such as erosion gulleys or incisions to drains, which would indicate large flows and energies within these drains;
7. Distinguish between the stormwater drains and irrigation canals where it was unclear at a desktop level;
8. Note where irrigation canals influence the stormwater drains;
9. Capture any additional points or drains to be surveyed and modelled;
10. Measure flows and cross-sections where necessary to refine the LiDAR survey; and
11. Engage with Nakambala personal, community members, and farm managers to develop historical site context.

As an additional assessment to the scope of work, GroundTruth undertook some minor water quality analysis exercises whilst on site, involving miniSASS and water clarity. This was to develop an understanding of the water quality impacts discharging to the Kafue River, which would directly influence downstream users, and to determine where these impacts are being generated. The assessment was done to inform the flood mitigation opportunities, where water quality enhancement tools or ecological infrastructure could be incorporated to the mitigation recommendations and future designs to achieve the sustainable objective of Zambia Sugar.

This project is ongoing, with final recommendations for Zambia Sugar to mitigate flood risk and design for a resilient, sustainable, and ecologically and socio-economically friendly landscape, still to be finalised. Watch this space!