Investigating the role of ocean currents and waves in securing freshwater for the Nelson Mandela Bay Metro

By Tommy Bornman (SAEON Elwandle Node), Wayne Goschen (SAEON Egagasini Node), Morné Pienaar (Aurecon), James Cullis (Aurecon), Marthinus Retief (WSP) and Edzard Verseput (NMBM) Tweet

Figure 1. Tommy Bornman servicing the Schoenmakerskop ADCP (Image courtesy of the Coastal and Marine Research Institute, NMMU)

The Nelson Mandela Bay Municipality (NMBM) identified the lack of traditional potable water sources as one of the major risks facing the metro in the medium to long term.

The majority of freshwater is supplied by large impoundments on the Kromme River, over 100 km to the west of the city, with additional water being supplied from the Orange River via the Orange-Fish transfer tunnel.

Unpredictable rainfall, aging infrastructure, an ever-increasing demand for water and an unknown future climate make supplying water to the 1,5 million residents a challenging task. To ensure a more reliable source of freshwater, particularly to the western half of the metro, the NMBM is investigating the feasibility of establishing a desalination plant on the southern coast of the metro.

Feasibility study

SAEON is assisting Aurecon (consulting engineers on the project appointed by the NMBM), WSP (marine engineers) and Nelson Mandela Metropolitan University (NMMU) in their feasibility study by means of a new ADCP (current meter) instrument deployed in 22 m of water to the east of Schoenmakerskop. This ADCP was the first SAEON current meter set up to measure wave height, an additional function that reduces the battery life from 10 months (measuring currents only) to less than one month (measuring currents and waves).

Although the reduced battery life means more regular service intervals (Figure 1), the data being produced is of immense value, not only for this project, but also to oceanographers, modellers and coastal managers. Current and wave data is valuable to determine the energy of the coastline (to inform appropriate infrastructure engineering) and to determine the mixing and dispersal capability of the ocean, which is critical in terms of the environmental impact assessment (EIA) and for the design of the marine intake and discharge pipeline for brine dispersal from the desalination plant.

Figure 2. Stick-vectors of the currents measured at Schoenmakerskop during September 2015

The data received to date indicates that the current flows parallel to the coastline (east-west), are the strongest at the surface and regularly change direction (eastwards or westwards) depending on the direction of the wind (Figure 2). The strongest currents measured during this period were in an easterly direction and were in all likelihood related to strong south-westerly winds. The significant wave height measured during September 2015 indicates a mean height in excess of two metres and eight instances where the wave height exceeded three metres (Figure 3).

Figure 3 : Wave height during the month of September 2015

This partnership between NMBM, Aurecon, WSP, NMMU and SAEON is a good example of what can be achieved through sharing of resources and expertise for enhanced societal benefit.

Algoa Bay Sentinel Site

Although the feasibility study will be completed in a few months’ time, SAEON will maintain its ADCP indefinitely into the future as part of the network of instruments in the Algoa Bay Sentinel Site for Long-Term Ecological Research.

This ADCP will enable SAEON to measure the predicted increase in storminess and storm surges associated with climate and global change, as well as the impact it will have on the marine ecosystems and the coastal zone in general.