Salt intrusion in the Pungue estuary, Mozambique

Abstract

Additional thesis - The Pungue river originates in Zimbabwe and drains into the Indian Ocean creating a large alluvial estuary. The river discharge generated in the Eastern Highlands of Zimbabwe is an important source of fresh water for water consumers in Mozambique. In Mozambique the fresh water is used for drinking water production for the city of Beira and for irrigation and process purposes of the sugarcane estate of Mafambisse. The Mafambisse sugarcane estate strongly depends on the fresh water supply of the Pungue river. In the dry season when a low discharge is combined with a high tidal range salt intrusion can lead to high salinity values at the water intake of the estate. If the salinity exceeds the threshold of 0.16 kg/m3 the water cannot be taken in, without seriously affecting the yield or the operation of the sugarcane factory. The objective of this research is to obtain a thorough insight in the characteristics of the salt intrusion in the Pungue estuary in order to give preliminary solutions to the sugarcane estate. To get insight in the characteristics of salt intrusion data is collected during November and December of 2016. In order to describe the parameters in the steady and unsteady state salt intrusion models the estuary geometry is determined, the river discharge has been measured and salinity measurements have been conducted. The steady state salt intrusion model is calibrated on salinity measurements carried out with the moving-boat method. The salinity measurements show a sudden increase in salinity values around 60 kilometres from the estuary mouth. From the calibration it becomes clear that the steady state model is not able to represent these irregularities in the salinity distribution. This research shows that it is likely that an additional source of salt is causing the increase in salinity values and that the unsteady state salt intrusion is able to represent this. To see the effect of extreme situations of the tide and river discharge on the salinity distribution in the Pungue estuary four scenarios have been developed and compared with a reference scenario. The following scenarios are chosen: expansion of FIPAG (drinking water company), an increased extraction rate of Mafambisse, a drought and a bend cutoff. From the scenarios it can be concluded that with exception of the expansion of FIPAG all scenarios exceed the salinity threshold at the intake causing serious water shortages. The most extreme situation is reached in case of a drought. The salinity value modelled at the intake becomes 2.63 kg/m3 and the salt intrudes up to 84.5 km. In this situation no extractions are possible and Mafambisse will suffer from severe water shortages. To provide the sugarcane estate with preliminary solutions in preventing salinity incidents at the intake this research elaborates on three concrete recommendations. A straightforward solution to reduce salt incidents at the intake would be the relocation of the pumping station. This research suggests relocating the pumping station to 90 km or further from the estuary mouth. Another solution offered would be an alternated pumping scheme. This research shows that alternated pumping can be beneficial under certain conditions; a high salt intrusion length combined with a relatively high extraction rate. Under these conditions an alternated pumping scheme can lead to a larger extraction volume compared to the situation where the pumps are in constant operation and suddenly needs to be turned off. This is the case because the advective transport of salt by the river is much faster than the process of effective dispersion. The last recommendation is to monitor the additional salt sources, which causes irregularities in salinity distribution. Once these sources are located it is possible to control the flushing of saline water into the river by making use of small structures (e.g. sluices).