Salt intrusion in the Pungue estuary, Mozambique: A case study on modelling the salinity distribution in the Pungue estuary

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).Civil Engineering and GeosciencesWater Managemen

Evaluation and improvement of remote sensing-based methods for river flow management

Rapid advancements in technologies open up possibilities for water resource authorities to increase their ability to accurately, safely and efficiently establish river flow observation through remote and non-intrusive observation methods. Low-cost Unmanned Aerial Vehicles (UAVS) in combination with Global Navigation Satellite Systems (GNSS) can be used to collect geometrical information of the riverbed and floodplain. Such information, in combination with hydraulic modelling tools, can be used to establish physically based relationships between river flows and permanent proxy. This study proposes a framework for monitoring volatile, dangerous and difficult to access rivers using only affordable and easy to maintain new technologies. The framework consists of four main components: i) establishment of geometry using airborne photogrammetry and bathymetry; ii) physically based rating curve development through hydraulic modelling of surveyed river sections; iii) determination of non-intrusive observations with for instance simple cameras or satellite observations; and iv) evaluating the institutional and societal impacts of using new technology. To establish this framework, a number of research questions require addressing. First, the factors impacting on accuracy of geometrical information of the floodplain terrain and bathymetry need to be investigated. Second the accuracy of a physically based rating curve compared to a traditional rating curve needs to be established. Third, for rapidly changing river segments, it should be investigated if the collection of occasional snapshots of multiple proxies for flow can be used to assess the uncertainty of river flows. The study finally explores the social and institutional impact of using new technologies for remote river monitoring. If these research gaps are addressed, this may strengthen water manager's ability to observe flows and extend observation networks.Water Resource