Ivar Abas and Hugo Hagedooren Salt intrusion in the Pungue estuary, Mozambique A case study on modelling the salinity distribution in the Pungue estuary Delft, March 2017 2 Salt intrusion in the Pungue estuary, Mozambique A case study on modelling the salinity distribution in the Pungue estuary By Ivar Abas and Hugo Hagedooren In fulfilment of the requirements of an Additional Master Thesis Master Civil Engineering Track Water Management at the Delft University of Technology, Supervisors: Prof. Dr. Ir. H.H.G. Savenije, Dr. Ir. S.G.J. Heijman, Ir. W.M.J. Luxemburg An electronic version of this thesis is available at http://repository.tudelft.nl/ 3 Notation 푎 Cross-sectional convergence length [m] 퐴 Tidal average cross-sectional area [m2] 2 퐴0 Cross-sectional area at the estuary mouth [m ] 2 퐴푟 Cross-sectional area of the river [m ] 푏 Width convergence length [m] 퐵 Tidal average estuary width [m] 퐵0 Width at the estuary mouth [m] 퐵푟 Width of the river [m] 퐶 Chézy coefficient [m0.5/s] 퐷 Tidal average longitudinal dispersion [m2/s] 2 퐷0 Dispersion coefficient at the estuary mouth [m /s] 2 퐷 Dispersion coefficient during HWS, TA or LWS [m /s] 퐷(푥) Dispersion coefficient as a function of 푥 [m2/s] 퐸 Tidal excursion [m] 퐸0 Tidal excursion at the estuary mouth [m] Gravitational acceleration [m/s2] ℎ̅ Tidal average depth [m] ℎ0 Depth at the estuary mouth [m] 퐻 Tidal range [m] 퐾 Van der Burgh’s coefficient [-] 퐿 Salt intrusion length [m] 푃 Wetted perimeter [m] 푃 Tidal prism [m3] 푃푛 Net rainfall in an estuary [m/s] 푞 Coefficient of the advective term [m/s] 3 푄푓 Fresh water discharge [m /s] 3 푄0 Fresh water discharge during dry season [m /s] 3 푄푟 River discharge [m /s] 3 푄푝 Extraction rate [m /s] 3 푄푡 Tidal discharge [m /s] 푟푆 Storage width ratio [-] 2 푅푆 Source term [m /s] 푠 푠 (푥, 푡) is the salinity [kg/m3] 3 푆푓 Fresh water salinity [kg/m ] 3 푆 Salinity during HWS, TA or LWS condition [kg/m ] 푆(푥) Salinity as a function of 푥 [kg/m3] 3 푆0 Sea salinity [kg/m ]’ 3 푆max _푛푡푎푘푒 Salinity threshold of the intake [kg/m ] 푡 Time [s] 푇 Tidal period [s] 푥 Distance from the estuary mouth [m] 훽 Dispersion reduction rate [-] 훿 Damping number [-] η Tidal amplitude [m] 휌 Density of fresh water [kg/m3] ∆휌 Density difference between salt and fresh water [kg/m3] 4 Preface Before reading our research we would like to introduce ourselves. We are Ivar Abas and Hugo Hagedooren, two students of the master Watermanagement at the faculty of Civil Engineering at the Delft University of Technology. This research is conducted to fulfil the requirements of an additional master thesis, which is part of our master programme. This research is done in the period September 2016 until March 2017. In this period there was a field visit of six weeks to Mozambique. This visit to Mozambique was a very interesting and educational experience. Our stay was not without some practical problems such as: engine failure, getting stuck on a sandbank, hippos blocking our way and malaria. So at the end we are very happy that we were able to conduct enough measurements. We hope you enjoy reading our additional master thesis. Acknowledgements As authors we want to thank everyone who contributed in any way to our research. First of all we want to thank the Mafambisse sugarcane estate (Tongaat Hulett Sugar) for their hospitality during our stay in Mozambique. In particular we want to thank Fungai Nyemba and Alasdair Harris for arranging everything in advance. We also want to thank Fungai Nyemba for arranging pleasant accommodation and helping us out with all the necessary transport. We want to thank Collert Moyo for providing technical information on all kinds of issues we encountered during our research and the answers he gave to all our questions. Special thanks go to Len of the irrigation workshop for building the diver installation cages and for the enjoyable evenings in the restaurant of the estate. Of course we want to warmly thank our professor Huub Savenije for inspiring us with his knowledge of alluvial estuaries and his corresponding enthusiasm. Our discussions were very fruitful and we enjoyed working together. We also would like to thank Willem Luxemburg for his advice and help with the preparation of our measurement equipment and methods. We would like to thank Van Essen instruments for providing us with CTD – divers and advise how to use them. Last but certainly not least we have to thank ARA-Centro and in particular Antonio Melembe, Castro Júnior and Bimbo for all their help. They provided us with a fantastic boat, hydrological data and accompanied us on all our field trips. Even if we had to get up very early in the morning you guys stood ready to help us out. We cannot thank you enough and it is hard to express our appreciation in words. We hope we get the opportunity to meet you guys again and we will remember the special moments. Special thanks are given to the TBI fund and the Lamminga fund for supporting us financially and making it possible to go on field visit in Mozambique. 5 Contents Notation .................................................................................................................................................. 4 Preface .................................................................................................................................................... 5 Acknowledgements ................................................................................................................................. 5 Abstract ................................................................................................................................................... 8 1. Introduction ......................................................................................................................................... 9 1.1 Estuary description ........................................................................................................................ 9 1.2 Problem statement ....................................................................................................................... 10 1.3 Research objective ....................................................................................................................... 10 2. Theory on salt intrusion ..................................................................................................................... 11 2.1 Geometry alluvial estuaries .......................................................................................................... 11 2.2 The tidal wave ............................................................................................................................. 12 2.3 Mixing processes in alluvial estuaries .......................................................................................... 13 2.4 Steady state model ....................................................................................................................... 16 2.5 Unsteady state model ................................................................................................................... 17 2.5.1 Theory of unsteady state model ............................................................................................. 17 2.5.2 Application of unsteady state model ...................................................................................... 19 3. Description of the study ..................................................................................................................... 20 3.1 Estuary geometry ......................................................................................................................... 20 3.2 River discharge ............................................................................................................................ 20 3.3 Salinity measurements ................................................................................................................. 20 4. Elaboration on salt intrusion models .................................................................................................. 22 4.1 Determination estuary geometry................................................................................................... 22 4.2 Application steady state model ..................................................................................................... 24 4.2.1 Calibration steady state model ............................................................................................... 24 4.2.2 Irregularities in salinity measurements ................................................................................... 25 4.3 Application unsteady state model ................................................................................................. 28 4.3.1 Calibration unsteady state model ........................................................................................... 28 4.3.2 Additional salt source in unsteady state model ....................................................................... 29 4.3.3 Calculation additional salinity source .................................................................................... 30 4.3.4 Virtual experiment alternated pumping .................................................................................. 31 6 5. Development of scenarios .................................................................................................................. 32 5.1 Set up of the scenarios ................................................................................................................