MSC. THESIS RAPID ASSESSMENT OF FLUSHING OF COASTAL EMBAYMENTS - WITH APPLICATION TO DOHA BAY - M.E.A. VAN DER VEN JULY 8, 2014 I Milou Elisabeth Anne van der Ven RAPID ASSESSMENT OF THE FLUSHING OF COASTAL EMBAYMENTS - With application to Doha Bay - THESIS For the degree Master of Science (MSc) in Hydraulic Engineering, Faculty Civil Engineering, Delft University of Technology AND For the degree Master of Science (MSc) in Water Resources Management, Faculty of Civil and Environmental Engineering, National University of Singapore In cooperation with Royal HaskoningDHV (Amersfoort) and Deltares (Delft) Amsterdam, July 8, 2014 SUPERVISORS Prof. dr. ir. M.J.F. Stive TU Delft Dr. ir. R.J. Labeur TU Delft S. Pande TU Delft Dr. V. Shua NUS Dr. L. Y. Min NUS Ir. J.H. ter Hoeven RoyalHaskoningDHV Ir. R. Morelissen Deltares II III ACKNOWLEDGEMENTS With the completion of this Master Thesis, the Double Degree Program has come to an end. These past years in which I have studied both at the TU Delft and at the National University of Singapore were very intensive, yet most rewarding. My semester in Singapore was a challenging and unique experience from which I have learned a lot about science and hydraulic engineering in particular, but also about other cultures and about myself. I would hereby like to show my gratitude to Paul Visser, who made this incredible journey possible. I sincerely appreciate it that you had faith in me and gave me the chance to participate in this great program. The past months of which I have spent most of my time at Royal HaskoningDHV have also been an interesting experience in which I have learned a lot about Royal HaskoningDHV and the work of engineering consultants. I would like to thank my supervisor Joost ter Hoeven for giving me the opportunity to graduate within the department of ‘Hydraulica en Morfologie’. Moreover, throughout these months you have given me the guidance and support that were necessary to make this project to a success. You have particularly helped me in distinguishing the relevant matters from those of less importance. Furthermore, I would like to thank my colleagues at Royal HaskoningDHV, who made feel at home and were always willing to help. Special thanks go to Lars, who helped me a lot with the numerical flow model. In addition, I would like to thank my supervisor at Deltares: Robin Morelissen. Your enthusiasm for this project was very encouraging and our weekly discussions were often necessary to put me back on track in times of struggle. Being the voice of the future user of my developed method, you have taught me to look at the problem from a more practical or engineering point of view. I would also like to express my sincere gratitude to my TU Delft mentor Robert Jan Labeur, who pushed me to raise the project to a higher level. Your guidance and extensive feedback were extremely valuable. You have taught me to approach the problem scientifically. Moreover, you have helped me enormously with the structure and contents of the report. I truly appreciate all the time and effort you have spent to help me to bring this report to a higher level. Furthermore, I would like to thank my professor Marcel Stive, who has had a crucial role particularly in defining the project objective and in making the approach more concrete. I greatly appreciate it that, despite the pressure on your agenda, you really took the time to help me out through this difficult project phase. I would also like to thank Saket Pande, Dr. Vivien Chua and Dr. Low Ying Min for being part of my graduation committee and making this graduation happen. Last but certainly not least, I would like to thank Carel, my parents and my sisters. Your endless support, encouragements and expressions of faith were necessary to bring this project to an end. I truly appreciate this. Milou van der Ven Amsterdam, July 2014 IV EXECUTIVE SUMMARY Extensive developments in coastal waters form a potential threat to the water quality worldwide. Most developments result not only in an increase of waste discharges but they might also influence the bay’s flushing conditions. The bay’s flushing can be described as the removal of pollutant concentrations by means of hydrodynamic transport, governed by the exchange of bay water with the external water body (CHOI et al., 2004). Because water is the carrier of (polluted) material, this exchange or flushing is a crucial determinant in the water quality (TAKEOKA, 1984). To make sure that the developments do not negatively affect the flushing conditions and inherently the water quality, it is therefore important to take the possible consequences of the intended changes to the bay’s flushing into account in the designs. However, no rapid assessment method is available yet, which can be used for engineering practises to obtain quick estimates regarding the flushing conditions in coastal bays under the influence of human developments in the bay. Either the computational time and amount of data required are too large or the level of accuracy is too low. Therefore the objective of this study was to develop a method for the rapid assessment of the influence of intended changes to a coastal bay on its flushing conditions. The study was focused on tidally dominated, small-scale, shallow water coastal bays. Through an extensive literature review, first the relevant physical processes to flushing were detected. Next, possible indicators were described and analysed in order to find the most suitable way to quantify the flushing conditions. It was found that for applications to environmental problems, it is common and suitable to quantify the exchange process by the use of the relevant transport time scale(s). Furthermore, when the total bay’s flushing conditions are concerned, the average residence time as defined by TAKEOKA (1984) was shown to be the most suitable transport time parameter. The average residence time represents the average time a single particle is situated in the bay. To define the approach towards the development of the rapid assessment method, the available methods to calculate the average residence time were elaborated. Methods based on a box, one-dimensional or two- dimensional representation of the bay were distinguished. It was decided that the rapid assessment method should be of the box type because this leads to the most rapid calculations. The relevant flushing processes which were not represented in the original box models can be accounted for through basic bay parameters of which the influence on flushing is fundamentally derived by the use of a more detailed, two-dimensional model. To assess the relation between the chosen relevant basic parameters (e.g. average bay depth, average tidal amplitude) to the average residence time of the bay, numerical calculations are carried out using a schematic 2D model of a coastal bay. Following the systematic approach of the dimensional analysis, the relevant relations between the dimensionless bay parameters and the dimensionless flushing parameter were successfully detected. Based on these relations the following analytical expression of the average residence time was derived: ̅ ( ) ( ) ( ) ( ) ( ) ( ) √ √ √ In which ̅ [days] is the average residence time, [days] the tidal period, [m] the bay’s width, [m] the 2 2 bay’s length, [m] the average bay depth, [m ] the total bay’s surface area, [m ] the cross-sectional 2 inlet area, [m] the Nikuradse roughness number, [m /s] the dispersion coefficient and [m] the tidal amplitude. V Because the value of the dispersion coefficient remains a factor of uncertainty, the most practical approach for the use of this proposed rapid assessment method (RAM) is to estimate the value of the dispersion coefficient and keep this value the same for the different cases of which the flushing conditions are compared. Inherently, this approach leads to the exclusion of the rate of change of internal mixing due to the development (or bay change) of interest. The use of the derived formulation as a RAM was first validated by application of the RAM to three real cases (Kuwait Bay, Boston Harbour and Venice Lagoon), using the results from detailed flushing studies of the areas in question as a reference. From the resulting values of the dispersion coefficients that were found to reflect the ‘realistic’ average residence times, it was concluded that the RAM performs reasonable in the cases of Kuwait Bay and Boston Harbour. However, the RAM appeared inapplicable to Venice Lagoon, which is attributed to its very shallow water depths. Finally, the performance of the RAM was further examined by application to Doha Bay. The influence of a number of bay changes on the bay’s flushing conditions is assessed, under which the future planned developments Sharq Crossing and Oryx Island. The predictive value of the rapid assessment method is analysed by comparing the estimates of (the relative change of) the average residence times by the RAM to those values obtained by the use a detailed numerical (depth-averaged) flow model. From the results of the test cases it was concluded that the RAM overestimates the effect of the tidal pumping mechanism which leads to relatively low absolute values of the calculated average residence times resulting from tidal flushing. This is one of the reasons why one should be very careful in putting any value on the absolute numbers derived by the RAM. However, the RAM has shown to be a useful tool for the estimation of the relative rate of change of the average residence time of the bay as a result of bay developments, under the following assumptions: The development is of such a large scale that it has a significant influence on the value one or several of the basic bay parameters in the RAM.