TY - THES
T1 - Nature-based solutions to mitigate salt intrusion
AU - Hendrickx, G.G.
PY - 2024
Y1 - 2024
N2 - Worldwide, estuaries are centres of life. Their connection with the open seas while providing fertile lands and freshwater has resulted in these regions becoming major hubs of human settlement and activities, which is expected to continue growing in magnitude. However economically useful, the open connection to the seas also poses a challenge: freshwater availability. Via this open connection saline water can enter the estuary and contaminate the freshwater reserves - so-called (estuarine) salt intrusion. This may cause freshwater shortages and damage to all living beings that depend on this invaluable resource.This research aims to develop mitigation measures to this estuarine salt intrusion following the Building with Nature-philosophy. Thus, the goal of this research is to develop nature-based solutions to mitigate salt intrusion. This is achieved by (1) exploring the potential of estuary-scale interventions to affect salt intrusion; (2) conceptualising various nature-based solutions that mitigate salt intrusion; (3) evaluating nature-based solutions in a multidisciplinary context; and, to conclude, (4) reflecting on the role of nature-based solutions.The exploration of potential estuary-scale modifications is a computationally expensive endeavour for which a novel simulation strategy is developed. This strategy is presented in Chapter 2 and proposes the use of machine learning techniques to determine the input space - i.e., which model simulations to execute, and which to exclude. The aim of the strategy is to put more focus on exploring the output space instead of exploring the input space.Subsequently, the model simulations are analysed in Chapter 3. Thus, Chapters 2 and 3 present respectively the method and the results of an extensive sensitivity analysis of estuarine salt intrusion to estuary-scale modifications. The end-result of Chapter 3 includes a shortlist of potential nature-based solutions to mitigate salt intrusion, including a ranking based on the sensitivity analysis.The conceptualisation of nature-based solutions focuses mainly on two potential options: (1) a (temporary) sill, or submerged dam (Ch. 4); and (2) enhancement of intertidal area (Ch. 5). In addition to these novel nature-based solutions, Chapter 6 evaluates a third nature-based solution: shallowing of an estuary. From a physical perspective, this mitigation measure is well-known but mainly poses challenges in the socio-economic domain, which is why it is not extensively covered in the conceptualisation-phase of this dissertation but the evaluation-phase instead.Both the sill and the intertidal area show a dependency on estuary class in how effectively the mitigation measures are. For the sill it holds that the weaker the tide, the more effective the sill mitigates landward salt transport - i.e., salt intrusion. When the tide is limited, salt intrusion is largely (if not fully) driven by gravitational circulation, for which the sill functions as a wall beyond which the formed salt wedge can hardly penetrate. However, with tidal energy increasing, the tidal momentum to push the saline water over the sill also increases resulting in more salt intrusion - the sill functions more like a speed-bump than a wall. All in all, a sill is most effective for estuaries with little tidal influence.Enhancement of intertidal area increases the mixing in the estuary. When the dominant salt transport mechanism is related to the estuarine circulation, this enhanced mixing by increasing the intertidal area reduces the salt intrusion. However, the opposite holds for estuaries in which the salt transport is dominated by the tidal oscillation. This means that in case of salt wedge and partially mixed estuaries, enhancement of the intertidal area reduces salt intrusion; and in case of well-mixed estuaries, the intertidal area increase promotes salt intrusion. Thus, the additional (vertical) mixing caused by the intertidal areas reduces salt intrusion as long as there is something to mix - i.e., as long as there is a (sufficient) vertical salinity gradient.The evaluation of nature-based solutions is inherently multidisciplinary. In this dissertation, two different perspectives are considered: (1) socio-economy (Ch. 6), and (2) socio-ecology (Ch. 7). In both cases, the Rhine-Meuse Delta is considered as case study.The socio-economic evaluation addresses the effect of water depth on two major stakeholders in an estuary: a port, and water boards. These stakeholders have opposing interests regarding the water depth: a port benefits from enhanced water depth to facilitate larger vessels, but the resulting contamination of freshwater reserves via salt intrusion is negatively affecting water intakes, and everyone that depends on them. Chapter 6 presents a multidisciplinary evaluation method based on Pareto-fronts, which are to inform decision- and policy-makers. The Pareto-front in Chapter 6 shows that the port performance remains relatively unaffected for shallowing until two metres with the current bed levels, while the freshwater availability improves. However, beyond this level of shallowing, port performance drops substantially against limited gains in freshwater availability.The socio-ecological evaluation focuses on the effects of reopening a closed-off estuary on two opposing interests as well: freshwater availability, and estuarine ecosystem functioning. Reopening is beneficial for the ecological diversity in the former estuary, but comes at the costs of returning saline influences into a freshwater lake - i.e., freshwater availability is hampered. The ecological implications are mapped by translating hydrodynamic (model) data to an ecotopes-potential map due to which the ecological impact of interventions can be quantified. As in Chapter 6, Pareto-fronts are used as method to inform decision- and policy-makers. The Pareto-front in Chapter 7 shows an increased ecological diversity in the former-estuary without impeding the freshwater availability when partially opening the gates. However, once the salt intrusion reaches the most western water intakes, the freshwater availability drops without major gains in diversity.This dissertation has shown the complexity of developing future-proof nature-based solutions to mitigate salt intrusion. Building on the lessons learned, the reflection of this dissertation proposes a next step for nature-based solutions: DARE (diverse, adaptive, and robust engineering; Ch. 8). DARE is based on nature's own approach to dealing with uncertainties. The three axes of DARE show how to deal with uncertainties in the three dimensions of engineering: (1) a diverse set of solutions to deal with uncertainty in forcing conditions; (2) an adaptive approach with room to change course in response to the uncertainty that comes with time; and (3) robust - or even antifragile - solutions regarding the uncertainty in performance. With DARE, the focus shifts from the input to the output: Which part of the output is desirable, and what are the options to get there?The nature-based solutions presented in this dissertation form a starting point for further explorations, with work in this dissertation already being used as a stepping stone for other studies. Besides further exploring the opportunities of nature-based solutions to mitigate salt intrusion, next steps also include bringing these findings into practice. This includes reassessing past estuarine modifications from a new perspective, one that is less susceptible to the challenges that the future may have in store.
AB - Worldwide, estuaries are centres of life. Their connection with the open seas while providing fertile lands and freshwater has resulted in these regions becoming major hubs of human settlement and activities, which is expected to continue growing in magnitude. However economically useful, the open connection to the seas also poses a challenge: freshwater availability. Via this open connection saline water can enter the estuary and contaminate the freshwater reserves - so-called (estuarine) salt intrusion. This may cause freshwater shortages and damage to all living beings that depend on this invaluable resource.This research aims to develop mitigation measures to this estuarine salt intrusion following the Building with Nature-philosophy. Thus, the goal of this research is to develop nature-based solutions to mitigate salt intrusion. This is achieved by (1) exploring the potential of estuary-scale interventions to affect salt intrusion; (2) conceptualising various nature-based solutions that mitigate salt intrusion; (3) evaluating nature-based solutions in a multidisciplinary context; and, to conclude, (4) reflecting on the role of nature-based solutions.The exploration of potential estuary-scale modifications is a computationally expensive endeavour for which a novel simulation strategy is developed. This strategy is presented in Chapter 2 and proposes the use of machine learning techniques to determine the input space - i.e., which model simulations to execute, and which to exclude. The aim of the strategy is to put more focus on exploring the output space instead of exploring the input space.Subsequently, the model simulations are analysed in Chapter 3. Thus, Chapters 2 and 3 present respectively the method and the results of an extensive sensitivity analysis of estuarine salt intrusion to estuary-scale modifications. The end-result of Chapter 3 includes a shortlist of potential nature-based solutions to mitigate salt intrusion, including a ranking based on the sensitivity analysis.The conceptualisation of nature-based solutions focuses mainly on two potential options: (1) a (temporary) sill, or submerged dam (Ch. 4); and (2) enhancement of intertidal area (Ch. 5). In addition to these novel nature-based solutions, Chapter 6 evaluates a third nature-based solution: shallowing of an estuary. From a physical perspective, this mitigation measure is well-known but mainly poses challenges in the socio-economic domain, which is why it is not extensively covered in the conceptualisation-phase of this dissertation but the evaluation-phase instead.Both the sill and the intertidal area show a dependency on estuary class in how effectively the mitigation measures are. For the sill it holds that the weaker the tide, the more effective the sill mitigates landward salt transport - i.e., salt intrusion. When the tide is limited, salt intrusion is largely (if not fully) driven by gravitational circulation, for which the sill functions as a wall beyond which the formed salt wedge can hardly penetrate. However, with tidal energy increasing, the tidal momentum to push the saline water over the sill also increases resulting in more salt intrusion - the sill functions more like a speed-bump than a wall. All in all, a sill is most effective for estuaries with little tidal influence.Enhancement of intertidal area increases the mixing in the estuary. When the dominant salt transport mechanism is related to the estuarine circulation, this enhanced mixing by increasing the intertidal area reduces the salt intrusion. However, the opposite holds for estuaries in which the salt transport is dominated by the tidal oscillation. This means that in case of salt wedge and partially mixed estuaries, enhancement of the intertidal area reduces salt intrusion; and in case of well-mixed estuaries, the intertidal area increase promotes salt intrusion. Thus, the additional (vertical) mixing caused by the intertidal areas reduces salt intrusion as long as there is something to mix - i.e., as long as there is a (sufficient) vertical salinity gradient.The evaluation of nature-based solutions is inherently multidisciplinary. In this dissertation, two different perspectives are considered: (1) socio-economy (Ch. 6), and (2) socio-ecology (Ch. 7). In both cases, the Rhine-Meuse Delta is considered as case study.The socio-economic evaluation addresses the effect of water depth on two major stakeholders in an estuary: a port, and water boards. These stakeholders have opposing interests regarding the water depth: a port benefits from enhanced water depth to facilitate larger vessels, but the resulting contamination of freshwater reserves via salt intrusion is negatively affecting water intakes, and everyone that depends on them. Chapter 6 presents a multidisciplinary evaluation method based on Pareto-fronts, which are to inform decision- and policy-makers. The Pareto-front in Chapter 6 shows that the port performance remains relatively unaffected for shallowing until two metres with the current bed levels, while the freshwater availability improves. However, beyond this level of shallowing, port performance drops substantially against limited gains in freshwater availability.The socio-ecological evaluation focuses on the effects of reopening a closed-off estuary on two opposing interests as well: freshwater availability, and estuarine ecosystem functioning. Reopening is beneficial for the ecological diversity in the former estuary, but comes at the costs of returning saline influences into a freshwater lake - i.e., freshwater availability is hampered. The ecological implications are mapped by translating hydrodynamic (model) data to an ecotopes-potential map due to which the ecological impact of interventions can be quantified. As in Chapter 6, Pareto-fronts are used as method to inform decision- and policy-makers. The Pareto-front in Chapter 7 shows an increased ecological diversity in the former-estuary without impeding the freshwater availability when partially opening the gates. However, once the salt intrusion reaches the most western water intakes, the freshwater availability drops without major gains in diversity.This dissertation has shown the complexity of developing future-proof nature-based solutions to mitigate salt intrusion. Building on the lessons learned, the reflection of this dissertation proposes a next step for nature-based solutions: DARE (diverse, adaptive, and robust engineering; Ch. 8). DARE is based on nature's own approach to dealing with uncertainties. The three axes of DARE show how to deal with uncertainties in the three dimensions of engineering: (1) a diverse set of solutions to deal with uncertainty in forcing conditions; (2) an adaptive approach with room to change course in response to the uncertainty that comes with time; and (3) robust - or even antifragile - solutions regarding the uncertainty in performance. With DARE, the focus shifts from the input to the output: Which part of the output is desirable, and what are the options to get there?The nature-based solutions presented in this dissertation form a starting point for further explorations, with work in this dissertation already being used as a stepping stone for other studies. Besides further exploring the opportunities of nature-based solutions to mitigate salt intrusion, next steps also include bringing these findings into practice. This includes reassessing past estuarine modifications from a new perspective, one that is less susceptible to the challenges that the future may have in store.
KW - estuaries
KW - salt intrusion
KW - nature-based solutions
KW - Building with Nature
U2 - 10.4233/uuid:12e26179-4ebb-4f05-91bc-590d4c575e98
DO - 10.4233/uuid:12e26179-4ebb-4f05-91bc-590d4c575e98
M3 - Dissertation (TU Delft)
SN - 978-94-6506-771-1
ER -