Climate change impacts on the stability of small tidal inlets

Research output: ThesisDissertation (TU Delft)

Abstract

The coastal zones in the vicinity of tidal inlets, which are commonly utilized for navigation, sand mining, waterfront developments, fishing and recreation, are under particularly high population pressure. The intensive population concentration and excessive natural resources exploitation in these areas could lead to biodiversity loss, destruction of habitats, pollution, as well as conflicts between potential uses, and space congestion problems, which will only be exacerbated by foreshadowed climate change (CC). Although a very few recent studies have investigated CC impacts on very large tidal inlet/basin systems, the nature and magnitude of CC impacts on the more commonly found small tidal inlet/estuary systems remains practically undnvestigated to date. These relatively small estuaries/lagoons (also known as "bar-built" or "barrier" estuaries, and hereon referred to as Small Tidal Inlets or STIs) are common along wave-dominated, micrcrotidal mainland coasts comprising about 50% of the world's coastline.
Due to their common occurrence in the tropical and sub-tropical zones, most STIs are found in developing countries, where data availability is generally poor (i.e. data poor environments) and community resilience to coastal change is low. Furthermore, STI environs in developing countries especially host a number of economic activities (and thousands of associated livelihoods) which connibute significantly to the national GDPs. The combination of pre-dominant occurrence in developing countries, socio-economic relevance and low community resilience, general lack of data, and high sensitivity to seasonal forcing makes STIs potentially very vulnerable to CC impacts and thus a high priority area of research. This study was therefore undertaken with the overarching objective of (a) developing methods and tools that can provide insights on potential CC impacts on STIs, and (b) demonstrating their application to assess CC impacts on the main types of STIs.
Throughout this Thesis, 3 case study STIs representing the 3 main STI Types are used:
- Negombo lagoon, Sri Lanka: Permanently open, locationally stable inlet (Type 1)
- Kalutara lagoon, Sri Lanka: Permanently open, alongshore migrating inlet (Type 2)
- Maha Oya river, Sri Lanka: Seasonally/Intermittently open, locationally stable inlet (Type 3)
To circumnavigate the inability of contemporary process based coastal area morphodynamic models to accurately simulate the morphological evolution of STIs over typical CC impact assessment time scales (e.g. 100 yrs) with concurrent tide, wave and riverflow forcing, 2 different snap-shot modelling approaches for data poor and data rich environments are proposed. The data poor approach uses schematized flat bed bathymetries that follow real world STIs and CC forcing derived from freely available coarse resolution global models while the data rich approach requires detailed bathymetries and downscaled CC forcing. Furthermore, to enable rapid assessments of CC impacts on STI stability, particularly to aid frontline coastal zone managers/planners, a reduced complexity model is developed based on existing knowledge and physical formulations. The model, which is capable of simulating 100 years in under 3 seconds on a standard PC, provides predictions of STI stability based on the Bruun inlet stability criterion.
Although CC driven STI Type changes appear to be rather unlikely in the 21th century, model results do show that CC is likely to change the level of stability of STIs, indicated by significant future changes of the value from its present value. At Type 1 STIs, future CC driven increases/decreases in longshore sediment transport may result in decreases/increases in their level of stability. At Type 2 and Type 3 STIs concurrent increases (decreases) in longshore sediment transport and decreases (increases) in riverflow may result in decreasing (increasing) the level of inlet stability. Sea level rise (SLR) appears not to be the main driver of change in the level of STI stability, with CC driven variations in wave direction emerging as the major driver of potential change in STI stability.
For future CC impacts assessment at STIs, an initial assessment using the reduced complexity model is recommended. If Type changes are predicted at any time (or if r drops below 10 for a Type 2 STI), or if specific insights (e.g. migration distance at Type 2 STIs, inlet closure time at Type 3 STIs) are desired, then it is essential that the (data poor or data rich, depending on which is feasible in the study area) process based snap-shot modelling approach be adopted.
Original languageEnglish
Supervisors/Advisors
  • Roelvink, J.A., Supervisor
  • Ranasinghe, Ranasinghe, Advisor
Award date1 Dec 2015
Publisher
Print ISBNs978-1-138-02944-6
Publication statusPublished - 2015

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