Coastal flood risk reduction by mangroves - An engineering perspective
Rik Gijsman is a PhD student in the department of Coastal Systems and Nature-Based Engineering. (Co)Promotors are prof.dr. K.M. Wijnberg and dr.ir. E.M. Horstman from the faculty ET and prof.dr. D. van der Wal from the faculty ITC.
Natural mangrove ecosystems are recognised for their contribution to coastal flood risk reduction. Salt-tolerant mangrove trees occupy sheltered coastlines in for instance estuaries and lagoons in the tropics, subtropics and warm temperate regions globally. Mangrove trees with their characteristic aerial root systems dissipate hydrodynamic energy of storms, thereby each year protecting millions of people from being flooded. Mangrove trees attenuate waves and dampen water flows by enhancing drag with their above-ground biomass, while they stabilise the sediment bed with their below-ground biomass. These ecosystem engineering effects can enhance sediment trapping and seedling recruitment, enabling mangroves to stabilise shorelines, recover from storm damage and adapt to changing environmental conditions. These resilient capacities are lacking in traditional hard-engineering infrastructure to reduce coastal flood risk, such as seawalls or dikes, potentially making the contribution of mangroves to coastal flood risk reduction comparatively more sustainable and cost-effective.
Although the interest in the application of mangroves for the reduction of coastal flood risk is increasing, their implementation in coastal engineering and management remains challenging. The application of mangroves requires quantitative assessments of the functionality of mangroves: the ability to attenuate flood events, and their persistence: the ability to maintain functionality for a certain period of time. While quantitative knowledge of mangrove functionality is increasing, knowledge on how biogeomorphological interactions influence mangrove forest development remains insufficient to guarantee mangrove persistence at seasonal to decadal timescales. Moreover, since mangrove functionality and persistence are determined by biogeomorphological interactions, an engineering and management approach that deals with the intrinsic variability and uncertainty in mangroves' contribution to coastal flood risk reduction is required.
This dissertation develops an adaptive management approach for coastal engineering and management with mangroves. The approach presents how process knowledge, monitoring tools and models can contribute to the application of mangroves for the reduction of coastal flood risk. The approach depends on assessments and predictions of mangrove functionality and persistence. This dissertation also increases quantitative knowledge of mangrove ecosystem engineering effects, in support of assessments of mangrove persistence. Firstly, the obtained insights provide a conceptual understanding of how mangrove ecosystem engineering influences the morphological and ecological development of mangrove forests. It was found that the seaward extent of mangrove forests may be governed by external processes (i.e., water levels and waves), but mangrove tree presence may increase sediment accretion and seedling recruitment in the forest fringe. Mangrove ecosystem engineering effects thus contribute to forest fringe development and mangrove persistence. Secondly, these insights provide quantitative design guidance on the implications of mangrove green-belt widths for mangrove persistence. The findings suggest that, for the specific settings of the study site (i.e., the Firth of Thames in Aotearoa New Zealand), a green-belt width of at least 190 m (for sediment accretion) and 400 m (for seedling recruitment) is required to optimally benefit from mangrove ecosystem engineering.
Key words: mangroves, ecosystem engineering, functionality, persistence, biogeomorphological interactions, ecosystem services
