Reclamation of Tidal Flats Within Tidal Basins Alters Centennial Morphodynamic Adaptation to Sea‐Level Rise. Issue 6 (23rd June 2022)

• Record Type: Journal Article
• Authors: Guo, Leicheng; Zhu, Chunyan; Xu, Fan; Xie, Weiming; van der Wegen, Mick; Townend, Ian; Wang, Zheng Bing; He, Qing
• Is Part Of: Journal of geophysical research. Volume 127:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-23
• Subjects: tidal basin -- embankment -- sea‐level rise -- morphodynamic modeling; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006556
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Reclamation of low‐lying tidal flats and floodplains adjacent to present shorelines has been implemented worldwide for both coastal defense and development. While it is technically feasible to monitor the short‐term impact of tidal flat embankments, it is challenging to identify long‐term and cumulative morphodynamic impact, particularly considering centennial sea‐level rise (SLR). In this study, we construct a process‐based hydro‐morphodynamic model for a schematized tidal basin and examine its morphodynamic evolution under the combined influence of SLR and tidal flat embankments. We see that rising sea levels lead to inundation of low‐lying floodplains just above high water, creating new intertidal flats that mitigate the drowning impact of SLR. This mitigation effect is lost if the low‐lying floodplains and tidal flats are reclaimed, preventing any shoreline migration under SLR. Removing a large portion of intertidal flats within the tidal basin induces significant changes in basin hypsometry and potentially, a reversal of flood/ebb dominance. The resulting hydro‐morphodynamic impact of large‐scale tidal flat embankment is more significant than SLR at a centennial time scale. This suggests a need for much greater management awareness regarding the cumulative impact of human activities. These findings imply that allowing lateral shoreline migration under SLR sustains tidal basin's inherent morphodynamic buffering capacity, whereas reclaiming tidal flats significantly alters hydro‐morphodynamic adaptation at the decadal to centennial time scales. It highlights the importance of conserving low‐lying floodplains and tidal flats in tide‐dominated systems to counteract the drowning impact of SLR. Plain Language Summary: Tidal estuaries and deltas host valuable wetlands and ecosystems wordwidely. They are also vulnerable to sea level rise (SLR) and human intervention when tidal flats are reclaimed to generate new land and to defend coasts from erosion and flooding. Recently, nature‐based solutions like wetland restoration have become increasingly popular in coastal management practice. However, the benefits of these approaches are not yet fully explored. Using a large‐scale morphodynamic model of a schematized tide‐dominated basin, we find that morphodynamic adaptation of an unconstrained basin under SLR mitigates the drowning effect by lateral shoreline migration, extending the tidal flats landwards. Reclamation of tidal flats, however, reduces the surface area, tidal prism and intertidal storage volumes within the tidal basin, which may even induce a regime shift from sediment export to import. SLR then exacerbates the impact of tidal flat embankments. These findings suggest that tidal flat losses can fundamentally change tidal systems' hydro‐morphodynamic adaptation at decadal to centennial time scales, and confirm the importance of conserving and restoring tidal wetlands. Key Points: Progressive tidal inundation of low‐lying land adjacent to existing tidal flats enables them to migrate landwards in response to sea level rise (SLR) Whether this maintains, extends or reduces the area of tidal flat, depends on the topography adjacent to the tidal basin Large‐scale reclamation of intertidal areas can have a more significant impact on morphology than SLR at the centennial time scale