Current and future mudflat losses in the southern Huanghe Delta due to coastal hard structures and shoreline retreat

Abstract

Coastal wetlands, especially broad mudflats, are facing an unprecedented crisis due to combined influence of intensified coastal development activities, drastic river sediment reduction and rapid relative sea level rise. However, it is still a huge challenge to accurately assess the evolution of mudflats for the lack of reliable shoreline indicators on satellite images, especially the indicators for shoreline at low tide. This study, based on instantaneous shoreline positions derived from 430 Landsat satellite images during 1983–2019, uses sea level correction and statistical analysis methods to study the evolution of Laizhou Mudflat, and the intertidal mudflat loss in Laizhou Mudflat was accurately assessed and the effects of multiple factors, especially the effects of various coastal hard structures, on the evolution of Laizhou Mudflat were quantitatively estimated for the first time. The results show that the intertidal mudflat loss caused by the construction of cofferdams was 98 km2, 93% of the total intertidal mudflat loss. Although the previous intertidal mudflat loss caused by shore-parallel structures was less, the shore-parallel structures will cause great mudflat loss in the future, and the broad Laizhou Mudflat will disappear in 110 years owing to the barrier effect of the widespread shore-parallel structures if the current rapid shoreline retreat is not improved. The construction of shore-normal structures can slow the shoreline retreat of adjacent mudflat but accelerate downstream shoreline retreat. As far as coastal protection is concerned, the construction of shore-normal structures in Laizhou Mudflat is inadvisable. Diverting 13 M ton/year of Huanghe sediment can totally offset the mudflat loss on the upstream of Dongying Pier, however, the mudflat located on the downstream of Dongying Pier is hard to restore owing to the block effect of the shore-normal structures to the alongshore transported sediment. Another option is demolishing the shore-parallel structures to recover the landward movement of shoreline, avoiding the mudflat loss at the expense of land loss.