Exploratory modelling of Nature-Based Island development at the Abandoned Yellow River Delta

Abstract

The People's Republic of China experienced large socio-economic development over the last decades, which was mainly concentrated in coastal regions. Jiangsu Province is among these regions, which shows large socio-economic differences between north and south. The Chinese Central Government has allocated Binhai County as a new coastal development zone to lessen inequality. New infrastructure (e.g. Binhai Port) was constructed and industrialisation increased in order to boost local development. Binhai County is located at the Abandoned Yellow River Delta. In 1855, the Yellow River shifted its course which cut off sediment supply to the delta. Major land retreat was stopped by the construction of sea dikes from the 1930's. Ongoing erosion is undermining the present sea defence and endangers the hinterland. The erosion along the delta has engaged the disappearance of mudflat area. Furthermore, large development in southern Jiangsu has led to constraints in usable land for which large mudflat areas were reclaimed. Multiple research pointed the negative effects of reclamation activities on ecology (e.g. decline of shorebird populations). After recognition of the importance of mudflats by the Chinese Central Government, the national policy was set to protect and restore mudflats along the Chinese coast. The erosive features of the Abandoned Yellow River Delta (AYRD) needs counteracting measures. Within this research it is aimed to integrate a multi-purpose design in which engineering and ecology are mutually enhanced. Following the Building with Nature guidelines multiple innovative solutions can be presented. Within this research it is aimed to protect the delta by implementation of islands in front of the coast. Furthermore, it is aimed to enhance mudflat growth which adds ecological value to the delta area. It is expected that by implementation of islands a reduced energetic environment is created in which fine sediment can settle. The Jiangsu Regional Model (JRM) was made available for this research, which is based on Delft3D software. An outcrop has been made at the area of interest which was refined (3:1) and further referred to as the Abandoned Yellow River Delta Model (AYRDM). The hydrodynamical forcing was completed by the addition of waves. The AYRDM was set-up with three sediment fractions from which one added cohesive fraction. The production simulation was set-up for ten morphological years by using a morphological factor of 50. The better obtained understanding of the governing processes optimised island configuration design. Fifteen configurations were designed and subdivided over two series: the Basic- and Exploratory series of eight- and seven configurations respectively. The Basic series was set-up with a systematic approach in order to represent the concept of barrier islands. The Exploratory series comprehend designs based on the entrapment of sediment by using sediment transport patterns. The definition of a mudflat was set from -0.88 m to all above, relative to mean sea level in order to measure mudflat growth. The findings within the reference production simulation (without the implementation of islands) showed a flood dominant environment in which flood direction is south. Hence, south- and offshore directed residual sediment transport were observed along the AYRD. The outgoing sediment flux from the AYRDM increased by 2.2E7 ton/year when waves were included. The addition of waves increased the average grain size by 26 µm at siltation areas due to sediment stirring and cross-shore transport. The effect of island configurations showed most favourable sediment entrapment when elongated islands and small inlet widths were applied. The shallows induce larger friction on tidal movement for which current velocities and herewith sediment transport capacity is reduced over distance. Hence, depositional areas were found to be radially deflected to the south. Secondary flow, due to eddy formation, showed significant streaming induced sediment transport behind the islands ends. Small bed slopes induce larger distances between islands and mainland China, which holds for relatively larger mudflat areas. Wave penetration was reduced significantly when islands were implemented, which created a less energetic environment. The largest increase of mudflat relative to the reference simulation is 59.8 km2 after ten morphological years by implementing 16 km2 of island area. The findings within this study are representative for the Abandoned Yellow River Delta area and show the potential of islands in order to protect the delta and create additional ecological/recreational benefits alongside.