Mangrove forests can be an effective coastal defence in the Pearl River Delta, China

Nature-based solutions, particularly nature-based coastal defenses, are increasingly recognized as crucial for mitigating the impacts of climate change and coastal hazards. Mangroves, intertidal wetlands found globally, provide natural coastal protection against flooding and erosion. The loss of mangroves significantly increases flood risk, highlighting the importance of conservation and restoration efforts. While the local-scale wave height reduction by mangroves is well-established, the factors controlling storm surge reduction remain less understood, with most studies focusing on salt marshes. The effectiveness of mangrove protection is dependent on factors like vegetation density, height, width, fragmentation, storm intensity, duration, and speed, as well as broader landscape characteristics. Most studies primarily address local (within-wetland) surge attenuation, with fewer investigating non-local (upstream) effects, particularly in complex estuarine or delta systems. The Pearl River Delta in China, a densely populated region with major cities like Guangzhou and Shenzhen, presents a critical case study. This delta has experienced both significant mangrove loss and recent restoration efforts, making it an ideal location to investigate the large-scale effectiveness of mangroves as coastal defenses. This study uses numerical modeling to address this gap in understanding by simulating the hydrodynamic effects of mangrove wetlands in the Pearl River Delta during Typhoon Hato, considering both local and non-local impacts.

Existing literature extensively documents the wave attenuation capabilities of coastal vegetation, particularly salt marshes and mangroves, at the local scale. Studies show that vegetation density, height, width, and fragmentation all affect wave reduction. However, understanding the role of these factors in storm surge mitigation remains incomplete, with a bias towards salt marsh studies. The influence of storm characteristics (intensity, duration, speed) and surrounding landscape features on surge reduction has also been investigated. Quantifying within-wetland surge attenuation (the rate of vertical surge reduction per horizontal distance) has yielded varying rates across different vegetation types and locations. While local effects are well-studied, non-local effects (upstream surge attenuation in estuaries) are less understood, especially for mangroves in complex delta environments. The limited large-scale implementation of nature-based coastal defenses reflects this knowledge gap, emphasizing the need for detailed analysis of vegetation interactions within estuaries and deltas.

This study employed a Finite Volume Community Ocean Model (FVCOM) implementation for the South China Sea and Pearl River Delta, using an unstructured grid with high resolution (100 m) in the delta. The model was previously validated against observed water level and tidal data from coastal stations and tide gauges. To assess mangrove effectiveness, multiple scenarios were modeled with varying mangrove locations (Shenzhen Bay and upper estuary) and properties. In Shenzhen Bay, simulations included mangrove forests of 300 m, 600 m, and 900 m widths, reflecting actual and potential extents. In the upper estuary, two hypothetical mangrove forests of 300 m width and different lengths (6 km and 16 km) were modeled based on existing plantations. Vegetation resistance was modeled using a momentum sink approach, adding a drag force to the three-dimensional momentum equation, a more accurate approach than simply increasing bottom friction. The drag force was calculated using a quadratic drag law, incorporating mangrove drag coefficient (C_D), flow-facing area per tree (A), and plant density. C_D values were derived from flume experiments on Pearl River Delta mangrove species, distinguishing between mangroves with and without leaves. Projected frontal surface area per tree (A) was estimated based on average trunk diameter and branch structure. The model accounted for the variable submersion of mangroves in intertidal areas. Typhoon Hato (2017) was simulated using wind velocity and air pressure data from the IBTrACS database, using the Holland parametric model to recreate radial wind and pressure profiles. The effectiveness of mangroves under sea-level rise (SLR) scenarios (30 cm, 50 cm, 90 cm) was also investigated by imposing increased mean sea level at the model boundary, simulating a fully protected coastline with mangroves in front of a seawall.

The study revealed that mangrove water level attenuation varies spatially and temporally, highlighting the limitations of a single linear reduction factor. In Shenzhen Bay, a 600 m wide mangrove patch with high drag reduced the maximum total water level from 3.3 m to 2.1 m during Typhoon Hato, and the maximum surge from 2.8 m to 1.4 m. A 900 m patch with high drag further reduced the total water level to 0.5 m and eliminated the surge. Narrower patches or those with low drag were less effective at reducing the peak water level but still provided some attenuation before the peak. The effect of mangroves on tides included time delays, with high-drag patches showing the greatest delays. Under SLR scenarios, the effectiveness of mangroves in Shenzhen Bay decreased, with surge reduction diminishing significantly at higher SLR levels (50 cm and 90 cm). In the upper estuary, hypothetical mangrove forests in the river channels contributed to upstream surge attenuation, reducing water levels in areas near Guangzhou. A 16 km long, 300 m wide patch with high drag caused a surge reduction exceeding 0.2 m upstream. This long patch also created a blockage effect, leading to temporary surge amplification downstream and in eastern channels, however the overall effect was a reduction in maximum surge. The spatial analysis showed localized current speed decreases within the mangrove patches, potentially reducing erosion, but with increases in current speed in some areas, potentially increasing erosion elsewhere. Analysis of different patch lengths and drag coefficients in the upper estuary demonstrated the impact of cumulative vegetation drag on upstream surge attenuation. The high-drag vegetation was generally more effective than low-drag vegetation, and longer patches yielded greater surge reduction. Under SLR scenarios, the upper estuary mangroves still reduced water levels upstream, although the interaction of SLR with river discharge and tides produced complex effects on total water level.

This research provides crucial insights into the effectiveness of mangroves as coastal defenses in complex delta systems. The findings demonstrate that the protective function of mangroves is not uniform but highly dependent on mangrove properties (width, drag coefficient) and location within the delta. While wide mangrove patches are highly effective in local surge attenuation, strategically placed, narrower patches can provide upstream attenuation in estuarine channels. The observed spatial variability in surge reduction and current speed underscores the importance of location-specific modeling for effective coastal management. The study highlights that the benefits of mangrove restoration can be significant, particularly for mitigating the impacts of extreme events. However, the limitations of mangrove effectiveness under SLR conditions emphasize the need to integrate mangrove restoration with other adaptation strategies. Future research could focus on refining the model to account for more complex interactions between mangroves and SLR and the implications of sediment accretion. A cost-benefit analysis comparing mangrove-based coastal protection with traditional hard engineering approaches would provide valuable information for decision-makers.

This study demonstrates the significant potential of mangrove forests for coastal protection in the Pearl River Delta. The effectiveness of mangroves varies greatly depending on their location and characteristics, with wide, high-drag patches offering the greatest local protection, while narrower patches can be effective in upstream attenuation. Sea level rise diminishes the effectiveness of mangroves, highlighting the need for integrated coastal management strategies. Future research should focus on cost-benefit analyses and the integration of sediment accretion dynamics to improve the effectiveness of mangrove-based coastal protection strategies.

This study uses a numerical model with inherent limitations. The accuracy of the model relies on the accuracy of input data and parameterizations, including vegetation properties and storm characteristics. The model doesn't explicitly account for all ecological factors (such as sediment accretion and mangrove resilience) which may influence long-term protection. The SLR scenarios presented focus on a fully protected coastline and do not consider the potential for mangrove inland migration or vertical growth in response to SLR.