The physical solution space for mangrove restoration and conservation for coastal flood protection in the Mekong River Delta

Abstract

The Mekong River Delta (MRD) in Vietnam is currently under severe threat of coastal flooding due to rapid growth and industrialisation of the delta population and climate change. Mangrove restoration and conservation (MRaC) is known to protect against coastal flooding, among other valuable ecosystem services and is thus a promising nature based solution to this problem in the MRD. However, uncertainty exist about the long-term effectiveness of mangroves to protect against coastal flooding under large SLR, subsidence and low sediment input conditions that are present in the MRD. Defining “solution space” can help decision makers tackle future uncertainty of adaptation measures (Haasnoot et al. 2020). The solution space represents the boundaries of what is economically, politically, socially, and technically possible ("room to manoeuvre”). It consists of the different possible ways of action regarding climate adaptation and their estimated costs, risks, difficulties and benefits in the longterm future, and is constantly changing its form due to new insights, views and opportunities. This study’s goal is to identify and describe the “physical” solution space for MRaC in the MRD until 2100. Physical is defined here as the climatic and environmental conditions that will allow effective protection from mangroves against flooding. The results could support decision makers to (1) work towards (a) the total solution space of MRaC in the MRD and (b) the solution space of climate adaptation measures in the MRD as a whole. Finally, a better understanding of the physical solution space will support decision makers in the MRD with legislation for mangrove protection and restoration. To this end I made a spreadsheet model that calculates, along 1D profiles, where and if mangroves survive and can protect against flooding in the future: the Dynamic Mangroves Model (DMM). The model is based on the 6 most influential physical factors for MRaC identified in this study: 1) current elevation of the MRD coastal profile (geomorphology), (2) SLR, (3) subsidence (natural and humaninduced), (4) tidal range, (5) human-induced mangrove barriers, (6) sedimentation within mangroves that is dependent on suspended sediment input (SSC), and organic matter accumulation. Nine selected profiles along the southeastern (SE) MRD coast were modelled according to 3 linked scenarios for SLR, land subsidence, sediment supply and placement of embankments: (S1) sustainable: SSP1-2.6 with an embankment retreat of 5 km, a gradual sediment supply increase to +50% in 2100 and no more groundwater extraction, (S2) middle of the road: SSP2-4.5 with an embankment retreat of 2.5 km, a stable sediment supply and without increasing groundwater extraction, (S3) fossil fuel development: SSP5-8.5 with no embankment retreat, a gradual sediment supply decrease to -50% in 2100 and an increasing groundwater extraction (3% per year increase). The average flooding date of the simulated MRD profiles ranged from the year ~2065 in S3 to ~2095 in S2 to ~2110+ in S1 (2110 being maximum in the simulation). However, even in the sustainable best case scenario and using all adaptation measures considered in this research, RSLR will still occur at the SE MRD coast. Eventually, without extra mitigation measures the SE MRD will drown. The most important physical factors that impede MRaC in the MRD are 1) SLR, 2) decreased sediment input, 3) human-induced subsidence, and 4) limited space for mangroves to retreat. Because of sedimentation feedback effects, combining multiple mitigation measures will have a greater total positive influence on MRaC solution space than the sum of the effect of each individual measure. It is thus recommended to implement MRaC in conjunction with other mitigation strategies to maximally extend the MRD lifetime and buy crucial time to adapt.