Plastic is an emerging pollutant, and the quantities in rivers and oceans are expected to increase. Rivers are assumed to transport land-based plastic into the ocean, and the fluvial and marine transport processes have been relatively well studied to date. However, the processes controlling the transport in tidal rivers and estuaries, the interface between fluvial and marine systems, remain largely unresolved. For this reason, current estimates of riverine plastic pollution and export into the ocean remain highly uncertain. Hydrodynamics in tidal rivers and estuaries are influenced by tides and freshwater discharge. As a consequence, flow velocity direction and magnitude can change diurnally. In turn, this impacts the transport dynamics of solutes and pollutants, including plastics. Plastic transport dynamics in tidal rivers and estuaries remain understudied, yet the available observations suggest that plastics can be retained here for long time periods, especially during periods of low net discharge. Additional factors such as riparian vegetation and riverbank characteristics, in combination with bi-directional flows and varying water levels, can lead to an even higher likelihood of long-term retention. Here, we provide a first observation-based estimate of net plastic transport on a daily timescale in tidal rivers. For this purpose, we developed a simple Eulerian approach using sub-hourly observations of plastic transport and discharge during full tidal cycles. We applied our method to the highly polluted Saigon River, Vietnam, throughout six full tidal cycles in May 2022. We show that the net plastic transport is about 20%-33% of the total plastic transport. We found that plastic transport and river discharge are positively and significantly correlated (Pearson's R2Combining double low line0.76). The net transport of plastic is higher than the net discharge (20%-33% and 16%, respectively), suggesting that plastic transport is governed by factors other than water flow. Such factors include wind, varying plastic concentrations in the water, and entrapment of plastics downstream of the measurement site. The plastic net transport rates alternate between positive (seaward) net transport and negative (landward) net transport as a result of the diurnal inequality in the tidal cycles. We found that soft and neutrally buoyant items had considerably lower net transport rates than rigid and highly buoyant items (10%-16% vs. 30%-38%), suggesting that transport dynamics strongly depend on item characteristics. Our results demonstrate the crucial role of tidal dynamics and bi-directional flows in plastic transport dynamics. With this paper we emphasize the importance of understanding fundamental transport dynamics in tidal rivers and estuaries to ultimately reduce the uncertainties of plastic emission estimates into the ocean.

@en

Strategic delta planning focuses on strategic, long-term choices to stimulate sustainable development in deltas. Strategic delta plans outline a long-term vision to be embedded into the plans and activities of government agencies and semi-public actors at multiple levels. This implies a form of coordinated, yet decentralized, implementation. Although, its importance is widely acknowledged, there are few analytical approaches to assess the feasibility and possible bottlenecks of such implementation processes. This article applies a motivation and ability (MOTA) framework to assess the implementation feasibility of the Mekong Delta Plan in Ben Tre province, Vietnam. The results reveal diverging motivations and a perceived lack of ability among government actors at local and regional level. When not well-managed, this could hamper the translation of the strategic goals and visions into local and regional actions. This suggests the usefulness of the MOTA framework as a tool to help manage implementation processes for strategic delta planning.

@en

The Vietnamese Mekong Delta has been recognized as a hotspot of vulnerability to climate change and its effects. Under some climate change projections, the delta could face 40% inundation by the end of the century, presenting serious challenges to the sustainability of agricultural systems in the delta and suggesting the need for transformations to more climate-resilient livelihood models. The Vietnamese government acknowledges these threats and has initiated a policy process to offer solutions; the Mekong Delta Plan. A central goal for the coastal region is ‘adaptation to salinity’. Adaptation to salinity does not only require an adjustment in the hydrological setting, but includes changes in livelihood practices as well. Therefore, this study aimed to add a bottom-up perspective to inform planning practices based on a behavioural perspective, centralizing the motivations and abilities of actors to act (MOTA). In this this study, we conducted a MOTA assessment at two levels; an assessment of motivations and abilities at farm level and an assessment of local government actors to adapt their planning and implementation practices. The empirical work was conducted in coastal Ben Tre province and included 100 structured interviews of farmer households; three focus groups with a total of 27 farmers; and semi-structured interviews with 28 representatives of provincial and district level government agencies. The analysis showed that the local setting is a very pluralistic one; the abilities to change differed greatly among the communes included in the analysis. The analysis discovered the underlying factors behind farmers’ self-perceived motivation and abilities. Furthermore, the analysis shows that each of the communes has its own preferred livelihood model; from the safe rice model in Phu Ngai to oyster, mushroom and Lingzhi in Vinh Hoa. These differences can partly be explained by the physical (hydrological) conditions, such as the availability of fresh water. However, there are also differences that cannot be explained by the physical conditions, such as the (un)ability of farmers to cooperate in farmer cooperatives or the difference in motivation to change. The analysis of the governance setting shows that salinity intrusion is primarily perceived as a threat. Coping with salinity is therefore not an opportunity for livelihood transformations, but salinity levels should be controlled and monitored. There is a high motivation to change, but this is a change towards ‘controlling salinity’ rather than ‘adapting to salinity’. An additional threat is the (uncontrolled) shift towards brackish water aquaculture. Not only do these farms interfere in the fresh water system, but availability of fresh water stays important, for drinking water supply, but also to maintain the right levels of brackish water. The ability assessment shows a low ability to change for all actors. The actors state to a lack of finances, techniques, and institutional abilities to improve current situation. This study also showed that with the MOTA bottom – up approach, the social adoptability i.e. whether or not farmers adopt plans, should be appropriately supported by governments and other organizations given their limited abilities. @en

In the context of climate adaptation planning there are relationships between adaptation drivers and adaptation measures, which makes the selection and implementation of the adaptation measures a challenging task. This challenge may be addressed by: structuring the adaptation problem using a multiple perspective adaptation framework; and applying a context specific precedence grammar logic for selecting and evaluating adaptation measures. Precedence grammar logic is a set of rule based algorithms (grammar) that are based on the relationships in a local adaptation context. This paper demonstrates the application of a context specific precedence grammar logic in an adaptation context in Can Tho, Vietnam. Adaptation pathways comprising flood adaptation measures (i.e. dike heightening) for this case were generated using rule based algorithms based on the relationships between the drivers and the adaptation measures. The results show that complex adaptation issues that are structured, can be resolved using a context specific adaptation grammar approach.@en