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A new wave-vegetation model is implemented in an open-source code, SWASH (Simulating WAves till SHore). The governing equations are the nonlinear shallow water equations, including non-hydrostatic pressure. Besides the commonly considered drag force induced by vertical vegetation cylinders, drag force induced by horizontal vegetation cylinders in complex mangrove root systems, as well as porosity and inertia effects, are included in the vegetation model, providing a logical supplement to the existing models. The vegetation model is tested against lab measurements and existing models. Good model performance is found in simulating wave height distribution and maximum water level in vegetation fields. The relevance of including the additional effects is demonstrated by illustrative model runs. We show that the difference between vertical and horizontal vegetation cylinders in wave dissipation is larger when exposed to shorter waves, because in these wave conditions the vertical component of orbital velocity is more prominent. Both porosity and inertia effects are more pronounced with higher vegetation density. Porosity effects can cause wave reflection and lead to reduced wave height in and behind vegetation fields, while inertia force leads to negative energy dissipation that reduces the wavedamping capacity of vegetation. Overall, the inclusion of both effects leads to greater wave reduction compared to common modeling practice that ignores these effects, but the maximum water level is increased due to porosity. With good model performance and extended functions, the new vegetation model in SWASH code is a solid advancement toward refined simulation of wave propagation over vegetation fields.

This study examined phytotoxic activities of several Vietnam localised OM rice (Oryza sativa L. OM) cultivars on junglerice (Echinochloa colona (L.) Link), an invasive weed threatening rice production, and identified allelochemicals and their activities for OM 4498, the most allelopathic cultivar. The aqueous MeOH extracts of all rice cultivars inhibited root and shoot growth of junglerice at a concentration as low as 0.01 g ml−1. Inhibitory effects were greatest in OM 380, OM 2395 and OM 4498, with the average effective doses (EDs) of extracts required for 90% inhibition (ED90) on junglerice as low as 0.981, 0.632 and 0.702 g ml−1, respectively. The phytotoxic substances salicylic acid, vanillic acid, p‐coumaric acid, 2,4‐dimethoxybenzoic acid, benzoic acid, p‐hydrobenzoic acid, trans‐cinnamic acid and coumarin were identified in OM 4498 at concentrations ranging from 0.066 to 11.412 mg in 100 g fresh weight of rice tissue. Two of these substances, 2,4‐dimethoxybenzoic acid and coumarin, have never previously been identified in rice. Compared to other compounds, p‐coumaric acid may have the greatest weed‐suppressant potential on junglerice, with the average ED50 being 0.28 mM. The total activities of salicylic acid, vanillic acid, p‐coumaric acid, trans‐cinnamic acid and 2,4‐dimethoxybenzoic acid in an OM 4498 rice plant on junglerice seedlings were 59.60, 0.78, 3.47, 2.52 and 0.61 mM/mM, respectively. The results clearly suggest that OM 4498 rice extract is an excellent source for allelochemicals against junglerice, and this rice cultivar should be considered in rice production areas invaded by junglerice.

Over the last few decades, the Mekong Delta Coast has undergone many physical changes that have increased its vulnerability. Issues that have grown in importance are erosion, human occupation of coastal and estuarine mangroves, decreased sediment supply by the Mekong River and subsidence due to groundwater extraction. These issues have led to the loss of coastal and estuarine land and mangroves, increasing flood vulnerability and salinity intrusion. Recently, young and promising Vietnamese researchers have undertaken a number of in-depth studies to increase our understanding of the above issues. The objective of the present paper is to give a concise description of their work and place it into a broader context. The topics concerned are satellite mapping of coastal landuse changes, numerical simulation of the tide and wave climate and of coastal erosion, coastal and estuarine mangrove squeeze, wave and current damping in mangroves and wave transmission through bamboo fences. The main findings are that (1) coastal landuse has changed significantly over the last decades with the largest change due to conversion of mangroves to aquaculture and a modest change due to coastal erosion, (2) the understanding of the tide and wave climate and of the erosion has increased due to successful numerical modelling, (3) the role of mangrove squeeze along the coast and along the estuaries has been assessed, (4) the understanding of wave and current damping in mangroves and of wave transmission through bamboo fences has increased through the combined effort of laboratory and numerical modelling.