H.J. Verheij

THE ERODIBILITY OF GRASSED INNER DIKE SLOPES AGAINST WAVE OVERTOPPING

INTRODUCTION Dutch river dikes, the core material of which usually consists of clay are covered by grass on both the inner and outer slopes. Sea dikes with hard revetments in the wave impact zone of the seaward slope also have a grass cover on the crest and the inner slope. The grass cover includes grassland vegetation rooted in soil, where two layers can be distinguished: the top soil and the subsoil (Fig. 1, Muijs 1999). The porous turf has a high root density and is elastic in moist conditions. The root structure holds the small clay aggregates together and prevents them from being washed out, whereas the underlying substrate, is stiff or plastic and less permeable. For clay, relatively large forces are necessary to break the aggregates, while relatively small forces are necessary to transport the aggregates.

FIELD TESTS ON SEA DEFENCES SUBJECT TO WAVE OVERTOPPING

acceptance number 215 (in book of abstracts) FIELD TESTS ON SEA DEFENCES SUBJECT TO WAVE OVERTOPPING Akkerman, Gert Jan Bernardini, Patrizia, Van der Meer, Jentsje W., Hoffmans, Gijs J.C.M. Erosion Wave Overtopping Grass slope Grass reinforcement Clay slope Dike Levee Embankment

Resistance versus resilience approaches in road planning and design in delta areas: Mekong floodplains in Cambodia and Vietnam

Engineering works in river basins that explicitly take into account floodplain hydraulic processes and dynamics, demonstrate a move towards more sustainable development in riparian areas. In this paper, this concept is applied to road planning and design in floodplains. The paper suggests that although a resilience strategy might require higher initial investments than a resistance strategy, in the longer term it will result in lower costs in terms of road damage and ecological damage. Results are presented from four cases in the Mekong floodplains where different strategies towards road planning, varying in resilience, have been assessed for their hydraulic, ecological, social and economic impacts based on a combination of modelling results, expert judgement and secondary data sources. The study finds that, with the exception of extreme cases, the impact of roads has a limited impact on the floodplain hydraulics. However, even small changes in flood dynamics (arrival of the peak, duration) may have large ecological impacts, especially if cumulative impacts of more road developments are taken into account. The results illustrate that road planning and design in floodplains is a complicated task that requires an integrated approach.

Instability of Grass caused by Wave Overtopping

This paper presents a method to deal with the instability of turf in a grass cover that could be considered as an engineering approach, in which both turbulence and two strength models (root model and turf-element model) are discussed in a heuristic way. Turbulence is discussed because of its essential role in erosion. The root model determines the shear strength of rooted soil in the vertical direction. The turf-element model based on a force balance predicts not only the instability of the grass cover but also the initiation of motion of grass. The results of the turf-element model are compared with experimental data of some prototype experiments at Dutch sea dikes.