Flood defences are in practice often multi-used, multi-managed and multi-financed. Flood defence asset management contains technical, organizational and spatial complex issues involving multiple organizations. In the literature, little attention has been given to the conditions for successful cooperation between organizations in flood defence asset management. This paper elaborates on this aspect of mature asset management from a practical point of view. Although the importance of a fit-for-purpose cooperation seems trivial, practice shows that the shape of cooperation is often the coincidental result of implicit or ad-hoc choices and is not deliberately designed. This paper reports on empirical data gathered in a case consisting of five different situations related to collaboration in flood defence management. The management context consists of three main tasks: performance assessment, reinforcement and daily management, and three decision levels: strategic, tactical and operational, resulting in nine different management environments and related interfaces. For effectively achieving desired outcomes, the shape of cooperation has to be explicitly chosen dependent on the complexity of content and organizational context, and relevant external circumstances: situational cooperation.

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Dredged cohesive sediment is progressively being used for wetland construction. However, little is known about the effect of plant growth during the self-weight consolidation of this sediment. In order to check the feasibility of such a study, a new experimental setup has been constructed. As an example, the effect of Phragmites australis on the consolidation and drainage of dredged sediment from Lake Markermeer, the Netherlands was investigated. The changes in pore water pressures at 10 cm depth intervals during a 129-day period in a column with and without plants were measured, while the water level was fixed at a constant level. Water loss via evaporation and plant transpiration was measured using Mariotte bottles and the photosynthetic processes — including plant transpiration — were measured with a LI-COR photosynthesis system. The results show that several processes initiated by Phragmites australis interfere with the physical processes involved in sediment drainage and consolidation. Phragmites australis effectively altered the pore pressure gradient via water extraction, especially between 40 and 60 cm from the bottom of the column. In this zone, daily cycles in pore pressures were observed which could directly be linked to the diurnal cycle of stomatal gas exchange. On average, water loss via evaporation and transpiration of leaves of Phragmites australis amounted to 3.9 mm day⁻¹, whereas evaporation of bare soil amounted on average to 0.6 mm day⁻¹. The depth-averaged hydraulic conductivity increased on average by 40% in presence of Phragmites australis. This pilot experiment confirms that the pressures sensors coupled with the new set-up enable to study pore pressure development over time and to link the effect of plant growth with alterations in water pressures profiles. A more systematic study with this set-up will in the future enable to quantify the effects of plant growth on consolidation.@en

Progressively, more dredged sediments are being reused for engineering projects. For example, the Marker Wadden is a new wetland constructed in lake Markermeer, the Netherlands, with dredged cohesive sediments originating from the bed of the lake. Such dredged sediments are often dominated by cohesive sediment particles with varying amounts of sand and organic matter. In addition, during and after the construction process, the material consolidates and is often compressed from a very loose state into a significantly denser condition. To assess the mechanical behavior of this material, the compressibility of the Markermeer dredged sediment samples with various sand and organic matter contents was analyzed with incremental loading oedometer tests, whereas the undrained shear strength was studied using the fall cone test. The behavior was theoretically analyzed assuming a fractal structure of the sediment and applying power law constitutive equations for effective stress, hydraulic conductivity, and undrained shear strength. These constitutive equations, usually used at low initial sediment densities, worked well at the relatively high initial densities studied and proved to be a useful tool to identify the transition fines content TFC. The constitutive equations were put in context with indicators traditionally used in soil mechanics. Samples, each with an identical composition of the fines fraction (particles< 63 μm), but with a sand content varying from 9 to 40%, showed the same compressibility and undrained shear strength behavior when considering the sand a filler material. For a natural sand content of 70%, the behavior was dominated by sand. The organic matter oxidation was observed to drastically decrease the compressibility and the shear strength, and even to decrease the amount of sand needed to exhibit sand-dominated behavior, showing the importance of the reactivity or state of organic matter on the TFC@en

We studied the drying behavior of slurries of Markermeer sediments in the Netherlands having different solid compositions. Natural processes such as sand–mud segregation and oxidation of organic matter were mimicked to analyze the effect of changes in sediment composition. Evaporation experiments were performed with soft slurry samples using the Hyprop setup. Soil water retention curves (SWRCs) and hydraulic conductivity curves (HCCs) were determined as a function of the water ratio (WR, defined as volume of water/volume of solids). The sediment remained close to saturation until the end of the experiments. The Atterberg limits reduced significantly after sediment treatment involving drying at 50 °C, rewetting, and chemical oxidation. Furthermore, the oxidized sediment lost capacity to retain water. The SWRCs of sandy and oxidized clays were steeper, and fine-textured sediments showed large water ratios. At low matric suctions, the water retention capacity of the upper sediment samples containing more labile organic matter was larger than that of the sediment underneath. Clear correlations were found between van Genuchten parameters and the degree of degradation of the organic matter. The hydraulic conductivity of fine-textured samples with less labile organics was larger. The results give insight into the drying behavior of Markermeer sediment, currently used to build wetlands.

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The Marker Wadden project aims to improve the ecosystem of Lake Markermeer (The Netherlands) by constructing a wetland with sediment from the lake. Sediment is dredged from the bed, and the resulting slurries are pumped into the project area. During this process, segregation and oxidation of the sediment may occur. The native sediment composition and changes induced by the construction process affect the mechanical behavior of the wetland. The initial stress state of the sediment is another variable affecting the behavior. Over time, vegetation may colonize the wetland, also influencing the mechanical properties of the sediment. These factors were studied in physical and numerical experiments as part of this thesis. First, consolidation experiments in settling columns at low initial concentrations below the gel point (virgin consolidation) were performed, and the material parameters were obtained. These parameters were different from the parameters obtained from the Seepage Induced Consolidation (SIC) test because of over-consolidated initial conditions induced by mixing. Numerical simulations were performed with a 1DV consolidation model to quantify the effect of over-consolidation and material parameters on the consolidation behavior. Incremental Loading and Constant Rate of Strain (CRS) tests were performed to analyze the compressibility behavior, and Fall Cone tests were used to determine the undrained shear strength. The fractal theory was found to be a useful tool to normalize and identify the different behavior of samples across all tests. The drying behavior was analyzed with using a Hyprop test, and the Soil Water Retention Curves obtained were fitted with a van Genuchten model. The model parameters were found to be more influenced by the type of organic matter than by its total amount. Finally the effect of Phragmites australis (i.e. common reed) on the consolidation and drying was assessed with a newly-designed column device. The reed acted as an ecological engineer, draining the sediment. However, no differences in the thickness of the sediment layer were found presumably because of armoring by roots. The general conclusion is that over-consolidated initial conditions can be induced by different processes such as mixing and atmospheric drying. Furthermore, the composition of the sediment may change when exposed to segregation and oxidation. In particular, the type of organic matter affects the mechanical behavior of fine sediment at all stages (settling, consolidation, drying) and needs to be characterized. Consequently, the material parameters need to be determined for actual project conditions.@en

Sediment is becoming scarce and fine sediments are progressively used for reclamation projects. Therefore there is an increasing need to use cohesive fine sediments (mud) for land reclamation and nature building. These sediments exhibit larger deformations and consolidation time than sandy sediment, and are therefore more challenging building materials to use. The MarkerWadden (Lake Markermeer, The Netherlands) is one of the first projects using fresh soft mud (with a low-strength and high water content) for wetland construction. In the research, the material properties of natural sediment from the Markermeer were determined and, the consolidation, drying and undrained shear strength was studied for varying solid compositions. Furthermore, the influence of vegetation and drainage during consolidation and drying was investigated.@en

Sediment is becoming scarce. Therefore, soft sediments are progressively being used for nature building. The MarkerWadden is an example of an ongoing Building with Nature (BwN) project which aims to improve the ecology of Lake Markermeer (The Netherlands) by creating a wetland with the cohesive sediments from the bed of the lake. It represents one of the first projects using fresh unconsolidated mud as a filling material. However, building with these fine sediments represents a great challenge, because of their complex properties.@en