Stability Analysis of Dike to Impound Freshwater in Brackish Water Estuarine Environment

This paper presents the stability analysis of a dike proposed to impound the river flood waters within an estuary near the west coast of south India. The proposed dike will be located within Ashtamudi Lake, the second largest lake in the state of Kerala, India, separating fresh water from brackish water. Constructing a dike at Munro Island portion of Ashtamudi, which is about 9.23 km from Arabian Sea coast, can meet the water demands of the region as well as prevent the sinking of Island during high tide. The river dike is designed to construct artificially as rock filled structures. The stratigraphy in the top 2 m consists of clayey mud underlined by fine to medium sand up to 6 m followed by alternate layers of clay and sand up the investigated depth of 15 m. In this paper, the stability of the dike located in an estuarine environment has been investigated and the factor of safety values have been determined with varying water level conditions and varying properties of the underlying soil. A detailed parametric study using RocScience software is presented, considering all the design conditions. The expected settlement of the dike is estimated considering the soil profile at the location both manually as well as using software Geo5. Both were in good agreement and the maximum possible settlement was found to be less than half a meter.

High-resolution 3D geological modelling of the Lek River dike for enhanced flood protection

<p>The Netherlands is a low-lying country: a large, densely populated and urbanised part lies below mean sea-level. The risk of flooding is therefore omnipresent and flood protection measures, such as dikes along rivers, are vital for the safety of the population and their economy. The regional water authorities apply high safety standards when monitoring and maintaining the dikes.</p><p>The water authority Hoogheemraadschap Stichtse Rijnlanden (HDSR) has launched a maintenance program to investigate and reinforce the northern Lek River dike between Schoonhoven and Amerongen. The geology of this area has been shaped by fluvial activity during the Holocene, resulting in a heterogeneous composition of the shallow subsurface. The strength and stability of the dike depend on both its design and the geology in its subsurface. A sandy channel deposit may lead to piping and undercutting of the dike while weak (e.g. peat) or layered strata under certain hydraulic pressures could potentially lead to collapse and catastrophic failure of the dike. Detailed knowledge of the subsurface in the area is therefore essential to design fit-for-purpose reinforcement measures.</p><p>The national GeoTOP model, built and maintained by TNO - Geological Survey Netherlands, is a 3D stochastic geological voxel model that provides insight in the lithostratigraphy and lithology up to a depth of 50 meters below MSL with voxels (3D cells) measuring 100x100x0.5 m. However, to estimate the risk of piping and other forms of instability, HDSR needs a higher level of detail. In this study we therefore constructed a high-resolution voxel model for three sections along the Lek River dike.</p><p>To model the lithology of each voxel we used borehole descriptions, cone penetration test (CPT) data and paleogeographic maps of the Rhine-Meuse Delta. Using CPT data as well as borehole descriptions allowed for higher-resolution modelling. To use the CPT’s for calculation of the lithology, the CPT measurements were translated into lithological classes using an Artificial Neural Network. Special attention was paid to the shape and position of the buried paleo channels, as their presence is a potential risk for piping, and to the mapping of man-made features in the landscape. The resulting 3D geological model has a voxel cell size of 25x25x0.25 m, a resolution that is 32x higher than the GeoTOP model.</p><p>The new high-resolution model is now used by HDSR for:</p><ul><li>identification of dike segments that need further investigation</li> <li>designing location-specific and fit-for-purpose dike reinforcement measures</li> <li>explaining proposed measures to local stakeholders.</li> </ul><p>The first two applications potentially reduce costs significantly; whereas the third application aids creating social foundation for reinforcement measures. Most importantly, the new high resolution model helps HDSR to enhance safety behind the dikes.</p>

Re-evaluating safety risks of multifunctional dikes with a probabilistic risk framework

It is not uncommon for a flood defence to be combined with other societal uses as a multifunctional flood defence, from housing in urban areas to nature conservation in rural areas. The assessment of the safety of multifunctional flood defences is often done using conservative estimates. This study synthesizes new probabilistic approaches to evaluate the safety of multifunctional flood defences employed in the Netherlands and explores the results of these approaches. In this paper a case representing a typical Dutch river dike combining a flood safety function with a nature and housing function is assessed by its probability of failure for multiple reinforcement strategies considering multiple relevant failure mechanisms. Results show how the conservative estimates of multifunctional flood defences lead to a systematic underestimation of the reliability of these dikes. Furthermore, in a probabilistic assessment uncertainties introduced by multifunctional elements affect the level of safety of the dike proportional to the reliability of the dike itself. Hence, dikes with higher protection levels are more suitable to be combined with potentially harmful uses for safety, whereas dikes with low protection levels can benefit most from uses that contribute to safety.