Modelling dike breach formation

Abstract

Dikes constructed of a sand core, clay cover and natural vegetation as (landside) slope protection are common flood defence structures in the Netherlands. The design parameters, such as the crest height of dikes, are determined through risk assessment to reduce the remaining probability of a failure mechanism, such as overflow. Little is known about the exact process leading to failure of the typical Dutch dikes in case the failure conditions actually occur. The most notable floods in the Netherlands have all been caused by dike breaching, where local damage in the dike lead to breach formation. This research describes the process of breach formation due to overflow and determines what design parameters affect the resistance of a dike against breaching. The proposed model (BRAM, a Breach Resistance Analysis Model) combines existing methods to determine the time-to-failure of the grass and clay cover with a newly developed process-oriented headcut erosion model. The model assesses the total time-to-failure of a dike through sequential failure of its components. The following process is simulated. Overflow over a dike causes a turbulent flow along the landside slope, resulting in local damage to the grass cover on either a prescribed weak spot or slope transition. The clay cover is subsequently eroded and the granular core material becomes exposed to the flow. A key assumption in the applied method lies in the fact that every newly exposed layer is less resistant to erosion than the layer above, resulting in slope steepening. Therefore, a near-vertical cliff, referred to as a headcut, is formed. When the overflowing water no longer follows the profile of the steep slope, a cascading flow, resembling a waterfall, forms a scour pit. The turbulent flow in this scour pit also erodes the core material of the dike, allowing horizontal erosion to undercut the slope. The undercutting of the slope causes the headcut to tumble over, moving the headcut and jet impact point to move towards the waterside slope, essentially restarting the scour process. This iterative process continues until the invert height of the dike is lowered to zero and the overflow rate is increased further. Model validation shows good agreement with test results on both large and small scale, as long as the assumptions regarding the structure of the dike hold (a dike profile consisting of a granular core under a clay layer and grass cover). When applied to three case studies, the BRAM-model predicts a similar total time-to-failure as the most advanced current model, posed by d’Eliso (2007). The predicted time-to-failure for the grass cover shows a significant difference. The BRAM-model predicts the grass cover to fail significantly faster than the model by d’Eliso (2007). This difference is offset by a longer predicted time-to-failure of the clay cover. Finally, the proposed model predicts a 50% slower headcut migration rate, as compared to the results by D’Eliso. This is a result of the more advanced description of the headcut erosion process. As the time-to-failure of the headcut migration phase is relatively short compared to the total breach formation process (between 10% and 25%), the total predicted time-to-failure is quite similar between both models. The main advantage of the BRAM-model is its suitability for design testing. The main weakness remains the unpredictable location of headcut initiation, which depends too strongly on spatial variation of the resistance of the dike to be predicted accurately, based on available data. Design parameters to increase the time-to-breaching have been identified. The most relevant were the steepness of the dike slope and thickness of the clay cover. Sensitivity analysis showed that the steepness and thickness of the clay cover, but also the porosity of the core material can be adapted to increase the time-to-failure of a dike. A breach-resistant dike may be designed for a limited overflow duration. Case studies showed three test dikes to sustain an overflow rate of 100 l/m/s for various hours by the grass and clay cover. As soon as the core material becomes exposed, the breach formation process accelerates significantly. The exact duration depends on the profile and construction material of the dike.