Stability of near-bed structures and bed protections

Abstract

In river and coastal engineering bed protections and near-bed rubble mound structures are often used to protect other structures such as river groins, pipelines and intake- and outfall structures for power-stations and desalination plants. From river engineering relatively extensive information is available for bed protections with currents as primary load. From coastal engineering relatively extensive information is available for low-crested rubble mound structures with waves as primary load. Information on the stability of bed protections with waves, or a combination of waves and currents , is relatively scarce. Also information on the stability of rubble mound structures with a very low-crest (i.e., near bed structures) under loading of waves and currents is scarce. Most available data concerns data related to start of damage, providing little information on damage levels related to failure of these structures. In the present study physical model tests have been performed to contribute to the understanding of relevant processes and to fill relevant gaps in the existing information on near-bed structures and bed protections. Based on the new data and a re-analysis of existing data, several methods to predict the stability of near-bed structures have been analysed. One of these methods was found to be the most appropriate. This method was calibrated to relate the erosion of near-bed structures to a mobility parameter. It was found that for low-to-moderate currents in combination with waves, the waves dominate the stability of the rubble mound material. For waves in combination with a strong current insufficient data was available to draw firm conclusions. For the tests with a combination of waves and a low-to-moderate current the stability of the near-bed structures can be predicted without taking the influence of the current into account; the scatter related to conditions with waves in combination with a current is within the scatter for conditions with waves only. The data-set obtained on the stability of bed protections behind near-bed structures is too small to obtain a reliable prediction method based on this data-set only. The data can however be used to verify hypotheses and numerical model results on this topic. In the present report most emphasis is put on the analysis of the data on near-bed structures. It is recommended to analyse the present data on bed protections in more detail in combination with other data and results from numerical models.