Stabilization of Angular-Shaped Riprap under Overtopping Flows
Riprap is mostly used to prevent erosion by flows down the steep slopes in river engineering. A total of 53 stability tests performed on angular riprap with a median stone size ranging from 15 to 278 mm and slope ranging from 1 to 40% are used in this study. The existing equations for the prediction of medium size of angular stones are checked for their accuracy using the available data. Predictions of median size using these equations are not satisfactory and results show deviation by more than ±20% from the observed values. A multivariable power regression analysis is performed to propose a new equation relating the median size with unit discharge, bed slope, riprap thickness and coefficient of uniformity. The proposed relationship satisfactorily predicts the median angular stone size with ±20% error. Further, the required size of the rounded stone is more than the angular stone for the same unit discharge and the ratio increases with unit discharge and also with embankment slope of the riprap. Keywords—Angularity, Gradation, Riprap, Stabilization
Riprap design on the downstream slopes of rockfill dams
Riprap is widely used as erosion protection. To prevent erosion in case of accidental overtopping or trough flow, the downstream slopes of rockfill dams can be secured with riprap. The present thesis focuses on the stability and design of dumped and placed riprap made of natural stone. Placed riprap for the tests was constructed by setting stones in an interlocking pattern, with their longest axes inclined towards a slope of 1:1.5 (vertical: horizontal). Riprap parameters, failure mechanisms, hydraulic parameters and existing stability approaches were addressed to optimize riprap design on steep slopes. Riprap stability was investigated with physical model tests in different scales and was expressed through the critical stone-related Froude number. In order to quantify the forces acting on a specific stone, the novel ’Smartstone’ monitoring equipment was tested. A Smartstone probe was mounted in a riprap stone to measure the acceleration in the moment when the stone was eroded from the riprap. However, the current version of the Smartstone equipment needs further development to allow for the desired application. Physical model tests and field tests with large-scale riprap stones were carried out to investigate riprap stability. For dumped riprap, similarity was found between the field and model tests in terms of riprap stability, packing density and visually observed flow pattern. The model tests and field tests with placed riprap showed good agreement with regard to flow pattern and overtopping height. However, the placed ripraps in the model had a higher packing density and were more stable. Displacements within the riprap layer were monitored during the model tests. The displacements lead to a gap at the transition between the horizontal crest and the downstream slope. The riprap became unstable when the gap exceeded approximately one stone length. Accumulating displacements were hence identified as failure mechanism for placed riprap on steep slopes. Due to the gradual development of the displacements, the dimension of time is recommended to be included in stability analyses in addition to discharge. Placed riprap had on average a seven times higher critical stone-related Froude number than dumped riprap. However, dumped riprap might provide sufficient stability for certain applications, and placed riprap should just be considered if increased stability is required. For an optimized riprap design, the requirements about riprap type, stone size and packing density as well as the corresponding costs have to be balanced with the desired level of stability.
The role of dissolved oxygen in supercritical water in the oxidation of ferritic–martensitic steel
Abstract To assess the role of dissolved oxygen in supercritical water on the growth processes of oxide scales, a marker experiment of ferritic–martensitic steel was performed by exposing to supercritical water (H 2 16 O) containing marked 18 O 2 at 600 °C under 25 MPa. The oxide scale was analysed by using Secondary Ion Mass Spectrometry (SIMS). The oxygen isotope profile shows that dissolved oxygen and supercritical water react with metals simultaneously. Dissolved oxygen changes the oxidation/reduction potential of supercritical water leading to an increase in the oxidation rate. The oxidation rate increases with increase in dissolved oxygen content in supercritical water. The mechanism of the effect of dissolved oxygen on oxidation is discussed.