Last days a video of a bridge collapse in Lima (Peru) became viral. The video shows the moment when the scour of a bridge gets scoured and the bridge collapses. Discussions about the collapse of this bridge became a trending topic in several social networks, especially in Peru. Some discussions tried to find someone or something to blame and others focussed on whether the bridge was properly designed or not. In this post we will not discuss such topics. Whether there is someone to blame or not; whether it was properly designed or not, the fact is that the video a good example of stream bed erosion. Due to space limitation, this post will be an introduction to stream bank erosion. In a future post we may simulate in detail the erosion process of the video and analyse whether a properly designed rip rap would have prevented the erosion (and collapse) or not.
Fig 1. Bridge collapse due to stream bank erosion in Peru (Source youtube)
Rivers are dynamic systems that change over time. One of the processes defining such change is stream bank erosion. All rivers have stream bank erosion. Even the so-called stable rivers have eroding banks. Certain events such a flooding usually trigger stream bank erosion
Stream bank erosion becomes even more complicated when the steam has bends inducing secondary flows. Even small bends create a vertical velocities profile and a helicoidally flow known as centre region cell. Besides this helicoidally flow, the outer bank of the river also experiences a so called outer bank cell flow. This outer bank cell flow is a small helicoidally flow with a direction that opposes the main helicoidally flow. Although the outer bank cell is small and weak, it plays an important role in stream bank erosion.
Fig 2. Secondary flows (Source: Blanckaert and Vriend 2004)
The full solution of the stream bank erosion is quite complicated. Thus, simplified approaches were developed based on the main mechanisms controlling stream bank erosion. The mechanisms controlling stream bank erosion may be divided in two groups: Scour and Mass failure
- Mass failure is the process when large chunks of bank material collapse into the river. Sometimes mass failure is a consequence of local scour removal.
- Scour failure is the direct removal of bank material
There are two main modelling approaches for analysing bank erosion due to the mentioned processes:
- Bank failure. Bank failure is a geotechnical based model that evaluates the bank stability and its critical failure plane. Once the soil gets saturated due to the flood, several theoretical failure planes are analysed considering its resisting forces and the driving forces, in order to define the one with the lowest safety factor. If the critical safety factor is lower than one, then all the soil above the plane will be eroded by mass failure.
Fig 3. Mass failure (Source: Carey B 2014)
- Toe scour. Toe scour considers the bank material removed by the flow. The shear stress between the flow and the bank toe is evaluated considering the critical share stress and the erodibility of the bank. If the shear stress is higher than the critical shear stress, a portion of the bank will be eroded. After the toe gets eroded, a second failure mechanism may occur. Due to the toe erosion, the soil will look like a cantilever. Depending on the size of toe scour and the volume of soil above the scour whole, a cantilever shear failure may occur.
Fig 4. Toe scour (Source: Carey B 2014)
If we get more information about the physical characteristics of the bridge and the river, in a further post we may simulate the scour process that collapsed this bridge, and analyze in more detail this case.