SIMULATIONS OF BANK EROSION PROCESSES

Massimo RINALDI - Department of Civil Engineering, University of Florence

SIEVE RIVER PROJECT

Pore water pressures (positive and negative) were monitored for four years (1996-1999) using a series of tensiometer-piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (Central Italy), with the overall objective of investigating pore pressure changes in response to flow events and their effects on bank stability. The saturated/unsaturated flow was modelled using a finite element seepage analysis, for the main flow events occurring during the four year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events.  

SIMULATION 1: Role of pore water pressures

River bank: Sieve River at Fornacina  

Methods of analysis: Pore water pressure changes are modelled by SEEP/W in transient conditions (using the hydrograph and rainfall as boundary conditions); bank stability is modelled by SLOPE/W. The simulation is with ‘constant geometry’ (bank deformation due to fluvial erosion is not taken into account, but bank geometry changes only after the bank failure).  

Author: S.Dapporto

Main references: Rinaldi et al. (2001); Rinaldi et al. (2004)  

 

SIMULATION 2: Seepage and stability analysis with deforming bank profile

River bank: Sieve River at Fornacina  

Methods of analysis: Modelling of the riverbank retreat during the 19/11/1999 flow event includes for each step the following components (see Flow chart): a) fluvial erosion and consequent changes in bank geometry; b) finite element seepage analysis; c) stability analysis by limit equilibrium method.  

Author: S.Dapporto

Main references: Dapporto (2003); Dapporto & Rinaldi (2003)

 

CECINA & ASKER RIVER PROJECT

Research Project: Modelling hydraulic and geotechnical river bank erosion processes interactions”, funded by the Royal Society (UK) (European Science Exchange Programme) - Dr. S.Darby (Department of Geography, University of Southampton, UK) & Dr. M.Rinaldi (University of Florence, Italy).

The general aim of the project is to apply hydraulic & geotechnical modelling techniques to investigate the interaction between two bank erosion processes: (1) the erosive action of the flow & (2) mass failure of banks undermined by fluvial erosion.

The research was undertaken at two diverse study sites (Cecina River, Central Italy, and the River Asker, Southern UK), representing end-members in a spectrum ranging from very active (Cecina bank erosion rate ~ 7-17 m/yr) to mildly active (Asker bank erosion rate ~ 0.2 m/yr) eroding river meanders. We have devised a new methodological approach that involves undertaking a combination of monitoring and hydraulic modelling investigations. Our rationale is to employ Computational Fluid Dynamics (CFD) techniques to obtain simulated high-resolution near-bank boundary shear stress data as a substitute for empirical data.

Main references: Rossi Romanelli et al. (2004); Darby & Rinaldi (2004 a, b); Darby et al. (2004 a, b)

SIMULATION 3: Seepage and stability with ‘constant geometry’  

River bank: Cecina River at Sterza confluence  

Methods of analysis: Pore water pressure changes are modelled by SEEP/W in transient conditions (using the hydrograph and rainfall as boundary conditions); bank stability is modelled by SLOPE/W. The simulation with ‘constant geometry’ (similarly to simulation 1) represents the ‘control simulation scenario’ in which the bank is not deformed. Comparison with the scenario of bank deformation by fluvial erosion will enable the effect of bank deformation on bank stability to be isolated.  

Author: L.Luppi

Main references: Luppi (2004); Rossi Romanelli et al. (2004)

 

SIMULATION 4: Seepage and stability with deforming bank profile  

River bank: Cecina River at Sterza confluence  

Methods of analysis: In this preliminary analysis, near-bank boundary shear stress distribution have also been evaluated at low resolution by applying a 1-dimensional HEC-RAS model (simulation). The near-bank shear stress distribution is then estimated using the Simons & Senturk (1977) distribution. Water table position is assumed from simulation 3 (SEEP with constant geometry) while a simplified pore water pressure distribution is assumed above the water table. Safety factor for slide and cantilever failures is calculated for each time step (see Flow chart).  

Author: L.Nardi

Main references: Nardi (2004); Canovaro et al. (2004)

 

SIMULATION 5: Hydrodynamic - Seepage - Stability modelling with deforming bank profile  

River bank: Cecina River at Sterza confluence  

Methods of analysis: Near-bank boundary shear stresses are computed by a hydrodynamic simulation using DELFT3D (2D depth averaged simulation) (Author: Beatrice Mengoni, University of Florence; Supervisor: Erik Mosselman, WL|Delft Hydraulics, NL). Near-bank shear stresses are used to deform the bank toe by using existing excess shear stress model for fluvial erosion. Pore water pressure changes are modelled by SEEP/W in transient conditions (using the hydrograph and rainfall as boundary conditions). Safety factor for slide and cantilever failures is then calculated for each time step.  

Author: L.Luppi & F.Agresti

Main references: Mengoni (2004); Agresti (2005)

 

References

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