Flow and Sediment Transport in Nature-Based Solutions for River Restoration
Open Access
- Author:
- Mousavi, Azadeh
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 23, 2024
- Committee Members:
- Farshad Rajabipour, Program Head/Chair
Anastasia Piliouras, Outside Unit Member
Xiaofeng Liu, Chair, Minor Member & Dissertation Advisor
Xiang Yang, Outside Field Member
Roberto Fernández, Major Field Member - Keywords:
- Sediment Transport
Scour
Porous in-Stream Structures
Hydrodynamics
Numerical Simulations
Engineered Log Jams
Large Woody Debris
Nature-Based Solutions
River Restoration
CFD
RANS
Immersed Boundary Method
Erosion
Hydraulic Flume Experiment
Scour Equation
Porosity - Abstract:
- This dissertation investigates the hydrodynamic and sediment transport processes around nature-based river restoration structures, with a focus on engineered log jams (ELJs) and large woody debris (LWDs). Although adding wood to rivers is a popular practice to mimic Nature, most designs have been based on experience rather than a thorough understanding of their impact on flow dynamics and sediment transport. Scour, a stability-threatening process around in-stream structures, also contributes significantly to their ecological benefits. While it has mostly been studied for solid, simple geometric structures like bridge piers, it is essential to investigate scour around nature-based solutions (NBS) to ensure that river restoration projects achieve their objectives. This research addresses this gap by analyzing the complex interactions between porous, geometrically complex structures and their environment, focusing on both hydrodynamics and morphodynamics. The primary hypothesis of this work is that the porosity and shape complexity of these structures are key factors controlling sediment transport. This hypothesis is explored through a combination of flume experiments and high-fidelity computational fluid dynamics (CFD) simulations. The aim is to uncover the fundamental physical processes that occur around and within these NBSs and to develop a quantitative framework for predicting sediment transport and scour in river restoration. Understanding the mechanisms of scour requires a thorough investigation of turbulent flow dynamics, bathymetric features, wall shear stress, and precise quantification of these factors. By quantifying these physical processes around complex porous in-stream structures, appropriate equations for predicting maximum scour depth and its temporal evolution are developed. This work is organized into four main parts. First, flume experiments were conducted to study the flow and sediment transport around engineered log jams (ELJs) with varying porosities and placements. These experiments provided detailed measurements of flow patterns, scour depth, and sediment transport, which were used to derive new equations for estimating scour depth. Specifically, modifications were made to existing solid structure equations to account for the effect of porosity. The experiments also introduced two definitions of porosity, i.e., surface and volumetric porosities, in an accurate general formula for equilibrium scour depth. The findings were validated against real-world river restoration projects, ensuring their practical relevance. The second part of this work focuses on high-fidelity computational fluid dynamics (CFD) simulations to investigate the coupled flow and sediment transport processes around ELJs. The simulations emphasized the quantification of flow characteristics, particularly turbulent flow patterns and bleeding flow through porous structures. Using the immersed boundary method, the model provided detailed insights into flow contraction, vortex formation, and velocity profiles. These simulations helped reveal how porosity alters the hydrodynamic behavior around the structures, offering a more thorough understanding of the differences between porous and solid structures. The third part extends the CFD simulations to emphasize the quantification of wall shear stress, scour depth, and sediment transport rate around porous and solid ELJs. These simulations enabled the development of a semi-theoretical model for predicting the temporal evolution of scour depth, incorporating porosity as a key factor. The model integrates physical processes such as bed shear stress distribution and sediment transport dynamics, offering a more generalized and accurate prediction tool than previous models purely based on empirical data fitting. The final part focuses on experiments and simulations of large wood structures with porous rootwads, capturing the flow and sediment transport around these complex geometries. The experiments were conducted to shed light on the differences in flow and equilibrium bathymetric patterns around a log with a rootwad and a single cylindrical log. Previous studies often simplified LWDs to simplified shapes like cylinders or blocks, missing their true complexity. Hence, this research also examines how much geometric detail is needed for accurate simulations of in-stream structures, comparing fully resolved, solid, and porous rootwad models to assess their effects on flow dynamics, sediment transport, and the temporal evolution of these processes.