Sand mining is a growing environmental and socioeconomic concern worldwide. As urbanisation and infrastructure development continue to increase, the demand for sand has skyrocketed. When mined on or near rivers, it alters the river's pathway, eroding riverbanks, damaging housing, infrastructure and livelihoods. This thesis examines the role of sand mining in the river-delta system, by examining the influence of dredging duration, dredging intensity, location and pit size on the river delta system.

A 2-dimensional depth averaged Delft3D model is made. Here a river-delta system is modelled and run for 600 years. Over the last 100 years, different sand mining scenarios have been modelled. With varying duration, intensity, locations, and pit geometry, each of these scenarios is then analysed using various method of analyses. Though changing the dredging scenarios, changes the downstream morphology and hypothe-
sised trends—such as pit migration, increased erosion, and reduced delta growth—were partially
observed. Furthermore, in the five scenarios, dredging influenced the river-delta system in complex, non-linear ways.

Some configurations (e.g., 30-year duration, 5.0x intensity, 200 m width) led to pronounced short-term changes, but long-term outcomes returned toward control-like conditions. In general, the results highlight high internal variability and limited predictability based solely on single dredging parameters. It is recommended to include a cluster of slightly varied control scenarios in future research to distinguish the effect of dredging from the natural variability of the river.

Predicting and modelling meandering river migration is necessary for river engineering, land development, and risk assessment. One chaotic process that makes predicting the migration more difficult is the cutoff of a river bend, which is the subject of this report. This report describes the research process and results of a study on overbank flood effects on chute and neck cutoffs in single-thread meandering alluvial rivers. The following research question is addressed in this report: “What is the relationship between the duration of overbank flooding and the formation of chute versus neck cutoffs?”. The relationships between the overbank flood shear stress and the frequency of chute versus neck cutoffs, and between the soil type of the floodplain and the frequency of chute versus neck cutoffs are evaluated.

An overbank flood exceeds the bankfull limit of a river, flowing over the floodplain. When a river bend is cut off, two cutoff types are visually distinguished: neck cutoffs and chute cutoffs. Cutoffs of both types were analysed in four rivers in the United States of America: Cheyenne River in North Dakota, Powder River in Wyoming and Montana, Pearl River in Mississippi, and Trinity River in Texas. These rivers have a chute cutoff regime, a mixed cutoff regime, and two neck cutoff regimes, respectively.

For these four rivers satellite imagery on Google Earth Pro was combined with measurements from USGS measurement stations and soil data from SoilWeb. The bankfull river discharge was converted to bankfull river depth to calculate the overbank flood heights and the overbank flood shear stresses acting on the floodplain during floods. Subtraction of the critical shear stresses of the soils resulted in residual shear stresses. These were combined with the corresponding flood durations to calculate the flood impulses of each flood.

The floods associated with chute cutoffs showed larger overbank flood shear stresses than those for neck cutoffs. Rivers with steeper slopes seem to be more prone to a chute cutoff regime than rivers with gentler slopes. The soils in which chute cutoffs were formed contained high sand percentages, while the neck cutoffs occurred in a wider range of soil types. The overbank floods creating chute cutoffs exerted larger residual shear stresses for shorter durations, as opposed to smaller residual shear stresses for longer durations of floods associated with neck cutoffs. The overbank flood impulses associated with chute cutoffs show a larger range and higher values, but are on average not significantly different to those of neck cutoffs.

The processes related to river bend cutoffs are very complex and not entirely understood yet and more research is needed on a local and a greater scale. The formulation of general relationships for chaotic events such as cutoffs will improve the prediction and modelling of meandering rivers. This will be increasingly useful, especially now with more extreme weather events and river floods all over the world.