Modelling the effects of normal faulting on alluvial river morphodynamics.

<p>Normal faulting acts as a forcing on the morphodynamics of alluvial rivers by changing the topographic gradient of the river valley and channel around the fault zone. Normal faulting affects river morphodynamics either instantaneously by surface rupturing earthquakes, or gradually by continuous vertical displacement. The morphodynamic responses to normal faulting range from longitudinal bed profile adjustments to channel pattern changes. However, the effect of faulting on river morphodynamics and morphology is complex, as they also depend on numerous local, non-tectonic characteristics of flow, river bed/bank composition and vegetation cover. Moreover, river response to faulting is often transient. Such time-dependent river response is important to consider when deriving relationships between faulting and river dynamics from a morphological and sedimentological record. To enhance our understanding of river response to tectonic faulting, we used the physics-based, two-dimensional morphodynamic model Nays2D to simulate the responses of a laboratory-scale alluvial river to various faulting and offset scenarios. Our model focusses on the morphodynamic responses at the scale of multiple meander bends around a normal fault zone. Channel sinuosity increases as the downstream meander bend expands as a result of the faulting-enhanced valley gradient, after which a chute cutoff reduces channel sinuosity to a new dynamic equilibrium that is generally higher than the pre-faulting sinuosity. Relative uplift of the downstream part of the river due to a fault leads to reduced fluvial activity upstream, caused by a backwater effect. The position along a meander bend at which faulting occurs has a profound influence on channel sinuosity; fault locations that enhance flow velocities over the point bar result in a faster sinuosity increase and subsequent chute cutoff than locations that cause increased flow velocity directed towards the outer floodplain. Our study shows that inclusion of process-based reasoning in the interpretation of geomorphological and sedimentological observations of fluvial response to faulting improves our understanding of the natural processes involved and, therefore, contributes to better prediction of faulting effects on river morphodynamics.</p>

Development of Chute Cutoff in the Lower Course of River Mayo-Inne, Yola South, Nigeria

The study investigates the development of chute cutoff in the lower course of River Mayo-Inne, Yola South LGA, Adamawa State, Nigeria. The study employed the integrated approach of Remote Sensing, Geographic Information System, Field Survey, Laboratory Analysis, Oral Interview and Personal Observation in examining the influences of some relevant channel planform parameters (Sinuosity Index, Cutoff Ratio and Braiding Index), land use/land cover, channel bank materials, water stage and channel depth on the development of the chute cutoff over a period of Twenty five years (1990-2015). Results revealed the drastic reduction of Sinuosity Index from 1.57 in 1990 to 1.46 in 2015, changing the channel from meandering to the straight pattern. The analysis of changes in cut-off ratio unveiled the development of chute cutoff in bend II, which ultimately separated the river flow, forming a weak braided channel with a braiding index of 0.43. These developments were attributed to incessant flooding in the study area and floodplain characteristics such as floodplain elevation, bank strength and changes in vegetal cover.