A general framework for using the rate law to simulate morphological response to disturbance in the fluvial system

Abstract

A general framework for modelling morphological responses to perturbation is proposed, based on the underpinning principle that the rates of morphological response in alluvial channels are initially high and then decrease through time as the system relaxes following disturbance. The framework includes three morphological response models, each developed from the fundamental rate law, which has the form of an exponential decay function. These models consider the possibility that characteristic behaviours of the fluvial system, such as delayed response and/or cumulative effects, may affect morphological responses, making them capable of representing relaxation paths and times for a range of morphological response variables, whatever their initial states. To test their utility, the models in the framework were applied to simulate the sequence of geomorphological responses to disruption observed in selected rivers with well-documented histories of morphological perturbation, adjustment and recovery. The results demonstrate that the models in the general framework can successfully simulate temporal and spatial patterns of morphological response in the fluvial system under a range of different circumstances, while also indicating how their reliability could be further improved.

Geomorphic response of the Jingjiang Reach to the Three Gorges Project operation

“…Previous studies indicate that morphological responses to perturbations in alluvial rivers are usually the result of the cumulative effect of earlier flow and sediment conditions (Knighton, ; Wu et al ., , ). In this study, the previous n year average fluvial erosion intensity during flood seasons ( ) can be expressed by: where n is the number of years for the moving average.…”

“…Recent studies by Wu et al . (, ) and Xia et al . () indicate that the adjustment in the channel geometry of an alluvial river is closely related to the previous years' flow and sediment conditions during flood seasons; and these conditions are usually represented by the average discharge and incoming sediment coefficient during flood seasons, in the Yellow River carrying high sediment concentrations.…”

“…Therefore, the channel adjustments in the Jingjiang Reach occurred mainly during flood seasons, and the intensity of the channel evolution during non-flood seasons was negligible in this investigation, compared with that during flood seasons. Recent studies by Wu et al (2008Wu et al ( , 2012 and Xia et al (2014a) indicate that the adjustment in the channel geometry of an alluvial river is closely related to the previous years' flow and sediment conditions during flood seasons; and these conditions are usually represented by the average discharge and incoming sediment coefficient during flood seasons, in the Yellow River carrying high sediment concentrations. However, in an alluvial river with low sediment concentrations, such as the Jingjiang Reach, the average fluvial erosion intensity during a flood season is usually used to represent the incoming flow and sediment conditions (Xia et al, 2014b).…”

“…Bankfull channel geometry in an alluvial river is described by the channel width, cross-sectional area, and the corresponding mean depth at bankfull level, and these bankfull dimensions are important parameters in river training works and flood control engineering (Leopold and Maddock, 1953;Williams 1978;Harman et al, 2008). Various changes in the downstream channel geometry can be predicted using empirical relations or conceptual models in the context of river dynamics or geomorphology (Wu et al, 2008(Wu et al, , 2012Shin and Julien, 2010;Xia et al, 2014a, b). Based on a general delayed response equation, Wu et al (2008) developed a methodology for the prediction of bankfull areas, with the cumulative effect of flow and sediment conditions in previous years on the channel evolution being considered.…”

River sensitivity: a lost foundation concept in fluvial geomorphology

“…Reaction time is the time taken for a system to react to a change in conditions (or disturbance event), whereas relaxation time is the time taken for the system to attain a characteristic state after disturbance (Brunsden, ; Knighton, ). If the frequency of disturbance is less than the reaction + relaxation time, then the system is in a constant state of adjustment (Renwick, ; Wu et al ., ). If however, the frequency of disturbance exceeds reaction + relaxation time then the system may be able to ‘bounce back’ following disturbance.…”