DEBRIS-FAN CONSTRICTIONS AND FLOOD HYDRAULICS IN RIVER CANYONS - SOME IMPLICATIONS FROM 2-DIMENSIONAL FLOW MODELING

Rapids in river canyons are frequently found at sites where debris fans constrict flow along the channel. Whereas some fans may have persisted in the same location with unchanging geometry for centuries to millennia, others have changed in response to flow conditions imposed by successive floods. Such a change in boundary conditions may alter local flow hydraulics. This paper utilizes two-dimensional flow modelling to compare flood hydraulics along two alternative versions of an idealized reach of a river canyon: one with uniform width, gradient and cross-section, and a second perturbed by a prominent debris fan along the valley wall. The flow pattern along the reach with the fan is far more complex than the pattern along the uniform reach. Maximum velocity along the debris-fan reach is up to 50 per cent higher than along the uniform reach, maximum bed shear stress is up to three or four times higher, and an area of supercritical flow is predicted extending from the nose of the fan into the zone of flow expansion immediately downstream. Comparison of model output along longitudinal profiles of the two reaches indicates that the backwater effect of the fan extends several valley widths upstream. Predicted flows based on the same stage are as much as 190 to 230 per cent greater along the uniform reach than along the debris-fan reach. Reconstruction of palaeoflood discharge based on remnant flood marks in the vicinity of the fan would be sensitive to assumptions about boundary conditions that existed in the past; this effect relaxes over a longitudinal distance of several hundred metres. Furthermore there are significant cross-stream gradients that change slope and direction several times in the vicinity of the fan, calling into question the utility of one-dimensional step-backwater hydraulic models for predicting high-water marks in areas of complex valley morphology.