Morphology of riffle-pool sequences in the River Severn, England

Despite the occurrence of riffle-pool sequences in many rivers there are fe w data concerning riffle-pool unit morphology. Of many criteria proposed to identify riffle-pool units, only three methods can be regarded as objectiv e and robust. These are the 'zero-crossing', the 'spectral analysis' and th e 'control-point' methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 ri ffles and 285 pools which forma continuous 32.1 lan reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit lengt h (lambda(p)) and channel width ((W) over bar), lambda(p) = 3 (W) over bar, applies to the River Severn. Reach-average riffle height ((H) over bar) is a constant proportion of bankfull depth ((h) over bar); typicaily (H) over bar congruent to 0.16h. Riffle height is a positive function of riffle len gth. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed-level amplitude, such that long riffles or pools are relatively 'flat'. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee side s and pool distal slopes. The statistical relationships between riffle stee pness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating r iffle bedform length (L-r) to water depth, i.e. L-r = alpha 2 pi h, wherein alpha congruent to 1 for steep near-equilibrium bedforms but alpha congrue nt to 2 to 3 as the relative depth decreases and riffles become long, low f eatures. Theoretical consideration and turbulence data indicate that the fr equency of coherent turbulent-flow structures associated with the riffle-po ol mixing length in the Severn should be of the order of 50 to 100 s. The m orphological similarity of the steepest River Severn riffles with dunes rai ses intriguing questions with respect to self-similar, convergent organizat ion of periodic alluvial bedforms and to bedform dynamic classification par ticularly. Copyright (C) 2000 John Wiley & Sons, Ltd.