P. A. Carling

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

Despite the occurrence of pool-riffle sequences in many rivers there are few data concerning pool-riffle unit morphology. Of many criteria proposed to identify pool-riffle units, only two methods can be regarded as objective and robust. These are the â��zero-crossingâ�� and the â��control-pointâ�� methods. In this paper statistics are developed from the â��zero-crossingâ�� method to describe the streamwise morphology of 275 riffles and 285 pools which form a near-continuous 32.1km of the bed of the River Severn in Shropshire, England. Yalinâ��s theoretical relationship between pool:riffle unit length (i�¬p) and channel width (W): i�¬p = 3W applies to the River Severn. Reach-average riffle height (H) is a constant proportion of bankfull depth (h); typically H i�� 0.16h. Riffle height is a positive function of riffle length. 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 â��flatâ��. 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 sides and pool distal slopes. The statistical relationships between riffle height, length and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are partially consistent with Yalinâ��s theoretical analysis relating riffle bedform length (Lr) to water depth i.e. Lr = i�i2i�°h wherein i�i i�� 1 for steep near-equilibrium bedforms but i�i i�� 2 to 3 as the relative depth decreases and riffles become long-low features. A consideration of turbulence data indicates that the frequency of coherent turbulent-flow structures associated with the riffle-pool mixing length is of the order of 60 seconds. The morphological and dynamic similarity of the River Severn riffles with dunes raises intriguing questions with respect to self-similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly.

The erodibility of upland soils and the design of preafforestation drainage networks in the United Kingdom

Hydraulic thresholds for erosion of fourteen upland mineral and organic soils were determined in a hydraulic flume. These soils are from areas to be afforested in the United Kingdom. Some of the group are erosion resistant but others are susceptible to erosion once denuded of vegetation; for example, by preafforestation ploughing. These threshold data were required to calibrate a hydraulic model for effective design of preafforestation drainage networks on a variety of soils. However, simple field measures of soil properties indicative of erosion potential would be of value to the forestry industry for management purposes. Consequently, hydraulic threshold data were related by linear regression methods to basic soil properties, including organic content, grain size, bulk density, compression strength and penetration resistance. The investigation concluded that four peat soils are not eroded by clear water velocities up to 5·7 m s−1, although a mineral bedload might induce erosion at lesser current speeds. Penetration resistance is a good field indicator of the degree of humification of the peat soils. Although selected physical parameters contribute resistance to water erosion, an increased organic content is pre-eminent in reducing erosion susceptibility in both organic and mineral soils. Although compressive strength was not indicative of soil erodibility, field measurements of penetration resistance on a variety of soils could be related to hydraulic thresholds of erosion; albeit through the construction of discriminant functions interpolated by eye. Consequently, organic content (laboratory) or penetration resistance (field) might form the basis of classifying upland soils in terms of erodibility. Mineral soils differ widely in terms of their erodibility, so that subject to further consideration, the use of ploughing for forestry cultivation might be appropriate in wider circumstances than presently recommended by the Forests and Water Guidelines. Ploughing should be acceptable on deep peat providing the underlying mineral soil is not exposed in the bottom of the furrow, and furrows are not led from mineral soils on to deep peat.

Coarse bedload transport in a mountain river

Coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in Squaw Creek, Montana, USA. During 1991 and 1992, a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated. Data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters. For larger discharges, coarser fractions of the bed material mobilized, and bedload transport rates and average hydraulic parameters stabilized. As discharge reduced, mobile coarse particles became less frequent and deposited fine particles were removed, resulting in an increase in bed roughness. These observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph. Bedload pulses were aperiodic but spatially variable. Flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches.

Computational fluid dynamics modelling of flow and energy fluxes for a natural fluvial dead zone

This study links together information that characterizes the energy and momentum fluxes across the shear zone between the main flow and a dead zone in the UK on the River Severn [1-3]. The depth-averaged flow in and around the dead zone is modelled using TELEMAC-2D, with semi-distributed eddy viscosity and bed roughness, and compares well with some distributed field measurements within the reach. The resulting velocity field is then used to provide momentum fluxes for a finite difference model incorporating finite volumes (FDFV model) numerical scheme that has been developed to solve depth averaged advection-diffusion of thermal energy in a body fitted co-ordinate system. Assuming that buoyancy forces can be neglected, the gross hydraulics of the system explains much of the temperature distribution that was observed using infra-red aerial imagery.

Criticality in the planform behavior of the Ganges River meanders

The critical point of planform transition from straight to meandering in the wandering Ganges River is identifiable. Recent remote-sensing data indicate that four similar meanders cut off, or attempted to cut off, after ∼31–35 yr, primarily due to channel aggradation. As main channels aggrade, sinuosity is maximized for broad channel widths and small radii of curvature and relaxes for bends of greater radii. Maximized form resistance occurs close to self-organized criticality and promotes cutoffs. Avulsions lead to main channel narrowing and prevent further bend tightening, relaxing the system by reducing sinuosity. Thus, the wandering river oscillates in space and time across the transition from a more ordered to a more chaotic state. Planform behavior is described by the Jerolmack-Mohrig mobility number and the Parker stability criterion, which well define meanders behavior as they approach criticality and then relax via partial or completed avulsions. The results have significance for river engineering and river network and stratigraphic modeling. Such an approach could be of practical value when predicting the behaviors of other major wandering rivers.