Braid bar

Abstract: The South Saskatchewan River has a long term average discharge of 275 m3/sec, with flood peaks in the range of 1500 to 3800 m3/sec. South of Saskatoon, the four major types of geomorphological elements recognised are channels, slipface-bounded bars, sand flats and vegetated islands and floodplains. Major channels are 3-5 m deep, up to 200 m wide, and flow around sand flats which are 50-2000 m long, and around vegetated islands up to 1 km long. At areas of flow expansion, long straight-crested cross-channel bars form. During falling stage, a small part of the crest of the cross-channel bar may become emergent, and act as a nucleus for downstream and lateral growth of a new sand flat. The dominant channel bedforms are dunes, which deposit trough cross bedding. Cross-channel bars deposit large sets of planar tabular cross bedding. Sand flats that grow from a nucleus on a cross-channel bar are mostly composed of smaller planar tabular sets, with some parallel lamination, trough cross-bedding, and ripple cross-lamination. A typical facies sequence related to sand flat growth would consist of in-channel trough cross-bedding, overlain by a large (1-2 m) planar tabular set (cross-channel bar), overlain in turn by a complex association mostly of small planar tabular cross-beds, trough cross-beds and ripple cross-lamination. By contrast, a second stratigraphic sequence can be proposed, related only to channel aggradation. It would consist dominantly of trough cross-beds, decreasing in scale upward, and possible interrupted by isolated sets of planar tabular cross-bedding if a cross-channel bar formed, but failed to grow into a sand flat. During final filling of the channel, ripple cross-lamination and thin clay layers may be deposited. In the S. Saskatchewan, these sequences are a minimum of 5 m thick, and are overlain by 0.5-1 m of silty and muddy vertical accretion deposits.

Abstract: The initiation and evolution of a kilometre-scale, sand braid-bar was monitored during a 28-month survey period from 1993 to 1996 in one of the world's largest braided rivers, the Jamuna River, Bangladesh. Repeated bathymetric surveys through two monsoon flood seasons, combined with bar-top surveys during exposure of the bar at low flow, provide the most detailed chronology of braid-bar growth yet compiled for a large sand-bed river. During rising and peak flow of the 1994 monsoon flood, a 1.5-km-long, 0.5-km-wide, 12-m-high, symmetrical mid-channel bar was deposited in the centre of a major channel downstream of a zone of flow convergence and significant bank erosion. Initial deposition and growth of the bar core were probably caused by amalgamation of dunes that are present in the Jamuna channels at all flow stages. Bar-top aggradation continued through downstream migration of an `accretionary dune front', a 3-m-high, angle-of-repose slipface that was composed of amalgamated, 0.5- to 1-m-high dunes. At waning and low flow, the mid-channel bar widened by up to 1 km through the lateral accretion of dunes onto the margins of the initial bar core. A low-velocity zone in the sheltered wake region of the bar-tail led to the accumulation of substantial volumes of silts and clays. During the rising and peak flows of the next monsoon flood, the mid-channel bar extended its bar-tail by up to 1.5 km, as one of the anabranches became dominant, and flow was deflected across the bar-tail. Accretion at the bar-tail generated a lobate, transverse bar-front with a 10-m-high, angle-of-repose avalanche face. Emergence of several smaller bars along this depositional front produced an overall reach morphology that more closely resembled an alternate bar rather than several mid-channel bars. The conversion of a mid-channel bar to an alternate bar is contrary to many previous descriptions of the braiding process.

Abstract: The three-dimensional subsurface alluvial architecture of a large (approximately 3 km long, 1 km wide, 12 m high), mid-channel sand braid bar in the Jamuna River, Bangladesh is described. Evolution of the bar and its depositional characteristics are assessed from a unique combination of ground-penetrating radar surveys, vibracoring, and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29-month period. This methodology permits identification of the formative processes of different packages of braid-bar sedimentation and provides a facies model for deposition within the entire bar. Mid-channel bar growth occurred in a region of flow expansion and was probably initiated by the stalling and amalgamation of large dunes. These dunes created a bar-core that grew by (i) propagation of a downstream-accreting slipface, (ii) vertical accretion through stacking of dunes on both bar stoss and top, and (iii) lateral accretion on the bar margins during recession of the flood hydrograph. Braid-bar sedimentation is dominated by four radar facies: (1) large-scale, predominantly planar, dipping reflections interpreted as cross-stratification, up to 8 m in height and greater than 100 m in width, that is produced by the cross-channel migration of bar margins, (2) medium-scale, trough-shaped and planar discontinuous reflections interpreted as cross-stratification up to 4 m in height and 300 m wide, that is deposited from large, sinuous-crested sand dunes, (3) discontinuous reflections, up to 2 m high and 30 m wide, interpreted as small-scale trough cross-stratification, that is the product of smaller sinuous-crested dunes, and (4) high-amplitude, undulating reflections interpreted as mud drapes, deposited in regions of slow flow, often in the bar-tail region at low stage. Dune size decreases vertically within the bar, in response to the progressively shallower flows on the emerging bar top. Later evolution of the bar, as one anabranch channel became dominant, created a 1.5 km extension to the bar tail with an 8 m high, angle-of-repose, bar-margin slipface, formed by flow transverse to the long axis of the bar. Seven styles of deposition can be defined that constitute the alluvial architecture: bar-margin slipface, vertical accretion in channel, bar-top vertical accretion, upstream accretion, lateral accretion, downstream accretion, and low-stage mud drapes. A model of braid-bar sedimentation is presented that shares many similarities with previous studies of smaller sand-bed braid bars with the dominance of dune-scale cross-stratification, the presence of large-scale, bar-margin cross-stratification, and the occurrence of lateral, vertical, upstream, and downstream accretion. However, the contribution of the bar-margin facies to the preserved stratigraphy highlighted herein may have been underestimated in previous models of braided rivers in which the braid bars were migrating slowly. This study suggests a scale invariance in several aspects of mid-channel bar sedimentation in sand-bed rivers and proposes a model of braid-bar sedimentation that may be applied widely within studies of braided alluvial architecture.

Abstract: The evolution, migration and deposits of a gravelly braid bar in the Sagavanirktok River, northern Alaska, are described in unprecedented detail using annual aerial photographs, ground-penetrating radar (GPR) profiles, trenches and cores. Compound braid bars in the Sagavanirktok River form by chute cut-off of point bars and by growth of mid-channel unit bars. Subsequent growth is primarily by accretion of unit bars onto their lateral and downstream margins. The upstream ends of braid bars may be sites of erosion or unit bar deposition. Compound braid bar deposits vary in thickness laterally and are thickest in medial sections and near cut banks. Compound bar deposits are typically composed of three to seven sets of simple large-scale inclined strata, each simple set formed by a unit bar. The simple large-scale strata contain medium-scale cross-strata (from dune migration) and planar strata (from migration of bedload sheets). The upstream and medial parts of compound braid bar deposits show very little vertical variation in grain size, but downstream and lateral margins tend to fine upwards. The deposits are mostly poorly sorted sands and gravels, although sands tend to be deposited at the top of the braid bar, and open-framework gravels preferentially occur near the top and base of the braid bar. The patterns of braid bar growth and migration, and the nature of the deposits, described from the Sagavanirktok River are generally similar to other sandy and gravelly braided rivers, and consistent with the theoretical braid bar model of Bridge (1993).