ABSTRACT. A ne twork of p assages situa ted along three-grain intersections enabl es water to p ercolate through temperate glacier ice. The d eformability of the ice a llows the passag es to expand and con tract in response to ch a nges in pressure, a nd melting o f the p assage walls by heat generated by viscous dissip a tion and ca rri ed b y above-freez ing water causes the larger passages gradually to in crease in size a t the expense of the sma ll er ones. Thus, th e behavior of the passages is prima rily the result o f three basic ch a racteristics: ( I) the capacity o f th e system continua ll y adj usts, though not instantly, to flu ctu ations in the suppl y o f melt water; (2) the direc tion of movement of the wa ter is d etermin ed mainly by the a mbient pressure in the ice, which in turn is governed primarily b y the slo pe of the ice surface a nd secondaril y b y the local topogra ph y of the glac ier bed ; and, most importan t, (3) th e ne twork of passages tends in tim e to become a rborescent, with a superglacia l part much like an ordin a r y river sys tem in a karst region , an englacial part com prised of Iree-like systems of passages penetrating th e ice from bed to surface, and a subglacia l p a r t consisting of tunn els in the ice carrying water a nd sedimen t a long the glacier b ed. These ch a rac teristics indica te that a sh ee t-like basa l water layer under a glacier wou ld normally be unstab le, th e stable form being tunnels: a nd they expl a in, among o ther things, why ice-marginal melt-water stream s and la kes a re so com mon , why eskers, which are generally considered to ha ve form ed in subglac ia l passages, trend in the general direction of ice fl ow with a tendency to follow va ll ey fl oors and to cross divides at their lowes t points, why they a re typicall y discontinuous where t hey cross ridge crests, why they sometimes con tain fragments from bedrock outcrops near the esker but no t actuall y crossed by it, a nd why they seem to be formed mostly during th e later stages of glaciation.RES UME. Ces ca ra cteristiques indiquen t qu 'u n n iveau d 'eau e tendu comme un drap su r le lit d ' un gl acier d evra it n orm a lement etre une formation insta ble, la form e stabl e e ta nt cell e d es tunnel s; ceci explique, en tre a utres, p ourquoi les torrents et lacs d 'eau d e fu sio n sont si communs le long d es ri ves d es glaciers, pourquoi les eskers, qui son t genera lement consideres comm e ayant e te formes par d es canaux sous-glaciaires, tend en t a s'aligner dans la direction genera le d e I'ecoul ement d e la glace avec une tendance a suivre le fond d e la vallee et a traverser les o bstacles en leur point le plus bas, pourquoi ils sont typiqu em ent interrom pus lo rsq u ' ils traversent la crete d'une ond ul a ti on, p ourquoi ils contien nen t parfois d es fragments pre leves sur le lit pres de I'esker mais non exactemen t sur sa trajectoire et pourquoi il s semblen t se former surtout a u cours des d erniers stades d 'un e glac iation. ZUSAMMENFASSUNG . B ew...

ABSTRACT. A ne twork of p assages situa ted along three-grain intersections enabl es water to p ercolate through temperate glacier ice. The d eformability of the ice a llows the passag es to expand and con tract in response to ch a nges in pressure, a nd melting o f the p assage walls by heat generated by viscous dissip a tion and ca rri ed b y above-freez ing water causes the larger passages gradually to in crease in size a t the expense of the sma ll er ones. Thus, th e behavior of the passages is prima rily the result o f three basic ch a racteristics: ( I) the capacity o f th e system continua ll y adj usts, though not instantly, to flu ctu ations in the suppl y o f melt water; (2) the direc tion of movement of the wa ter is d etermin ed mainly by the a mbient pressure in the ice, which in turn is governed primarily b y the slo pe of the ice surface a nd secondaril y b y the local topogra ph y of the glac ier bed ; and, most importan t, (3) th e ne twork of passages tends in tim e to become a rborescent, with a superglacia l part much like an ordin a r y river sys tem in a karst region , an englacial part com prised of Iree-like systems of passages penetrating th e ice from bed to surface, and a subglacia l p a r t consisting of tunn els in the ice carrying water a nd sedimen t a long the glacier b ed. These ch a rac teristics indica te that a sh ee t-like basa l water layer under a glacier wou ld normally be unstab le, th e stable form being tunnels: a nd they expl a in, among o ther things, why ice-marginal melt-water stream s and la kes a re so com mon , why eskers, which are generally considered to ha ve form ed in subglac ia l passages, trend in the general direction of ice fl ow with a tendency to follow va ll ey fl oors and to cross divides at their lowes t points, why they a re typicall y discontinuous where t hey cross ridge crests, why they sometimes con tain fragments from bedrock outcrops near the esker but no t actuall y crossed by it, a nd why they seem to be formed mostly during th e later stages of glaciation.RES UME. Ces ca ra cteristiques indiquen t qu 'u n n iveau d 'eau e tendu comme un drap su r le lit d ' un gl acier d evra it n orm a lement etre une formation insta ble, la form e stabl e e ta nt cell e d es tunnel s; ceci explique, en tre a utres, p ourquoi les torrents et lacs d 'eau d e fu sio n sont si communs le long d es ri ves d es glaciers, pourquoi les eskers, qui son t genera lement consideres comm e ayant e te formes par d es canaux sous-glaciaires, tend en t a s'aligner dans la direction genera le d e I'ecoul ement d e la glace avec une tendance a suivre le fond d e la vallee et a traverser les o bstacles en leur point le plus bas, pourquoi ils sont typiqu em ent interrom pus lo rsq u ' ils traversent la crete d'une ond ul a ti on, p ourquoi ils contien nen t parfois d es fragments pre leves sur le lit pres de I'esker mais non exactemen t sur sa trajectoire et pourquoi il s semblen t se former surtout a u cours des d erniers stades d 'un e glac iation. ZUSAMMENFASSUNG . B ew...

No abstract

The interactions between turbulence events and sediment motions during bed load transport were studied by means of laser-Doppler velocimetry and high-speed cinematography. Sweeps (u' > O, w' < 0), which contribute positively to the mean bed shear stress, collectively move the majority of th e sediment, primarily because they are extremely common. Outward interactions (u' > O, w' > 0), which contribute negatively to the bed shear stress and are relatively rare, individually move as much sediment as sweeps of comparable magnitude and duration, however, and much more than bursts (u' < O, w' > 0) and inward interactions (u' < O, w' < 0). When the magnitude of the outward interactions increases relative to the other events, therefore, the sediment flux increases even though the bed shear stress decreases. Thus, although bed shear stress can be used to estimate bed load transport by flows with well-developed boundary layers, in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity, it is not accurate for flows with developing boundary layers, such as those over sufficiently nonuniform topography or roughness, in which significant spatial variations in the magnitudes and durations of the sweeps, bursts, outward interactions, and inward interactions occur. These variations produce significant peaks in bed load transport downstream of separation points, thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms. NELSON ET AL.' NEAR-BED TURBULENCE STRUCTURE 2073

Motion pictures taken at Duck Creek, a clear stream 6.5 m wide and 35 cm deep near Pinedale, Wyoming, provide detailed, quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them. Bed shear stress was approximately 6 Pa (60 dynes cm−2), which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material; and transport was almost entirely as bedload. The displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones, and rarely by brief sliding of large, angular ones. Entrainment was principally by rollover of the larger particles and liftoff of the smaller ones, and infrequently by ejection caused by impacts, whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones. The displacement times averaged about 0.2−0.4 s and generally were much shorter than the intervening repose times. The collective motions of the particles were characterized by frequent, brief, localized, random sweep-transport events of very high rates of entrainment and transport, which in the aggregate transported approximately 70% of the total load moved. These events occurred 9% of the time at any particular point of the bed, lasted 1–2 s, affected areas typically 20–50 cm long by 10–20 cm wide, and involved bedload concentrations approximately 10 times greater than background. The distances travelled during displacements averaged about 15 times the particle diameter. Despite the differences in their dominant modes of movement, the 8–16 mm particles typically travelled only about 30% slower during displacement than the 2–4 mm ones, whose speeds averaged 21 cm s−1. Particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely, with diffusivities of 4.6 and 0.26 cm2 s−1, respectively. The bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler. The 2–4 mm particles were entrained 6 times faster on finer areas of the bed, where 8–16 mm particles covered 6% of the surface area, than on coarser ones, where they covered 12%, even though 2–4 and 4–8 mm particles covered practically the same percentage areas in both cases. The 4–8 and 8–16 mm particles, in contrast, were entrained at the same rates in both cases. To within the statistical uncertainty, the rates of distrainment balanced the rates of entrainment for all three sizes, and were approximately proportional to the corresponding concentrations of bedload.