Scour Hole Depth Research Articles

Characterizing large river history with shallow geophysics: Middle Yukon River, Yukon Territory and Alaska

Changes in longitudinal valley-fill thickness can provide inferences about fluvial behaviour over millennia. Increased thickness of channel deposits may reveal climate-induced accelerated sediment supply, basin subsidence, tectonic displacements, or rising downstream base level. Decreases in thickness, expressed as terraces, may suggest active uplift, decreased sediment flux, or falling base level within a reach. To date, methods to characterize valley fills have depended largely on borehole data. However, recent developments in shallow geophysics make such investigations possible and provide a practical methodology to investigate alluvial rivers. We carried out a longitudinal geophysical survey, coupled with floodplain chronologies to characterize a 1000 km reach of the middle Yukon River between western Yukon Territory and central Alaska. Ground penetrating radar (GPR) and electrical resistivity ground imaging (ERGI) profiles along the middle Yukon River between central Yukon and Alaska, coupled with channel-bottom echo profiles, demonstrates that there is little variation in thickness of gravel fill and depth of active scour holes. GPR surveys near the White River, the largest sediment source in the upper Yukon, show no significant longitudinal variation in gravel thickness (8–16 m) up to near Circle, Alaska (530 km of wandering channels). Geophysical profiles across the Yukon Flats sedimentary basin (430 km of braided and wandering channels) show an increase in gravel thickness (25–30 m), but this is consistent with compaction of underlying lacustrine sediment and maximum depths of contemporary scour holes. From the western margin of the Yukon Flats to the Dalton Highway bridge (30 km, confined valley with no floodplain or braid bars), borehole data at the bridge indicates 6–10 m of gravel on bedrock beneath 3–6 m of water. Floodplain stratigraphy indicates there is little evidence of fluvial aggradation of the middle Yukon during the Holocene. Radiocarbon ages from the gravel–overbank silt contact near present river level demonstrate a general vertical stability of the river with a period of rapid eolian accumulation during the late Pleistocene and early Holocene. The consistency of gravel-fill and scour hole depth, and lack of Holocene terraces along the 1000 km reach of the Yukon, indicate that the river has been in a state of mass-balance equilibrium of sediment transport (input=output) over the Holocene. Despite massive sediment influxes from the glacial-fed White River and crossing major faults, variation in valley-fill depth is not significant. And further, the Yukon Flats reach shows no evidence of bed aggradation during the Pleistocene and has probably existed in equilibrium since the late Pliocene.

Sediment erosion by Görtler vortices: the scour-hole problem

Experimental results on sediment erosion (scour) by a plane turbulent wall jet, issuing from a sluice gate, are presented which show clearly – it seems for the first time – that the turbulent wall layer is destabilized by the concave curvature of the water/sediment interface. The streamwise Görtler vortices which emerge create sediment streaks or longitudinal sediment ridges. The analysis of the results in terms of Görtler instability of the wall layer indicates that the strength of these curvature-excited streamwise vortices is such that the sediment transport is primarily due to turbulence created by these vortices. Their contribution to the wall shear stress is taken to be of the same form as the normal turbulent wall shear stress. For this reason, the model developed by Hogg et al. (J. Fluid Mech. Vol. 338, 1997, p. 317) remains valid; only the numerical coefficients are affected. The logarithmic dependency of the time evolution of the scour-hole depth predicted by this model is shown to be in good agreement with experiments. New scaling laws for the quasi-steady state depth and the associated time, inspired by the Hogg et al. (1997) model are proposed. Furthermore, it is emphasized that at least two scouring regimes must be distinguished: a short-time regime after which a quasi-steady state is reached, followed by a long-time regime, leading to an asymptotic state of virtually no sediment transport.

Deflector designs for fish habitat restoration.

Paired current deflectors are structures that are installed on each bank of a river to locally reduce the width of the channel, thereby creating flow acceleration and promoting scouring. These instream habitat structures have been used extensively in restoration projects to create pool habitat for fish, but there are many discrepancies in deflector design recommendations in terms of orientation, height, and length. Our objectives were to (1) examine how the angle, height, and length of paired deflectors affect scour hole dimensions and potential for bank erosion; and (2) test the applicability to paired deflectors of existing equations for scour hole depth and volume. Three deflector angles (45 degrees, 90 degrees, and 135 degrees), two deflector heights (with flow under and over the deflector height), and two lengths (reducing the width by 25% and 50%) were investigated using uniform sand in a laboratory flume. Results showed a 26-30% smaller scour depth resulting from 45 degrees deflectors than from 90 degrees deflectors and a 5-10% smaller scour depth for 135 degrees deflectors compared to 90 degrees deflectors. The volume of scour and the potential for bank erosion were greater when flow was under the height of the deflectors rather than overtopping and when the length of deflector was increased. When flow was under the deflector height, 135 degrees deflectors had the highest amount of bank erosion; whereas during overtopping flow conditions, 90 degrees deflectors had the greatest bank erosion potential. Values predicted by the model of Kuhnle and others were closest to observed scour depth and volume measurements. The assumption that upstream-oriented deflectors always generate the largest scour should be revised.

Time evolution of scouring downstream of bed sills

Results of laboratory experiments conducted on time evolution of the scouring phenomenon at bed sills are presented. Starting from previous findings on long-term depth and length of the local scour hole downstream of bed sills, 12 long-duration tests were designed and performed in the Sloping Sediment Duct at HR Wallingford Ltd., using uniform sand. No sediment feeding system was adopted. Video-recordings of the scour holes through the glass-sided duct allowed successive image analysis to obtain water surface and bed profiles over time. Results confirmed the validity of the assessment method of the maximum scour depth, y s, at equilibrium; uncertainties arose about the length of the scour hole, l s. A cyclic phenomenon perturbing the scour hole was also observed and described. The investigation of the time evolution of \\\\ showed that the scour hole develops rapidly and conditions close to equilibrium are achieved in a short time. The introduction of a morphological time. t s permitted the non-dimensional description of the increasing of y s over time through a unique curve. The short- and the long-term local scour regions are clearly distinguished. An application of the results is shown in a numerical example.

Distorted Froude‐scaled flume analysis of large woody debris

AbstractThis paper presents the results of a movable‐boundary, distorted, Froude‐scaled hydraulic model based on Abiaca Creek, a sand‐bedded channel in northern Mississippi. The model was used to examine the geomorphic and hydraulic impact of simplified large woody debris (LWD) elements. The theory of physical scale models is discussed and the method used to construct the LWD test channel is developed. The channel model had bed and banks moulded from 0·8 mm sand, and flow conditions were just below the threshold of motion so that any sediment transport and channel adjustment were the result of the debris element. Dimensions and positions of LWD elements were determined using a debris jam classification model. Elements were attached to a dynamometer to measure element drag forces, and channel adjustment was determined through detailed topographic surveys.The fluid drag force on the elements decreased asymptotically over time as the channel boundary eroded around the elements due to locally increased boundary shear stress. Total time for geomorphic adjustment computed for the prototype channel at theQ2discharge (discharge occurring once every two years on average) was as short as 45 hours. The size, depth and position of scour holes, bank erosion and bars created by flow acceleration past the elements were found to be related to element length and position within the channel cross‐section. Morphologies created by each debris element in the model channel were comparable with similar jams observed in the prototype channel. Published in 2001 John Wiley & Sons, Ltd.

Drop Height Influence on Outlet Scour

A study was conducted to determine how fall distance, or drop height, beneath a culvert outlet affects the dimensions of localized scour. Eighteen experiments were performed in which flow discharged from a circular, 4‐in.‐diameter pipe onto a near‐horizontal bed for 316 min. The bed material was relatively uniform sand with median grain diameter, d50=1.86mm. The scour hole dimensions, depth, width, length, and volume, were documented for drop heights of 0, 1, 2, and 4 times the culvert diameter above the bed. A series of expressions were developed correlating the dimensionless scour hold parameters of depth, width, length, and volume to the modified discharge intensity, QA-1g-0.5RH-0.5, and to the drop height. It was determined that culverts placed above the bed result in deeper, wider, and shorter scour holes than culverts placed adjacent to the bed. Criteria for adjusting the existing U.S. Department of Transportation scour hole prediction procedure for drop height is presented in this paper.

Initial habitat response to incised channel rehabilitation

Abstract Incised stream channel aquatic habitats typically are severely degraded. After the primary knickpoints or knickzones have passed, base flows are limited to shallow channels flanked by sandy berms within the enlarged high‐flow channel. Riparian vegetation, woody debris and pool habitat are in short supply, and stream systems become disengaged from their floodplains. We hypothesized that habitat recovery might be accelerated in channels that have incised and are regaining equilibrium through deposition of sandy berms by placing rock spurs in the channel and by planting woody vegetation on the berms. On the basis of literature review and a pilot study, planting designs were developed for a large‐scale field experiment: 2550 1.5 m long cuttings of native willow (Salix spp.) 2–25 cm in diameter were planted 1–1.2 m deep along the base‐flow channel of an incised stream. A ridge of stone was placed on the water side of the plantings, and 17 rock spurs were constructed by extending existing spur dikes from the opposite bank. Woody cover along the treated bank increased from 38% to 66% of bankline after one growing season. Survival of individual plantings was reduced from an estimated 60% to an observed 34% by competition from the exotic kudzu vine, Pueraria lobata. Mean depth and mean scour hole depth, corrected for stage variation, increased 44% and 82%, respectively. Mean scour hole width increased 130%. The mean length of fish and the number of fish species approximately doubled, while the total weight of fish captured by a unit of sampling effort increased by an order of magnitude.

Headwall Influence on Scour at Culvert Outlets

This study investigates headwall influence on local scour downstream of culvert outlets. Scour holes produced by discharging jets of varying duration and magnitude through circular culverts onto a horizontal sand bed were observed and mapped. The bed material was a uniform sand with a mean grain diameter d50 = 1.86 mm and a standard deviation σ = 1.33. The culvert diameter was 102 mm (4 in.). A series of expressions are developed describing the depth, width, length, and volume of scour hole as a function of time, discharge intensity, and headwall conditions. Results are presented in equation and graphical form. A criterion is presented for the design depth of a headwall at culvert outlets.

An Experimental Study of Channel Confluences

The evolution and morphology of the confluences of model channels in a small flume were documented. Scour holes developed that were similar to features described at the confluences of branch channels in the braided North Saskatchewan River, North Platte River, and Medano Creek. The scour holes were maintained by turbulence and helicoidal flow cells generated by the converging flows; their depths and cross-sectional areas increase as turbulence increases. Scour hole depth increases rapidly as confluence angle increases from 15° to 90°, and more slowly up to 180°. Depth also increases as the difference between the discharges of the two confluent tributaries declines, and is a maximum when discharges are precisely equal. Depth decreases, other things being equal, as total sediment load increases. Apparently, an increase in sediment load requires an increase in shear stress for it to be transported, and therefore a constriction of flow and commensurate increase in flow velocity. Scour at the confluences of no...