2D soil and engineering-seismic bedrock modeling of eastern part of Izmir inner bay/Turkey
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Bedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief. Using digital elevation models, topographic maps, and field reconnaissance, we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the Romanche watershed, French Western Alps. Longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches. We reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat. From morphometric analyses, we find that mean channel gradients and widths, as well as knickpoint retreat rates, display a drainage area dependence modulated by bedrock lithology. However, there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints. Assuming a postglacial origin of these gorges, our results imply high postglacial fluvial incision (0.5–15 mm yr −1 ) and knickpoint retreat (1–200 mm yr −1 ) rates that are, however, consistent with previous estimates. Numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges. Results from simple end‐member models suggest transport‐limited behavior of the bedrock gorges. A more sophisticated model including dynamic width adjustment and sediment‐dependent incision rates predicts present‐day channel geometry only if a significant supply of sediment from the gorge sidewalls (∼10 mm yr −1 ) is triggered by gorge deepening, combined with pronounced inhibition of bedrock incision by sediment transport and deposition.
Bedrock
Lithology
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Bay mud
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Postglacial bedrock river erosion is likely to be a major control on the evolution of deglaciated landscapes. This study provides a quantitative assessment of bedrock channel change in a postglacial, post-orogenic terrain, encompassing the long-term evolution of bedrock channel distribution, geometry and the timing and rate of fluvial incision.
In the NW Scottish Highlands, fluvial incision is focused on steep valley headwalls and at knickpoints formed at inherited glacial valley-floor steps (riegels). Holocene average incision rates of 0.4 – 1.3 m/kyr were measured using cosmogenic surface exposure dating (10-Be) at five strath terrace sites. Incision rates of 0.1 m/kyr were quantified from active channel beds and are lower than the Holocene average. This finding is consistent with a paraglacial decline in sediment supply being responsible for a reduction in fluvial incision in detachment-limited channels. Further support for a paraglacial sediment influence on bedrock channels is found in the long-term increase in the proportion of bedrock-exposure, reflecting a decrease in the critical slope threshold for the alluvial to bedrock channel transition. In reaches that have undergone a switch from alluvial to bedrock channel conditions, the onset of fluvial incision into bedrock was found to lag deglaciation by 2 – 4 kyr, suggesting that a substantial reduction in sediment availability occurred within several thousand years of ice retreat.
Hydraulic conditions and substrate resistance are also major controls on the distribution and geometry of bedrock channels, and the rate of fluvial incision, in the NW Highlands. The geometry of both bedrock and alluvial channels was found to be strongly hydraulically scaled, with bedrock channels significantly narrower than coarse-grained alluvial channels. Lithology also governs the critical slope for alluvial to bedrock channel-transition; resistant metasedimentary bedrock produces relatively coarse-grained bed material with a high threshold for sediment entrainment, meaning that alluvial channels occur up to comparatively steep channel slopes. Lithological resistance also constrains the process and rate of fluvial incision. A new lithological resistance index, the ratio of joint spacing to intact rock strength, successfully discriminates between abrasion and plucking dominated channels and is non-linearly related to incision rate.
The pulse of postglacial incision in bedrock channels has resulted in 2 – 8 m of entrenchment into valley floors since deglaciation. Bedrock channels narrow during entrenchment, achieving a consistent hydraulic geometry when entrenched to between 1 and 2 times the bankfull flow depth. Width adjustment occurs within 8 – 17 kyr of ice retreat, but adjustment of channel slope takes considerably longer and the long profiles of NW Highland rivers remain strongly glacially conditioned. Entrenchment disconnects channels from floodplains and may have contributed to the decline in paraglacial sediment flux, suggesting that fluvial incision may be a self-limiting process in post-orogenic postglacial terrains.
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Deglaciation
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Abstract Recent studies reveal that low‐slope bedrock reaches (bedrock surface slope milder than ~5 m/km) are more common than previously thought and can be found in engineered rivers and densely populated deltas. Here we present a novel formulation of alluvial morphodynamics of low‐slope bedrock rivers transporting nonuniform bed material that accounts for the nonuniformity of the sediment size and the presence of small scale bedforms such as dunes and can thus be of aid to solve management/restoration problems in low‐slope bedrock rivers. The formulation is implemented in a one‐dimensional morphodynamic model. Numerical results are compared with laboratory experiments on equilibrium bedrock reaches downstream of stable alluvial‐bedrock transitions. The differences between experimental and numerical results are comparable with those obtained in the alluvial case. Model applications simulate (1) bedrock reaches with a stable bedrock‐alluvial transitions, (2) an alluvial‐bedrock transition subject to sea level rise, and (3) steep bedrock reaches. Upstream of a stable bedrock‐alluvial transition the flow decelerates in the streamwise direction with the formation of a stable pattern of downstream coarsening of bed surface sediment. In response to sea level rise, alluvial‐bedrock transitions migrate downstream and bedrock‐alluvial transitions migrate upstream. Opposite migration directions are expected in the case of sea level fall. When applied to steep channels, the model predicts gradual alluviation, but it fails to reproduce runaway alluviation.
Bedrock
Beach morphodynamics
Bedform
Alluvion
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Bedrock
Bedform
Flume
Abrasion (mechanical)
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Debris flow erosion into bedrock helps to set the pace of mountain denudation, but there are few empirical observations of this process. We studied the effects of debris flows on bedrock erosion using Structure-From-Motion photogrammetry and multiple real-time monitoring measurements. We found that the distribution of bedrock erosion across the channel cross-section could be generalized as an exponentially decreasing function of height above the channel thalweg. Using this empirical function, we simulated the erosion at a cross-section after the theoretical passage of a migrating knickpoint effectively matching the upstream pre-knickpoint cross-sectional shape to the downstream post-knickpoint cross-sectional shape via debris-flow bedrock erosion.
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Debris flow
Denudation
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<p>Sediment grains in bedrock-alluvial channels can be entrained from bedrock surfaces or from alluvial patches. Field tracer data has shown that grains entrained from different surfaces can have very different critical shear stresses, which will affect bedload transport rates, the stability of sediment cover and bedrock incision. We hypothesise that the topography of the bedrock surface affects the critical shear stress of a sediment grain in at least three ways: the pivot angle through which the grain must move to be mobilised; the extent to which the grain is sheltered by upstream bedrock protrusions; and the impact on the flow profile via the roughness length z<sub>0</sub>. Here we quantify how bedrock topography affects these three different components, and their overall impact on critical shear stress.</p><p>Our analysis is based around six samples of bedrock river topography, from rivers with different degrees of roughness and structural characteristics. Each surface was 3D printed at a reduced scale, and pivot angles were measured by dropping grains of different sizes at different locations, and tilting the surface until the grain moved. For the surface with bedrock ribs, experiments were repeated with the ribs parallel and perpendicular to the downslope direction. Further experiments were performed after incrementally covering 25% through to 100% of the surface with fixed sediment cover. Bedrock sheltering and z<sub>0</sub> were estimated from analysis of surface topography.</p><p>Overall, we find that measured pivot angles decrease with increasing surface roughness, similar to previous relationships from alluvial channels. However, we find that the pivot angle for a grain at any particular location cannot be predicted from the local surface topography, because of the complex interaction between grain shape and the different scales of roughness present on the surface. Rib direction also has a significant influence on mean pivot angle. The impact of sediment cover depends on the relative roughness of the cover and the bedrock surface.</p><p>We calculate critical shear stress using Kirchner&#8217;s force balance model, parameterised using our measurements of pivot angle, sheltering and z<sub>0</sub>. We find that z<sub>0</sub> has the largest impact on the predicted median values of critical shear stress. Including the measured pivot angles reduces the lowest values of critical shear stress, with implications for the onset of sediment transport. Overall, our data represent the first attempt to quantify fully how bedrock topography influences the critical shear stress of sediment grains in bedrock-alluvial channels.</p>
Bedrock
Hydraulic roughness
Entrainment (biomusicology)
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To understand the pattern of sedimentation rates as fundamental physical parameter of coastal environment, the 210Pb dating method was applied to core samples collected from Kagoshima Bay, Southwestern Japan. The sedimentation rate varied at each location within the bay (0.08–0.30 g·cm−2·y−1), and the rate at the bay-head area was less than that at the centre of the bay. The inventory of ex210Pb has a lower value in the bay-head area. The low ex210Pb inventory at Stn.5' is considered to be due to physical, and chemical conditions in the bay-head area.
Sedimentation
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Field observations from western Washington and eastern Tibet indicate a strong lithologic influence on strath terrace formation and highlight the response of bedrock channel width to spatial gradients in bedrock erodibility and/or rock uplift rates. Measurements of local bedrock bed and bank erosion rates together with observations of the role of weathering processes on erosion of siltstones and sandstones illustrate a mechanism that underlies a conceptual model that predicts a strong lithological control on strath terrace formation. Direct measurements of bedrock erosion rates in lithologies susceptible to accelerated erosion upon sub-aerial exposure show that lateral rates of bedrock bank erosion can substantially exceed vertical incision below the perennial flow level due to an asymmetry in erodibility between perennially submerged rock and rock exposed to cyclic wetting and drying on bedrock channel walls. Under such conditions, the rapid, weathering-mediated lowering of bedrock exposed above baseflow promotes development of a beveled bedrock platform that could become a strath terrace upon abandonment. In contrast to previous studies, channel widths measured across a substantial gradient in lithology, slope, and long-term erosion rates in eastern Tibet do not follow conventional relations where channel width scales as a power law function of drainage area. Instead, as they flow across a zone of rapid uplift these channels systematically narrow relative to the predicted width extrapolated from traditional power-law relations. Field studies from a wide range of geological settings and lithologies support the interpretation that, in general, strath terraces tend to be more extensive in rivers flowing over weak sedimentary rock and tend to be poorly developed and/or preserved where rivers flow over hard, erosion-resistant rock.
Bedrock
Lithology
Terrace (agriculture)
Bank erosion
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