Valley Geometry Research Articles

Gravity Modelling of Ice Thickness and Valley Geometry on Taku Glacier (T'aakú Kwáan Sít'i), Alaska

Gravity Modelling of Ice Thickness and Valley Geometry on Taku Glacier (T'aakú Kwáan Sít'i), Alaska

Autogenic evolution of valley‐confined deltas during sea‐level rise: Insights from numerical and physical modelling

AbstractNearshore incised valleys are important conduits for the transport of sediment, nutrients, pollutants and organic carbon from the continents to the sea. Therefore, it is essential to understand the autogenic evolution of deltas confined within incised valleys and how such evolution is affected by relative sea‐level rise. To date, limited research has focused on how deltas constrained by incised valleys or other forms of antecedent topography respond to rising sea level. An existing theory of autostratigraphy envisages scenarios in which two‐dimensional or unconfined three‐dimensional fan deltas can experience three evolutionary stages under constant rates of relative sea‐level rise and sediment supply: progradation, autoretreat and post‐autobreak transgression. In this work, an integrated study of geometric numerical models and physical experiments is undertaken to investigate autostratigraphic delta evolution for a variety of incised‐valley geometries, under conditions of constant rates of relative sea‐level rise and sediment supply. Results indicate that interplays of antecedent topography (valley geometries) and sediment mass balance expressed in resultant deltaic geometries can result in autogenic changes in shoreline dynamics and river avulsion frequency on deltas. The following primary findings arise. (i) Compared to valleys with rectangular and trapezoidal cross‐sectional profiles, valleys with triangular cross‐sections tend to contain deltas that experience faster rates of progradation, autoretreat and post‐autobreak transgression under rising sea level, and exhibit a more prominent convex‐seaward shoreline trajectory. (ii) The shoreline trajectory is also related to delta topset geometry, becoming more convex‐seaward under decreasing topset slopes. (iii) River avulsion frequency on deltas with rising sea level varies markedly across valleys with different geometries, even under the same rate of relative sea‐level rise; this is attributed to the difference in temporal evolution of shoreline migration for different valley geometries and the resultant difference in the delta topset aggradation. This study highlights complexities in responses of sedimentary systems under the confinement of different topographic configurations that have hitherto largely been overlooked in sequence‐stratigraphic models. The findings provide insight into future shoreline behaviour and river avulsion hazard on confined deltas, and for decoding the stratigraphic record.

High‐Resolution Large‐Eddy Simulations of Flow in the Complex Terrain of the Canadian Rockies

AbstractImproving the calculation of land‐atmosphere fluxes of heat and water vapor in mountain terrain requires better resolution of thermally driven diurnal winds (i.e., valley, slope winds) due to differential heating by terrain and radiative fluxes. In this study, the Weather Research and Forecasting model is used to simulate flow in large‐eddy simulation (LES) mode over the complex terrain of the Fortress Mountain and Marmot Creek research basins, Kananaskis Valley, Canadian Rockies, Alberta in mid‐summer. The model was used to examine the temporal and spatial evolution of local winds and near‐surface boundary layer processes with variability in topography and elevation. Numerically resolving complex terrain wind flow effects require smaller grid cell size. However, the use of terrain‐following coordinates in most numerical weather prediction models results in large numerical errors when flow over steep terrain is simulated. These errors propagate through the domain and can result in numerical instability. To avoid this issue when simulating flow over steep terrain a local smoothing approach was used, where smoothing is applied only where slope exceeds some predetermined threshold. LES results from local smoothing were compared with a mesoscale model and LES with global smoothing. Simulations are evaluated using sounding data and meteorological stations. The differences in flow patterns and reversals in two mountain basins suggest that valley geometry and volume is relevant to the break up of inversion layers, removal of cold‐air pools, and strength of thermally driven winds.

Paleoglaciological reconstruction and geomorphological mapping of Dobrowolski Glacier, King George Island, Antarctica

The paper aims to reconstruct the fluctuations of Dobrowolski Glacier, a tidewater glacier located in the inner position of the Admiralty Bay (King George Island, Antarctica), from the Little Ice Age (1400-1700) until the present. Measurements of the glacier’s area and length were based on multitemporal satellite imagery and submarine glacial landforms. The glacier surface area variations between the Little Ice Age and 2014 A.D were estimated. Morainic banks and paleoglacial reconstructions provided evidence of fluctuations in the surface area of the glacier between PIG and 2014 AD. Therefore, four stages of analysis were established: Stage I (Part I) (1400 to 1700), Stage I (Part II) (1700 until the mid-20th century), Stage II (mid-20th century until the 1980s), Stage III (1980 to 2000), and Stage IV (2000 to 2020). The climate during the Little Ice Age triggered the last major glacial advance, and their grounding line position was recorded by an external and prominent morainic bank. After the major glacial advance position of the grounding line, the ice-margin has undergone higher retreat rates (stage I) as response to the warming trend and the loss of anchoring point. The stage II (Unit B) is recorded by distal and discontinuous morainal ridges and glacial lineations formed in the context of an active ice flow at a deeper point in the fjord. During stage III (Unit C) glacial lineations and steep slopes occur, while landforms are less preserved, revealing a fast shrinkage phase. Stage IV is characterized by discontinuous morainic ridges (Unit D), when the glacier presents the highest annual glacial area loss. Currently, the accelerated shrinkage may be linked to the loss of anchorage on seamounts (serving as pinning points) and increased warming. The U-shaped valley geometry has also influenced the glacier shrinkage processes and the redirection of glacial paleoflow during the last 300 years. The retreat rate to mid-20th century-2020 period is higher than Little Ice Age- mid-20th century period.

Geomorphic complexity influences coarse particulate organic matter transport and storage in headwater streams

Coarse particulate organic matter (CPOM; organic matter 1–100 mm in diameter, excluding small wood) stored in streams provides an important energy source for aquatic ecosystems, and CPOM transport provides downstream energy subsidies and is a pathway for watershed carbon export. However, we lack understanding of the magnitude of and processes influencing CPOM storage and transport in headwater streams. We assessed how geomorphic complexity and hydrologic regime influence CPOM transport and storage in the Colorado Front Range, USA. We compared CPOM transport during snowmelt in a stream reach with high retentive feature (e.g., wood, cobbles, and other features) frequency to a reach with low retentive feature frequency, assessing how within-a-reach geomorphic context influences CPOM transport. We also compared CPOM transport in reaches with differing valley geometry (two confined reaches versus a wide, multi-thread river bead) to assess the influence of geomorphic variations occurring over larger spatial extents. Additionally, we compared CPOM storage in accumulations in reaches (n = 14) with flowing water or dry conditions in late summer and investigated how small pieces of organic matter [e.g., woody CPOM and small wood (>1 min length and 0.05–1 min diameter or 0.5–1 min length and >0.1 min diameter)] influence CPOM storage. We found that within-a-reach retentive feature frequency did not influence CPOM transport. However, valley geometry influenced CPOM transport, with a higher CPOM transport rate (median: 1.53 g min−1) downstream of a confined stream reach and a lower CPOM transport rate (median: 0.13 g min−1) downstream of a low gradient, multi-thread river bead. Additionally, we found that particulate organic carbon (POC) export (0.063 Mg C) in the form of CPOM was substantially lower than dissolved organic carbon (DOC) export (12.3 Mg C) in one of these headwater streams during the 2022 water year. Dry reaches stored a higher volume of CPOM (mean = 29.18 m3 ha−1) compared to reaches with flowing water (15.75 m3 ha−1), and woody CPOM pieces trapped 37% of CPOM accumulations. Our results demonstrate that the influence of geomorphic context on CPOM transport depends on the scale and type of geomorphic complexity, POC may be lower than DOC export in some headwater streams, and small woody organic material is important for trapping CPOM small streams.

Beyond the Case Study: Characterizing Natural Floodplain Heterogeneity in the United States

AbstractWe use the five landscape ecology metrics of aggregation index, percentage of like adjacencies, interspersion and juxtaposition index, patch density, and Shannon's evenness index to assess spatial heterogeneity at 15 floodplains in the continental United States. Assessments are based on floodplain classes and patches delineated remotely using topography and vegetation. Floodplain reaches examined here represent diverse drainage areas, flow regimes, valley geometries, channel planforms, and biomes. We selected sites with minimal direct human alteration. Our objectives are to quantify floodplain spatial heterogeneity; evaluate whether statistically significant patterns are present; and interpret the statistical analyses with respect to the influence of lateral channel mobility and valley‐floor space available. We develop a conceptual model of the influences on lateral mobility and space available, and then test specific hypotheses derived from this conceptual model. These natural floodplains have a median aggregation index of 58.8%, median percentage of like adjacencies of 58.5%, median interspersion and juxtaposition index of 74.9%, median density of 1,241 patches/ha, and median Shannon's evenness index of 0.934 (n = 15). In other words, natural floodplains have moderate aggregation of classes, high evenness and intermixing of classes, and a wide range of patch densities. Drainage area, the ratio of floodplain/channel width, elevation, precipitation, total sinuosity, large wood volume, planform, and flow regime emerge as important variables to floodplain heterogeneity. These results highlight the influence of biotic‐abiotic interactions in shaping floodplain heterogeneity across diverse river corridors.

From landslide characterization to nature reserve management: The “Scialimata Grande di Torre Alfina” landslide Geosite (Central Apennines, Italy)

Italy is characterized by widespread geomorphological instability, among which landslides leave impressive marks on the landscape. Nevertheless, landslide bodies may represent key sites for thematic and educational itineraries, especially in protected areas, where their management becomes an important issue. Our study focuses on the “Monte Rufeno Nature Reserve” (Central Apennines, Italy), where iconic landslides are present. Here, the “Scialimata Grande di Torre Alfina” landslide (SGTA) is listed in the regional Geosite database. This work aims to propose a multiscale procedure for landslide analysis, in terms of both hazard sources but also educational and geoheritage enhancement opportunities in natural reserves. After performing a Landslide Susceptibility conditional Analysis (LSA) for the reserve territory, attention was focused on the SGTA, to define properly its features and morphodynamics. A multi-disciplinary approach was adopted, by applying both remote sensing (UAV structure from motion, Photointerpretation) and field survey (geomorphological and GPS monitoring). From the LSA, based on drainage density, curvature, and slope triggering factors, the road and trail susceptibility maps were derived, as base tools for future risk assessments and trail paths management within the reserve. At the SGTA scale, the monitoring showed a displacement of up to 23 m during the time interval between 2015 and 2018. The landslide dynamics seem to be driven by alternating dry and extremely wet periods; moreover, leaks from the aqueduct in the detachment area and piping effects through clays may have also decreased the substrate cohesion. The SGTA complex influence on the Paglia River valley geometry was also hypothesized, underlining the action of landslide through different spatial scales (on-site and off-site) and on different environment features (sediment connectivity, hydrology). Finally, the SGTA appears highly representative of the geomorphic dynamics within the Nature Reserve (i.e., scientific value) and it could be classified as an active geosite. Since the site was featured by a tourist trail, adequate management strategies must be adopted, considering the educational value and safety issues.

Modeling How Interface Geometry and Mechanical Stress Affect Li Metal/Solid Electrolyte Current Distributions

We develop a coupled electrochemical-mechanical model to assess the current distributions at Li/single-ion conducting solid ceramic electrolyte interfaces containing a parameterized interfacial geometric asperity, and carefully distinguish between the thermodynamic and kinetic effects of interfacial mechanics on the current distribution. We find that with an elastic-perfectly plastic model for Li metal, and experimentally relevant mechanical initial and boundary conditions, the stress variations along the interface for experimentally relevant stack pressures and interfacial geometries are small (e.g., <1 MPa), resulting in a small or negligible influence of the interfacial mechanical state on the interfacial current distribution for both plating and stripping. However, we find that the current distribution is sensitive to interface geometry, with sharper (i.e., smaller tip radius of curvature) asperities experiencing greater current focusing. In addition, the effect on the current distribution of an identically sized lithium peak vs valley geometry is not the same. These interfacial geometry effects may lead to void formation on both stripping and plating and at both Li peaks and valleys. The presence of high-curvature interface geometry asperities provides an additional perspective on the superior cycling performance of flat, film-based separators (e.g., sputtered LiPON) versus particle-based separators (e.g., polycrystalline LLZO) in some conditions.

Simulations of Wind Formation in Idealised Mountain–Valley Systems Using OpenFOAM

An OpenFOAM computational fluid dynamics model setup is proposed for simulating thermally driven winds in mountain–valley systems. As a first step, the choice of Reynolds Averaged Navier–Stokes k−ε turbulence model is validated on a 3D geometry by comparing its results vs. large-eddy simulations reported in the literature. Then, a numerical model of an idealised 2D mountain–valley system with mountain slope angle of 20° is developed to simulate thermally driven winds. A couple of top surface boundary conditions (BC) and various combinations of temperature initial conditions (IC) are tested. A transient solver for buoyant, turbulent flow of incompressible fluids is used. Contrary to classical approaches where buoyancy is set as a variable of the problem, here temperature linearly dependent with altitude is imposed as BC on the slope and successfully leads to thermally driven wind generation. The minimum fluid domain height needed to properly simulate the thermally driven winds and the effects of the different setups on the results are discussed. Slip wall BC on the top surface of the fluid domain and uniform temperature IC are found to be the most adequate choices. Finally, valleys with different widths are simulated to see how the mountain–valley geometry affects the flow behaviour, both for anabatic (daytime, up-slope) and katabatic (nighttime, down-slope) winds. The simulations correctly reproduce the acceleration and deceleration of the flow along the slope. Increasing the valley width does not significantly affect the magnitude of the thermally driven wind but does produce a displacement of the generated convective cell.

Multi‐scale drivers of spatial patterns in floodplain sediment and phosphorus deposition

AbstractThe capacity for floodplains to capture sediment and filter pollutants is spatially variable and depends on the complex interactions of geomorphic, geologic, and hydrologic variables that operate at multiple scales. In this study, we integrated watershed‐scale and local assessments to improve our understanding of floodplain depositional patterns. We developed a dataset of event‐scale observations of sediment and phosphorus deposition rates distributed at 129 plots across large environmental gradients of floodplain topography, valley geometry, and watershed characteristics in the Lake Champlain Basin, Vermont. Plot‐scale observations were used to evaluate the cross‐scale influence of environmental factors and were summarized into site‐scale averages to explore regional trends. Consistent with other studies, floodplain deposition generally scaled with drainage area, but trends were longitudinally discontinuous and depended on variations in valley width and slope. While variability in deposition patterns at the watershed‐scale was large (average of 2.0 (0.2–9.8) kg sediment m−2 yr−1; average of 1.4 (0.2–6.5) g phosphorus m−2 yr−1), the range in deposition rates locally across a floodplain was greater (average of 4.6 (0.06–21.7) kg sediment m−2 yr−1; average of 6.4 (0.1–41.1) g phosphorus m−2 yr−1). Local variables that described the proximity to water and sediment sources, and frequency with which the plot was activated by a flood, had the greatest relative contribution to boosted regression tree models of phosphorus deposition rates, highlighting the importance of river–floodplain connectivity for floodplain functioning and the profound impact of human activities that limit such connectivity. Patterns identified in our study may guide prioritization of restoration and conservation practices designed to capture sediment and phosphorus on floodplains.

Hillslope Sediment Supply Limits Alluvial Valley Width

AbstractRiver‐valley morphology preserves information on tectonic and climatic conditions that shape landscapes. Observations suggest that river discharge and valley‐wall lithology are the main controls on valley width. Yet, current models based on these observations fail to explain the full range of cross‐sectional valley shapes in nature, suggesting hitherto unquantified controls on valley width. In particular, current models cannot explain the existence of paired terrace sequences that form under cyclic climate forcing. Paired river terraces are staircases of abandoned floodplains on both valley sides, and hence preserve past valley widths. Their formation requires alternating phases of predominantly river incision and predominantly lateral planation, plus progressive valley narrowing. While cyclic Quaternary climate changes can explain shifts between incision and lateral erosion, the driving mechanism of valley narrowing is unknown. Here, we extract valley geometries from climatically formed, alluvial river‐terrace sequences and show that across our dataset, the total cumulative terrace height (here: total valley height) explains 90%–99% of the variance in valley width at the terrace sites. This finding suggests that valley height, or a parameter that scales linearly with valley height, controls valley width in addition to river discharge and lithology. To explain this valley‐width‐height relationship, we reformulate existing valley‐width models and suggest that, when adjusting to new boundary conditions, alluvial valleys evolve to a width at which sediment removal from valley walls matches lateral sediment supply from hillslope erosion. Such a hillslope‐channel coupling is not captured in current valley‐evolution models. Our model can explain the existence of paired terrace sequences under cyclic climate forcing and relates valley width to measurable field parameters. Therefore, it facilitates the reconstruction of past climatic and tectonic conditions from valley topography.

The fracture-controlled carbonate Brejões Karst System mapped with UAV, LiDAR, and electroresistivity in the Irecê Basin - Brazil

Understanding how fractures influence fluid flow in carbonate rocks is essential to understanding karst evolution, oil reservoirs, and aquifer exploitation. The present study investigates karst fracture geometry at the surface and subsurface in the Irecê Basin, a Neoproterozoic epicontinental cratonic basin in the São Francisco Craton, Brazil. We used an Unmanned Aerial Vehicle (UAV) to map both fractures in three outcrops and valley geometries, Light Detection And Ranging (LiDAR) to map 3500 m of cave conduits, and Electrical Resistivity Tomography (ERT) to identify karst zones between 90 and 110 m deep. Our results indicate that the Brejões Karst System is comprised of dissolution and collapse features such as caves, dolines, pavements, and karst valley. The quantitative analysis of subseismic fractures (<50 m long) identified 4398 fractures in Outcrop A, 2458 in Outcrop B, 4374 in Outcrop C, and 7172 in 18 cross-sections of the Brejões Cave. These fractures form a corridor with four main sets striking N–S, NNE–SSW, NW–SE, and E–W, where the fracture system is essential to cave connectivity, permeability, and development. The highest frequency of fractures coincides with the main cave axes, where the intersection of the fracture/fault are sites prone to dissolution. The ERT profiles indicated a high density of geophysical anomalies that correspond to enlarged fractures in the epikarstic zone that extend between the surface and approximately 30 m deep. The cave infill impacts the cave characterization and leads to underestimating cave dimensions. This study may contribute to an underestimation of the role played by subseismic scale fractures in karst evolution in gently deformed settings.

Kinematic evolution of kilometre-scale fold trains in surge-type glaciers explored with a numerical model

Glacier surging provides a window into the processes responsible for some of the fastest ice flow on Earth. Surge-type glaciers are festooned with fold trains—many visible from space—which encode the history of polyphase deformation associated with this form of episodic fast flow. We conduct the first investigation of the kinematic evolution of these kilometre-scale folds using a full-Stokes numerical ice-flow model. We model the folds through multiple surge cycles within a set of synthetic glacier confluence configurations, and identify how differences in glacier flow regimes imprint themselves on three-dimensional fold geometry. Based on simulation results across parameter space, we present an archetype of kinematic evolution that describes the transition from cylindrical vertically plunging gentle folds emplaced during the surge phase, to complex depth-varying folds following multiple cycles of surging and quiescent flow. A detailed examination of the surface trace of these folds highlights the links between glacier flow regime and folding. The initial fold geometry is controlled by longitudinal and lateral shear stress regimes during surging, while fold evolution is governed primarily by lateral shearing after emplacement. This reflects the influence of valley geometry and glacier dynamics on the variability of flow regimes during both surging and quiescent flow. Finally, we illustrate the potential of our approach to reconstruct more complex fold geometries as observed in nature.

The Effects of the Width of an Isolated Valley on Near-Surface Turbulence

With the aim of ascertaining the effects of the widths (A) of valleys on near-surface turbulence, flows over an isolated symmetric three-dimensional valley of constant depth (H) and slopes are characterized in a large-boundary-layer wind tunnel. Starting at A = 4H, valley widths were systematically varied to A = 12H with constant increments of 2H. High-resolution laser-Doppler velocimetry measurements were made at several equivalent locations above each of the resulting valley geometries and compared with data from undisturbed flows over flat terrain. Flow separation caused by the first ridges generated inner-valley recirculation bubbles with lengths dependent on the valley widths. Secondary recirculation zones were also observed downstream from the crests of the second ridges. Results show that the width modifications exert the strongest effects on turbulence within the valleys and the vicinities of the second ridges. Above these locations, maximal magnitudes of turbulence are generally found for the larger width geometries. Furthermore, lateral turbulence overpowers the longitudinal counterparts nearest to the surface, with maximal gains occurring for the smaller widths. Our data indicate that valley widths are impactful on near-surface flows and should be considered together with other more established geometric parameters of influence.

The Influence of Slopes of Isolated Three-Dimensional Valleys on Near-Surface Turbulence

Motivated by a limited understanding of how valleys affect near-surface turbulence, characterizations of neutrally stable atmospheric-boundary-layer flows over isolated valleys are presented. In particular, the influence of the slopes of the three-dimensional ridges that form the idealized valleys are investigated. Flows over three distinct symmetric valley geometries were modelled in a large boundary-layer wind tunnel. For each valley geometry, the high-resolution measurements from the crests of each of the ridges and the midpoint between them are compared with an undisturbed moderately rough classed boundary-layer flow over flat terrain with homogeneous surface roughness. Flow separation originates above the crests of the first ridges of all geometries and generates recirculation zones. These are characterized by slope-dependent increases in three-dimensional near-surface turbulence when compared with the attached flows further upstream. The recirculation zones longitudinally extend to roughly half the valley width. Above the crests of the second ridges, the longitudinal velocity component decreases and turbulence intensity increases when compared with the flows above the crests of the first ridges. Results also exhibit significant increases of turbulence above the inner-valley regions of all geometries.

Controls on the formation and size of potential landslide dams and dammed lakes in the Austrian Alps

Abstract. Controls on landsliding have long been studied, but the potential for landslide-induced dam and lake formation has received less attention. Here, we model possible landslides and the formation of landslide dams and lakes in the Austrian Alps. We combine a slope criterion with a probabilistic approach to determine landslide release areas and volumes. We then simulate the progression and deposition of the landslides with a fluid dynamic model. We characterize the resulting landslide deposits with commonly used metrics, investigate their relation to glacial land-forming and tectonic units, and discuss the roles of the drainage system and valley shape. We discover that modeled landslide dams and lakes cover a wide volume range. In line with real-world inventories, we further found that lake volume increases linearly with landslide volume in the case of efficient damming – when an exceptionally large lake is dammed by a relatively small landslide deposit. The distribution and size of potential landslide dams and lakes depends strongly on local topographic relief. For a given landslide volume, lake size depends on drainage area and valley geometry. The largest lakes form in glacial troughs, while the most efficient damming occurs where landslides block a gorge downstream of a wide valley, a situation preferentially encountered at the transition between two different tectonic units. Our results also contain inefficient damming events, a damming type that exhibits different scaling of landslide and lake metrics than efficient damming and is hardly reported in inventories. We assume that such events also occur in the real world and emphasize that their documentation is needed to better understand the effects of landsliding on the drainage system.

Impact of Shelf Valleys on the Spread of Surface-Trapped River Plumes

AbstractThis study focuses on mechanisms of shelf valley bathymetry affecting the spread of riverine freshwater in the nearshore region. In the context of Changjiang River, a numerical model is used with different no-tide idealized configurations to simulate development of unforced river plumes over a sloping bottom, with and without a shelf valley off the estuary mouth. All simulated freshwater plumes are surface-trapped with continuously growing bulges near the estuary mouth and narrow coastal currents downstream. The simulations indicate that a shelf valley tends to compress the bulge along the direction of the valley long axis and modify the incident angle of the bulge flow impinging toward the coast, which then affects the strength of the coastal current. The bulge compression results from geostrophic adjustment and isobath-following tendency of the depth-averaged flow in the bulge region. Generally, the resulting change in the direction of the bulge impinging flow enhances down-shelf momentum advection and freshwater delivery into the coastal current. Sensitivity simulations with altered river discharges Q, Coriolis parameter, shelf bottom slope, valley geometry, and ambient stratification show that enhancement of down-shelf freshwater transport in the coastal current, ΔQc, increases with increasing valley depth within the bulge region and decreasing slope Burger number of the ambient shelf. Assuming potential vorticity conservation, a scaling formula of ΔQc/Q is developed, and it agrees well with results of the sensitivity simulations. Mechanisms of valley influences on unforced river plumes revealed here will help future studies of topographic influence on river plumes under more realistic conditions.

The resilience of logjams to floods

AbstractLogjams that span the bankfull channel strongly influence hydraulics and downstream fluxes of diverse materials. Several studies quantify the longitudinal distribution of channel‐spanning logjams, but fewer studies examine changes in longitudinal distribution in response to disturbances such as floods. We use 10 years of annual surveys of a population of channel‐spanning logjams along mountain streams in the Southern Rocky Mountains. Surveys from 2010 to 2019 bracket substantial interannual variability in the snowmelt peak flow as well as a rainfall flood in 2013. We characterised the number of logjams per unit length of valley (logjam distribution density) within and between reaches designated based on longitudinally consistent channel and valley geometry. Our primary objectives are to evaluate the influences on logjam distribution density of (i) spatial variations in valley and channel geometry and (ii) temporal variations in peak annual flow. We hypothesized that logjam distribution densities are resilient to disturbance at both spatial scales. At the creek scale, logjam distribution density correlates significantly with increasing ratio of floodplain width to channel width and wood piece length to channel width. Wide, low gradient reaches with greater distribution density exhibit greater interannual variation in distribution density. These reaches lost jams during the 2013 flood but returned to pre‐flood distribution density values by the end of the study. The pattern of greater logjam distribution density in unconfined reaches relative to confined and partially confined reaches is also consistent over the period of the study. We interpret these results as indicating the resilience of logjam distributions to disturbance. The persistence of greater numbers of logjams in wide, low gradient reaches suggests that river restoration employing engineered logjams and wood reintroduction can focus most effectively on these reaches.

The effects of longitudinal variations in valley geometry and wood load on flood response

AbstractWe use field measurements and airborne LiDAR data to quantify the potential effects of valley geometry and large wood on channel erosional and depositional response to a large flood (estimated 150‐year recurrence interval) in 2011 along a mountain stream. Topographic data along 3 km of Biscuit Brook in the Catskill Mountains, New York, USA reveal repeated downstream alternations between steep, narrow bedrock reaches and alluvial reaches that retain large wood, with wood loads as high as 1261 m3 ha−1. We hypothesized that, within alluvial reaches, geomorphic response to the flood, in the form of changes in bed elevation, net volume of sediment eroded or aggraded, and grain size, correlates with wood load. We hypothesized that greater wood load corresponds to lower modelled average velocity and less channel‐bed erosion during the flood, and finer median bed grain size and a lower gradation coefficient of bed sediment. The results partly support this hypothesis. Wood results in lower reach‐average modelled velocity for the 2011 flood, but the magnitude of change in channel‐bed elevation after the 2011 flood among alluvial and bedrock reaches does not correlate with wood load. Wood load does correlate with changes in sediment volume and bed substrate, with finer grain size and smaller sediment gradation in reaches with more wood. The proportion of wood in jams is a stronger predictor of bed grain‐size characteristics than is total wood load. We also see evidence of a threshold: greater wood load correlates with channel aggradation at wood loads exceeding approximately 200 m3 ha−1. In this mountain stream, abundant large wood in channel reaches with alluvial substrate creates lower velocity that results in finer bed material and, when wood load exceeds a threshold, reach scale increases in aggradation. This suggests that reintroducing small amounts of wood or one logjam for river restoration will have limited geomorphic effects. © 2020 John Wiley &amp; Sons, Ltd.

Geospatial simulations of airborne ice-penetrating radar surveying reveal elevation under-measurement bias for ice-sheet bed topography

AbstractAirborne radio-echo sounding (RES) surveys are widely used to measure ice-sheet bed topography. Measuring bed topography as accurately and widely as possible is of critical importance to modelling ice dynamics and hence to constraining better future ice response to climate change. Measurement accuracy of RES surveys is influenced both by the geometry of bed topography and the survey design. Here we develop a novel approach for simulating RES surveys over glaciated terrain, to quantify the sensitivity of derived bed elevation to topographic geometry. Furthermore, we investigate how measurement errors influence the quantification of glacial valley geometry. We find a negative bias across RES measurements, where off-nadir return measurement error is typically −1.8 ± 11.6 m. Topographic highlands are under-measured an order of magnitude more than lowlands. Consequently, valley depth and cross-sectional area are largely under-estimated. While overall estimates of ice thickness are likely too high, we find large glacier valley cross-sectional area to be under-estimated by −2.8 ± 18.1%. Therefore, estimates of ice flux through large outlet glaciers are likely too low when this effect is not taken into account. Additionally, bed mismeasurements potentially impact our appreciation of outlet-glacier stability.