查看更多>>摘要：Abstract Current geomorphological research tends to give preference to landforms rather than landscapes. This commentary brings these notions closer together and incorporates varied views of ‘landscape’ (such as landscapes in art and poetry) as well as showing the difficulties of communicating them to audiences wider than geomorphology. The paper promotes the use of decimal Latitude Longitude (dLL) values to assist better location of samples, analyses and especially landforms, to produce ‘information surfaces’ within landscape and landsystem approaches to geomorphology. This will also help the development of geomorphic visual literacy.
Mark G. MacklinJonathan BoothPaul A. BrewerStephen Tooth...
20页
查看更多>>摘要：Abstract ‘Complex response’ (Schumm, 1973, Geomorphic thresholds and complex response of drainage systems. In Morisawa, M. (ed.), Fluvial Geomorphology. Binghamton: New York State University Publications: 299‐310) describes situations in which a single event triggers a series of progressively damped morphological and sedimentary adjustments within a catchment. Schumm and Parker's (1973, Implications of complex response of drainage systems for Quaternary alluvial stratigraphy. Nature 243: 99–100) classic stream table experiment of drainage system development showed that one baselevel fall event could result in formation of two sets of paired river terraces that need not be related to additional external (e.g., climate) influences. Despite its enduring popularity in fluvial geomorphology, large‐scale and long‐term field evaluations of Schumm and Parker's complex response model are very limited. Here, we report on a multi‐millennial, multi‐catchment field experiment in south‐western Crete where a high‐magnitude earthquake (estimated magnitude 8.3–8.5) on 21 July 365?ce resulted in up to 9?m of instantaneous uplift over a land area exceeding 6000?km2. Geomorphological, sedimentological, and chronological investigations were used to investigate the erosional and depositional histories in three catchments with outlets uplifted by the 365?ce event. These catchments were compared with the Anapodaris catchment in south central Crete where baselevel was not significantly affected by the earthquake. Although all uplifted catchments experienced valley floor incision, this occurred hundreds of years after 365?ce during a period of wetter climate. The number and age of trunk stream incision and aggradation phases are similar in both uplifted and non‐uplifted catchments, indicating that river responses following the 365?ce uplift event have not followed complex response trajectories in the form documented by Schumm and Parker (1973). This finding highlights the need for rigorous evaluation of other catchment or river response concepts, including through the combined use of laboratory experimental results, field data, and geochronology. In an era of rapid environmental change, characterizing and anticipating catchment and river system response increasingly will depend on a healthy interplay between different investigative approaches.
查看更多>>摘要：Abstract Glaciers fluctuate in response to climate change and record these changes by building sedimentary landforms, including moraines. Therefore, glacial landscapes are a potentially valuable archive of terrestrial palaeoclimate change. Typically, a cooling climate causes glaciers to expand and a warming climate causes glaciers to shrink. However, the glacier response time and the influence of mountainous topography on glacier dynamics complicates this behaviour, such that moraines are not always a straightforward indicator of glacier change in response to climate change. We used a glacial landscape evolution model to simulate the response of a hypothetical mountain glacier to simple changes in climate and the resulting formation and preservation of moraines. These results show that the rate of climate change relative to the glacier response time determines the geometry, number, and position of moraines. Glaciers can build distinct moraines in the absence of climate change. The distance from the maximum ice extent may not represent the chronological order of moraine formation. Moraines can be preserved after being overrun and eroded by subsequent glaciations, but moraine sequences may also contain gaps that are unidentifiable in the field.
查看更多>>摘要：Abstract Given the actual trajectory of greenhouse gases emissions, in situ actions are needed to limit rivers water warming. Riparian tree planting has been shown as a successful mitigation strategy, but most studies have been conducted on upland forested rivers. The hydraulic, geomorphic and thermal conditions of lowland rivers, subject to straightening and fragmentation, are very different from upland systems. In this study, the role of riparian cover relative to the presence of weirs was investigated in lowland slow‐flowing rivers within the agricultural context of the Bresse region, France. Using airborne thermal infrared (TIR) imaging, nine reaches of 20 to 30?km each were mapped to quantify water temperature, riparian vegetation and assess weir location. Campaigns were carried out during the hot summer 2018. Results have comforted that riparian vegetation is capable of limiting increase in temperature during warm events by 2 to 3°C, but with varying success depending on the rivers' thermal regimes. Rivers with a ‘colder’ regime (i.e., ground/karstwater‐fed) are thermally less influenced by riparian vegetation but also more resilient to the presence of weirs. Rivers with the highest degree of thermal alteration showed the highest positive response to riparian shading. Riparian vegetation prevents stagnant water from extreme warming but does not prevent alteration of the thermal regime, for example thermal stratification, likely to occur on a daily basis and to large extents (> 80%) during warm and low‐flow events in impounded rivers. Thermal conditions were sub‐optimal for most fish species that could be expected in these streams. Although the direct relation with thermal alteration is not proved, hydrobiological scores of flow alteration are correlated with our metric of thermo‐physical alteration. Management strategies to limit temperature increase should carefully consider restoring the thermal regime of rivers through weir removal when it is possible, combined with riparian tree planting as an additional mitigation measure.
查看更多>>摘要：Abstract Sediment grain size has a first‐order control on aeolian transport rates due to threshold effects. However, there remain limited in‐situ approaches to measuring properties of sediment size within the saltation layer in field environments. Here, a demonstration of in‐line holography for generating quasi‐3D images of saltating particles near the bed during an aeolian transport event is presented. A vertical fining of sediment grain size in the 50?mm above the bed is found for time‐averaged results. At shorter timescales this fining trend is not unconditionally true as the instantaneous positions of individual saltating particles is dependent on their trajectories. Whereas both the number of particles and sediment concentrations increase with rising wind speeds, the measurements suggest that median grain size coarsened with time despite reducing wind speeds—likely due to winnowing of fine sands from the surface bed layer. Trends found using the holographic sensor are confirmed with traditional measurement techniques, with deviations between these various measurement approaches analyzed and discussed. The data here collectively demonstrate the ability of off‐the‐shelf in‐line holographic technology to provide new insights into detailed particle‐scale aeolian sediment transport dynamics.
查看更多>>摘要：Abstract In order to investigate the effect of intervention structures like dams and barrage on the sediment grain size distribution, we examined a?~75?km stretch of the Tista River in the foothill alluvial plain of the eastern Himalayas. However, studies on the impact of dams on streamflow and sediment in the Himalayan rivers are constrained due to inadequate historical gauge data. Therefore, we employed a simple method to track the initial formation of some of the existing river landforms that formed before the dam/barrage operations from yearly satellite images (1987–2017) for in situ sediment sample collection. Hence, the collected sediment grain sizes analysis is used here as a proxy to comprehend the pre‐, syn‐ and post‐dam river sediment‐transport pattern. We have grouped 23 landforms into three temporal clusters: (i) those formed before 1997, earlier than the dams/barrage interventions (TOL); (ii) formed between 1998 and 2010 during the upstream dam construction (TML) and (iii) formed after 2010 (TNL) when upstream dams and the barrage were fully operational. A total of 116 sediment samples from these landforms indicate the overall grain size reduction for TML and TNL sediment due to reservoir sequestration of coarser grains. In comparison, the coarsest TOL sediment with downstream fining trend represents the pristine river condition. The coarsening trend with a poor correlation coefficient of TML is primarily assigned to partial trapping of the coarse bedload and massive sediment generation by the upstream under‐construction dams, resulting in the downstream erosion of the riverbed and transport of the coarser bedload further down. Similarly, post‐dam sediment trapping and controlled flow were responsible for the weaker trend with a distinctly finer size of TNL. The historical satellite images and data on streamflow and suspended sediment reveal that contemporary river behaviour is consistent with the earlier interpretation.
查看更多>>摘要：Abstract Locating the epicenters and quantifying the magnitudes of historical earthquakes are important, yet difficult, tasks. These tasks often entail estimating seismic intensities based on written documentation, which suffers from biases and uncertainties that are difficult to evaluate. However, past strong earthquakes may trigger numerous landslides that remain in the landscape to the present day, whose number and distribution are correlated with the magnitude and location of the earthquakes. Thus, mapping historical coseismic landslides may provide a useful tool for re‐evaluating the magnitudes of historical earthquakes. We use remote sensing images to map landslides in the southern Loess Plateau, China. We suggested that c. 5000 preserved landslides are related to the 734 ce Tianshui earthquake. These landslides are densely distributed along a?c.?70‐km‐long section of the West Qinling Fault. Based on the assumption that the length of the zone in which substantial landslides occurred is equal to the fault rupture length, we estimated a Mw of 7.2 for the 734 ce Tianshui event, which is similar to previous estimates (Mw?=?6.8–7.5). We confirmed that the 1920 ce Haiyuan, 1718 ce Tongwei, and 1654 ce Lixian earthquakes did not contribute to the historical landslides observed in the study area by combination of the mapping and Chinese related literature. We estimated an Mw of 7.0 for the 1654 ce Lixian earthquake, which is lower than previous estimates (Mw?=?8.0). We suggest that coseismic landslide of medium and large sizes with areas > 104?m2 can be used as a criterion to locating and quantifying historical earthquakes, thereby reducing uncertainties in the estimated magnitudes of historical earthquakes that lack instrument records.
Miles J. ClarkGeorgina L. BennettSandra E. Ryan‐BurkettDavid A. Sear...
16页
查看更多>>摘要：Abstract Bedload transport is a fundamental process by which coarse sediment is transferred through landscapes by river networks and may be well described stochastically by distributions of grain step length and rest time obtained through tracer studies. To date, none of these published tracer studies have specifically investigated the influence of large wood in the river channel on sediment transport dynamics, limiting the applicability of stochastic sediment transport models in these settings. Large wood is a major component of many forested rivers and is increasing due to anthropogenic ‘Natural Flood Management’ (NFM) practices. This study aims to investigate and model the influence of large wood on grain‐scale bedload transport. We tagged 957 cobble to pebble sized particles (D50?=?73?mm) and 28 pieces of large wood (> 1?m in length) with radio frequency identification (RFID) tracers in an alpine mountain stream. We monitored the transport distance of tracers annually over 3 years, building distributions of tracer transport distances with which to compare with published distributions from wood free settings. We also applied linear mixed modelling (LMM), to tease out the influence of wood from other controls on likelihood of entrainment, deposition, and the transport distances of sediments. Tracer sediments accumulated both upstream and downstream of large wood pieces, with LMM analysis confirming a reduction in the probability of entrainment of tracers closer to wood in all 3 years. Upon remobilization, tracers entrained from positions closer to large wood had shorter subsequent transport distances in each year. In 2019, large wood also had a trapping effect, significantly reducing the transport distances of tracer particles entrained from upstream, that is forcing premature deposition of tracers. This study demonstrates the role of large wood in influencing bedload transport in alpine stream environments, with implications for both natural and anthropogenic addition of wood debris in fluvial environments.
查看更多>>摘要：Abstract Several models of classical aeolian sand transport mechanism have been developed to predict the rate of sand transport. However, the values predicted through modeling almost always display large and significant differences from the measured values. The reason for such discrepancies is not fully understood. By studying the effects of wind velocities and fetch lengths on the C values in the model proposed by Lettau and Lettau in 1978, we found that the saturation level of the measured sand flow can significantly affect the accuracy of the predicted value. To solve this problem, we introduced an index (δ) to represent the saturation level. The closer the δ value is to 1, the greater the chance that the sand flow is saturated. This new approach only requires measurement of the saturated sand flow data to verify the robustness of the aeolian sand transport model, a step that has been neglected by previous researchers. Our results suggest that the value of C?=?6.7 proposed in the Lettau and Lettau model is sound and reliable if the sand flow is saturated. We proposed the conditions required for the sand flow to reach saturation and generated an equation to predict the sand transport saturation length (Lsat). Accordingly, we estimated Lsat to be approximately 92?m when u*?=?0.47?m?s?1 for the studied sand surface, which is also the minimum wind shear velocity at which the sand flow can potentially reach saturation.
NSTL
Wiley