High-resolution Topographic Surveys Research Articles

Inferring on fluvial resilience from multi-temporal high-resolution topography and geomorphic unit diversity

The resilience of a river corridor represents its ability to withstand and recover from disturbances. Quantifying fluvial resilience in the face of various stressors is essential for integrating ecology and geomorphology in a context of river management. Geomorphic unit diversity analysis emerges as a valuable tool for characterizing and quantifying fluvial resilience to disturbances due to its inherent connection with fluvial dynamics. This paper aims to analyse and quantify the fluvial resilience of a wandering gravel-bed river affected by natural (e.g., floods) and human-induced (e.g., instream gravel mining) stressors. To achieve this goal, we use multi-temporal high-resolution topographic surveys of the Upper River Cinca (South-Central Pyrenees) spanning from 2014 to 2020. By employing the Geomorphic Unit Tool (GUT) on these surveys, we can map geomorphic units over time, quantify their diversity, and study geomorphic adjustments through morphodynamic signatures, altogether allowing inferring fluvial resilience.Our findings reveal that topographic changes (i.e., erosion and sedimentation) correlate with the type of stressor: maintenance works and gravel mining lead to degradation, while floods induce aggradation. Geomorphic Unit Diversity decreases following channel disturbances caused by gravel mining but rebounds after periods primarily stressed by floods, returning to pre-impact levels within six years. Geomorphic adjustments, such as channel incision and mid bar development, reflect erosion and sedimentation processes respectively, with channel maintenance works and floods being the main drivers. Despite the recovery of the geomorphic unit diversity within the reach, the deficit resulting from gravel extraction remains unresolved, perpetuating a degrading trend that poses risks in reducing lateral connectivity and could potentially catalyse future vegetation encroachment in bars and floodplains, causing changes on flood conveyance and hydraulics. Monitoring the river's geomorphic diversity provides crucial insights for effective conservation and management decisions regarding land use, development, and conservation along riverbanks, thereby sustaining or enhancing fluvial system resilience.

Improving vegetation spatial distribution mapping in arid and on coastal dune systems using GPR in Tottori Prefecture (Japan)

In this article, desertification and dune progression over vegetation was quantified using remote sensing data. However, vegetation buried under sand blowout could not be counted using this method. Therefore, to estimate the extent of buried vegetation, a GPR campaign was conducted over the coastal sand-dune of Tottori Prefecture (Japan) in combination with a high-resolution topographic UAV-based survey of the topography. The results show that buried vegetation exists underneath sand-blowout, especially near the dune ridges, and can extend from 20 to 30 meters further than the estimate based on airborne remote sensing. Furthermore, the presence of palaeo-vegetation in palaeodune layers also provides the information on the long-term evolution of sand dunes, which can be used to reconstruct Quaternary coastal environments.

Multi-temporal analysis of morphological changes in an Alpine proglacial area and their effect on sediment transfer

Proglacial areas in high-mountain environments are among the most dynamic landscape zones in terms of morphological, sedimentological, and hydrological changes. In fact, in such areas both glacial and paraglacial dynamics exert a strong control on water fluxes (and thus on bedload transport capacity) and on sediment supply. Because of the rapid changes undergoing in these environments, the understanding of the dynamics of sediment transfer and the prediction of how this will change in future still remain a challenge. In this study, we used four different high-resolution topographic surveys to quantitatively analyse the spatio-temporal patterns of sediment transport that occurred in the proglacial area of the Sulden glacier (South Tirol, Italy). The topographic surveys covered the period 2013–2019 and – by building DEMs of difference (DoDs) – we calculated the volumes of erosion and deposition occurring at the glacier terminus and along its outflow channels. Our results indicate that, within the analysed time-period and for different areas, erosion and deposition generally show similar values, suggesting that the analysed proglacial areas has been acting both as a sediment trap and as a sediment source for the downstream river reaches. In addition, the results indicate a complex morphological dynamics, which have strongly impacted coarse sediment connectivity in the upper catchment. In particular, high temperatures and the recent and rapid glacier melt, have promoted a direct control on the structural connectivity within the proglacial area. Morphological changes have increased or decreased sediment connectivity depending on pre-existing morphological features. In contrast, extreme precipitation events, both in term of rainfall and snowfall, have locally and temporally increased the functional connectivity, sometimes leading to long-term modifications in the proglacial area. Overall, in the analysed time-period sediment transfer has been extremely dynamic, suggesting that sediment in the downstream river channel may have been supplied with irregular pulses associated with the morphological changes taking place in the proglacial area.

Assessing the Role of Extreme Mediterranean Events on Coastal River Outlet Dynamics

River mouths are highly dynamic environments responding very rapidly to changes in wave energy or river floods. While the morphological response during floods or during marine storm events has been widely documented in the literature, little is known about the mechanisms acting during the co-occurrence of fluvial and marine hazards. This concomitance of river flood and marine storm is quite common in the western Mediterranean Sea, and was the case for the Gloria event, considered to be the most extreme event in recent decades. During this event, monitoring of hydrodynamics and morphological evolution was implemented, making it possible to better understand the impact of concomitant marine storm and fluvial flood during an extreme meteorological event on spit breaching of a small Mediterranean river mouth. Monitoring using a combination of high-resolution hydrodynamic measurements, topographic and bathymetric surveys, and sediment cores was used before, during, and after the storm “Gloria”. The results suggest an amplification of the morphological impact of the events and a different morphogenic response than if each of the events had acted independently on the system. The marine storm, occurring first, weakened the spit and initiated its breaching, which was continued by the extreme fluvial flood, thus leading to the complete destruction of the mouth. The destruction of the spit acted as a sediment source for subaqueous large delta deposition amounting to 50% of the total volume. The contribution of the river, estimated at 30%, was quite low for an exceptional event, showing the importance of locating rainfall in a catchment area controlled by a dam. For this event, extreme morphological evolution was observed, as well as the importance of water levels in the river mouth, which probably increased flood hazards, demonstrating the importance of including the compounding effect of extreme coastal water levels in river flood risk management.

Potentiality of high-resolution topographic survey using unmanned aerial vehicle in Bangladesh

Topographic surveys are generally used for the representation of earth's undulating topography, commonly known as relief features of the earth's surface. It is generally determined by the difference in elevation between two points. Although several satellite imagery and field-based methods are available for representation of earth's relief but both of them are either high-cost or time-consuming method with lower accuracy. High-resolution UAVs have the potentiality of drawing very accurate earth's relief features with lower cost and time. Primary data was key to conduct the research work. DroneDeploy software was utilized for the planning and acquisition of imageries using UAV with a side and frontal overlap of 80%. The vertical accuracy was ± 0.1 m, and the horizontal accuracy was ± 0.3 m. Georeferencing, Geocorrection, Building Tie Points, Dense Cloud, Depth Maps, 3D Model, DSM, and Orthomosaic operations conducted with Agisoft Metashape Professional software. Geomatica software was used to subtract false heights. Finally, ArcMap was used to represent the DSM and DTM surface analysis and representation of analyzed results in the form of spatial distribution and spatial organization. The research reveals that UAVs have the tremendous potentiality for topographic mapping with real-time elevation.

Holocene morpho-sedimentary evolution of Marambaia Barrier Island (SE Brazil)

AbstractBarrier islands are sedimentary bodies susceptible to changes in sediment supply, dominant physical processes, and sea level. The aim of this work was to study the sedimentary processes that established Marambaia Barrier Island (SE Brazil) as an elongated sandy body that created Sepetiba Bay. For this purpose, barrier and back-barrier bay environments were analyzed using high-resolution satellite imagery, geophysical and topographic surveys, surface sediment samples and short cores, and radiocarbon and optically stimulated luminescence (OSL) dating techniques. Seven morpho-sedimentary domains were identified: coastal beach ridges, overland flow features, inter-ridge paleo lagoon, bayside beach ridges, marshlands, dune field and tidal wetlands. The results show that Marambaia Barrier Island evolved throughout the Holocene, first under normal regression conditions during sea-level rise, and then by forced regression as sea level lowered to its present position. Concurrent processes related to longshore drift, onshore transport, reworked barrier deposits, eolian transport, bay circulation, and pedogenesis influenced its morpho-sedimentary evolution. Morphological features such as truncated beach ridges, flying spits, and filled channels attest to the occurrence of alternating periods of erosion and accretion, evincing how the morphology of barrier island systems preserves an important archive of environmental changes.

Combining Instrumental Monitoring and High-Resolution Topography for Estimating Sediment Yield in a Debris-Flow Catchment

ABSTRACT In mountain basins, long-term instrumental monitoring coupled with high-resolution topographic surveys can provide important information on sediment yield. The Gadria catchment, located in the eastern Italian Alps, typically features several low-magnitude flood episodes and a few debris-flow events per year, from late spring to late summer. Beginning in 2011, sensors devoted to debris-flow detection (geophones, video cameras, flow stage sensors) were installed along the main channel, upstream of a retention basin. In case of debris flows, high-resolution topographical surveys of the retention basin are carried out multiple times per year. Rainfall is measured in the lower part of the catchment and at the headwaters, while passive integrated transponder tracing of bedload was performed in the main channel during spring and summer 2014. In this work, we present the reconstruction of the sediment dynamics at the catchment scale from 2011 to 2017. Results show that (i) coarse sediment yield is dominated by the few debris flows occurring per year; (ii) debris-flow volume estimations may be significantly different—up to 30 percent lower—when performed through a digital elevation model of difference analysis, compared to the time-integration of the debris-flow discharge estimates; (iii) using this latter method, the volumes are affected by significant uncertainties, particularly for small values of flow depth; and (iv) rainfall analysis permits us to characterize debris-flow initiation but also highlights difficulties in discriminating triggering from non-triggering rainstorms if based on rainfall duration and intensity only.

Quantification of mudcracks-driven erosion using terrestrial laser scanning in laboratory runoff experiment

Understanding rill development and erosion rates is crucial for understanding changes in hillslope morphology and soil erosion. Mudcracks, a widely distributed feature on the land surface, play an important role in erosion, but the quantitative relationship between mudcracks and erosion has seldom been reported. Thanks to advances in terrestrial laser scanning (TLS, also known as terrestrial LiDAR), topographic changes can be observed at high resolution. This study is designed to quantify the effects of mudcracks on erosion. Mudstone specimens with and without mudcracks were analyzed. Using TLS, we quantified the millimeter-scale effects of mudcracks on slope erosion and rill development in laboratory experiments with well-controlled samples and water discharge. The point cloud data collected by TLS were then converted to digital elevation models (DEMs) for DEM of difference (DoD) analysis. The results show that mudcracks facilitate rill development. In the early stage of the experiment, the erosion depth of the cracked mudstone is twice that of the surface without cracks, and the total erosion amount is more than twice. During the experiments, the dominant erosion pattern changes from sheet erosion to rill erosion and then lateral erosion. Moreover, our study highlights the potential of TLS and DoD for high-resolution spatio-temporal topographic surveys to identify and characterize micro-relief-induced erosion.

Talus slope geomorphology investigated at multiple time scales from high‐resolution topographic surveys and historical aerial photographs (Sanetsch Pass, Switzerland)

AbstractTalus slopes are common places for debris storage in high‐mountain environments and form an important step in the alpine sediment cascade. To understand slope instabilities and sediment transfers, detailed investigations of talus slope geomorphology are needed. Therefore, this study presents a detailed analysis of a talus slope on Col du Sanetsch (Swiss Alps), which is investigated at multiple time scales using high‐resolution topographic (HRT) surveys and historical aerial photographs. HRT surveys were collected during three consecutive summers (2017–2019), using uncrewed aerial vehicle (UAV) and terrestrial laser scanning (TLS) measurements. To date, very few studies exist that use HRT methods on talus slopes, especially to the extent of our study area (2 km2). Data acquisition from ground control and in situ field observations is challenging on a talus slope due to the steep terrain (30–37°) and high surface roughness. This results in a poor spatial distribution of ground control points (GCPs), causing unwanted deformation of up to 2 m in the gathered UAV‐derived HRT data. The co‐alignment of UAV imagery from different survey dates improved this deformation significantly, as validated by the TLS data. Sediment transfer is dominated by small‐scale but widespread snow push processes. Pre‐existing debris flow channels are prone to erosion and redeposition of material within the channel. A debris flow event of high magnitude occurred in the summer of 2019, as a result of several convective thunderstorms. While low‐magnitude (<5,000 m3) debris flow events are frequent throughout the historical record with a return period of 10–20 years, this 2019 event exceeded all historical debris flow events since 1946 in both extent and volume. Future climate predictions show an increase of such intense precipitation events in the region, potentially altering the frequency of debris flows in the study area and changing the dominant geomorphic process which are active on such talus slopes. © 2020 John Wiley & Sons, Ltd.

Coarse sediment dynamics in a large glaciated river system: Holocene history and storage dynamics dictate contemporary climate sensitivity

Abstract The gravel-bedded White River drains a 1279 km2 basin in Washington State, with lowlands sculpted by continental glaciation and headwaters on an actively glaciated stratovolcano. Chronic aggradation along an alluvial fan near the river’s mouth has progressively reduced flood conveyance. In order to better understand how forecasted climate change may influence coarse sediment delivery and aggradation rates in this lowland depositional setting, we assessed the contemporary delivery and routing of coarse sediment through the watershed; this assessment was based on a rich set of topographic, sedimentologic, and hydrologic data from the past century, with a focus on repeat high-resolution topographic surveys from the past decade. We found that most of the lower river’s contemporary bed-load flux originates from persistent erosion of alluvial deposits in the lower watershed. This erosion is a response to a drop in local base level caused by a major avulsion across the fan in 1906 and then augmented by subsequent dredging. The 1906 avulsion and modern disequilibrium valley profiles reflect landscape conditioning by continental glaciation and a massive mid-Holocene lahar. In the proglacial headwaters, infrequent large sediment pulses have accomplished most of the observed coarse sediment export, with exported material blanketing downstream valley floors; during typical floods, transported bed material is largely sourced from erosion of these valley floor floods. Throughout the watershed, we observe decadal-scale coarse sediment dynamics strongly related to the filling or emptying of valley-scale sediment storage over 102–104 yr time scales, often in response to major disturbances that either emplace large deposits or influence their redistribution. Paraglacial responses in large watersheds are suggested to be inherently complicated and punctuated as a result of internal landform interactions and stochastic/threshold-dependent events. We argue that, in combination, Holocene disturbance, storage dynamics, and human flow modification make coarse sediment fluxes in the lower White River relatively insensitive to decadal climate variability. Results highlight the degree to which river sensitivity to contemporary disturbance, climatic or otherwise, may be contingent on local and idiosyncratic watershed histories, underscoring the need to unpack those histories while demonstrating the utility of watershed-scale high-resolution topography toward that end.

Paleoseismic patterns of Quaternary tectonic and magmatic surface deformation in the eastern Basin and Range, USA

AbstractThe competing contributions of tectonic and magmatic processes in accommodating continental extension are commonly obscured by a lack of on-fault paleoseismic information. This is especially true of the Sevier Desert, located at the eastern margin of the Basin and Range in central Utah (USA), where surface-rupturing faults are spatially associated with both regional detachment faults and Quaternary volcanism. Here, we use high-resolution topographic surveys (terrestrial lidar scans and real-time kinematic GPS), terrestrial cosmogenic nuclide (10Be and 3He) exposure dating, 40Ar/39Ar geochronology, and new neotectonic mapping to distinguish between modes of faulting and extension in a transect across the Sevier Desert. In the western Sevier Desert, the House Range and Cricket Mountains faults each have evidence of a single surface-rupturing earthquake in the last 20–30 k.y. and have time-integrated slip and extension rates of <0.1 and ∼0.05 mm yr−1, respectively, since ca. 15–30 ka. These rates are similar to near-negligible modern geodetic extension estimates. Despite relatively low geologic, paleoseismic, and modern extension rates, both faults show evidence of contributing to the long-term growth of topographic relief and the structural development of the region. In the eastern Sevier Desert, the intrabasin Tabernacle, Pavant, and Deseret fault systems show markedly different surface expressions and behavior from the range-bounding normal faults farther west. Pleistocene to Holocene extension rates on faults in the eastern Sevier Desert are >10× higher than those on their western counterparts. Faults here are co-located with Late Pleistocene to Holocene volcanic centers, have events temporally clustered around the timing of Pleistocene volcanism in at least one instance, and have accommodated extension ∼2×–10× above geodetic and long-term geologic rates. We propose a model whereby Pliocene to recent extension in the Sevier Desert is spatially partitioned into an eastern magma-assisted rifting domain, characterized by transient episodes of higher extension rates during volcanism, and a western tectonic-dominated domain, characterized by slower-paced faulting in the Cricket Mountains and House Range and more typical of the “Basin and Range style” that continues westward into Nevada. The Sevier Desert, with near-complete exposure and the opportunity to utilize a range of geophysical instrumentation, provides a globally significant laboratory for understanding the different modes of faulting in regions of continental extension.

Parametric Analysis of Urban Flood Risk Based on ‘Shallow Water’ Model; a Real Case at Small Scale

In this paper, we examined the increased risk of urban flooding due to incorrect or insufficient maintenance of small hydrological basins. In particular, the effects of a peak rainfall event on a water drainage canal near to a town in central Italy were considered. By means of the RiverFlow2D commercial computational software, the Shallow water technique was chosen, which is based on the Finite Volume Element and on the Godunov-Riemann technique. We also experimented with parallel computing, by performing the same calculations with GPUs (Graphics Processing Units) and we were able to significantly reduce the total time by 80 times. The study area is located in Pianello di Ostra district (Ancona, Marche, Italy). The peak rain data, which is publicly available, were recorded by the Corinaldo pluviometric station (9.1 km from the area) between April 26th and May 2nd 2014 and it caused a subsequent flooding event lasting two days. We integrated pluviometric data with available cartography, a rigorous site inspection, interviews with inhabitants and a high-resolution topographic survey (30 x 30 cm) acquired with a drone. To process rainfall data, we selected the Curve-Number (CN) empirical method, developed by the USDA Natural Resources Conservation Service (SCS). The parametric simulations on Fosso della Trocca basin were performed considering both pre-flooding maintenance state, with obstructions of the channel and the presence of a small bridge, and optimal maintenance state, with no obstructions and no bridge. The computer-simulated depth of the flooding water was compatible with what was observed during the real flood. Thus, the effects of increasingly intense rainfall events were estimated. However, we found that threshold values exist above which no ordinary maintenance is sufficient to avoid flooding phenomena.

Reconstruction of a Segment of the UNESCO World Heritage Hadrian’s Villa Tunnel Network by Integrated GPR, Magnetic–Paleomagnetic, and Electric Resistivity Prospections

Hadrian’s Villa is an ancient Roman archaeological site built over an ignimbritic tuff and characterized by abundant iron oxides, strong remnant magnetization, and elevated magnetic susceptibility. These properties account for the high-amplitude magnetic anomalies observed in this site and were used as a primary tool to detect deep archaeological features consisting of air-filled and soil-filled cavities of the tuff. An integrated magnetic, paleomagnetic, radar, and electric resistivity survey was performed in the Plutonium-Inferi sector of Hadrian’s Villa to outline a segment of the underground system of tunnels that link different zones of the villa. A preliminary paleomagnetic analysis of the bedrock unit and a high-resolution topographic survey by aerial photogrammetry allowed us to perform a computer-assisted modelling of the observed magnetic anomalies, with respect to the archaeological sources. The intrinsic ambiguity of this procedure was reduced through the analysis of ground penetrating radar and electric resistivity profiles, while a comprehensive picture of the buried archaeological features was built by integration of the magnetization model with radar amplitude maps. The final subsurface model of the Plutonium-Inferi complex shows that the observed anomalies are mostly due to the presence of tunnels, skylights, and a system of ditches excavated in the tuff.

Estimation of sediment compaction and its relationship with river channel distributions in the Yellow River delta, China

The geomechanical properties of the stratigraphic sequences strongly influence the land surface subsidence, as the latter can be caused by the sediment compaction. Depositional units of transitional environments such as coastal plain and deltas are characterized by a high lithological variability due to the history and complexity of the sedimentary processes. Therefore, detailed modeling of the stratigraphic architecture is necessary to better understand the land surface subsidence process and accurately estimate its rates. The frequent avulsion and rearrangement of the channel network in the modern Yellow River delta since 1855 has led to the rapid accumulation and deposition of large amounts of sediment in the delta plain. The compaction of the substratum sediment due to the new sediment load and the consolidation of younger deposits caused land surface subsidence. We used 108 sediment core data covering the delta and obtained detailed stratigraphic structural information. A detailed distribution of clay and soft clayey soils, which are the primary contributors to land subsidence, was identified. Furthermore, the compaction rates and the consolidation at time of the subaerial area and neritic area for the entire modern Yellow River delta were estimated. We also analyzed the characteristics of present land subsidence by combining a ground movement dataset obtained by high-resolution topographic surveys with Synthetic Aperture Radar (SAR) Interferometry (InSAR) of satellite images. The results show that during the early stages of the development of the active deltaic lobe, the compaction of neritic sediments was the primary contributor to initial subsidence. Furthermore, combined with the relevant sedimentary age and environment determined by previous studies, we found that the spatial variation in compaction-induced subsidence in the subaerial deltaic area over time is closely related to the historical distributions of the river channels in the modern Yellow River delta. Our findings, providing a better characterization of the spatial and time heterogeneity of the land surface subsidence of both subaerial and submarine areas, significantly improved the previous knowledge on land subsidence of the Yellow River delta. Our results also demonstrate that the subaerial delta becomes increasingly stable over time with the gradual decrease in the average subsidence rate of each deltaic lobe.

An evaluation of a low-cost pole aerial photography (PAP) and structure from motion (SfM) approach for topographic surveying of small rivers

ABSTRACTTo better understand fluvial forms and processes, geomorphologists have a need for high-resolution fluvial topographic surveys. Continuous data collection approaches such as airborne laser scanning (ALS), terrestrial laser scanning (TLS), and structure from motion (SfM) photogrammetry provide methods to collect such data sets. Comparisons of SfM and laser-based approaches for topographic surveys have demonstrated site-specific benefits and drawbacks. Survey preference is largely dependent on specific project requirements, indicating a need for examples of best practice for different applications. This study demonstrates how pole aerial photography (PAP) provides specific advantages for SfM data collection when mapping the topography of small river corridors. Digital elevation models (DEMs) were created using three different surveying approaches for a 100-m reach of Coledale Beck, a small upland stream in Cumbria, United Kingdom. This included (a) imagery collected from a 5-m telescopic pole processed using SfM photogrammetry, (b) imagery collected using an unmanned aircraft system (UAS), also processed using SfM photogrammetry, and (c) point cloud data collected using a TLS. All three approaches produce DEMs of sufficient quality to enhance our understanding of fluvial forms and processes, with DEM/point cloud resolutions of 0.01 m for the close-range methods (TLS and PAP) and c. 0.02 m for the longer-range UAS method. Overall, TLS mean errors (0.123–0.135 m) are almost twice as large as the UAS and PAP (0.037–0.103 m) errors, and the standard deviation is approximately 25% higher. However, results vary significantly for different surface cover types (i.e. vegetated, exposed and submerged surfaces), with TLS outperforming the other approaches for exposed gravel surfaces. Data acquisition rates for the PAP approach are approximately half those of the two other methods (430 m2/hour versus 845 and 730 m2/hour for PAP, TLS, and UAS, respectively). When equipment costs and ease of use are taken into consideration, the PAP approach provides an effective way of collecting topographic data from small rivers.

Using a 2D shallow water model to assess Large-Scale Particle Image Velocimetry (LSPIV) and Structure from Motion (SfM) techniques in a street-scale urban drainage physical model

Physically-based numerical modelling of surface processes in urban drainage, such as pollutant wash-off or the assessment of flood risks, requires appropriate calibration and terrain elevation data to properly simulate the overland flows and thus to achieve useful results. Accordingly, this study aims to obtain an accurate representation of the runoff generated by three different rain intensities, 30, 50 and 80 mm/h, in a full-scale urban drainage physical model of 36 m2. The study focuses firstly on applying the Structure from Motion (SfM) photogrammetric technique to carry out a high-resolution and accurate topographic survey. This topography was implemented in a 2D shallow water model and the results were compared with those obtained using traditional data point measured topography. Negligible differences were found when comparing the two models with measured discharges at the physical model gully pots. However, significant differences were obtained in the velocity distributions, especially in the shallowest flow areas where drainage channels of a few millimeters’ depth appeared in the high resolution topographic survey. Results from the numerical model were compared with overland flow velocities, determined by applying a modified Large Scale Particle Image Velocimetry (LSPIV) methodology using fluorescent particles. With the SfM topography, the 2D model was able to obtain a better representation of the experimental data, since small scale irregularities of the pavement surface could be represented in the model domain. At the same time, LSPIV was presented as a very suitable tool for the accurate measurement of runoff velocities in urban drainage models, avoiding the interference of raindrop features in the recorded images and with overland water depths in the order of few millimeters.

Intertidal boulder-based wave hindcasting can underestimate wave size: Evidence from Yorkshire, UK

Large boulder-size clasts can represent important archives of high energy erosional wave activity at the coast. From tropical coral reefs to high-latitude eroding cliffs, boulders have been used to hindcast the frequency and magnitude (height) of waves produced by both storms and tsunami. Such reconstructions are based on the balance between the hydrodynamic forces acting on individual boulders and the counteracting resistive forces of friction and gravity. We test published models on over 900 intertidal boulders on the coast at Staithes in North Yorkshire, U.K., using repeat high-resolution topographic survey data. We quantified the predicted versus actual rates of boulder movement in the field over a 7-year period (2007–2014). We assessed the degree to which local geomorphology and biology may affect the resistive forces inhibiting transport. We found that 84% of intertidal boulders predicted to be mobile remained stationary over 7 years, suggesting that in some contexts boulder movement may be significantly less than predicted, regardless of boulder volume, shape or location. In situ cementation of boulders to the substrate by marine organisms appears to play a key role in retarding or preventing boulder transport. The implication is that boulder characteristics may not always provide a reliable estimation of wave height on the coast and reliance solely on hindcasting relationships may result in under prediction of the frequency and magnitude of past storm wave activity. We suggest here that more field studies that consider boulder movement are needed, set within specific site contexts, and that these data are used to improve the applicability and robustness of boulder transport models.

High-resolution monitoring of beach topography and its change using unmanned aerial vehicle imagery

Beaches suffer from considerable degradation due to the dual influences from natural and human-induced factors. Higher resolution topographic data are frequently needed for assessment of the rapid changes and for building the models that can predict the evolution of these natural environments. In this paper, a low-altitude quad-rotor unmanned aerial vehicle (UAV), equipped with a non-metric camera, was used to acquire the two periods of UAV images in a complex beach of Wujiao Bay where the significant geomorphic changes occurred mainly due to the strong scouring of the drainage gullies. A detailed practical technique workflow was presented for deploying the low-altitude UAV for high-resolution monitoring the topography and the changes of the beach. The high resolution orthomosaic with geo-reference accuracy less than 1 pixel and the Digital Surface Model data with high vertical accuracy of approximately 10 cm were achieved by applying the UAV photogrammetry process based on Structure from Motion method to numerous overlapped UAV images. These data were further applied to the topographic change monitoring and analysis of the beach. This study greatly enhanced the applicability of the UAV remote sensing and enriched the existing tools for the high resolution topographic survey and geomorphic change detection in the complex beach.

Long–term variability of supratidal coastal boulder activation in Brittany (France)

High–energy supratidal coastal boulder deposit (SCBD) dynamics were investigated on Vierge Island and Pors Carn Point, north and south of western Brittany, France, respectively. Morphological changes induced by boulder transport and quarrying were quantified using high–resolution topographic survey data taken between 2012 and 2017. Additional in–situ wave parameters and water levels were also recorded over this period (2014–2017) in order to compute the maximum water levels and assess the relationship between SCBD morphological changes and specific hydrodynamic conditions. During extreme water levels (for maximum water levels exceeding a one in ten year event), SCBDs were broadly reworked (up to 40% of the total volume). During lower intensity events, for which maximum water levels were still very high, morphological changes represented 1% to 5% of the total volume. These morphological and hydrodynamic observations were then used to calibrate a chronology of SCBD activation events based on 70years of hindcast winter maximum water levels. These long–term time–series showed great interannual variability in SCBD activation but no significant long–term trend. Winter–frequency SCBD activation was better correlated to the WEPA index (r=0.46) than the NAO index (r=0.1). Therefore, the WEPA index can be considered to be a more significant climate proxy for assessing storm–related geomorphic changes in the temperate latitudes of the N–E Atlantic basin (36°–52° N), including the Brittany coast. The potential of SCBDs as a morphological storm proxy for macrotidal high–energy rocky coasts is addressed.

Evolution of a beach nourishment project using dredged sand from navigation channel, Dunkirk, northern France

The largest beach replenishment project ever in France was completed in February 2014 in Dunkirk on the coast of northern France. A volume of 1.5 × 106 m3 of sand extracted from a navigation channel was placed on the beach to build up a 150 to 300 m wide supratidal platform in front of a dike, called « Digue des Allies », which protects several residential districts of Dunkirk from marine flooding. High resolution topographic surveys were carried out during 2½ years to monitor beach morphological changes, completed by a hydrodynamic field experiment conducted in February 2016. Approximately −138,200 m3 of sand, corresponding to 9.2% of the initial nourishment volume, were eroded over the nourishment area in about 2 years. An obvious decrease in erosion eastward with a shift from erosion to accumulation was observed, suggesting an eastward redistribution of sand. This longshore sand drift is beneficial for the eastward beach of Malo-les-Bains where most of the recreational activities are concentrated. Hydrodynamic measurements showed that waves and wave-induced currents play a major role on the longshore sand redistribution compared to tidal flows. Strong relationships were observed between cumulative offshore wave power and beach volume change during distinct beach survey periods (R2 = 0.79 to 0.87), with more significant correlations for northerly waves. A slight decrease in erosion during the second year compared to the first year after nourishment suggests that the loss of sand should decrease after an initial phase of rapid readjustment of the beach shape towards equilibrium.