Multiplatform-SfM and TLS Data Fusion for Monitoring Agricultural Terraces in Complex Topographic and Landcover Conditions

Agricultural terraced landscapes, which are important historical heritage sites (e.g., UNESCO or Globally Important Agricultural Heritage Systems (GIAHS) sites) are under threat from increased soil degradation due to climate change and land abandonment. Remote sensing can assist in the assessment and monitoring of such cultural ecosystem services. However, due to the limitations imposed by rugged topography and the occurrence of vegetation, the application of a single high-resolution topography (HRT) technique is challenging in these particular agricultural environments. Therefore, data fusion of HRT techniques (terrestrial laser scanning (TLS) and aerial/terrestrial structure from motion (SfM)) was tested for the first time in this context (terraces), to the best of our knowledge, to overcome specific detection problems such as the complex topographic and landcover conditions of the terrace systems. SfM–TLS data fusion methodology was trialed in order to produce very high-resolution digital terrain models (DTMs) of two agricultural terrace areas, both characterized by the presence of vegetation that covers parts of the subvertical surfaces, complex morphology, and inaccessible areas. In the unreachable areas, it was necessary to find effective solutions to carry out HRT surveys; therefore, we tested the direct georeferencing (DG) method, exploiting onboard multifrequency GNSS receivers for unmanned aerial vehicles (UAVs) and postprocessing kinematic (PPK) data. The results showed that the fusion of data based on different methods and acquisition platforms is required to obtain accurate DTMs that reflect the real surface roughness of terrace systems without gaps in data. Moreover, in inaccessible or hazardous terrains, a combination of direct and indirect georeferencing was a useful solution to reduce the substantial inconvenience and cost of ground control point (GCP) placement. We show that in order to obtain a precise data fusion in these complex conditions, it is essential to utilize a complete and specific workflow. This workflow must incorporate all data merging issues and landcover condition problems, encompassing the survey planning step, the coregistration process, and the error analysis of the outputs. The high-resolution DTMs realized can provide a starting point for land degradation process assessment of these agriculture environments and supplies useful information to stakeholders for better management and protection of such important heritage landscapes

Volume estimation of soil stored in agricultural terrace systems : a geomorphometric approach

High-resolution topographic (HRT) techniques allow the mapping and characterization of geomorphological features with wide-ranging perspectives at multiple scales. We can exploit geomorphometric information in the study of the most extensive and common landforms that humans have ever produced: agricultural terraces. We can only develop an understanding of these historical landform through in-depth knowledge of their origin, evolution and current state in the landscape. These factors can ultimately assist in the future preservation of such landforms in a world increasingly affected by anthropogenic activities. From HRT surveys, it is possible to produce high-resolution Digital Terrain Models (DTMs) from which important geomorphometric parameters such as topographic curvature, to identify terrace edges can be extracted, even if abandoned or covered by uncontrolled vegetation. By using riser bases as well as terrace edges (riser tops) and through the computation of minimum curvature, it is possible to obtain environmentally useful information on these agricultural systems such as terrace soil thickness and volumes. The quantification of terrace volumes can provide new benchmarks for soil erosion models, new perspectives to stakeholders for terrace management in terms of natural hazard and offer a measure of the effect of these agricultural systems on soil organic carbon sequestration. This paper presents the realization and testing of an innovative and rapid methodological workflow to estimate the anthropogenic reworked and moved soil of terrace systems in different landscapes. We start with remote terrace mapping at large scale and then utilize more detailed HRT surveys to extract geomorphological features, from which the original theoretical slope-surface of terrace systems were derived. These last elements were compared with sub-surface information obtained from the excavations across the study sites that confirm the reliability of the methodology used. The results of this work have produced accurate DTMs of Difference (DoD) for three terrace sites in central Europe in Italy and Belgium. Differences between actual and theoretical terraces from DTM and excavation evidence have been used to estimate the soil volumes and masses used to remould slopes. The utilization of terrace and lynchet volumetric data, enriched by geomorphometric analysis through indices such as sediment conductivity provides a unique and efficient methodology for the greater understanding of these globally important landforms, in a period of increasing land pressure

Ending the Cinderella Status of Terraces and Lynchets in Europe : The Geomorphology of Agricultural Terraces and Implications for Ecosystem Services and Climate Adaptation

Terraces and lynchets are ubiquitous worldwide and can provide increasingly important Ecosystem Services (ESs), which may be able to mitigate aspects of climate change. They are also a major cause of non-linearity between climate and erosion rates in agricultural systems as noted from alluvial and colluvial studies. New research in the ‘critical zone’ has shown that we must now treat soil production as an ecologically sensitive variable with implications for soil carbon sequestration. In this review and synthesis paper we present a modified classification of agricultural terraces, review the theoretical background of both terraces and lynchets, and show how new techniques are transforming the study of these widespread and often ancient anthropogenic landforms. The problems of dating terraces and the time-consuming nature of costly surveys has held back the geomorphological and geoarchaeological study of terraces until now. The suite of techniques now available, and reviewed here,includes Digital Elevation Models (DEMs) - Structure from Motion (SfM) photogrammetry, Airborne and Terrestrial Laser Scanning (ALS-TLS); optically stimulated luminescence (OSL and pOSL), portable x-ray fluorescence (pXRF), Fourier-transform infra-red analysis (FTIR), phytoliths from plants, and potentially environmental DNA. Three process-related geomorphological questions arise from using this suite of methods; a) can they provide both a chronology of formation and use history, b) can we identify the sources of all the soil components? c) can terrace soil formation and ecosystem services be modelled at the slope to catchment scale? The answers to these questions can also inform the management of the large areas of abandoned and under-used terraces that are resulting from both the economics of farming and rural population changes. Where possible, examples are drawn from a recently started ERC project (TerrACE; ERC-2018-2023; https://www.terrace.no/) that is working at over 15 sites in Europe ranging from Norway to Greece

A model-based early warning system for runoff-generated debris-flow occurrence: Preliminary results

Early warning systems for debris flows are low cost measures for mitigating this kind of hazard. The early warning systems provide a timely alert for upcoming events in order to take protective measures, such as closing railways-roads, evacuating people from the threatened areas, and put rescue forces into readiness. These systems usually are sensor-based, and the alert time is the interval between the timing of the first detachment of debris flow by a sensor and its arrival into the threatened area. At the purpose of increasing the alert time, we propose an early warning system based on a model-cascade: nowcasting, hydrological- and triggering models. Nowcasting anticipates rainfall pattern that is transformed into runoff by the hydrological model. The triggering model estimates the volume of sediments that the runoff can entrain, and compares it with a critical threshold. If this is exceeded the alert is launched. The proposed early warning system is tested against the available data of the Rovina di Cancia (Northeast Italy) site

Early to Middle Bronze Age agricultural terraces in north-east England : morphology, dating and cultural implications

Terracing is found widely in the Mediterranean and in other hilly and mountainous regions of the world. Yet while archaeological attention to these ‘mundane' landscape features has grown, they remain understudied, particularly in Northern Europe. Here, the authors present a multidisciplinary study of terraces in the Breamish Valley, Northumberland. The results date their construction to the Early to Middle Bronze Age, when they were built by cutting back the hillside, stone clearance and wall construction. Environmental evidence points to their use for cereal cultivation. The authors suggest that the construction and use of these terraces formed part of an Early to Middle Bronze Age agricultural intensification, which may have been both demographically and culturally driven

Assessing sediment dynamics and check dams efficiency in a debris-flow catchment using multi-temporal topographic surveys

Torrent control works have always been a fundamental tool for preventing torrential hazard in mountain catchments, where the sediment transport phenomena as debris flows are one of the most dangerous geomorphic processes affecting small steep basins. The linkages between sediment source areas on the hillslopes and channel network, along with the temporal and spatial distributions of channel storage, are key controls of debris-flow occurrence and magnitude. Consequently, the prevention of natural hazards related to debris-flows requires a better understanding of sediment dynamic. Among the hydraulic engineering structures, grade control dams and sediment retention dams are the most effective and common technique to manage debris flows and debris floods hazard. These structures could have important effects on sediment dynamic. Therefore, an integrated approach that analyses the debris-flow dynamic and its interactions with torrent control works, is needed to assess the efficiency of the realized structures and to improve the long-term hazard management at catchment scale. In spite of the widespread presence of such hydraulic structures in steep mountain streams worldwide, very little researches considered the role of check dams on sediment dynamics in debris-flow environments over time to enhance the planning of the torrent control works. The monitoring of debris-flow events, the estimation of debris-flow magnitude and frequency, and the analysis of spatial patterns in terms of eroded and deposited volumes, are fundamental to improve the sediment dynamic understanding. In the last two decades, High-Resolution Topography (HRT) has provided new opportunities to characterize debris-flow activity at different scales. Between these, the application of Structure from Motion (SfM) photogrammetry paired with Multi-View Stereo (MVS) algorithms has become a low-cost method to collect HRT at multiple temporal and spatial scales, also in rugged or inaccessible environments like those observed in debris-flow catchments. SfM allows carrying out HRT with high frequency; nevertheless, the SfM technique is limited at broad spatial scales. Therefore, other technologies as LiDAR surveys could be used to assess the sediment dynamic also at catchment scale. However, the use of HRT required the design of appropriate workflows for data post-processing and uncertainty assessment to compare multi-temporal surveys, especially in a topographically complex environment. In this research, the effects of torrent control works on debris-flow dynamics were investigated by means of multi-temporal SfM and LiDAR surveys in the Moscardo torrent (eastern Italian Alps) where several check dams have been built over time. Methodological workflows enabled the realization of multi-temporal Digital Elevation Models (DEMs) which were compared (i.e., DoD) to quantify the debris mobilized and the time evolution of erosion and deposition patterns in debris-flow channels equipped with check dams. The DoDs data were integrated with a sediment connectivity analysis to have a whole assessment of debris-flow dynamic. The results show that the check dams considerably modified debris-flow dynamics in the studied channel but their performance cannot be considered satisfactory. They temporary stored volumes of debris just after their construction, but soon when the structures were filled the check dams acted as sediment sources that increased debris-flow magnitude. Moreover, the sediment paths flowed around some check dams. These processes triggered the slope foot erosion and activated shallow landslides, further sediment source areas for debris-flow process. The analysis proposed in this work could help to improve design approaches and to obtain more realistic cost-benefit ratios of the adopted strategies and, in this way, select the best solutions