Sediment management is an important aspect of river reconnection projects, often driving costs and influencing community acceptance. At sites with uncontaminated sediments, downstream release is an attractive option because it is often the cheapest and most practical approach and the sediment can be ecologically beneficial to downstream areas deprived of it for years by the dam. To employ this option, project proponents must estimate the sediment quantity to be released and, if substantial, estimate how long it will take to erode, where it will go, and how long it will stay there. We investigated these issues for sediments released by the 2018 removal of Bloede Dam on the Patapsco River in Maryland, USA. The dam was about 10 m high and its impoundment filled with sand and mud. Taking the surface elevations of these sediments surveyed immediately before removal and subtracting estimates of the pre-dam valley elevations derived from 21 cores and post-removal surveys of exhumed pre-dam surfaces, we estimate there was approximately 186,600 m³ of stored sediment composed of 70% sand and 30% mud. These proportions match estimates made during pre-removal engineering studies, but our total stored sediment estimate is about 20% less. The difference between estimates reflects a real change in stored sediment quantity between 2018 and 2012 when the engineering studies were completed, additional data available to us after removal, and different estimation methods. After removal, using elevation surveys generated by traditional methods as well as UAS-based aerial imagery and structure-from-motion (SfM) at high temporal resolution, we documented rapid erosion of the stored sediments in the first six months (~60%) followed by greatly reduced erosion rates for the next couple of years. A stable channel was developed in the impoundment during the rapid erosion phase. These results are similar to a two-phased erosion response reported for sediment releases at dam removals around the world across a range of dam and watershed scales, indicating what practitioners and communities should expect when reconnecting rivers in similar settings. Downstream, repeat surveys combined with discharge and sediment gaging show rapid transport of eroded sediments through a 5 km reach, especially during the first year when discharges were above normal, and little overbank storage.

How to cite: Collins, M., Baker, M., Cashman, M., Miller, A., and Van Ryswick, S.: Releasing sediments while reconnecting rivers: how do channels respond and how long does it take?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7225, https://doi.org/10.5194/egusphere-egu23-7225, 2023.

Connectivity is crucial for the functioning of river corridors as it determines the natural flow and sediment regime as well as the ability of species to migrate. Thus, it is a property highly relevant for the development of riverine landscapes and their potential of ecosystem service provision. Today, the connectivity of most (large) rivers is affected by anthropogenic infrastructure such as hydropower dams. This is also true for the Aral Sea Basin in Central Asia. The importance of rivers as freshwater resource led to an intensive exploitation of water resources and to the construction of a large number of dams and thus to a fragmentation of the river network. Despite its relevance for the functioning of the river corridors, connectivity remains unexplored for this  basin. This is partly due to the fact that the large scale assessment of connectivity for such data-scarce regions is challenging. For instance, there is the need to delineate a robust and accurate river network from globally available digital elevation models (DEM) as readily available datasets like the Hydrosheds river network suffer from significant errors in this region. In this study, we present a first assessment of the connectivity of the river network in the Aral Sea Basin. In addition, we discuss the challenges associated with large scale modeling of structural connectivity of river networks in data-scarce regions and how to overcome them.

We take as a basis a channel network delineated from the 30 m Copernicus DEM along with geomorphon-based major geomorphological units to derive landscape-specific channel initiation thresholds. We use a least-cost path approach for flow routing to avoid artifacts resulting from sink filling. Multispectral satellite time series from the Landsat mission are used to remove abandoned channels and to correct the river network. Additional input are the barriers in the Aral Sea Basin. We use the dam data from Global Dam Watch and complement it by mapping from high resolution Google Earth imagery. The river network and the barrier locations are used to create a graph representation of the river network where river reaches are represented by edges and confluences as well as dam locations by nodes. This river graph is used to compute connectivity metrics such as the dendritic connectivity index, for both the whole network and at the subcatchment scale.

The results of our study deliver the first analysis of connectivity of the river network in the Aral Sea Basin. Along with the insights in this particular river basin, we present an approach which is optimized for the application in large, data-scarce study areas. Such static analysis of structural connectivity is of course a first indicator only, and further analysis is required to understand the impact of hydrological and sediment connectivity on the riverine landscapes of the river corridors of the region. Thus, rather than a final result, we see our study on river network connectivity as an important basis for assessing sediment dynamics across the network, natural flow regime and its impairment as well as river and floodplain habitat integrity.

How to cite: Betz, F., Schmitt, R., Lauermann, M., Chymyrov, A., and Heckmann, T.: First Assessment of the Connectivity of the River Network in the Aral Sea Basin in Central Asia: Challenges of Large Scale Connectivity Modeling in Data-Scarce Regions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12365, https://doi.org/10.5194/egusphere-egu23-12365, 2023.

In the scope of soil erosion modeling few attempts have been made to assess the coarse-grained sediments at steep slopes. The current approach results in a lack of correspondence in mountain watersheds because most models perform validation for different scales, topography, and land use, especially in the agricultural field. Available models normally prioritize the estimation of fine sediment in the field while the contribution of course-grained sediment is simplified or even neglected, especially in areas that require an analysis of hillslope failure events. 

Herein, we selected the Upper Llobregat River Basin (Pre-Pyrenees, Spain) as a study area and focus on the 1982 rainstorm, which triggered more than 1000 mass movements. The present investigation aims to allow an assessment of sediment production that focuses on the characterization of coarse sediment source areas from a mountain range.

The methodology includes analysis through geomatic methods, field surveys, and historic landslide inventories, followed by an index connectivity calculation to main rivers, morphometric parameters, and triggering factors. Preliminary results of this ongoing study confirm the importance of hillslope processes for the assessment of the sediment budget in mountainous areas. The outcomes will be incorporated into a modular sediment-budget model, which will be calibrated in our study area and tested in other watersheds in the Pyrenees. In addition, the impact of future changes (climate and land use) will be analyzed since extreme weather scenarios strongly affect both the production and transport of sediment in mountainous areas.

How to cite: Rodriguez, S., Hürlimann, M., Medina, V., Torra, O., Oorthuis, R., and Puig Polo, C.: Characterization of coarse-grained sediment supply for the assessment of soil erosion in mountainous areas. Application to the Upper Llobregat River Basin (Pyrenees, Spain), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-470, https://doi.org/10.5194/egusphere-egu23-470, 2023.

Small mountainous rivers (SMRs) produce about 40% of the world’s sediment discharge from land to ocean. The dominant sources of this sediment are earthquake and typhoon-driven landslides. It is widely accepted that earthquake-induced landslides tend to locate at hillslopes, while typhoon-induced landslides tend to cluster at hillslope toe areas. These differences in landslide drivers and landslide location together determine the connectivity of resulting sediment transfers from the source (landslides) to the river network. Therefore, understanding when and where earthquake and typhoon-driven landslides occur, and the pathways and timescales over which these sediment sources are connected to river channels can help us determine the relative controls of earthquakes and typhoons on sediment discharge in SMRs. Here, we illustrate how detailed spatial and temporal mapping of landslides, using Landsat imagery within Google Earth Engine, enables us to better understand sediment connectivity in SMRs, and improve our understanding of the controls on sediment discharge in SMRs. We focus our analysis on the period between 1999 and 2020, which includes the 1999 Chi-Chi earthquake (M_w=7.7) and typhoon Morakot, which generated over 3000 mm of rainfall between 5th and 10th August 2009. The results show that we can identify event-induced landslide areas at a higher temporal resolution than the open-source landslide dataset from the Forest Bureau, Taiwan, which enables us to refine our understanding of the relative controls of discrete events (i.e. earthquakes and typhoons) on landslides and connected sediment transport pathways, and determine the timescales over which they lead to elevated sediment discharge in SMRs. Refining our understanding of earthquake and typhoon-driven controls on landslides in SMR catchments and cascading impacts on sediment export from SMRs is particularly important given the recent intensification of rainfall intensities that are anticipated to continue in the future.

How to cite: Lin, Y.-T., Turnbull, L., and Wainwright, J.: Landslide impacts on sediment dynamics in mountainous rivers after large earthquake and typhoon events: A sediment connectivity approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-498, https://doi.org/10.5194/egusphere-egu23-498, 2023.

The flowing portion of river networks experiences never-ending event-based and seasonal expansion/retraction cycles, which mirror the unsteady nature of the climatic forcing.  These "temporary" rivers constitute a major fraction of the global river network, and are found across a variety of settings, especially in the headwaters. A dominant feature of these channel network dynamics is represented by the fact that the active streams are dynamically fragmented, i.e. they do not simply expand upstream when the catchment wets up, to dry down in the downstream direction during the recessions. Instead, the wetting/drying processes frequently happen in complicated spatio-temporal patterns, either activating disconnected reaches first, that will only eventually get connected to the main channel, or generating disconnections by drying out segments in the middle of the river network before the flow heads start retracting. This contribution analyzes the spatial patterns of local persistency along river networks (i.e. the fraction of time for which flowing water can be observed at each location, which is related to the wetting/drying order of the different reaches) combining field data on the spatiotemporal evolution of flowing channels and theoretical analyses, with the aim of elucidating the physical drivers of stream connectivity and disconnectivity in temporary streams. The analysis reveals that river fragmentation is related to the spatial heterogeneity of subsurface properties other than the contributing area (e.g. slope, local width, permeability). The proposed framework provides a clue for analyzing the impact of the spatial and temporal heterogeneity of streamflow presence for a variety of morphologic and biogeochemical processes (sediment transport, ecological dispersion and stream metabolism).

How to cite: Botter, G. and Durighetto, N.: Dynamically fragmented stream networks: field observations and physical drivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7271, https://doi.org/10.5194/egusphere-egu23-7271, 2023.

Fine-sediment input into river networks presents one of the biggest environmental pressures in agricultural catchment systems due to accelerating soil erosion rates associated with agricultural practices. To understand and effectively manage this problem, it is essential to identify sediment source areas and the pattern of linkages between different landscape compartments along the sediment pathways that ultimately end up in the river channel. Sediment connectivity is an increasingly used concept to help assess both on-site and off-site impacts of soil erosion by describing the efficiency of fine-sediment transfer through these different zones. Natural Water Retention Measures (NWRM) that emulate natural processes to enhance or restore the water retention capacity of ecosystems, available to be applied at agricultural fields, can be understood within this framework as they modify connectivity and provide benefits that include erosion and sediment control. The aim of this study is to assess the differences in sediment connectivity associated with scenarios of different NWRM (e.g. buffer strips, strip cropping, terracing, mulching) in a selected hillslope of the Fugnitz Catchment (Austria). Using a process-based sediment-transport model (MAHLERAN; Wainwright et al., 2008), runoff and sediment transport were dynamically simulated. Simulation results of water and sediment fluxes were then translated into a graph representing the flow network, consisting of nodes and edges, where connectivity and network properties can be quantified using graph-theoretical metrics (e.g. betweenness centrality). The same workflow was used for other NWRM scenarios, modifying parameters of the sediment transport model to capture the NWRM conditions. The results show notable changes in connectivity between scenarios as well as varying patterns of hot spots where sediment delivery is high and where interventions may be targeted, thus providing options of agricultural practices that can be implemented to improve sustainability in agricultural catchments.

How to cite: Perez, J. E., Poeppl, R., Turnbull, L., Tiwari, S., and Wainwright, J.: How do Natural Water Retention Measures modify connectivity on agricultural hillslopes?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12616, https://doi.org/10.5194/egusphere-egu23-12616, 2023.

Soil erosion is a process accelerated by natural and anthropogenic disturbances over time and space, leading to land degradation and causing geomorphological change. It is difficult to investigate the spatial and temporal distribution of soil erosion and sedimentation in data-scare areas, in that case, the use of simplified methods to analyze soil erosion and sediment connectivity variations over time and space can help. Sediment connectivity denotes the transfer of sediment from source to sink areas through channel systems of landscape compartments within a watershed. In this study, we aimed to investigate sediment yield (SY) variation over time and space and understand the link between hillslope soil erosion and sediment connectivity to identify hotspot areas in the Rogativa catchment (∼53 km²) in Southeast Spain. The (specific) sediment yield (S)SY was estimated by combining the Revised Universal Soil Loss Equation (RUSLE) model with the sediment delivery ratio (SDR). The SDR was calculated based on the Index of Connectivity (IC). In the channels, 100% delivery was assumed. In the Rogativa catchment, 58 check dams were constructed in 1976/77. Their trapping efficiency, obtained from field observations of sediment retained behind the checkdams in 2003, was included in the SDR estimation of the checkdams. SY was estimated from accumulated hillslope soil erosion in the local stream network while accounting for sedimentation through the SDR. Soil erosion, IC, SDR, and (S)SY were quantified and compared for the years 1956, 1977, 2001, and 2016, for which different land use maps were available. SY model results for the year 2001 were compared with observed SY (in 2003) behind the check dams. Only for about half of the checkdams, model results were comparable. This is investigated further and could be explained by complex sediment dynamics within the channels and between checkdams (i.e. one check dam retaining part of the sediment, the next downstream checkdam as well, etc) – these dynamics are not included in the RUSLE-SDR model. The RUSLE-generated soil erosion and sediment connectivity signatures (IC, SDR, and (S) SY) showed higher values in the channels and croplands than in hillslopes and decreased over time due to significant changes in land use and construction of check dams in the catchment. Moreover, the combined proportion of erosion-connectivity patterns showed about 7% of the area adjacent to some of the streams was found both highly erodible and highly connected, which indicates an adverse erosion-prone part. It is possible to apply this method to understand SY amount and distribution and identify hotspot locations in drainage systems with limited field data in data-scarce semi-arid areas like the Rogativa catchment. However, more field observations to validate the models to identify hotspot locations and investigate river network systems rather than focusing only on hillslopes, which could help to know where to intervene in the catchment.

Keywords: Soil erosion-RUSLE, Sediment connectivity, Sediment delivery ratio, Sediment yield, hotspot location

How to cite: Abebe, N., Baartman, J., Eekhout, J., Vermeulen, B., Boix-Fayos, C., de Vente, J., Grum, B., and Hoitink, T.: Quantifying hillslope erosion and sediment connectivity in the Rogativa catchment, Southeast Spain, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12681, https://doi.org/10.5194/egusphere-egu23-12681, 2023.

Debris flows are major geophysical processes which are able to modify the landscape. Structural sediment connectivity describes the physical coupling of landscape units, and it may be affected by the occurrence of single large-magnitude debris flows and/or by the cumulative changes determined by frequent, small-magnitude events. Understanding the coupling of hillslopes to the main channel during and after debris flows is essential for comprehending catchments sediment transfer at different timescales. Debris flows might provoke sudden changes in the landscape through processes such as bed and bank erosion, overbank deposition and natural dam formation. While debris flows may modify the landscape, their characteristics (e.g., path, runout) are strongly affected by the geomorphological settings. Indeed, there is an interplay between landscape morphology and debris flows, one conditioning the other and vice versa. In this regard, determining how much structural connectivity influences the coupling of debris flow with the channel network remains a challenge. An evaluation of the structural connectivity before and after storm events that triggered debris flow has been carried out on multi-temporal DTMs available for the Stolla basin (Autonomous Province of Bozen-Bolzano, Italian Alps) utilizing the Index of Connectivity (IC) and on a pre-event DTM for the Revolver basin (Santa Catarina state, Southern Brazil). To understand whether the morphological changes caused by the debris flows had an impact on flow routing during the event, some of the debris flow events were simulated by a physically based model. The topographic changes caused by the simulated scenarios have been used to compare pre- vs post-event sediment connectivity.

How to cite: Paul, L. R., Scorpio, V., Michel, G. P., Comiti, F., Zanandrea, F., and Schwarz, H.: The effects of debris flow on structural sediment connectivity: case studies in the Italian Alps and in Southern Brazil, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12999, https://doi.org/10.5194/egusphere-egu23-12999, 2023.

Rainfall-induced landslides are natural processes inherent to the sediment dynamics, fundamentally acting in landscape evolution, while favoring the occurrence of hazardous conditions. Given their potential to mobilize large volumes of sediments from the release zones, they play a major role in sediment production in mountain basins, while affecting sediment connectivity through the deformations produced on the landscape.

These landslides are triggered out by the imbalance of shear strength and shear stress acting on a soil layer, promoted by the presence of water. The soil wetting through infiltration, namely, the vertical transfer of water from the surface layer of the soil to its interior, during intense precipitation events, has thus historically been associated with landslide triggering.

Whereas precipitation intensity and/or volume are recurrently related, the failure of a slope can occasionally be caused by precipitation events with magnitudes that are lower than those that have previously occurred. The triggering is dependent on the overlay of hydrological processes with different spatial and timescales, including the antecedent moisture conditions of the soil and the water storage in the watershed. Thus, different processes that act in the filling, storing, and draining of water in the soil, from seconds to weeks prior to the triggering, as preferential flow or bedrock infiltration and exfiltration are all equally relevant to understand and quantify landslide behavior and sediment connectivity. Furthermore, the soil moisture conditions not only govern landslide triggering but also their potential to fluidize and travel across longer distances, influencing their connectivity.

In this way, we analyzed how soil-water processes affect the filling, storing and draining dynamics during landslide triggering and their effects on sediment connectivity. Two study areas with distinct climatic and geomorphological characteristics located in southern Brazil (Mascarada River Catchment, Rio Grande do Sul) and northern Italy (Gadria River Catchment, Autonomous Province of Bozen-Bolzano) were analyzed. Field collected data was used, together with physical-based models to simulate slope stability as well as saturated and unsaturated flow and water retention in the soil layer. Scenarios including infiltration, preferential flow and bedrock exfiltration were simulated to assess whether this increase in uncertainty and complexity brings better results to the representation of the processes.

How to cite: Schwarz, H., Comiti, F., Michel, G. P., and Paul, L. R.: SOIL-WATER PROCESSES AND SEDIMENT CONNECTIVITY IN RAINFALL-TRIGGERED LANDSLIDES: A comparative analysis between small catchments in Italy and Brazil, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16272, https://doi.org/10.5194/egusphere-egu23-16272, 2023.