GM4.16

Chilika is a large brackish water lagoon in the eastern coast of India and received the distinction of a Ramsar wetland in 1971. It was put under ‘The Montreux Record’ in 1993 because of large scale degradation, but the remarkable restoration activities led to regaining its Ramsar status in 2002. The Chilika receives an influx of saline water from the Bay of Bengal and that of freshwater from the terrestrial systems from its catchment as well as from a distributary channel of Mahanadi River. Having these two very different types of water influx is a characteristic feature of this wetland which defines its biophysical as well as hydrogeomorphic attributes. There are at least three connectivity-related factors controlling the physical attributes of Chilika Lagoon – its connectivity with the sea, the lagoon-catchment connectivity, and lagoon-river connectivity. In the present work, we have assessed the lagoon-catchment hydrological connectivity for 30 years (1990-2020) by calculating connectivity indices (IC) annually. Using the Connectivity Response Unit (CRU) approach, we evaluated the dynamic hydrological connectivity of the lagoon with its catchment as well. The ICs are calculated as a function of topographic and land-cover factors. Since grasslands and shrubs are the primary land-cover in the catchment, we used NDVI to model the vegetation-induced impedance to the hydrological connectivity. The algebraic sum (IC_sum) of the IC values of each pixel of the catchment in a given year was used to compare the overall connectivity at the inter-annual scale. Since connectivity increases with increasing IC values, a higher IC_sum represents a relatively higher hydrological connectivity. The IC_sum is showing a strong decreasing trend with time, which implies that the overall hydrological connectivity of the lagoon with its catchment is decreasing since the 1990s. Further, the CRU assessment has demarcated the specific regions of the catchment which are showing dynamicity in the hydrological connectivity. As expected, the CRUs with high connectivity potential are in proximity to the lagoon. Nevertheless, there are large patches of CRUs with increasing connectivity in the distal parts of the catchment as well. A total of 13.5% area of the catchment is showing either high or increasing connectivity pattern, however, an overwhelming 67.6% of the catchment is exhibiting either low or decreasing connectivity pattern. In 14% area of the catchment, the connectivity was high in the past, but it is diminishing with time, whereas in 20% of the catchment, the low connectivity is intensifying. The changing lagoon-catchment hydrological connectivity is expected to impact the biophysical and hydrogeomorphic characteristics of the Chilika wetland by impacting the freshwater inflows in a brackish water lagoon such as the Chilika, and therefore, this is an important management consideration for the wetland.

How to cite: Patel, U., Singh, M., and Sinha, R.: Evaluating the dynamic hydrological connectivity of a coastal wetland: an application of connectivity response unit (CRU) approach on Chilika Lagoon, Odisha, India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4180, https://doi.org/10.5194/egusphere-egu21-4180, 2021.

Surface runoff (dis)connectivity manifests across scales, spawning from different spatial flow patterns, which are dominated by both topography (structural connectivity) and hydrodynamics (dynamic connectivity). How the connectivity builds and evolves throughout rainfall events is integrated into observable hydrological signatures (namely, hydrographs and water balance). 

In this contribution we explore the connectivity properties of runoff generation processes across spatial scales. We revisit three case studies of runoff generation during rainfall, numerically simulated by solving shallow-water equations. This approach provides a full description of the hydrodynamic flow fields, allowing to study both the connectivity properties, as well as the domain-integrated hydrological signatures (namely, hydrographs) that build up in response to flow phenomena. 

We discuss and link the runoff generation processes arising from (i) runoff generation at the plot scale (20 m2 at cm resolution) with explicit microtopographies, (ii) runoff generation at the hillslope or first-order catchment scale with overland and (ephemeral) rill flows in the Hühnerwasser experimental catchment (4000 m2 at m resolution), and (iii) runoff generation at the catchment scale in the Lower Triangle catchment (15 km2 at m resolution).

The detailed study of runoff generation dynamics highlights the needs to use time-evolving connectivity metrics, which are particularly useful to understand spatiotemporal model output. We computed the number of disconnected flooded clusters (and Euler characteristic) as the main connectivity metric.

The results of the three different systems suggest similar qualitative behaviours of connectivity across scales, from plot to catchment scales, and therefore also offer the possible use of connectivity to understand how fluxes are re-scaled across the landscape, and as a multiscale indicator of hydrological function. The relationship between the connectivity response at a given scale (e.g., plot) and the hydrological signature observed at the next larger scale (e.g., hillslope) may lead into a hierarchical relationship of connectivities and signatures, in which the time-continuous nature of the connectivity signal may give rise to non-linear and threshold behaviours in the larger scale signature. 

Additionally, in the context of assessing model quality, connectivity is a feature of the natural system which models (and modellers) should strive to ensure. In this sense, we argue that model formulations, meshing (including resolution/topology and preprocessing/smoothing of the terrain model) and parameterisations should be evaluated not only using integrated signatures (e.g., water balance, hydrographs) or point data (water depth, velocities) but also using (dis)connectivity metrics. In this way, it is possible to evaluate to which extent a model and its setup can simulate natural flow paths and landscape functions.

How to cite: Caviedes-Voullième, D., Özgen-Xian, I., and Hinz, C.: Surface runoff connectivity across scales: revisiting three simulation studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5004, https://doi.org/10.5194/egusphere-egu21-5004, 2021.

The ability of identifying –based on numerical analysis– disconnected areas –in terms of overland flow pathways– depends on the digital elevation model (DEM) resolution, type of flow accumulation algorithm and DEM accuracy. On the other hand, tillage practices (in lowlands) and terrain preparation (at any slope gradient) may condition the occurrence of permanent/ temporal disconnected areas. In this study, the effect of DEM resolution and the presence of a drainage ditch and forest trails on the number, location and characteristics of disconnected areas is evaluated in a steep (mean slope gradient of 29%) farmland area of the Spanish Pyrenees. A new vineyard plantation (3785 m² and 5120 m² including the transit area; espalier system) and its upslope drainage area are evaluated. This site is located near Barbenuta village (Huesca province), at high elevation (1184-1260 m a.s.l.). Abandoned terraced fields and patches of natural vegetation (trees and shrubs) occupy the upslope area, where several forest trails cross from east to west. To protect soil against water soil erosion, farmers built a drainage ditch (total length of 137 m; ca. 0.30 m width; ca. 0.15 m depth) upslope the vineyard boundary, which minimizes runoff entrance into the field. A professional drone (senseFly© eBee X) was used to obtain –after point cloud processing– Structure-from-Motion (SfM)-derived DEMs at different spatial resolution, namely: 1, 0.5, 0.2, 0.1 and 0.05 m. We used combined information of the DEMs before and after filling the local sinks. As expected, the number (n=34, 341, 1079, 1272 and 1907) and size (mean=500, 60, 21, 18 and 12 m²; median=68, 15, 5, 4 and 2 m²; σ=920, 178, 69, 71 and 49 m²) of sub-basins increased and decreased, respectively, with decreasing the pixel size, due to fractal geometry and higher influence of micro-topography components (e.g. soil roughness, random local sinks) –higher ratios of 'residual topography (σ of slope) / pixel size': 0.2 (at coarser resolution), 1.8, 20.3, 113.6 and 636.8 (at finer resolution)–. The total area also varied with the different DEMs: 17010, 20514, 22398, 22852 and 22807 m². The number (n=21, 292, 903, 928 and 1283) and area (41, 143, 118, 58 and 44 m²) of disconnected areas increased and decreased, respectively, with decreasing the pixel size, representing 0.24%, 0.70%, 0.53%, 0.25% and 0.19% of the total drainage area. Similar differences were observed in other topographic metrics like the drainage-boundary perimeter and maximum flow length. These results prove the impossibility of defining a unique overland flow pattern. Further research should be focused on the role of runoff depth and how the effect of man-made landscape elements (drainage ditch, forest trail) and practices (tillage) on disconnectivity may depend on rainfall depth and intensity, and indirectly on plant growth.

How to cite: López-Vicente, M., Montenegro-Rodríguez, J., Antolín, M. C., and Gogorcena, Y.: Overland flow (dis)connectivity in a new vineyard under steep slope conditions in the Spanish Pyrenees: Effect of DEM resolution and terrain preparation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7287, https://doi.org/10.5194/egusphere-egu21-7287, 2021.

Sediment transport and connectivity are key factors for the functioning of fluvial eco-systems, and variations to these drivers deeply affect the geomorphology of the river system. Given that lags often occur in river systems, these changes may appear displaced in time and space from the disturbances that generated them. Modelling sediment (dis)connectivity and its reaction to anthropic pressures with a network-scale perspective is thus necessary to increase the understanding of river processes, to quantify real impacts and estimate future evolutionary trajectories. The CASCADE model (Schmitt et al., 2016) is a sediment connectivity model developed to address this type of research question: it combines concepts of network modelling with empirical sediment transport formulas to quantitatively describe sediment (dis)connectivity in river networks.

In this work, we present a new version of the CASCADE model which expands on the original model by featuring a dynamic simulation of sediment transport processes in the network (D-CASCADE). This new framework describes sediment connectivity in term of transfer rates through space and time. It takes into consideration multiple factors that can affect sediment transport, such as spatial and temporal variations in water discharge and river geomorphological features (i.e., river gradient and width), different sediment grainsizes, sediment entraining and deposition from and in the river bed and interactions between materials coming from different sources.

We apply the new D-CASCADE on the Bega River, New South Wales, Australia, which due to anthropic alterations post European colonization after 1850 including large-scale deforestation, removal of riparian vegetation and swamp drainage, has experienced significant alteration to the character and behaviour of streams, widespread channel erosion and massive sediment mobilization (Fryirs and Brierley, 2001). Our objective is to reproduce the historical sediment transfers that occurred across the network and associated river reach sediment budgets. First, we reconstructed the pre-settlement geomorphic features of the river network and the past hydrology from historical observations and expert-based reconstruction, and then modelled the sediment transport processes in the network in the last two centuries introducing the different drivers of change observed historically in the proper chronological sequence. Due to the uncertainty in the reconstruction of the historical conditions, multiple scenarios have been used.

The D-CASCADE model successfully reproduces the timing and magnitude of the major sediment transfers of the last two centuries in the Bega River network from headwaters swamps to lowland river reaches and associated channel geomorphic adjustments. Using the knowledge acquired by these historical simulations, the model was also applied to provide estimations on future trajectories of sediment transport and sediment budgets at the river reach scale.

With this research, we demonstrate the potential of the new D-CASCADE model to simulate and quantify at the network-scale sediment transport events generating information on sediment budget transfers from a single event to historical trajectories of the last centuries. Such knowledge paves the way to aid predictions of future impacts of basin-scale management measures and can support decision-making when designing sediment management strategies or river restoration initiatives.

How to cite: Tangi, M., Bizzi, S., Fryirs, K., and Castelletti, A.: A dynamic, network scale sediment (dis)connectivity model to reconstruct historical sediment transfers and river reach sediment budgets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8668, https://doi.org/10.5194/egusphere-egu21-8668, 2021.

Natural flood management (NFM) promotes the sustainable enhancement of natural fluvial processes to reduce flooding (SEPA, 2015; Wilkinson et al., 2019), and is increasingly popular for use by community groups, contractors and governments (Kay et al., 2019). Reintroduction of wood to a river channel is a popular form of NFM often achieved through seeding natural logjams, or with an emphasis on engineering through installing woody dams (WDs). WDs are currently installed or being installed in catchments in an effort to reduce flood risk, through hydrograph attenuation, increase biodiversity and improve geomorphic heterogeneity (Wenzel et al., 2014; Burgess-Gamble et al., 2017; Grabowski et al., 2019). A further objective is to emulate the effect of natural wood found in river channels by partially, or completely, blocking the channel to accelerate the recruitment of natural wood as part of the natural wood cycle (Addy & Wilkinson, 2016).

There is a growing body of evidence supporting the benefits of NFM, however, the hydrogeomorphic effects of WDs are less well understood (Dadson et al., 2017). There is little scientific underpinning concerning the long-term impact of these features upon hydrogeomorphology at reach and catchment-scales. Very few numerically based studies consider the influence of sediment transport on WDs, and how changes in local bed morphology influence their effectiveness. Most NFM research to date has focused upon modelling the effectiveness of local NFM measures in small catchments (<10 km²) (Dadson et al., 2017), with less work evident at larger spatial and temporal scales (Kay et al., 2019; Wilkinson et al., 2019).

There is a need for a verified tool that is able to represent WDs accounting for geomorphic processes and interactions between the dams and morphodynamics, different design specifications of dams, and changing efficacy due to geomorphic evolution. We present the new CAESAR-Lisflood (Coulthard et al., 2013) “Working with Natural Processes” toolkit, capable of representing WDs across a digital experimental environment. Global sensitivity testing was conducted using the Morris method (Morris, 1991) to assess the sensitivity of five aspects of the toolkit, and their potentially influences on geomorphology and flood risk reduction.

How to cite: Wolstenholme, J., Skinner, C., Milan, D., and Parsons, D.: Geomorphological numerical modelling of woody dams in CAESAR-Lisflood, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9636, https://doi.org/10.5194/egusphere-egu21-9636, 2021.

Land use on rural drylands has as its occupation process developing a hydric security and transportations infrastructure system. Dryland tropical systems present fluvial hydrological regime controlled by precipitation inputs, with few or none springs, showing ephemeral and intermittent rivers. Floodway crossings are a widespread infrastructure, on the countryside road network, to cross small creeks, especially intermittent and ephemeral rivers during the rainy seasons. Floodways are concrete or rock block structure, with or without culverts, that allows the river flow goes through or over it. They are part of a set of small longitudinal impediments, like small earth dams and check-dams, and can significantly impact the connectivity, notably by the high density of these impediments on rural areas. This research analysed the effect of floodways crossing on longitudinal connectivity of intermittent small rivers, focusing on morphological and sedimentological impacts in Brazilian Dryland. We analysed four floodways crossing with culverts installed on sandbed intermittent rivers, with upstream catchment are from 10 Km² to 130 Km². The analyses were based on rainfall data, lateral and longitudinal topographic profiles, generate by UAV surveys; and sedimentological samples of upstream and downstream of each structure. The Effective Catchment Area (ECA) was the first step to understand that several dams, and other longitudinal disconnect elements, decrease the ECA sharply, from 2 Km² to 38 Km² of the floodways analysed. Consequently, it affects the magnitude and frequency of water and especially sediment that reaches the floodway crossings. The results reveal the increase of upstream local base level, affecting 500 to 1000 meters, and coarse sediment retention, which is 1.7 to 3.6 times the standard percentage of very coarse sand and gravel. The retained sediment can be re-worked (reconnect) by extreme rainfall/discharges events, recurrence 0,22/year, and when the silting surface reaches the culvert level. The evolution of the upstream silting process is controlled not only by construction age but also by ECA spatiality and changes, and frequency and magnitude of rainfall/discharges events. The results discussion enabled developing an evolution model based on four stages: Installation, Adaptation, Coexistence, and Silting up. The Installation stage is the building process that locally deconfigures the channel morphology and, sometimes, inserts unfamiliar materials on channels. The Adaptation Stage starts with the first flow events that recreate a channel morphology but affected by the floodway, with the beginning of enhanced upstream sedimentation and downstream erosion. The Coexistence stage the disconnectivity effect is evident with the upstream sedimentation moving upstream. The downstream erosion creates a pool, expanding the floodway/riverbed height gap, and progressively increasing the vertical incision downstream. Lastly, when the sedimentation reaches the culvert level or the floodway, sediment retention decreases, and most of the transported sediment overpass the impediment. The frequency and magnitude of flow events control the time to progress through each stage, remembering the ECA analysis importance over space and time. This proposed model that still on initial development stage can help the integrative environmental management on areas impacted by widespread small longitudinal impediments.

How to cite: Branco, A. and Souza, J.: Effect of small impediments on channel morphology – intermittent rivers in Brazilian drylands., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13534, https://doi.org/10.5194/egusphere-egu21-13534, 2021.

The Ganga-Brahmaputra river system in the Himalayan Foreland supports diverse aquatic fauna. Decades of flow regulation through dams and barrages have affected their habitat suitability. To evaluate the impacts of large barrages on the morphology and habitat ecology we studied two different reaches (middle and lower) of the Ganga River. These reaches are the habitat of the endangered Ganga River dolphin (Platanista gangetica). In a reach in the middle Ganga between Bijnor and Narora barrage, a reported rise in dolphin population has been documented. In contrast, near the Farakka barrage in the lower reach of the Ganga River, a significant decline in the dolphin population has been observed.

We use Corona and time-series Landsat satellite images along with flow discharge data to assess the morphological and ecological impact of the barrages. In middle Ganga, the dolphin habitat is isolated between the Bijnor and Narora barrage where the minimum flow is available throughout the year for the dolphins to thrive.  On the other hand, in the lower Ganga, contrasting impacts are observed in the proximity (upstream/downstream) of the Farakka barrage. In the downstream, reduction in water (by one-third in the pre-monsoon discharge) and sediment discharge has decoupled the channel belt to its floodplain resulting in a loss of lateral connectivity. The presence of minimum flow between the Bijnor and Narora barrage has aided the dolphin population rise while the loss of lateral connectivity and excess siltation at the Farakka barrage has made the river reach unsuitable for habitation.   

How to cite: Sonkar, G. K. and Gaurav, K.: Impact assessment of structural barriers and flow regulation on the Ganga River morphology and ecology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14018, https://doi.org/10.5194/egusphere-egu21-14018, 2021.

The concept of geomorphic connectivity is being widely used since last two decades to understand and explain the various earth surface processes and dynamics. Its applicability to understand inter- and cross-scale process-response systems is now well established. In the present work, we have evaluated the applicability of the geomorphic connectivity framework (Singh et al., 2020, ESPL) for managing and mitigating various geological hazards. For an effective hazard mitigation and management planning, we need to know (a) source of hazard, (b) hazard propagation pathways, (c) probable affected areas, and (d) identification of escape routes/pathways. The connectivity concept can be effectively utilised to satisfy aforementioned requirements. For example, sediment and hydrological connectivity can be used to evaluate the potential pathways, identify sources and affected areas, and to assess return periods of fluvial-related hazards such as debris flow and riverine flash floods. Similarly, the potential sites of landslide, stream congestions (and hence, flash flood)- can be identified by evaluating the channel-slope sediment connectivity and longitudinal hydrological connectivity. The concept of landscape connectivity can play a pivotal role in understanding the forest fire probabilities by evaluating the connectivity between various fire-prone patches of forests, fuel, and the spatial positions of fire-breaking landscape patches. Based on connectivity concepts, the potential paths of forest fire propagation can be demarcated in advance and can play a crucial role in forest fire mitigation. Other than identifying the risk-prone zones with respect to various hazards, connectivity concept can also be used to plan evacuation routes as well. Therefore, we propose that the geomorphic connectivity framework can be a robust tool to manage and mitigate various geological hazards.

How to cite: Singh, M. N., Singh, S., Laha, A., and Mishra, K.: Applicability of connectivity concept for disaster management and hazard mitigation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14040, https://doi.org/10.5194/egusphere-egu21-14040, 2021.

Changes in hydro-geomorphic connectivity have been previously linked to catastrophic shifts in landscape structures and function leading to irreversible degradation. Here we present evidence and new observations to better understand the link between connectivity of water and sediments and possible phase transitions for the case of semiarid ecosystems at the catchment and hillslope scales.  We first focus on rangelands, where coevolving vegetation and landform structures lead to a distinct connectivity pattern responsible for the healthy functioning of the system. Positive feedbacks, triggered by disturbances in vegetation, water or sediment structures can alter the hydro-geomorphic connectivity leading to degradation. Our results for rangelands in Australia, from both simulations and observations, suggest that an increase in connectivity beyond a threshold may lead to irreversible degradation, meaning that the system return to a functional state is unlikely without extensive management interventions. We also analyse the case of semi-arid floodplain wetlands of the Murray-Darling Basin, where we observe that dis-connectivity during droughts promote terrestrial vegetation encroachment and degradation. Simulations and observations also indicate the presence of thresholds beyond which the recovery of the system is unlikely without interventions.

How to cite: Saco, P., Moreno-de las Heras, M., Rodriguez, J., Sandi, S., Azadi, S., and Quijano, J.: Applications of a hydro-geomorphic (dis)connectivity framework to study vegetation transitions in semiarid ecosystems., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14057, https://doi.org/10.5194/egusphere-egu21-14057, 2021.

The highland cirques mostly created by nivation and glacial exaration take large areas in mountains and have a significant role in the sediment transit of the basins. The approximate view on the connection of cirques and low levels in the sediment flow could be given with the sediment connectivity index analysis. We study the spatial distribution of the index for typical ice cirque – the Koiyavgan cirque near the join of the Main Caucasus Range and its offshoot (the Gumachy range). This area is located in the tops of the Adyl-Su valley (left side of the Baksan river basin). In August 2020, we got a high-resolution orthophoto image (13+ cm) and digital elevation model (27+ cm) from aerial photography. The territory located in the elevation range from 3230 to 4022 m. Geological conditions: gneiss, metamorphic shale and basic dark coloured igneous rocks. There is no developed vegetation cover. Typical post-glacial cirques topography includes (top-down): mountain tops, very steep bedrock slopes, colluvial footslopes and fans, cirques bottom (moraine ridges with dividing valleys, craters from melting of the in-moraine covered ice etc.) with fluvial, avalanche and creep post-shaping, and bottom surface break as analogue of riegels in glacial trough valleys. The connectivity index (CI) after Cavalli et al. [2013] is very dependent on initial DEM resolution, from the method for filling mistaken depressions, from window size for computing intermediate geomorphometric variables (e.g. roughness index), from choice in flow impedance variable, from area coverage and terrain diversity and others. We compute connectivity index with the parameters: 1) DEM resolution – 27 cm; 2) impedance variable – terrain roughness index (standard deviation of elevation) with window 7*7 cells; 3) standard filling method used in the ArcMap (filling local depression without any limitations on maximum depth); 4) range of impedance values before normalization (partially related to area coverage) is from 0 to 72 m. In the some buffers from the channel network the connectivity index generally grows in the top-down direction. Greatest spurt of the CI values relates to the cirques low border - the riegel (3300 m asl). There are two levels characterised with low values of the CI: 3550 m and 3750 m. The first one is backside of cirques bottom with relatively low flow accumulation area and low-moderate slopes (0-25°), the second one is mountain tops with high steep slopes, but with lowest flow accumulation. For different geomorphodynamical zones the threshold of IC where sediment transit turns into sediment accumulation has differ values: for example, -2.3 for colluvial fans and -2.5 for alluvial fans (p-value for differences significance « 0.01). Maximum values of CI (quantile: the top-95%) for accumulative positions again are -1.27 and -0.72. Its means, those accumulative processes areas with different mechanics of the deposition may be delineated with using non-constant CI values only. The potential of sediment flow connectivity modelling for high mountain isn’t exhausted, but its application needs wide discussion and calibration.
The study was supported by the Russian Science Foundation (project No. 19-17-00181).

How to cite: Kedich, A., Uspensky, M., Tsyplenkov, A., Kharchenko, S., and Golosov, V.: Sediment connectivity in the Koiyavgan glacier's cirques (Adyl-Su river basin, Caucasus, Russia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16175, https://doi.org/10.5194/egusphere-egu21-16175, 2021.