Property:CSDMS meeting abstract
From CSDMS
This is a property of type Text.
2
A barrier aware Riemann solver is developed for the shallow water equations in the presence of the sub-grid-scale barriers using an explicit finite volume scheme. Our algorithm guarantees that the barrier-containing cell can be split into two effective cells that are maintained outside of the reset of the grid structure. To avoid time-steps constrained by the size of small cut cells, we redistribute the fluxes computed on those cells engaging a modified h-box method. The solver ensures that water does not cross the barrier when it is not supposed to, maintain large time-steps relative to the cells being cut through and retain the desirable properties. Also, the wet-dry interface, the boundary between cells that are wet (or flooded) and dry are well handled so that quantities going to zero and conservation are carefully integrated into the method. The work is built off of the GeoClaw package so inherits various extensions to tsunami and storm surge simulations. +
A critical part of predicting and representing coastal responses during large storms is to represent the areas of compound flooding where both oceanic factors (tides and surge), and hydrologic factors (i.e., rainfall-runoff processes), as well as their interactions impact water levels and flow velocities. During extreme events, these compound flood waters can remobilize thick layers of sediment, exposing material that had been buried for many years. One such event was Hurricane Harvey which made a landfall along the Texas Gulf Coast (US) on August 26, 2017. Within Galveston Bay, compound flooding occurred and persisted for weeks as the result of the interaction between the storm surges created as storm approached the coast, and a subsequent long-lasting flood pulse induced by the torrential rainfall associated with the hurricane. The flooding mobilized thick layers of sediment, including contaminated sediment from the Buffalo Bayou shipping channel. Sediment core data taken after the storm showed that these contaminants were transported several 10s of km. To predict the response of this type of event requires numerical models that can account for the rainfall-runoff processes, sediment erosion and transport, and oceanographic processes including storm surges, tides and wind-driven currents.
These types of coupled models are currently being developed and tested as part of the NOAA funded Coastal Ocean Modeling Testbed (COMT). The eventual goal of our project is to develop the capability to represent compound flooding and the associated particulate and contaminant fluxes across the river – to ocean continuum. Specifically, we plan to link a hydrological model (WRF-Hydro) to a Galveston Bay hydrodynamic model (ROMS) and apply it to large compound flooding events such as Hurricane Harvey. A higher resolution model capable of representing the Buffalo Bayou shipping channel will then be nested within the Galveston Bay model and used to reproduce sediment erosion and contaminant exposure patterns that were observed in the wake of Hurricane Harvey. In this poster we present preliminary model development that links the high – resolution (~20m) hydrodynamic model of Buffalo Bayou with the lower – resolution (~100 m) hydrodynamic model of Galveston Bay. The 20-m model grid resolves the relatively deep shipping channels and is being used in initial model runs to represent typical conditions in the upper Galveston Bay and Buffalo Bayou shipping channel.
A dynamic framework coupling social and ecological sub-systems while aligning management, policy, governance, science, legal and decision-making elements under an overarching goal will be presented and described. A nested set of conceptual models is used to represent and analyze the general internal organization and functioning of a federal agency. External connectivities are also addressed while the conceptual model is able to generate testable hypotheses. The selection of managing for resilience as the main goal of the framework as well as their underpinning elements will be illustrated and explained. The overall functioning of the proposed resilience framework seeks to mimic and anticipate environmental change and is aligned with commonly used elements of resilience-thinking. Dynamic management frameworks addressing socio-ecological dynamics can facilitate the efficient and effective utilization of resources, reduce uncertainty for decision and policy makers, and lead to more defensible decisions on resources. +
A goal of the geomorphology community is to translate our understanding of past and present processes to predict landscape change in the future. Here we present our knowledge about relict permafrost landscapes across central Appalachia, and we propose a framework through which the geologic record and landscape models may be used to predict change in modern permafrost settings. The onset of Quaternary glacial cycles profoundly influenced the pace and pattern of erosion in mid-latitude settings through the development and subsequent degradation of perennially-frozen soils. Lidar-based mapping documents extensive periglacial alteration of the central Appalachian landscape, including solifluction lobes and other mass-wasting features. These features appear aspect-modulated, implying microclimate control. Geomorphic mapping, shallow geophysical imaging and cosmogenic nuclide dating reveal that periglacial erosion sets regolith patterns, subsurface architecture and erosion rates for multiple glacial cycles. Moreover, a combination of slow erosion rates and structural traps means headwater valleys and basins preserve direct records of upland erosional response to climate change, and planned work to core modern peat bogs may provide paleoclimate and paleoecological markers like pollen and leaf waxes in addition to quartz-rich debris for cosmogenic dating. Geologic data can be supplemented by permafrost hydrology models for an improved understanding of both the microclimate and long-term climate controls on periglacial hillslope processes. Informative models pair realistic active layer flow paths, accounting for both infiltration and permafrost thaw, with effective stress calculations to develop more accurate failure depth estimates. Such process-based models will be key to predicting future periglacial landscape change as warming exceeds historical trends. +
A
A major challenge of geophysics today is addressing the problems of general interest through intense collaboration that bridges disciplinary boundaries. Such collaborations are greatly complicated by the fact that Earth Sciences have steadily diverged and evolved to the point of the Tower of Babel. Scientific jargon makes it difficult to meaningfully explore ideas across disciplines, while lack of cyberinfrastructure for sharing causes poor reproducibility and code reuse.<br>
My vision for an EarthCube frontend is that of a maximally simple API that could be run from any platform or in a browser. At it's core, it would support the following functionality:
# make it really simple for someone to submit their own data, models and software with provenance and descriptive metadata;
# support data discovery in 4D space, at a range of scales, through semantically-enabled metadata (and the data might - and will - be stored in one of the existing databases);
# have potential for elaborate visualization capabilities;
# build up upon a social network of some sort (so that there's a face behind each data component); and, finally,
# make it easy to create, modify and run workflows remotely through intelligent combination of software and data components.
The last point seems critical for long term useability of EarthCube, and requires upfront thinking and code coupling capabilities.<br>
Specifically, the plug-and-play component programming approach used by CSDMS could be adapted by the larger solid Earth geophysics community with great long-term benefits, hopefully resulting in better scientific reproducibility, code reuse and, eventually, streamlined collaboration. +
A major issue, hampering our understanding about the human impacts on sediment fluxes is our limited knowledge about the magnitude and controlling factors of catchment sediment yields (SY, t km-2 y-1) under ‘baseline’ conditions, i.e. the SY that could be expected from a catchment if it was unaffected by human impacts.
To address this problem, a dataset was set up with measured SY-data from 146 catchments in Europe that are little or not affected by humans in terms of land use and have no significant reservoirs, lakes, impoundments or glaciers in their upstream area. The considered catchments span a wide range in catchment areas (0.3 – 4,000km²) and observed SY-values (0.5 – 3,100 t km-2 y-1). Analyses of these data indicates that climate exerts little control on the observed range of SY-values. However, strong correlations were found between SY and average catchment slope, lithology and tectonic activity (as derived from a globally available earthquake hazard map). Based on these findings, a regression model was developed that allows predicting baseline SY. Model calibration and validation results indicate that this model is able to provide robust approximations of the baseline SY, with >95% of predictions deviating less than one order of magnitude from the measured SY-values.
This model can therefore significantly improve our understanding about the controlling factors of SY and their sensitivity to human impacts. However, it is also the first model that explicitly considers the effect of tectonic activity on catchment SY. Despite the relatively limited tectonic activity in many of these catchments, differences in earthquake sensitivity alone was found to explain already more than 40% of the observed variation in SY. Our results therefore illustrate that tectonic activity has a strong, but hitherto largely neglected, influence on SY. +
A mathematical model of carbonate platform sedimentation is presented in which the depth-dependent carbonate growth rate determines the depositional rate of a platform top responding to relative sea-level rise. This model predicts that carbonate platform evolution is primarily controlled by the initial water depth and the sediment production rate at the initial depth, rather than by the maximum potential production rate and imposed rate of relative sea-level rise. A long-standing paradox in the understanding of drowned carbonate platforms in the geological record is based on comparing relatively slow long-term rates of relative sea-level rise with maximum growth potentials of healthy platforms. The model presented here demonstrates that a carbonate platform could be paradoxically drowned by a constant relative sea-level rise when the rate is still less than the maximum carbonate production potential. This does not require other external controls of environmental change, such as nutrient supply or siliciclastic sedimentation. If the rate of relative sea-level rise is higher than the production rate at the initial water depth, the top of the carbonate platform gradually drops below the active photic zone and drowns even if the rate of relative sea-level rise is lower than the maximum carbonate accumulation growth potential. This result effectively resolves the paradox of a drowned carbonate platform. Test runs conducted at bracketed rates of relative sea-level rise have determined how fast the system catches up and maintains the “keep-up” phase, which is a measure of the time necessary for the basin to respond fully to the external forcing. The duration of the “catch-up” phase of platform response (termed carbonate response time) scales with the initial seawater depth and the platform-top aggradation rate. The catch-up duration can be significantly elongated with an increase in the rate of relative sea-level rise. The transition from the catch-up to the keep-up phases can also be delayed by a time interval associated with ecological reestablishment after platform flooding. The carbonate model here employs a logistic equation to model the colonization of carbonate-producing marine organisms and captures the initial time interval for full ecological reestablishment. The increase in delay time due to the carbonate response time and self-organized processes associated with biological colonization, implies a greater likelihood of autogenic origin for high-frequency cyclic strata than has been previously estimated.
2
A new approach for mapping landslide hazard is developed by combining probabilities of landslide probability derived from a data-driven statistical approach and process-based model of shallow landsliding. Our statistical approach integrates the influence of seven site attributes on observed landslides using a frequency ratio method. Influential attributes and resulting susceptibility maps depend on the observations of landslides considered: all types of landslides, debris avalanches only, or source areas of debris avalanches. For each landslide type the frequency ration (FR) classification is converted into a Stability Index (SI), mapped across our study domain in the North Cascades National Park, WA. Using distributed landslide observations a continuous function is developed to relate local SI values to landslide probability. This probability is combined with spatially distributed probability of landsliding obtained from Landlab using a two-dimensional binning method that employs empirical and modeled based probabilities as indices and calculates empirical probability of landsliding at the intersections of bin ranges of the empirical and process-based probability domains. Based on this we developed a probabilistic correction factor to modeled local landslide probability. Improvements in distinguishing potantially unstable domain with the proposed model is quantified statistically. +
A
A new class of models based on population ecology, nutrient-geochemistry, and sedimentology is able to simulate carbonate accretion in reef and shelf environments. Unlike previous models for carbonates, they produce very detailed simulations of facies variabilities in space and time. With adjustments to the model runs the range of variabilities can be explored and described statistically. We look at comparisons of the statistics from the models and in outcrops/drillcores of carbonate rocks and ecological transects of present-day seabed areas. +
2
A new submerged aquatic vegetation (SAV) model is developed and incorporated into the fully coupled hydrodynamic-water quality framework of SCHISM-ICM in order to account for the impacts of SAV to the aquatic system. This multidisciplinary study incorporates biogeochemistry, hydrodynamics, numerical computing and field survey data. The interactions between SAV, hydrodynamics and biogeochemistry contain several complex nonlinear feedback loops, which had not previously been explored. My research uses a fully coupled hydrodynamic-biogeochemistry-SAV model to quantitatively explore the relative contributions of each process associated with SAV (from purely physical processes such as dragging, to purely biological processes such as growth) to its total impact on the system. Through applications, we find that SAV generally encourages phytoplankton accumulation by increasing the residence time, while suppressing local primary production of the phytoplankton through competition for light and nutrients. The dynamic feedback of SAV to hydrodynamics is significant, accounting for up to 80% of the changes of the water quality variables. Our results highlight the importance of incorporating the nonlinear feedback loops in a model in order to correctly account for complex hydrodynamic and biogeochemical processes. This new SAV model has immediate applications in the monitoring and guidance of SAV removal (e.g. San Francisco Bay and Delta) or recovery (e.g. Chesapeake Bay) in different systems over the world. +
A
A number of two- and three-dimensional models are currently available to calculate sediment transport and channel change in rivers. These three-dimensional models rely on time-averaging and parameterization of the turbulence. Available depth-averaged, two-dimensional models also rely on simple boundary stress closures. In relatively simple channels these models have predictive capability, but they often perform poorly when there is large-scale flow separation or when secondary circulation is strong. Sharp meanders, channel constrictions, many engineering structures, vegetation, and certain types of bedforms all cause flow separation, secondary circulation, and free shear layers. Turbulence-resolving flow and sediment transport models may do better at predicting channel change in complex channels, but at a substantially larger computational cost. With parallelization, turbulence-resolving models can now be developed and applied to refractory fluvial morphodynamic problems.
Detached-Eddy Simulation (DES) is a hybrid large eddy simulation (LES) and Reynolds-averaged Navier Stokes (RANS) method. RANS is applied to the near-bed grid cells, where grid resolution is not sufficient to fully resolve wall turbulence. LES is applied further from the bed and banks. A one equation turbulence closure model with a wall-distance dependence, such as that of Spalart and Allmaras (SA), is ideally suited for the DES approach. The rough wall extension of the SA model is utilized herein. Our river DES numerical modeling system was developed in OpenFOAM. The model resolves large-scale turbulence using DES and simultaneously integrates the suspended sediment advection-diffusion equation, wherein advection is provided by the DES velocity field minus particle settling, and diffusion is provided by the sub-grid or RANS eddy viscosity. As such, turbulent suspension throughout most of the flow depth results from resolved turbulent motions. A two-dimensional, depth-averaged flow model, also written in OpenFOAM, determines the local water surface elevation. A separate program was written to automatically construct the block-hexagonal, computational grid between the calculated water surface and a triangulated surface of a digital elevation model of the given river reach. Domain decomposition of the grid is employed to break up the integration between multiple processors, and Open MPI provides communication between the processors. The model has shown very good scalability up to at least 128 processors.
Results of the modeling system will be shown of flow and suspended sediment model in lateral separation eddies in the Colorado River in Grand Canyon. The eddy recirculation zones exist downstream of channel constrictions from tributary debris fans. The modeling system is currently being developed and validated to be used in designing discharges from Glen Canyon Dam for the preservation of sandbar beaches, which are critical habitat for endangered fish.
Keywords: fluvial geomorphology, sediment transport, lateral separation zones.
Movie at:
https://csdms.colorado.edu/mediawiki/images/GrandCanyonDES.avi
A series of controlled laboratory experiments were conducted at the St. Anthony Falls laboratory of the University of Minnesota to study the effect of changing precipitation patterns on landscape evolution over long-time scales. High resolution digital elevation (DEM) both in space and time along with instantaneous sediment transport rates were measured over a range of rainfall and uplift rates. These experiments were designed to develop a complete drainage network by growth and propagation of erosional instabilities in response to tectonic uplift. We focus our study to the investigation of how changes in the frequency and magnitude of large-scale rainfall patterns (e.g. monsoonal variability) might influence the development of mountainous landscapes. Preliminary analysis suggests that the statistics of topographic signatures, for example, evolution of drainage network, slopes, curvatures, etc., show dependence on both rainfall patterns and uplift rate. The implications of these results for predictive modeling of landscapes and the resulting sediment transport are discussed. +
2
A series of controlled laboratory experiments were conducted to study the effect of changing precipitation patterns on landscape evolution at the short and long-time scales. High resolution digital elevation (DEM) both in space and time were measured for a range of rainfall patterns and uplift rates. Results from our study show distinct signatures of extreme climatic fluctuations on the statistical and geometrical structure of landscape features. These signatures are evident in widening and deepening of channels and valleys, change in drainage patterns within a basin and change in the probabilistic structure of erosional events, such as, landslides and debris flows. Our results suggest a change in scale-dependent behavior of erosion rates at the transient state resulting in a regime shift in the transport processes in channels from supply-limited to sediment-flux dependent. This regime shift causes variation in sediment supply, and thus in water to sediment flux ratio (Qs/Qw), in channels of different sub-drainage basins which is further manifested in the longitudinal river profiles as the abrupt changes in their gradients (knickpoints), advecting upstream on the river network as the time proceeds. +
A
A two-dimensional numerical model was developed for simulating free surface flow. The model is based on the solutions of two-dimensional depth averaged Navier-Stokes equations. A finite volume method is applied such that mass conservation is satisfied both locally and globally. The model adopted the two-step, high resolution MUSCL-Hancock scheme. This Godunov type scheme is used together with the approximate Riemann solver. The boundary cells are treated as cut-cells in order to accommodate arbitrarily geometries of natural rivers. There are sixteen types of cut-cells depending on the slope of the boundary intersection with the cell. A cell merging technique is incorporated in the model that combines small cells with neighboring cells to create a larger cell, helps keeping the CFL condition. The cut-cells approach permits a fully boundary-fitted mesh without implementing a complex mesh generation procedure for irregular geometries. The model is verified by several laboratory experiments including unsteady flow passing through cylindrical piers and dam break flow in a rectangular channel. The model is also applied to simulate a 100-year flood event occurred at the Huron Island reach of the Mississippi River, where flow paths in the island formed a complex channel network as flood propagates. +
2
Acoustic sediment monitoring technology provides a practical means to obtain high resolution estimates of suspended sand flux. However, bed bedload flux can be a significant component of total load and remains difficult to measure directly. In most cases, bedload is treated as a power-law function of water discharge, a constant fraction of suspended sand flux, or ignored. However, bedload flux may vary independently from water discharge or suspended sediment flux in supply-limited rivers due to systematic grain size and reach-geometric effects. We propose a model for bedload flux that enables improved prediction using variables that are routinely measured at acoustic sediment monitoring stations.<br><br>Though this model is rooted in causal physical theory, it contains several scaling parameters that must be constrained empirically. To this end, we propose a Bayesian statistical procedure that facilitates propagation of uncertainty from multiple sources of information. Application of this procedure is demonstrated at one monitoring station on the Colorado River in Grand Canyon National Park. Repeat bathymetric surveys of dune migration in the reach adjacent to the monitoring station are used to estimate bedload flux, providing an observational basis for statistical analysis. Parameter estimation and prediction are also informed by data from other rivers, which is incorporated in a hierarchical framework.<br><br>We find that conventional methods of estimating bedload flux fail to capture fluctuations driven by the interaction between flow strength and sediment supply, and can introduce large persistent biases to estimates of total load. Our model is applicable in a wide range of scenarios, and substantially reduces the uncertainty associated with estimating bedload flux in sand bed rivers. +
Addressing society's water and energy challenges requires sustainable use of the Earth's critical zones and subsurface environment, as well as technological innovations in treatment and other engineered systems. Reactive transport models (RTMs) provide a powerful tool to inform engineering design and provide solutions for these critical challenges. In this keynote, I will showcase the flexibility and value of RTMs using real-world applications that focus on (1) assessing groundwater quality management with respect to nitrate under agricultural managed aquifer recharge, and (2) systematically investigating the physical, chemical and biological conditions that enhance CO2 drawdown rates in agricultural settings using enhanced weathering. The keynote will conclude with a discussion of the possibilities to advance the use of reactive transport models and future research opportunities therein. +
After Glacial Lake Agassiz drained ~8.5 ka, the Red River (North Dakota, USA) formed, flowing northward into Lake Winnipeg and incising into paleolacustrine sediment as it meandered. The Red River provides a natural experiment to interrogate the role of slope change on river meandering and morphologic evolution as it is characterized by shallow bed slopes (~0.0001), which have been controlled by crustal deformation due to glacial isostatic adjustment (GIA) in response to ice sheet unloading since the river’s inception. GIA has changed Red River channel elevation by 10s of meters, reducing slopes by up to 60% in the downstream reaches. We isolate the role of slope in order to explore its importance to lateral migration rate relative to other factors such as bank strength, sediment supply, and fluid flow.
We quantified the impact of GIA-induced slope changes on the Red River’s morphology by performing an analysis of river meanders and cutoffs (Kodama et al., 2023). We constructed a dataset that quantified the number of meanders, cutoffs, and modeled change in slopes caused by GIA along the Red River. Notably, the abundance of cutoffs normalized for channel width (a proxy for time-averaged meander rate) statistically significantly correlates with changes in slope, with far fewer cutoffs in the downstream reaches of the river, where the largest slope reduction occurred. We expanded this analysis to two tributaries of the Red River, and found that this relationship holds in all three river systems regardless of sign (negative or positive) of the GIA-induced slope change. We infer that slope drives changes in lateral migration rate for these detachment-limited systems by modulating the magnitude of shear stress on riverbanks.
We next developed a modeling framework by modifying a simple kinematic model of meander migration (Howard & Knutson, 1984) to explore the impact of GIA-induced slope change on the temporal trends of meander migration rate along the Red River. Previous work showed that the meander rate of two Red River meander scrolls exponentially decayed over the Holocene (Brooks 2003), which we are able to simulate with our GIA forced meandering model. Our study isolates the role of slope on river lateral migration and highlights how rivers near former ice sheets can respond to changes in slope that occur over thousands of years.
After Superstorm Sandy impacted the New Jersey coastline in 2012, the state’s primary coastal resiliency plan was to fortify the entire shoreline by constructing large-scale berm-dune systems along the beach. These large artificial dunes, funded entirely by Congress, were constructed with the goal of mitigating future storm damage to houses and infrastructure. Two long-term management questions are 1) is it feasible for a beachfront community to maintain these projects over the long term?; and 2) if not, what fraction of the cost would need to be subsidized? To tackle these questions, we use a “geo-economic” model that captures the natural processes of beach and dune erosion and migration via storm overwash coupled with engineering interventions of beach nourishment and dune construction. The economic portion of the model accounts for the relationship between property values and berm-dune geometry. Previous work suggests that due to their protective and recreational value, higher dunes and wider beaches increase that property values. However, it is unclear whether this relationship holds true for dune protection some years after a storm has occurred as lags in major storm events may lead to perceptions of lower risks. Thus, beachfront communities may place greater value upon viewership and private property, rather than on protection by artificial dunes. By deriving mathematical expressions for optimal berm and dune size as a function of geologic and economic parameters, our model suggests that changes in risk perception can lower property values and therefore reduce the ability of a community to keep up with the costs of maintaining these structures. We are currently testing this hypothesis by analyzing past and present LiDAR imagery (i.e. 2010, 2014, and 2018) and real-estate data from Long Beach Island, NJ. +
Agricultural expansion has led to high rates of deforestation and land-use change in tropical ecosystems, relegating many of the remaining native forests to networks of fragmented patches. As a result, large forest-dwelling ungulates may alter movement and habitat-use patterns to accommodate for the changed spatial orientation of essential resources. In turn, some native patches may be subjected to increased ungulate impacts (e.g. trampling, bioturbation, and seed dispersal/ predation), while others may be devoid of these influences. We created an individual-based model utilizing empirical ungulate movement data from white-lipped peccaries (WLP) in the Brazilian Cerrado to evaluate variations in habitat use with degree of fragmentation (e.g. connectivity and number of patches) and percent of native forest cover (FC). In the model, a peccary herd moves across a landscape with a percent FC between 10% and 100% and one to four native forest patches. We then quantified the distribution of habitat-use intensity and percent of unused native habitat after five years. To empirically quantify impacts of white-lipped peccary habitat use, we measured seedling density in 72 1x1 plots in the Cerrado, 44 with and 28 without WLP.
Results indicate that in a fully-connected landscape (one-patch simulations), as percent FC decreases, the frequency distribution of habitat use goes from narrow and left-skewed (low use in the majority of the habitat) to widely and evenly distributed (no use to high use in distinct parts of the habitat), reflecting a more heterogeneous use of the habitat with less FC. In a fragmented landscape (two-four patch simulations) below 30% FC, habitat use is driven by the degree of connectivity between forest patches. However, above 60% FC, the percent of unused forest is negligible (similar to one-patch simulations), indicating that patch spatial configuration is no longer the driving factor of habitat use past a 60% FC threshold. Between 40% and 60% FC, habitat use is a function of both connectivity and percent FC. Preliminary empirical results suggest riparian forests have the greatest difference in mean seedling density between areas with or without WLP, while palm swamps have the least. Collectively, these results suggest conservation measures in agricultural landscapes should emphasize percent FC, connectivity, or both, depending on the amount of forest remaining and that riparian zones may be most adversely affected by the loss of large ungulates.
Alluvial megafans are sensitive recorders of landscape evolution: the influence of both autogenic processes and allogenic forcing and of the coupled dynamics of the fan with its mountainous catchment can often be deciphered from the megafan sediment record and the system’s morphometric characteristics. The Lannemezan megafan in the northern Pyrenean foreland was abandoned by its mountainous feeder stream during the Quaternary and subsequently incised. During the incision, a flight of alluvial terraces was left along the stream network. We use numerical models (CIDRE model, Carretier et al. 2015) to explore the relative roles of autogenic processes and external forcing in the building, abandonment and incision of a foreland megafan. We then compare the results with the inferred evolution of the Lannemezan megafan. We conclude that autogenic processes are sufficient to explain the building of a megafan and the long-term entrenchment of its feeding river at the time and space scales that match the Lannemezan setting. In the case of the Lannemezan megafan, climate, through temporal variations in precipitation rate, may have played a second-order role in the pattern of incision at a shorter time-scale. In contrast, base-level changes, tectonic activity in the mountain range or tilting of the foreland through flexural isostatic rebound do not appear to have played a role in the abandonment of the Lannemezan megafan. +
