Property:CSDMS meeting abstract
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Tropical Montane Cloud Forests (TMCFs) are located at the headwaters of biodiversity-rich ecosystems like the Amazon and regulate the release of water downstream. Epiphytes are vascular and non-vascular plants that grow in the canopies of TMCFs and hold large amounts of water, regulating temperature and humidity. In order to investigate how epiphytes control canopy microclimate, we developed an uncalibrated, numerical water balance model where we considered epiphytes as a water tank. Water enters the epiphyte tank via fog, precipitation, and dew deposition, and exits the tank primarily through evapotranspiration. The model also considers the role of host tree aerial roots uptaking water from the epiphyte tank. We validate the model against field data collected from cloud forests in Monteverde, Costa Rica. Preliminary results demonstrate that epiphyte temperatures have a phase lag with air temperature, and this lag is responsible for regulating diurnal conditions within the canopy. Under clear sky conditions, epiphytes increase humidity in the canopy during the afternoon and reduce evapotranspiration at night. This work improves our understanding of the hydrologic cycle of TMCFs and will help us understand how resilient these ecosystems are to climate change. +
Tropical montane cloud forests (TMCFs) are ecosystems at high elevation where fog and low clouds regularly occur. These forests are routinely wet, which promotes biodiversity and plant richness not seen in other parts of the world. A key feature of TMCFs is the abundance of epiphytes, plants that live along the branches and trunks of trees and collect their water and nutrients from the atmosphere. Epiphytes can be vascular and non-vascular plants and usually store water about 4 times their dry weight in water. A critical gap in our understanding of hydrologic processes in TMCFs is the role that epiphytes play as water stores in the canopy. Deforestation and land use changes threaten the boundaries of TMCFs, frequency and amount of fog and low cloud immersion, and the abundance of epiphytes. In order to investigate how the loss of epiphytes or changes in patterns of fog and low clouds will affect TMCFs, we developed an epiphytes water balance model (EWB). The EWB model conceptualizes epiphytes in the host tree as a water store inside the canopy that is filled via vertical and horizontal precipitation, and depleted via evapotranspiration and host tree water uptake. We tested the model using idealized and observed dry season conditions for TMCFs in Monteverde, Costa Rica. Results from the idealized and real model simulations capture how epiphytes regulate water and energy fluxes in the canopy at diurnal scales, and are consistent with field observations. A key result is that dew deposition may recharge up to 34% of epiphyte water storage lost due to evapotranspiration over a 3-day dry-down event. We also found that energy and water mass balances are sensitive to the water storage size, i.e. the maximum water content and the abundance of epiphytes in the canopy. Our results provide the first quantitative demonstration of how epiphytes regulate temperature in TMCFs. This work sets the foundation for developing a process-based understanding of the effects of climate change on TMCF eco-hydrology.
Tsunami currents can be very dangerous even if there is no onshore inundation, and can create whirlpools and other small-scale structures. With inundation, ships can be carried onshore and become part of the debris field. In this work, the open source GeoClaw tsunami modeling code (www.geoclaw.org) is used to model tsunami tsunami generation, propagation and inundation. The depth-averaged shallow water equations are used to compute the water depth and fluid velocities, which are saved on a fine grid near the region of interest every few seconds. Postprocessing scripts are then used to track the motion of particles in this flow field, making it easy to experiment with different initial particle locations, masses, grounding depths, etc. Developing better algorithms for debris tracking and using them in probabilistic models is a very active research topic for the Cascadia CoPes Hub (cascadiacopeshub.org) and others in the tsunami modeling community. +
Tsunami modeling often combines the need for an ocean-wide simulation with the requirement that a small region of the coast (some community of interest) be simulated with a fine level of resolution (often ⅓ arcsecond, less than 10 meters). In the open ocean we might need 1-4 arcminute resolution, but only in regions that the waves have reached. This is addressed in the GeoClaw software package by using adaptive mesh refinement to place higher resolution grids around the waves based on where the water surface height is significant. We present a method of placing higher resolution grids when there is a small region of interest (say, a single coastal community) by using the adjoint equation. Advantages of this new method are presented, including reduced computational times and the capability to refine only the waves that will impact the specific community during a given time range of interest. +
Two numerical forward stratigraphic models are used to explore the origin of carbonate lacustrine strata characteristics at various scales. The large-scale model (Carbo-CAT; Burgess, 2013) focuses on exploring kilometre scale carbonate stratal heterogeneity developing in extensional settings. New developments include spatial distribution of dissolved carbonate in water controlling carbonate production and subsidence produced by various 3D fault configurations. The small-scale model (Mounds3D) investigates the controls on microbial mound development in the metre to decametre scale. Modelled microbial growth includes precipitation and trapping and binding processes. These are affected by energy, slope and spatial distribution of the microbial community. The model incorporates a depth-averaged hydrodynamic model to assess the impact of transported sediment deposition, erosion and trapping and binding in mound development.
Numerical experiments using these models show the complex relationship between initial conditions, processes and resulting stratal geometry. For example, large-scale models of carbonate systems developing over relay ramps show that their size, shape and facies distribution is controlled by the combined effect of varying basement surface, the platform ability to keep up with relative water-level rise and sediment transport processes. The tests performed with the small-scale model show that, although the initial bathymetry exerts a first order control on initial location of microbial mounds, the dynamic behaviour of the systems suggests that mound spacing is also controlled by local variations on the hydrodynamic and depositional conditions. +
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Understanding gravel bed river morphology over decadal to centennial timescales is vital to making informed stream management and restoration decisions. Factors such as land use change and climate shifts over such timescales may drastically alter river evolution – with major implications for in-channel and riparian habitat. Given these longer timescales of influence, field-based studies may be unable to fully capture such morphologic shifts. Scenario-based morphodynamic modeling is emerging as a means of quantifying gravel bed river evolution, yet current models are unable to predict changes in stream morphology over the timescales in question and with adequate spatial resolution, a problem due largely to the computational overhead they require. Since the computational overhead required to drive sediment transport has hindered previous modeling efforts, field-based research suggests a potential improvement, in that sediment is often mobilized downstream with characteristic step-lengths. Here we introduce a morphodynamic model which drives sediment transport using a step-length based approach. Such a technique negates the need for frequent recalculation of sediment dynamics in the flow, and correspondingly reduces computational overhead. Upon application of this model to the River Feshie (UK), we observe that it accurately reproduces many bed morphologies observed during annual high-resolution topographic surveys. By employing step-length based sediment transport distributions, the formation and preservation of bed morphologies can be accurately predicted with less computational overhead than was available in previous morphodynamic models. Using this new model, a better understanding of gravel-bed river morphodynamics over longer-term timescales (decades to centuries) may aid in the management of gravel bed streams under shifting discharge and sediment regimes. +
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Understanding how channel geometry influences dynamic connectivity in river networks is crucial for predicting environmental flux transport. Here, we investigate the impact of channel link-lengths on the dynamic connectivity time (ΔT) that describes difference between time periods when network is connected by structural extent (DCs) and network connectivity based on total flux aggregation (DCT). We show that the observed exponential link-lengths distribution such that link-lengths decreases with increasing stream order impacts the ΔT. Additionally, these hierarchical variation in link lengths is more evident in humid channels compared to dry channels.
We explicitly analyze the role of this hierarchy in determining ΔT by comparing geometrically derived ΔT_geom (using actual link lengths) and with topologically derived ΔT_topo (assuming uniform link lengths), we find that ΔT_geom is consistently lower than ΔT_topo. The difference between ΔT_geom and ΔT_topo is more evident in humid basins compared to dry. Furthermore, when link lengths are randomized, ΔT_geom converges to ΔT_topo, highlighting the dominant influence of channel geometry on connectivity.
Our findings highlight that the hierarchical distribution of link lengths governs dynamic connectivity time, with humid channels exhibiting more efficient connectivity than dry channels and emphasize the significant role of channel geometry in flux aggregation and transport dynamics. +
Understanding how landscapes acquire their form is complicated by evolution across a large range of spatial and temporal scales. Disentangling causes of landscape evolution should carefully consider scale and scalings, but how best to do so? I summarise the work we have been doing to make use of observations, spectral analyses and forward and inverse modelling to address the following questions. Where and at what scales do fluvial landscapes acquire their physical and chemical properties? How can we use the geometries of landscapes and their compositions to recover information about driving and responsive processes? Demonstrations of how observations of landscape form and chemical concentrations can be combined with simple theory to identify where and how landscapes acquire their geometries and material provenance are given. Spectral analyses of landscape geometries are used to identify scales at which they acquire their form and scaling regimes. Consequently, it is possible to assess whether it is reasonable to ‘stitch together’ observations or theory used to understand geomorphic processes operating at small scales to determine how landscapes acquire their form. In short, that proposition is highly unlikely to be successful because of the existence of erosional ‘shockwaves’ and stochasticity. However, despite local (spatio-temporal) complexity, fluvial landscapes appear to possess emergent, deterministic, simplicity at scales > 100 km, such that processes operating at these scales (e.g. dynamic topography) are manifest in drainage networks with simple, self-similar, scalings (e.g. Brownian noise). The use of upscaling of simple physical models to generate appropriate scaling regimes and statistical insights into how fluvial landscapes acquire their form is explored. Finally, a demonstration of how statistical measures based on Optimal Transport theory can be used to identify optimal landscape evolution models is presented. Wasserstein distances are shown to have significant benefits over more widely used Euclidean measures of misfit, especially when local noise is prevalent.
Understanding sediment dynamics during storms and hurricanes is vital for predicting coastal morphodynamics and improving resilience strategies, especially for the Texas–Louisiana coast.
This study presents preliminary results from an integrated hydrodynamic-sediment transport model of Galveston Bay during Hurricane Harvey. To capture the complex interplay of different hydrological forces, the hydrodynamic model incorporates the combined impacts of wind, precipitation, river, wave, tide, and current. A three-dimensional sediment transport model with a 100-m resolution is developed in the Regional Ocean Modeling System (ROMS) for Galveston Bay. The open boundary conditions are generated from ROMS model (100m) and river discharges of Buffalo Bayou and San Jacinto River will be derived from WRF-Hydro model. The bay bottom sediment input parameters are derived from the Texas Sediment Geodatabase (TxSed), which includes a comprehensive inventory of sediment properties, ensuring simulations with an enhanced level of accuracy and regional specificity. For model modification, river discharge data from the United States Geological Survey (USGS) and/or a WRF-Hydro model will be employed to calibrate and adjust the hydrodynamic model. This study will eventually provide open boundaries and initial sediment conditions for a higher resolution (20m) bayou model focusing on Buffalo Bayou and other rivers feeding into Galveston Bay and will contribute to the development of a detailed river-estuary-ocean continuum model. The outcomes of this research are anticipated to inform future coastal management and resilience planning against storm-induced sediment and contaminant fluxes. +
Understanding the factors that control lateral erosion rates in bedrock channels is a frontier in geomorphology. Lateral erosion rates and the evolution of wide bedrock valleys are linked to bedrock lithology, sediment supply in the stream, and shear stress exerted on channel walls. I use a newly-developed lateral erosion component in the Landlab modeling framework to explore how model results compare with recently published field examples of downstream sweep erosion as a mechanism for gorge eradication and bedrock valley widening. The lateral erosion component dictates that lateral erosion rate is proportional to shear stress exerted on the channel walls in a bend in the river; therefore sharp bends with a smaller radius of curvature will produce faster lateral erosion. Cook et al. (2014) identified a similar mechanism they call downstream sweep erosion (DSE). They suggest that bedrock gorges can be rapidly eroded by DSE when a wide flood plain with a laterally mobile stream exists upstream of the gorge, requiring a sharp bend in the channel to enter the gorge. I set up the model domain to recreate conditions of a low relief area with a mobile channel in the upper part of the model domain and a narrow, high relief gorge in the downstream end of the model domain. I ran modeling experiments under a range of water flux and sediment mobility conditions. The model results show gorge widening that propagates downstream as described by Cook et al. (2014) and preferential erosion of blocks that protrude into the channel. The enhanced lateral erosion at channel bends and the resulting downstream sweep erosion emerge naturally from the models, matching observations in many field areas. Together this suggests that channel curvature is of fundamental importance to lateral erosion rates in bedrock channels. +
Understanding the response of coastal barrier systems to sea level rise is a crucial societal need. Despite the problem having been studied extensively, major knowledge gaps remain. For example, neither the sedimentary record nor existing numerical models have been conclusive in explaining the formation of barrier islands. Here I present a comprehensive 2D model that seamlessly couples cross-shore and along-shore transport, tidal transport, storm surges, and wind waves, and use it to simulate an idealized passive margin during the last 7,000 years. In the early Holocene, when sea level was rising ~20 mm/yr, shoals and ephemeral barrier islands formed, periodically drowned, and then formed again at a landward location. Shoal emergence was triggered by the disequilibrium of the recently submerged shelf, especially for large waves and mild shelf slopes. About 5000 years ago, as sea level rise slowed down to ~1 mm/yr, barriers stabilized and even prograded seaward. The combination of excess sediment in the nearshore and storm surges allowed barriers to accrete above mean high water. When barriers eventually equilibrated to the new sea level rise rate and started to retreat, their retreat rate was highly variable in space and time due to autogenic processes such as inlet formation and backbarrier channel interception. This variability also included multi-decadal periods of localized progradation. Both lag dynamics and autogenic processes confound the relationship between barrier retreat and sea level rise rate. +
Understanding the sensitivities of preserved environmental signals to erosional and transport processes within the sediment generation portion of landscapes is vital in constraining uncertainties within provenance analysis. Here our focus is on populations of detrital zircon U-Pb ages as one of the most ubiquitous sediment provenance methods. Many studies often assume uniform parameters upstream of the sampling site, potentially overlooking variations in erosion rates, zircon size and zircon fertility across landscapes. To tease these uncertainties out, we model synthetic landscapes along with their expected provenance evolution.
Employing the Concentration Tracker component, we systematically manipulate erodibility and zircon fertility within synthetic landscapes to simulate sediment provenance evolution and incorporate zircon concentration into a statistical analysis comparing a null hypothesis of an area-dependent contribution to fractions dependent on mass and zircon abundance. While these scenarios do not necessarily reflect “real” landscapes, we use these simple experiments to understand basic controls on the magnitude of potential biases imparted to the simulated detrital zircon U-Pb datasets.
By exploring these potential biases, we provide valuable insights into the uncertainties inherent in provenance analysis and thus their utility and fidelity in reconstructing histories of past landscape evolution. Ultimately, this research contributes to refining the methodologies used in detrital zircon provenance analysis and enriches our understanding of the processes shaping sedimentary records. +
Understanding trends in water table dynamics is critical for closing the global water budget and for water resources management and environmental sustainability. Continental-scale hydrological simulations typically assume that the water table is at steady-state, despite the fact that this is unlikely to be true under changing climate. Here, we present monthly water table fields for the year 2020 across North America based on a simulation using the Water Table Model (WTM). To obtain these, we initialised the WTM using a transiently simulated water table from 500 years before present, and performed a model spin-up to obtain our monthly temporal resolution. The WTM integrates climate variables, topography, and hydrogeological characteristics to simulate depth to the water table, including groundwater and lakes. Our results offer insights into spatial and temporal patterns of water table response to seasonal climatic conditions. Results indicate significant regional variations in water table fluctuations driven by seasonal precipitation and evapotranspiration. This study shows a lag time of approximately 3 to 4 months between measured changes in climate variables and the corresponding response in the water table level. Our study emphasizes the need for targeted, regional management practices to mitigate potential adverse impacts and to optimize water resources under climatic changing conditions. +
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Urbanization and global climate change will severely stress our water resources. One potential unforeseen consequence of these stressors, which is neglected in channel evolution models, is accelerated stream channel erosion due to change in stream water temperature, pH and salinity which affect the surface potential and hence stability of soil colloids. Summer thunderstorms in urban watersheds can increase stream temperature >7 °C and the impact of global warming on average stream temperature is already evident in some stream systems. Initial estimates indicate a 2 °C rise in stream temperature could increase erosion by 30%. Urbanization has significant effects on the pH and salinity of stormwater runoff and as a result on the water quality of headwater streams. Channel erosion and the resulting sediment pollution threaten the sustainability of water resources and urban infrastructure. The goal of this research is to assess the impact of changes in stream water temperature, pH and salinity on stream channel erosion rates and to explore changes in the electrical surface potential of clay colloids as a potential soil stability mechanism. This exploratory research utilizes two reference clays with different permanent surface charges: montmorillonite, and vermiculite. Samples will be eroded in a recirculating sediment flume to determine soil critical shear stress and erodibility. Three water temperatures (12 °C, 20 °C, 27 °C), two pH (5 and 7), and two salinity levels (5 and 50 mg/l NaCl) will be analyzed. Three replicates of each treatment will be conducted for each clay. Additionally, the zeta potential of the clays will be determined under each condition. Research has demonstrated that variations in zeta potential affect liquid limit and shear stress of soil colloids. Results of this research could lead to a reassessment of stream channel stability modelling in urban watersheds and a paradigm shift in urban stormwater management. +
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Variation in bedrock erodibility along a river profile gives rise to differences in vertical incision rate and influences sediment characteristics such as clast lithology, coarse sediment generation rate, and grain size. In rivers whose beds are eroded, in part, through sediment abrasion, these streamwise sediment dynamics are part of a crucial feedback that sets the dominant fluvial erosion process and determines whether a river exhibits transport-limited or detachment-limited behavior. The role that sediment plays in setting the shape of a river profile is of particular interest in the case of a river’s transient response to external forcing.
Here we present a model that explores river profile evolution in a setting with streamwise bedrock variability. Our model combines theory for five interrelated processes: bedload sediment transport in equilibrium gravel-bed channels, channel width adjustment to flow and sediment characteristics, abrasion of bedrock by mobile sediment, plucking of bedrock, and progressive loss of gravel-sized sediment due to grain abrasion. We envision a generic “range-foreland” system that consists of erosion-resistant, crystalline rocks in the upstream reaches, juxtaposed with softer, more erodible rocks downstream. In this setting, coarse sediment generation is confined to the upstream part of the fluvial system. As the sediment is transported downstream, it creates an alluvial blanket across the soft, fine-grained unit. Bedrock erosion is modulated by the thickness of the alluvial layer.
We use the model to explore the range of transient forms that can occur in such a setting in response to changes in tectonic or climatic regime. We pay special attention to the conditions under which the upstream gravel source either increases the downstream fluvial gradient (by partially shielding the underlying material from incision) or decreases the gradient (by providing tools that amplify the efficiency of abrasion). We also examine the conditions under which erosion is concentrated at the downstream-most reaches of the river profile, versus at the lithologic boundary.
While our work takes its motivation from the Southern Rocky Mountains and High Plains of North America, the model is applicable generally to settings in which a bedrock-incising river traverses multiple lithologies. This work aims to improve our interpretations of the history of river profiles in lithologically heterogeneous environments and inform our understanding of landscape evolution in these settings.
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Very few in-situ measurements of runoff from the Greenland Ice Sheet (GrIS) exist, though melt water runoff from the GrIS is important to global eustatic sea level, ocean salinity, thermohaline circulation and sea ice dynamics and the transport of sediment and nutrients to fjords and the ocean. We continue to develop the use of NASA MODIS imagery to gauge river discharge of sediment and freshwater into fjords hydrologically linked to the GrIS.
Essential to this remote sensing proxy are accurate models of fjord and plume dynamics. We compare Hutton and Syvitski’s PLUME model results to in situ oceanographic and sedimentological measurements of Greenlandic river sediment/freshwater plumes towards the end goal of exploring the suitability of inverting the PLUME model and combining it with remotely sensed MODIS imagery to estimate river discharge.
Within our study fjords a range of estuarine conditions present a robust test for our plume method, and in turn conditions present a range of complexities to test the suitability of inverting the PLUME model. Fjord conditions range from ocean to river dominated. Some plumes mix very quickly from fresh to near full ocean salinities (22 – 28 PSU). Other plumes maintain low salinities (0 – 10 PSU) to depths exceeding six meters and down fjord over 65 km. Fjord geometries, tidal range, and other conditions impact sediment plume dynamics. These dynamics must be accounted for to link plume imagery to discharge into fjords. +
WBMsed is a spatially and temporally explicit global riverine model predicting suspended and bedload sediment fluxes based on the WBMplus water balance and transport model (part of the FrAMES biogeochemical modeling framework). The model incorporates climate input forcings to calculate surface and subsurface runoff for each grid cell. The prediction of fluvial sediment fluxes is highly dependent on how well its transport medium, riverine water, is simulated. Our analyses indicate that average water discharges are well predicted by the WBMplus model. However, daily freshwater predictions are often over or under predicted by up to an order of magnitude, significantly affecting the accuracy of sediment flux simulation capabilities of WBMsed and indicating that certain hydrological processes are less captured within the model. One of these processes could be temporal storage of water discharge on floodplains, dampening the water hydrograph significantly. In WBMsedv2.0 we incorporate a floodplain reservoir component to improve daily water discharge simulations. The Floodplain reservoir component is used in WBMsedv2.0 to store overbank water flow which are refurbished back to the river once its water level has subsided. Here we compare two methods for determining overbank flow: (1) the log-Pearson III (flood frequency analysis) 5-year maximum discharge recurrence and (2) an empirical relationship between mean river discharge and river width and bank height. +
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Wave Boundary Layer (WBL) plays an important role in sediment offshore transport and material exchange between seafloor and overlying water, especially during strong wave events when fluid mud (concentration > 10g/L) is formed. We incorporated wave-supported fluid mud (WSFM) processes into the Community Sediment Model System (CSTMS) on the platform of the Coupled Ocean-Atmosphere-Wave-and-Sediment Transport Modeling system (COAWST). A new WBL was introduced between the bottom sigma layer (water) and top sediment bed layer, which accounted for the key sediment exchange processes (e.g., resuspension, vertical settling, diffusion, and horizontal advection) at the water-WBL and WBL-sediment bed boundaries. To test its robustness, we adapted the updated model (CSTMS+WBL) to the Atchafalaya Shelf in the northern Gulf of Mexico and successfully reproduced the sediment dynamics in March 2008, during which active WSFM processes were reported. The CSTMS+WBL model simulated a lutocline between the WBL and overlaid water as well as a stronger onshore/offshore erosion/deposition. Sensitivity tests of free settling, flocculation and hindered settling effects suggested sediments were transported further offshore due to reduced settling velocity in the WBL once fluid mud was formed. +
Wave- and current-supported turbidity currents are new class of turbidity flows that has been discovered over the last three decades. Its significance as a carrying agent of fine sediments over low-gradient shelves has been recognized with growing evidence. Due to their vertical length scales, which are on the order of decimeters, understanding the full range of mechanisms that are responsible for and/or affect these currents cannot proceed without turbulence-resolving numerical simulations and/or high-resolution sensor deployment in a laboratory/field experiments. In this talk the culmination of two-phase, turbulence-resolving simulations, i.e. Direct Numerical Simulations (DNS), of wave- and alongshore current-supported fine sediment turbidity currents across mild bathymetric slopes will be presented. Simulation results show that such turbidity currents follow a logarithmic velocity profile across the shelf whose parameters depend on the sediment concentration, across-shore bathymetric slope, and Reynolds number while it is independent of the settling velocity of the sediments. The numerical simulations also provide significant insights on modelling these turbidities in a regional-scale model which can be used to estimate the location of mud depocenters and the dynamics of submarine geomorphology such as in the clinoform development at the continental margin. +
Wave-resolving, Boussinesq nearshore wave models such as FUNWAVE-TVD are capable of providing nonlinear hydrodynamic outputs that wave-averaged models cannot directly provide. Understanding such nonlinear nearshore processes is crucial to deepen our understanding of complex coastal processes, such as morphodynamics and sediment transport. However, the computational cost of wave-resolving models has made them prohibitive to use for many such applications.
To bridge this gap, a machine learning model trained on thousands of FUNWAVE-TVD models using synthetic, experimental, and field data was developed to estimate nonlinear nearshore wave statistics. Given boundary conditions and forcing terms, the model can “learn” the statistics associated with nonlinear nearshore processes. Such a model is broadly useful for other coastal models that rely on accurate measures of these nonlinear wave properties to parameterize other processes of interest. +
