H43D-0516
The relation between 20th century dune migration and wetland formation at Cape Cod National Sea Shore, MA
Approximately 1,800 ha of outer Cape Cod is dominated by active and stabilizing parabolic and transverse dunes, interspersed with numerous interdunal wetlands that are centers of biologic diversity. A time series of mosaiced and georeferenced aerial photographs for 1938, 1960, 1977, 1986,2001 and 2003 were analyzed to track individual dune movement and subsequent wetland propagation and expansion. This time series analysis indicates that parabolic dunes have migrated 150 to 250 m since 1938, with >60% of the movement between 1938 and 1977. There is noticeable stabilization of dunes in the 1980s and 1990s, with renewed movement in the 21st century. Wetlands expand following dune movement, particularly in the 1950s to the 1980s. The enlargement and formation of blowouts lowers surfaces that intersect, at least seasonally, with the water table. In turn, dune movement increases the catchment area for enhanced focusing of overland and through flow. A surprising conclusion is that the majority of wetlands are very young, <200 years old, which has been confirmed by optically stimulated dating of subwetland eolian quartz grains. The dynamic and coupled Cape Cod wetland and dune system may primarily reflect landscape non-equilibrium response to human disturbance post the 18th century.
H43D-0517
GroundwaterGeochemistry,SeasonalChangesandControlsonEolianMorphology.WhiteSandsNationalMonument,NewMexi co,TX
The White Sands of New Mexico is composed of transverse, barchan and parabolic dunes formed of gypsum sand. Transverse dunes pass downwind into barchans and then vegetated parabolic dunes. This study focuses on the transition from barchan dunes that migrate rapidly (12 m/year) through an unvegetated landscape, to isolated parabolic dunes migrating at 1 to 2 m/year through vegetated, stabilized dunes and sand sheets. One theory is the vegetated parabolic dunes form where enough sand has been deposited above the permanent saline water table to allow the formation of a fresh water lens. Conversely, where there is enough vegetation, due to the permanent and highly saline groundwater table, the sand blows away as barchan dunes, deflating the dune field. In a 5 km long swath through the dunes, six sampling sites were established: two barchan, two parabolic and two in the transition zone between the two dune types. Groundwater as well as soil samples were collected to measure the seasonal variations on water salinity and of sand movement in the dune field. The water table is less than 1.5 m at all sites. During successive sampling during the first half of 2005, at the barchan sites, the water is three times more saline than the parabolic sites at (20,000 g/ml vs. 3,000-8000 g/ml). Sampling each two months from early December through June suggests the influx of fresh water into the basin has caused a salinity decrease in the soil water in both the transition and parabolic sites during the spring and summer from about 2,000-3,000 mg/L to about 1,000 mg/L). March results show an overall decrease in salinity readings although in May the results are more varied, showing some sites high and some showing low salinity readings but mostly ranging about 1,000 mg/L. The parabolic dunes specifically show an increase, in salinity from 1,000 mg/L in December to 8,000 mg/L in May. During the winter, thin zones of high salinity, or spikes, are found between 40 and 80 cm deep in the soil column in the parabolic dune area. These may correlate with very fine grained layers (mean 5 microns) in the soil. In the Transition sites, salinity decreases from December to May but hovers around 10,000 mg/L. The Barchan sites have relatively low salinity readings above the groundwater, whereas the groundwater is very saline. The values at the barchans sites remained relatively the same throughout the six months at around 20,000 mg/L. TDS (total dissolved solids) decreases from 7 g/L in the barchan to 2g/L in the parabolic dunes. In the barchan area, the amount of DO (dissolved oxygen) is also greater. It is presumed the vegetation and the associated ecology withdraw soil water, creating higher salinities in the soil column. These layers may have formed through precipitation from groundwater or through soil alteration of the eolian sand. The dramatic changes in groundwater salinity suggest dune fields can be shaped by dynamic feedback between eolian dynamics, vegetation and groundwater chemistry.
H43D-0518
The control of salt flat topography by water-mediated halite transport - A case study at the salar de Uyuni, Bolivia
The salar de Uyuni is a massive salt flat in the Bolivian Altiplano that is periodically inundated by a shallow lake during the rainy season and whose water table never drops far beneath the salt surface. Using available gravity data, we have confirmed that the entire salar surface lies within several centimeters of the equipotential surface to which the water table conforms. This close correlation appears to be due to the control of salar topography by the water-mediated salt transport. The salar surface is almost pure halite (sodium chloride), which forms a tough crust that resists mechanical erosion by wind. Halite is readily transported in aqueous solution, however, which means that water plays a critical role in the evolution of the salar surface. Dissolved salt is transported by the flow of surface and subsurface water until it is precipitated by evaporation, with complicating effects due to spatial and temporal variation in dissolution rates, saturation state, evaporation, and wind and Coriolis forcing on overland flow. We separately consider the effect of salt transport for groundwater, surface runoff and rainwater, focusing most of our attention on direct precipitation because of its large associated water flux. We develop a coupled overland-flow/salt-transport model to show how the action of rain alone is sufficient to preserve the equipotential shape of the salar surface despite constant non-equilibrium forcing due to tectonics and isostatic rebound.
H43D-0519
Two dimensional hydrological simulation in elastic swelling/shrinking peat soils
Peatlands respond to natural hydrologic cycles of precipitation and evapotranspiration with reversible deformations due to variations of water content in both the unsaturated and saturated zone. This phenomenon results in short-term vertical displacements of the soil surface that superimpose to the irreversible long-term subsidence naturally occurring in drained cropped peatlands because of bio-oxidation of the organic matter. The yearly sinking rates due to the irreversible process are usually comparable with the short-term deformation (swelling/shrinkage) and the latter must be evaluated to achieve a thorough understanding of the whole phenomenon. A mathematical model describing swelling/shrinkage dynamics in peat soils under unsaturated conditions has been derived from simple physical considerations, and validated by comparison with laboratory shrinkage data. The two-parameter model relates together the void and moisture ratios of the soil. This approach is implemented in a subsurface flow model describing variably saturated porous media flow (Richards' equation), by means of an appropriate modification of the general storage term. The contribution of the saturated zone to total deformation is considered by using information from the elastic storage coefficient. Simulations have been carried out for a drained cropped peatland south of the Venice Lagoon (Italy), for which a large data set of hydrological and deformation measurements has been collected since the end of 2001. The considered domain is representative of a field section bounded by ditches, subject to rainfall and evapotranspiration. The comparison between simulated and measured quantities demonstrates the capability of the model to accurately reproduce both the hydrological and deformation dynamics of peat, with values of the relevant parameters that are in good agreement with the literature.
H43D-0520
Modeling Soil Salinity Distribution Along An Elevational Gradient In Tidal Salt Marshes In Atlantic And Gulf Coastal Regions
Soil pore water salinity plays a very important role in determining the distribution of vegetation, plant productivity and biogeochemical processes in estuarine ecosystems. Pore water salinity gradients and salinity-vegetation associations in salt marshes have often been observed but rarely explained. A quantitative and systematic study on the pore water salinity distribution in salt marshes is not only critical to the understanding of the phenomenon itself but also to the use of the phenomenon as a convenient ecological and environmental change indicator. In this research, we developed a salt marsh pore water salinity model based on a salt and water balance model with modifications to several key features (e.g., applying the Penman-Monteith equation to calculate ET for different climate zones) to examine the impacts of climate, tidal forcing, soil, vegetation, and topography on pore water salinity distribution along elevation in the Atlantic and Gulf coastal regions. This model was calibrated and validated using field observations from the St. Marks National Wildlife Refuge (NWR) of northwestern Florida, USA. The results showed that the model had good agreement (r2=0.84, n=15, P<0.001) with field observations. We found that the mean higher high water (MHHW) level determines the location of the salinity maximum along an elevational gradient, and the salinity maximum most likely occurs at an elevation approximately 25 cm above MHHW. Simulations indicate that tidal irregularity (defined as the standard deviation of tides in this study area) primarily controls the width of the salinity variation band (i.e., elevation range with soil salinity dramatically > incoming tidal salinity) along elevation. A standard deviation increase of 10 cm in tidal heights could result in an increase in salinity variation band by approximately 40 cm (mostly seaward). Moreover, ET, temperature, hydraulic conductivity, and incoming tidal salinity are the dominant factors determining the magnitude of the salinity maximum, which may lead to the occurrence of salt barrens/flats when reaching a threshold level (e.g., >70 ppt). Our analyses are important to understanding the effects of climate change and sea-level rise on the productivity and biogeochemical processes of salt marsh ecosystems by monitoring soil pore water salinity, an effective environmental indicator, over a salt marsh elevational gradient. Key words: Pore water salinity, Tide, Salt marsh, Elevational gradient, Model simulation, Atlantic and Gulf coasts
H43D-0521
A Probabilistic Model of Rainfall-triggered Shallow Landslides in Hollows: Long-term Analysis and Dependence on Hyetograph Characteristics
The long-term temporal evolution of soil thickness in hollows depends on the processes controlling the rates of colluvium accumulation and erosion. Accumulation is due to soil creep and mass-wasting processes from the adjacent slopes, while erosion of colluvial deposits is mainly due to debris flow and landsliding. An analysis of the long-term evolution of colluvial deposits is developed through a stochastic model of soil mass balance at a point accounting for colluvium infilling, expressed as a deterministic function of the deposit thickness, and soil erosion by shallow landslides, modeled as a random (Poisson) process. Landsliding is related to the characteristics of the triggering precipitation through an infinite-slope stability analysis, a kinematic model of hollow response to rainfall, and the intensity-duration-frequency curves characterizing the regime of extreme precipitation. This analysis provides a probabilistic representation of the long-term dynamics at a point of colluvium thickness as a function of the timescale of hollow infilling and of the frequency of triggering rainfalls. The model is solved both numerically and (under simplified conditions) analytically, showing the existence of different regimes in the temporal evolution of soil thickness. The effects of hyetograph shape on the potential for landsliding are also analyzed. An existing pore pressure response model is used to study the effects of unsteady rainfall infiltration in hillslopes and is coupled with simple hyetograph models and to intensity-duration-frequency functions to determine the return period of landslide-triggering rainfall. Results show that hyetographs with a peak at the end of a rainfall event have a stronger destabilizing effect than hyetographs with a constant rainfall or with a peak at the beginning of a storm. Thus the variability of hyetograph shapes adds uncertainty to the assessment of landsliding triggered by rainfall.
H43D-0522
Long-Term Interactions of Streamflow Generation and River Basin Morphology
It is well known that the spatial patterns and dynamics of streamflow generation processes depend on river basin topography, but the impact of streamflow generation processes on the long-term evolution of river basins has not drawn as much attention. Fluvial erosion processes are driven by streamflow, which can be produced by Horton runoff, Dunne runoff, and groundwater discharge. In this analysis, we hypothesize that the dominant streamflow generation process in a basin affects the spatial patterns of fluvial erosion and that the nature of these patterns changes for storm events with differing return periods. Furthermore, we hypothesize that differences in the erosion patterns modify the topography over the long term in a way that promotes and/or inhibits the other streamflow generation mechanisms. In order to test these hypotheses, a detailed hydrologic model is imbedded into an existing landscape evolution model. Precipitation events are simulated with a Poisson process and have random intensities and durations. The precipitation is partitioned between Horton runoff and infiltration to groundwater using a specified infiltration capacity. Groundwater flow is described by a two-dimensional Dupuit equation for a homogeneous, isotropic, unconfined aquifer with an irregular underlying impervious layer. Dunne runoff occurs when precipitation falls on locations where the water table reaches the land surface. The combined hydrologic/geomorphic model is applied to the WE-38 basin, an experimental watershed in Pennsylvania that has substantial available hydrologic data. First, the hydrologic model is calibrated to reproduce the observed streamflow for 1990 using the observed rainfall as the input. Then, the relative roles of Horton runoff, Dunne runoff, and groundwater discharge are controlled by varying the infiltration capacity of the soil. For each infiltration capacity, the hydrologic and geomorphic behavior of the current topography is analyzed and the long-term evolution of the basin is simulated. The results indicate that the topography can be divided into three types of locations (unsaturated, saturated, and intermittently saturated) which control the patterns of streamflow generation for events with different return periods. The results also indicate that the streamflow generation processes can produce different geomorphic effective events at upstream and downstream locations. The model also suggests that a topography dominated by groundwater discharge evolves over a long period of time to a shape that tends to inhibit the development of saturated areas and Dunne runoff.
H43D-0523
Impact of Extreme Events and Soil Hydraulic Conductivity on the Evolution of a Mesa-top Waste Repository Cover
The Siberia model was used to optimize the design of a mesa-top waste repository cover at Los Alamos National Laboratory on the Pajarito Plateau in Northern New Mexico, USA. The cover was designed to meet criteria that the depth to waste from the cover surface would be greater than 1 meter after 1000 years of erosion. The model was run using two steady-state landscape forming events (2 and 5 year return periods) derived from a 20 year data set at the Santa Rita Experimental Watershed in Arizona, and hydraulic properties of two soils, loam and sandy loam. Although we were able to show that the final design cover met the performance criteria for both high and moderate erosion scenarios, concerns remained about the impact of extreme events. In addition, Hydrus simulations, based on saturated hydraulic conductivity (Ksat) measurements from cores of cover material comprised of crushed tuff and a bentonite admixture, suggested that surface runoff on the cover might be orders of magnitude higher than the landscape forming runoff events used for the Siberia simulations. The Siberia runoff events were based on Ksat values for loam and sandy loam soils with identical texture (% sand , silt and clay) to the engineered cover soil, but these values assume soil structure that may or may not develop in the engineered cover. This work summarizes the impacts of both 1) the timing and size of extreme events and 2) the impact of soil structure and Ksat, on long-term repository cover evolution.
H43D-0524
Normal and Anomalous Dispersion in Fluvial Sediment Transport
Understanding the rate of motion and pattern of dispersion in fluvial sediment transport is essential for a variety of applications, including predicting the fate and transport of solid-phase contaminants and modeling the cosmogenic-Nuclide inheritance of water-borne sediment. In order to create a probabilistic model of sediment particle motion, it is necessary to characterize the statistical properties of fluvial sediment dispersion. In general, two modes of behavior have been observed in advective-diffusive transport systems: normal and anomalous dispersion. Normal dispersion is characterized by a well-defined mean position and spatial variance and the time evolution of particle concentration is described by a simple advection-diffusion equation. In contrast, a transport system that exhibits anomalous dispersion will tend to have a heavy-tailed spatial distribution, a mean position that is different from the peak concentration, and a large variance. The fundamental difference lies in the probability distribution of individual particle velocities. When the distribution is sufficiently heavy-tailed, the resulting dispersion pattern will be anomalous. Anomalous dispersion has been observed in geophysical systems ranging from turbulent flow to transport in heterogeneous porous media. Several lines of evidence from the sediment transport literature suggest that fluvial sediment may undergo anomalous dispersion. Tracer experiments show a preference for right-skewed travel distance distributions, a characteristic of anomalous diffusion. Studies suggest that large inputs of sediment to rivers (such as a landslide) tend to disperse in place rather than translate downstream. In addition, the fact that sediment grains can become trapped in flood plains and bars for long periods of time and then move long distances in rare, short duration events such as floods suggests a potential for anomalous dispersion due to a broad distribution of particle residence times. We develop a random-walk model for bedload and suspended-load motion. Particles alternate between entrainment and deposition and the transition between these states is governed by a probability distribution of instantaneous shear stress and an entrainment threshold. Once a particle is entrained, its motion is governed by probability distributions describing turbulent velocity fluctuations. This model provides a framework to explore the conditions under which rivers are likely to exhibit anomalous sediment dispersion.
H43D-0525
Typhoons, Extreme Discharge, and Bedrock River Meanders: a Quantitative Relationship Between Regional Climatology and DEM-Derived Sinuosity
It is notoriously difficult to extract from landscapes any \textit{quantitatively} meaningful signatures of climate or tectonics. While some topographic studies have been able to infer basic tectonic boundary conditions, they have shown little success so far in inferring the values of key climatic variables that shape the relief. We set ourselves the challenge of doing just that, and we chose as our focus the sinuosity of mountain rivers as measured using the SRTM digital elevation model. Bedrock rivers adopt a sinuous, sometimes meandering form, just as do alluvial rivers, and this sinuosity is achieved through both lateral and vertical erosion. Measurement of DEM-derived mountain river sinuosity, together with analysis of discharge statistics, could therefore provide constraints on channel incision processes. Our analysis of mountain catchments across the western North Pacific cyclone basin has revealed that sinuosity is not controlled by \textit{mean} flow conditions. Rather, for geologically similar terrain, sinuosity appears to relate directly to the shape of the discharge distribution. Rivers that have more relative variance in discharge tend to have greater lateral mobility. Discharge variability is a function of rainfall variability, which is dominated in our study area by the climatology of tropical cyclones. We conclude that mountain river sinuosity in the western North Pacific is controlled by typhoon strike frequency.
H43D-0526
A New Method for Computing Flow Paths and Contributing Areas Over Both Convergent and Divergent Topography
Contributing area grids play a central role in spatial hydrologic modeling and are required for landscape evolution models. A variety of algorithms have been introduced over the last twenty years for the grid-based flow routing and computation of watershed contributing area. The first was the well-known D8 method, which permits flow to only one neighborpixel, and while still useful for certain purposes it does a very poor job on divergent hillslopes. Several different multiple flow direction algorithms were proposed in the nineties, including the D-Infinity method which partitions flow between two neighbor pixels using a slope-based rule, and other methods which use slope-based rules to partition flow between more than two neighbors. A difficulty with these slope-based rules is that they are merely intuitive and do not follow from a conservation law or any other physical principle. Another method called DEMON was proposed that allows flow to at most two neighbor pixels but which attempted to avoid arbitrary slope-based partitioning rules. Unfortunately, this algorithm was incomplete and could not handle all of the different scenarios that occur in real DEMs, particularly in the vicinity of drainage divides. As a result, it appears that it has never been used in an operational capacity. The author will introduce a new method that is similar in some respects to the DEMON method but that provides an improved solution to this important problem. This new ''mass flux" method is based on mass conservation and uses quarter-pixels to avoid various ambiguities in the assignment of continuous flow angles near peaks and divides. Results for test surfaces and the complex topography near Mount Sopris, Colorado will be shown to be superior to results from both the D8 and D-Infinity methods.
H43D-0527
Impacts of landscape evolution on hydrology: headwater channels in deglaciated terrain.
We examined nine small headwater channels originating within three vegetation communities on different landforms: sloping bogs, forested wetlands, and upland forests, to quantify relationships between stream type and flow characteristics. Discharge was measured with flumes instrumented with pressure transducers. Average cross-sectional area of the bankfull flow channel was estimated from cross-sections made above and below flumes. Stream hydrographs for forested wetlands and uplands were similar with rapid response to rainfall and rapid return to baseflow. Bog hydrographs were damped relative to the other stream types, presumably due to differences in soil hydrologic conductivity, groundwater input, and slope gradient. Preliminary results suggest that most of our study channels have not acquired their present shape by bankfull flows associated with recent floods. The morphology of some channels may be attributed to debris flow occurrence subsequent to glacial retreat. Other headwater channels, lacking adequate channel incision, overtop banks frequently, and thus, appear to have been formed recently. This work supports concurrent work aimed at calculation of chemical flux rates within a range of wetland and non-wetland settings.
H43D-0528
Insight on watershed development along the actively uplifting Mount Lebanon range (Lebanon) from marine and fluvial terraces
Active uplift in the Mt. Lebanon range results from regional transpression along a ~200-km-long restraining bend within the Dead Sea fault system. Thus, the resultant landscape is characterized by the combined influences of tectonic, eustatic, and climatic controls. Marine terraces in northern Mt. Lebanon range provide significant constraints on regional uplift and, consequently, base level control on watershed development. Detailed geologic mapping reveals at least six coastal terrace levels between the cities of Tripoli and Batroun in northern Lebanon, ranging in elevation from 5 m to 113 m above sea level. The marine terraces are primarily abrasional platforms with little to no sediment cover. However, at certain locations, the terraces comprise of a thick (up to 20 m towards the coast) sedimentary cover that are the result of episodic periods of cut and fill into older Pliocene deposits. The majority of these sediments are well-rounded, cobble-size clasts of limestone cemented by a calcite matrix with occasional clasts of basalt and marine fossils. Travertine formations, fossil remnants, and limestone clasts are available to constrain ages on terrace formations and, in turn, coastal uplift rates. Correlation of terrace heights with Pleistocene sea level variations suggests an average, regional uplift rate of 0.3 m/ka. Fluvial terraces in the northern Mt. Lebanon allow reconstruction of longitudinal profiles that grade into base levels represented by the corresponding marine terraces. Hence, this correlation constrains the ages of fluvial terraces and consequently permits estimates of fluvial erosion. Temporal variations in fluvial transport capacity are suggested by episodic aggradation of massive boulder-size clasts of basalt and dolomite that originate over 20 km upstream. Furthermore, knickpoints in the present-day drainage also appear to correlate with the former base levels. Hence, the retreat of these knickpoints permits assessing the lag time in the response of the fluvial system to base level changes.
H43D-0529
Co-evolution of plants and topography - a modeling approach
Topography controls much of the soil moisture distribution over a landscape. Vegetation depends on the soil moisture. Hence topography and vegetation are tightly coupled. Vegetation also controls geomorphic and hydrologic processes. Plant cover protects the surface against erosion, reducing its erodibility. Vegetation cover also increases soil's infiltration capacity and thus reduces overland flow, but on the other hand, it also may create heterogeneities in the surface, patches, that may result in differential erosion of the surface and can result in an augmented channelization of the flow along bare surfaces. Here we present the preliminary findings of exploring the multidimensional interaction between vegetation, hydrology and erosion in a 3D landscape evolution model where climate, topography, soil moisture and plants interact and reshape the landscape through time.
H43D-0530
Coupling Between Hydrogeology And Progressive Failure Of Mountainous Rock Slopes: Field And Modelling Results From La Clapiere Valley (Southern Alps, France)
Hydromechanical effects of water flow within fractures are predominant effects that induce strength decrease of rock slopes and progressive failure propagation. A multi-parametric approach was conducted on the 70 km long La Clapière valley (Southern French Alps) consisting in mapping geology, hydrogeology and gravitational features, dating gravitational scarps and in monitoring slope springs yields, water chemistry and slope deformation for more than 10 years. First a hydromechanical model of rock slope behaviour was established and compared to bibliography. Second, taking this model as a reference, relationships between slope failure and hydrogeology were parametrically investigated using the two-dimensional distinct element method program UDEC. Rock slopes general structure consists in a superficial weathered zone (a few hundreds of meters thick) overlapping a deep intact zone. Penetrative discontinuities cut both zones. In the weathered zone tensile cracks scatter from the middle to the top of the slope. Large landslides are located at the slope foot. A perched saturated-with-water zone nested within the cracks is drained towards the slope foot through the landslides. Annual precipitation infiltrations induce hydromechanical effects that participate to the rock strength decrease through tilting and diffuse shear plane development. Such progressive failure propagation lasts over thousands of years (10 000 years in the studied area). Numerical study shows that, at the early stage of slope alteration, hydrostatic pressures are concentrated in tensile features of the upper part of the slope for moderate infiltration yield (mean inter-annual value). Pressure increase induces fracture shear dilation and traction opening that in reverse modifies flow paths and pressure. Consequence is tilting of rock columns with progressive diffuse failure at the columns' foot. A thick high porosity and high permeability weathered layer (up to hundreds meters thick) is generated between the theoretical bottom of the perched aquifer and the slope surface. When the strength of the weathered layer is low enough, state of principal stresses is very low and the entire layer is in tensile stress. Because rock porosity and permeability became very high, interstitial pressure variations are low. To the contrary, the saturated volume of rock is very high and seasonal precipitation recharge of the perched aquifer induces large density variations. At this late evolution stage, rock remains un-failed only in sparse volumes that are located at intersections between major discontinuous surfaces and even moderate recharge can induce failure of those residual volumes and large landslides triggering.