Margin lateral erosion is arguably the main mechanism leading to marsh loss in estuaries and lagoons worldwide. Our understanding of the mechanisms controlling marsh edge erosion is currently quite ...limited and current predictive models rely on empirical laws with limited general applicability. We propose here a simple theoretical treatment of the problem based on dimensional analysis. The identification of the variables controlling the problem and the application of Buckingham's theorem show, purely on dimensional grounds, that the rate of edge erosion and the incident wave power density are linearly related. The predictive ability of the derived relationship is then evaluated, positively, using new long‐term observations from the Venice lagoon (Italy) and by re‐interpreting data available in previous literature.
Key Points
Marsh edge erosion rate is a linear function of wave power density
Marsh edge erosion rate is a function of marsh cliff height
Traditional, power‐law, formulas are inconsistent with theory and observations
The fluid dynamics of stony debris flows generated in two small tributaries adjacent to each other and flowing into a main receiving channel was analyzed experimentally at a laboratory scale. The ...analysis on the propagation along the tributaries and deposition in the main channel provide information about sediment‐water mobility, dangerous damming, and potential hazard. Debris flows were generated by releasing a preset water discharge over an erodible layer of saturated gravels material. As a consequence, the debris flow sediment concentration varied accordingly to the entrainment rate which, in turn, was strongly controlled by the tributary slope. The data collected by acoustic level sensors, pore fluid pressure transducers, and a load cell were used to characterize the evolution of bulk density and solid concentration of the sediment‐water mixture. These two parameters were relevant to assess the stony debris flow mobility which contributes to determine the shape of sediment deposits in the main channel. The detailed bed topography surveys carried out in the main channel at the end of each experiment provided information on the morphology of these deposits and on the interplay of adjacent confluences. The influences of confluence angle, tributary slopes, and triggering conditions have been investigated, for a total of 18 different configurations. Within the investigated range of parameters, the slope angle was the parameter that mainly influences the stony debris flow mobility while, for adjacent confluences, the degree of obstruction within the receiving channel was strongly influenced by the triggering scenario.
Key Points:
Stony debris flow dynamics in confluence setting
Slope angle and triggering scenario control, respectively, mobility and morphology
Main channel morphology at confluences
In dolomitic headwater catchments, intense rainstorms of short duration produce runoff discharges that often trigger debris flows on the scree slopes at the base of rock cliffs. In order to measure ...these discharges, we placed a measuring facility at the outlet (elevation 1770 m a.s.l.) of a small, rocky headwater catchment (area ∼0.032 km2, average slope ∼320%) located in the Venetian Dolomites (North Eastern Italian Alps). The facility consists of an approximately rectangular basin, ending with a sharp‐crested weir. Six runoff events were recorded in the period 2011–2014, providing a unique opportunity for characterizing the hydrological response of the catchment. The measured hydrographs display impulsive shapes, with an abrupt raise up to the peak, followed by a rapidly decreasing tail, until a nearly constant plateau is eventually reached. This behavior can be simulated by means of a distributed hydrological model if the excess rainfall is determined accurately. We show that using the Soil Conservation Service Curve‐Number (SCS‐CN) method and assuming a constant routing velocity invariably results in an underestimated peak flow and a delayed peak time. A satisfactory prediction of the impulsive hydrograph shape, including peak value and timing, is obtained only by combining the SCS‐CN procedure with a simplified version of the Horton equation, and simulating runoff routing along the channel network through a matched diffusivity kinematic wave model. The robustness of the proposed methodology is tested through a comparison between simulated and observed timings of runoff or debris flow occurrence in two neighboring alpine basins.
Key Points:
Hydrological response to convective rainfalls in headwater rocky catchments is impulsive and mainly ruled by excess infiltration
Combining SCS‐CN method for estimating excess rainfall and constant velocity for routing, the observed hydrographs cannot be reproduced
Observed peaked hydrographs need a simplified Hortonian equation and the Muskingum‐Cunge scheme along network to be simulated
Deterioration of salt marshes may be due to several factors related to increased anthropic pressure, sea level rise, and erosive processes. While salt marshes can reach equilibrium in the vertical ...direction, adapting to sea level rise, they are inherently unstable in the horizontal direction. Marsh boundaries are characterized by scarps with bare sediment below the vegetated surface layer that can be easily removed by wave‐induced erosion. In this work, we explore the different mechanisms involved in the erosion of marsh borders through the interpretation of field data. The analysis is based on a systematic field monitoring of a salt marsh in the Venice Lagoon subject to lateral erosion. Measurements included horizontal retreat of the scarp at various locations and wave height in front of the marsh during three storm surges. Continuous erosion and mass failures alternated during the observed period, leading to an average retreat up to 80 cm/yr. The data, collected roughly every month for 1.5 year, indicate that the linear relation that links the observed erosion rate to the impinging wave power exhibits a larger slope than that already estimated in literature on the basis of long‐term surveys. Moreover, an increase in the gradient of erodibility is detected along the marsh scarp, due to the combined action of soil strengthening by vegetation on the marsh surface and the impact of wave breaking at the bank toe, which promote cantilever failures and increase the lateral erosion rate.
Key Points:
Lateral erosion and wave height at a salt marsh from field measurements
The toe of the bank is more erodible than the top, promoting mass failures
A less erodible vegetated bank top may lead to relatively higher lateral retreat
In this paper we present the results of a first series of laboratory experiments carried out in a large experimental apparatus, aimed at reproducing a typical lagoonal environment subject to tidal ...forcings. The experiments were designed in order to improve our understanding of the main processes governing tidal network initiation and its progressive morphodynamic evolution. During the experiments we observed the growth and development of tidal networks and analyzed their most relevant geomorphic features, taking into account the role played by the characteristics of the tidal forcing in driving the development of channeled patterns. The synthetic networks displayed geomorphic features which compare favorably with those of actual networks, showing that our experimental framework proves useful for analyzing the processes governing the formation and evolution of tidal channel networks. In particular, the synthetic networks develop via headward growth driven by the exceedance of a critical bottom shear stress, and display width-to-depth ratios and seaward exponential widening in accordance with observational evidence. Furthermore experimental networks reproduce statistical network characteristics of geomorphic relevance, such as the exponential probability distribution of unchanneled path lengths.
A two‐dimensional numerical model is used to study tidal hydrodynamics and distribution of bed shear stresses in the Fly River delta, Papua New Guinea. The model describes the propagation of the ...tidal wave within the delta and along the river. Model results indicate that tidal discharge at the mouths of the distributary channels is between 10 and 30 times larger than the river discharge, and that the upstream part of the delta is flood‐dominated, whereas near the mouth, the delta is ebb‐dominated. Numerical simulations allow us to investigate the sensitivity of fluxes and bottom stresses with respect to the variations of sea level and the area of delta islands. The results suggest that a decrease in the total area of the delta occupied by islands increases the tidal prism and, therefore, the bed shear stresses. Similarly, an increase in sea level reduces the dissipation of the tidal signal and speeds up the propagation of the tidal wave within the delta, thus yielding higher discharges and increased bed shear stresses.
On funneling of tidal channels Lanzoni, S.; D'Alpaos, A.
Journal of geophysical research. Earth surface,
03/2015, Volume:
120, Issue:
3
Journal Article
Peer reviewed
Open access
Tidal channels dissect the tidal landscape and exert a crucial control on the morphodynamic evolution of these landscapes. Improving our understanding of channel equilibrium morphology is therefore ...an important issue for both theoretical and practical reasons. We analyze the case of a tidal channel dissecting a relatively short, unvegetated tidal flat characterized by microtidal conditions and a negligible external sediment supply. The three‐dimensional equilibrium configuration of the channel is determined on the basis of a hydrodynamic model, describing the cross‐sectional distribution of the longitudinal bed shear stresses, coupled with a morphodynamic model retaining the description of the main physical processes shaping the channel and the adjacent intertidal platform. Both channel bed and width are allowed to adapt to the flow field so that an equilibrium altimetric and planimetric configuration is eventually obtained, when erosion becomes negligibly small, and asymptotically constant elevations are reached everywhere within the domain. Model results reproduce several observed channel characteristics of geomorphic relevance, such as the relationship between channel cross‐sectional area and the flowing tidal prism, the scaling of the width‐to‐depth ratio with channel width, and the longitudinal distributions of bed elevations and channel widths. In analogy with empirical evidence from estuaries, tidal channel funneling is usually assumed to be described by an exponential trend. Our theoretical analyses, modeling results, and observational evidence suggest that a linear relationship also provides a good approximation to describe longitudinal variations in channel width for short tidal channels. Longitudinal bed profiles characterized by a strong planform funneling tend to attain an upward concavity, whereas a low degree of convergence implies an almost linear profile. Finally, the model allows one to analyze the influence of environmental conditions (sediment characteristics, basin size, tidal amplitude, etc.) on the geomorphological features of tidal channels (equilibrium cross‐sectional area and bottom profile, width‐to‐depth ratios, and planform shape). Wider and deeper channels develop as the width of the domain increases, as the tidal amplitude increases, or as the mean platform elevation decreases. Conversely, narrower and shallower channels result from an increase in the critical shear stress for erosion or a decrease in the flow conductance. We thus believe that this model provides a useful tool for quantitative analyses of long‐term morphodynamics of tidal landscapes.
Key Points
A morphodynamic model for the 3‐D equilibrium morphology of tidal channels
A linear landward decrease in channel width can describe channel funneling
Model provides a useful tool for quantitative long‐term tidal morphodynamics
How long are tidal channels? SEMINARA, G.; LANZONI, S.; TAMBRONI, N. ...
Journal of fluid mechanics,
01/2010, Volume:
643
Journal Article
Peer reviewed
Do tidal channels have a characteristic length? Given the sediment characteristics, the inlet conditions and the degree of channel convergence, can we predict it? And how is this length affected by ...the presence of tidal flats adjacent to the channel? We answer the above questions on the basis of a fully analytical treatment, appropriate for the short channels typically observed in coastal wetlands. The equilibrium length of non-convergent tidal channels is found to be proportional to the critical flow speed for channel erosion. Channel convergence causes concavity of the bed profile. Tidal flats shorten equilibrium channels significantly. Laboratory and field observations substantiate our findings.
Predicting the equilibrium cross section of natural rivers has been widely investigated in fluvial morphology. Several approaches have been developed to meet this aim, starting from regime equations ...to the empirical formulations of Parker et al. (2007) and Wilkerson and Parker (2011), who proposed quasi‐universal relations for describing bankfull conditions in sand and gravel bed rivers. Nevertheless, a general physics‐based framework is still missing, and it remains an open issue to better clarify the basic mechanisms whereby a river selects its width. In this contribution we focus our attention on lowland rivers with cohesive banks, whose resistance to erosion is crucial to control the river width. In particular, we formulate a theoretical model that evaluates the equilibrium width of river cross sections modeling the interaction between the core flow in the central part of the section and the boundary layer that forms in the vicinity of the cohesive banks. The model computes the cross‐section equilibrium configuration by which the shear stresses on the banks equal a critical threshold value. These stresses are computed by partitioning the total shear stress into an effective grain roughness component and a form component (Kean and Smith, 2006a). The model is applied to a large data set, concerning both sand and gravel bed rivers, and it is used to determine the relations expressing the channel width and the bankfull flow depth to the bankfull discharge, which appear to provide a unitary description of bankfull hydraulic geometry.
Key Points
Bank roughness in the shape of periodic bumps provides the controlling factor for river widening
We develop a physics‐based model that is able to predict the equilibrium cross section for rivers at the bankfull condition
The model provides an unifying interpretation for both gravel and sand bed rivers
In tidal environments, channel networks act as essential drainage pathways. Although the complex interactions between environmental factors have been studied extensively, the effects of the initial ...bathymetry on tidal network ontogeny are poorly understood. In this contribution, we used a numerical model to mimic a schematic tidal basin subjected to tidal forcing. The effects of the initial bathymetry and vegetation growth are analyzed by changing the features of randomly generated bed perturbation and the intertidal platform slope. Different perturbation densities mildly affect the growth of tidal networks, which, at equilibrium, share similar values in terms of channel length, tidal prism, and cross‐sectional area. The complexity and structure of channel networks are more sensitive to variations in the perturbation distribution. Increasing the initial bathymetry slope can shorten channels and reduce the tidal prism and drainage efficiency. Vegetation growth is found to invariably promote channel lengthening and narrowing, increasing the complexity and drainage efficiency of the system. An asymmetrical tidal forcing generally leads to longer channels and smaller unchanneled lengths. Under ebb‐dominant conditions, channels get deeper, and the increased channel length ensures a higher drainage efficiency. The insights of our study provide a deeper understanding of the environmental factors controlling the equilibrium morphology of tidal channel systems and their overall resilience. Further implications concern the restoration and management of coastal areas through the informed use of topographic changes and planting arrangements. Finally, accounting for the uncertainties associated with initial conditions is relevant when modeling other earth systems and comparing them with real systems.
Plain Language Summary
Tidal channels form networks connecting the sea to the inner land and act as essential pathways in coastal landscapes to exchange water, sediments, and nutrients. The channel network morphology and its efficiency in draining the basin exhibit different sensitivities to environmental factors. In this study, the evolution of a tidal network is simulated using a modeling framework to analyze the effects of small topographic features, vegetation, and tidal conditions on tidal channel characteristics. Small topographic features can significantly enhance the complexity of the channel network and increase the number of branches. The presence of vegetation generally promotes channel expansion and improves the drainage efficiency. However, the effect of vegetation on channel drainage efficiency may become weaker when the basin is sloping to the sea. The drainage efficiency can be further enhanced under flood‐ or ebb‐dominant conditions. Furthermore, under ebb‐dominant conditions, the tidal channel system will dissect the tidal basin more efficiently, resulting in a higher drainage efficiency than in flood‐dominant conditions. These findings are relevant for assessing the resilience of tidal channel systems and could be useful for the design of restoration projects and the management of coastal areas.
Key Points
Randomly distributed bed perturbations may increase channel drainage efficiency by changing the channel network structure
Vegetation promotes channel elongation and drainage efficiency, while a sloping basin has opposite effects due to reduced tidal prism
Ebb‐dominant conditions produce a rearrangement of total channel length and mean unchanneled length, ensuring a higher drainage efficiency
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