A nine-year whole-ecosystem experiment demonstrates that nutrient enrichment, a global problem in coastal ecosystems, can be a driver of salt-marsh loss. Salt marshes provide important ecosystem services such as storm protection for coastal cities, nutrient removal and carbon sequestration, but despite protective measures these ecosystems are in decline. Nine years of data from a whole-ecosystem nutrient-enrichment experiment now demonstrate that current levels of coastal nutrient loading can alter key salt-marsh-ecosystem properties, leading to the collapse of creek banks and, ultimately, the conversion of salt marsh into mudflat. The potential deterioration of coastal marshes owing to eutrophication adds another dimension to the challenge of managing nitrogen while meeting food-production demands in the twenty-first century. Salt marshes are highly productive coastal wetlands that provide important ecosystem services such as storm protection for coastal cities, nutrient removal and carbon sequestration. Despite protective measures, however, worldwide losses of these ecosystems have accelerated in recent decades1. Here we present data from a nine-year whole-ecosystem nutrient-enrichment experiment. Our study demonstrates that nutrient enrichment, a global problem for coastal ecosystems2,3,4, can be a driver of salt marsh loss. We show that nutrient levels commonly associated with coastal eutrophication increased above-ground leaf biomass, decreased the dense, below-ground biomass of bank-stabilizing roots, and increased microbial decomposition of organic matter. Alterations in these key ecosystem properties reduced geomorphic stability, resulting in creek-bank collapse with significant areas of creek-bank marsh converted to unvegetated mud. This pattern of marsh loss parallels observations for anthropogenically nutrient-enriched marshes worldwide, with creek-edge and bay-edge marsh evolving into mudflats and wider creeks5,6,7. Our work suggests that current nutrient loading rates to many coastal ecosystems have overwhelmed the capacity of marshes to remove nitrogen without deleterious effects. Projected increases in nitrogen flux to the coast, related to increased fertilizer use required to feed an expanding human population, may rapidly result in a coastal landscape with less marsh, which would reduce the capacity of coastal regions to provide important ecological and economic services.

Coastal eutrophication as a driver of salt marsh loss

Boundary conditions for displacement experiments through short laboratory soil columns [Dispersion coefficients]

We present an integrated analysis of bank erosion in a high-curvature bend of the gravel bed Cecina River (central Italy). Our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes, the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations. The results identify two mechanisms that explain how most bank retreat usually occurs after, rather than during, flood peaks. First, in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases, reducing sidewall boundary shear stress and fluvial erosion at peak flow stages. Second, bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events.

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Numerical simulation of hydrodynamics and bank erosion in a river bend

Many models of river meander migration rely upon a simple formalism, whereby the eroding bank is cut back at a rate that is dictated by the flow, and the depositing bank then migrates passively in response, so as to maintain a constant bankfull channel width. Here a new model is presented, in which separate relations are developed for the migration of the eroding bank and the depositing bank. It is assumed that the eroding bank consists of a layer of fine-grained sediment that is cohesive and/or densely riddled with roots, underlain by a purely noncohesive layer of sand and/or gravel. Following erosion of the noncohesive layer, the cohesive layer fails in the form of slump blocks, which armor the noncohesive layer and thereby moderate the erosion rate. If the slump block material breaks down or is fluvially entrained, the protection it provides for the noncohesive layer diminishes and bank erosion is renewed. Renewed bank erosion, however, rejuvenates slump block armoring. At the depositing bank, it is assumed that all the sediment delivered to the edge of vegetation due to the transverse component of sediment transport is captured by encroaching vegetation, which is not removed by successive floods. Separate equations describing the migration of the eroding and depositing banks are tied to a standard morphodynamic formulation for the evolution of the flow and bed in the central region of the channel. In this model, the river evolves toward maintenance of roughly constant bankfull width as it migrates only to the extent that the eroding bank and depositing bank ‘talk’ to each other via the medium of the morphodynamics of the channel center region. The model allows for both (a) migration for which erosion widens the channel, forcing deposition at the opposite bank, and (b) migration for which deposition narrows the channel forcing erosion at the opposite bank. Copyright © 2010 John Wiley & Sons, Ltd.

A new framework for modeling the migration of meandering rivers

Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (Ks) with depth, but differences between lateral Ks of a concretion layer and the vertical Ks of the underlying layers contributed to the propensity for lateral flow. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low-flow (LF) seeps (flow rates typically less than 0·05 L min−1), persistent high-flow (HF) seeps (average flow rate of 0·39 L min−1) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than −5 kPa). Seep discharge from HF seeps reached a maximum of 1·0 L min−1 and sediment concentrations commonly approached 100 g L−1. Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min−1) and sediment concentrations (989 g L−1). In cases where perched water table conditions exist and persistent HF seeps occur, seepage erosion and bank collapse of streambank sediment may be significant. Copyright © 2007 John Wiley & Sons, Ltd.

Measuring streambank erosion due to ground water seepage: correlation to bank pore water pressure, precipitation and stream stage

Pore water pressures (positive and negative) were monitored for four years (1996–1999) using a series of tensiometerpiezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to flow events and their effects on bank stability. The saturated/unsaturated flow was modelled using a finite element seepage analysis, for the main flow events occurring during the four-year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern-Price), using pore water pressure distributions obtained by the seepage analysis. The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of flow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the confining pressure of the river during the initial drawdown were responsible for triggering the mass failure. Results deriving from the seepage and stability analysis of nine selected flow events were then used to investigate the role of the flow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main one. Copyright © 2004 John Wiley & Sons, Ltd.

Monitoring and modelling of pore water pressure changes and riverbank stability during flow events

The erosion of sediment from riverbanks affects a range of physical and ecological issues.
Bank retreat often involves combinations of fluvial erosion and mass-wasting, and in recent
years bank retreat models have been developed that combine hydraulic erosion and limit
equilibrium stability models. In related work, finite element seepage analyses have also been
used to account for the influence of pore-water pressure in controlling the onset of masswasting.
This paper builds on these previous studies by developing a simulation modeling
approach in which the hydraulic erosion, finite element seepage and limit equilibrium
stability models are, for the first time, fully coupled. Application of the model is
demonstrated by undertaking simulations of a single flow event at a single study site for
scenarios where (i) there is no fluvial erosion and the bank geometry profile remains constant
throughout, (ii) there is no fluvial erosion but the bank profile is deformed by simulated
mass-wasting, and (iii) the bank profile is allowed to freely deform in response to both
simulated fluvial erosion and mass-wasting. The results are limited in scope to the specific
conditions encountered at the study site, but they nevertheless demonstrate the significant
role that fluvial erosion plays in steepening the bank profile, or creating overhangs, thereby
triggering mass-wasting. However, feedbacks between the various processes also lead to
unexpected outcomes. Specifically, fluvial erosion also affects bank stability indirectly, as
deformation of the bank profile alters the hydraulic gradients driving infiltration into the
bank, thereby modulating the evolution of the pore-water pressure field. Consequently, the
frequency, magnitude and mode of bank erosion events in the fully coupled scenario differ
from the two scenarios in which not all the relevant bank process interactions are included.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JF000722

Coupled simulations of fluvial erosion and mass wasting for cohesive river banks

Riverbanks along the Arno River have been investigated with the aims of deﬁning the main mechanisms of failure and retreat, their spatial distribution, and their causes. Geomorphological aspects were investigated by a reconnaissance of riverbank processes, for a number (26) of representative sites. Laboratory and in situ tests were then performed on a selected number of riverbanks (15). Based on the material characteristics, six main typologies of riverbanks have been deﬁned, with homogeneous ﬁne-grained and composite banks representing the most frequent types. Slab-type failures are the most frequent mechanism observed on ﬁne-grained banks, while cantilever failures prevail on composite banks.



The role of river stage and related pore water pressure distributions in triggering the main observed mechanisms of failure has been investigated using two different types of stability analysis. The ﬁrst was conducted for 15 riverbanks, using the limit equilibrium method and considering simpliﬁed hypotheses for pore water pressure distribution (annulment of negative pore pressures in the portion of the bank between low water stage and peak stage). Stability conditions and predicted mechanisms of failure are shown to be in reasonably good agreement with ﬁeld observations. Three riverbanks, representative of the main alluvial reaches of the river, were then selected for a more detailed bank stability analysis, consisting of: (a) deﬁnition of characteristic hydrographs of the reach with different return periods; (b) modelling of saturated and unsaturated ﬂow using ﬁnite element seepage analysis; and (c) stability analysis with the limit equilibrium method, by adopting pore water pressure values derived from the seepage analysis. The results are compared to those obtained from the previous simpliﬁed analysis, and are used to investigate the different responses, in terms of stability, to different hydrological and riverbank conditions. Copyright © 2003 John Wiley & Sons, Ltd.

Mechanisms of Riverbank Failure Along the Arno River, Central Italy

A severe rainstorm of high intensity occurred on 20th–21st November 2000, in the region of Pistoia, Tuscany, Italy, which triggered, within the entire province, over 50 landslides. These landslides can be broadly defined as complex earth slides—earth flows, originating as rotational slides that develop downslope into a flow. In this paper, two such landslides have been investigated by modelling the process of rainwater infiltration, the variations in both the positive and negative pore water pressures and their effect on slope stability during the storm. For both sites, results from morphometric and geotechnical analyses were used as a direct input to the numerical modelling. A modified Chu, 1978 approach was used to estimate the surface infiltration rate by adapting the original Green and Ampt, 1911 equations for unsteady rainfall intensity in conjunction with the surficial water balance. For transient conditions, a finite element analysis was used to model the fluctuations in pore water pressure during the storm, with the computed surface infiltration rate as the surface boundary condition. This was then followed by the application of the limit equilibrium Morgenstern and Price, 1965 slope-stability method, using the temporal pore water pressure distributions derived from the seepage analysis. From this methodology, a trend for the factor of safety was produced for both landslide sites. These results indicate that the most critical time step for failure was a few hours following the rainfall peak.

S. Dapporto

Papers

Monitoring and modelling of pore water pressure changes and riverbank stability during flow events

Coupled simulations of fluvial erosion and mass wasting for cohesive river banks

Mechanisms of Riverbank Failure Along the Arno River, Central Italy

Analysis of the landslide triggering mechanism during the storm of 20th–21st November 2000, in Northern Tuscany

Failure mechanisms and pore water pressure conditions: analysis of a riverbank along the Arno River (Central Italy)