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Wave Damping in Reed: Field Measurements and Mathematical Modeling
Abstract
Wave damping in vegetation in shallow lakes reduces resuspension and thereby improves the light climate and decreases nutrient recycling. In this study, wave transformation in reed (Phragmites australis) was measured in a shallow lake. Theoretical models of wave height decay, based on linear wave theory, and transformation of the probability density function (PDF), using a wave-by-wave approach, were developed and compared to the collected data. Field data showed an average decrease in wave height of 4-5% m(-1) within the first 5-14 m of the vegetation. Incident root-mean-square wave height was 1-8 cm. A species-specific drag coefficient C(D) was found to be about 9 (most probable range: 3-25). C(D) showed little correlation with a Reynolds number or a Keulegan-Carpenter number. The PDF for the wave heights did not change significantly, but for longer distances into the vegetation and higher waves it tended to be more similar to the developed transformed distribution than to a Rayleigh distribution. Relationships developed in this study can be employed for management purposes to reduce resuspension and erosion.
... We know that the presence of a 'rough' bed with vegetation elements protruding into or through the water column significantly and instantaneously attenuates waves (top arrow in Figure 1A). In a true-to-scale experiment in which dense 70 cm tall salt marsh vegetation was submerged to water depths of 2 m, for example, Möller et al. (2014) recorded dissipation rates of 15-20% over only a 20 m distance for storm surge magnitude waves of significant wave height (H s ) 0.2 to 0.9 m, Fully emergent wetland vegetation, such as reed, of a much lower density (434-483 stems m -2 with a diameter of 14-16 mm at 25 cm above the bed) has also been shown to affect wave heights within the first 3-4 m of a reedbed, with both Lövstedt and Larson (2010) and Möller et al. (2011) recording H s reduction within the range 2.6 -11.8% m -1 (i.e. at least 40% over a 20 m distance) at separate locations on the shores of the Baltic Sea. ...
... Few observations of wave propagation within reed beds exist, but those that do typically capture high temporal variability in both wave conditions and dissipation rates (Lövstedt and Larson, 2010;Möller et al., 2011). Such temporal variability is also characteristic of the wave decay measured at Histria and Cape Dolosman Figures 5, 6). ...
... The two key field studies available, that have measured wave height dissipation in reed stands in similar settings, can be compared to this study (i.e. mean H s reduction of 1.5% m -1 (Cape Dolosman) and 2.7% m -1 (Histria): First, Lövstedt and Larson (2010) report mean rates of root-mean-square wave height reduction of 4-5% m -1 at the margin (4-15 m into the vegetation) of a Phragmites australis reed bed on the northern shores of Lake Krakesjön in southern Sweden. Second, Möller et al. (2011) report a wider range (mean rates of H s reduction of 2.6% m -1 and 11.8% m -1 ) at a sheltered and exposed site, on the southern shores of the Baltic sea in east Germany. ...
Coastal wetlands are dynamic bio-physical systems in which vegetation affects the movement of water and sediment, which in turn build and maintain the landform and ecosystem. Wetlands are an effective buffer against coastal erosion and flooding, enhance water quality and human health and wellbeing. Numerous field and laboratory experiments have quantified the reduction of waves by coastal ecosystems. Numerical models, however, are only able to capture observed reduction in wave energy when calibration coefficients are obtained by comparison with measured dissipation rates. A deeper understanding of how wave attenuation varies over time, with local flow conditions and ecosystem properties, is still lacking and should be acquired from a greater range of ecosystem types and geographical settings. Few studies have observed the detailed seasonal variations in how coastal wetlands function as wave buffers and how such seasonal variations might be explained. Equally, few studies have focused on the effect of coastal reed beds on wave dynamics. This study addresses both: i) seasonal variability in wave dissipation through reed vegetation and ii) intricate connections between reed vegetation and the physical context (meteorological and topographical) that might explain such variability. We present observations of wind generated wave transformation through two Phragmites australis reed beds in the Razelm-Sinoe Lagoon System, Danube Delta, Romania. We find that seasonal changes in vegetation density and biomass, as well as meteorological conditions, affect observed wave conditions within the first few meters of the reed beds. Our results also show a preferential reduction of higher frequency waves, irrespective of reed stem diameter or density and suggest the potential importance of seasonal vegetation debris to observed wave dissipation. Such complex and non-linear biogeomorphic effects on wave dissipation are not currently well understood or captured in the parameterisation of vegetation-induced wave dissipation. Our study highlights the importance of an accurate and temporally granular quantification of nearshore bathymetry, wetland topography, and vegetation to fully understand, model, and manage bio-physical interactions in coastal wetlands. More specifically, our results point towards the need for spatially and temporally explicit wave decay functions in emergent reed vegetation. This is particularly critical where the accurate evaluation of the flood and erosion risk contribution of any wetland is required as part of nature-based coastal protection solutions.
... Coastal wetlands provide key ecosystem goods and services at local, national, and international levels (Millennium Ecosystem Assessment, 2005;Koch et al., 2009). One such service is that of providing a natural wave energy buffer for wind-generated waves, particularly on open, exposed coasts (Leonard and Reed, 2002;Neumeier and Ciavola, 2004;Möller, 2006). This buffering function provides a benefit in socio-economic terms at the local level, due to the reduced wave impact on (and thus reduced cost of maintaining) landward lying sea defences. ...
... Coops et al., 1996a;Blindow et al., 2002;Houwing et al., 2002). Lövstedt and Larson (2010) have provided the first systematic field measurements of wave conditions offshore and within lake margin Phragmites australis reed beds in southern Sweden, with a wave height reduction of 4e5% m À1 observed over a distance of 4e15 m of vegetation. The low incident RMS wave heights (1.0e6.1 cm) and high root-mean-square error (0.39 cm) reported in their study, however, do not allow conclusive statements to be made about the applicability of their wave height probability distribution transformation method. ...
... Prior observations of wave dissipation across, and/or drag forces induced by, reed (Lövstedt and Larson, 2010), mangrove (Mazda et al., 1997), and saltmarsh (Möller et al., 1999) vegetation have all shown a dependency on water depth conditions. This is because vegetation properties are vertically non-uniform and the influence of vegetation on waves becomes much reduced when the upper surface of the vegetation canopy is located some distance below the water surface. ...
Global environmental change poses a growing challenge for the management of
low-lying coastal environments. The challenge is to (a) recognise and quantify
the ecological functions of such environments, and (b) develop management
approaches that allow those functions to be maintained in the context of global
change. Meeting this challenge is particularly important on micro-tidal
shorelines, where the ecological sensitivity to sea level rise and changing
climatic conditions (e.g. storm frequency and intensity) is likely to be high.
This study addresses the need to quantify the wave-dissipating function of
these types of coastal wetland. Previous studies have focused on tidal coasts and
salt marsh or mangrove vegetation and have highlighted relationships between
coastal wetland vegetation type, water depths, and observed wave energy
reduction. Prior to this study, however, no data was available on the sea-defence
function of coastal grasslands and reed beds, where irregular inundation by
meteorologically driven storm surges dominates over tidal inundation.
Results are presented of wave and vegetation monitoring along three crossshore
transects at the fringes of reed beds and coastal brackish grasslands on the
German Baltic shoreline. The data highlight significant differences in the seadefence
function of these two types of micro-tidal coastal habitat, highlighting
important differences in the likely response to future climatic (and sea level)
changes and raising questions around how these functions might be maintained,
enhanced, or restored in the context of environmental change.
Keywords: sea level rise, wave dissipation, baltic coastal wetlands, coastal
management.
... Coastal wetlands provide key ecosystem goods and services at local, national, and international levels (Millennium Ecosystem Assessment, 2005;Koch et al., 2009). One such service is that of providing a natural wave energy buffer for wind-generated waves, particularly on open, exposed coasts (Leonard and Reed, 2002;Neumeier and Ciavola, 2004;Möller, 2006). This buffering function provides a benefit in socio-economic terms at the local level, due to the reduced wave impact on (and thus reduced cost of maintaining) landward lying sea defences. ...
... Coops et al., 1996a;Blindow et al., 2002;Houwing et al., 2002). Lövstedt and Larson (2010) have provided the first systematic field measurements of wave conditions offshore and within lake margin Phragmites australis reed beds in southern Sweden, with a wave height reduction of 4e5% m À1 observed over a distance of 4e15 m of vegetation. The low incident RMS wave heights (1.0e6.1 cm) and high root-mean-square error (0.39 cm) reported in their study, however, do not allow conclusive statements to be made about the applicability of their wave height probability distribution transformation method. ...
... Prior observations of wave dissipation across, and/or drag forces induced by, reed (Lövstedt and Larson, 2010), mangrove (Mazda et al., 1997), and saltmarsh (Möller et al., 1999) vegetation have all shown a dependency on water depth conditions. This is because vegetation properties are vertically non-uniform and the influence of vegetation on waves becomes much reduced when the upper surface of the vegetation canopy is located some distance below the water surface. ...
... Along many coastal regions of the world, wetlands and coastal forests provide a natural defence line against storm surge and waves. Surge reduction and wave attenuation have been recognized in previous laboratory experiments and numerical modelling studies (Resio and Westerink 2008;Lövstedt and Larson 2010;Gedan et al. 2011;Shepard et al. 2011). Numerical models have been employed to investigate the effects of vegetation and routinely utilized to predict storm surge and waves for emergency preparedness and for coastal resources management (Loder et al. 2009;Wamsley et al. 2009;Wamsley et al. 2010). ...
... Moreover, the HC-based model estimates an increased spectral width as the waves propagate over the vegetation whereas the RD-based model results in a decrease in the spectral width parameter. A field study of wave damping by reeds (Lövstedt and Larson 2010) shows the transformation of the probability distribution function (PDF) of wave heights and their observed change in the PDF suggests that the spectral width of the wave field may also change. The change of spectral width due to vegetation damping has been confirmed by recent field measurements collected during a tropical storm (Jadhav and Chen 2012). ...
The study utilizes a coupled wave-surge-vegetation modeling system to quantify the effects of salt marsh vegetation on hurricane-generated waves. The wave model incorporates the energy dissipation model of Chen and Zhao (2012) for random waves over vegetation. The storm surge model incorporates the vegetal drag for both rigid and flexible types of vegetation. The surge and wave models with the vegetation effects are coupled, allowing the spatially and temporally varying vegetation heights, water levels and depth-averaged currents from the storm surge model to be fed into the wave model. Numerical experiments have revealed that vegetation can change the surge height and a storm surge can change the vegetation height. Both control the wave reduction rate in flooded wetlands. The impact of vegetation on hurricane-generated waves consists of indirect and direct effects. The former is caused by the changes in surge height due to vegetation. The latter comes from the direct interaction between vegetation and the oscillatory motion of surface waves. It has been found that flexible marsh vegetation deflects under the hydrodynamic force produced by a hurricane. The deflected height not only reduces the flow resistance in the surge model, but also decreases the energy dissipation caused by vegetation in the wave model. Consequently, neglecting plant flexibility may lead to overestimates of vegetation effects and exaggeration of wetland potential for flood risk reduction.
... Our results support this, since the variables "walls" and "reed" showed the lowest erosion prediction rates compared to the variable "natural banks" (Fig. 6B). It is well known that waves are strongly damped in reed stands preventing shorelines from erosion (Coops et al., 1996;Rolletschek, 1999;Borell Lövstedt and Larson, 2010). Moreover, shore walls are mainly constructed to prevent shorelines or land fills from erosion (Schiereck, 2001;Nordstrom, 2014), thus our results simply confirm that they fulfill their intended function. ...
Population growth and tourism exert tremendous pressures on the lakeshore habitats of Lake Constance. However, great efforts have been put into shore restoration measures over the last few decades to generate a more natural state. These activities, and the importance of the littoral zone as a UNESCO world cultural heritage, increase the need for comprehensive shore typology data to provide a decision base for littoral zone management. Our approach comprises the delineation and classification of different regions of the littoral zone based on morphological features as a first step towards a comprehensive shore typology. We applied a combination of spatial and statistical analyses to classify the Lake Constance shore, based on width and slopes at different depths. We classified ten different hydro-morphological shore types, each representing homogenous areas in the littoral zone separated into sublittoral zones and potential flood areas. Validation of the typology revealed that the shorelines with steep inclinations or embankments were the shore types most likely to erode. Our findings show that the typology based on morphological features can be a useful predictor and method to link the structures and processes that interact in the shore area with the shore morphology.
... Nepf (2012) has reviewed the mean and turbulent flow structures influenced by AV in detail, and identified the canopy-and stem-scale turbulences generated by AV and their effects on mass transport. Many previous studies have also investigated the interaction between water motion and AV under pure-waves, and most of them were mainly on the wave damping by vegetation (Bradley & Houser, 2009;Lovstedt & Larson, 2010;Luhar et al., 2017;Mendez & Losada, 2004), the mean and turbulent flow structures within canopy (Abdolahpour et al., 2017;Lowe et al., 2005;Luhar et al., 2010Luhar et al., , 2013Pujol et al., 2010Pujol et al., , 2013. For example, Lowe et al. (2005) studied the velocity attenuation within a model rigid canopy and a theoretical model was developed to predict the magnitude of in-canopy wave orbital velocity under oscillatory flow. ...
Field measurements were conducted to study the influence of aquatic vegetation on flow structures in floodplains under combined currents and wind‐driven waves. Wave and turbulent velocities were decomposed from the time series of instantaneous velocity and analyzed separately. In the present study, the wind waves were small, leading to the ratios of wave excursion (Ew) to stem spacing (S) for all cases tested here were less than 0.5. This caused the vertical distributions of time‐averaged velocity (Uhoriz) and turbulent kinetic energy (TKE) impacted by vegetation similar with the vegetated flow structures under pure current conditions. For emergent vegetation, Uhoriz and TKE distributed uniformly through the entire water column or increased slightly from bed to water surface. Similar distributions were present in the lower part of submerged vegetation. In the upper part of submerged vegetation, Uhoriz and TKE increased rapidly toward water surface and TKE reached its maximum near the top of vegetation. The measured wave orbital velocity (Uw) fitted linear wave theory well through the entire water depth for both the emergent and submerged cases, so that with small Ew/S the wave velocity was not attenuated within vegetation and Uw within canopy can be predicted by the linear wave theory under combined currents and waves. However, wind‐driven waves made the turbulence generated near the top of canopy penetrate a deeper depth into vegetation than predictions under pure current conditions.
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... (14)) was used to compare the plant scale drag on the species listed in Table 3 for a range of water depth (0.2 to 3 m). C s = 0.6 was used because the mature marsh plants have similar morphology as the geometrically-and dynamically-scaled model plant (see photos of plants in Lövstedt and Larson (2010), Rupprecht (2015), Schoutens et al. (2020) and Fig. 2c), so that it is reasonable to assume the same sheltering. The wave amplitude varied with water depth. ...
Salt marshes are a common feature in coastal regions and have been noted for their ability to attenuate wave energy, providing an important first line of coastal defense. Marsh plants usually consist of multiple leaves distributed along a central stem. This paper constructed a model predicting wave force on a marsh plant by modeling the reconfiguration of both the leaves and stem in waves. The individual leaf and stem models and the full plant model were validated with experimental measurements of drag and plant motion using both live and dynamically-similar model plants under a range of wave conditions. Although the leaves exhibited greater reconfiguration than the stem, they contributed more than 70% of the plant drag. Plant reconfiguration produced a drag force that had a weaker than quadratic dependence on wave velocity. A simplified model, which combines scaling laws for the stem and individual leaves, is proposed and validated. Wave drag on a variety of marsh species with different morphology and rigidity were estimated and compared.
... Nepf (2012) has reviewed the mean and turbulent flow structures in detail, and emphasized on the canopy-and stemscale turbulence generated by AV and their effects on mass transport. Many previous studies have also investigated the interaction between water motion and AV under pure-waves, and most of them were mainly on the damping of waves by vegetation (e.g., Bradley & Houser, 2009;Lovstedt & Larson, 2010;Luhar, Infantes, & Nepf, 2017;Mendez & Losada, 2004 ), the mean and turbulent flow structures within canopy (e.g., Abdolahpour, Hambleton, & Ghisalberti, 2017;Lowe, Koseff, & Monismith, 2005;Luhar, Coutu, Infantes, Fox, & Nepf, 2010;Luhar, Infantes, Orfila, Terrados, & Nepf , 2013;Pujol, Serra, Colomer, & Casamitjana, 2013 ). For example, Lowe et al. (2005) studied the velocity attenuation within a model rigid canopy and a theoretical model was developed to predict the magnitude of in-canopy wave orbital velocity under oscillatory flow. ...
... In the literature, it is noticed that the evaluation of the wave attenuation according to K t appears to be not consistent. For example, the wave attenuation was calculated per 100 m (Mazda et al., 1997), per 1 m (Mazda et al., 2006;Massel, 2006;Quartel et al., 2007;Bao, 2011;Möller et al., 1999;Möller and Spencer, 2002;Möller, 2006;Cooper, 2005;Bradley and Houser, 2009;Lövstedt and Larson, 2009) or per length of the forest (Brinkman et al., 1997;Brinkman, 2006). As a result, the rate of wave attenuation in the literature is hardly informative. ...
Wave attenuation through mangrove forests has received more and more attention, especially in the context of increasing coastal erosion and sea-level-rise. Numerous studies have focused on studying the reduction of wave height in a mangrove forest. However, the understanding of this attenuation process is still in its infancy. In order to obtain more insight, a laboratory experiment, mimicking the processes of wave attenuation by coastal mangroves in the Mekong Delta, Vietnam was conducted. The reduction of wave height for different scenarios of mangrove densities and wave conditions was investigated. A new method to quantify vegetation attenuation induced by vegetation is presented. The wave height reduction is presented over a relative length scale (viz. the number of wavelengths), instead of an absolute length scale of the forest (e.g per meter or per 100 m). The effects of wave non-linearity on the wave height attenuation over the mangrove forest were investigated using the Ursell number. It is suggested that the non-linear character of waves has a strong influence on the attenuation of the waves inside the mangrove forest. A numerical model, mimicking the experiment was constructed in SWASH and validated using the experimental data. Finally, the data set was extended through numerical modelling so that a larger ranging relationship between wave attenuation per wave length and the Ursell number could be formulated.
... In large shallow lakes similar situations may exist where vegetated foreshores, consisting of reed beds dampen wave attack on the shoreline [5][6][7]. However, in the Netherlands the large shallow lakes are bordered by large dikes to protect the low lying land behind it. ...
Vegetated foreshores in front of existing dikes can contribute to the overall reduction of wave loads on the dike. In order to test this concept in large shallow lakes a field pilot was constructed along the Houtribdijk in Lake Markermeer (the Netherlands) in 2014 to gain experience with construction, stability, maintenance and governance aspects. A large scale monitoring programme was set up to follow the hydrodynamic forcing, morphological changes and vegetation development on the pilot. The pilot is located on an exposed south-westerly direction, and experiences substantial wave impact. As a result the desired vegetation on the land-water interface has not been able to establish, but a rather dynamic sandy beach is currently the main feature along the waterline of the site. Higher up the slope planted reeds, and a mixture of willows has well established itself in the first growing season. The exposed position of the location makes that hardly any natural pioneer vegetation has settled, only in small sheltered areas some annuals were able to germinate and maintain themselves.
... Although this effect of vegetation has been acknowledged by several authors [e.g. Lövstedt and Larson, 2010;Stratigaki et al., 2011;Akgul et al., 2013], only a limited number of studies aimed to quantify these effects [e.g. Ma et al., 2013;Guannel et al., 2015], and thus the effect of vegetation on the mean water level through wave setup is often ignored in flood risk assessments. ...
Aquatic vegetation in the coastal zone attenuates wave energy and reduces the risk of coastal hazards, e.g. flooding. Besides the attenuation of sea-swell waves, vegetation may also affect infragravity-band (IG) waves and wave setup. To date, knowledge on the effect of vegetation on IG waves and wave setup is lacking, while they are potentially important parameters for coastal risk assessment. In this study, the storm impact model XBeach is extended with formulations for attenuation of sea-swell and IG waves, and wave setup effects in two modes: the sea-swell wave phase-resolving (non-hydrostatic) and the phase-averaged (surfbeat) mode. In surfbeat mode a wave shape model is implemented to capture the effect of nonlinear wave-vegetation interaction processes on wave setup. Both modeling modes are verified using data from two flume experiments with mimic vegetation and show good skill in computing the sea-swell and IG wave transformation, and wave setup. In surfbeat mode, the wave setup prediction greatly improves when using the wave shape model, while in non-hydrostatic mode (nonlinear) intra-wave effects are directly accounted for. Subsequently, the model is used for a range of coastal geomorphological configurations by varying bed slope and vegetation extent. The results indicate that the effect of wave-vegetation interaction on wave setup may be relevant for a range of typical coastal geomorphological configurations (e.g. relatively steep to gentle slope coasts fronted by vegetation).
... The SPM formulas (CERC, 1984) for fetch-limited shallow and transitional water conditions, which take into account wave damping by bottom friction, were used to compute H rms and T a . These equations model the transition to deep water conditions, so that they capture the residual wave-bottom interaction persisting for h / L w > 1 / 2 (Borell Lövstedt and Larson, 2010), as is the case of the Superior Lake of Mantua. ...
This paper presents a numerical modelling framework developed to simulate circulations and to generally characterise the hydrodynamics of the Superior Lake of Mantua, a shallow fluvial lake in Northern Italy. Such eutrophied basin is characterised by low winds, reduced discharges during the summer and by the presence of large lotus flower (Nelumbo nucifera) meadows, all contributing to water stagnation. A hydrodynamic numerical model was built to understand how physical drivers shape basic circulation dynamics, selecting appropriate methodologies for the lake. These include a 3D code to reproduce the interaction between wind and through-flowing current, a fetch-dependent wind stress model, a porous media approach for canopy flow resistance and the consideration of wave-current interaction. The model allowed to estimate the circulation modes and water residence time distributions under identified typical ordinary, storm and drought conditions, the hydrodynamic influence of the newly-opened secondary outlet of the lake, the surface wave parameters, their influence on circulations and the bottom stress they originate, and the adaptation time scales of circulations to storm events. Some probable effects of the obtained hydrodynamic characteristics of the Superior Lake of Mantua on its biochemical processes are also introduced.
... The study of aquatic vegetation attenuating wave is investigated and quantified in several laboratory studies with both natural and artificial vegetation (e.g. Dubi, Tørum 1996;Tschirky et al. 2000;Augustin et al. 2009;Cavallaro et al. 2010) and also field measurements in various different environments (Möller, Spencer 2002, Möller 2006Quartel et al. 2007;Bradley, Houser 2009;Lövstedt, Larson 2010). In addition, modeling studies were also performed to investigate wave-vegetation interactions (Dalrymple et al. 1984;Asano et al. 1992;Mendez et al. 1999;Mendez and Losada 2004;Mullarney, Henderson 2010). ...
This paper presents a well-controlled laboratory experimental study to evaluate wave attenuation by artificial emergent plants (Phragmites australis) under different wave conditions and plant stem densities. Results showed substantial wave damping under investigated regular and irregular wave conditions and also the different rates of wave height and within canopy wave-induced flows as they travelled through the vegetated field under all tested conditions. The wave height decreased by 6%–25% at the insertion of the vegetation field and towards the downstream at a mean of 0.2 cm and 0.32 cm for regular and irregular waves respectively. The significant wave height along the vegetation field ranged from 0.89–1.76 cm and 0.8–1.28 cm with time mean height of 1.38 cm and 1.11 cm respectively for regular and irregular waves. This patterns as affected by plant density and also location from the leading edge of vegetation is investigated in the study. The wave energy attenuated by plant induced friction was predicted in terms of energy dissipation factor (fe) by Nielsen’s (1992) empirical model. Shear stress as a driving force of particle resuspension and the implication of the wave attenuation on near shore protection from erosion and sedimentation was discussed. The results and findings in this study will advance our understanding of wave attenuation by an emergent vegetation of Phragmites australis, in water system engineering like near shore and bank protection and restoration projects and also be employed for management purposes to reduce resuspension and erosion in shallow lakes.
First published online: 21 Oct 2015
... As these equations were specifically tested to properly model the transition to deep water conditions, they should capture this residual interaction when used just outside their natural field of application (e.g. Borell Lövstedt & Larson, 2010). ...
Shallow fluvial lakes are inland water bodies in which circulations are shaped by both the wind and the through-flowing current. Usually, these lakes are the product of damming operations and are located in the lowland reach of river basins, where nutrients and pollutants loads and water abstraction are maximum, leading to hypereutrophic conditions, with risks of ecosystem collapse. Ecological processes in fluvial lakes are strictly dependent on the hydrodynamic phenomena, whose understanding is then essential to plan effective safeguard and remediation measures for the environment.
The Mantua Lakes are an exceptional case study of renaturalised fluvial lakes, dating back to the end of the XII century. This thesis focuses on the most upstream and largest of the three basins, the Superior Lake of Mantua, of which a 3D numerical model was developed, reproducing the relevant features of its physics. This lake hosts large patches of lotus flower, the widest island covering ~10% of the water surface, influencing both the physical and the biogeochemical phenomena. An hydrodynamic characterisation of the lake was performed through simulations, defining the circulation processes occurring under the different typical meteorological and hydrological conditions. The model was applied to the study of phytoplankton distribution, comparing its results to remote-sensed Chlorophyll-a distribution maps, providing also an indirect validation.
Results of this thesis represent a solid basis for future studies on the relations between the hydrodynamics and the ecosystem in the Mantua Lakes, in addition to contributing to the knowledge on the modelling of shallow fluvial lakes.
... In particular, dense reed populations in aquatic habitats act an effective filter against pollutants, thus preserving water quality (Bonanno, 2011;Zhao et al., 2012). Furthermore, reed stands stabilize river and lake margins (Ostendorp, 1999;Lövstedt and Larson, 2010) and represent key habitats for wildlife, especially nesting birds (Poulin et al., 2002(Poulin et al., , 2010. For these reasons, researchers are paying much attention to investigate the causes of extensive reed die-back observed in reed-dominated freshwater ecosystems (Brix, 1999;Reale et al., 2012), but also in brackish coastal wetlands (Fogli et al., 2002). ...
... However, for freshwater aquatic macrophytes communities in relation to wave attenuation only limited information is available, predominantly focusing on rigid (shoreline) vegetation (e.g. Coops et al. 1999;Lövstedt and Larson 2010). Especially flexible, obligate freshwater macrophytes exhibit a wide variety in morphological shapes that is both species specific, but can also be a phenotypic plasticity resulting from adapation to stress (Puijalon et al, 2008). ...
This thesis shows the role of hydrodynamic and ecohydraulic processes in shallow lakes and how this knowledge can be used to better understand and manage these ecosystems. Wind driven resuspension is at the centre of this work and links between (re-)suspended sediments, bivalve communities and wave attenuation by plants are discussed. Measures to limit resuspension are addressed and a framework for such an assessment is described.
... A number of recent studies have underscored the importance of accounting for the stem and blade motion of flexible vegetation, and have proposed models that account for it (Bradley and Houser, 2009;Mullarney and Henderson, 2010;Riffe et al., 2011). Wave attenuation has been studied in a controlled laboratory environment (Augustin et al., 2009;Dubi and Tørum, 1996;Løvås and Tørum, 2001;Stratigaki et al., 2011), in field conditions involving salt marshes (Bradley and Houser, 2009;Cooper, 2005;Jadhav and Chen, in review;Möller, 2006;Möller and Spencer, 2002;Möller et al., 1999;Riffe et al., 2011), coastal mangrove forests (Mazda et al., 2006;Quartel et al., 2007), and vegetated lakeshores (Lövstedt and Larson, 2010). Most of these studies primarily focused on the attenuation of integral wave heights or wave energy, and estimation of integral bulk vegetation drag coefficients. ...
Spectral energy dissipation of random waves due to salt marsh vegetation (Spartina alterniflora) was analyzed using field data collected during a tropical storm. Wave data (significant wave heights up to 0.4 m in 0.8 m depth) were measured over a two-day period along a 28 m transect using 3 pressure transducers. The storm produced largely bimodal spectra on the wetland, consisting of low-frequency swell (7–10 s) and high-frequency (2–4.5 s) wind-sea. The energy dissipation varied across the frequency scales with the largest magnitude observed near the spectral peaks, above which the dissipation gradually decreased. The wind-sea energy dissipated largely in the leading section of the instrument array in the wetland, but the low-frequency swell propagated to the subsequent section with limited energy loss. Across a spectrum, dissipation did not linearly follow incident energy, and the degree of non-linearity varied with the dominant wave frequency. A rigid-type vegetation model was used to estimate the frequency-dependent bulk drag coefficient. For a given spectrum, this drag coefficient increased gradually up to the peak frequency and remained generally at a stable value at the higher frequencies. This spectral variation was parameterized by employing a frequency-dependent velocity attenuation parameter inside the canopy. This parameter had much less variability among incident wave conditions, compared to the variability of the bulk drag coefficient, allowing its standardization into a single, frequency-dependent curve for velocity attenuation inside a canopy. It is demonstrated that the spectral drag coefficient predicts the frequency-dependent energy dissipation with more accuracy than the integral coefficient.
... Extensive field and laboratory studies of flows in vegetated watercourses have been carried out recently [17][18][19][20][21][22][23][24][25][26]. In particular, it was observed that under certain flow conditions organized vertical flow structures occur at the tip of the vegetation. ...
Vegetation in rivers, estuaries and coastal areas is often submerged and highly flexible. The study of its interaction with the ambient flow environment is important for the determination of the discharge capacity, morphological characteristics and ecological conditions of the water course where it grows. In this work the hydrodynamics of submerged flexible vegetation with or without foliage is investigated by using a 3D numerical model. Flexible vegetation is modeled by momentum sink terms, with the velocity-dependent stem height determined by a large deflection analysis which is more accurate than the previously used small deflection analysis. The effect of foliage on flow resistance is expressed in terms of the change in the product of the drag coefficient and the projected area, which is supported by available experimental data. The computed results show that the vertical profiles of the mean horizontal velocity and the vertical Reynolds shear stress are correctly simulated. The temporal variation of the stem deflection follows closely that of the velocity and the ‘Honami’ phenomenon can be reproduced. The numerical simulations also confirm that the flexibility of vegetation decreases both the vegetation-induced flow resistance force and the vertical Reynolds shear stress, while the presence of foliage further enhances these reduction effects.
One of the beach protection techniques is using natural methods based on the coastal ecosystem. Studies show the reducing effect of forest covers on wave destruction intensity in different areas. However, it is not yet well understood how various densities of terrestrial coastal forest (TCF) affect the wave attenuation and reduce their strength. Studying the impact of various forest parameters, such as density, distance, and arrangement type on the wave force attenuation, this research measures the wave forces directly. TCF model was installed in a knife edge flume, which equipped with a load cell and an acoustic Doppler velocimeter. The experiments were performed in two staggered and parallel arrangements consisting of different densities from 12 to 273 stems per unit area. Based on obtained results, TCF had significant effects on the wave force absorption. An increase in the number of trees (density) increased TCF resistance force and the absorbed wave force. In its best, the TCF could absorb the wave force 3.76 times more than the no-TCF case. It could reduce the wave height by up to 81% at the highest density and maximum wave height. The absorbed wave force and drag coefficient rose as the number of rows of trees opposing the flow decreased and the intervals between trees were shortened. Increasing tree density from 12 to 273 stems per unit area increased the drag coefficient by the average of 61.82% for parallel and staggered arrangements, which means an average increase of 9.7% for each TCF row.
Quantitative investigation of wave attenuation by flexible vegetation is an increasingly prominent area in wave mechanics. Given that the existing XBeach non-hydrostatic wave model can only simulate wave attenuation by rigid vegetation, a mechanical model for simulating submerged flexible vegetation dynamics driven by waves and a coupling simulation method between the flexible vegetation dynamic model and the XBeach non-hydrostatic wave model were proposed. To assess the performance of the proposed coupled model, a wave flume experiment was conducted. This study demonstrates the validity of the new flexible vegetation dynamic model in simulating forces and motions of a single submerged flexible stem. The validation between modeled results and experimental data shown that the coupled model is capable of reproducing the wave attenuation in the presence of flexible vegetation. The above results further indicate that the coupling implementation is valid and reliable. Besides, this study further confirms that flexible vegetation is able to attenuate wave height, and the elasticity modulus of flexible vegetation plays a vital role in wave attenuation when all the other variables are invariant. This study can broaden the application of the XBeach model and provide technical support for the engineering practice of coastal nature-based solutions.
With the increase in coastal hazards induced by global change, estuarine deltas are urgently required to enhance nature-based coastal defenses. This study sets an example at the Nanhui nearshore salt marshes in the Yangtze Estuary, China, with a one-dimension wave attenuation empirical model and Landscape Resistance Index (LRI). We explored wave attenuation by salt marshes at site and transect scale, and put forward potential approaches to improve coastal defenses. Results showed that 1) the two 10 m-wide stripes of typical vegetation (Spartina alterniflora and Scirpus mariqueter) could attenuate wave height mostly when incident wave height was 0.22 m and 0.20 m, respectively; 2) under the condition of 0.6 m initial wave height, wave attenuation rate among all transects ranged from 32.50% to 98.30% (71.30 ± 16.09% on average); and 3) Land cover-Position-Length weighted LRI was an effective indicator for the relationship between wave attenuation and landscape pattern. Wave attenuation by salt marshes differ in various environments and are species specific. Wave attenuation along transects was also affected by both land cover and spatial configuration. Thus, we proposed some salt marsh restoration strategies, such as preferential restoration of long transects, differentiation of vegetation configuration, and engineering protections like low levees or breakwaters in front of salt marsh edges. These findings could be used to promote ecological functions in the Yangtze Estuary and other coastal marshes in the world.
Coastal areas are prone to natural disasters like tsunami and earthquake. The losses occuring due to these disasters are voluminous, since high population densities are generally located along the coastal region. The massive velocity and salinity of waves causes soil erosion and affect the structures present along the coastal belt. Coastal vegetation such as seagrass canopies acts as a natural barrier to soil erosion and to the wave impact. Seagrass is the most abundantly found marine species along the Indian coast. It is located at an ideal depth to dissipate the waves before reaching the shore. The use of seagrass as a buffer zone is gaining momentum in the field of coastal engineering as it also helps in conserving the ecosystem. This paper presents the wave attenuation due to seagrass by numerical modelling and experimental investigation. The Cymodocea Serrulata species (CSS) was selected for the study which is found in coastal regions of India like Palk Bay and Gulf of Mannar. Wave attenuation by the CSS vegetation for different wave heights and wave periods was studied. The numerical model for wave attenuation was created using Flow 3D software and artificial vegetation (silicon rubber tubes) was used for the experimental investigation carried out at wave flume in National Institute of Technology, Tiruchirappalli (NITT). Multiple waves were created from the numerical simulation by varying the wave heights, wave periods and transmitted wave heights at different meadow widths were recorded and analysed. The results of numerical modelling were compared with the experimental investigation. The submergence ratio increases from 0.47 to 0.53. The wave attenuation increases from 60 to 54% of that of original wave height. The model exhibits increased efficiency (the relative plant height (h/d)) in wave height reduction.
With the experience of storm surges along Persian Gulf of Iran it is needed to simulate storms in coastal areas and study the protection techniques in order to protect communities and industrial facilities in coastal areas. Mangrove forests play a unique role in attenuating storm surge during tropical cyclones. MIKE21 FM is an applicable software for simulating cyclones within coastal environments. A hydrodynamic model is used to analyze the attenuating role of the protected area of mangroves between Gheshm Island and Khamir Port mangroves under a realistic conditions by updating the calibrated Manning's coefficient based on the land cover data to incorporate the mangrove effect with modifying bottom friction and characteristics of cyclone Gonu 2007 which is the strongest cyclone recorded in the Arabian Sea and Persian Gulf. In this research we measured the maximum surge levels in both scenarios (in situ conditions and the condition which mangrove forest is neglected); maximum storm tide reduction by mangroves (MSTRM) by mangroves and maximum storm tide reduction rate by mangroves (MSTRRM). Also inundation maps are presented aiming to measure the areas rescued from inundation and the areas inundated caused by mangroves. The simulations shows that the minimum and maximum storm tide reduction rate by mangroves are 5.32% and 34.88% respectively.
The main features of wave properties in relatively large but shallow Lake Peipsi (Estonia/Russia) are determined based on wave measurements at its western coast (58.75 degrees N 27.1 degrees E) in summer and autumn 2005-2007. Although the data set is relatively limited, it still covers 263 days and characterizes well the basic properties of wave climate in this water body. The wave regime is mostly calm, with the long-term average significant wave height below 0.3 m and seas with H-s < 1.2 m covering at least 2/3 of the ice-free time. The seasonal variation in wave properties mimics the analogous variation in the wind speed, with the most stormy months October-December. Wave heights are, on average, considerably lower in summer (July-August) than in autumn (October-November). Significant wave heights >1 m were recorded in autumn and covered 3% or the measurement time. The maximum recorded wave height H-s = 1.98 m occurred on October 27, 2005. The mean periods are mostly concentrated in a range of 1.5-2.5 s and exhibit an almost Gaussian distribution.
The paper presents theoretical and numerical analyses of random wave attenuation attributable to vegetation. Existing models based on Rayleigh distribution of wave heights are critically examined followed by the development of two new models for random waves over vegetation. The first model is derived on the basis of Hasselmann and Collins' treatment of energy dissipation of random waves attributable to the bottom friction. The second model is derived on the basis of Longuet-Higgins' probability density function for the joint distribution of wave heights and wave periods, which recovers to the model that uses the Rayleigh distribution of wave heights if the spectrum becomes narrow banded. Such a model allows for quantifying the effects of the spectral width on the model performances. Comparisons of the modeled and measured root-mean-square wave heights over vegetation show good agreement. Moreover, the Joint distribution-based model provides insight into the spectral distribution of the energy dissipation, which is different from other dissipation models that follow exactly the wave energy spectrum. DOI: 10.1061/(ASCE)EM.1943-7889.0000318. (C) 2012 American Society of Civil Engineers.
Wave data were measured along a 28 m transect using 3 pressure transducers over a 2-day period during a tropical storm. The tropical storm force winds produced waves up to 0.4 m high (zero-moment) that propagated over vegetation of Spartina alterniflora submerged under a surge of over 1 m above the marsh floor. Measured wave heights, energy losses between gages and spectral energy dissipation models of rigid vegetation were utilized to estimate wave height decay rates, integral and frequency-dependent bulk drag coefficients, and frequency distribution of energy dissipation induced by the vegetation. Measurements showed that incident waves attenuated exponentially over the vegetation. The exponential wave height decay rate decreased as Reynolds number increased. The swell was observed to decay at a slower rate than the wind sea regardless of the wave height. The linear spatial wave height reduction rate increased from 1.5% to 4% /m as incident wave height decreased. The bulk drag coefficient estimated from the field measurement decreased with increasing Reynolds and Keulegan-Carpenter numbers. The energy dissipation varied across the frequency scales with the largest magnitude observed near the spectral peaks, above which the dissipation gradually decreased. The wave energy dissipation did not linearly follow the incident energy, and the degree of non-linearity varied with the frequency. For a given spectrum, the frequency-distributed drag coefficient increased gradually up to the peak frequency and remained approximately at a stable value at the higher frequencies. This spectral variation was parameterized by introducing a frequency-dependent velocity attenuation parameter inside the canopy. The spectral drag coefficient is shown to predict the distribution of energy dissipation with more accuracy than the integral coefficients, which results in a more accurate prediction of the mean wave period and spectral width of a wave field with vegetation.
Coastal wetlands have, for many decades, fascinated ecologists and geomorphologists alike. The existence of terrestrial vegetation communities in highly saline and hydraulically extremely dynamic environments has provided an ideal opportunity to study both ecological adaptation mechanisms to physical stressors as well as the importance of vegetation to landform evolution. In recent years, however, the importance of understanding the linkages between the biological and physical factors that control coastal wetland functioning and evolution has been brought into focus within the conservation, engineering, and policy sector. This is largely the result of a rising awareness of the value of coastal wetlands resulting from the services they provide to society. Those services include their role as natural sea defenses, a role that is becoming increasingly significant in the context of ever increasing coastal population densities alongside environmental pressures (e.g. sea level rise and increasing storm frequencies) arising from climate change. This paper reviews how, over the past quarter of a century, advances in field, laboratory, and numerical modeling approaches have made particular inroads into the quantification of the sea defense role of coastal wetlands. It is becoming increasingly clear that the sea defense function itself is complex and highly context dependent. Although there is now an urgent need for improved ecologically-informed engineering solutions, these are unlikely to be successful without future research finding appropriate ways of scaling up hydraulically important parameters to the landscape scale and defining the physical and biological process thresholds that control the continued provisioning of the sea defense function of coastal wetlands in the face of potential extreme events and sea level rise.
Zero-crossing wave heights, obtained from the field measurement of random waves propagating through salt marsh vegetation (Spartina alterniflora) during a tropical storm, were analyzed to examine their probability distribution. Wave data (significant wave heights up to 0.4 m in 0.8 m depth) were collected over a two-day period along a 28 m transect using three pressure transducers sampling at 10 Hz. Wave height distribution was observed to deviate from the Rayleigh distribution. The observed probability densities of the larger wave heights were reduced significantly by vegetation, producing wave heights lower than those predicted by the Rayleigh distribution. Assuming Rayleigh distributed wave heights for the incident waves to the vegetation patch, existing vegetation-induced wave attenuation formulations are used to derive a special form of two-parameter Weibull distribution for wave heights in the inundated wetland. The scale parameter of the distribution is theoretically shown to be a function of the shape parameter, which agrees with the measurements, effectively reducing the proposed distribution to a one-parameter type. The derived distribution depends on the local parameters only and fits well to the observed distribution of wave heights attenuated by vegetation. Empirical relationships are developed to estimate the shape parameter from the local wave parameters.
A description is given of a model developed for the prediction of the dissipation of energy in random waves breaking on a beach. The dissipation rate per breaking wave is estimated from that in a bore of corresponding height, while the probability of occurrence of breaking waves is estimated on the basis of a wave height distribution with an upper cut-off which in shallow water is determined mainly by the local depth. A comparison with measurements of wave height decay and set-up, on a plane beach and on a beach with a bar-trough profile, indicates that the model is capable of predicting qualitatively and quantitatively all the main features of the data.
This paper presents an analytical solution of water waves propagating over submerged vegetation and a mathematical expression for vegetation motion swayed by the water waves.
Aquatic plants convert mean,kinetic energy into turbulent kinetic energy at the scale of the plant stems and branches. This energy transfer, linked to wake generation, affects vegetative drag and turbulence intensity. Drawing on this physical link, a model is developed to describe the drag, turbulence and diffusion for flow through emergent vegetation which for the first time captures the relevant underlying physics, and covers the natural range of vegetation density and stem Reynolds’ numbers. The model,is supported by laboratory and field observations. In addition, this work extends the cylinder-based model for vegetative resistance by including the dependence of the drag coefficient, CD, on the stem population density, and introduces the importance of mechanical diffusion in vegetated flows.
A parabolic model for calculating the combined refraction/diffraction of monochromatic linear waves is developed, including a term which allows for the dissipation of wave energy. The coefficient of the dissipation term is related to a number of dissipative models. Wave calculations are performed for a localized area of dissipation, based on a friction model for a spatial distribution of rigid vertical cylinders. The region of localized dissipation creates a shadow region of low wave energy, which may have important implications for the response of neighboring shore lines.
To achieve sustainable coastal management and planning, the interaction between fine-grained - in particular , vegetated - intertidal environments and incoming waves needs to be better understood. Previous studies have established that wave attenuation over saltmarshes can be significantly greater than over unvegetated intertidal surfaces. However, detailed, quantitative information on the effect of marsh elevation in the tidal frame, marsh width, seaward marsh edge configuration (e.g. cliffed versus ramped marshes), seasonal changes in marsh surface roughness (e.g. creek density, vegetation composition) and incident wave conditions, however, has been lacking. Based on a 10-month-long wave/tide dataset from two sites on the Dengie marshes, eastern England, this study addresses the effect of (i) marsh edge topography; (ii) marsh width; (iii) inundation depths; and (iv) seasonal changes in marsh surface vegetation cover on wave height and wave energy dissipation. Directional waves and water levels were recorded at 21 locations across both shallow-sloping and cliffed (cliff height of ca. 1.5m) intertidal profiles. In addition, changes in marsh surface vegetation cover and composition were recorded on a seasonal basis. Wave height attenuation over 310m of the shallow-sloping profile averaged 92 % over the monitoring period. Further analysis shows that the most rapid reduction in wave heights occurs over the most seaward 10 meters of permanent saltmarsh vegetation, where wave height attenuation averaged 2.1% and 1.1% per meter at the shallow- sloping and cliffed site respectively. Across the mudflat and the saltmarsh as a whole, wave height dissipation rates were significantly lower with an average of 0.1% and 0.5% per meter respectively. The presence of a saltmarsh cliff increased average wave heights by up to 0.5% per meter. Observed wave height attenuation showed a seasonal pattern at both sites (average wave energy attenuation near the marsh edge was highest in September - November and lowest in March - July) and appears to be linked to the cycle of seasonal vegetation growth. The study provides criteria for the assessment of the wave dissipation potential of marshes characterised by different widths, edge configurations and slopes, variability of water depths, and seasonal variations in vegetation cover/density. ADDITIONALINDEXWORDS: wave attenuation, coastal management, intertidal hydrodynamics, coastal geomorphology
A description is given of a model developed for the prediction of the dissipation of energy in random waves breaking on a beach. The dissipation rate per breaking wave is estimated from that in a bore of corresponding height, while the probability of occurrence of breaking waves is estimated on the basis of a wave height distribution with an upper cut-off which in shallow water is determined mainly by the local depth. A comparison with measurements of wave height decay and set-up, on a plane beach and on a beach with a bar-trough profile, indicates that the model is capable of predicting qualitatively and quantitatively all the main features of the data.
Earlier models of random wave transformation are reviewed in the first section. Then the transformation of waves, including dissipation due to breaking and bottom friction, is described by an energy flux balance model. The wave height pdf of all waves (broken and unbroken) is shown by the field data to be well described by the Rayleigh distribution everywhere. The observed distributions of breaking and broken wave heights are fitted to simple analytical forms, and breaking wave dissipation is calculated by using a periodic bore formulation. The energy flux equation is integrated to yield local values of Hrms as a function of offshore wave conditions. Both analytical and numerical models are developed. In the last section the models are compared with results from random wave experiments in the laboratory and from an extensive set of field measurements.
The results of a fetch limited wind wave growth experiment in water of finite depth are presented. The experiment involved measurements of wind wave spectra, wind speed and direction at eight stations along the fetch. The site for the experiment was a lake with an almost constant water depth of 2 m. The results clearly show the evolution of both the non-dimensional total energy and peak frequency as a function of non-dimensional fetch. At short fetch, the waves are in deep water and the results are comparable to previous deep water measurements. As the fetch increases, the results diverge from the deep water case. Both the growth of total energy and the migration of the peak frequency to lower values are reduced in comparison to results from deep water conditions. At large fetch, evolution of both total energy and peak frequency ceases, both parameters becoming depth limited. Rather than there being a single growth law formula, as in deep water, a family of curves are developed, one for each value of non-dimensional depth.
Modeling capabilities for shallow, vegetated, systems are reviewed to assess hydrodynamic, wind and wave, submersed plant friction, and sediment transport aspects. Typically, ecosystems with submersed aquatic vegetation are relatively shallow, physically stable and of moderate hydrodynamic energy. Wind-waves are often important to sediment resuspension. These are open systems that receive flows of material and energy to various degrees around their boundaries. Bed shear-stress, erosion, light extinction and submersed aquatic vegetation influence each other. Therefore, it is difficult to uncouple these components in model systems. Spatial changes in temperature, salinity, dissolved and particulate material depend on hydrodynamics. Water motions range from wind-wave scales on the small end, which might be important to erosion, to sub-tidal or seasonal scales on the large end, which are generally important to flushing. Seagrass modifies waves and, therefore, affects the relationships among the non-dimensional scaling parameters commonly used in wave analysis. Seagrass shelters the bed, often causing aggradation and changes in grain size, while increasing total resistance to flow. Hydrodynamic friction can not be well characterized by a single-parameter equation in seagrass beds, and models need appropriate enhancement when applied to these systems.
Presently, modeling is limited by computational power, which is, however, improving. Other limitations include information on seagrass effects expressed in frictional resistance to currents, bed-sheltering, and wave damping in very shallow water under conditions of both normal and high bed roughness. Moreover, quantitative information on atmospheric friction and shear stress in shallow water and seagrass areas are needed. So far, various empirical equations have been used with wind or wave forcing to describe resuspension in shallow water. Although these equations have been reasonably successful in predicting suspended sediment concentrations, they require site-specific data. More detailed laboratory and field measurements are needed to improve the resuspension equations and model formulation pertaining to seagrass beds.
Based on a field observation at the Vinh Quang coast in northern Vietnam, the characteristics of wave reduction due to the
drag force of one mangrove species, Sonneratia sp., were quantitatively analyzed. The reduction rate of sea waves in this area changed substantially with the tidal phase,
due to the unique vertical configuration of Sonneratia sp. At the shallow range of water depth, since the shape of pneumatophores
of Sonneratia sp. tapers off upward, the effect of drag force by these roots on the wave reduction decreased with the increase in the water
level, resulting in a decrease in the rate of wave reduction. On the other hand, when water levels rose above the height of
thickly spread branches and leaves of these trees, the rate of wave reduction increased again with an increase in the water
level. Further, at this high range of water level, the rate of wave reduction depended strongly on the incident wave height.
These results indicate that the thickly grown mangrove leaves effectively dissipate huge wave energy which occurs during storms
such as typhoons, and protect coastal areas. Referring to the past studies, our results suggest that the hydrodynamic knowledge
in various mangrove conditions such as the vertical configuration of mangrove species, their vegetation conditions, water
depth, incident wave condition and the relationships between these factors should be further accumulated and then quantitatively
formulated in order to protect coastal areas from severe sea waves.
Suspended solids concentrations were measured at routine 2–3 week intervals and on additional windy days for at least one year in each of seven shallow (mean depth < 2="" m)="" south="" island,="" new="" zealand="" lakes.="" surface="" wave="" characteristics="" were="" estimated="" from="" water="" depths="" and="" local="" meteorological="" data="" using="" a="" shallow-water="" wave="" forecasting="" model="" for="" fetch-limited="" waves.="" bottom="" shear="" stresses="" were="" computed="" from="" surface="" wave="" characteristics="" for="" the="" sampling="" stations="" and="" for="" a="" hypothetical="" lake-average="" station.="" the="" calculated="" shear="" stresses="" were,="" on="" average,="" much="" better="" predictors="" of="" suspended="" solids="" concentrations="" than="" alternative="" models="" using="" two="" different="" functions="" of="" wind="" speed,="" wave="">2/depth or wavelength/depth. A combination of the sample station and lake average shear stresses provided slightly better predictions than the sample station values alone, suggesting that currents also contribute significantly to the concentration at a given point. Regressions of suspended solids on the combined functions had r
2 values ranging from 0.74–0.73 in the seven lakes. The slopes of these regressions were negatively related to the settling velocity of the lowest quartile of the sediment, and to macrophyte biomass, in multiple regression (r
2 = 0.94, p
Based on the assumption that in the surf zone, random waves behave as a collection of individual regular waves, a closed-form transformation of random variable is performed to yield the probability density function for wave height across a beach of uniform slope. Starting from a Rayleigh distribution well seaward of the surf zone, the transformation is accomplished by using linear wave theory for shoaling and an analytical solution of a model for decay of regular waves due to breaking. Comparisons of the solution to histograms from the DUCK'85 field experiment demonstrate the model's ability to reproduce salient changes in shape of the histogram as the surf zone is traversed. Both data and model indicate that relative position in the surf zone, denoted by the local proportion of waves that are breaking, and bottom slope are the parameters which have the greatest effect on the shape of the probability density function. General expressions for characteristic wave heights (e.g. root-mean-square wave height) are also derived, and their transformation across the surf zone is found to depend distinctly on beach slope and mean wave steepness behavior that has been previously reported as trends in laboratory and field data. Although some facets not included in the model can be important, such as nonlinear shoaling or a distribution in wave period, the closed-form solution made possible by this simpler formulation inherently contains much of the behavior observed in field data, and thereby serves to edify the problem of random breaking waves in the surf zone.
Flow dynamics on the vegetated surfaces of coastal wetlands may impact a wide range of processes including geochemical exchanges at the sediment water interface, larval recruitment and dispersion, and sediment deposition and retention. Nevertheless, little field data exist which describe flow behavior through emergent vegetated wetlands and its control over sediment transport and deposition. The goal of this paper is to describe canopy flow dynamics and suspended particulate transport for a variety of marshes that differ with respect to vegetation type and tidal regime. hi situ measurements of tidal currents were collected in micro-, meso-, and macrotidal marshes of the Pacific, Gulf of Mexico and Atlantic coasts of the US and in a UK marsh on the North Sea. Mean flow speeds, vertical velocity profiles, and turbulence intensities were evaluated as were canopy characteristics and total suspended solid (TSS) levels. Broad scale flow characteristics exhibited little variation among sites. Mean flow speeds were almost always less than 10 cm s(-1) regardless of tidal regime. The presence of vegetation (regardless of type) significantly reduced both flow speed and turbulence intensity relative to adjacent open water areas. Variations in canopy morphology and the physical structure of individual plants control fine scale hydrodynamics, and influence particle advection, and particle settling. Flow speed magnitude and the importance of creek channel processes, however, appear to be most strongly influenced by the tidal regime in each of the marsh types examined.
This paper investigates the drag exerted by randomly distributed, rigid, emergent circular cylinders of uniform diameter d. Laboratory measurements are presented for solid volume fraction phi=0.091, 0.15, 0.20, 0.27, and 0.35 and cylinder Reynolds number Re-p U(p)d/nu=25 to 685, where U-p=temporally and cross-sectionally averaged pore velocity and nu=kinematic viscosity. These ranges coincide with conditions in aquatic plant canopies. The temporally and cross-sectionally averaged drag coefficient, C-D, decreased with increasing Re-p and increased with increasing phi under the flow conditions investigated. The dimensionless ratio of the mean drag per unit cylinder length <(f(D)) over bar >(H) to the product of the viscosity, mu, and U-p exhibits a linear Re-p dependence of the form <(f(D)) over bar >(H)/(mu U-p)=alpha(0)+alpha Re-1(p), consistent with Ergun's formulation for packed columns. In the range of experimental conditions, alpha(1), increases monotonically with phi. In contrast, alpha(0) is constant within uncertainty for 0.15 <= phi <= 0.35, which suggests that viscous drag per unit cylinder length is independent of phi in this range.
A model for the decay of random waves in the surf zone that requires transformation of only one representative wave height [root-mean square (RMS)] without making any assumption about the shape of the probability-density function is presented. The breaker decay model proposed by Dally is used as a starting point in the derivation of the new model. It is assumed that random wave properties in the surf zone may be obtained by individually transforming a large number of waves across shore and adding together the effect from each single-wave component. In deeper water, outside the surf zone, where wave breaking is negligible, a Rayleigh distribution is employed to characterize the randomness of the sea. This semianalytic model is compared with a complete Monte Carlo-simulation approach for different beach profile shapes, including barred profiles, and macrofeatures of wave height and energy flux transformation across the surf zone are similar for the two approaches. Comparisons are made in the paper with laboratory data from the SUPERTANK project and field data from the DELILAH project.
The vertically two-dimensional problem of small-amplitude waves propagating over submerged vegetation is formulated using the continuity and linearized momentum equations for the regions above and within the vegetation. The effects of the vegetation on the flow field are assumed to be expressible in terms of the drag force acting on the vegetation. An analytical solution is obtained for the monochromatic wave whose height decays exponentially. The expressions for the wave number and the exponential decay coefficient derived for arbitrary damping are shown to reduce to those based on linear wave theory and the conservation equation of energy if the damping is small. The analytical solution is compared with 60 test runs conducted using deeply submerged artificial kelp. The calibrated drag coefficients for these runs are found to vary in a wide range and appear to be affected by the kelp motion and viscous effects that are neglected in the analysis. The analytical solution is also shown to be applicable to subaerial vegetation, which is predicted to be much more effective in dissipating wave energy.
Wave setup can contribute significantly to elevated water levels during severe storms. In Florida we have found that wave setup can be 30% to 60% of the total 100-year storm surge. In areas with relatively narrow continental shelves, such as many locations along the Pacific Coast of the United States, wave setup can be an even larger proportionate contributor of anomalous water levels during major storms. Wave setup can be considered as comprising two components, with the first being the well-known static wave setup resulting from the transfer of breaking wave momentum to the water column. The second, oscillating component, is a result of nonlinear transfer of energy and momentum from the primary (linear) spectrum to waves with length and time scales on the order of the wave groups.
Though there is widespread agreement thatSpartina
alterniflora marshes absorb some wave energy, there is considerable question regarding the magnitude and importance of this process. It
has been suggested that marshes are much like an array of vertical cylinders in a water column. Based upon empirical estimates
of the fluid drag forces occurring on vertical cylinders and laboratory observations of various arrays of cylinders, a model
was developed to describe wave decay in marshes. In 1981, a series of field experiments were conducted to test and calibrate
this empirical model in a series of naturalS.
alterniflora marshes. The model with some modification was found to be very useful for describing wave decay in coastal marshes.
This book covers the hydrodynamic effects of offshore structures. The subject of hydrodynamics applied to the structures is so vast that it is impossible to incorporate every aspect of this subject in one book. This monograph introduces various types of offshore structures with reference to their actual installation in various parts of the world. It describes wave mechanics and how to choose wave theories and design waves. After a choice of design is made, the resulting wave is used to calculate forces acting on a fixed offshore structure. Various methods for determining the ensuing motions of these structures are presented. Short- and long-term responses are derived, and model tests to verify these methods are carefully explained.
Mangroves are a special form of vegetation as they exist at the boundary of terrestrial and marine environment. They have a special role in supporting fisheries and in the stabilizing the tropical coastal zones. Biochemical and trophodynamic processes in the mangroves are strongly linked to water movement, due to tides and waves. In this paper we present the theoretical attempt to predict the attenuation of wind-induced random surface waves in the mangrove forest. The energy dissipation in the frequency domain is determined by treating the mangrove forest as a random media with certain characteristics determined using the geometry of mangrove trunks and their locations. Initial nonlinear governing equations are linearized using the concept of minimalization in the stochastic sense and interactions between mangrove trunks and roots have been introduced through the modification of the drag coefficients. The resulting rate of wave energy attenuation depends strongly on the density of the mangrove forest, diameter of mangrove roots and trunks, and on the spectral characteristics of the incident waves. Examples of numerical calculations as well as preliminary results from observation of wave attenuation through mangrove forests at Townsville (Australia) and Iriomote Island (Japan) are given.
1. Sediment resuspension dynamics were investigated in relation to changes in water column nutrients (TP, TN, PO4-P, NO3-N and NH4-N), chlorophyll a and phaeopigment in seven shallow (Zm < 1.5 m) lakes in South Island, New Zealand, ranging in area from 0.1 to 180 km2.
2. Benthic shear stress, calculated from wind speed, effective fetch and depth, was a considerably better predictor of nutrient and pigment concentrations than wind speed.
3. For TP, TN, chlorophyll a and phaeopigment, sixteen of the possible twenty-eight linear correlations with benthic shear stress were significant at P < 0.05, with 16–87% of the variation being explained by shear stress.
4. Wind decreased the ratios of TN : TP, with ratios exponentially approaching those of the sediments as shear stress increased in four of the lakes.
5. Relationships of dissolved inorganic nutrients to shear stress were considerably weaker than those for total nutrients and showed no consistent trend over the seven lakes.
6. Estimated annual mean TP inclusive of resuspension was over four times higher than that derived from measured calm samples in two lakes.
7. The number of nutrient and pigment parameters that were significantly correlated with shear stress and the strengths of the relationships varied widely from lake to lake. We could establish no simple relationships between these effects and any single characteristic of the lake, sediment, or water.
8. A function is developed to predict the rate of entrainment of TN and TP in response to an applied shear stress, where the independent variables are sediment nutrient content and particle size, and the macrophyte density in the lake.
Aim To investigate the long-term changes in aquatic vegetation in a lowland, shallow lake, and to assess the relationship between aquatic vegetation and natural and anthropogenic catchment changes.
Location Gundsømagle Sø, Zealand, Denmark: a shallow (mean depth 1.2 m), hypereutrophic lake (mean annual total phosphorus (TP) c. 700 μg TP L−1) located in a predominantly agricultural catchment (88% cultivated land). The lake is presently devoid of macrophytes.
Methods One hundred and forty-seven contiguous samples from a sediment core (taken in 2000) were analysed for macrofossil remains together with loss-on-ignition and dry weight. From an earlier sediment core (taken in 1992), 67 samples were analysed for pollen and the two cores were correlated using the ignition residue profiles. Core chronology was determined by 210Pb and 137Cs dating of the recent lake sediments, while older sediments were dated by pollen-stratigraphical correlation, as 14C dating proved problematical. Aquatic macrofossil abundance was used to reconstruct past changes in the lake's plant community and water-level. The contemporary catchment land-use change was inferred from sedimentary pollen data, and soil erosion to the lake was deduced from the minerogenic content of the lake sediments.
Results The macrofossil record covers the last 7000 years, but aquatic plant remains were scarce prior to c. 1300 bc. After this date the abundance of submerged and emergent macrophyte remains increased dramatically, paralleled by an increase in sediment minerogenic matter and non-arboreal pollen (NAP). Aquatic plant remains were abundant for more than 3000 years until the mid 1900s. Macrofossils of Linum usitatissimum (L.) (flax) and high pollen percentages of ‘Cannabis type’ (hemp) were recorded in periods between c. 1150 bc and 1800 ad.
Main conclusions Our study suggests that, between c. 5000 bc and 1300 bc, the submerged plant community was confined to the littoral zone. From 1300 bc onwards, the submerged macrophyte vegetation expanded rapidly across the lake bed, presumably as a response to lake shallowing caused by a combination of climatic-induced water-level lowering and enhanced erosional infilling of the lake basin due to intensified anthropogenic activities in the catchment. The lake was meso-eutrophic and had an extensive and diverse aquatic flora for more than 3000 years, until the middle of the twentieth century. In periods between c. 1150 bc and 1800 ad, the lake experienced direct anthropogenic impact from retting of fibre plants (Linum and Cannabis). Over the last 200 years, erosional infilling of the lake basin increased drastically, probably as a result of agricultural intensification. In the twentieth century, the lake was strongly affected by nutrient enrichment from both point sources (sewage from built-up areas) and diffuse agricultural run-off which led to hypertrophic conditions and the collapse of the submerged vegetation c. 1950–60. The concept of ‘naturalness’ and the implications for lake conservation are discussed.
1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia.2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m−2 of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m−2, respectively.3. With the resuspended sediment, 39.4 mg P m−2 day−1 was brought to the water column outside the stand, 22.4 mg P m−2 day−1 at the edge and 13.4 mg P m−2 day−1 within the stand.4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.
The study is based on monitoring data on the seasonal variation during four (1996–1999) vege-tation periods, as well as long-term summer data on submerged vegetation, nutrients, light, phyto-plankton and zooplankton in Lake Krankesjön, a shallow, calcium-rich, moderately eutrophic lake in southern Sweden. The lake has been in the clear water state with abundant submerged vegetation since the end of the 1980s. Somewhat lower summer biomass of submerged macrophytes during 1997 and 1999 indicates a temporary instability of the clear water state. During these 2 years, summer transparency was about 1.2–2.1 m, while concentrations of total phosphorus and chlorophyll (Chl) a were about 26–40 and 8–18 g l −1 , respectively.
Field studies of tidal flows in largely pristine mangrove swamps suggestthat the momentum equation simplifies to a balance between the water surfaceslope and the drag force. The controlling parameter is the vegetation lengthscale LE, which is a function of the projected area ofmangrove vegetation and the volume of the vegetation. The value ofLE varies greatly with mangrove species and water depth. It isfound that the drag coefficient is related to the Reynolds number Re definedusing LE. The drag coefficient decreases with increasingvalues of Re from a maximum value of 10 at low value of Re (4), and converges towards 0.4 for Re < 5="">4.
In this work, a model for wave transformation on vegetation fields is presented. The formulation includes wave damping and wave breaking over vegetation fields at variable depths. Based on a nonlinear formulation of the drag force, either the transformation of monochromatic waves or irregular waves can be modelled considering geometric and physical characteristics of the vegetation field. The model depends on a single parameter similar to the drag coefficient, which is parameterized as a function of the local Keulegan–Carpenter number for a specific type of plant. Given this parameterization, determined with laboratory experiments for each plant type, the model is able to reproduce the root-mean-square wave height transformation observed in experimental data with reasonable accuracy.
The degree to which incident wind waves are attenuated over intertidal surfaces is critical to the development of coastal wetlands, which are, amongst other processes, affected by the delivery, erosion, and/or resuspension of sediment due to wave action. Knowledge on wave attenuation over saltmarsh surfaces is also essential for accurate assessments of their natural sea-defence value to be made and incorporated into sea defence and management schemes. The aim of this paper is to evaluate the use of a digital photographic method for the quantification of marsh vegetation density and then to investigate the relative roles played by hydrodynamic controls and vegetation density/type in causing the attenuation of incident waves over a macro-tidal saltmarsh.
Wave attenuation was studied in a coastal mangrove system in the Red River Delta, Vietnam on the coast north of Do Son. From sea towards land the study area consisted of a bare mudflat, covered by a sandy layer with embryonic cheniers, abruptly changing into a muddy tidal flat overgrown with mangroves. Three instrumented tripods (A–C) placed in a cross-shore profile, were used to measure current velocity and water level, at the open tidal flat, at the beginning of the mangrove vegetation, and in the mangrove vegetation, respectively. Measurements were conducted in the wet season in July and August 2000. The elevation of the area was surveyed using a levelling instrument. Over the bare sandy surface of the mudflat, the incoming waves are reduced in height (and energy density) due to bottom friction. This reduction decreases with increasing water depth. In the mangrove vegetation, the bottom friction exerted by the clay particles is very low. However, the dense network of trunks, branches and above ground roots of the mangrove vegetation causes a much higher drag force. For the mangrove vegetation which mainly consists of Kandelia candel, the drag force can be approached by the function CD = 0.6e0.15A (with A being the projected cross-sectional area of the under water obstacles at a certain water depth). For the same muddy surface without mangroves the function would be CD = 0.6.
Emergent vegetation development, wave extinction and soil erosion are strongly interrelated processes in exposed riparian zones. The above-ground parts of the vegetation reduce wave energy, while the below-ground parts strengthen the soil. On the other hand, vegetation development may be restricted as a result of wave stress. Interactions between waves, soil erosion, and emergent vegetation were studied during three consecutive years. Two helophyte species, Phragmites australis (Cav.) Trin. ex Steudel and Scirpus lacustris L., were planted in separate bank sections on two types of sediment, sand and silty sand, in a wave tank. Regular waves were transmitted through 4 m wide bank sections with and without helophytes growing on a horizontal part. Bank profiles, wave transmission patterns and vegetation parameters were measured after exposure to waves with a height of 10 cm (Year 1) and 23 cm (Years 2 and 3). Both 10 cm and 23 cm waves affected bank profiles. Erosion of the banks occurred due to downslope transport of sediment. Soil erosion patterns closely reflected the patterns of standing waves over the horizontal part of the bank. Emergent vegetation influenced the erosive impact of waves by both sediment reinforcement and wave attenuation. A smaller amount of net erosion was measured in the wave-exposed sections covered by vegetation than in the unplanted sections. The stands of Scirpus lacustris were damaged due to uprooting of rhizome parts by 23 cm waves, followed by increased erosion of the soil. No damage occurred to the Phragmites australis stands. The greatest wave attenuation (measured as relative wave height reduction) was measured in the fully developed vegetation in August of each year in both types of vegetation.
Seagrasses are able to modify current flow and sediment composition, yet little information exists describing their effect on waves. Four species of seagrass, Halodule wrightii, Syringodium filiforme, Thalassia testudinum and Zostera marina were evaluated for their ability to reduce wave energy under various combinations of shoot density and water depths over a 1 m test section in a wave tank. Percent wave energy reduction per meter of seagrass bed equaled 40% when the length of these seagrasses was similar to the water depth. Seagrasses are approximately equal to saltmarshes in reducing wave energy on a unit distance basis, but only when water depth is scaled to plant size. When seagrass beds occur as broad, shallow meadows, the influence of seagrasses on wave energy will be substantial.
The turbidity of lakes is generally considered to be a smooth function of their nutrient status. However, recent results suggest that over a range of nutrient concentrations, shallow lakes can have two alternative equilibria: a clear state dominated by aquatic vegetation, and a turbid state characterized by high algal biomass. This bi-stability has important implications for the possibilities of restoring eutrophied shallow lakes. Nutrient reduction alone may have little impact on water clarity, but an ecosystem disturbance like foodweb manipulation can bring the lake back to a stable clear state. We discuss the reasons why alternative equilibria are theoretically expected in shallow lakes, review evidence from the field and evaluate recent applications of this insight in lake management.
Both field studies and laboratory experiments were carried out in order to identify relevant processes that cause the phenomenon of a clear water phase above submerged vegetation fields, as commonly observed in lakes in The Netherlands. Results from the field study revealed that an increase in the turbidity level of lake waters is due to local wind induced wave activity. Advective transport of suspended sediment is shown not to contribute to changes in the turbidity level. Resuspension of bed material by waves is likely confined to a so-called 'fluffy layer'. Results from the laboratory study showed that submerged vegetation decreased the eddy diffusivity by affecting both the turbulent kinetic energy and the sizes of the turbulent structures. However, this did not result in an increase in sedimentation within the vegetation field. Waves were effectively damped by the vegetation. This effect is a function of plant morphology (stiffness and plant length). Results from the laboratory experiments therefore corroborate the findings from the field study: the phenomenon of a clear water phase above submerged vegetation canopy is most likely due to the dampening effect of waves by the vegetation, which inhibits local resuspension of the sediment bed.
Hydro-physical conditions in kelp forests and the effect on wave damping and dune erosion
- S M Løvås
Løvås, S. M. 2000. " Hydro-physical conditions in kelp forests and the
effect on wave damping and dune erosion. " Ph.D. thesis, The Norwegian Univ. of Science and Technology, Trondheim.
Shore protection manual, Dept. of the Army, Waterways Experiment Station, Corps of Engineers
U.S. Army Coastal Engineering Research Center. 1984. Shore protection manual, Dept. of the Army, Waterways Experiment Station,
Corps of Engineers, Coastal Engineering Research Center, Washington, D.C.