«Habilitationsschrift zur Erlangung der venia legendi im Fachbereich Geowissenschaften der Universität Bremen Dr. Christian Winter Bremen, 2011 ...»
Observation- and Modelling of
Morphodynamics in Sandy Coastal Environments
zur Erlangung der venia legendi
der Universität Bremen
Dr. Christian Winter
The topic of sandy coast morphodynamics involves the mutual influences of coastal
topography, local sedimentology, the driving meteorological and hydrodynamic boundary
conditions, flora and fauna, and the activities of human beings: The latter as direct actors through coastal constructions and other interventions, as indirect actors through possible contributions to global change, but also as receiving agents - as living individuals confronted with the forces of the sea.
The general aim of coastal research is to gain an as comprehensive as possible understanding of the different systems and their interaction in order to be able to evaluate their current state, assess their stability, explain past changes (in the geological record), and predict future developments under different conditions. Such systems dynamics involve a large bandwidth of spatial and temporal scales: from the microscopic interaction of turbulent fluid motions with single particles to meso-scale tidal dynamics of subaqueous bedforms to macro-scale seasonal adaptations of beach profiles or the meandering of tidal channels, to the mega-scale evolution of shorelines and shelf systems over decades to centuries.
The process of understanding involves a continuous feedback of observations, abstractions, mathematical formulations, model development (ranging from conceptual models to mathematical formulations of processes, and to complex, process-based numerical modelling systems), and the testing of models on the basis of observations, new abstractions, and so forth. In the case of the morphodynamics of sandy coasts, the interaction of the physical processes involved in hydrodynamics, sediment dynamics, and their mutual adjustment to changing bed topographies seem most relevant, although biogeochemical processes play a (commonly underrated) additional role.
This discourse presents an extended summary of the current state in the continuous process of gaining knowledge on coastal morphodynamics. It focuses on the dynamics of tidal channels and their main roughness elements: subaqueous compound bedforms. Methodological approaches involved are field measurements and numerical modelling, which are introduced and discussed.
Contents 1 Introduction
2 Observation of coastal morphodynamics
2.1 Observation of micro- to meso-scale morphodynamics
2.2 Observation of macro- to mega-scale morphodynamics
3 Morphodynamic modelling
3.1 Morphodynamic model concepts
3.2 The modelling process
3.3 Model evaluation
3.4 A short biography of process based morphodynamic modelling
4 Synthesis: Combining field data and models
4.1 Papers on sediment transport processes and model reduction
4.2 Papers on data reduction and model applications
Land and sea meet in the coastal zone - it is ‘the part of the land most affected by its proximity to the sea and that part of the ocean most affected by its proximity to the land’ (Hinrichsen, 1998). Coasts are among the most dynamic environments on the planet because here terrestrial and marine processes continuously interact over a broad bandwith of spatial and temporal scales. In its vast variety of geographical forms, ecological richness, physical relations, and economical values the coast has for eons fascinated humankind from casual admirers to dedicated scientists of various disciplines who try to understand the interaction of the complex processes controlling the evolution of coastlines and ecosystems under natural forcing and anthropogenic influence.
With increasing human exploitation, coastal zones in particular have come under increasing socio-economic pressure. The exploitation potential of the land-sea interface in terms of settlement, traffic, constructions, harbour development, tourism, fisheries, offshore structures, etc., goes along with severe impacts on the natural environment. Past experiences have shown the vulnerability of marine systems, e.g. in the form of large-scale responses to small-scale coastal engineering interventions (Capobianco et al., 1999; Pilkey and Dixon, 1998), and the disastrous effects of extreme events (Pilkey and Young, 2005; Schiermeier, 2005). The often detrimental effects of human interferences with the coast have led to the development of management strategies which aim at a sustainable development of coastal zones based on detailed knowledge of the natural systems, the underlying processes, and their response to external forcing.
Various coastal systems can be defined which encompass the interaction, interrelation, and
interdependency of their associated entities:
• Socio-economic systems that link demographic and economic characteristics of a wide variety of coastal management issues such as, for example, environmental protection, coastal constructions, recreation, exploitation of natural resources.
• Ecosystems on micro-, local- and regional-scales that involve interdependent organisms such as plants and animals within the same habitat, and which are linked together through nutrient cycles and energy flow, and are individually and together influenced by chemical and physical factors of their environment.
• Physical systems that describe the dynamics of acting forces and their effects. The main entities of these systems encompass driving hydrodynamic processes, resulting transports, and their geomorphologic effects.
The above systems are characterised by their parts and composition, their drivers, processes and output, and their interconnectivity: The various parts of a system, and also the different systems by themselves, have functional and structural relationships between each other. These may be analysed as closed structures, but must take into account environmental aspects that act across the system boundaries.
Definition of Morphodynamics The above-mentioned physical systems can be understood as primary drivers of the other systems. The interplay between hydrodynamics (meteorological forcing, short waves and wave-, wind- and tide-induced currents) and sediment dynamics (erosion, transport, deposition and resulting morphological change) both drives and is influenced by coastal morphology. The main processes are commonly schematised as a looped series of fluid motion, sediment transport, and topographic change. The commonly cited work of Wright and Thom (1977) termed the ‘mutual adjustment of fluid dynamics and topography involving sediment transport’ as morphodynamics.
This scheme, however, can be misinterpreted as a closed system, tending towards a stable equilibrium because the most crucial external drivers (the open boundary conditions) are not mentioned. This certainly does not hold in natural systems that are continuously exposed to unsteady forcing. De Vriend (1991) understands the term morphodynamics more generally as the 'dynamic behaviour of alluvial boundaries'. The dynamic behaviour is the result of the feedback loop of hydrology, sediment transport and resulting bed evolution driven by timevariant or stationary boundary conditions (Figure 1).
Figure 1: Scheme of coastal morphodynamics: Loop of the mutual adjustment of fluid motions, topography and sediment transport under the influence of conditions at the system boundaries (b.c.).
The morphodynamic scales
The morphodynamic loop described above holds on several temporal and spatial scales:
Kraus et al. (1991) classified morphodynamics into micro-, meso-, macro-, and megatemporal and -spatial scales. In that sense, micro scales cover the interaction of waves, turbulence and single grains, and the formation of small ripples on the seabed, or the formation and destruction of flocs and aggregates in less than seconds to minutes on millimetre to centimetre length scales. Meso-scales cover processes acting on meters to kilometres in minutes to months. However, a further differentiation of the meso-scale into a small-scale (decimenters to tens of meters; minutes to days) and a large-scale (hundreds of meters to kilometres; days to months) seems appropriate in this context (Figure 2). Thus, small-scale morphodynamic processes comprise the formation, dynamics and hydraulics of bedforms such as small dunes or scours produced in the instantaneous response to tide- or wind- and wave-driven currents. The large meso–scale, for example, covers the migration of large dunes in tidal channels or the adaptation of beach profiles to storm conditions. Sediment pathways, tidal channel migration, or the vertically oscillating behaviour of tidal flats cover macro-length (kilometres) and -time scales (months to years). Mega-scale morphodynamics, in turn, comprises coastal features exceeding 10 km in length and dynamics over decades to centuries. Finally, sub-regional and regional morphodynamics (mega-spatial scale 10 km) occurs within macro- to mega-time scales (Kraus et al., 1991). The morphodynamics on yet larger scales, e.g. the Holocene evolution of coastlines or the formation of barrier islands, are beyond the scope of this discourse and are therefore not covered here.
Figure 2: Spatial and temporal scales and typical coastal morphological features.
Cowell and Thom (1997) group time scales at which coastal processes operate into four classes: Instantaneous time scales involve the evolution of morphology during a single cycle of the forces that drive morphological change (waves, tides) from a few seconds to many days or weeks. Event time scales are concerned with coastal evolution as a response to forcing processes operating across time spans ranging from that of an individual event through seasonal variation from a few days to many years. Engineering time scales describe coastal evolution under natural forcing and its response to human impact from a few months to decades. Geological or geomorphological time scales operate over decades to millennia and cover the evolution of landforms in response to long-term mean trends in the forces (sea level, climate).
It should be noted, that the above classifications draw rather arbitrary lines through a continuous time-space system. The listed features certainly rely on processes over different scales which interact and depend on each other. Scientific understanding of morphodynamic systems thus not only requires the study of processes and interactions according to the spatiotemporal equilibrium, but also the bridging of temporal and spatial process scales and classifications.
Understanding the system The visualisation and interpretation of coastal morphodynamics are commonly based on timeseries of morphological states. The inter-comparison of data of different times for the same area reveals a residual morphological evolution between the states. Topographic (‘dry morphology’) and bathymetric (‘wet morphology’) data usually are derived from land- or ship-based surveying (Ehlers, 1988) or remote sensing products like aerial photography, lidar or satellite altimetry (Chu et al., 2006; Kroon et al., 2007; Niedermeier et al., 2005).
Observed morphological changes can often be directly related to external (e.g. hydrodynamic) forcing factors. Beach erosion due to storm wave action, scouring around offshore foundations, or the breaching of dikes in a storm surge are examples of forced behaviour, also called ‘hydrodynamic templates’. In contrast to the obvious direct effect of the forcing agent to morphology, freely- or self-organised behaviour describes cases in which no obvious relation between assumed forcing factors and observed patterns can be recognised.
Patterns like bed ripples, dunes, or features like beach cusps and rip currents are often related to and explained by self-organisation. Obviously, the analysis of coastal morphodynamics requires more than just information on morphological states: The description of the relevant drivers is of prime importance. However, coherent hydro- and sediment dynamics are only rarely measured simultaneously with the morphological measurements and a completely synoptic analysis, i.e. the simultaneous observation of all relevant parameters of a multidimensional morphodynamic system, is certainly not feasible.