Practical Applications
- Lake Veluwe
- Schematised tidal basin
- Salt marsh restoration in Nisqually estuary, Puget Sound, USA
- Impact of salt marshes on tidal channel(s) (formation)
1. Lake Veluwe
For Lake Veluwe, a semi-enclosed lake between the old mainland and the Flevopolders in the Netherlands, spatially distributed measurements of the amount of macrophytes are available yearly from 1994 to 1999. This dataset was used to validate the ecological component of the model system. Figure 2 shows measured and computed Chara aspera population densities and distributions from year 1993 to 1997, indicated as population density classes from low (1) to high (7) (differences in initial conditions are due to data interpolation).
Although little effort was put into the model calibration, the observed spatial patterns of two macrophyte species in the lake are represented reasonably well by the model.
2. Schematized tidal basin
A schematized tidal inlet system was considered in relation to a salt marsh restoration project in San Francisco Bay – USA (schematized to make it more general). In Figure 3, the influence of saltmarsh formation on the morphological evolution is clearly visible: please note the modelled morphological evolution over time, with (center) and without (left) saltmarsh effects (z-level (m)). The vegetation distributions are shown in the right panels (from no vegetation (0%) to fully occupied (100%)).
Qualitatively, this influence could be described as a decrease in channel depth, which are also more narrow. This is well related to physical reasoning (increase in critical shear stress near vegetation hampers channel evolution). Furthermore, it was found that sediment exchange (export for this specific case) through the tidal inlet decreased significantly (approx. 50%). For additional information, refer to Ye, 2012.
3. Salt marsh restoration in Nisqually estuary, Puget Sound, USA
In the Nisqually estuary a saltmarsh restoration project was executed in 2009. By removing a dike, an area of 308 ha was exposed to salt water. Soon after removal of the dike the existing freshwater vegetation died off, old channels reopened and grew further inland, flushing almost all dead vegetation. The biogeomorphological modelling system should be able to reproduce this scenario. Figure 4 shows the observed morphological changes in the first 7 months after the dike removal. Although the parameters of the ecological processes are far from perfect, spatial patterns of the vegetation distribution are reproduced, qualitatively. The effects of vegetation on morphology appears significant. An increase in sedimentation on tidal flats (where vegetation grows and spreads) is observed, furthermore, sedimentation in channels decreased. Unfortunately, due to a lack of measured data, only qualitative comparisons could be made. As an example of the modelled vegetation growth, Figure 5 is shown, which shows the modelled vegetation pattern (Phalaris arundinacea – reed canarygrass) after three years, which follows tidal channels as (qualitatively) observed (for initial conditions, refer to Ye (2012)).
For more information on these (and some additional) cases we refer to Ye (2012).
4. Impact of salt marshes on tidal channel(s) (formation)
A study by Schwarz et. al. (in prep. 2013) focusses on the initial development of saltmarsh channel networks in relation to vegetation. Not only was the influence of the presence of vegetation on tidal channel formation shown again, but also the influence of the vegetation characteristics (e.g. stiffness, stress tolerance, spatial expansion velocity, etc.) on morphological development. Figure 6 shows the channel network formation (after three years) for different vegetation species (Spartina alterniflora (cordgrass) and Scirpus mariqueter, both typical saltmarsh vegetation species)). The modelling results species comparison shows: (a) Initial plant cover (%); (b) Plant cover after 3 years (%); (c) Drainage basins and networks; (d) Cumulative sedimentation (pos.)/erosion (neg.) after 3 years (m); (e) Initial bed level (m); (f) Bed level after 3 years (m).
Finally, Schwarz et. al. (2012) concluded that the characteristics of existing channels, in relation to vegetation-flow interaction, strongly determine the nature of morphological changes (e.g. in tidal flats and channels).