Abstract
Coastal lagoons are shallow water bodies separated from the sea by shoreparallel
barriers, which dampen wave and current energy. Marine ora and
fauna typically inhabit coastal lagoons and contribute to the individual lagoon
characteristics and functioning. The ecosystem health of coastal lagoons is vulnerable
to increasing turbidity levels caused by e.g. sediment suspension, due
to hindered light penetration to the bed. This thesis investigates the hydroand
ne-grained sediment dynamics of the shallow coastal lagoon, Rødsand lagoon,
in southeast Denmark. Three dierent research methodologies were used
in order to assess the natural processes and the lagoonal system response to future
disturbances. The methodologies included eld investigations, laboratory
experiments and numerical modelling.
Rødsand lagoon is protected by the European Commission as a Natura 2000
area, a designation that prohibits a permanent disruption of the natural habitat.
Projected future increases in storm intensity due to climate change could
exacerbate the local turbidity levels and likewise, a sediment spill related to
oshore construction work may pose a threat to the lagoonal ecosystem health.
Improved understanding and management of the lagoonal sediment dynamics
are therefore urged. It is concluded that the bed erosion was inuenced by
aggregation, and that the settling velocity of the aggregates was high compared
to previous studies. The bed was eroded during high stress conditions
primarily caused by wind-induced wave activity. Succeeding storms decreased
the shear stress threshold for sediment suspension. A high wind speed event in
the winter season caused a seagrass cover reduction, which led to an increase in
the sediment suspension relative to the bed shear stress. These processes and
interactions are important for the understanding of the sediment dynamics of
lagoonal systems.
The lagoonal response to a regional increase in the wind speed was amplied
suspended sediment levels in the shallow parts of Rødsand lagoon. The lagoonal
response to a sediment spill outside of Rødsand lagoon was increased
suspended sediment levels according to the distance to the spill source. The regional
response was 10-fold larger than the sediment spill where only advective
sediment transport took place. This highlights the importance of the internal
lagoonal processes as opposed to exterior disturbances. Numerical modelling
showed that implementing a mussel reef outside of Rødsand lagoon reduced the
sediment import to the lagoon and thus lowered the suspended sediment levels
within the lagoon. The results improved the understanding of the hydro- and
ne-grained sediment dynamics of Rødsand lagoon, and led to the denition of
a new coastal lagoon type.
barriers, which dampen wave and current energy. Marine ora and
fauna typically inhabit coastal lagoons and contribute to the individual lagoon
characteristics and functioning. The ecosystem health of coastal lagoons is vulnerable
to increasing turbidity levels caused by e.g. sediment suspension, due
to hindered light penetration to the bed. This thesis investigates the hydroand
ne-grained sediment dynamics of the shallow coastal lagoon, Rødsand lagoon,
in southeast Denmark. Three dierent research methodologies were used
in order to assess the natural processes and the lagoonal system response to future
disturbances. The methodologies included eld investigations, laboratory
experiments and numerical modelling.
Rødsand lagoon is protected by the European Commission as a Natura 2000
area, a designation that prohibits a permanent disruption of the natural habitat.
Projected future increases in storm intensity due to climate change could
exacerbate the local turbidity levels and likewise, a sediment spill related to
oshore construction work may pose a threat to the lagoonal ecosystem health.
Improved understanding and management of the lagoonal sediment dynamics
are therefore urged. It is concluded that the bed erosion was inuenced by
aggregation, and that the settling velocity of the aggregates was high compared
to previous studies. The bed was eroded during high stress conditions
primarily caused by wind-induced wave activity. Succeeding storms decreased
the shear stress threshold for sediment suspension. A high wind speed event in
the winter season caused a seagrass cover reduction, which led to an increase in
the sediment suspension relative to the bed shear stress. These processes and
interactions are important for the understanding of the sediment dynamics of
lagoonal systems.
The lagoonal response to a regional increase in the wind speed was amplied
suspended sediment levels in the shallow parts of Rødsand lagoon. The lagoonal
response to a sediment spill outside of Rødsand lagoon was increased
suspended sediment levels according to the distance to the spill source. The regional
response was 10-fold larger than the sediment spill where only advective
sediment transport took place. This highlights the importance of the internal
lagoonal processes as opposed to exterior disturbances. Numerical modelling
showed that implementing a mussel reef outside of Rødsand lagoon reduced the
sediment import to the lagoon and thus lowered the suspended sediment levels
within the lagoon. The results improved the understanding of the hydro- and
ne-grained sediment dynamics of Rødsand lagoon, and led to the denition of
a new coastal lagoon type.
| Originalsprog | Engelsk |
|---|
| Forlag | Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen |
|---|---|
| Status | Udgivet - 2017 |
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