4.2 Impacts of the Sea on Coastal Fresh Water Systems
53
Salt concentration influences the specific gravity of water. It can therefore cause a
stratification of different waters with different salinities.
What impacts do water-level fluctuations and differences in composition have on the
hydrological processes of shallow coastal regions?
4.2.1 Impacts on Water Quantity
Tidal Influence on Coastal Rivers
The oscillation of the sea water level leads to a fluctuating gradient of the water level in the
estuaries, influencing the flow velocity of coastal rivers. During high tides, river water is
impounded, and if the sea level exceeds the water level of the river, sea water can even flow
upstream (see Sect. 5.4). Rivers at shallow coasts can therefore also experience a change
in flow direction: while during low tide water flows downstream, water can flow upstream
during high tides (Fig. 4.5). The described flow pattern is superimposed by a mass densitydriven flow caused by the different mass densities of salt water and fresh water. The mass
density-driven flow leads to an upstream salt water flow at the river bed while fresh water
flows downstream at the water surface. The result is that during slack water conditions,
meaning that the net water flux in the river or estuary is zero, upstream and downstream
fluxes of salt and fresh water may occur (Fig. 4.5).
Impoundment and upstream water flow lead to an oscillation in estuaries and coastal
rivers. That phenomenon can be observed in the hinterland up to the tidal limit, which
is the spatial boundary of tidal influence. There, the tidal oscillation of the surface water
level is similar to that at coastal gauges (Fig. 4.3) while the morphology of the estuary may
induce asymmetries of the rising and falling limbs.
The propagation of the tides in estuaries and coastal rivers can be described by tidal
waves, moving upstream into the estuaries. Tides therefore are delayed at upstream sites
compared to the coastline. In case of funnel-shaped estuaries and rivers, the amplitude of
this wave (tidal range) can even increase upstream, as observed for the Elbe and Weser
rivers in Germany, for example. Dredging of rivers and estuaries can amplify such effects
(see Sect. 5.4).
Ground Water Connected to Tidal River or to the Sea
Generally, surface water and shallow ground water systems are interacting. Changes in
river and/or sea water levels therefore also induce changes in the water levels and the
dynamic of shallow ground water systems. Such influence is strongest close to the river
and the coastline, and it decreases with increasing distance from the surface water body
(Fig. 4.6). The range of such ground water oscillations in the neighborhood of tidal rivers
is proportional to the ground water flow velocity. According to Darcy’s law,
2 the velocity
2 Darcy’s law in general: v = k f ∗ I, where v = rate of water flow (volume per time), k f = coefficient
of permeability and I= hydraulic gradient.
53
Salt concentration influences the specific gravity of water. It can therefore cause a
stratification of different waters with different salinities.
What impacts do water-level fluctuations and differences in composition have on the
hydrological processes of shallow coastal regions?
4.2.1 Impacts on Water Quantity
Tidal Influence on Coastal Rivers
The oscillation of the sea water level leads to a fluctuating gradient of the water level in the
estuaries, influencing the flow velocity of coastal rivers. During high tides, river water is
impounded, and if the sea level exceeds the water level of the river, sea water can even flow
upstream (see Sect. 5.4). Rivers at shallow coasts can therefore also experience a change
in flow direction: while during low tide water flows downstream, water can flow upstream
during high tides (Fig. 4.5). The described flow pattern is superimposed by a mass densitydriven flow caused by the different mass densities of salt water and fresh water. The mass
density-driven flow leads to an upstream salt water flow at the river bed while fresh water
flows downstream at the water surface. The result is that during slack water conditions,
meaning that the net water flux in the river or estuary is zero, upstream and downstream
fluxes of salt and fresh water may occur (Fig. 4.5).
Impoundment and upstream water flow lead to an oscillation in estuaries and coastal
rivers. That phenomenon can be observed in the hinterland up to the tidal limit, which
is the spatial boundary of tidal influence. There, the tidal oscillation of the surface water
level is similar to that at coastal gauges (Fig. 4.3) while the morphology of the estuary may
induce asymmetries of the rising and falling limbs.
The propagation of the tides in estuaries and coastal rivers can be described by tidal
waves, moving upstream into the estuaries. Tides therefore are delayed at upstream sites
compared to the coastline. In case of funnel-shaped estuaries and rivers, the amplitude of
this wave (tidal range) can even increase upstream, as observed for the Elbe and Weser
rivers in Germany, for example. Dredging of rivers and estuaries can amplify such effects
(see Sect. 5.4).
Ground Water Connected to Tidal River or to the Sea
Generally, surface water and shallow ground water systems are interacting. Changes in
river and/or sea water levels therefore also induce changes in the water levels and the
dynamic of shallow ground water systems. Such influence is strongest close to the river
and the coastline, and it decreases with increasing distance from the surface water body
(Fig. 4.6). The range of such ground water oscillations in the neighborhood of tidal rivers
is proportional to the ground water flow velocity. According to Darcy’s law,
2 the velocity
2 Darcy’s law in general: v = k f ∗ I, where v = rate of water flow (volume per time), k f = coefficient
of permeability and I= hydraulic gradient.