Environmental Engineering Reference
In-Depth Information
3
Water Exchange Mechanisms in Coastal Environments
In estuaries many physical and chemical processes take place that have an
impact on the concentration of a contaminant in the environment. The hydro-
logical conditions in the estuary are, however, the major determining factor.
Having a good grasp of the order of magnitude of the hydrological exchange of
for example a harbor in an estuary with it's surroundings is essential in order to
predict the concentration. In general, the exchange of water between a harbor
basin and an estuary is caused by three phenomena [27, 28], that is by:
•
filling and emptying by the tide;
•
the horizontal eddy generated in the harbor entrance by the passing main
flow;
vertical circulation currents in the harbor generated by density differences
between the water inside and outside the basin.
In some cases the above picture is complicated by the extra effects of a wa-
ter discharge through the harbor basin to the estuary or sea. On the one hand
such a discharge has a positive effect by flushing the basin, but on the other
hand it has a negative effect by introducing or enhancing water exchange by
density currents (as in e.g. some Dutch harbors, such as Delfzijl, IJmuiden,
Harlingen, and Terneuzen). It even may introduce a serious sediment influx
contributing to shoaling in the harbor basin. This mechanism is addressed
as flushing with withdrawal of water (e.g. cooling water intake) defined as
a negative flushing discharge rate.
Most quick assessment models only incorporate an empirical exchange
(REMA, EUSES, Simplebox) or use only the tidal exchange. The Mam-Pec
model is an exception as it incorporates all phenomena and allows for em-
pirical exchange volumes as well. Most current true 3D models, such as
Delft3D [29, 30], Mike-3 [31], or Telemac [32], incorporate all processes but
require very experienced users with a high level of hydrological knowledge to
use the models.
•
3.1
Tidal Exchange
The exchange by the first mechanism over a tidal period m, i.e. the tidal prism
can easily be determined as:
V
t
=2
η
A
b
(5)
where:
V
t
= tidal prism of the harbor basin,
η
= tidal amplitude (Fig. 2),
A
b
=
(storage) area of the basin.
The total water exchange volume is the sum of the tidal prism and the
exchange volumes due to the horizontal eddy in the harbor entrance (
V
h
),
