Water Quality Modelling in the Channel Network of Venice
by Lucia Zampato, Georg Umgiesser, Martina Bocci and Giovanni Coffaro
In Venice a new activity has started aimed at the hydrodynamic and
water quality modelling of the city's inner channels. This project is considered
to be important because of the problems the city of Venice has with pollution
of its water and sediments. A two-dimensional finite element hydrodynamic
model is used to simulate the behaviour of the lagoon surrounding the city;
the modelling system WASP is used to describe the hydrodynamics of the
inner channels and the quality of the water. A team of scientists from
the Italian National Research Council (CNR) and the University of Padua
carry out the modelling activity within the framework of the 'Inner Channels
of Venice' Project, which begun in 1994. The project is coordinated by
UNESCO's UVO-ROSTE Office in Venice and financed by the Italian Ministry
for Scientific Research.
The city of Venice is uniquely structured, since it is constituted by
small islands surrounded by a complicated network of channels that communicate
with the lagoon. Even today, this channel system constitutes the principal
way of communication for people, goods and public services.
These channels are also the collectors of pollutants; the most important
of them are the sewage waters. Stopping the dredging of channels between
the end of the 1960's and 1994 has worsened this problem and makes the
pollution of water and sediments a very pressing problem for the city.
Inquiry into the possible solutions to the problem of cleaning the channels
is the general objective of the project 'Inner Channels of Venice'. The
concluding part of this project was started at the beginning of 1998 and
is expected to terminate at the end of 1998.
Main research subjects in the project include:
a) the quality of water and sediments
b) the health of the inner channels
c) water quality modelling.
Topics a) and b) are performed through field measurements and laboratory
analyses; they concern hydrodynamic quantities (velocity, water level),
ecological parameters (temperature, salinity, pH, dissolved oxygen, nitrogen
and phosphorus compounds and others), and biological indicators (virological
parameters and pathogenic bacteria).
The activities in which the authors are directly involved regard the
implementation of the water quality model (WASP) to the channel network
of Venice. The possibility to simulate the hydrodynamic behaviour of the
channels, and the evolution of biological parameters with different tidal
regimes, meteorological conditions and morphological configurations, give
an important contribution to this study. The research, co-ordinated by
a team of international experts, is carried out in Venice by a group of
local scientists.
Modelling Activities
The aim of the modelling activity is to obtain a description and therefore
a better understanding of the hydrodynamic and eutrophic processes in the
inner channels of the city of Venice. The activities planned are:
- the implementation of a 1-D hydro-dynamic model to the channel network
and its coupling with a 2-D hydrodynamic model for the lagoon of Venice,
that provides the boundary conditions around the city
- the application of a water quality model to the inner channels.
To accomplish these tasks the following models will be used:
- a 2-D finite element model of the Venice Lagoon (Umgiesser, 1986):
the grid of the model is made up of 4359 nodes and 7842 triangular elements
describing the varying bathymetry. Forcing functions of the model are height
of sea level at the three lagoon inlets, wind blowing over the lagoon and
the discharges from rivers into the lagoon. The grid of the model will
be modified in the area around Venice to allow the coupling with the 1-D
model of the inner channels.
- The WASP-EPA model will be used to simulate water circulation and water
quality of the channel network. DYNHYD, the hydrodynamic module of WASP,
will be used to describe the water circulation; the model is based on a
link-node approach and provides water velocity and waterlevel values. The
model will be calibrated by means of the Manning coefficient with data
on water level and velocity recorded during the field campaigns. The results
of DYNHYD will be used in simulating the water quality. EUTRO, the water
quality module of WASP, will be used to model the dynamics of the following
water quality parameters: dissolved oxygen; organic carbon; nitrogen (as
nitrate and ammonia); phosphorus (ortho-phosphate); chlorophyll-a; and
suspended matter (modelled by means of the TOXI sub-model). Nitrogen, phosphorus,
and suspended matter loading in the channels will be computed from data
on the resident population and tourists and from water consumption data
and specific conversion factors. Boundary conditions for the water quality
will be extracted from the available water quality data in the area of
the lagoon surrounding Venice. Initial conditions will be set according
to field data gathered in the framework of this project and considering
other data from the UNESCO database.
Conclusions
The model will be calibrated through the comparison with data already
available, or collected in the field measurements during 1998. Future activities
are planned in the framework of the project regarding the dredging of channels
carried out by the municipality of Venice:modelling of hydrodynamics and
water quality in the channels is an effective tool to better plan the excavation
works and to predict the consequence of these on the water circulation
and environmental health conditions. For further information, see: http://www.isdgm.ve.cnr.it/~georg/
Please contact:
Georg Umgiesser - ISDGM-CNR
Tel: +39 041 521 6875
E-mail: georg@lagoon.isdgm.ve.cnr.it
