DEVELOPMENT OF HYDRODYNAMIC, WATER QUALITY AND EUTROPHICATION MODELS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 131

7.1. INTRODUCTION
As explained in the previous chapters, agricultural irrigated watersheds have a complex physical
nature in that they include interacting irrigation and drainage networks which may be connected to
lakes or lagoons. Therefore, studying surface water quality problems in such watersheds, calls for an
integrated modeling approach that takes into consideration the different components of the watershed
in both problem assessment and the development of solutions. The developed models are aimed at a
better understanding of the hydrodynamics of a watershed where the drainage catchment is connected
to a shallow coastal lake, which is considered the main outlet of the drainage water to the sea. A 1D-
2D model of the drainage network and the shallow lake is developed using the Sobek 1D-2D
mathematical modeling package. This model reveals the basic hydrodynamic properties of the
drainage network and its link with the lake water body. For a more detailed description of the
hydrodynamics and water quality, a detailed 2D hydrodynamic model is developed for the lake area
using the Delft 3D modeling tool. The detailed 2D model of the lake shows the effect of the tidal
forces and wind on the lake hydrodynamics and the corresponding effect on the spatial distribution of
pollutants in the lake. A 2D water quality model is coupled with the 2D hydrodynamic model of the
lake to study the effect of different chemical and biological pollutants discharged into the lake, in
addition to the effect of nutrients on the lake eutrophication.
7.2. MODELS BUILDING
The overall hydrodynamic modelling approach used in this research is based on the Sobek and Delft-
3D modeling software developed by WL|Delft Hydraulics. Sobek is an integrated package for
modeling river, urban or rural water networks, and the Sobek 1D channel flow module can be
integrated with the 2D Delft-FLS model. It is therefore possible to implement a 1D-2D implicitly
coupled modeling system. This provides a high-quality tool for modeling irrigation systems, drainage
networks, natural streams and rivers in lowland and hilly areas. The functionality contained in the
hydrodynamics of two-dimensional flow is part of the Sobek-Rural model package. This functionality
includes the one dimensional river/channel flow and two-dimensional flow and is called “ Sobek 1D-
2D ”. The numerical aspects of 1D modeling include the water flow and pollutants transported by the
flow. The water flow is computed by solving the complete Saint Venant equations. The mass
conservation equation is applied at the calculation and connection points (1D nodes), and the
momentum equation is applied at the reaches between two calculation nodes. The Saint Venant
equations (7-1) , (7-2) expressed in their non-conservative form per unit width of channel are as
follows:
Continuity equation
(7-1)
Momentum equation
(7-2)
where
: Water level (based on reference level) defined as = h+zb [m];
h : Local water depth [m];
zb : Local bottom level [m]
u : Flow velocity [m/s]
cf : Friction coefficient [-]