## 510 Mathematik

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- Method of lines (5) (remove)

In this paper an overview on modelling techniques and numerical methods applied to problems in water network simulation is given. The considered applications cover river alarm systems (Rentrop and Steinebach, Surv Math Ind 6:245–265, 1997), water level forecast methods (Steinebach and Wilke, J CIWEM 14(1):39–44, 2000) up to sewer and water supply networks (Steinebach et al., Mathematical Optimization of Water Networks Martin. Springer, Basel, 2012).
The hyperbolic modelling equations are derived from mass and momentum conservation laws. A typical example are the well known Saint-Venant equations. For their numerical solution a conservative semi-discretisation in space by finite differences is proposed. A new well-balanced space discretisation scheme is presented which improves the local Lax-Friedrichs approach applied so far. Higher order discretisations are achieved by WENO methods (Kurganov and Levy, SIAM J Sci Comput 22(4):1461–1488, 2000).
Together with appropriate boundary and coupling conditions this method of lines approach leads to an index-one DAE system. Efficient solution of the DAE system is the topic of Jax and Steinebach (ROW methods adapted to network simulation for fluid flow, in preparation).

Hydrological processes, like the water flow or transport of soluble substances in rivers, are usually described by partial differential equations (PDEs). The numerical solution of these equations offer various fields of important applications, e.g., the prediction of water stages during floods or low water periods (optimal load capacity of the ships) and the prediction of the fate of pollution plumes in case of an accident. The aim is to develop numerical schemes that are applicable to mostly all those simulations and well-tested with respect to their stability and efficiency. The method of lines (MOL)-approach was found to be a suitable numerical technique to solve the whole class of the discussed problems. The subject of this paper is the comparison of different numerical schemes for the space discretizations in the MOL-approach, in particular the ENO-discretizations which are found to be very useful even when sharp gradients occur.

For many practical problems an efficient solution of the one-dimensional shallow-water equations (Saint–Venant equations) is important, especially when large networks of rivers, channels or pipes are considered. In order to test and develop numerical methods four test problems are formulated. These tests include the well-known dam break and hydraulic jump problems and two steady state problems with varying channel bottom, channel width and friction.
The space discretization of the partial differential equations is based on a finite volume approach with central WENO interpolation and local Lax–Friedrich fluxes (Kurganov and Levy, 2000) [7]. For time-integration new linearly-implicit two-step peer methods of orders three and four are developed. These methods are especially adapted to the usage within the method of lines framework. They show a good performance compared to the well-established methods like ode15s, radau5 or rodasp.

For many practical problems an efficient solution of the one-dimensional shallow water equations (Saint-Venant equations) is important, especially when large networks of rivers, channels or pipes are considered. In order to test and develop numerical methods four test problems are formulated. These tests include the well known dam break and hydraulic jump problems and two steady state problems with varying channel bottom, channel width and friction.

Integrated modelling approaches for whole river catchments require the coupling of different types of models. As an example, river flow and forecast models in one- and two-space dimensions are discussed. Usually, these models are based on the hyperbolic shallow water equations and require special discretizations like ENO or Godunov-type methods.
The basic coupling mechanisms like coupling via source terms, via boundary conditions, via state variables and simulator coupling are introduced by examples. Their properties with respect to performance and accuracy requirements and implementation issues are presented.
If coupling conditions are considered, additional algebraic equations arise. By the method of lines approach it is possible to translate the partial differential equations and the algebraic equations into a large system of differential algebraic equations (DAEs). The DAEs can efficiently be solved if the special structure of the Jacobian of the coupled model components is taken into account.