Theoretical Basis of the 1D Water Quality Solver

Introduction

The 1D Water Quality Solver is a computer program used to model water quality in open channels. Flood Modeller should be installed on the user’s PC before the 1D Water Quality Solver can be used. A special licence for the 1D Water Quality Solver is required which differs from the standard licence of the 1D Solver. The river model simulations and the Water Quality simulations are run separately. Any sensible looped or branched network can be modelled using 1D Solver, which incorporates a wide range of hydraulic units including a variety of conduit types, hydraulic structures, etc. After the hydraulic information is computed by 1D Solver, 1D Water Quality Solver utilises the flow information and models the concentrations of water quality variables within the network.

The separation of the 1D Solver and the 1D Water Quality Solver codes reduces the computational cost as multiple runs of the 1D Water Quality Solver can be undertaken using the same hydrodynamic results (Figure 1 ).

Figure 1. The schema of information flows for calculating the pollutant flow information using Flood Modeller.

The 1D Water Quality Solver computes concentrations using a finite difference approximation to the advection-diffusion equation. An explicit implementation of the SMART algorithm, developed by Gaskell and Lau (1988), is used to approximate the advection term. Two alternative algorithms, QUICK and first order upwinding, are also available but tend to suffer from numerical diffusion giving unrealistic concentrations. The user is given the opportunity to decide which of the three algorithms they wish to use. The user is referred to the Flood Modeller manual for information on the Flood Modeller 1D Solver.

Although the 1D Water Quality is a depth averaged model, for mud transport and water quality modelling an element is divided into four vertical sub-components (Figure 2 ).

Figure 2. The vertical sub-components of the 1D Water Quality Solver.

The four components are:

• Water column. This is the main body of water through which dissolved and suspended substances are transported.
• Fluffy layer. This is a layer of mud that lies on top of the consolidated bed and is less dense. Settled matter initially falls into this layer. It is limited to a maximum thickness. Once the layer has filled to its maximum thickness, any additional settled material causes an equal amount to pass into the bed.
• Bed. This represents the consolidated mud that has settled out of the water column and can be re-suspended.
• Pore water. As mud consolidates into the bed layer, water is trapped within its pores. The rate of transfer of dissolved substances into the pore water is proportional to the deposition rate.

The contents of the fluffy layer can interact biochemically and biologically with the water column. The material in the bed and pore water can interact but are isolated from the water column until resuspended. Erosion of the fluffy layer and bed material returns their contents and that of pore water to the water column (see the section on Sediment for more details).