Numerical Simulation of Unsteady Mobile-Bed Hydrodynamics and Contaminant Transport in River Systems

Author

Dr. Forrest M. Holly, Jr.

General Description

CHARIMA is a general-purpose computer code for the simulation of steady or unsteady water, sediment, and contaminant movement in simple or complex systems of channels. Water movement (hydrodynamics) forms the core of a CHARIMA simulation; mobile-bed (sediment) and contaminant dynamics can be activated or deactivated. Mobile-bed capabilities include bedload and/or suspended-load transport of mixtures of noncohesive or cohesive sediment, along with the associated short- or long-term bed-level changes (aggradation and degradation), bed-sediment sorting, and armoring. Subsurface layering, pre-existing or as the result of persistent deposition, is included. Contaminant capabilities include transport and fate of any number of conservative contaminants, and heat. Transport of radionuclides and their selective sorption interaction with sediment size classes, including subsurface sediments, has recently been developed. Nitrates and dissolved oxygen are currently being implemented. Other nonconservative and interacting constituents are easily introduced through implementation of the appropriate source/sink relations in the code. Channel systems may comprise a single channel, branched systems, or looped networks of arbitrary connectivity and flow direction.

Data Requirements

Hydrodynamic simulation requires essentially the same data as a backwater calculation, i.e.: channel cross sections and roughnesses, plan-view connectivity, hydrographs of inflow at upstream boundary points (possibly steady flow), and stage hydrographs (or rating curves) at downstream boundary points. Mobile-bed simulation additionally requires the initial distribution of bed sediments, as well as sediment inflow (rate and distribution) at upstream inflow points. Contaminant simulation requires contaminant inflow rates at upstream inflow points. Initial distributions of contaminant or suspended-sediment concentrations are useful but not required.

Required Hardware and Software

CHARIMA is written exclusively in FORTRAN, and thus requires a FORTRAN compiler. Modest applications can be performed in a 640- k DOS environment. Larger models are effectively run in expanded DOS memory (with an appropriate compiler) or workstations (UNIX, Apollo-Domain, etc.) Input and output data sets are in formatted ASCII form. Graphical support must be provided by the user.

Recent Applications Include

• Thermal regime and hydrodynamics in the Upper Illinois Waterway (Commonwealth Edison Company)
• Cohesive sediment and radionuclide transport and unsteady hydrodynamics in the Watts Barr Reservoir (Tennessee Valley Authority)
• Radioisotope transport and hydrodynamics in the Columbia River (Battelle Pacific Northwest Laboratories)
• Thermal regime and hydrodynamics in the Missouri River (Iowa Consortium of Electric Power Utilities)
• Mobile-bed dynamics in the Cho-Shui River (National Science Foundation)
• Mobile-bed dynamics in the Missouri River from Gavins Point Dam to Rulo (National Science Foundation)
• Mobile-bed dynamics in the Missouri River from Ft. Randall to Gavins Point Dam (Corps of Engineers)
• Reservoir flushing operations in Warsak Dam, Pakistan (Michael A. Stevens, Consultant)

Mathematical Basis

The hydrodynamic computation (steady or unsteady flow) is based on the full dynamic de St. Venant equations. Contaminant and suspended-sediment transport is based on the advection-diffusion equation with specified longitudinal dispersion coefficient and source/sink term appropriate for the transported constituent. Mobile-bed dynamics, including bed elevation changes, sorting, and armoring, are based on the Exner equation with a suspended-sediment source/sink term; subsurface layering is included. All equations and their numerical solution procedures accommodate reversing unsteady flow, e.g. in estuaries.

Numerical Basis

The Preissmann four-point implicit finite-difference scheme is used for the hydrodynamic computation. Contaminant transport is computed using a hybrid Lagrangian scheme for advection (Holly-Preissmann scheme) and a finite-difference diffusion operator. Full dynamic memory allocation obviates any built-in restrictions on numbers of channels, junctions, constituents, etc.

Limitations

Subcritical flow. Fully-mixed transport (one-dimensional). Fixed bank location and plan-view channel layout.

Availability

CHARIMA is under continuous development and generalization, and therefore is not generally available to outside users. However the Institute has worked directly with users to effect code transfer and training in conjunction with specific user projects. The contact is: IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, IA 52242 (Phone 319-335-5229; Fax 319-335-5238), Attn: Forrest M. Holly Jr.