Modelling of pollution dispersion in natural stream during dry period Yvetta Velísková Institute of Hydrology SAS, Racianska 75, Bratislava, Slovakia

Drought consequences environmentalenvironmental economiceconomic socialsocial reduction of water quality

Environmental problems Water pollution determination of transport, dispersion and self-purifying characteristics of channels, optimum location of outlet structures in streams, delineation of mixing zones, prediction of spreading of accidental contaminant waves, etc. determination of pollutant distribution in natural streams in which the bed stream roughness, as results of flow conditions during low flows, forcefully impacts on a flow structure Mathematical and numerical modelling

Mathematical models of natural processes energy and matter conservation law non-uniform velocity distribution (low flow condition) pollutant distribution changes caused by trapping and slow release of pollutant from eddies that are trapped behind roughnesses in an open channel DEAD ZONES

Dead zone model - scheme the mass transfer across the main flow and dead zone interface is proportional to the concentration difference across them andto the concentration difference across them and to the discharge velocityto the discharge velocity

The influence of dead zones was formed to the equations, which represent the conception: transported mass – pollutant from the main flow (with the concentration C) gets into a side or a bottom dead zones as into a mixing tank; after that pollutant is mixed within the tank volume and then is exported back to the main flow zone with concentration C DZ. The residence time of the fluid in the tank depends on the mass exchange coefficient between the dead zones and the main flow and also on ratio of interfacial area to the dead zone volume.

numerical models condition for application range of input data rate of precision etc. ( B>30h ) two-dimensional model

Hydrodynamic approach c - mass concentration of pollutant [kg.m -3 ], h - depth [m], t - time [s], x,y,z - longitudinal, transverse and vertical coordinates [m], u,v,w - depth-averaged longitudinal, transverse and vertical velocities [m.s -1 ], ε x, ε y, ε z - longitudinal, transverse and vertical dispersion coefficients [m 2.s -1 ] Input data: geomorphological characteristics of river, data connected with discharge and water level regime dispersion characteristics at simulated part of river data about pollution sources + influence of „dead zones“

Model MODI: 2-D simulation model for determination of pollutant transport in natural 2-D simulation model for determination of pollutant transport in natural (non-prismatic) channels (non-prismatic) channels based on the solution of advection-diffusion equation based on the solution of advection-diffusion equation simulates under conditions of steady flow in non-prismatic channels simulates under conditions of steady flow in non-prismatic channels possibility of simulation: possibility of simulation: arbitrarily situated unsteady sources of pollution influence of “dead zones” self-purification effect  numerical solution ADQ stream-tube conception stream-tube conception Input data: geometric characteristics of cross-sections and velocity profiles in selected location of computed part of stream quantity and location of pollutant sources coefficients of transverse dispersion self-purification coefficient diffusion coefficient with “dead zones” and the interfacial area between the main flow and “dead zones” (the volume of “dead zones”)

t= 1000s t= 2000s t= 4000s the mass exchange coefficient between the dead zones and the main flow (from m 2 s -1 to 0.03 m 2 s -1 ), the interfacial area of dead zones (from 10% to 30% of active part) and their volume (from 20% to 50% of active part)

dz eps A mz V mz T

dz 0,03 A mz V mz T

Conclusion  drought - a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance in the affected area. It is a period of unusually persistant dry weather that persists long enough to cause serious problems such as crop loss or damage, water supply shortages, soil erosion reduction of water quality, because low water flows change conditions in a natural stream, reduce dilution of pollutants and so increase contamination of water. low flow roughness dead zone

Conclusion dead zonemodels  there are existed the dead zone models which divide the flow into two distinct zones: the main stream, in which advection-diffusion equation could be applied, and well mixed separation zones along the bed and banks  the mass transfer across the main flow and dead zone interface is proportional to the concentration difference across them and to the discharge velocity  this idea and conception was successfully applied into the two-dimensional dispersion model MODI  the results of numerical experiments show that the curve of mass distribution modifies its form and its peaks delay; this decrease and shifting of pollutant distribution curve depends on the values of the dead zones parameters (with increased time the shifting and decreasing grow) It is very important to give higher attention to prediction and solution of ecological accidents also in future, because from the outputs of climate scenarios it is clear that occurrence of minimum discharges at streams would be more and more frequent, the industry and land-use of regions near streams would mean possibility of new pollutant sources and from point of view of environment it is necessary to know how we can solve such problems.

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