Reference & User Manual
|Last modified on: Thursday, 20-May-10 18:34 CEST|
WaterWare models: STREAM Water Quality ModelSTREAM is a dynamic (daily) water quality model that utilizes WRM scenarios, sharing the network and daily flow data generated by WRM. Please note that STREAM is a very simple, but efficient model: its main purpose is one of screening the overall water quality situation in a complex basin/network, without any pretensions for very detailed local scale results. For those, and alternative model with a high spatial and temporal resolution (BLTM, the Branched Lagrangian Transport Model) is available).
STREAM describes water quality in the open channels of the network in terms of:
Model representationSTREAM work with network scenarios it shares with WRM. The additional data that are required for a STREAM scenario are defined with a special editor, based on the WRM scenario editor.
Geometry and hydraulicsReaches are assumed to be homogeneous in tertms of the their hydrogeometric properties, and all rate constant are assumed to be constant within a reach. Reaches are subdivided into computational elements, for which the conservation and contiunity laws apply. Computational elements are assumed to be completely mixed.
The basic equation is the one-dimensional advection mass transport equation, including advection, dilution, reactions, interactions between constituents and sources and sinks.
STREAM operates under a steady-state flow regime - within a computational time step. Average stream velocity is either obtained from the rating curve associated with a reach, or using a discharge velocity relationship of the type:
where a and b are empirical constants, and Q is the stream flow in the reach.
Alternatively we can use Manning's equation:
where N is Maning's roughness (between 0.025 and 0.15), R the hydraulic radius (cross-sectional area /wetted perimeter), and S is the slope of the reach, and satisfying the continuity equation for consecutive reaches. Where the cross section is not derived from the rating curve table for the reach, a rectangular, trapeciodal, or parabolic cross section can be used.
which assumes a decay rate (K1) of 0.23/day. All BOD input (assumed to be expressed as BOD5 is converted accordingly.
The mass budget for BOD in a reach or computational segment considers inputs (including diffuse, lateral inflow), oxygenation, and loss due to setlling (the latter should be a function of the speed of low and thus turbulence within a segment).
DO or dissolved oxygen is the counterpart of BOD and a central water quality parameter, as the availability of (dissolved) oxygen controls all life (and in particular fish) in the water. The mass budget for oxygen includes the concentration in al incoming waters, the demand by BOD, photosynthesis of algae and plants in the water, and, usually dominantly, the re-aeration through the water surface from the atmosphere. Re-aeration is described in terms of the concnetration difference Cs-C where Cs is the temperature dependent saturation concentration of DO (in mg/l), times a reaeration coefficient K2 (in /day).
pollutant the additional arbitrary pollutant uses the basic elements for a mass budgets including a first order decay term (that may be set to zero for a conservative substance like salinity): decay = -Kn*Cn where Kn is the decay rate /day.
To simplify scenario editing, the value is entered as a half time constant, describing the period it would take to reduce and initial concentration to half.
The Solution of the coupled (DO and BOD) equations is using a fourth order Runge-Kutta method.
Saturated Dissolved OxygenRe-aeration, the central process to replenish oxygen in the water, depends on the difference between current DO concentration and saturation. DO Saturation is primarily a function of temperature, atmospheric pressure, and dissolved substances (e.g., salinity) of the water.
STREAM uses a fourth order polynomial to estimate saturation levels of DO, ignoring salinity effects.
Re-aeration (K2)Re-aeration is expressed as an empirical function of flow and average depth of the reach, increasing with flow and thus turbulence.
Deoxygenation coefficient (K1)K1 depends on the pollutant (waste) type and composition; a typical value for domestic, raw sewage would be 0.5/day. Please noe that with the level of waste water treatment, the values of K1 go down, i.e., the remaining pollutants are more difficult (slow) to oxydize. Also please note that field estimates may tend to overestimate, as they will include other processes (losses) such as settling.
As any (at least in part) biological process, deoxygenation also depends on temperature:
where T is the water temperature, and K120 is the rate constant at 20 degree Centigrade.