PROJECT TOPIC- DISPERSION STUDIES IN RIVER EBERE
1.1 BACKGROUND OF STUDY
The possibility of a contaminant being accidentally or intentionally 6:pilled upstream from a water supply is a consran! cmcern to those diverting
time or dispersion is needed for pollution cor,t;c! or warning systems on streams where data are limited. As greater demands are placed on streams, the evaluation of significant f ~ r c e s of self-purification, such as deoxygenation and reaerstion properties, becomes increasingly necessary.
Therefore the ability to simulate potential pollution build up in streams, lakes and estuaries becomes increasing important.
Travel time and mixing of water within a stream are the basic stream flow characteristics for a wide range of flows are basic data needed to
address a11 of t h e concerns with the widespead availability of coinputsrs lotlay, it is natwal to think of numerical models as a rncans of arlsweiing
these questions i:lthough many excellent models are available 10 make these types of calculations needed. none can he used with confidence without calibration and verir’katiun tr, the p<wticular river reach in q;iestrm.
That is say. all models must be provided with information from +n,hich flow velocities and mixing rates can be computed. In general there are no reliable methods of predicting dispersion coefficients (mixing rates) from commonly available hydraulic information. Stream velocities typically predicted by use of a flow model, generally require very detailed channel geometry and flow resistance coefficients which are seldom available. The availability of reliable input information ic therefore. almost always the weakest link in the c h i n of events needed to predict thc rate i)l. movement. dilution and mining of pollute~nti n Fvers and streams.
Soluble tracer (,an he used to simuln~eth e tianyport and dispersion of solutes in surface water because they have virtually, the same Laidjaw
1977). This is the case in either a steady flowing river or in the unsteady oscillatory stage and flow of a tidal estuary.
Measured tracer – response curves produced from the injection of a known quantity of soluble tracer provides an efficient method of obtaining
the data necessary to calibrate and verify pollutant transport models. Thcse data can also be used in conjunction with th to stimulate potential pollution build up in strearno lakes and estuaries without 1 1 7 ~n eed to use numerical mod&. Extensive use of fluorescent dyes and solutes!e tracers to quantify the tmnsport and dispersion in streams and rivers h3s been used by various authors. Disper~ion number is however usually detrimental by tracer studies (Polpraselt & others. 1983: Mwecos do Mof~tean d M:ua, 194 1 ) .
Thin involves the injection of a tracer in the river and then Sampling for the concentration a! other points in the river downstream until the concenVation reduces to an insignificant level. Turbulent dispersion and mixing will cause an expansion of the cloud of the slug of tracer traveling down stream according to convective movement of the fluid and consequently the conceive~t;;~fiction the tracer will decrease.
PROJECT TOPIC- DISPERSION STUDIES IN RIVER EBERE
t.I:>u.evel-. the determimticm of a singlc v<tlt~co f the dirnersian nuinher in !+lC ponds, streams and rivers usin? tmcer qtw.11es usually involves
extensive sampling which is tedious, time consuming and expensive. (Pdprasert and Rhaliarai, 1985; A p w a m b a & Others 1992; a Agunwam
1992b). This is attributable to the need to sample continously until the outlet concentration reduces to an insignificant Ievel. Hut this acluevable at large tlmes. especially in field ponds or rivers where slow release of tracer particles from the sub-layer into the main flow results in long tailed timeconcentration curves (Fischer 1967, Smith, 1986, Agunwamba 1992b).
The conventional manner of displaying the response of a stream to a slug injection of tracer is to plot the variation of concentration with time
it!:icer-response curve) as observed at two CJr more cross secti0r.s downstream or the injection as il!ustratd in fi,ou:e 1. I . ‘The tracer responve
curve, defined by the a!taylsis of Rater samples teken at selected time ii~tcrvalsd uring the tracev-cloud passape is the h a c~sfo r determining t! me-of travel studies and dispersion chxacteristics of streams. A &tailed explanation of the analysis and presentation of time-of-travel data are
covered in the report by Kilpatrick and Wilson (1989).
The characteristics of the trace response curves shown in figure 1.1 are described in terms of elapsed time after an instanatneous tracer injection.
The following terms are so defined for fig 1.2. C, = peal: conce?tration of the tracer cloud.
Ti = ~-.?:irwttilm e to t ! a~rril r31o f the leading edpe of a tracer clouci :~ta s m ~ p l i nlo~ca tion.
T, = eiapsed time :o the p e ~ kco ncentration of tracer clouti. Td = duration of the tracer cloud (T, – TI)
T, = elapsed time t o the trailing edge of the tracer cloud Tlod = duration from lcading edge until tracer concentration has reduced to within 10 percent of the peak concentration. N = number of sampling site down stream of injection The mass of trace1 to pass a cross section Mr is competed as Mr. = l’ 5: q p h + d . (1.1)
Where w is the total width of the river, C, is the vertically averaged tracer concentration, and q is the unit discharge (discharge per u ~ iwt idth) Brth C, and q are given at time t and distance w from one bark. Mer rnixi~g is compiete in the croqs section, the cquc?tim simnlifies to Mr = (‘Q dl
– ( 1.2) If mixing is not clmqlete, Equation (1: .2) can still be used as long as the concentratior! C is the discharge-weighted; cross qectional average
concentration, if the discharge is constant during the passage of a tracer cloud, it can also be factol-ed out of the integrai, the shape and magnitude of the tracer-response curves shown in figure 1.1 and 1.2 are determined by four factors.
I . The quantity of tracer injected – -. ‘l’he J q e e to n+ich !lie tracer is conseri.ative
3. The ~n a g n i~u docr t !w strelam dischzrge
4. The longitudinal dispersion.
Hence, douhli~g the amount of tracer, would double the observed concentrations but the shape and duration of the tracer response curves
would remain constant.