near the bottom boundary. To overcome these limitations, experiments were 

 conducted to combine the surface tracing of dyed waters with the velocity 

 measurements in the vertical profile at a stationary point. The purpose 

 is to correlate V^ and u^, corresponding to the dye velocity and the 

 velocity measured by metering, respectively, and requires that the sled- 

 mounted meters be positioned in the field where the dye is dispersing. 

 Correlations can only be carried out in the area immediate to the fixed- 

 point sensors, irrespective of the type of dye injection used (Fig. 34). 

 However, this method can be improved by spacing two sets of current meters 

 at the upstream and downstream boundaries of the area where dyes are 

 traced, correlate the Eulerian velocity scales for the Lagrangian domain, 

 then use the results as a check on the dispersion and diffusion of dye 

 through the control area. 



Dye releases require preselecting the algorithm which will later 

 govern the analytical procedures. Three injection techniques will satisfy 

 this: (a) Continuous injection at a point, (b) continuous line injection, 

 and (c) point source (slug) injection. Choice of injection is influenced 

 by geographical, physical, or operational constraints and whether the 

 experiment is designed to study convective properties of the flow. 



Continuous injection requires flow conditions in the control (measure- 

 ment) area to be homogeneous. The technique is based on the principle of 

 "steady dilution," i.e., the tracer concentration stabilizes in time as 

 its properties become representative of the flow properties themselves. 

 Steady dilution requires a complete mixing over time- length of the system 

 and a single sample from the well-mixed (equilibrium) condition. The 

 disadvantage of this algorithm for use in coastal areas where the time 

 scales associated with molecular diffusion, advection, and turbulent 

 diffusion vary in space and time, is that the additional requirements of 

 steady, unidirectional flow and high discharge rates cannot be met; thus, 

 the mixing length cannot be estimated. Use of a line injection will not 

 materially alter the analytical problems experienced with point injections. 

 However, this technique is useful for charting flow lines near coastal 

 engineering works because the variability in surface flow velocities, the 

 mixing zone, and the pulsating and meandering nature of coastal currents 

 can be observed. 



If a single-point injection is used, time integration may be applied. 

 The drawbacks associated with the algorithm are that complete lateral 

 mixing and high-discharge steady flow are still required, and the dye 

 field must be sampled continuously at a fixed point and fixed depth for 

 the estimation of the mixing length. This method is best applied to 

 tidal inlet flow studies where the physical boundaries present and the 

 quasi-steady, high-discharge tidal flows staisfy the minimum requirements. 

 Another algorithm built around single-point injections is the spatial- 

 integration technique where the center of gravity of a diffusing-advecting 

 cloud is found and the concentration is measured. If the instantaneous 

 position of the centroid is denoted by Xi(t ), the difference 

 x i (t ) " x i( t o) becomes a measure of V^. Therefore, it is critical to 

 know the depth to which the tracer cloud has diffused at each position and 



59 



