Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 94

green band near 560 nm (nanometers or millimicrons) and emit light in the yellow-orange band
near 580 nm. The relative intensity of the emitted light is also a function of the amount of luo-
rescent material present. Therefore, the combination of wavelengths and the intensity of the light
absorbed and emitted can be used to measure the amount of material present using a luorometer
(Martin and McCutcheon 1999).
Two common types of dye studies are based on the manner in which the dye is injected: slug or
continuous releases. The slug injection is more commonly used to estimate the time of travel and
dispersion, while continuous injections offer advantages for estimating dilutions. Kilpatrick and
Wilson (1989) provide guidance on conducting time of travel and dispersion studies. Story et al.
(1994) discussed some of the considerations for planning and conducting dye studies in estuaries.
The dye can be monitored either by measuring the dye concentrations as the water parcel passes a
point in the river, or by periodically running through the dye cloud to determine the centroid and
spread of the dye cloud (Figure 4.14).
The resulting time-concentration curves are generally Gaussian or bell-shaped but are skewed
toward the leading edge. The curve shown in Figure 4.15 illustrates a time-concentration curve for
a Mississippi river as reported by the Mississippi Department of Environmental Quality as part of
“Time-of-travel of water tracing dyes in MS streams” (MDEQ 2007).
The time-concentration curve at a particular location downstream of the injection can be used
to determine the time of travel, simply by measuring the distance downstream to that location
and dividing by the elapsed time (described in more detail by Kilpatrick and Wilson [1989] and
Jobson [1996]). The location of the centroid of the dye cloud is used to compute the time of travel,
but the time of the leading and trailing edges of the dye cloud is also useful in dispersion and spill
studies. The low can also be estimated by irst integrating under the time-concentration curve

Maximum
concentration

Longitudinal
dispersion

Slug injection
of tracer

Lateral mixing
and longitudinal
dispersion

Vertical and lateral
mixing, longitudinal
dispersion
(vertical not shown)

IV
long distance

Stream
boundary

III
Optimum
distance

Flow II

Short
distance

I
Very short
distance

FIGURE 4.14 Lateral mixing and longitudinal dispersion patterns from a center slug injection of a tracer.
(From Kilpatrick, F.A. and Wilson Jr. J.F., Techniques of Water-Resources Investigations Reports , Book 3
Applications of Hydraulics, United States Geological Survey, Washington, DC, p. 34, 1989.)