Persistence of Skewness in Longitudinal Dispersion Data: Can the Dead Zone Model Explain It After All?

Field studies reporting coefficients of temporal skewness that do not decrease in the main flow direction have cast doubt on the transient storage (TS) or dead zone model of longitudinal dispersion in rivers and streams. In this study, the conditions under which the TS model predicts persistent or growing skewness coefficients are investigated. The findings clearly show that, though not outright impossible, an instantaneous slug release into a uniform channel reach is, indeed, extremely unlikely to result in persistent or growing skewness coefficients. In contrast, the passage of a tracer or pollutant along a sequence of (hydraulically) different subreaches may easily give rise to nondecaying skewness coefficients, the occurrence of which is governed by the parameter sets of the subreaches concerned. Thus, the TS model does show a certain potential to explain the persistence of skewness. The findings reported here are expected to be useful in guiding future field studies on the subject. An application of the newly derived criterion to stream tracer data has been successful.