Configuration and properties of a breakup jam

Abstract

Favourable weather conditions in January, 1986, resulted in a breakup jam on the Thames River that froze in place. This afforded safe access and performance of detailed thickness measurements that are normally not possible to obtain. The thickness of the jam is highly variable in the lateral direction but without consistent trends, which, in a crude sense, justifies the assumption of lateral uniformity made for analytical purposes. In the downstream direction, the thickness increased, except for a short reach downstream of the toe where it rapidly decreased to zero. Approximate assessment of the absolute roughness of the jam indicated fair agreement with earlier deductions based on hydraulic resistance data. Using existing theoretical concepts, it was deduced that the downstream portion of the jam was likely formed by "shoves" or internal collapse ("wide") whereas the upstream portion was likely formed by frontal progression ("narrow") and juxtaposition. For the former reach, the present data enabled approximate evaluation of coefficients utilized in pertinent theories. Agreement with previously published values was good, though in one instance there are no previous field determinations. To study the decline of jam thickness past the toe, the "wide" jam theory was adapted to describe a jam overlain by solid ice cover. This enabled approximate determination of the friction coefficient between the jam and the cover. Key words: breakup, coefficients, hydraulics, ice, internal collapse, jam, river, roughness, theory, thickness.