Sewer exfiltration and the colmation layer.

Sewer rig studies demonstrate a rapid exponential decline in exfiltration rates from gaps and joints to establish an ultimate steady-state equilibrium varying between 10(-3)-10(-6) l s(-1), with minimum average daily rates per standardised leak area and sewer length varying between 0.02-9.0 l d(-1)cm(-2) and 0.0002-2.0 l s(-1) km(-1) respectively. These loss rates are much larger than those derived from indirect monitoring/modelling studies which suggest losses between 1.4 x 10(-5)-0.179 l s(-1) km(-1). The confusion regarding conflicting definitions of the colmation, transition, bridging and biofilm layers is addressed, and the significance of these clogging layers in terms of both hydraulic and matrix potential on the exfiltration loss is evaluated. The influence of variability and instability of flow and bed turbulence on determining critical leakage conditions following the onset of equilibrium steady-state is assessed. This challenges the generally held assumption that elevated head pressure condition alone is a necessary precursor for rupture of the clogging layers.