Sewer exfiltration and the colmation layer

摘要

Sewer rig studies demonstrate a rapid exponential decline in exfiltration rates from gaps andnjoints to establish an ultimate steady-state equilibrium varying between 1023–1026 l s21, withnminimum average daily rates per standardised leak area and sewer length varying betweenn0.02–9.0 l d21cm22 and 0.0002–2.0 l s21km21 respectively. These loss rates are much larger thannthose derived from indirect monitoring/modelling studies which suggest losses betweenn1.4 ￡ 1025–0.179 l s21km21. The confusion regarding conflicting definitions of the colmation,ntransition, bridging and biofilm layers is addressed, and the significance of these clogging layersnin terms of both hydraulic and matrix potential on the exfiltration loss is evaluated. The influencenof variability and instability of flow and bed turbulence on determining critical leakage conditionsnfollowing the onset of equilibrium steady-state is assessed. This challenges the generally heldnassumption that elevated head pressure condition alone is a necessary precursor for rupture ofnthe clogging layers.