Stream restoration: when are timber structures strong enough?

Posted on September 23, 2013

It’s well known that fallen tree logs and branches play an important geomorphological and ecological role in streams by providing refuge and feeding opportunities for invertebrates and fish, by acting as local stores of carbon, and by creating scour pools that increase habitat complexity.   Although there’s been an historical trend to remove large wood from streams with the aim of reducing flood hazards or improving navigation, a growing  appreciation of the ecological value of large wood has stimulated widespread efforts to reintroduce it into stream channels in recent years.  To ensure that such rehabilitation designs are physically stable, it’s important to be able to predict the fluid forces acting on the introduced woody material.  However, to date most studies have been carried out under simplified experimental conditions using small, smooth-sided models and flumes.  Engineers in Oklahoma, U.S.A. used an outdoor experimental channel (1 m bankful depth, 1.8 m wide, bed slope 0.33%, with grassy banks) to measure drag and lift on logs and log-like structures, most of which were about 1.8 m long and 0.2 m in diameter.  Two tree species (hackberry, Celtis occidentalis and rough-barked oak, Quercus sp.) were used.   Stream velocities ranged from 74 to 123 cm/sec.  When simple cylinders were tested in steady flow conditions, drag and lift varied systematically with the log’s position: drag was highest when the log was at right-angles to the flow and well above the stream bed, while lift was highest when logs were close to the bed.  With branched logs and root wads, drag and lift were both much higher than with simple cylinders, but there was no systematic variation with submergence or orientation.  Under unsteady flows and a rising water level, drag increased sharply to reach a maximum when water overtopped the log, and then decreased when the log was fully submerged.  Lift showed a similar, though much less marked pattern of rise and fall.   The measurements showed that fluid forces on woody structures in small flashy streams are likely to be highest – around 2-3 times greater than in steady flows – in the early stages of a high flow event.

Reference:   Shields, F.D. Jr. & Alonso, C.V.  2012.  Assessment of flow forces on large wood in rivers.  Water Resources Research  vol. 48, W04516, doi:10.1029/2011WR011547.