The influences of large woody debris on British headwater streams.
This thesis examines the influence of large woody debris (LWD) on the physical habitat of British
headwater streams. The distribution and density of LWD accumulations throughout catchments is
considered using data from the River Habitat Survey (RHS) and more detailed catchment scale
surveys. The effect of accumulations of LWD on stream hydraulics and physical habitat at a reach
scale is examined using transect based measurements of depth and velocity over a range of
discharges before and after LWD removal for two reaches, measurement of reach average hydraulic
parameters for 25 reaches with differing levels of LWD and the application of the Aggregated Dead
Zone (ADZ) and Physical Habitat Simulation (PHABSIM) models.
The RHS and catchment scale surveys showed that LWD accumulations exhibit systematic patterns
in abundance within catchments, smaller streams having a higher density of LWD and a greater
number of LWD accumulations. The most hydraulically active type of LWD accumulations were
found to reduce flow velocity by an average of 55%, increase channel roughness by 149% and
increase depth by 165%. This impact was, however, found to vary with discharge. Application of
the Aggregated Dead Zone (ADZ) model showed that LWD increases the volume of ADZ, which
may indicate greater ecological refuge potential. Using the Physical Habitat Simulation
(PHABSIM) model, it was found that LWD improved habitat quality and overall habitat diversity.