The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC
discharge) had the effect of reducing flood peak by 1-8% at Alma Weir with up to half an
hour delay compared with baseline. In terms of design events, it was found that a
moorland improvement scenario (of grip blocking) had little effect on the magnitude of
the 10 year peak but reduced the peak of the 50 year and 100 year events by 5-7%. It
must be noted that the same ‘moorland improvement’ scenario did produce a more
gradual recession limb on the hydrograph, indicating a longer flood duration at Alma
Weir.
Blocking drains in peatsoil wetlands may therefore have the same effect as reducing the
runoff from floodplains, i.e., a reduction in speed of runoff so attenuating downstream
peak discharges. The role of peat saturation levels may thus be of secondary
importance to the role of drains facilitating runoff. Overall, therefore, the effectiveness
of restoring drained peatlands in order to reduce runoff and attenuate downstream
flooding is still uncertain. Whilst there is evidence of local scale attenuation, it is not yet
known how peatland restoration impacts flood peaks at a catchment scale.
An example of hydrometric response to catchment scale land management change has
been reported for the Munster Blackwater, Ireland, as part of the Mallow Drainage
Scheme Engineering Report (OPW 2003b). Since EU accession in 1973, wide scale
intensification of land drainage and forestry has occurred within the catchment. Using
historical data available from Killevullen hydrometric station, the periods 1955-1973
(pre- EU accession) and 1973-2003 (post- EU accession) were compared. Revealingly,
only a minor difference was detected in mean annual flooding between the periods,
which was attributed to higher average rainfall in the 1990s. No evidence was found
that catchment-scale land management change had contributed to an increase in peak
flood flows. This type of study requires replication using data sets from other
catchments with varying soil types and should include a more rigorous land
management correlation for any strong conclusions to be drawn. In addition, whilst
there were no significant changes to flood peak height at Killevullen, the study could not
account for the timing or frequency of flood peaks.
4.3 Variables affecting wetland management and flood attenuation
From the above account of how management of wetlands may affect their ability to
attenuate floods, it is clear that there are two main variables driving the impact of a
particular wetland to mitigate flooding – surface storage (soil moisture deficit and
storage of water in pools and small depressions) and runoff rate (affected by surface
‘roughness’ and vegetation cover). For effective mitigation, the former should be
enhanced and the latter reduced. The issue of drainage ditches, then, represents a
particular management problem. Removing or blocking drains may retard the runoff of
water during a flood, but will also tend to make the wetland ‘wetter’ outside of rainfall
periods, leading to a greater soil water and, potentially, surface water content, in turn
reducing the water storage capacity of the wetland. On the other hand, coverage and
diversity of bog vegetation has been shown to increase as a result of drain-blocking
(Holden, 2009, Section 4.2) which, as well as having biodiversity benefits (Section 6), is
believed to contribute to the attenuation of surface water runoff.
FINAL REPORT, February, 2012 40