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

The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC
On the Weinfluss (River) near Vienna a test section of “wooded” “floodplain” was
constructed. It was an outdoor flume with channel and floodplain into which artificial
floods were released from an upstream reservoir. Experiments using tracer solutions
showed that the flood wave becomes attenuated as it flows across the floodplain and
moves downstream. The experiments also showed that wooded floodplain can have
the effect of increasing flood water levels upstream of the reach (FLOBAR2, 2003).
In a simulation modelling study conducted on Australian rivers, the additional resistence
to flow provided by natural riparian vegetation was found to reduce peak discharge, so
reducing the incidence of downstream flooding. Importantly, this effect is more marked
during smaller floods, which are more sensitive to vegetation conditions than larger
floods (Anderson et al., 2005, 2006). Similarly, Thomas and Nisbet (2007) conducted
hydrological simulation studies on the ability of a floodplain woodland to attenuate
flooding. They found that water velocities decreased within the woodland during floods,
so leading to higher water levels and the creation of a backwater effect extending
considerable distances upstream. Flood storage was increased and peak discharge
downstream reduced. They concluded that strategically placed floodplain woodland
could potentially alleviate downstream flooding.
3.2 Peatlands
The hydrology of peatlands is very different to alluvial floodplains and they generally
receive much of their water either as rainwater (particularly bogs) or groundwater and
hillslope (fens) (Fig. 2). Very little water flows onto peatlands from channel-fed overland
flow because of the small size of channels owing to their headwater nature. Critical to
understanding the flood attenuation ability of peatlands is knowledge of how surface
water in the wetland, derived from rainwater, hillslope or groundwater interacts with
the peat soil and surface vegetation (Gibson, 2000).
Within peatlands, water flow can include vertical and horizontal movement within the
various layers and sheet flow and channel flow over the surface as well as water
exchanges with upland systems (Kværner and Kløve, 2008). Whilst water retention by
peatland surfaces may attenuate and delay runoff events, the flood mitigation role of
peatsoil wetlands is often overstated (Keddy, 2000; Bullock and Acreman, 2003) and it
has been recognised for many years that not all peatlands reduce storm flows nor
provide higher flows in summer (Kay, 1960). In contrast to the knowledge of the
hydrological properties of floodplains, the role of headwater peatlands in runoff
generation and flood attenuation is still contentious (Kværner and Kløve, 2008).
3.2.1 Raised and blanket bogs
Raised- and blanket bog peatlands consist of two layers with distinctly different
hydrological properties - the upper aerated ‘acrotelm’ and lower, anoxic ‘catotelm’
(Holden and Burt, 2003). The catotelm is darker and more humified than acrotelm and
consists of more well-decomposed and denser peat. The catotelm is also constantly
saturated, whereas the water level in the acrotelm can fluctuate more. Water flow
through peat is related to the pore size between particles of peat. In poorly
decomposed peat, consisting of larger particles of woody vegetation or stems and leaves
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