Project funded
by theEU–DGEnvironment
L
andsurfacerelevant
forapplication
Artificial surface
Agriculture land
Forest and semi-natural areas
Wetlands
F
inancialcosts
(C
apital
,
operation
&
maintenance
)
Typically there are no costs of land acquisition for overland flow areas as they are
situated in the forest itself. Other costs may be linked to investigations or studies
to determine the likely amount and timing of runoff to be processed and the
precautions needed to avoid any damage. There may be
opportunity costs
if the overland flow area is sited on productive forest land. Using an area for
overland flows can influence the choice of tree species and temporary water cover
can increase operation costs; that in turn may impact the income which could
potentially be obtained from forest harvesting.
D
esign
Overland flow areas will typically be located within the ditch network of
man-
aged boreal forests
. Their dimensions will be dependent on the size of the
upstream catchment. Ideally, the space needed will not impact on productive forest
areas. This measure can be part of a bundle of measures designed to minimize
forestry impacts on water quality, including water sensitive driving, sediment
capture ponds, appropriate design of roads and stream crossings and peak flow
control structures.
S
cale
Overland flow areas are only suitable for
application in relatively small areas such
as those drained by a single ditch or
small ditch network.
Overland flow areas are set aside to be used to
minimise the negative impacts
of forest management on
water quality
: they collect some of the
excess sediment produced during ditch maintenance and other forest management operations. Overland flow areas are created by building a semi-permeable
dam
in a forest ditch and
lateral ditches
upstream of the dam (to transport water into the surrounding catchment). At periods of high flow, water will overflow
the lateral ditches; its velocity will be slowed and much of the sediment will be deposited. At periods of low flows, the permeable dam will slow water flow and
cause deposition of sediment.
Case studies:
RestorationofDurrowalluvialwoodland, Ireland;Habitat reconstruction inKörösValley,Hungary
Existing wetlands may function as overland
flow areas under some circumstances.
FORESTRY
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