Environmental Engineering Reference
In-Depth Information
The effects on runoff of subsurface drainage are
soil dependent and impacts on downstream flood-
ing are difficult to interpret from field data. River
channel improvements can have a much greater
effect on peak flows
runoff associated with modern practices are wide-
spread (Souchere et al. 1998; Holman et al. 2001;
Hollis et al. 2003; Palmer 2003a, 2003b).
Field-drainage and associated subsoil treat-
ments can increase or decrease peak drain flows
and the time to peak flow by as much as two
to three times either way; the behaviour appears
to depend on the soil type and wetness regime
(Leeds-Harrison et al. 1982; Armstrong and
Harris 1996; Robinson and Rycroft 1999).
Enhanced surface runoff generation as a result
of some of the abovemodern farming practices can
generate local-scale flooding. For example, long-
term studies in small catchments in the South
Downs of southeast England show that there is
a significant relationship between the presence of
autumn-sown cereal fields and local 'muddy
floods' in autumn (Boardman et al. 2003). This
relationship has also been observed in France
(Papy and Douyer 1991; Souchere et al. 1998) and
Belgium (Bielders et al. 2003). The frequency of
these floods can be reduced using appropriate
arable land management practices (Evans and
Boardman 2003). Muddy floods, and the erosion
and subsequent deposition of substantial amounts
of eroded soil, generate substantial economic da-
mages each year, most of which occur off-farm
(Evans 1996).
There is, in contrast, very little direct evidence
of how such changes affect the flow in surface
water networks, and evidence that is available is
for small catchments (
<
10 km
2
), and mainly re-
lates to the impacts of forests, which are gener-
ally considered to reduce flood peaks, except for
the effects of drainage and forest roads (McCul-
loch and Robinson 1993). However, peak flows
can increase in the period after forest planting,
mainly as a result of plough drainage and ditch-
ing (Robinson 1986; Robinson et al. 1998). In
a review of results from 28 monitoring sites
located throughout Europe, Robinson et al.
(2003) concluded that forests probably have a
relatively small role to play in managing regional
or large-scale flood risk, and significant local-
scale impacts are likely only for the particular
case of managed plantations on poorly drained
soils.
than field drainage
(Robinson 1990).
Catchment-scale impacts
National analyses of flooding trends have not
shown significant impacts of either climate or
land use change, largely because of the overriding
influence of year-to-year climatic variations,
which make trends associated with climate and
land use difficult to identify (Robson et al. 1998;
Institute of Hydrology 1999). The UK Flood Esti-
mation Handbook (Institute of Hydrology 1999) is
based on two methods of flood estimation, the
Statistical Approach and the Rainfall-Runoff
Approach. Regression relationships linking flood
statistics (e.g. the median annual flood) or rain-
fall-runoff method parameters (e.g. the time to
peak of the unit hydrograph) with catchment
characteristics did not reveal any significant
relationship with land cover. It should be noted,
however, that the records used in the analysis
were mainly from catchments not experiencing
major land use change (see UK Flood Estimation
Handbook, Vol. 3, p. 234), and that land cover
data cannot alone reflect land use management
practices.
River channels in the UK have also undergone
substantial modifications over the past 70 years
as a result of land drainage schemes and flood
protection works for urban and rural floodplain
areas (NewsonandRobinson1983; Robinson1990;
Robinson and Rycroft 1999; Sears et al. 2000).
Channels have been subject to a number of
different modifications, depending on the circum-
stances, for example straightening, resectioning,
embanking, culverting and the construction of
weirs and sluices. More recently, there has been
a move towards the restoration of channels and
floodplains to their natural states and functions, as
part of biodiversity and natural flood mitigation
schemes. It is clear that such modifications will
have changed the natural routing processes in
