Environmental Engineering Reference
In-Depth Information
forms in fluvial catchments and channels, where sediment is transported as either
dissolved
,
suspended
or
bed
load.
EROSION
Continental denudation occurs through net erosion in upper catchment areas and the
removal of debris, eventually to the oceans. Distinctions are drawn between sediment
derived by fluvial erosion of bedrock, debris
URBAN DEVELOPMENT ON FLOOD PLAINS
new developments
Large tracts of urban land in central and southern Britain were subject to repeated
episodes of flooding during the winter of 2000-2001, and to a lesser extent early in 2002,
leading to little loss of life but well in excess of £2.1bn of damage and economic losses.
It was claimed initially that 'freak weather' conditions and the early impact of global
climate change were responsible. Blame was attached to the Environment Agency for
poor flood warning, although extra time could only have mitigated impacts and not
prevented the flooding. We are 'reassured' that forecast return periods of more than once
in a hundred years mean they are unlikely to recur during our lifetimes, even though
climate is changing.
There is some truth in each claim but an emerging and undeniable element is the
extent to which modern urban development places people and infrastructure in the flood
path, inevitably increasing flood hazard. Hydrological models have a place in flood
avoidance by forecasting the
probable maximum precipitation
for any given geographical
area. This, and other hydrometeorological data, is transformed into the probable
maximum flood by factoring in the hydrogeological character of the catchment. Updated
with predictions of global climate change emanating from AOGCMs and IPCC, forecasts
should improve future flood adjustment or abatement schemes. It is also recognized,
however, that some climatic shifts could considerably shorten recurrence intervals.
Building trends, and recent events, clearly challenge the wisdom of urban
development on flood plains. This calls for changes in planning policy, to remove
confusion in flood perception and enhance flood protection or proofing. Faster evacuation
of potential flood waters from urban areas through channelization provides a false sense
of security and overlooks the fact that river channels and the flood plain itself are natural
stores which reduce flood risk elsewhere. Economic pressures on building land and the
attraction of waterside living and recreation have drawn urban development increasingly
into the hazard zone. The time has come to consider stricter regulation and zoning on
future building (Figure 1). This can be less expensive than
hard
protection measures such
as embankment, channelization, flood relief routeing or rebuilding above a threshold
level storage reservoirs, diversionary channels and other flood routeing schemes, the
emplacement of control structures (dams, etc.) upstream and the socioeconomic cost of
failure. Integrated catchment management policies also aid flood abatement by assessing
activity in every part of the catchment for its contribution to flood risk. Current trends
towards planning zonation, de channelization, meander restoration, etc., are more
sustainable options.
