flooding and the jubilee river (2010)
TRANSCRIPT
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Describe the factors and processes controlling flooding in river catchments and for a
case of study of your choice critically assess the effectiveness of flood protection and
alleviation [by Ramiro Aznar Ballarín]
Introduction
Floods are one of the most damaging and dangerous natural hazards (Wheater, 2006).
According to recent climate change scenarios (Hulme et al., 2002), flood risk in UK can be
expected to increase. Therefore, it seems highly important to understand the agents and
dynamic of this hydro-geological process as well as the negative impacts on human settings
and infrastructures in order to manage it sustainably.
In the first section of this work, I will describe the main factors and processes controlling
flooding in river catchments. Whilst in the second part of the assignment, I will discuss the
effectiveness of the Jubilee River, the artificial river channel created to reduce flood risk in the
area of Windsor, Eton and Maidenhead (Berkshire, UK).
Factors and processes controlling flooding in river catchments
In rivers, floods can be generated by three main causes. First, river floods are caused
almost entirely by excessively heavy and or/prolonged rainfall or, in areas of snow or ice
accumulation, by periods of prolonged and/or intense melt (Ward and Robinson 1990). Other
climatological-related floods are triggered by rain-on-snow and glacier breaches (jökulhlaups).
Thus, precipitation and temperature are decisive meteorological variables for producing
floods. According to Frei et al. (2000), the basin and time-integrated precipitation amounts as
well as the temporal evolution and spatial distribution within the catchment play an essential role
for the development of flooding. A good example of this is the catastrophic flood of June 2000 in
the Upper Guil catchment (Queyras, Southern French Alps). Arnaud-Fassetta et al. (2005)
points out that one of the main aggravating factors was the storm movement along the drainage
system. The rainfall started from the southernmost part of the catchment and then moved
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northwest, propagating downstream at the same pace as the flood wave. Surface air
temperature, on the other hand, determines the partitioning into snow and rain during the
precipitation event and partially controls the runoff from snow and ice melt and determines the
amount of soils and plants evapotranspiration (Frei et al., 2000).
Estuarine (and coastal) floods are caused by the second major ecological factor, storm
surges. These natural events are created by the interaction of very high tides, onshore winds
and low atmospheric pressure (Jones, 1997). Finally, there are other infrequent causes of
flooding which are indirectly related with climatological events: tsunami produced by
earthquakes, landslides into enclosed or semi-enclosed water bodies (Fig. 2) and the failure of
dams and other water control human structures (Ward, 1978).
Figure 2. Photograph of the landslide dam which blocked the Poerua River (Westland, New Zealand) in October
1999 (a); oblique photograph of Poerua in October 1999 - 2 days after the dam
break (b) (based on Hancox et al., 2005).
The magnitude of a particular flood is determined by the interaction between the
triggering factors mentioned above and the river basin characteristics. Thus, the final
expression of a flood in a specific catchment depends on the basin properties, and channel and
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channel network characteristics (Ward, 1978). First, according to Ward and Robinson (1990),
the major basin features can be divided in area, shape, slope, aspect and altitude. They argue
that the area affects both the time of concentration (the shortest time in which the whole of the
drainage basin contributes to the streamflow) and the total volume of runoff; shape is
associated with the concept of bifurcation ratio (Rb); water movement increase with slope; and
aspect and altitude affect both the amount and type of precipitation as well as the extent to
which is effectiveness is altered by evaporation.
Moreover, the properties of the hillslope materials, especially infiltration capacities and
lateral permeabilities in the topsoil as well as the preexisting conditions of temperatures and
humidity of the soils are considered key factors in the development of floods (Jones, 1997). Frei
et al. (2000) illustrates that frozen or moisture-saturated soils have a limited infiltration and
moisture-storage capacity, which tends to accelerate the runoff process. In an investigation
about the 2000 floods carried out in England and Wales, Holman et al. (2003) demonstrates
that most soils suffered compactation and structural damage because they were wet during
critical times for land management (the spring and early summer of 2000 were particularly wet),
such as for ploughing or harvesting. Soil structural damage led to a significantly reduction in soil
water storage and infiltration capacity, thus increasing runoff and consequent flooding.
According to Ward (1978), drainage pattern is associated with Rb and basin shape. He
goes on to argue that dendritic drainage (low values of Rb), on the one hand, are usually related
to sharp high-magnitude floods at the lower catchment due to the coalescence of flows from a
number of major tributaries. Trellised drainage systems (high values of Rb), on the other hand,
are patterns that permit the evacuation from the catchment of flood flows from the downstream
tributaries before those from the upstream tributaries have arrived, thus having a more
moderated flood response. Ward and Robinson (1990) also points out the importance of the
total area of interconnected saturated surface within the catchment network. This determines
the volume of quick flow produced by a precipitation/melt event. Roughness, bed and bank
materials, vegetation and load are also channel aspects that determine the flood intensity within
a catchment.
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An important factor that usually aggravates flood conditions is wildfire. Moody and
Martin (2001) suggest that this natural disturbance alter some watershed characteristics in four
different interconnected ways. First, wildfire decreases the canopy interception, which increases
the percentage of rainfall available for runoff. Second, fires reduce the water normally lost as
evaporation, which increases the base flow. Third, wildfires also consume ground cover, litter,
duff, and debris, which increases runoff velocities and reduces interception and storage. Fourth,
wildfires alter the chemical properties of the soil that affect infiltration and, thus, the hydrologic
response. In the summer of 2006, Galicia (Spain) suffered the worst series of fires (Fig. 3 a) of
the last centuries. The immediate economic losses were about 300 millions; however, this
estimation was gauge without taking into consideration the losses resulted from floods (Fig. 3 b)
caused partially by post-fire effects such as erosion and decrease of soils infiltration capacity
(Barrio et al., 2007).
Figure 3. Satellite image of the Northwest of the Iberian Peninsula in the 2006 summer fires (a); picture of the
consequences of one of the subsequent floods (b).
The majority of these fire episodes were initiated by humans. In the case of floods,
human factors can both intensify them. Table 1 illustrates the three main human effects on
altering flood conditions within river catchments: water supply engineering, land surface
changes and channel modification. Aside from these alterations, Jones et al. (2000) highlight
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the importance of roads within watersheds. They suggest that roads interact with flows in four
different ways, they can act as a corridor, a barrier, a sink or a source, and, thus, roads can
significantly alter the direction and intensity of floods.
Table 1. Human effects on flooding within river catchments (based on Jones, 1997).
Water Supply Engineering Land Surface Changes Channel Modification
Dam construction, operating rules and failures Urbanization Land drainage
River regulation and conjuctive use of groundwater Deforestation/afforestation Channel straightening
Interbasin transfer Agriculture and husbandry Flood protection works
Water abstraction and irrigation
The Jubilee River
In 1992 the National Rivers Authority (NRA) promoted the latest major scheme, the
Maidenhead, Windsor and Eton Alleviation Scheme (MWEFAS), a substantial part of it is the
Jubilee River (Fig. 4 a), an 11.6 Km flood relief artificial river channel to the north-east of the
natural River Thames, with a design capacity of 215 m3/s (Onions, 2004) and for the
approximately 1 in 60 year event (Wheater, 2006). It was opened 20 years later and built at a
cost of £110 million. In addition, the area around the channel has been landscaped, new
riparian and aquatic habitats created and a footpath and cycleway added (Fig. 4 b).
During the winter 2003 floods, the River Thames catchment experienced the largest
flood event since 1947. To prevent flooding, 140 m3/s were diverted from the River Thames into
the Jubilee River. Despite protecting the area of Maidenhead, Windsor and Eton Water, Datchet
and Wraysbury were flooded and a number of structures and areas of embankment along the
Jubilee River suffered significant damage, e.g. Black Potts Viaduct (Fig. 4 a). Atkins, an
engineering consultancy commissioned by the Environmental Agency, concluded that the actual
capacity of the Jubilee River was below the anticipated capacity as identified in the original
planning application (Atkins, 2007). Thus, according to Atkins’ recommendations, flood defense
were increased in key locations. In contrast, Onions, a consultant civil engineer with Arup,
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stated that the operation of the Jubilee River did not increase levels by more than 2-3 mm at or
below the confluence (Onions, 2004).
Figure 4. Jubilee River at the Black Potts in December 2009 (a); and footpath and riparian vegetation planted in the
surrounding area (b).
It is difficult to determine the global effectiveness of the new flood defences in the
Jubilee River. However, there have been developed some environmental sustainable measures
such as the creation of riparian habitats and the use of sustainable drainage systems (SUDS)
which promote riverine biodiversity, create buffer zones for water (DEFRA, 2005) and reduce
the local runoff which move toward a more sustainable flood risk management within the River
Thames catchment. In regard to the debate about role played by the Jubilee River in the 2000
floods only can promote the emergence of more and alternative solutions.
References
Arnaud-Fassetta, G., Cossart, E. and Fort, M. (2005). Hydro-geomorphic hazards and impact of
man-made structures during the catastrophic flood of June 2000 in the Upper Guil catchment
(Queyras, Southern French Alps). Geomorphology 66, 41-67.
Atkins. (2007). Jubilee River Post 2003 Recommendations Closure Report . Prepared for the
Environment Agency, pp. 36.
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Barrio, M., Loureiro, M. and Chas, M.L. (2007). Aproximación a las pérdidas económicas
ocasionadas a corto plazo por los incendios forestales en Galicia en 2006. Economía Agraria y
Recursos Naturales 7(14), 45-64.
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and coastal erosion risk management in England. First Government response to the autumn
2004 Making space for water consultation exercise. London: Department of Environment Food
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precipation and flood events in Central Europe. Integrated Assessment 1, 281-299.
Hancox, G.T., McSaveney, M.J., Manville, V.R. and Davies, T.R. (2005). The October 1999 Mt
Adams rock avalanche and subsequent landslide dam-break flood and effects in Poerua River,
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Jones, J.A., Swanson, F.J., Wemple, B.C. and Snyder, K.U. (2000). Effects of Roads on
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Currey, UK, pp. 34.
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