|
|
|
|
|
|
|
|
The Borough of Dacorum sits within the Colne catchment. This catchment contains six main
rivers and their tributaries: the Colne, Ver, Gade, Bulbourne, Chess and Misbourne.
The catchment contains a unique collection of river systems, ranging from chalk rivers
in the upper catchment to the more urban clay-based systems in the lower catchment.
The headwaters of these rivers are in the Chilterns, a designated Area of Outstanding
Natural Beauty in recognition of its exceptional landscape qualities.
The upper reaches of the Colne’s main tributaries are dependent on the chalk groundwater
for a constant water supply, these groundwater levels fluctuate seasonally in response to
recharge from rainfall. The chalk is the most significant aquifer of southern England and
is of regional and national significance in terms of abstracted volume. Geological
features mean the majority of the rivers in the Colne catchment are susceptible to
low flows due to low groundwater levels
Drinking water is predominately supplied by Three Valleys Water. This is a drinking water
only provider and relies significantly on groundwater for its supply. The area is home to
some of the largest per capita demand in the country, approximately 177 l/h/d, compared to
national average of 155l//h/d. Groundwater abstraction for PWS is also limited within the
Colne Catchment due to contamination issues. We have not granted any new abstraction licenses
for public water supply in this area for many years. Nonetheless, the area is over-abstracted
due to abstraction licenses granted in the past, many of them being licenses of right (i.e.
without time-limits).
The Environment Agency is concerned about low flows in a number of rivers within
the catchment. The Restoring Sustainable Abstraction programme (RSA programme) is
a way of prioritising and progressively examining and resolving these concerns The
RSA programme is an umbrella for work required under the Habitats Directive, Sites
of Special Scientific Interest, Biodiversity Action Plans and undesignated sites of
local importance. The map below identifies those sites / river stretches where we are
currently undertaking investigations. Currently 11 sites are being investigated in the
catchment, all of which are associated with Water Company abstractions.
In the past, a number of Alleviation of Low Flow schemes (ALF, the predecessor to RSA) have
been implemented in an attempt to resolve the impacts of abstraction. Solutions include the
reduction of abstraction volumes, replacing abstractions with water from outside of the
region and relocating abstractions to sites lower down river catchments.
On the River Ver, a scheme to improve flows was implemented in 1993, resulting in the
closure of a pumping station. The deficit in water supply was compensated by supplies
being provided from Grafham Water which is outside of the catchment. Subsequent RSA
investigations have identified that the River Ver is still suffering from low flows.
It was concluded that the river continues to dry out and suffer form low flows. We are
currently reviewing options to improve flows in the River.
A number of sites along the upper reaches of the River Colne are perceived to be suffering
form low flows. Investigations were recently undertaken by the Agency to determine whether
the ecology of the Upper Colne was suffering any adverse impact from licensed abstractions.
The report identified that there was strong historical evidence that the Upper Colne was a
typical chalk stream before major abstraction was undertaken. The River looses water all
along its course and only gains significant amounts of groundwater when groundwater levels
are very high. The investigation also identified a number of areas of ecological concern.
Further investigations are proposed in the near future with the aim of trying to understand
the impact abstractions are having on flows and ecology.
Investigations into the causes of low flows on the River Gade have recently been completed.
The investigation identified that flows in the rural reach of the River are reduced by public
water supply abstractions and exacerbated by historical channel modifications. It was, however,
deemed unfeasible to relocate or stop the impacting public water supply abstraction. It was
therefore decided to look at alternative methods of enhancing and improving the flow regime
in the river through river restoration and improvement schemes. We are expecting the results
of this report to be available in the near future.
The River Bulbourne has a history of drying in the upper reaches, particularly associated
with periods of low rainfall. Abstractions from the ground for public water supply have
been identified as exacerbating low flows. Following an investigation, it was agreed that
the best solution to the low flow problems would be to cease abstraction from a nearby public
water supply borehole. Water quality issues have resulted in a delay in the complete shutdown
of the abstraction and we are therefore still assessing the impact of the reduction. Despite
the work so far undertaken to improve flows in the River, the impact of abstractions remain
a concern. The River Bulbourne continues to be listed on the RSA programme and we have begun
a monitoring strategy to help us investigate the potential impacts of other abstractions in
the catchment.
The River Chess has a history of low flows and drying through Chesham particularly during
periods of low rainfall. In the past the River has been perceived as suffering from reduced
flows possibly linked to local abstractions or historic management. Initial investigations
undertaken by the Agency concluded that during dry periods flows in the upper reaches of the
Chess were unable to maintain characteristic chalk stream habitats. There was, however,
insufficient data to asses whether this is related to abstraction from within the catchment.
The Agency is therefore undertaking further investigative work to understand what impact
abstractions may be having.
We are currently investigating the middle reaches of the River Colne between the Gade
confluence and Denham Green including the surrounding lakes, one of which is a designated
SSSI. These sites are currently on the RSA programme due to the perception that they are
suffering from low flows and low lake levels. Initial investigations undertaken by the
Environment Agency have focused on the identification of any abstraction related impacts
to the rivers or lakes. The investigations concluded that a further monitoring programme be
developed to gain a better understanding of the hydrology / hydrogeology of the lakes and River
Colne. Further work will also focus on understanding the impact that lake management has on
water levels and how other water bodies, such as the Grand Union Canal, interact with the
River and Lakes.
For a number of years the River Misbourne has been perceived as suffering from low flows
and various options for alleviating this have been assessed. In 1997-98 the preferred
alleviation scheme was completed. Thames Water reduced abstraction in the catchment by
an average of 7 Ml/d to 2 Ml/d and Three Valleys Water reduced pumping by an average of
8 Ml/d. The total reduction in abstraction for the whole catchment was 15 Ml/d. Despite
the completion of the scheme, the River Misbourne is believed to continue to suffer from
low flows. It is perceived locally that these continued low flows are exacerbated by the
remaining authorised abstraction from the catchment. Therefore the River Misbourne remains
listed on the RSA programme. Further investigative work has ascertained that the full
benefit of the ALF scheme has not been achieved. We are currently investigating why flows
have not returned to predicted levels.
We believe that demand for water in Hertfordshire and other parts of South East England
should be reduced. We believe that any new housing or other development in Hertfordshire
should be supplied within existing abstraction limits. The average rate of water use in
Hertfordshire is the highest in the country, and far higher than in similarly wealthy EU
countries. Demand can be reduced for example, by the installation of water meters. The
evidence so far suggests that simply installing meters delivers a 10% drop in water use.
Author: Miles Morgan, Water Resources Officer, Environment Agency August 2008
Further information on the water resources situation in the Colne Catchment can be
found in the Colne Catchment Abstraction Management Strategy( Colne CAMS) which can
be found at http://www.environment-agency.gov.uk/cams
|
|
