River Kelvin Flood Prevention Scheme
Conception to Completion
Alan McGowan, Halcrow Group
Background
During the 9 to 11 December
1994 prolonged rainfall over
central Scotland resulted in
exceptionally high flows in many
watercourses. The long duration
of the rainfall resulted in the whole
of the River Kelvin catchment
being saturated and contributing
to flow in the watercourses. The
Scottish Environment Protection
Agency (SEPA) operate seven
flow gauging stations within the
River Kelvin catchment area and
all except one recorded the
December 1994 flood event as the
highest on record. The water
levels at the Killermont gauge
were more than 0.7 metres higher
than the previous maximum
recorded flood level.
The peak flow at the Dryfield
gauge at Hayston was around 130
cumecs compared to a mean flow
of around 6.9 cumecs.
The
Meteorological
Office
identified that the rainfall event
that occurred over the River
Kelvin catchment on the 10 and
11 of December 1994 had a return
period of between 1 in 300 and 1
in 1000 years depending on the
location and altitude.
The catastrophic flood caused the
tragic loss of two lives and
extensive damage to over 300
business premises and residential
properties. The cost of the
damage has been estimated at
over £10 million.
Strathclyde
Water
Services
appointed the Halcrow Group to
carry out a hydrological study and
flood risk assessment for the
River Kelvin and its tributaries
between Kilsyth and Killermont
and to ascertain the nature and
frequency
of
inundation
throughout the study area and in
particular
obtain
record
information about the flood event
in December 1994.
and varied between 1 in 10 and 1
in 50 years in the upper reaches
and on the main tributaries, the
Luggie and Glazert Water.
The study also recommended that
a flood alleviation scheme was
implemented to protect vulnerable
areas in Kirkintilloch, Balmore,
Torrance and Lennoxtown.
Following
local
government
reorganisation the recommendations from the previous study
were
adopted
by
East
Dunbartonshire Council who were
committed to developing a flood
prevention scheme to alleviate the
risk of flooding in Kirkintilloch,
Torrance,
Balmore
and
Lennoxtown
Extensive surveys of the river
channel and flood plain (which is
over a kilometre wide in places)
were carried out to capture
sufficient data to develop a fully
calibrated
and
verified
hydrodynamic
model.
This
included over one hundred and
seventy five river cross sections
and surveys of all bridges and
other hydraulic structures.
The study concluded that the
return period of the December
1994 flood was around 1 in 200
years on the lower reaches of the
Kelvin in Glasgow and Kirkintilloch
1km
December 1994 Flood Event
Page 1
December 1994 Flood Event
-
Preparation of the Scheme
In order for the Council to
construct a flood prevention
scheme they must promote it in
accordance
with
the Flood
Prevention (Scotland) Act: 1961
(the Act).
The main objective of the Scheme
is to protect existing residential
and industrial properties from
inundation during the design
event. The proposed defences
were placed remote from the
water course, and close to the
protected urban areas to preserve
as far as practicable, the existing
floodplains. The floodplains serve
River Kelvin Flood Prevention Scheme
the
purpose
of
providing
temporary storage to a flood when
the volume of water passing
downstream is too large to be
contained in the river channel.
Consequently the removal of part
of the floodplain by constructing
flood defences to protect existing
properties or further development
on the flood plain would reduce
the storage volume available
during a flood event and increase
the flood level.
The Scheme ensures that the vast
majority of the flood plain is
retained thus minimising the
effective loss of flood plain
storage. This design philosophy
also preserves the existing river
ecology by locating the defences
in areas remote from the river
channel.
The Scheme has been designed
with the aim of minimising the
impact on flood water levels both
upstream or downstream of the
works due to the reduction in
channel conveyance and storage.
Hydraulic Modelling
Glasgow University carried out the
hydraulic modelling of the River
Kelvin and its tributaries as a subconsultant, building on the work
previously carried out in the
feasibility study. A hydrodynamic
model of the proposed channel
geometry was developed to
identify the height of the proposed
defences and determine the
impact on water levels and flows
upstream and downstream due to
the loss in flood storage and the
modified channel properties.
River Management Options
Several alternative options were
considered to assess whether a
reduction in flood water level in
the vulnerable areas was feasible.
These options included the
provision of additional flood
storage
and
the
widening,
deepening and regrading of the
river channel.
Design Standards
Flood Storage
In general, the provision of
additional flood storage within a
catchment can lead to a reduction
in flows passing downstream. The
flood response times on the main
tributaries the Glazert and Luggie
Water were significantly quicker
than the River Kelvin. It was found
that the mobilisation of additional
storage in the upper catchment
would delay the time to peak of
each of the tributary flood
hydrographs, bringing the time to
peak closer to that in the River
Kelvin and increasing the peak
flow
passing
downstream.
Therefore the use of additional
flood storage was discounted due
to an increased risk of flooding to
areas currently unaffected by the
design flood.
Channel
Widening
Deepening
and
A further option considered was
the improvement of the channel
conveyance. This could be
achieved by widening, deepening
or regrading the bed. The
improvements that could be
gained were very small in
comparison to the environmental
damage that would have been
caused by such a proposal. The
modification to the river channel
was also found to increase the
peak flows passing downstream
into
Glasgow,
which
was
undesirable.
Direct Defences
The form of defences adopted for
the Scheme were generally based
on a series of earth embankments
and retaining walls, the location of
which was chosen to preserve as
much of the existing flood plain as
practical and preserve property
boundaries where possible.
Page 2
The level of defence adopted for
the River Kelvin Flood Prevention
Scheme (the Scheme) was the
greater of the predicted 1 in 100
year return period flood event or
the December 1994 flood level. A
freeboard allowance of 0.5 metres
above the predicted 1 in 100 year
flood water levels and 0.3 metres
above the December 1994 flood
water levels were also to be
incorporated into the Scheme.
Future Raising
Given
the
uncertainties
in
predicting hydrological variations
and the current international
thinking on the effect of climate
change, the design of flood
defences had to take into account
the possibility of future raising to
be accommodated without major
works. In relation to retaining
walls, the foundations had to be
designed to withstand a potential
flood level 0.5 metres higher than
the current design level. In
addition, seepage cut offs had to
be installed deep enough to
withstand
the
increased
hydrostatic pressure without any
additional seepage.
Secondary Flooding
In addition to providing protection
against overland fluvial flooding,
the design also considered the
possibility of secondary flooding
from the drainage system. A
number of pumping stations were
required to maintain the discharge
of surface water during a flood
event, particularly where overland
flow was now prevented due to
the construction of flood defences.
River Kelvin Flood Prevention Scheme
downstream of the Scheme who
were concerned that the Scheme
would lead to an increase in the
frequency of flooding to their
course.
The Inquiry initially commenced in
September
1999
and
was
suspended until December 1999
to allow further studies to be
carried out. The Inquiry heard
evidence from the three objectors
together with evidence from
supporters
of
the
Scheme
including the local Flood Action
Group.
Public Consultation
Extensive consultation with the
public and the Scottish Executive
was carried out during the
preparation of the Scheme. This
took the form of meetings and
public exhibitions where feedback
was encouraged. Following these
detailed discussions the Scheme
was amended prior to the Scheme
being formally advertised.
Benefit: Cost Assessment
A benefit: cost assessment of the
Scheme was carried out in
accordance with MAFF guidelines.
Direct damages were calculated
using the methodology published
by the Middlesex University Flood
Hazard Research Centre and
updated in the FLAIR Manual.
The resultant ratio was 1.2 and is
in excess of the minimum
acceptable value set by the
Scottish Executive and entitles the
Council to financial support of the
costs
expended
following
confirmation by the Scottish
Ministers.
Flood Prevention Scheme
Submission
Under the Flood Prevention
(Scotland) Act, 1961, the Council
is required to publish the Scheme
and issue notices and copies of
the Scheme to all affected parties
and place advertisements in the
local
press
and
Edinburgh
Gazette.
Following advertisement of the
Scheme a number of objections
were received from individuals
and businesses affected by the
Scheme. The majority of these
objections
were
withdrawn
through negotiation although three
objections remained. The Council
exhausted all avenues in trying to
negotiate with the remaining
objectors and requested that a
Local Public Inquiry be held.
Local Public Inquiry
Two of the objections were from
developers who owned land within
the natural floodplain, which would
not be protected by the Scheme.
Both objectors had previously
been refused planning permission
for their proposed developments
in the flood plain and had lost their
appeals. They objected to the
Scheme on the basis that it would
affect their ability to develop the
sites should the necessary
approvals be granted in the future.
The final objection came from a
golf club several kilometres
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Following a period of six months
after the inquiry, the Reporters
recommended that the objections
be dismissed and the Scheme be
confirmed without modification.
Planning Approval
In addition to the approval granted
by the Scottish Ministers, the
Council was required to obtain
planning approval for the Scheme.
The process of obtaining planning
approval for the Scheme was
carried out by the Council and a
Notification of Intention to Develop
(NID) was lodged with the
Secretary of State.
The NID identified, in greater
detail, the landscaping and
architectural detailing which were
to form part of the overall
Scheme.
Planning approval was granted for
the Scheme in May 1999.
Form of Procurement
The
two
principal
options
considered were as noted below:


traditional form of contract
with a contractor appointed to
construct
the
Employer’s
design
design and build form of
contract with a contractor
appointed to carry out both
the detailed design and
construction
River Kelvin Flood Prevention Scheme
Following careful consideration,
the design and build form of
contract was adopted. This
achieved the Council’s main
objective
of
commencing
construction as soon as possible
to recover time lost as a result of
the public inquiry.
The Council were keen to promote
a partnering approach and benefit
from the principles laid out in the
Egan
Report.
The
NEC
Engineering and Construction
Contract (ECC) was adopted due
to its non-adversarial approach.
The Council were also keen to
establish a greater certainty of out
turn costs and Option A of the
ECC was adopted, which is a
lump sum contract. The transfer of
certain risks also formed a key
part of the contract.
Selection of a Contractor
The tender for the main contract
was advertised in the Official
Journal
of
the
European
Communities
(OJEC)
in
accordance with the Public Works
Directive
71/305/EEC
and
expressions of interest were
received
from
twenty
one
contractors. Following a three
stage selection process three
contractors were selected to
tender for the contract.
Following receipt of tenders and a
detailed evaluation process, the
contract was awarded to Harbour
and General Works Limited for the
sum of £8.9M with a completion
date of September 2002.
Construction
Detailed design and construction
of the Scheme commenced in
April 2001. The Scheme involved
the construction of:
2.8 km of flood defence walls
4.6 km of earth embankments
13 no. pumping stations
4 no. flood gates
The detailed design of the
Scheme was sub-contracted by
Harbour and General to Ove Arup
& Partners.
Prior to any construction work
commencing it was necessary to
remove a large number of trees.
This exercise was programmed
around the bird nesting and bat
roosting seasons in order to
minimise the disruption to these
animals.
During the construction phase,
both the affected land and
property owners and the general
public were kept informed of
progress at public meetings and
advertisements in the local press.
Construction of the Scheme
generally
progressed
well,
however,
as
with
any
development within a densely
populated area (and also a
floodplain) there were a few
unforeseen problems including the
significant rainfall event of July
2002, where minor surface water
flooding occurred in a few areas
as the required modifications to
the
drainage
system
were
incomplete.
The Scheme includes a mixture of
both soft and hard landscaping
and locally appropriate materials
were
incorporated
into
the
completed defences including
natural stone cladding to walls
and planting with indigenous
species.
The works were completed in
February 2003, some four months
behind programme. The Scheme
is now fully operational offering
protection to both those that were
affected by the devastating floods
in December 1994 and those who
narrowly escaped inundation in
this event.
References
‘River Kelvin – Kilsyth/ Killermont,
Hydrological Study and Effects of
Flooding’,
Crouch
Hogg
Waterman, October 1996
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