River Kelvin Flood Prevention Scheme

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.
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 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 recommend-ations 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

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 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 sub-consultant, 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.

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 and Deepening
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.
Design Standards
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.

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 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.
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
  

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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