Flood relief scheme to protect Bandon from future disaster
10 April 2014
Following the major flooding of Bandon town in November 2009, the Office of Public Works (OPW) began investigating the feasibility of developing a flood relief scheme to protect Bandon. The 2009 floods had damaged or affected 190 business properties in Bandon and resulted in approximately €140 million in insurance claims across Co Cork. During November 2009, Bandon had received approximately 20% of the total annual rainfall for 2009, resulting in significantly elevated levels in the local river.
The Bandon Flood Relief Scheme is one of the first drainage schemes to be commenced under the Arterial Drainage Act 1995, following the implementation of the Floods Directive. It is also one of the first schemes to be designed following the pilot Catchment Flood Risk Assessment and Management Studies (CFRAM Study). The CFRAM Study policy represents a shift, as recommended in the 2004 by the Flood Policy Review Group, towards catchment based, flood risk-management measures.
The scheme has been developed to provide Bandon town with localised flood relief. Byrne Looby PH McCarthy was appointed by the OPW as lead consultants on the scheme. Sub-consultants employed by Byrne Looby Partners on the Bandon Flood Relief Scheme include:
- CIT – sediment monitoring and analysis;
- Fishway Engineering – fish pass design;
- JBA Consulting – hydrology and hydraulics;
- Lane Purcell Archaeology – archaeological services;
- Survey Tech – topographical surveys
The project is funded by the OPW and the consultants have been managed by an in-house OPW project management team. There are five distinct stages in the project (listed below), with the project currently at tender stage.
- Stage I: Feasibility Study and Preparation of a Flood Risk-Management Plan;
- Stage II: Public Exhibition;
- Stage III: Detailed Design, Confirmation and Tender;
- Stage IV: Construction;
- Stage V: Handover of Works.
A key part of the scheme development was compiling a flood risk-management plan. Kieran Thornton, project director with Byrne Looby, explained: “Flooding can come from a number of sources and the flood risk-management plan considers the effects of flooding from rivers [fluvial flooding] and other sources of flooding such as pluvial [surface water], tidal and groundwater. It’s very important to identify the flood hazards correctly at the early stages of the project, as all of the subsequent analysis, modelling and ultimately the scheme design will stem from this.”
Fluvial flooding arises due to the river channel being unable to convey the flow of water in the river, which receives rainfall run-off from the surrounding catchment. The flow is influenced by a number of factors including rainfall intensity, catchment area, soil saturation index, response, season and land use such as urbanisation, among others.
When a flood event occurs, river flood extends into the natural floodplain to pass the flood at a particular point. The duration of a flood also depends on catchment size, rainfall intensity and duration and season. In general, runoff from sustained rainfall events tends to result in longer duration flood events and runoff from intense thunderstorms results in short duration flash floods. Smaller catchments are more prone to flash flooding, as evidenced by the flood events in Co Cork in June 2012. In Bandon, it was observed that the main flood hazards were fluvial and pluvial flooding.
FLOOD RISK MANAGEMENT PLAN
The Flood Risk Management Plan (FRMP) included the collection of survey data and the assembly and analysis of meteorological, hydrological and tidal data. The data was used to develop a suite of hydraulic computer models of the River Bandon from approximately 4.5km upstream of Bandon weir to Kinsale Harbour. The model was used for the development of a number of flood maps.
Thornton observed that “flood maps are one of the main outputs of the flood risk-management study and are a very effective method of communicating the model results to the end user quickly and easily”.
The key types of mapping developed for the Bandon scheme included:
- Flood extent maps – which show the extents of flood inundation, an indicator of degree of confidence associated with the flood extent and tables of peak flood flows for all modelled design flood-event probabilities;
- Flood zone maps – which show only three flood extents – zones A, B and C – representing high-, moderate- and low-risk areas respectively, in accordance with the Guidelines on the Planning System and Flood Risk Management;
- Flood depth maps – which show the modelled flood depth for a given flood event;
- Flood velocity maps – which show the velocities of floodplain flow;
- Flood hazard function maps – which show flood hazard (the ‘risk to life’) calculated as a function of flood depth, velocity and debris.
The flood maps allow identification of likely locations and receptors in the area at risk of flooding. Receptors are defined as people, places, objects or activities that would suffer harm or damage in the event of flood inundation. The impacts of flooding have been considered and mapped under four receptor categories: social, environmental, cultural heritage and economic.
The study identified a range of potential flood risk-management options to manage the flood risk, including structural options (e.g. flood walls, embankments, improved conveyance and stormwater drainage improvements) and non-structural options (e.g. flood forecasting and planning development control).
To appraise these options, a three-step optioneering process was used to assess the feasibility of the flood risk-management options against flood risk-management objectives, which were predefined by the OPW. The three steps comprised: I) screening; II) development of possible options; III) selection of preferred option using a multi-criteria analysis matrix which considered technical, social, environmental and economic criteria.
COMPREHENSIVE DAMAGES ASSESSMENT
An important consideration was the potential for the proposed options to be adapted in the future. A comprehensive damages assessment was also undertaken to determine the economic damages to property and infrastructure in the Bandon and the economic viability of a flood relief scheme. The final selection of measures to be adopted as part of the FRMP are:
- River channel dredging to remove 155,000m3 of material over a 3.6km stretch of river;
- Flood defences comprising 1,500 metres of walls and 1,200m of embankments;
- New pedestrian bridge;
- New fish passes;
- Drainage systems;
- Flood forecasting system;
- Land-use management plan
- Planning and control measures;
- Building controls on flood proofing of buildings;
- Public awareness and preparedness campaigns;
- Individual flood protection of buildings outside the benefiting area.
While the evaluation was quite systematic, public opinion was also important on the scheme. “The approach to selecting a final flood risk-management plan is quite analytical in terms of economics and risk evaluation. However, an important aspect is including the views of the public and the actual stakeholders. We carried out a number of public consultancy days during 2011 to collect feedback from local people who the scheme would ultimately impact,” explained Thornton.
The proposed scheme that emerged includes of a combination of river dredging and flood-defence walls and embankments. The flood defences includes new defences, upgrading of existing and improvement works to repair some of the infrastructure in place. In addition to dredging and flood defences, other minor works being undertaken. These would include local drainage with pumping stations to protect low-lying properties, drainage and channel improvement works. Non-structural measures currently in place include a flood early-warning system and sustainable urban-drainage systems.
The scheme is currently at procurement stage as part of a €16 million investment by the OPW and will ultimately lead to protection of 177 residential and 215 commercial properties in Bandon.
In light of the recent flooding, there are many challenges for government in deciding what areas should receive assistance. “One of the big challenge of a rural flooding project is justifying the cost benefit analysis,” said Thornton. “The scale of flooding at Bandon was such that the savings justify the expenditure involved; however, there are other areas which are affected by flooding where this may not be the case.
“A Government grant for retro-fitting houses in areas affected by flooding could be a very effective option for minimising flood damage in these cases. In some areas, modifications can be made to gates, doors and drains to prevent water entering buildings, while raising electrical sockets and using flood resistant building materials, finishes and furniture can reduce damage if water does enter a building.”
Kieran Thornton is technical director in Byrne Looby’s Cork office. He delivered a presentation on the Bandon Flood Relief Scheme to Engineers Ireland’s Cork Region on 18 March and to the Flood Risk Management Conference at Clyde Road on 12 February 2014, as part of Engineer’s Week. Byrne Looby employs 100 staff with Irish offices in Cork, Dublin and Galway. The company also trades in the UK and the Middle East.http://www.engineersjournal.ie/2014/04/10/flood-relief-scheme-to-protect-bandon-from-future-disaster/http://www.engineersjournal.ie/wp-content/uploads/2014/04/River-channel-in-town-1024x683.jpeghttp://www.engineersjournal.ie/wp-content/uploads/2014/04/River-channel-in-town-300x300.jpegCivilconstruction,Cork,flooding,OPW