Planning and construction of the Clonmel Flood Defence Scheme

Author: Cyril Mc Carthy, chartered engineer, Office of Public Works During the flooding of February 2014, residents of Clonmel, Co Tipperary, experienced a new and unfamiliar phenomenon. The prolonged and heavy rainfall saw the River Suir rise to flood levels that were four feet above the quays at the lower end of the town. This flood event differed from the all-too-familiar events of the past, in that the river was contained within its channel (see Fig 1). There was a palpable atmosphere of delight and awe as the river ran adjacent to the streets, but was held back by the flood defences completed by the Office of Public Works (OPW) in November 2012. [caption id="attachment_19583" align="aligncenter" width="437"] Fig 1: Clomnel's quays with river contained (courtesy of Mott MacDonald Ireland)[/caption] The flood event certainly provided a 'baptism of fire' for the project, which served to protect hundreds of properties that would otherwise have been inundated with the cold and turbid waters of the Suir. This alleviation of misery and loss serves to remind us why we do what we do and we should be cautiously proud of our achievement in Clonmel. From a civil engineering perspective, however, we shall remain vigilant against complacency as the complexities of such a scheme are appreciable and the delivery of hard infrastructure represents a phase in an ongoing process. This article aims to tell the story of the flood-relief scheme and how the OPW, in partnership with Clonmel Borough Council (now Tipperary County Council) and consulting engineers Mott MacDonald Ireland (MMI), has greatly improved the quality of life for so many citizens. I shall start below by attempting to outline some of the complexities presented by a project that seeks to manage such large-scale natural processes as flooding.

The design standard

The River Suir rises on the eastern slopes of Borrisnoe Hill to the north of the Devil’s Bit. Clonmel is situated about 94km downstream. As such, an area of 2,172 square km contributes rainwater to the river that enters Clonmel. To put the scale into context, some 1cm of rainfall over the whole catchment is equivalent to over 21 million tonnes of water. The rain that falls, however, will vary in its intensity, duration and distribution. Rainwater that does not soak into the ground and is not taken up by plants and evaporation will run off the land and make its way to the intricate network of streams and tributaries that feed the River Suir. In response to this, the river pulsates, constantly rising and falling as it meanders towards the sea. [caption id="attachment_19584" align="aligncenter" width="420"]

Fig 2: Aerial photograph of Clonmel flood of 2000[/caption] The first challenge for the designers of the scheme was to decide upon a standard to be applied to the defences. It is not possible or feasible to eliminate the risk of flooding at a given location. The approach taken is one of managing the risk of flooding. This is done by considering large pulses or flood events in terms of their probability of occurrence. Larger events have a reduced probability, giving people a sense of what normal river levels should be. The OPW Hydrometric Section maintains strategically positioned gauges that constantly monitor river levels. Data from such a station at Gashouse Bridge in Clonmel provided continuous data going back to 1953. This data, coupled with survey information of the river channel, enabled the designers to correlate the amount of water flowing in the river at that location with a probability of such an event occurring on any particular year. Using this statistical model, it is then possible to identify a flow rate corresponding with the required design standard. In this case, the OPW applied the international standard of designing to defend against a flood event with a probability of occurrence of 0.01. This may also be stated as a flood with a return period of 100 years (i.e. one would expect such a flood on average once every century if considered over a very long time) or a flood with a 1% chance of occurrence on any given year. This flood event was quantified as 495 cubic metres (i.e. tonnes) of water per second. Once the design flow was established, the next requirement was to identify the extent and level of such a flood and, in doing so, identify the problem to be addressed. By inputting survey information of the river channel to a computer programme, we were able to simulate a flood event whereby 495 tonnes of water flowed through the virtual channel. This model was calibrated until its output reflected observations on the ground. Fig 3 shows the model’s estimate of the design flood envelope. This may be compared to the photograph showing the real flood event of 2000. Having identified the extent of the problem to be addressed, the next stage in the process was identifying the optimum solution to the problem. [caption id="attachment_19586" align="aligncenter" width="442"]

Fig 3: Flood envelope of design event (courtesy of Mott MacDonald Ireland)[/caption]

Options considered and scheme design

The OPW reviewed several options with the potential to address Clonmel's flood problem. Among these were: a by-pass channel to divert river water around the town, the creation of an upstream storage area, river widening and dredging and a fully defended urban area. Each option was evaluated to a preliminary degree under the headings of environmental, economic and social impacts. If an option was deemed feasible under these headings, it was assessed further using the hydraulic model. The option of providing a fully defended urban area using constructed elements to effectively contain the river within its channel was chosen as the most cost effective and reliable way to manage the flood risk experienced by Clonmel. Having identified the preferred option, i.e. to use hard engineered defences to contain the flood water within the channel, the next challenge was to provide a designed solution that meets the specific requirements of the site. While concrete walls and clay embankments will hold back flood water, one has to consider the underlying geology. Our site investigation revealed the presence of gravels beneath the river and town. The risk that river water would seep through such strata and emerge at the town side of the defences needed to be addressed. This is managed by providing a curtain of steel, where required, using sheet piles that are placed to depths of up to 12m. [caption id="attachment_19587" align="aligncenter" width="213"]

Fig 4: Sheet piling at Stretches Island[/caption] Another challenge presented itself above ground level. The model that was developed estimated design flood levels to be around 2.5m above street level in places. Containing such a depth of water within the channel would require walls of heights up to 2.8m. Such construction would have a detrimental affect upon the town and the amenity value of the river. A solution to this problem was provided by constructing the river walls to a standard height of circa 1.2m and incorporating a removable wall on top of the permanent structure. This demountable element may then be put in place at times of flood risk and removed when not required (see Fig 5). During times of flood, the river will be above street level, so non-return valves are a key component precluding flood water pathways through gullies and other conduits. Storm water is discharged into the river using a series of 12 pumping stations at strategic locations. To further support storm-water management, a new network with gullies has been provided for the low-lying areas of the town identified in the design flood envelope. The drainage system is designed such that it will also manage any unavoidable seepage by collecting it and returning to the river through the pump stations. The pump stations contain submersible pumps removing up to 750l/sec when required. Given that the scheme design is a function of climatic occurrence, climate change is of course a consideration. The scheme has been designed such that it may be augmented at a future stage if required. Fig 5 gives an image of the temporary elements in place on top of the permanent river wall. The river wall, in turn, is built upon the sheet piles that provide an impermeable barrier extending up to 12m below the river bed. [caption id="attachment_19588" align="aligncenter" width="459"]

Fig 5: Quay wall with demountable defences in place (courtesy of Mott MacDonald Ireland)[/caption] Obviously, this solution will only work if the temporary elements are put in place prior to a flood arriving. To this end, a flood warning system was developed. This system includes 30 rain gauges that monitor rainfall throughout the catchment. There are also eight river gauges continually monitoring changes in river level upstream of the town. This information is fed into a mathematical model that allows the computer to estimate predictions of river level. If the estimated level is above a set threshold, then a series of text alerts are sent to the town engineer and his team. Once alerted, the town engineer will mobilise the crew and a flood response protocol is called into action. Of course, during times of flood risk, the team in Clonmel have years of experience responding to floods and will be watching the weather closely – human judgement will always play a central role in decision making. [caption id="attachment_19589" align="alignleft" width="237"]

Fig 6a: River gauges[/caption] [caption id="attachment_19590" align="alignright" width="240"]

Fig 6b: Rain gauges[/caption]

Building the scheme

Having designed the preferred solution, the next step in the process was to deliver it. This was done through a number of phased construction projects. Preliminary works commenced in July 2007. A series of embankments at the upstream (western) end of the town were completed in August 2008 by Tony Kirwan Civil Engineering Contractors Ltd. The next phase of work, Clonmel West, was carried out by SIAC Construction, which was awarded the contract in April 2008. This contract comprised sheet piled walls around Spring Gardens, Green Lane, Oldbridge, an area in Marlfield and at the Convent Road. The height of the existing quay wall was raised upstream of the Old Bridge into Irishtown. This phase of work also included the replacing of a culvert at the Whitening Stream. These works were substantially completed in April 2010 and provided a degree of protection to areas upstream of the Old Bridge. In August of that year, the final phase of work, the Clonmel North & East Scheme, was commenced by Ward & Burke Ltd. These works were substantially completed in November 2012. These works comprised a wall to be constructed along the existing sheet piled quay, running from Old Bridge to Gashouse Bridge. This line of defence continues, as wall and clay embankment, along the left bank as far as Silversprings on the Davis Road. Suir Island is defended by sheet piled wall and embankments where room allows. This phase of work also saw improvement work to the Auk Stream and isolated defences being placed about dwellings and a hotel downstream of Clonmel along the right bank of the River Suir. This phase of work includes improvement works to the medieval Old Bridge and the adjacent Skew Bridge. Two bridges to the south of this have been replaced with more hydraulically efficient structures. To give an idea of scale, the main elements of the project are tabulated below.

Defence Type	Length
RC Wall	3,800m
Impermeable Embankments	3,500m
Sheet Piles(Average depth 9m)	2,600m
Demountable Defences	950m
Bridges Replaced	2
Bridges Upgraded	3
Storm Water Sewers	5,900m
Main Pump Stations	12

Table 1: Overview of design elements

Looking to the future

The scheme, as it has been developed, is a dynamic one and the ongoing partnership between the OPW and the Council will remain central to its effective management. Given the scale and complexity of the problem being addressed by the scheme, continual learning will also be a key component of the project throughout its operational life. Each flood event that occurs will teach us something about our modelled predictions and our operational protocols. What is clear from the flood event of February 2014 is that Clonmel has entered a new chapter in its ongoing relationship with the River Suir. Our records show that the quays in the town flooded on average six out of every ten years prior to the construction of the scheme. This is no longer the case and the challenge for the future will be keeping alive an appreciation of the residual risk that now remains.