River Mall (Templemore) Flood Relief Scheme
- Aerial view of Templemore
The town of Templemore, Co. Tipperary, lies on the River Mall, which drains a catchment area of 22km2 that includes parts of the Kilduff and Devilsbit Mountains, before flowing into the River Suir two kilometres further downstream. The town has a long history of flooding, with a number of events having been reported over the last 150 years. The most significant of these occurred in December 1968 and, more recently, the November 2000 flood inundated approximately 40 properties.
The Office of Public Works (OPW) was requested to address the flooding problem and, in response to this, the flood relief section’s Design team undertook a pre-feasibility study that involved a preliminary investigation and an assessment as to whether a flood relief scheme for the town was likely to be technically achievable and economically viable. The study found that such a scheme was likely, and so a full feasibility study and outline design of alleviation options was then undertaken and those findings brought to Public Exhibition.
Since then, an improved understanding of the flow regime (in particular, during both drought and flood conditions) has driven design changes. This study has shown that the properties upstream of Templemore belong to a separate flooding problem. These are now being treated as such under their own Small Works and the Local Authority will be put in funds to build their remedial works. The flood situation has been subjected to extensive hydrological analysis. This used the ten Hydrometric Years of data from OPW’s Hydrometric Station at Small’s Bridge (16136) to find its Index Flood. This also estimated the flood with just 1% Annual Probability of exceedence (the so-called ‘100-year’ event) as 16.2 m3/s. However, the biggest event that may be reliably estimated from data has a return period equal to twice the length of data; in the case of Station 16136 in Templemore, this is 20 years. As the Mall catchment is small, other Index Flood methodologies based on catchment characteristics are not appropriate; as these were developed based on larger catchments. As such, a Factorial Standard Error (FSE), at the customary 68% confidence limit (Cl), is being applied in order to give additional confidence in the estimation of the 100-year design flood, i.e. this helps to account for the reduced certainty due to having only ten years of Hydrometric Data. The final 100-year design flow estimate is 18.02 m3/s.
The OPW Climate Change policy requires that a scheme is designed in a manner that, at a future date, defence levels or flow capacity could be enhanced at minimum cost and impact, should the expected change occur. In that eventuality, the cost of upgrading culverts is likely to be very significant. As such, it is both reasonable and practical to make allowances for Climate Change in culvert sizing at this time. For Section 50 applications, the OPW national policy is to add 20% to the flow estimate. This has been applied to the 100-year design flow to produce the Climate Change flow estimate of 21.63 m3/s.
The numerical, hydraulic model developed for the town and its upstream and downstream reaches has been calibrated against observed data from the November 2000, October 2004 and October 1997 events. The flooding problem has been evaluated in some detail through this model, and this has confirmed that a number of complex mechanisms exist in addition to direct flooding out from the river, and that inappropriate development and construction may have exacerbated this condition. Their impact is less than what it might otherwise have been due to flow on the floodplain and also along the roads. This realisation that out-of-bank flow is sizeable is key to understanding those complex hydraulic mechanisms. For instance, modelling of the November 2000 event estimates that, about, one quarter of flow upstream of Shortt’s Bridge passed around the reach of the restrictive bridges by flowing down the floodplain and also along the Blackcastle Road, over O’Dwyer’s Bridge and onto the Mall Road. This significantly reduced the height of floodwaters within the town.
Any flood relief option, therefore, that prevents flow from passing through or around the town by upstream defences and containment measures along the river will instantly increase the amount of flow in the river; in extreme events this could be as much as a third. That, naturally, results in what might seem, at first, unrealistically high water levels when compared against the present-day condition where flow passes through the town
The proposals that were exhibited in 2015 comprise the following -
- A 70m-long inlet channel starting in the Mall River just downstream of Abbey Cottage.
- A new outflow from the lake to run under Blackcastle Road to the inlet channel
- A defence line north of the town with Debris and Gravel Traps and a small Drop Weir.
River relocation by constructing a new 750m long channel (with a 7.5m base-width) that begins in Shortt’s field and finishes 230m downstream of Small’s Bridge. As this new channel will operate all the time, its Environmental needs are different to those of a traditional diversion, and in order to aid fish movement, there are no long culverts, unlike the proposals exhibited in 2009.
The scheme has since been confirmed and construction began in June 2017 by OPW direct labour force. To date approximately 100m of new channel has been excavated in the Talavera area, with works currently ongoing in this area.
Further updates will be provided under the “current documents” section on the right hand side of this page.
- Works downstream of Templemore
- Works near Talavera, Templemore
- Shortt's Field, bridge works 1, Templemore
- Shortt's Field, bridge works 2, Templemore