Comment on the Proposed Cork Tidal Barrier and its impact on Controlling Flooding Risk to Cork City
Updated: Feb 9, 2021
I am a native of Cork City, educated at UCC and then a Fulbright Scholar at the Massachusetts Institute of Technology in the late 1960s, where I later served on the Civil Engineering faculty. I have specialized in Hydrology for the past 50 years, with emphasis on simulation of urban runoff and measures to control flooding. My family has a deep connection to consulting engineering in Cork City (O’Connell & Harley), and I worked as a design engineer on the Cork County Hall during the 1960s, and multiple other civil engineering projects before leaving for the USA. Following my work at MIT, I founded a consulting firm specializing in hydrologic simulation models and have worked in that area during the 45 years since then. Of particular relevance to the problems facing Cork as it addresses how best to deal with the evolving flooding challenge of Climate Change, I worked since 1981 as a consultant in Singapore assisting that government plan and implement modern drainage systems as the city grew rapidly, living standards improved, and the urban runoff had to be managed to reduce the excessive frequent flooding of the downtown core city areas. As part of that work I led the development of initial studies for what ultimately was implemented as the Marina Barrage on the main river channel to the sea, just downstream of the old dock areas. Subsequent to the initial concept design the project evolved as its potential implementation offered the urban planners the opportunity to change the entire use of the waterfront/old quay area in the city, and to significantly enhance the economic redevelopment of the old city. The project also was adapted so it was integrated into the urban landscape of the ‘modern’ Singapore, and is now open for public use as enjoyment as part of the park ‘network’ around Marina Bay. More than 1 million people a year visit the facility. I served as Project Director, responsible for the entire project, during the construction phase and its commissioning in 2008, so I have had the professional experience of guiding the project from its original concept through final commissioning. Like Cork, Singapore is subject to both increases in river/canal flows – a consequence of urban development and densification – with overtopping of old canal banks, and backwater flooding during the monthly Spring high tides. While the 10,000 Ha drainage area in Singapore discharging to the Marina Barrage is small compared to the River Lee basin at Cork City, Singapore has an annual rainfall of more than 2,400 mm and with the dense urban development the storm flows in the drainage network discharging to the Marina Barrage can approach 1,000m3/s. That is significantly more than the projected flood flows on the River Lee through the city. The drainage simulation studies in Singapore indicated that while increasing the capacity of the main drainage network ‐ by increasing canal sizes and also by diverting large flows through new tunnels to an adjacent river basin ‐ the frequent flooding of the low lying sections of the old city during high tide conditions, particularly with coincident storms would continue. The Marina Barrage was proposed as a means of limiting the tidal impact on the city waterfront, by closing the main channel before high tide events and protecting the city from direct tidal flooding. Due to the probability of both monsoon storms and high tides, the Barrage was also designed with large drainage pumps which serve to pump excess flows to sea, and maintain the ‘pool’ upstream of the Barrage at a level low enough to limit flooding on the downstream reaches of the Singapore River, the Kallang River, and associated canals. The Marina Barrage later evolved to include full isolation of the channel from the sea, and the upstream reservoir converted to capture urban runoff as a source fresh water for potable supply. Cork does not currently need such water supply capability, but the fundamental function of a tidal barrier at Lough Mahon to limit the risk of flooding along the River Lee through Cork City is very similar to what the Marina Barrage has achieved. Making the decision to implement the Marina Barrage led to many considerations on the potential impact on what was then an active port area of closing access to the sea. Urban planners made the far‐ sighted decision that such commercial activities, valuable as they were even in the late 20th century, would ultimately need more modern and efficient port areas, so they were slowly moved away from Marina Bay. The freed‐up waterfront space has since been developed into very high value commercial and residential uses. The thriving activities around Marina Bay that are now experienced by residents and visitors are all due to the ability of Marina Barrage to control the water levels in the downtown area on a daily basis. The construction cost for the Marina Barrage including the 350m long 9‐gated structure, 7 large flood pumps with a capacity of 40m3/s each, housed in a large aesthetic building, a visitor centre, and other public use areas was about €200million. This illustrates that such a multi‐capability tidal‐barrier scheme can be very cost effective, and in many ways far more operationally beneficial than attempting to modify the fundamental historic drainage network of a low lying coastal city, and try to make it continue to operate while the water levels in the main river channels continue to increase within walled banks. I have reviewed the several reports of the Lower Lee relief scheme, and offer the following comments based on my long professional career in Urban Drainage and specifically my experience on the Marina Barrage. 1. It is unlikely that raising the quay walls alone along the branches of the River Lee will be an effective long term solution. The climate regime continues to evolve with increasing fluvial flows, and the city itself needs to develop the old docks, Marina and Tivoli stretches to enhance the urban city. 2. During my 20+ years in Cork I experienced flooding in the downtown area (including Patrick Street) several times, and I know how interconnected the branches of the Lee are with various major drainage pathways under many of the streets. Trying to limit flooding of the city by ‘forcing’ all flows into restricted flow paths on the North and South branches of the Lee, through higher quay wails, will lead to higher level in the main rivers and back flooding through the multiplicity of drains that connect to the river banks throughout the city. It is infeasible to provide effective pumping from all these outlets, so the city core will still be subject to back flooding during major storm events. 3. My experience suggests that the present single focus on raising the quay walls will not solve the persistent flooding risks to Cork City by itself. 4. I also note that slight modification to the operation of the Lee dams before and during major storm events can help reduce flows reaching the city centre. Their presence and potential benefits cannot be ignored. 5. However, no matter how much the upstream flows can be moderated, the downtown city will still be subject to backwater flooding under storm conditions that drive flows upstream from Lough Mahon. This risk will continually increase as sea level rises during the next century. 6. The city is fortunate that it has Lough Mahon downstream, since controlling tidal levels there can provide enhanced fluvial capacity through the city itself. Initial simulations suggest that controlling water levels in Lough Mahon has an influence upstream as far as the Mardyke. So, essentially the entire Lee channels through the city can be regulated if the level in Lough Mahon can be stabilized. 7. I have briefly reviewed the “Wallingford” scheme for a tidal barrier and strongly recommend it be pursued as an option that needed to be more fully investigated before Cork makes any final decision. But I also suggest that the scheme be considered for modification as noted in the flowing section. 8. I do note that the presently proposed tidal barrier does not include any flood discharge pumps built into the dikes across Lough Mahon, and the operation of the scheme would solely depend on keeping the tidal barrier closed during high tide events. The fluvial flows through the city will then be stored in the channel upstream of the tide barrier until the gates are reopened; this can result in the Lee channels filling as the water is stored. The scheme does include some smaller release gates in addition to the main channel barrier gate, and these auxiliary gates can be operated to discharge some flows as possible even while the main barrier is still closed. As we developed the Marina Barrage we understood that there can be long duration storms that overlap several tide cycles and it is not feasible to store all the excess water upstream of the Barrage without causing substantial urban flooding. The Marina Barrage thus incorporates a number of high capacity ‘flood pumps’ that can be operated as needed to pump water out of the basin during a storm event, or often to lower the upstream pool level before a storm occurs. Such high capacity, but low head, flood pumps are very efficient, and can readily be incorporated, now or in the future, into the tidal barrier system. Including such pumping capability will provide much flexibility to the regulation of flood levels up through the City in the future. 9. The implementation of an effective tidal barrier, and an operating policy that provides flexibility in limiting water levels upstream from the tidal barrier will also provide protection to critical transportation facilities, such as the Jack Lynch Tunnel, and the major highways and railways that also run along the bank of the Lee in the dock and Tivoli area. These facilities will need protection in the future. The ‘Walls’ scheme does absolutely nothing to provide such protection. 10. Finally, I suggest that the Cork City leaders might like to visit Singapore, view their drainage facilities, and be briefed by their urban planners on how they incorporated the then‐future benefits of the Marina Barrage into their urban planning. I would be pleased to provide the contact information and introduction for such a visit.
I trust that this additional information may be helpful as the City considers how to move forward. I recognize that some immediate flood protection works will still be necessary, but it would be very worthwhile for the City planners and engineers to consider what the long range solution should be. Mitigating and managing the impact of the ongoing and significant Climate change is a challenge for many coastal cities. I have had the unique opportunity to work on, and guide, a globally recognized project such as Marina Barrage, and strongly suggest that Cork consider expanding its set of alternatives before they commit to a very disruptive but limited long term scheme such as the proposed ‘Walls’ scheme.
I am providing this comment as a private individual with a strong interest in my ‘home’ city, and not as a representative of any firm or organization.
Dr. Brendan M. Harley, B.E, M.Eng.SC, ScD, P.H., FIEI Boston, Massachusetts, USA
9 May 2018