Tidal bar­ri­ers would cost over €1bn, says OPW

Irish Examiner - - News - Eoin English

The Of­fice of Pub­lic Works (OPW) has again ruled out the con­struc­tion of a down­stream tidal bar­rier to pro­tect Cork City from flood risk, say­ing that walls will be re­quired.

The de­fin­i­tive state­ment came as the OPW and Arup en­gi­neers, who are over­see­ing the €140m Cork City flood re­lief scheme, briefed city coun­cil­lors on the sta­tus of the project be­hind closed doors on Mon­day.

In doc­u­ments seen by the Ir­ish Ex­am­iner, they said they have con­ducted a de­tailed con­sid­er­a­tion of two tidal bar­rier al­ter­na­tives — one at Lit­tle Is­land and the other, a dou­ble-bar­rier op­tion ei­ther side of Great Is­land.

Both op­tions were considered un­der sev­eral head­ings, in­clud­ing nav­i­ga­tional im­pact and safety, en­vi­ron­men­tal im­pact on des­ig­nated sites, re­silience, re­li­a­bil­ity and risks, cli­mate change adapt­abil­ity, and cost.

The en­gi­neers said a tidal bar­rier at Lit­tle Is­land would in­crease peak ve­loc­ity flows by a fac­tor of three re­sult­ing in po­ten­tially un­safe con­di­tions, with these in­creased flows also im­pact­ing neg­a­tively on nearby en­vi­ron­men­tally sen­si­tive ar­eas.

They said it would re­quire the rais­ing of a 500m stretch of the N25 and the con­struc­tion of em­bank­ments along the Cork to Mi­dle­ton rail­way, would be ex­tremely un­likely to be ac­cept­able to the Port of Cork and other har­bour users, and would in­crease the po­ten­tial for on­go­ing dredg­ing.

This op­tion could cost at least €656m to build, with the price tag ris­ing to just over €1bn to build, main­tain, and de­liver all the ad­di­tional flu­vial flood de­fences.

They said while the Great Is­land tidal bar­rier op­tion did not have the same en­vi­ron­men­tal or safety im­pacts, it would cost €1.2bn to build with the price tag soar­ing to €1.86bn with the main­te­nance and an­cil­lary costs added in.

Nei­ther op­tion is fi­nan­cially vi­able and walls would still be needed to pro­tect the city from flu­vial flood­ing, they in­sisted.

They told coun­cil­lors that they have also ex­am­ined sev­eral up­stream stor­age al­ter­na­tives pro­posed by Save Cork City, the cam­paign group lead­ing the op­po­si­tion to the OPW scheme’s re­liance on raised quay walls, and which has ar­gued that a tidal bar­rier could be built at Lit­tle Is­land for just €140m.

The OPW and Arups said that com­puter mod­el­ling showed that stor­ing flood waters up­stream will not re­move the need for di­rect de­fences, and could de­lay peak flows on the Shour­nagh river, which would ac­tu­ally in­crease flood risk in the city.

They said sub­mis­sions which sug­gest that the ex­ist­ing dams at In­nis­carra have suf­fi­cient stor­age to avoid de­fences en­tirely are flawed and are premised on a num­ber of in­cor­rect as­sump­tions, based on a pro­posed dam op­er­at­ing regime which would ac­tu­ally jeop­ar­dise dam safety by sig­nif­i­cantly in­creas­ing the risk of over­top­ping of the dams.

They said tech­ni­cally vi­able op­tions to raise the dams would all im­pact on the pro­tected Gear­agh re­gion, would re­quire the dis­place­ment of 80 prop­er­ties, the rais­ing or relocation of a min­i­mum of 8km of na­tional or re­gional roads, would ster­ilise at least 5 sq km of land, and would not tackle tidal flood­ing risk in the city.

The en­gi­neers also con­firmed that they are ex­am­in­ing ground­wa­ter con­cerns in the city, raised by Save Cork City and its ex­perts.

“This is­sue is recog­nised and well un­der­stood and is be­ing as­sessed in de­tail. Ground­wa­ter mon­i­tor­ing, pump test­ing and seep­age anal­y­sis is on­go­ing,” they said.

“Pre­lim­i­nary as­sess­ment to date con­firms that ground­wa­ter flood risk will be man­age­able and that the scheme will not sig­nif­i­cantly im­pact the nor­mal ground­wa­ter regime.”

The brief­ing came as the OPW con­firmed its in­ten­tion to seek planning per­mis­sion for the first phase of the over­all flood-de­fence scheme — the €6m Mor­ri­son’s Is­land pub­lic realm project — by the end of the month.

An artist’s im­pres­sion of what the OPW’s draft plans for the Mor­ri­son’s Is­land pub­lic realm project could look like.

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