Collapsed Bridges: When Will They Be Restored?

Collapsed Bridges: When Will They Be Restored?
United Daily News editorial (Taipei, Taiwan, ROC)
A Translation
August 17, 2009

Summary: Typhoon Morakot brought with it a record-breaking 3000 mm of rainfall. But even more astonishing was the number of collapsed bridges. Fourteen provincial highway bridges collapsed. Add to that county and township bridges, railway bridges and yet to be confirmed local bridges, and the total number of collapsed bridges rises to over 70. This exceeds the previous record for collapsed bridges during a single typhoon by a factor of ten. If in the face of "extreme weather" one fails to fundamentally rethink bridge design, then this embarrassing record for collapsed bridges could well reach new highs.

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Typhoon Morakot brought with it a record-breaking 3000 mm of rainfall. But even more astonishing was the number of collapsed bridges. Fourteen provincial highway bridges collapsed. Add to that county and township bridges, railway bridges and yet to be confirmed local bridges, and the total number of collapsed bridges rises to over 70. This exceeds the previous record for collapsed bridges during a single typhoon by a factor of ten. If in the face of "extreme weather" one fails to fundamentally rethink bridge design, then this embarrassing record for collapsed bridges could well reach new highs.

"Bridges are the interface between man-made structures and rivers." This is Bridge Building 101. Bridge design must combine knowledge from civil engineering, structural engineering, hydrology, soil engineering, geology, and other disciplines. Only then can one ensure safety while aiding transportation. Bridge Building 101 may be clear and understandable. Yet it is sorely lacking on Taiwan. That is the reason for the high incidence of collapsed bridges.

Bridge design has long neglected hydrology. The most bridge builders are willing to do is to consult hydrological data during the design phase, when bridge designers calculate pier sizes, horizontal spans, and the depth and strength of the pile foundations, They design and built bridges to last an average of 50 years. Once the bridge is completed, hydrology and water management issues are ignored. The only factor considered is the water level under the bridge. The hydrology upstream and downstream of the bridge may have long changed. The chance to fix mistakes is squandered. Only when the pile footings are exposed, and the danger is obvious, do they realize they should have considered hydrology. By then of course it is too late.

Everyone knows that rivers and flowing waters constantly change in response to natural and man-made variables. Taiwan's rivers are short and steep. They transport huge volumes of sand, and are highly erosive. Human activities such as water and soil conservation measures and sand mining continuously impact hydrology and water management. They raise riverbeds in the upper reaches. They deepen riverbeds in the lower reaches. Major rivers undergo dramatic course changes. If bridge designers assume that the hydrology will not change over 50 years, then the collapsed bridges cannot be blamed on Typhoon Morak, but on the bridge designer's naivete.

Typhoon Morakot caused the collapse of the Shuangyuan Bridge and Chiwei Bridge near the mouth of the Kaoping Creek, and the Taching Bridge across the Laonung Creek, where it crosses Maoling. None of these bridges collapsed as a result of old age. They collapsed as a result of hydrological changes in the course of the river or the height of the river bed. None of these collapsed bridges were old. The Taching Bridge was renovated only a few years ago. The problem is not the bridge themselves, but external factors such as the hydrology and geology.

Bridge builders have long ignored hydrological changes. In 2000, Typhoon Bilis caused the collapse of the Kaoping Bridge, part of a major traffic artery. The Control Yuan investigated the highway department and water resources department, wondering why the two failed to coordinate with each other. Only then was a mechanism established to "ensure coordination on river and bridge safety." When necessary, it would enable construction firms and water resources experts to communicate. But the engineering and water resources realms have long mistrusted each other. Coordination was initially pro forma, and even included buck-passing. Only recently have communications improved ever so slightly.

Following the current disaster bridge builders urge the construction of cable-stayed bridges and suspension bridges costing several times the new bridge budget. This money should be spent. It is worth it. For one, it will keep up with aesthetic trends. For another, in principle at least, large span bridges reduce the number of piers subject to erosion, hence the attendant risk of bridge collapses. The collapse of bridges during Typhoon Morakot was actually caused by other factors, including hydrology. Yet bridge designers' recommendations fail to even mention them. This shows that although the interdisciplinary mechanism has been in place several years, bridge designers still do not appreciate the importance of hydrology.

Even when bridge owners have been frightened by the sight of exposed bridge piers, they still may not understand or appreciate river hydrology and water control issues. They may consult water resources experts through liaison units. But they don't trust them. They always consider the water resources experts' recommendations too restrictive. This makes it impossible to resolve the bridge building crisis. Bridge owners don't believe that consolidating the river bed and building weirs can prevent the exposure of bridge piles. They fall back on their own piecemeal reconstruction or new construction, in an effort to restore the bridges to safe status.

Water resources experts consider bridge owners short-sighted for believing that protecting bridge piers with cages and increasing the depth of the bridge's foundation piles will ensure safety. As an example, they cite the highly praised cable-stayed Liling Bridge on National Highway Route Three. A few years ago, even though the Liling Bridge had been completed not long ago, the piles were already exposed. Only after water resources experts consolidated the riverbed upstream, did the river sand gradually return. This simultaneously improved the overall safety of the Kaoping River riverbed and solved the problem of the exposed foundation piles.

The reconstruction of collapsed bridges is an important part of post-disaster reconstruction. Since it is a key issue, let's eliminate compartmentalization. Let's work together. After all, the direction we should be headed is the comprehensive management of bridges and rivers.

斷橋遺恨 何日方休
【聯合報╱社論】
2009.08.17 03:36 am

莫拉克颱風創下總雨量近三千毫米的驚人紀錄，但更驚人的紀錄其實是斷橋，僅僅省道公路橋梁就斷了十四座，若再加上縣鄉道橋梁、鐵路橋以及尚未確認的地方斷橋，估計在七十座以上，不僅超過以往單一颱風斷橋紀錄，甚至是十幾倍。面對「極端氣候」加劇，如果對橋梁的興建、維護思維不做根本的調整，不光彩的斷橋紀錄勢必再創新高。

「橋梁是人為構造物與河川界面的整合」。這是橋梁工程的入門第一課，意思是橋梁須結合土木、結構、水文、工程地質等多個領域的知識，才能夠確保安全及發揮輸運功能。入門第一課雖說得清楚易懂，卻是台灣橋梁工程最欠缺、也是斷橋頻仍的關鍵原因。

橋梁工程多年來一直不重視水文水理，頂多在顧問公司設計階段參考河川水文資料，就當時數據計算橋墩、跨距、基樁等的深度與強度，設計興建出平均五十年壽命的橋梁。之後不論是橋梁管理或安全維護，都不再納入水文、水理變化因素，只管橋下的水位變化，罔顧橋梁上下游的水文早就丕變，蹉跎了補救時機，直到驚覺橋墩裸露、險象環生，才想到該去借重水利，為時已晚。

任誰都知道河川、行水沒有一刻不因自然或人為因素而改變，尤其台灣河川短促、高陡降、輸砂多、沖刷強的先天特性，以及後天水土保持、採砂等人為活動，水文、水理沒有一刻不在變化，造成上游河床墊高、下游河道刷深、主河道移位等的巨大改變。如果以為設計當時的水文會五十年不變，那這次的斷橋遺恨就不該記在莫拉克帳上，而是橋梁業主太天真之故。

對照莫拉克造成高屏溪出海口雙園大橋、旗尾橋斷橋，進入茂林咽喉的跨荖濃溪大津橋斷橋等，都不是橋梁老化等自身因素，而是因河道移位或河床墊高帶來的水文、水理變化所致。尤其這些斷橋都不是老舊橋梁，大津橋甚至才改建沒幾年，這證明問題不在橋梁本身，是水文、地質等外在因素造成。

跨河構造物不注重水文變化的現象由來已久，直到民國八十九年碧利絲颱風吹斷運輸動脈高屏大橋，監院追究為什麼公路單位與水利單位互不聯繫後，才建立「維護河川保護橋梁安全聯繫會報」機制，有需要時由工程業主邀集水利等相關單位溝通。不過，在工程與水利界長期存在芥蒂下，初期這個會報基本上徒具形式，實質是各個單位劃清界線，甚至是互推責任的局面；直到最近，溝通雖略好一些，但也有限。

就拿這次災後橋梁界的建議來看，認為即使興建斜張橋、脊索橋等新式橋梁要多花幾倍預算，這個錢也應該花、值得花，不僅美觀跟得上「流行」，且大跨距的設計減少橋墩數，理論上就減少承受水流沖刷的力道，連帶減少斷橋風險。至於莫拉克造成斷橋，其實是橋面下包括水文等非橋梁因素所致，橋梁界的建議卻全未觸及，這反映形式上雖跨界聯繫會報已運作好幾年，但橋梁不重視水文的現象並未改變。

即便業主被嚴重橋墩裸露嚇到，瞭解再不重視河川水文水理會出問題，也透過聯繫會報去徵詢水利單位；但在缺乏信任下，始終認為對方的建議太侷限，不可能解決橋梁的危機，亦不認為固床工、攔河堰那套水利手法能夠改善橋梁基樁裸露，還是要靠自己那一套局部改建或新建，才是換得橋梁安全的正辦。

水利界則認為橋梁業主短視，以為只在橋墩圍些蛇籠、加強基樁深度橋就可保安全，還舉頗獲好評的國道三號里嶺斜張橋為例，前幾年里嶺橋雖才新建完工不久，很快就有基樁裸露現象，還不是靠水利專業的在上游做固床工，讓河砂慢慢回淤，整體改善高屏溪河床安全後，連帶才解決基樁裸露的問題。

斷橋是災後重建的重點工作，既然知道問題關鍵所在，就請彼此破除門戶矜持，攜手合作；畢竟，「橋河併治」才是正確的思考方向。