利用者:青子守歌/ワークスペース2

セント・フランシスダム
所在地	アメリカ合衆国カリフォルニア州ロサンゼルス郡サンタクラリタソーガス
位置	北緯34度32分50秒 西経118度30分45秒
河川	サン・フランシスキート川（英語: San Francisquito Creek）
ダム諸元
ダム型式	重力式コンクリートダム
堤高	56 m
堤頂長	210 m
堤体積	99.733 m³
流域面積	97 km²
総貯水容量	45,080,000 m³
利用目的	貯水池
事業主体	ロサンゼルス市水道電力局（英語: Los Angeles Department of Water and Power）
着手年/竣工年	1924年/1926年
備考
カリフォルニア州指定歴史建造物
登録名	St. Francis Dam Disaster Site
登録コード	919

セント・フランシスダム（英語: St. Francis Dam）は、ロサンゼルスにかつて存在した重力式コンクリートダムである。同地域への水源としての貯水池であり、ロサンゼルス上水路の一部であった。

シエラ・ペロナ山脈（英語: Sierra Pelona Mountains）のサン・フランシスキート峡谷（英語: San Francisquito Canyon）に位置しており、ロサンゼルスの下町から北西におよそ64km、現在で言えばサンタクラリタ市内から北におよそ16kmに位置していた。

1924年から1926年にかけてロサンゼルス市水道電力局（英語: Los Angeles Department of Water and Power）によって建設された。担当部署は水道部（英語: Bureau of Water Works and Supply）であり、当時の部責任者および主任技術者はウイリアム・ムルホランド（英語: William Mulholland）であった。

完成から2年後の1928年3月12日23時57分、このダムは突然崩落し、これにより引き起こされた洪水によって少なくとも431名が死亡した。このセント・フランシスダム崩落は、20世紀のアメリカ土木工学史における最悪の災害のひとつであり、カリフォルニア史においては、1906年のサンフランシスコ地震による死者数についで2番目に多くの死者を出したのものであった。また、この崩落により、ムルホランドは土木技術者を引退することとなった。

計画と設計

セント・フランシスダム以前のロサンゼルスでは、市内への水供給はザンジャス(zanjas)と呼ばれる水路を経て、ロサンゼルス川（英語: Los Angeles River）から得ていた。 この当時は、民間企業であるロサンゼルス市水道会社(Los Angeles City Water Company)が、市の上水道をリースして市内へ水を供給していた。 1878年に、その会社にウイリアム・ムルホランド（英語: William Mulholland）が雇われ、業績が評価された後に1886年には監督者に昇格した。

1902年、ロサンゼルス市は上水道のリースを終了し、ムルホランドを部長として公共水道部（英語: Water Department）を設立した。その後、1911年に水道部は水道局（英語: Bureau of Water Works and Supply）に改組され、ムルホランドは引き続き局長であると同時に主任技術者となった。

1913年、ムルホランドはロサンゼルス上水路を完成させた。これはオーエンズ・バレーからロサンゼルスまで375kmを結ぶものであり、当時では世界最長の上水路であった。この水路は現役である。

ムルホランドは、ロサンゼルス上水路が完成しても、乾季が長引いた場合や上水路が地震で損傷を受けた時のことを考え、ロサンゼルスに水を供給できる十分な大きさの貯水池が必要であると考えていた。 そこで、建設中に、ムルホランドはサン・フランシスキート峡谷（英語: San Francisquito Canyon）の断面がダムに適していることを発見した。具体的には、第1号水力発電所と第2号の間が、幅広い上流から自然に下流に狭くなる地形となっており、小さいダムで大きな貯水池となると考えた。

そこでムルホランドは、その地域に作られていた労働者のための大規模飯場に滞在し、空き時間を使ってその地域の地質学的特徴の調査に乗り出した。そして、ダム建設予定地から西側の丘陵の上部から中央部が、主に赤色の礫岩とその中に散在する石膏を含んだ砂岩で構成されていることを発見した。一方、西丘陵から峡谷の底および東側の崖の地質構成は劇的に変わっており、滑石が偏在する雲母片岩が主体となっており、層の数が多くいたるところで断面破壊を確認した。後世ではこの境界面は、正確な場所は間違っているものの、サン・フランシスキート不活断層のことであると考えられている。 そして、実際に赤色の礫岩丘陵に試掘坑を掘りその特性を調査し、また透水性試験を実施した結果、この西側丘陵はダムの取付部（アバットメント）に十分であると結論づけた。

一方、峡谷東側に存在する片岩の危険な性質については、1911年にムルホランドは公共事業委員会(Board of Public Works)への年次報告書で報告したのだが、後のダム建設現場監督であるスタンリー・ダンハム（英語: Stanley Dunham）にはその内容について誤解釈および無視されてしまう。ダンハムが自身が発注した試験では、東側丘陵もダム取付部（アバットメント）に十分なほど固く全体的に同じ性質であり、よって、この場所はダム建設に適していると結論づけた。

この間に、ロサンゼルスの人口は急速に増加しており、1900年には10万人を越え、1910年には3倍以上の32万人、1920年には58万人となった。この急速な人口拡大に伴って、1920年から1926年にかけて、新しく7つの小さな貯水池を建設したほか、当時最大の貯水池であった下サンフェルナンドダム（英語: Lower San Fernando Dam）では堤高を2m嵩上げしたのだが、水需要の大幅な増加に対応するため更に大きな貯水池が必要であることは明白であった。

この大型貯水池の計画予定地、元々は、サンフェルナンド渓谷の北東部で現在のサンランド（英語: Sunland）の北に位置するビッグ・タハンガ渓谷（英語: Big Tujunga Creek）であった。しかし、その場所にあった牧場と私有地が高額であったため（ムルホランドの私見では不当に釣り上げられたものだった）、ムルホランドは、私有地の価格がもう少し安いサン・フランシスキート峡谷（英語: San Francisquito Canyon）に予定地を変更した。しかし先述のようにその場所は、自身が12年前に危険であると報告していた場所であり、そのことを忘れていたか無視したと考えられる。

建設工事と改変

ダムによって作り出される貯水池の推定地図

ダムの正確な建設予定地を決めるための調査が開始されたのは1922年12月であった。建設予定地の土地精算も開始されたが、ロサンゼルス上水路がオーエンズ・バレーの農家と土地所有者からの執拗な妨害を受けていたこともあり、ロサンゼルス市は今回はそのような価格の釣り上げや時間の浪費を避けたかった。

セント・フランシス（サン・フランシスキートと呼ばれることもある）ダムは、市水道局によって2番目に設計・建設されたコンクリートダムであったが、最初のダムはムルホランドダム（英語: Mulholland Dam）であり、こちらの建設は1年先に開始されていた。セント・フランシスダムは、ムルホランドダムを地形に合うように変更されただけでほぼ同じものとして設計された。実際に、適用された設計方法や応力計算式はムルホランドダムの建設と同じものであり、これらは水道局の技術部によって設計された。。

セント・フランシスダムの形状は湾曲しているが、この湾曲は弧形であるため、今日ではこれはアーチダムであり、つまり重力式アーチダムと呼ばれる形式であった。しかしこの当時は重力式アーチダムに関する技術は未発達で、アーチによってどのように荷重が伝達され、安定性や基盤支持を与えるのかといった効果に関する技術的知見が不足していた。そのため設計にはアーチによってもたらされる恩恵が含まれておらず、その大きさに対して保守的な設計となった。。

水道局が毎年公共サービス委員会（英語: Board of Public Service Commissioners）に提出している年次財政報告書によると、水道局は、1923年6月には予定地となるセント・フランシスダムおよびダム湖の予備地形調査を完了させていた。これによれば、ダム水位は標高556m、河床から53mの高さに位置し、初期計算では貯水量はおよそ37,000,000m³と推定された。そして1924年7月1日、水道局の技術者ハールバット（英語: W. W. Hurlbut）は、ムルホランドに予備調査の完了を報告し、彼が委員会に提出した年次報告書には次のように書かれており、貯水量が39,000,000m³に訂正された。

...at the St. Francis Reservoir the dam site has been cleared and the foundation trench started. All concrete placing equipment has been contracted for and it is expected actual work of pouring the concrete will start in approximately ninety days. Additional topographic surveys have been completed and disclose a storage capacity of 32,000 acre feet at elevation 1825 feet above sea level.

（訳）…セント・フランシスダムおよびダム湖の建設予定地は全て精算され、基礎掘削がすでに始まっています。全てのコンクリート打設工事は契約済みで、実際の工事は90日以内には開始される予定です。追加の地形調査も完了しており、貯水容量は32,000エーカーで水位は海面から1825フィートとなります。

ダム自体の建設はその5週間後の8月上旬に開始され、コンクリート打設が始まった。1925年3月、ハールバットはムルホランドにダムの進捗報告の中で、貯水容量が47,000,000 m³になり水位は河床から56mになることを報告した。この3mの拡張により拡大した貯水池を収容するために、西側のダム取付部に約179m長のウイングダム（英語: Wing dam）が追加で建設されることとなった。

セント・フランシスダムの特徴は、階段状の下流側側面である。これは、各段の高さは1.5m刻みで、幅は各段が地形に合わせて1.7m（河床付近の標高500m）から、0.44m（洪水吐入口付近の標高556m）まで地形に合わせて変化するものであった。

ダムの完成は1926年5月4日であった。高さ46cm、幅6.1mの開口部を持つ11個の洪水吐と、上流面でスライドゲートによって制御された直径76cmの放水管路が中央部に設置された。

営業中の諸問題

湛水が始まったのは1926年3月12日であったが、いくつかも問題が発生した。まず、水位が上がるにつれ、温度低下によるコンクリートの収縮亀裂が発生し、取付部付近で少量の漏水が発生した。技術部がムルホランドダム建設時に策定した設計手法では、収縮目地が設置されていなかったからである。さらに、ダムを突き抜けるように大きな垂直亀裂が、流出ゲートから18m西と同距離東に2つ発生した。ただし、それらの亀裂と漏水箇所については、ムルホランドと主任技師補佐のハーヴィー・ヴァン・ノーマン（英語: Harvey Van Norman）が調査したところ、セント・フランシスダムの規模のダムであれば想定通りであると結論づけた。

4月の始め、水位は西側取付部のサン・フランシスキート不活断層の領域に到達し、すぐにこの部分からの漏水が発生しはじめた。作業者たちがこの漏水を止めようとしたが、完全に封じ込めるに至らず、ダム表面を漏水が伝い続けることになった。この漏水を集めるため5センチの管水路を、断層線からダム管理人のトニー・ハーニッシュフェガー（英語: Tony Harnischfeger）の家まで設置しすることで、この排水管によってダムの静水圧の上昇が少し緩和された。1927年4月には水位は洪水吐から3mのところに到達し、5月中は水位が概ね1m以内の範囲にあった。この間も漏水量は増減せず、毎月およそ管路の1/3程度の流量であった。漏水量は1926年から1927年の間記録され、この量はセント・フランシスダムの規模では十分に小さい量であり、ムルホランドは「自分が見たことのあるダムの中で、この規模では最も乾いたダムであった」と述べている。

一方、5月27日、オーエンズ・バレーの問題が激化し、ダイナマイトによりロサンゼルス上水道が大きく損傷した（これは後にカリフォルニア水戦争と呼ばれる騒動の一部である）。数日後には別の場所で大規模な破壊があり、その後さらにいくつかの場所がダイナマイト爆破されたため、ロサンゼルス上水道からの水供給は完全に絶えてしまった。この騒動により、北方近隣で活きている貯水池はセント・フランシス湖が唯一となったため、さっそくダムからの水供給が始まった。この時、ロサンゼルス保安官事務所に、インヨー郡から暴徒がセント・フランシスダムを破壊するために向かっており「早く保安官たちを戻したほうが良い」と匿名の電話があった。この情報は、すぐに水道局および現場人員に通知され、警察官および保安官達がかけつけた。結局、実際にこのような暴徒の兆しはなかったのだが、この警戒中の間、峡谷は武装キャンプのようであったという。

5月27日から6月30日の高水位日報によると、この期間でおよそ8,500,000m³から10,000,00m³程度が放流された。7月以降もオーエンズ・バレーの闘争は継続していており上水道からの供給が途絶えていたため、引き続きダムから水が供給されることとなった。そして8月上旬、反対運動の指導者の横領が発覚すると、ようやく沈静化し、市はすぐに上水道の修理保守作業を開始した.。

騒動の間にも湛水は続き、この年の後半には、新たに西取付部付近から中央部へ向かう亀裂が報告された。ムルホランドが調査したところ、これが新たな亀裂であることが確認されたため、槙皮（英語: Oakum）で隙間を埋めて止水した。同時期に、ダム東側頂上近くの洪水吐から山の中腹にかけて約20mの長さの別の亀裂が発見され、これも同様に止水された。これらの亀裂の特徴として、山肌に近いところは幅が広く、上に行くにつれて狭くなるものが共通であった。1928年2月には洪水吐まであと30cmというところまで水位は上昇した。この間も新たな亀裂が堤体の他の部分やウイングダム（英語: Wing dam）も発生し、取付部からは漏水が続いた。

2月も終わりに近くなった頃、主ダムから約46m西地点のウイングダム基部に問題となる亀裂が見つかった。漏水量は約17,000cm³/sで、ムルホランドはすぐに調査した。その結果、この亀裂は別の収縮または温度による切れるであると判断し、漏水させたまま放置した。 3月第1週の間に、漏水量はほぼ倍増していた。加えてその地点では侵食が発生したことも重なり、ムルホランドは直径20.3cmの排水管を設置した。管は堤体表面に沿って設置され、主ダムの取付部から排水した。 そのせいで、丘の中腹は水浸しになってしまい、さらにダム下流部の階段状斜面を水が流れ落ちる様子は、ダムから東に210m先にある道路を通行する峡谷の住民らを不安にさせた。 1928年3月7日には、貯水位が洪水吐の頭頂部から7.6cm下まで上昇し、ムルホランドはこれ以上ダムに水を貯めないように指示した。

3月13日の朝、ダム管理人が通常点検時に、西側取付部に新たな亀裂を発見した。同じ場所で過去に他にも漏水が発生していたことや、加えて今回は漏水の色が濁っていたことから、管理人はこれがダム基礎の侵食の兆候ではないかと考えた。 そこですぐにムルホランドに報告し、ムルホランドとヴァン・ノーマンが付近の漏水調査を開始した。 まもなくヴァン・ノーマンが流出元を見つけ流出先までを追跡した結果、水の濁りの原因はその漏水自体ではなく、先立って建設された新しい接続道路の軟弱地盤から発生していることを突き止めた。 そして、その漏水量は5,700から8,500cm³/s程度であると推定した。 これについては、確かに、漏水箇所に加えて漏水量自体が不安定であることで危険性の懸念は高まっていたと、後に事故調査官に語っている。 実際に、両者ともが観察中に2回の水量の増減に気づいており、ムルホランドは何かしらの修正が必要だとは感じた一方で、それは将来のいつかで良いと考えた。

その後、ムルホランドとヴァン・ノーマンに加えてハーニッシュフェガーの3人でダムの他の漏水や染み出しを調査したが、ダム本体においては異常な点を発見できず、懸念も特に見つけられなかった。 よって最終的に、ムルホランドとヴァン・ノーマンは、この新しい漏水は危険なものではなくダムは安全であると結論づけ、ロサンゼルスへ帰ったのであった。

Collapse and flood wave

Two and a half minutes before midnight on March 12, 1928, the St. Francis Dam catastrophically failed.

There were no surviving eyewitnesses to the collapse, but at least five people passed the dam within the hour prior without noticing anything unusual. The last, Ace Hopewell, a carpenter at Powerhouse No. 1, rode his motorcycle past the dam about ten minutes before midnight. He testified at the Coroner's Inquest that he had passed Powerhouse No. 2 without seeing anything there or at the dam that caused him concern. He went on to state that at approximately one and one-half miles (2.4 km) upstream he heard above the roar of his motorcycle a rumbling much like the sound of "rocks rolling on the hill." He stopped and got off, leaving the engine idling, and smoked a cigarette while checking the hillside above him. The rumble that had caught his attention earlier had begun to fade behind him. Assuming that it may have been a landslide, as these were common in the area, and satisfied that he was in no danger, he continued on.

At the Bureau of Power and Light at both Receiving Stations in Los Angeles and the Water Works and Supply at Powerhouse No. 1 there was a sharp voltage drop at 11:57:30 p.m. Simultaneously, a transformer at Southern California Edison's Saugus substation exploded, a situation investigators later determined was caused by wires up the western hillside of San Francisquito canyon about ninety feet above the dam's east abutment shorting.

St. Francis Dam

The same view post-collapse. The west (left) abutment was entirely swept away. The inactive San Francisquito Fault is clearly visible, being located along the contact zone of schist and conglomerate.

Given the known height of the flood wave, and that within seventy minutes or less after the collapse the reservoir was virtually empty, the failure must have been sudden and complete. Seconds after it began, little of what had been the dam remained standing, other than the center section and wing wall. The main dam, from west of the center section to the wing wall abutment atop the hillside, broke into several large pieces, and numerous smaller pieces. All of these were washed downstream as 12.4 billion gallons (47 million m³) of water began surging down San Francisquito Canyon. The largest piece, weighing approximately 10,000 tons (9,000 metric tons) was found about three-quarters of a mile (1.2 km) below the dam site.

In a somewhat similar phenomenon, the dam portion east of the center section had also broken into several larger and smaller pieces. Unlike the western side, most of these ended lying near the base of the standing section. The largest fragments fell across the lower portion of the standing section, coming to rest partially on its upstream face. Initially, the two remaining sections of the dam remained upright. As the reservoir lowered, water undercut the already undermined eastern portion, which twisted and fell backwards toward the eastern hillside, breaking into three sections.

The dam keeper and his family were most likely among the first casualties caught in the initially 140フィート (43 m) high flood wave, which swept over their cottage approximately a quarter of a mile (400 m) downstream from the dam. The body of a woman who lived with the family was found fully clothed and wedged between two blocks of concrete near the base of the dam. This led to the suggestion she and the dam keeper may have been inspecting the structure immediately before its failure. Neither his nor his six-year-old son's bodies were found.

Five minutes after the collapse, the then 120-フート-high (37 m) flood wave had traveled one and one-half miles (2.4 km) at an average speed of 18マイル毎時 (29 km/h), destroying the heavy concrete Powerhouse No. 2 there and taking the lives of 64 of the 67 workmen and their families who lived nearby. This cut power to much of Los Angeles and the San Fernando Valley. It was quickly restored via tie-lines with Southern California Edison Company, but as the floodwater entered the Santa Clara riverbed it overflowed the river's banks, flooding parts of present-day Valencia and Newhall. At about 12:40 a.m. Southern California Edison's two main lines into the city were destroyed by the flooding, re-darkening the areas that had earlier lost power, and spreading the outage to other areas served by Southern California Edison. Nonetheless power to most of the areas not flooded was restored with power from Edison's Long Beach steam electric generating plant.

Near 1:00 a.m. the mass of water, then 55 ft (17 m) high, followed the river bed west and demolished Edison's Saugus substation, cutting power to the entire Santa Clara River Valley and parts of Ventura and Oxnard. At least four miles of the state's main north–south highway was under water and the town of Castaic Junction was being washed away.

The 12 mph (19 km/h) flood entered the Santa Clarita valley. Approximately five miles downstream, near the Ventura–Los Angeles county line, a temporary construction camp the Edison Company had set up for its 150-man crew on the flats of the river bank was hit. In the confusion, Edison personnel had been unable to issue a warning and 84 workers perished.

Shortly before 1:30 a.m., a Santa Clara River Valley telephone operator learned from the Pacific Long Distance Telephone Company that the dam had failed. She called a California Highway Patrol officer who lived nearby, then began ringing the homes of those in danger. The officer and a fellow officer criss-crossed the streets in the danger zone with their sirens sounding. Within an hour the streets were empty, but little could be done for those on ranches and dairies in lowlands to the west of Santa Paula.

The flood heavily damaged the towns of Fillmore, Bardsdale, and Santa Paula, before emptying both victims and debris into the Pacific Ocean 54マイル (87 km) downstream south of Ventura at what is now the West Montalvo Oil Field around 5:30 a.m., at which point the wave was almost two miles (3 km) wide and still traveling at 6 mph (9.7 km/h). Bodies were recovered as far south as the Mexican border; many were never found.

Newspapers across the country carried accounts of the disaster. The front page of the Los Angeles Times ran four stories, including aerial photos of the collapsed dam and the city of Santa Paula. A Times Flood Relief Fund was set up to receive donations, mirrored by similar efforts by other publications. In a statement Mulholland said, "I would not venture at this time to express a positive opinion as to the cause of the St. Francis Dam disaster... Mr. Van Norman and I arrived at the scene of the break around 2:30 a.m. this morning. We saw at once that the dam was completely out and that the torrential flood of water from the reservoir had left an appalling record of death and destruction in the valley below." Mulholland stated that it appeared that there had been major movement in the hills forming the western buttress of the dam, adding that three eminent geologists, Robert T. Hill, C. F. Tolman and D. W. Murphy, had been hired by the Board of Water and Power Commissioners to determine if this was the cause. It was noted that no tremors had been reported at seismograph stations, ruling out an earthquake as the cause of the break.

Investigation

The collapse of the dam prompted the creation of over a dozen separate investigations into the cause of failure. With unprecedented speed, eight of these had begun by the weekend following the collapse. Almost all of these involved investigative panels of prominent engineers and geologists. The more notable of these groups and committees were those sponsored by California governor C. C. Young, headed by A. J. Wiley, the renowned dam engineer and consultant to the U.S. Bureau of Reclamation's Boulder (Hoover) Dam Board; the Los Angeles City Council, which was chaired by the Chief of the Reclamation Service, Elwood Mead; the Los Angeles County coroner, Frank Nance and Los Angeles County District Attorney Asa Keyes. Others were convened: the Water and Power Commissioners started their own inquiry, as did the Los Angeles County Board of Supervisors who hired J. B. Lippincott. The Santa Clara River Protective Association employed the geologist and Stanford University professor emeritus, Dr. Bailey Willis, and eminent San Francisco Civil Engineer and past president of the American Society of Civil Engineers, Carl E. Grunsky. There were others, such as the railroad commission and several political entities who only sent investigators or representatives.

Although they were not unanimous on all points, most commissions quickly reached their respective conclusions. The governor's commission met on March 19 and submitted their 79 page report to the governor on March 24, five days later, and only eleven days after the early-morning March 13 flood. Although this may have been sufficient time to answer what they had been directed to determine, they had been deprived of the sworn testimony at the Coroner's Inquest which was scheduled to be convened March 21, the only inquiry that took into consideration factors other than geology and engineering.

The need for nearly immediate answers was understandable, having its roots in the Swing–Johnson Bill in Congress. This bill, which had first been filed in 1922, and failed to be voted on in three successive Congresses, was again before Congress at the time. This bill would ultimately provide the funding for constructing the Hoover Dam. Supporters and responsible leaders alike realized the jeopardy in which the bill then stood. Although the water and electricity from the project were needed, the idea of the construction of such a massive dam of similar design, which would create a reservoir seven hundred times larger than the St. Francis, did not sit well with many in light of the recent disaster and the devastation. The bill was passed by Congress, and signed into law by President Coolidge on December 21, 1928.

The governor's commission was the first to release its findings, titled Report of the Commission appointed by Governor C. C. Young to investigate the causes leading to the failure of the St. Francis dam near Saugus, California. The report became the most widely distributed analysis. Along with most of the other investigators, they perceived the new leak as the key to understanding the collapse, although the commission believed that "the foundation under the entire dam left very much to be desired." The report stated, "With such a formation, the ultimate failure of this dam was inevitable, unless water could have been kept from reaching the foundation. Inspection galleries, pressure grouting, drainage wells and deep cut-off walls are commonly used to prevent or remove percolation, but it is improbable that any or all of these devices would have been adequately effective, though they would have ameliorated the conditions and postponed the final failure." They placed the cause of the failure on the western hillside. "The west end," the commission stated, "was founded upon a reddish conglomerate which, even when dry, was of decidedly inferior strength and which, when wet, became so soft that most of it lost almost all rock characteristics." The softening of the "reddish conglomerate" undermined the west side. "The rush of water released by failure of the west end caused a heavy scour against the easterly canyon wall ... and caused the failure of that part of the structure." There then "quickly followed ... the collapse of large sections of the dam."

The committee appointed by the Los Angeles City Council, for the most part concurred in attributing the collapse to "defective foundations", and wrote, "The manner of failure was that the first leak, however started, began under the concrete at that part of the dam which stood on the red conglomerate; this leak increased in volume as it scoured away the foundation material already greatly softened by infiltrated water from the reservoir which removed the support of the dam at this point and since no arch action could occur by reason of the yielding conglomerate abutment, made failure of the dam inevitable." Likewise, they concluded the failure most likely followed a pattern similar to that which was proposed by the governor's commission, although they did acknowledge that "the sequence of failure is uncertain."

The committee ended their report with, "...having examined all the evidence which it has been able to obtain to date reports its conclusions as follows:

1. The type and dimensions of the dam were amply sufficient if based on suitable foundation.
2. The concrete of which the dam was built was of ample strength to resist the stresses to which it would normally be subjected.
3. The failure cannot be laid to movement of the earth's crust.
4. The dam failed as a result of defective foundations.
5. This failure reflects in no way the stability of a well designed gravity dam properly founded on suitable bedrock."

Concrete block from the west abutment of the dam about half a mile below the dam site. Approximately 63 ft. long, 30 ft. high and 54 ft. wide. The wing wall is in the distance.

Standing section with fragments from east side of dam

The consensus of most of the investigating commissions was that the initial break took place at or near the fault line, which had been a problem area since water first covered the area, on the western abutment. The prevailing thought was that increasing water percolation through the fault line had either undermined or weakened the foundation to a point that a portion of the structure blew out or the dam collapsed from its own immense weight. This agreement among them was also due, in conjunction, with a chart which was made by the automatic water level recorder located on the dam's center section. This chart clearly showed that there had been no significant change in the reservoir level until forty minutes before the dam's failure when, during that forty minutes, a small though gradually increasing amount of loss was recorded. This controversial item would unfortunately, turn out to be another area they had handicapped themselves without the important information that would later be brought to light during the testimony which would be given at the Coroner's Inquest; the only investigation that took evidence other than engineering and geology into account.

The only theory to vary greatly from the others was that of Bailey Willis, Carl E. Grunsky and his son. They believed that the portion of the east abutment below the dam was the first to give way, clearing the way for the collapse to take place. Their investigations, while somewhat in collaboration, culminated in two reports, one by the Grunskys and the other by Dr. Willis, which were completed in April 1928. These reports, according to Carl Grunsky, "were reached independently" and "are in substantial agreement."

Dr. Willis and the Grunskys agreed with the other engineers and investigators on the poor quality and deteriorating conditions of the entire foundation, although they maintained that a critical situation developed on the east abutment. Dr. Willis, the geologist of the investigative team, was most likely the first to discover the "old landslide" within the mountains which had made the eastern abutment for the dam. In his report, he discussed it at great length and the Grunskys drew substantially on it, as they did his analysis of the schist, for their own report. The Grunskys, as civil engineers, took the lead in that area of the investigation, and in describing the role played by "hydrostatic uplift".

Uplift takes its name from its tendency to lift a dam upward. Although many designers and builders of dams had become aware of this phenomenon by the late 1890s to early 1900s, it was still not generally well understood or appreciated. Nevertheless, it was becoming a matter of debate and a concern to dam builders of this era that water from a reservoir could seep under a dam and exert pressure upward. Due for the most part to inadequate drainage of the base and side abutments, the phenomenon of uplift destabilizes gravity dams by reducing the structure's "effective weight", making it less able to resist horizontal water pressure. Uplift can act through the bedrock foundation: the condition most commonly develops where the bedrock foundation is strong enough to bear the weight of the dam, but is fractured or fissured and therefore susceptible to seepage and water saturation.

According to their theories, water from the reservoir had permeated far back into the schist formation of the eastern abutment. This lubricated the rock and it slowly began to move, exerting a tremendous amount of weight against the dam, which according to the Grunskys was already becoming less stable due to "uplift." Making the situation worse, Dr. Willis established, was that the conglomerate, on which the western abutment of the dam rested, reacted upon becoming wet by swelling. In fact, the amount of swelling was such that it would raise any structure built upon it. This hypothesis was reinforced when surveys taken of the wing wall after the failure were compared with those taken at the time it was built. They reveal that in some areas the wall was 2 to 6 inches higher than when built. Therefore, the dam was caught between forces that were acting on it much like a vise, as the red conglomerate swelled on one side, and the moving mountain pressed in on it from the other.

In his report, Grunsky concluded;

As soon as the dam was loosened on its base the toe of the structure spalled off. This was probably the beginning of its breaking up, and probably occurred sometime after 11:30 PM during the 23 minutes in which the water in the reservoir apparently fell 3/10 of a foot. Thereupon, quite likely, a part of the east end of the dam, meanwhile undermined, went out and the dam at this end lost its hillside support. Hydrostatic uplift at the already loose west and the weight of the remaining portion of the undermined east end caused a temporary tilting of the dam towards the east, accompanied by a rapid washing away of the hillside under the dam at its west end which then also began to break up. The reservoir water was now rushing with tremendous force against both ends and against the upstream face of all that was standing of the dam. This rush of water carried away huge blocks of concrete from both ends of the dam...

There was and remains a difference of professional opinions on the amount of time that elapsed, shown by the chart made by the Stevens automatic water level recorder, from when the line indicating the reservoir level broke sharply downward until it became perpendicular. Most investigating engineers feel the amount of time indicated on the chart is thirty to forty minutes, not the twenty-three minutes that Grunsky stated.

In support of his theory of the dam tilting, Grunsky pointed to an odd clue near the western lower edge of the standing section. Here a ladder had become wedged in a crack that had opened apparently during this rocking or tilting process and then had become tightly pinched in place as the section settled back on its foundation. Measurements taken proved the crack must have been much wider at the time that the ladder entered it. Further, surveys indeed showed the center section had been subjected to severe tilting or twisting. These surveys established that the center section had moved 5.5インチ (14 cm) downstream and 6インチ (15 cm) toward the eastern abutment.

Although this investigation was insightful and informative, the theory, along with others which hypothesized an appreciably increasing amount of seepage just prior to the failure, becomes less likely when it is compared against the eyewitness accounts of the conditions in the canyon and near the dam during the last thirty minutes before its collapse. Grunsky hypothesized, though failed to explain the action of the dam tilting as he described. This action would have the dam in motion as a singular unit while conversely, testimony given at the Coroner's Inquest indicates that the dam was fractured transversely in at least four places. Furthermore, the two cracks, which bordered each side of the standing center section, would have served as hinges to prevent this.

Aftermath

Map showing the location of the St. Francis Dam and reservoir north of Santa Clarita between two later, still extant reservoirs – Castaic and Bouquet.

Concrete ruins of the St. Francis Dam remain strewn about San Francisquito Canyon.

The center section, which had become known as "The Tombstone" due to a newspaper reporter's description of it as such, became an attraction for tourists and souvenir hunters.

In May 1929, the upright section was toppled with dynamite, and the remaining blocks were demolished with bulldozers and jackhammers to discourage the sightseers and souvenir hunters from exploring the ruins. The wing dike was used by Los Angeles firemen to gain experience in using explosives on building structures. The St. Francis Dam was not rebuilt, though Bouquet Reservoir in nearby Bouquet Canyon was built in 1934 as a replacement.

To this day, the exact number of victims remains unknown. The official death toll in August 1928 was 385, but the remains of victims continued to be discovered every few years until the mid-1950s. Many victims were swept out to sea when the flood reached the Pacific Ocean and were never recovered, while others were washed ashore, some as far south as the Mexican border. The remains of a victim were found deep underground near Newhall in 1992, and other bodies, believed to be victims of the disaster, were found in the late 1970s and 1994. The current death toll is estimated to be at least 431.

At the Coroner's Inquest, the leak that Tony Harnischfeger had spotted was cited as evidence that the dam was leaking on the day of the break, and that both the Bureau of Water Works and Supply and Mulholland were aware of it. Mulholland told the jury he had been at the dam the day of the break, due to the dam keeper's call, but neither he nor Van Norman had observed anything of concern, nor found any dangerous conditions. Mulholland further testified that leaks in dams, especially of the type and size of the St. Francis, were common. During the Inquest Mulholland said, "This inquest is a very painful thing for me to have to attend but it is the occasion of it that is painful. The only ones I envy about this thing are the ones who are dead." In subsequent testimony, after answering a question he added, "Whether it is good or bad, don't blame anyone else, you just fasten it on me. If there was an error in human judgment, I was the human, I won't try to fasten it on anyone else."

The Coroner's Inquest jury determined that one of the causative factors for the disaster lay in what they had termed as "an error in engineering judgment in determining the foundation at the St. Francis Dam site and deciding on the best type of dam to build there" and that "the responsibility for the error in engineering judgment rests upon the Bureau of Water Works and Supply, and the Chief Engineer thereof." They cleared Mulholland as well as others of the Bureau of Water Works and Supply of any criminal culpability, since neither he nor anyone else at the time could have known of the instability of the rock formations on which the dam was built. The hearings also recommended that "the construction and operation of a great dam should never be left to the sole judgment of one man, no matter how eminent."

Mulholland retired from the Bureau of Water Works and Supply in March 1929. His assistant, Harvey Van Norman, succeeded him as chief engineer and general manager. Mulholland was retained as Chief Consulting Engineer, with an office, and received a salary of $500 a month. In later years, he retreated into a life of semi-isolation. He died in 1935, at the age of 79.

Dam safety legislation

In response to the St. Francis Dam disaster, the California legislature created an updated dam safety program and eliminated the municipal exemption. Before this was added, a municipality having its own engineering department was completely exempt from regulation.

On August 14, 1929, the Department of Public Works, under the administrative oversight of the State Engineer, which was later assumed by the Division of Safety of Dams, was given authority to review all non-federal dams over 25 feet high or which would hold more than 50 acre-feet of water. The new legislation also allowed the state to employ consultants, as they deemed necessary.

Additionally, the state was given full authority to supervise the maintenance and operation of all non-federal dams.

Licensure of civil engineers

Having determined that the unregulated design of construction projects constituted a hazard to the public, the California legislature passed laws to regulate civil engineering and, in 1929, created the state Board of Registration for Civil Engineers (now the Board for Professional Engineers, Land Surveyors, and Geologists).

Looking across the canyon at the dam site in 2009; the outlines of landslides are visible on the far side of the canyon.

Analysis

The failure of the dam is now believed to have begun with the eastern abutment of the dam giving way, possibly due to a landslide. This scenario, having its roots in the works of Willis and Grunsky, was expanded upon by the author Charles Outland in his book Man-Made Disaster : The Story of St. Francis Dam which was first published in 1966. The material on which the eastern abutment of the dam had been built may itself have been part of an ancient landslide, but this would have been impossible for almost any geologists of the 1920s to detect. Indeed, the site had been inspected twice, at different times, by two of the leading geologists and civil engineers of the day, John C. Branner of Stanford University and Carl E. Grunsky; neither found fault with the San Francisquito rock.

J. David Rogers, inspired by the work of Outland, investigated the failure and published an extensive scenario, albeit somewhat controversial, of the possible geological and rock mechanic actions which may have led to the dam's failure. He attributed the failure to three major factors: the instability of the ancient landslide material on which the dam was built, the failure to compensate for the additional height added to the dam's design, and the design and construction being overseen by only one person.

A critique of Rogers's historical analysis of the dam's collapse was published in the journal California History in 2004 by historians Norris Hundley Jr. (Professor Emeritus, UCLA) and Donald C. Jackson (Professor, Lafayette College). While accepting most of his geological analysis of the failure, the article makes clearer the differences and deficiencies of the structure built in San Francisquito Canyon and how it fell short of the standards for large-scale concrete gravity dams as practiced by other prominent dam engineers in the 1920s.

Mulholland Dam reinforcement

Shortly after the disaster, many living below Mulholland Dam, which creates the Hollywood Reservoir, feared a similar disaster and began to protest and petitioned the City of Los Angeles to drain the reservoir and remove the dam.

A Committee of Engineers & Geologists to Assess Mulholland Dam was appointed to evaluate the safety of the Mulholland Dam. An External Review Panel to evaluate the structure, convened by the State of California, followed this in 1930. The same year, the City of Los Angeles Board of Water & Power Commissioners appointed their own Board of Review for the dam. Although the state's panel did not recommend modification of the dam, both panels came to similar conclusions that the dam lacked what was then considered sufficient uplift relief. That may have led to destabilization, and was unacceptable. Again in 1931, a fourth panel, the Board of Engineers to Evaluate Mulholland Dam, was appointed to assess the structure. As well, an external study group appointed by the Board of Water & Power Commissioners produced a "Geological Report of the Suitability of Foundations". Certain design deficiencies, which had been made by the engineering department during the planning phase of the dam, were uncovered. These had to do with the dam's base width in conjunction to its ability to resist uplift, sliding and to withstand earthquake loading.

The decision was made to permanently keep the Hollywood Reservoir drawn down. It was also decided to keep the amount stored in the reservoir to no more than 4,000 acre⋅ft (4,900,000 m³) and to place an enormous amount of earth, 330,000 cu yd (250,000 m³), on the dam's downstream face to increase its resistance against hydraulic uplift and earthquake forces, and to screen it from public view. This work was carried out in 1933–34.

Remains of the "Tombstone" section of the dam in 2009. The partially buried edges of the stair-stepped face of the dam are visible.

Present-day remains

The only visible remains of the St. Francis Dam are weathered, broken chunks of gray concrete and the rusted remnants of the handrails that lined the top of the dam and the wing dike. The ruins and the scar from the ancient landslide can be seen from San Francisquito Canyon Road. Large chunks of debris can still be found scattered about the creek bed south of the dam's original site.

The site of the disaster is registered as California Historical Landmark #919. The landmark is located on the grounds of Powerhouse No. 2 and is near San Francisquito Canyon Road.

The road sustained heavy storm damage in 2005, and when rebuilt, it was routed away from both the remains of the dam and the damaged portion of the roadway.

In popular culture

• Numerous fictionalized references are made to Mulholland, the California Water Wars, the aqueduct, and the St. Francis Dam disaster in the 1974 movie Chinatown.
• Rock musician Frank Black makes several references to the St. Francis Dam disaster in his songs "St. Francis Dam Disaster" and "Olé Mulholland."
• The 2014 young adult novel 100 Sideways Miles by Andrew A. Smith features the current site of the dam disaster as well as some discussion of the historical event.

See also

• List of dams and reservoirs in California
• Dam failure
• Baldwin Hills Reservoir
• Malpasset Dam
• Johnstown Flood

References

Notes

Bibliography

• Coroner's Inquest (1928年). “Transcript of Testimony and Verdict of the Coroner's Jury In the Inquest Over Victims of St. Francis Dam Disaster: Book 26902”. Los Angeles County Department of Coroner.
• Jackson, Donald C.; Hundley, Norris (Fall 2004). Privilege and Responsibility: William Mulholland and the St. Francis Dam Disaster. California History. http://ce.jhu.edu/perspectives/handouts_unprotected/stfrancis.pdf. 
• Kahrl, William. L. (1983). Water and Power: The Conflict over Los Angeles Water Supply in the Owens Valley. University of California. ISBN 0-520-05068-1 
• Mead, Elwood; Hill, Louis; Beach, Lansing (1928). Report of Committee appointed by the City Council of Los Angeles to investigate and report the cause of the failure of the St. Francis Dam. 
• Outland, Charles F. (2002). Man-Made Disaster: The Story of St Francis Dam (revised ed.). The Arthur H. Clark Company. ISBN 0-87062-322-2 
• Rogers, David J. (1995). “A Man, A Dam and A Disaster”. The St. Francis Dam Disaster Revisited Nunis Jr., Doyce B. Ed. Historical Society of Southern. ISBN 0-914421-13-1 
• Wiley, et al., A. J. (1928年). “Report of the Commission appointed by Governor C. C. Young to investigate the causes leading to the failure of the St. Francis dam near Saugus California”.

Further reading

• Horton, Pony R. "A Test of Integrity: The Original Story Upon Which The Docu-Drama is Based".
  • A popular article detailing the St. Francis Dam disaster. Based on Horton's 25 years of research into the story. Informational sources include Horton's interviews with Catherine Mulholland, Dr. J. David Rogers, and Robert V. Phillips, former Chief Engineer & General Manager, LADWP. A slightly lengthened version of the article was published in 2009 in The Raven and The Writing Desk; The 6th Antelope Valley Anthology by MousePrints Publishing, Lancaster, California. ISBN 0-9702112-7-9
• Wilkman, John (2016) Floodpath: The Deadliest Man-Made Disaster of 20th-Century America and the Making of Modern Los Angeles, New York: Bloomsbury. ISBN 978-1-62040-915-2.

External links

• “San Francisquito Canyon and the St. Francis Dam”. Santa Clarita Valley Historical Society. 2003年3月29日閲覧。. Several photographs of the dam under construction, completed, its ruins, and a list of the victims.
• St. Francis Dam Disaster 30-minute television program available online.
• Remembering the St. Francis Dam Disaster, by Michele E. Buttelman, The Signal March 11, 2001.
• Google Earth image of the St. Francis Dam ruins
• "Application of the Method of Characteristics to the Dam Break Wave Problem", Journal of Hydraulic Research, 47 (1), pp. 41–49.
• St Francis Dam Flood, image gallery at USGS
• Complete List of St. Francis Dam Disaster Victims, compiled by Ann C. Stansell, California State University–Northridge.