Flash Flooding: Response and Lessons Learned | Banjir Bandang: Respon dan Pelajaran yang Dipelajari

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01/01/2018
Environmental Issues

After a significant flood event affected three of its hydroelectric plants in Wisconsin, Xcel Energy used the lessons learned to better prepare its facilities for future floods.

On July 11 and 12, 2016, severe storms and torrential rainfall in northern Wisconsin resulted in record flooding on several streams in the region. Flash flooding caused four fatalities, as well as extensive damage to public infrastructure and private property, especially roads and bridges.

Xcel Energy operates 19 hydroelectric projects and five storage reservoirs in northern and western Wisconsin. Floods of record occurred at three of Xcel Energy’s hydroelectric projects near Lake Superior: 1.5-MW Superior Falls Hydro and 1.2-MW Saxon Falls Hydro on the Montreal River, and 600-kW White River Hydro on the White River. The Federal Energy Regulatory Commission classifies these three dams as low hazard.

Fortunately, the flash flooding caused only minor damage at the hydroelectric projects. The damage was primarily associated with surface erosion, scour and inundation. However, the flooding presented challenges in accessing and operating the dams. This article describes the storm, Xcel Energy’s response, impacts at the dam and in the region, and lessons learned from the event.

Significant woody debris accompanied the increase in flow at the White River project, and it accumulated at the White River spillway log boom.

Storm event

Severe storms and heavy rain affected east-central Minnesota and northern Wisconsin from the afternoon of July 11 to the early morning hours of July 12. Official 24-hour rainfall across the region ranged from 4 in to 10 in, but most of the rain fell over four to eight hours from the evening of July 11 to the early morning of July 12. A particularly heavy band of 8 in to 10 in fell in east-central Minnesota and northern Wisconsin (see Figure 1). Unofficial totals of more than 12 in were reported near Lake Superior along the Wisconsin-Michigan border, including 14.25 in recorded at the Superior Falls project rain gauge.

Figure 1: Total Precipitation July 11-12, 2016

During the severe storms, a particularly heavy band of rain – 8 in to 10 in – fell in east-central Minnesota and northern Wisconsin.

Most of the rain in Wisconsin fell in the first three to five hours of the event. At Superior Falls, the operator reported 13.5 in from 8:00 p.m. on July 11 to 4:00 a.m. on July 12. However, 11 in fell by 12:00 a.m. on July 12. The operator at White River Hydro reported similar conditions, with a lesser total of 8.5 in for the event. National Weather Service hourly precipitation maps for the event indicate that the storm produced hourly total rainfall amounts of up to 4 in.

Table 1 provides regional NOAA rainfall frequency and duration data. Rainfall of 7.9 in in six hours has a recurrence interval of 1,000 years. This storm produced more than 8 in in fewer than six hours at many locations. Figure 2 shows the annual exceedance probability for the storm throughout the region. The map indicates that a large area experienced rainfall totals with an annual exceedance probability of less than 1/1,000.

Figure 2: Annual Exceedance Probability

This National Weather Service map shows that a large area experienced rainfall totals with an annual exceedance probability of less than 1/1,000.

Flooding

The rainfall intensity, relatively steep topography and low-permeability soils resulted in flash flooding across the region. Area roads, including major highways, quickly became impassible due to overtopping and washed out culverts, bridges and shoulders. The flash flooding and aftermath resulted in four fatalities in northern Wisconsin. The area received federal disaster aid, with more than $30 million damage to public infrastructure. Saxon Harbor on Lake Superior was heavily damaged, with nearly all of the watercraft in the harbor destroyed by wind and flooding.

The Bad River, east of Ashland, experienced the most severe flooding in the region. A U.S. Geological Survey (USGS) stream gauge on the river recorded a peak flow of 39,300 cubic feet per second (cfs), exceeding the previous record flow of 27,700 cfs in 1960 by more than 40%. The gauge dates to 1949. The flood’s estimated recurrence interval is 2,000 years based on extrapolated flood frequency curves. There are no hydro plants on the Bad River, but the Village of Odanah, about 5 miles east of Ashland, experienced devastating flooding.

Montreal River

The Montreal River forms a portion of the border between Wisconsin and the Upper Peninsula of Michigan. The Superior Falls project is located near the mouth of the Montreal River at Lake Superior. Saxon Falls Hydro is located about 2 river miles upstream of Superior Falls. The Superior Falls spillway has three tainter gates and a small overflow section. The Saxon Falls spillway includes one tainter gate and an overflow spillway measuring about 145 ft long. Saxon Falls also has an earth embankment.

The Montreal River watershed covers 264 square miles at its mouth at the Superior Falls Hydro Project. The most significant rainfall occurred over the lower one-third of the watershed downstream of the confluence with the West Branch of the Montreal River near Ironwood, Mich. The drainage area just downstream of the confluence of the Montreal River and the West Branch of the Montreal River is 186 square miles.

The most intense rain fell on the lower part of the Montreal River watershed near Lake Superior, while the upper watershed received significantly less rain. The Superior Falls Hydro operator recorded more than 14 in of rain at the plant, with 11 in falling between about 8:00 p.m. and 12:00 a.m. Small streams and ditches quickly overflowed their banks, resulting in widespread road washouts, including to State Highway 122, a primary access road to Saxon Falls and Superior Falls.

The Superior Falls operator arrived at the dam at about 8:30 p.m. after reviewing radar and receiving a call from the White River Hydro operator regarding heavy rain at that location. The Saxon Falls operator proceeded to the White River project to assist with debris. The operator began opening gates at the Superior Falls dam to pass the flood waters and debris. At about 9:30 p.m., after opening one of the three gates about 4 feet, he proceeded to the Saxon Falls dam. However, when he arrived he found water flowing over the road and could not safely access the dam. Both the road to the spillway and the road to the plant had flowing water and erosion that made them too dangerous to traverse. Because the Saxon Falls dam has an overflow spillway along with one gate, he determined that it would be better to return to Superior Falls and continue operating the spillway gates there.

Between the time the operator left Superior Falls and attempted to access Saxon Falls, conditions deteriorated significantly. Water was over the highways at several locations, but the operator was able to safely return to Superior Falls. Subsequently, the roads between the plants washed out. In addition, the access road from the Michigan side had also washed out. At this point, the access and egress at Superior Falls was cut off.

The situation was further complicated by the loss of communications. The road washouts broke underground telephone and fiber-optic lines. There was no telephone communication to the plant. It was also no longer possible to remotely monitor the water levels at Superior Falls and Saxon Falls. Communications between the operator and other hydro operations personnel was limited to cell phone. Cell phone coverage is poor at the plant, so the operator had to get to nearby higher ground to make calls.

The flows continued to increase rapidly on the Montreal River. At 10:30, one gate was open 5 ft. By 11:15, one gate was fully open and another was half open. By 12:00 a.m., all three gates were fully open. The reservoir elevation at Superior Falls peaked about 1.5 ft above normal full elevation around 2:00 a.m. and began to recede.

Operators eventually gained access to the Saxon Falls dam at 7:00 a.m. on July 12. Observations indicated the reservoir may have been within 1 ft of overtopping the dam. The operator opened the gate to pass additional water and lower the reservoir.

Post-flood inspections conducted at Superior Falls noted areas of erosion near the surge tank and powerhouse. Erosion on the steep drive from the surge tank to the powerhouse made the road impassible and several large trees were uprooted. The powerhouse remained accessible by stairway from the surge tank. Erosion was also noted on the hillside adjacent to the penstocks. None of the erosion presented a dam safety concern. Surveys showed no movement at the spillway and no significant downstream scour.

Post-flood inspections at Saxon Falls noted areas of erosion at access roads and near the surge tank and powerhouse. A culvert washed out on the road to the spillway. The culvert has since been replaced with two culverts, resulting in significantly more discharge capacity. A gully formed near the downstream end of the penstock at the second penstock support upstream of the surge tank. The support is founded on rock. Erosion was also noted on the hillside adjacent to the penstocks. None of the erosion presented a dam safety concern.

Some scour occurred downstream of the Saxon Falls spillway. A training wall between the stream channel and penstock was undermined. Some scour and undermining were also noted downstream of the spillway apron. Surveys indicated no movement of the structures. None of the scour presented an immediate threat to the dam.

White River

The White River Hydro Project is about 5 miles south of Ashland on the White River (a tributary to the Bad River). The project’s drainage area is 301 square miles. The dam includes a spillway with two tainter gates and earth embankments on each side of the spillway. The river has historically been prone to flash flooding and debris due to relatively steep topography and heavy clay soils throughout the watershed.

The White River watershed received 5 in to 8.5 in of rain between 7:00 p.m. and 3:00 a.m. on July 11 and 12, 2016. The White River Hydro Project received 8.5 in. The majority of the rain fell in the first four hours of the event. The operator arrived at the site around 7:30 p.m. in response to the heavy rain and began opening gates. A second operator from Saxon Falls hydro arrived a short time later to assist. Flows increased from about 150 cfs to 6,700 cfs in just three hours. The USGS stream gauge just downstream of the dam recorded a peak discharge of 8,570 cfs at about 5:00 a.m. on July 12. The previous flood of record at the site since 1949 was 6,720 cfs in 2005. The 2016 flood event had an estimated recurrence interval of 100 years.

The rapid increase in flow was accompanied by significant woody debris at the dam. The operators worked to open gates to minimize the surcharge and manage debris. In spite of the operators’ efforts, the reservoir surcharged almost 3 ft above the normal full elevation. The log boom upstream of the spillway held the debris. Maintenance personnel were dispatched at about 1:00 a.m. on July 12 to assist with debris management. Maintenance personnel loaded trucks and proceeded to the White River project. This is normally a three-hour drive, but the heavy rain and local road washouts hindered travel and the maintenance personnel did not arrive at the site until about 9:00 a.m.

Only minor damage was incurred at the project. The powerhouse generator floor flooded, resulting in minor damage to the generators. A 60-ft portion of the downstream left river bank eroded near the buried penstock. In addition, some surface erosion occurred along the shoulders of the state highway that traverses the dam embankments.

Surveys and soundings indicated minor scour in the powerhouse tailrace. No significant changes were noted downstream of the spillway when compared to previous soundings and no movement was noted. An underwater inspection for the Wisconsin Department of Transportation for the State Highway 112 bridge across the spillway found no adverse structural issues.

Lessons learned

Prompt response to developing conditions and decisive action by the operators prevented potential dam failures. The rainfall intensity and watershed characteristics led to rapidly increasing flow, requiring fast response. There was little time to dispatch supplemental staff from other plants. In addition, the severe storms and record stream flow resulted in a heavy debris load. Delays in opening gates could have resulted in additional debris accumulation that could have blocked the spillways.

While the dams had adequate spillway capacity to handle the floods, loss of access to the dams to open spillway gates could have resulted in overtopping failures. Under extreme rainfall events, access to the dams may be impossible even when multiple access routes are available. The flash flooding exceeded the design flow for many bridges, culverts and drainage ways. Under these conditions, even major highways can overtop and become impassable. Widespread washouts that cut off multiple routes are possible. Xcel Energy is exploring adding remote or automated operation for these sites.

While power outages are common during severe weather, underground communication lines are generally protected from damage. No power outages were experienced at the dams during the event. However, when the roads washed out, many of the communication lines were broken, cutting off normal communications. Cell phone communication was difficult due to poor coverage at the dams. In addition, remote monitoring through telephone lines was also disrupted. Xcel Energy has since enhanced communications through cell phone boosters and radio improvements.

Inaccurate spillway rating curves created some confusion between plant operators and hydro management staff. One rating curve was based on an incorrect assumption of the maximum gate opening, resulting in underestimation of the actual discharge. At another project, the shallow approach to the spillway results in an upstream hydraulic control. The spillway rating curve did not account for this condition and therefore overestimated the discharge. Rating curves have since been reviewed and updated.

Finally, the public often misunderstands the role that dams play during flooding. The projects discussed in this article are run-of-river with small reservoirs. The public and even local government officials often incorrectly assume that these hydro projects provide downstream flood control or that they can be operated to alleviate upstream river flooding. Discussions with stakeholders were necessary after the events to explain our operations during high flow events.

Dean Steines is principal plant engineer, hydro operations with Xcel Energy.

Banjir Bandang: Respon dan Pelajaran yang harus diingat
01/01/2018

Isu yang berkaitan dengan lingkungan

Setelah peristiwa banjir yang signifikan mempengaruhi tiga pembangkit hidroelektrik di Wisconsin, Xcel Energy menggunakan pelajaran penting yang dipetik untuk lebih mempersiapkan fasilitasnya dari banjir di masa depan.

Oleh Dekan Steines

Pada tanggal 11 dan 12 Juli 2016, badai hebat dan hujan deras di Wisconsin utara mengakibatkan banjir di beberapa sungai di wilayah tersebut. Banjir bandang menyebabkan empat korban jiwa, serta kerusakan luas pada infrastruktur publik dan properti pribadi, terutama jalan dan jembatan.

Xcel Energy mengoperasikan 19 proyek hidroelektrik dan lima waduk penyimpanan di Wisconsin utara dan barat. Rekor banjir terjadi di tiga proyek hidroelektrik Xcel Energy dekat Danau Superior: 1,5-MW Superior Falls Hydro dan 1,2-MW Saxon Falls Hydro di Sungai Montreal, dan 600-kW White River Hydro di Sungai Putih. Komisi Pengaturan Energi Federal mengklasifikasikan ketiga bendungan ini sebagai bahaya rendah.

Untungnya, banjir bandang hanya menyebabkan kerusakan kecil pada proyek-proyek hidroelektrik. Kerusakan terutama terkait dengan erosi permukaan, gerusan dan genangan. Namun, banjir menghadirkan tantangan dalam mengakses dan mengoperasikan bendungan. Artikel ini menjelaskan tentang badai, respons Xcel Energy, dampak di bendungan dan di wilayah tersebut, serta pelajaran yang didapat dari peristiwa tersebut.

Puing kayu yang signifikan disertai peningkatan aliran di proyek Sungai Putih, dan itu terakumulasi pada boom log spillway Sungai Putih.

Peristiwa badai

Badai yang parah dan hujan lebat mempengaruhi Minnesota di bagian timur-tengah dan Wisconsin utara dari siang hari tanggal 11 Juli hingga pagi hari di 12 Juli. Curah hujan 24 jam resmi di seluruh wilayah itu berkisar antara 4 hingga 10 inci, tetapi sebagian besar hujan turun lebih dari empat hingga delapan jam dari malam tanggal 11 Juli hingga dini hari tanggal 12 Juli. Band yang sangat berat, 8 di antara 10 di antaranya jatuh di Minnesota tengah-timur dan Wisconsin utara (lihat Gambar 1). Total tidak resmi lebih dari 12 di dilaporkan dekat Danau Superior di sepanjang perbatasan Wisconsin-Michigan, termasuk 14.25 yang tercatat di pengukur hujan proyek Falls Superior.

Gambar 1: Total Presipitasi 11-12 Juli 2016


Selama badai yang parah, rentang curah hujan yang sangat berat – 8 ke 10 – terjadi di Minnesota timur-tengah dan Wisconsin utara.

Sebagian besar hujan di Wisconsin terjadi dalam tiga hingga lima jam pertama dari peristiwa itu. Di Superior Falls, operator melaporkan 13,5 dari jam 8:00 malam. pada 11 Juli hingga 4:00 pagi pada 12 Juli. Namun, 11 di musim gugur pada jam 12 pagi pada 12 Juli. Operator di White River Hydro melaporkan kondisi serupa, dengan total 8,5 lebih rendah untuk acara tersebut. Layanan Cuaca Nasional peta curah hujan setiap jam untuk acara tersebut menunjukkan bahwa badai menghasilkan jumlah curah hujan per jam hingga 4 jam.


Tabel 1 memberikan data curah hujan dan durasi durasi NOAA regional. Curah hujan 7,9 jam dalam enam jam memiliki interval kekambuhan 1.000 tahun. Badai ini menghasilkan lebih dari 8 dalam waktu kurang dari enam jam di banyak lokasi. Gambar 2 menunjukkan probabilitas pelampauan tahunan untuk badai di seluruh wilayah. Peta menunjukkan bahwa area yang luas mengalami curah hujan total dengan probabilitas pelampauan tahunan kurang dari 1 / 1.000.

Gambar 2: Peluang Eksekusi Tahunan


Peta Layanan Cuaca Nasional ini menunjukkan bahwa area yang luas mengalami curah hujan total dengan probabilitas pelampauan tahunan kurang dari 1 / 1.000.
Banjir

Intensitas curah hujan, topografi yang relatif curam dan permeabilitas rendah menyebabkan banjir bandang di seluruh wilayah. Jalan-jalan di daerah, termasuk jalan raya utama, dengan cepat menjadi tidak dapat dilewati karena gorong-gorong, jembatan dan bahu meluber. Banjir bandang dan akibatnya mengakibatkan empat korban jiwa di Wisconsin utara. Daerah tersebut menerima bantuan bencana federal, dengan kerusakan lebih dari $ 30 juta untuk infrastruktur publik. Pelabuhan Saxon di Danau Superior rusak berat, dengan hampir semua perahu di pelabuhan hancur oleh angin dan banjir.

Sungai Buruk, timur Ashland, mengalami banjir paling parah di wilayah tersebut. Pengukur aliran Geologi AS (USGS) di sungai mencatat aliran puncak 39.300 kaki kubik per detik (cfs), melebihi rekor sebelumnya yaitu 27.700 cfs pada tahun 1960 lebih dari 40%. Pengukur tanggal ke 1949. Perkiraan interval pengulangan banjir adalah 2.000 tahun berdasarkan kurva frekuensi banjir ekstrapolasi. Tidak ada tumbuhan hidro di Sungai Bad, tetapi Desa Odanah, sekitar 5 mil di sebelah timur Ashland, mengalami banjir yang dahsyat.
Sungai Montreal

Sungai Montreal membentuk sebagian dari perbatasan antara Wisconsin dan Upper Peninsula of Michigan. Proyek Air Terjun Superior terletak di dekat muara Sungai Montreal di Danau Superior. Saxon Falls Hydro terletak sekitar 2 mil sungai di hulu dari Air Terjun Superior. The Superior Falls spillway memiliki tiga gerbang tainter dan bagian luapan kecil. Air terjun Saxon Falls termasuk satu gerbang tainter dan sebuah luapan melimpah berukuran sekitar 145 kaki panjangnya. Air Terjun Saxon juga memiliki tanggul bumi. Sungai Montreal mencakup sekitar 264 mil persegi di mulutnya di Superior Falls Hydro Project. Curah hujan paling signifikan terjadi di sepertiga bawah hilir hilir pertemuan dengan Cabang Barat Sungai Montreal dekat Ironwood, Mich. Daerah drainase di hilir pertemuan Sungai Montreal dan Cabang Barat Montreal Sungai adalah 186 mil persegi. Hujan paling lebat turun di bagian bawah DAS Sungai Montreal dekat Danau Superior, sedangkan daerah aliran sungai bagian atas menerima hujan yang jauh lebih sedikit. Operator Air Terjun Superior Hydro mencatat lebih dari 14 hujan di pabrik, dengan 11 jatuh antara sekitar jam 8:00 malam. dan 12.00 Aliran kecil dan parit dengan cepat meluap-luap di tepian sungai mereka, yang mengakibatkan pencabutan jalan yang meluas, termasuk ke State Highway 122, jalan akses utama ke Saxon Falls dan Air Terjun Superior. Operator Falls Superior tiba di bendungan sekitar pukul 8:30. sore setelah meninjau radar dan menerima panggilan dari operator Hydro Sungai Putih mengenai hujan lebat di lokasi itu. Operator Saxon Falls melanjutkan ke proyek Sungai Putih untuk membantu dengan puing-puing. Operator mulai membuka gerbang di bendungan Superior Falls untuk melewati air banjir dan puing-puing. Sekitar jam 9:30 malam, setelah membuka salah satu dari tiga gerbang sekitar 4 kaki, dia melanjutkan ke bendungan Air Terjun Saxon. Namun, ketika dia tiba, dia menemukan air mengalir di atas jalan dan tidak dapat dengan aman mengakses bendungan. Baik jalan menuju pelimpah dan jalan menuju pabrik telah mengalir air dan erosi yang membuat mereka terlalu berbahaya untuk dilalui. Karena bendungan Air Terjun Saxon memiliki saluran pelimpahan melimpah bersama dengan satu gerbang, ia memutuskan bahwa akan lebih baik untuk kembali ke Air Terjun Superior dan terus mengoperasikan gerbang pelimpah di sana. Antara saat operator meninggalkan Superior Falls dan berusaha untuk mengakses Air Terjun Saxon, kondisi memburuk secara signifikan. Air berada di jalan raya di beberapa lokasi, tetapi operator dapat dengan aman kembali ke Superior Falls. Selanjutnya, jalan-jalan di antara tanaman-tanaman terhapus. Selain itu, jalan akses dari sisi Michigan juga tersapu bersih. Pada titik ini, akses dan jalan keluar di Superior Falls terputus. Situasi semakin diperumit oleh hilangnya komunikasi. Pencucian jalan merusak telepon bawah tanah dan jalur serat optik. Tidak ada komunikasi telepon ke pabrik. Itu juga tidak mungkin lagi untuk memantau ketinggian air di Superior Falls dan Saxon Falls. Komunikasi antara operator dan personil operasi hidro lainnya terbatas pada telepon seluler. Cakupan ponsel buruk di pabrik, sehingga operator harus pergi ke tempat yang lebih tinggi di dekatnya untuk membuat panggilan. Aliran terus meningkat dengan cepat di Sungai Montreal. Pukul 10:30, satu gerbang terbuka 5 kaki. Pada pukul 11:15, satu gerbang terbuka penuh dan yang lain setengah terbuka. Menjelang pukul 12:00 pagi, ketiga gerbang sepenuhnya terbuka. Elevasi waduk di Superior Falls memuncak sekitar 1,5 kaki di atas elevasi penuh normal sekitar pukul 02:00 dan mulai surut. Para operator akhirnya memperoleh akses ke bendungan Air Terjun Saxon pada pukul 07:00 pada 12 Juli. Observasi menunjukkan waduk mungkin telah berada di dalam 1 ft dari limpasan bendungan. Operator membuka gerbang untuk mengalirkan air tambahan dan menurunkan reservoir. Pemeriksaan banjir-banjir yang dilakukan di Air Terjun Superior mencatat area erosi di dekat tangki gelombang dan pembangkit tenaga listrik. Erosi pada drive curam dari tangki lonjakan ke pembangkit tenaga listrik membuat jalan tidak bisa dilewati dan beberapa pohon besar tumbang. Pembangkit tenaga listrik tetap dapat diakses dengan tangga dari tangki lonjakan. Erosi juga dicatat di sisi bukit yang berdekatan dengan penumpukan. Tak satu pun dari erosi yang disajikan sebagai masalah keamanan bendungan. Survei menunjukkan tidak ada pergerakan di jalur pelimpah dan tidak ada aliran hilir yang signifikan. Inspeksi banjir di Saxon Falls mencatat area erosi di jalan akses dan dekat tangki gelombang dan pembangkit tenaga listrik. Gorong-gorong yang tersapu bersih di jalan menuju pelimpah. Gorong-gorong tersebut telah diganti dengan dua gorong-gorong, sehingga kapasitas debit menjadi lebih besar. Sebuah selokan yang terbentuk di dekat ujung hilir penstock di penstock kedua mendukung hulu dari tangki lonjakan. Dukungan didirikan di atas batu. Erosi juga dicatat di sisi bukit yang berdekatan dengan penumpukan. Tak satu pun dari erosi yang memunculkan kekhawatiran keamanan bendungan. Beberapa kali terjadi hilir aliran air terjun Saxon Falls. Sebuah dinding pelatihan antara saluran sungai dan penstock telah dirusak. Beberapa gerusan dan penggerogotan juga dicatat di bagian hilir apron spillway. Survei menunjukkan tidak ada pergerakan struktur. Tak satu pun dari gerusan itu menghadirkan ancaman langsung ke bendungan. Sungai Putih Proyek Hidro Sungai Putih berjarak sekitar 5 mil di selatan Ashland di Sungai Putih (sebuah anak sungai ke Sungai Buruk). Daerah drainase proyek adalah 301 mil persegi. Bendungan tersebut mencakup sebuah spillway dengan dua gerbang tambang dan tanggul-tanggul bumi di setiap sisi saluran pembuangan.

Sungai secara historis telah rentan terhadap banjir dan puing-puing karena topografi yang relatif curam dan tanah liat berat di seluruh DAS. Sungai Putih menerima 5 hingga 8,5 hujan antara jam 7 malam. dan 3:00 pagi pada 11 dan 12 Juli 2016. Proyek Hydro Sungai Putih menerima 8,5 masuk. Sebagian besar hujan turun dalam empat jam pertama dari acara tersebut. Operator tiba di lokasi sekitar jam 7.30 malam. dalam menanggapi hujan lebat dan mulai membuka gerbang. Operator kedua dari Saxon Falls hydro tiba beberapa saat kemudian untuk membantu. Arus meningkat dari sekitar 150 cfs menjadi 6.700 cfs hanya dalam tiga jam. Pengukur aliran USGS di bagian hilir bendungan mencatat debit puncak sebesar 8.570 cfs sekitar pukul 05:00 pada tanggal 12 Juli. Rekor banjir sebelumnya di lokasi tersebut sejak tahun 1949 adalah 6.720 cfs pada tahun 2005. Peristiwa banjir tahun 2016 memiliki perkiraan kekambuhan interval 100 tahun. Peningkatan aliran yang cepat disertai dengan serpihan kayu yang signifikan di bendungan. Operator bekerja untuk membuka gerbang untuk meminimalkan biaya tambahan dan mengelola puing-puing. Terlepas dari upaya operator, waduk tersebut dijahit hampir 3 kaki di atas elevasi penuh normal. Log boom hulu dari spillway menahan puing-puing. Personil pemeliharaan dikirim sekitar pukul 01.00 pada 12 Juli untuk membantu pengelolaan puing. Pemeliharaan personil memuat truk dan melanjutkan ke proyek Sungai Putih. Ini biasanya tiga jam perjalanan, tetapi hujan deras dan pencucian jalan setempat menghambat perjalanan dan personel pemeliharaan tidak tiba di lokasi sampai sekitar jam 9 pagi. Hanya sedikit kerusakan yang terjadi pada proyek tersebut. Lantai pembangkit pembangkit tenaga listrik banjir, menyebabkan kerusakan kecil pada generator. Bagian 60-ft dari tepi sungai kiri hilir terkikis dekat penstock terkubur. Selain itu, beberapa erosi permukaan terjadi di sepanjang bahu jalan raya negara bagian yang melintasi tanggul bendungan. Permukaan dan bunyi-bunyi menunjukkan gerusan kecil di pusat pembangkit tenaga listrik. Tidak ada perubahan signifikan yang dicatat di bagian hilir spillway bila dibandingkan dengan bunyi sebelumnya dan tidak ada pergerakan yang tercatat. Inspeksi bawah laut untuk Departemen Transportasi Wisconsin untuk jembatan State Highway 112 di seberang spillway tidak menemukan masalah struktural yang merugikan. Pelajaran yang didapat Tanggapan yang tepat untuk mengembangkan kondisi dan tindakan tegas oleh operator mencegah potensi kegagalan bendungan. Intensitas curah hujan dan karakteristik DAS menyebabkan aliran meningkat dengan cepat, membutuhkan respon cepat. Ada sedikit waktu untuk mengirim staf tambahan dari tanaman lain. Selain itu, badai yang parah dan aliran sungai yang tercatat menghasilkan beban puing-puing yang berat. Keterlambatan di gerbang pembukaan dapat mengakibatkan akumulasi puing tambahan yang dapat memblokir jalan keluar. Sementara bendungan memiliki kapasitas spillway yang memadai untuk menangani banjir, hilangnya akses ke bendungan untuk membuka gerbang spillway dapat mengakibatkan kegagalan limpasan. Di bawah peristiwa hujan ekstrim, akses ke bendungan mungkin tidak mungkin bahkan ketika beberapa rute akses tersedia. Banjir banjir melebihi aliran desain untuk banyak jembatan, gorong-gorong dan drainase. Di bawah kondisi ini, bahkan jalan raya utama dapat melampaui dan menjadi tidak dapat dilewati. Pencucian yang luas yang memotong beberapa rute adalah mungkin. Xcel Energy sedang menjajaki penambahan operasi jarak jauh atau otomatis untuk situs-situs ini. Sementara pemadaman listrik biasa terjadi selama cuaca buruk, jalur komunikasi bawah tanah umumnya terlindung dari kerusakan. Tidak ada pemadaman listrik yang dialami di bendungan selama acara. Namun, ketika jalan-jalannya rusak, banyak jalur komunikasi rusak, memutus komunikasi normal. Komunikasi telepon seluler sulit karena cakupan yang buruk di bendungan. Selain itu, pemantauan jarak jauh melalui saluran telepon juga terganggu. Xcel Energy sejak itu meningkatkan komunikasi melalui penguat ponsel dan peningkatan radio. Laju penilaian spillway yang tidak akurat menciptakan beberapa kebingungan antara operator pabrik dan staf manajemen hidro. Satu kurva penilaian didasarkan pada asumsi yang salah tentang pembukaan gerbang maksimum, yang mengakibatkan perkiraan yang kurang dari debit sebenarnya. Pada proyek lain, pendekatan dangkal terhadap spillway menghasilkan kontrol hidrolik hulu. Kurva penilaian spillway tidak menjelaskan kondisi ini dan karena itu melebih-lebihkan debit. Kurva penilaian telah ditinjau dan diperbarui. Akhirnya, publik sering salah memahami peran yang dimainkan oleh permainan selama banjir. Proyek-proyek yang dibahas dalam artikel ini adalah run-of-river dengan waduk kecil. Para pejabat pemerintah dan bahkan pemerintah lokal sering salah mengira bahwa proyek-proyek hidro ini menyediakan pengendalian banjir hilir atau bahwa mereka dapat dioperasikan untuk meringankan banjir sungai hulu. Diskusi dengan pemangku kepentingan diperlukan setelah kejadian untuk menjelaskan operasi kami selama peristiwa aliran tinggi.

Steine adalah insinyur pabrik utama, operasi hidro dengan Xcel Energy.

Diterjemahkan bebas dan ditulis oleh Ir. Russel Effandy AAAI-K. IPGDI. DIPL.CII. – http://www.cii.co.uk/membership/local-and-global/goodwill-ambassadors/russel-effandy-biography/

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+628129992037 (Russel Effandy)
+6281293617419 (Cepa Jasa Indonesia Center – CJIC)

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