Flash Flooding: Response and Lessons Learned

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.

 


 

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