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Flooding around the world

Cross-posted at Highly Allochthonous

Based on information from The Flood Observatory and other news sources, here are some tidbits about on-going and recent flood events around the world. Every one of these floods is having significant local and regional impacts, even if they don’t make the international news circuit. Common threads amongst these floods are the impact of the La Nina climate pattern and the unequal distribution of flood risks across the economic spectrum.

New Zealand

Cyclone Wilma hit the northern end of New Zealand’s North Island on Friday and Saturday 28-29 January, bringing with it intense rains, flooding, and landslides. Wilma unleashed about 28 cm of precipitation in just 12-14 hours, resulting in damage to homes, roads, and water and sewer treatment infrastructure. This was the fourth tropical low to impact New Zealand in just three weeks. The New Zealand Herald has a nice collection of reader-submitted images showing flooding and damage in various areas. My particular favorites are this flooded river valley and this road closed by a landslide. The New Zealand National Institute of Water and Atmospheric Research (NIWA) provides near real-time hydrologic, sea level, and climatic data through their Environmental Data Explorer, so I can show you quantitatively what this cyclone meant for a couple of rivers.

Mangakahi River at Gorge stream discharge data from NIWA

Mangakahi River at Gorge stream discharge data from NIWA

Waitangi River at Wakelins stream discharge data from NIWA

Waitangi River at Wakelins stream discharge data from NIWA

While the graphs above show discharge (flow volume per time), which is the unit of currency for hydrologists who want to compare multiple rivers to each other, local flooding impacts depend also on the depth(or stage) of the water. For reference, the Waitangi River goes from ~0.4 m before the storm to 6.2 m at the end of the record shown above. If you click through to this image on the New Zealand Herald website, you’ll see why the record for the Waitangi River ends when it does. That gaging station wasn’t meant for those flow conditions.


While Queensland begins to tally its losses and recover from massive flooding earlier this month, tropical cyclones aren’t about to make the job any easier. Cyclone Anthony brought mostly heavy winds to the Queensland coast south of Townsville Sunday night, and damage is reported to be minimal. But much bigger and much stronger Cyclone Yasi is expected to make landfall in the same area as a Category 4 storm later this week. This cyclone is expected to produce widespread, heavy rain, a strong storm surge along the coast, and winds up to 260 km per hour.

Meanwhile, in the southeastern state of Victoria, tributaries to the Murray River are also flooding. These floodwaters are still rising and are expected to take weeks to months to recede. Increasing my sympathy for the Australians, Victoria and South Australia are also experiencing a ridiculous heat wave, with temperatures reaching or exceeding 40 C for several days in a row.

Saudia Arabia

Flooding occurred around the city of Jeddah over the weekend, killing at least 10 people. Three hours of rain produced 11 cm of precipitation, cars were washed away, and the video below shows the failure of a dam, which the videographer says contained a lake used for dumping untreated sewage.


South Africa

Flooding in South Africa has gotten almost no international attention, despite the fact that floods have killed 120 people there and have caused disaster declarations in 8 of 9 provinces. Flooding has also affected Mozambique, where 13 people have died, and forecasts for continued heavy rains over the next several months have much of the southern part of the continent on alert. In some areas, up to 10 times as much rain as normal has fallen in the month of January. Tens of thousands of homes have been destroyed. Many of the lost homes are shacks belonging to poor Africans, because informal settlements are often located in low lying areas.


The clouds have cleared over the area around Rio that was hard-hit by floods and landsliding earlier this month. The death toll now exceeds 840 people, and the Brazilian federal and state governments have promised to provide up to 8000 homes for people that lost theirs in the disaster. The government also plans to immediately begin increasing its disaster preparedness, including mapping of high risk areas and better weather data collection. Dave Petley did a great analysis using before and after aerial imagery in one of the slide-affected area.

Edible debris flow

Cross-posted at Highly Allochthonous</em>

Steep hillslopes with loose sediment are at risk from debris flows triggered by heavy rain or rapid snowmelt. As water is added to the hillslope, surface runoff or positive pore water pressure catastrophically destabilizes a portion of the slope. Pulled by gravity, the water and sediment mixture moves downslope – picking up trees, boulders, gravel, and more mud and water along the way. Usually nothing stops these flowing masses of debris until they reach a relatively flat surface.

Debris flows have the power to reshape mountainsides and valley bottoms, and they can cause tragic devastation to people and property in their way. From North Carolina and California to Japan and Brazil, debris flows are a significant natural hazard and an area of active research by geoscientists and engineers.

In the spirit of the Accretionary Wedge, I decided to undertake my own research and investigate the possibilities for an edible analog for debris flows. First, I assembled sediments of a range of sizes, shapes, and natural cohesions.

Ingredients of my debris flow pilaf

Loose sediments (clockwise from top right) of the onion, rice, lentil, potato, portabella, garlic, pepper, salt, coriander, ginger, and barley varieties.

Then I added water to saturate the mixture, and placed it on a slope. Voila, debris flow pilaf! From the view below, you can see a bunch of features of debris flows.

Debris flow pilaf

Debris flow pilaf

  • At the top, there is the area of initial failure. In this case, it appears to be in the midst of a broccoli clear cut, where root strength had been weakened, reducing cohesion in the soils.
  • The debris flow then moved downslope in a somewhat confined manner. Usually the flow will move down an existing channel on the slope, but sometimes debris flows have to start from scratch and may not leave much of an erosional impression.
  • There is some evidence that the debris flow bulked up by lateral accumulation of material on its downslope track (i.e., places where sediment appears to accrete along the sides of the flow).
  • At least one large boulder has been rafted along the top of the flow, thanks to the quirks of fluid mechanics in very viscous fluids.
  • When the flow moved off the hillslope and onto the valley floor, the potential energy disappeared and the flow quickly stopped moving. Sometimes, debris flows will form a fan shape deposit at their front. But in our case, while there was some lateral spreading, it just stopped moving a short distance out onto the flat.
  • At the flow front, there is a significant accumulation of woody debris. (Amazing that it has kept its leaves on!) This debris has either been rafted on the top of the flow or been pushed along ahead of it.
  • There is a higher concentration of the coarsest grain sizes at the flow front. This sort of bouldery front is typical of debris flows where coarse material is available. You can see this better in the image below.
Overhead view of the debris flow runout.

Overhead view of the debris flow runout.

Of course, there are some limitations to using kitchen ingredients as analogs for debris flows. I highly encourage you to watch this classic USGS video on debris flows for its incredible footage of a whole range of debris flow materials and behaviors, all set to the most wonderful classical music. (Seriously, try the first 40 seconds of part 1 and see if you are not hooked.) Then, after you’ve watched the videos, I encourage you to use the comment section to make suggestions for improvements to the physical realism of future experiments with edible debris flows.


If that’s not enough debris flow video goodness to satisfy your appetite, check out these USGS videos of debris flows at their experimental flume in the Oregon Cascades. The recipe for barley pilaf with lentil confetti (or as it shall always be known in my mind: “debris flow pilaf”) came from Didi Evans’ Vegetarian Planet.

When it rains a lot and the mountains fall down

Cross-posted at Highly Allochthonous

2006 debris flow deposit in the Eliot Glacier drainage, north flank of Mount Hood (Photo by Anne Jefferson)

The geo-image bonanza of this month’s Accretionary Wedge gives me a good reason to make good on a promise I made a few months ago. I promised to write about what can happen on the flanks of Pacific Northwest volcanoes when a warm, heavy rainfall hits glacial ice at the end of a long melt season. The image above shows the result…warm heavy rainfall + glaciers + steep mountain flanks + exposed unconsolidated sediments are a recipe for debris flows in the Cascades. Let me tell you the story of this one.

It was the first week of November 2006, and a “pineapple express” (warm, wet air from the tropic Pacific) had moved into the Pacific Northwest. This warm front increased temperatures and brought rain to the Cascades…a lot of rain. In the vicinity of Mt. Hood, there was more than 34 cm in 6 days, and that’s at elevations where we have rain gages. Higher on the mountain, there may even have been more rain…and because it was warm, it was *all* rain. Normally, at this time of year, the high mountain areas would only get snow.

While it was raining, my collaborators and I were sitting in our cozy, dry offices in Corvallis, planning a really cool project to look at the impact of climate change on glacial meltwater contributions to the agriculturally-important Hood River valley. Outside, nature was opting to make our on-next field season a bit more tricky. We planned to install stream gages at the toe of the Eliot and Coe glaciers on the north flank of Mt. Hood, as well as farther downstream where water is diverted for irrigation. But instead of nice, neat, stable stream channels, when we went out to scout field sites the following spring, we were greeted by scenes like the one above.

Because sometime on 6 or 7 November, the mountain flank below Eliot Glacier gave way…triggering a massive debris flow that roared down Eliot Creek, bulking up with sediment along the way and completely obliterating any signs of the pre-existing stream channel. By the time the flow reached the area where the irrigation diversion occur, it had traveled 7 km in length and 1000 m in elevation, and it had finally reached the point where the valley opens up and the slope decreases. So the sediment began to drop out. And debris flows can carry some big stuff (like the picture below) and like the bridge that was washed out, carried downstream 100 m and turned sideways.

2006 Eliot Glacier debris flow deposit (photo by Anne Jefferson)

2006 Eliot Glacier debris flow deposit (photo by Anne Jefferson)

In this area, the deposit is at least 300 m wide and at least a few meters deep.

Eliot Creek, April 2007 (photo by Anne Jefferson)

Eliot Creek, April 2007 (photo by Anne Jefferson)

With all the big debris settling out, farther downstream the river was content to just flood…

Youtube video from dankleinsmith of the Hood River flooding at the Farmers Irrigation Headgates

and flood…

West Fork Hood River flood, November 2006 from

West Fork Hood River flood, November 2006 from For the same view during normal flows, take a look at my picture from April 2007:

and create a new delta where Hood River enters the Columbia.

Hood River delta created in November 2006 (photo found at

Hood River delta created in November 2006 (photo found at

And it wasn’t just Mt. Hood’s Eliot Glacier drainage that took a beating in this event. Of the 11 drainages on Mt. Hood, seven experienced debris flows, including a rather spectacular one at White River that closed the main access to a popular ski resort. And every major volcano from Mt. Jefferson to Mt. Rainier experienced debris flows, with repercussions ranging from downstream turbidity affecting the water supply for the city of Salem to the destruction of popular trails, roads, and campgrounds in Mt. Rainier National Park (pdf, but very cool photos).

In the end, our project on climate change and glacial meltwater was funded, we managed to collect some neat data in the Eliot and Coe watersheds in the summer of 2007, and the resulting paper is wending its way through review. The November 2006 debris flows triggered at least two MS thesis projects and some serious public attention to debris flow hazards in the Pacific Northwest. They also gave me some really cool pictures.

Flood risks in the aftermath of the Sichuan earthquake

As the casualty count continues to climb in China’s Sichuan province following the May 12th M 7.9 earthquake, authorities are struggling to provide shelter and prevent disease amongst the 5 million people displaced by the quake. Seismologists are warning that there is still the potential for large aftershocks, and many people are still jittery. But there’s also another potential danger lurking in the mountain valleys of Sichuan province – that of floods of water released by the failure of dams.

Some of these dams are man-made and suffered structural damage in the earthquake, like the 150 m Zipingpu dam near Dujiangyan. NPR reported:

One of its abutments sank 10 centimeters (4 inches). The force of the earthquake opened cracks in the dam wall. But, officials say,
Zipingpu remains structurally stable and safe.

Still, here’s an ominous thought: The reservoir at Zipingpu can hold up to 1.1 billion cubic meters of water. The Water Resources
Ministry says the city of Dujiangyan, with a population of more than 600,000 people, “would be swamped” if the dam failed.

An even more ominous threat is from landslide dams. During the earthquake, large landslides were knocked loose from the steep mountain slopes and came to rest in the narrow valleys below. The landslide deposits can block river flow and create a reservoir upstream. Eventually the water level will overtop the dam, and the reservoir will stabilize. Unless, of course, the erosive power of the overtopping is enough to cause dam failure, the pressure of the water is stronger than the unstable dam can support, or an aftershock destabilizes the deposits.

In the week following the quake, 24 lakes had formed in the area affected by the earthquake. NASA has incredible images of one of those lakes, filling the valley and flooding two villages. The image is shown below.

NASA Image

The largest dam appears to be 3.5 km upstream from Beichuan, a town of 30,000 that has been the hardest hit by the earthquake and resulting landslides This dam is apparently still inaccessible because of blocked roads in mountain passes, but it is reported to be 2 km long and blocking the Qingjiang River. Another dam is 70 m high and 300 m wide, and it blocked the Chaping River and destroyed a hydropower station. =A smaller dam is 7 m high, 33 m wide, and 100 m long, and it holds back 606,000 cubic meters of water. At least one evacuation has already taken place when a landslide dam threatened to burst. Researchers are trying to develop plans to safely drain the lakes before catastrophe occurs.

This isn’t the first time Sichuan province has faced this threat. In 1786, a M 7.75 earthquake triggered a landslide that blocked the Dadu River. F.C. Dai and colleagues (2005) did meticulous historical and geomorphic research to reconstruct the events that followed. The landslide dam was 70 m high and held back 50 million cubic meters of water in a reservoir area of 1.7 square kilometers. A large aftershock hit the area 10 days after the main quake, and it caused the dam to fail. The resulting flood had a peak discharge of ~37,000 cubic meters per second, but the real tragedy is that the flood killed 100,000 people living downstream. The landslide dam on the Dadu was not spectacularly large (the largest existing landslide dam is in Tajikstan and is 550 m high), but the deaths from downstream flooding make it the most catastrophic dam failure on record.

Journal articles cited:

Dai et al. 2005. “The 1786 earthquake-triggered landslide dam and subsequent dam-break flood on the Dadu River, southwestern China” Geomorphology 65: 205-221. doi:10.1016/j.geomorph.2004.08.011

Stone, R. 2008. Landslides, flooding pose threats as experts survey quake’s impact. Science. 320:996-997.

Other news sources are linked above.