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floods

Why does the Red River of the North have so many floods?

Cross-posted at Highly Allochthonous

Communities along the Minnesota-North Dakota border are watching the water levels, listening to the weather forecasts, and preparing for another season of flooding. It must be a disconcertingly familiar routine, as this will be the third year in a row in which the Red River of the North reaches major flooding levels. But this isn’t merely a run of bad luck for residents in the Red River Valley, major floods are to be expected in a place with an unfortunate combination of extremely low relief and a river at the whim of snowmelt and ice jams.

The Red River of the North begins in Minnesota, near the border with North and South Dakota, and it flows northward through Fargo/Moorhead, Grand Forks, and Winnipeg before emptying into Lake Winnipeg, Manitoba. The landscape around the Red River is excruciatingly flat (Figure 1), for the Red River Valley isn’t a stream-formed feature at all, but is the remnant landscape of Glacial Lake Agassiz, which held meltwaters from the Laurentide Ice Sheet for more than 5000 years. The modern Red River has barely managed to incise into this flat, flat surface, because it slopes only very gently to the north (~17 cm/km). Instead, the river tightly meanders across the old lake bed, slowly carrying its water to the north. Topographically, this is a pretty bad setting for a flood, because floodwaters spread out over large areas and take a long time to drain away.

Topography of the US portion of the Red River Valley from SRTM data as displayed by NASA's Earth Observatoryredriver_srtm_palette

Figure 1. Topography of the US portion of the Red River Valley from SRTM data as displayed by NASA's Earth Observatory

The climate of the Red River watershed makes it prone to flooding during the spring, usually peaking in about mid-April. The area receives about 1 m of snow between October and May, and the river freezes over. In late March to early April, the temperatures generally rise above freezing, triggering the start of snowmelt. Temperatures warm soonest in the southern, upstream end of the watershed and they get above freezing the latest near the mouth of the river. This means that snowmelt drains into the river’s upper reaches while downstream the river is still frozen, impeding flow (Figure 2). As the ice goes out, jams can temporarily occur and dam or back up the river, exacerbating local flooding problems.

Red River near Oslo, Minnesota, 3 April 2009, photo by David Willis

Figure 2. Red River near Oslo, Minnesota, 3 April 2009. Here the main river channel is still clogged with ice, while surrounding farmland is underwater. Photo by David Willis of http://www.cropnet.com/.

Together the topography and climate of the Red River watershed are a recipe for large-scale flooding, and the historical record shows that floods are a frequent occurrence on the river. Usually, hydrologists talk about rivers in terms of their flow, or discharge, which is the volume of water per second that passes a point. But, when talking about floods like those on the Red River, it’s not so much volume that matters as how high the water rises (“stage”). The National Weather Service is responsible for flood prediction in the US, and they define flood stage as “the stage at which overflow of the natural streambanks begins to cause damage in the reach in which the elevation is measured.” If the water level continues to rise, “moderate flooding” occurs when “some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary.” Further increases in water levels can bring a river to “major flooding“, when “extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.” That’s the sort of flooding that will happen in places along the Red River this spring, as it has many springs in the historical record (Figure 3).

Annual peak stage on the Red River at Grand Forks, North Dakota

Figure 3. Annual peak stage on the Red River at Grand Forks, North Dakota. Data replotted from the USGS, with local NWS flood stages shown.

Already, flood warnings are being issued for the Red River and its tributaries. As I’ll discuss in my next post, the long-range forecast for this spring’s floods on the Red is looking pretty grim. But as the communities along the river brace for the on-coming flood, it is important to remember that the geology and climate of the region make repeated major floods inevitable.

Pakistan floods: Predicted or Predictable, but a disaster nonetheless

Cross-posted at Highly Allochthonous

ResearchBlogging.org

Unusually heavy monsoon rains in July and August 2010 left large swaths of Pakistan underwater. At least 18 million people were affected by the flood, and it is estimated that, more than six months later, several hundred thousand remain without even temporary shelter. As a result of lost crops and livelihoods from the flood and inadequate relief supplies, malnutrition continues to kill people. Like most floods, the Pakistani poor have suffered far more than those with resources to avoid the flood, or at least its aftermath.

Remains of a school destroyed by flooding, near Jacobabad by DFID - UK Department for International Development, on Flickr

Remains of a school destroyed by flooding, near Jacobabad by UK Department for International Development, on Flickr. Used under a Creative Commons license.

A paper in press in Geophysical Research Letters shows that the 2010 floods were extraordinary. Monsoonal rains tend to occur in pulses, with multi-day wet periods followed by multi-day dry periods, and while the total rainfall over Pakistan during the 2010 monsoon season was not unprecedented, the number and intensity of extremely heavy rains over northern Pakistan was very unusual. The authors are working with very limited historical and satellite data, but they estimate that the number of intense rain bursts that occurred in 2010 had a probability of less than 3% in any given year.

Using data from the European Centre for Medium Range Weather Forecasts collection of meteorological models, the authors of the new paper show that the timing and intensity of northern Pakistan’s monsoon rain bursts are predictable up to 6 to 8 days in advance – including the rains that caused the flooding in 2010.

Lead author, Peter Webster, and his coauthors from the Georgia Institute of Technology, draw the following conclusion from their analysis:

We conclude that if these extended quantitative precipitation forecasts had been available in Pakistan, the high risk of flooding could have been foreseen. If these rainfall forecasts had been coupled to a hydrological model then the high risk of extensive and prolonged flooding could have anticipated and actions taken to mitigate their impact.

The floods really kicked off with a burst of rain on 28-29 July 2010, and according to Webster’s reanalysis, that rainfall was predictable with good skill 7 days in advance (21 July). Webster and colleagues argue that if that forecast was available in Pakistan, lives would have been saved and the immensity of the disaster reduced. But, C. Christine Fair, writing on the Foreign Policy magazine website suggests that the flood was forecast in Pakistan.

In the middle of July, the PMD began tracking a storm brewing in the Bay of Bengal. This eastern weather system developed interactively with a western weather system to produce the massive rains and the subsequent super flood of 2010. On July 24, the PMD issued a flood warning to the provincial government of Khyber-Pakhtunkhwa (KPK). Despite these increasingly severe warnings, KPK’s citizenry did not believe them. … The PMD kept issuing warnings to KPK as the rains began to fall. However, as fate would have it, on July 28, … a passenger jet coming to Islamabad from Karachi crashed …With the media beset upon this tragic spectacle, the PMD’s warnings went unheeded as the rain began to fall.

So the Pakistani government did forecast the flood – at least four days out – in plenty of time to get people in northern Pakistan’s valleys out of the way. The problem was not with the meteorological and hydrologic science either internationally or in Pakistan. Instead, disaster was ensured when flood warnings were not taken sufficiently seriously by regional authorities, media, and residents.

Why wouldn’t flood warnings be heeded? Perhaps more could have been done to communicate to Pakistanis through channels whose authority they respected. Webster cites an example of flood warnings in Bangladesh being disseminated by imams at local mosques. The Foreign Policy article quoted above places some blame on media distractedness.

But there was also a more insidious reason the forecasted flood was ignored. It was a rare event, but it was also part of a new climatic pattern for Pakistan. As the Foreign Policy article describes it:

in recent years there has been a slow but steady change in the location where Pakistan’s major rainfalls concentrate. In the past, monsoon rains fell most intensely over the Punjab. Slowly and steadily, the concentration of rainfall has moved north and west to KPK. This redistribution of concentrated rainfall away from the Punjab and towards KPK explains why no one in KPK had any reason to believe the predicted weather.

Flooding frequency and intensity have increased in Pakistan in the last 30-40 years compared to earlier in the 20th century. Webster and coauthors state, “This recent increase is consistent with the increase in intensity of the global monsoon accompanying the last three decades of general global warming.” The flood warnings were ignored, in part, because the statistics of monsoon rain patterns are changing. Human memory and historical records are not good guidance if the weather system is changing. In situations like this one, the past is not the key to the present.

There are lots of things that should have been improved to lessen the magnitude of the Pakistani flood disaster – reservoir management should have been altered; emergency relief supplies should have been distributed more equitably, broadly, and consistently; international assistance should have been much more generous – but the two big lessons for hazard mitigation coming out of the Pakistan floods seem to be: “find a system for making sure that warnings are issued and that they actually make it to people in harm’s way” and “don’t assume the climate of living memory is a very good indicator of the weather of the present and future.”

Webster, P. J., Toma, V.E., & Kim, H.-M. (2011). Were the 2010 Pakistan floods predictable? Geophysical Research Letters : 10.1029/2010GL046346

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.

Australia

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.

[youtube=http://www.youtube.com/watch?v=iiW1ChsOhSs&w=640&h=510]

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.

Brazil

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.

Flooding in Pakistan

A post by Anne Jefferson For the past two weeks, unusually heavy monsoon rains have deluged Pakistan, resulting in flooding and landslides. Pakistan is heavily populated all along the Indus River valley, so this is a slow-moving disaster of epic proportions. The latest news reports estimate that flooding has displaced 14 million people – more than the number of people affected by the 2010 Haiti earthquake, 2005 Kashmir earthquake, and 2004 Indian Ocean tsunami combined.

In the first 10 days of August, parts of Pakistan received almost 24 mm per day more rainfall than usual – during what is usually the wettest part of the year, when the monsoon rains fall. During June-September, the relatively cool Indian Ocean has high atmospheric pressure system, while the intense summer sun heats up the Indian subcontinent and forms a low pressure system. Much like a river flows downhill, air in the atmosphere flows from high pressure to low pressure areas. So moisture rich air from the ocean flows toward India and Pakistan – bringing months of cloudy weather and intense rain. The images below are from true color images from NASA’s MODIS satellite showing the Indus and Chenab rivers in the area around Islamabad. The top image shows the clearest day from recent weeks (9 August), while the bottom image shows a more typical day (6 August).

NASA MODIS image on Northern Pakistan from 9 August 2010

NASA MODIS image on Northern Pakistan from 9 August 2010

NASA MODIS image of northern Pakistan from 6 August 2010

NASA MODIS image of northern Pakistan from 6 August 2010

This year, the monsoon precipitation has been especially intense in Pakistan, because the jet stream is experiencing a blocking event – when the normal eastward progression of weather patterns in the midlatitudes gets stalled out and you get the same weather for weeks on end. This created an additional low pressure zone over Pakistan’s northern mountains, bringing even more moisture to the headwaters of the Indus River. The rain also seems to be exacerbating the landslide and landslide dam problems in the region of the Hunza River, a tributary to the Indus in the northern mountains. (Update: Jeff Masters has a nice explanation of this jet stream blocking event and how it links the Russian heat wave and Pakistan floods.)

Flooding on the Indus is not an instantaneous disaster – it is one that will continue to occur for weeks, with consequences that last years. Because the flooding is being caused by prolonged intense precipitation, there can be multiple flood peaks – where the water level crests, starts to fall, and then rises again. There are already two flood peaks moving downstream, and if further rain falls, there may be three peaks to the flooding, and it could last through the end of August. Also, since much of the rain has fallen in the north, closer to the headwaters of the Indus, the flooding began in the north, and the flood waves are transmitted downstream over a matter of days to weeks. While river levels are now slowly declining in the north, the first flood peak is just reaching Hyderabad – the largest city along the river, with a population of 1.5 million. How fast the flood moves downstream depends on the storage properties of the channel and floodplain. There is some possibility that if the first flood peak stalls out in an area with lots of floodplain storage or obstruction of flow by debris-choked bridges, the second flood peak could catch up, creating an even larger disaster. NASA has a series of wonderful images showing the flood progressing downstream from Sukkur, north of Hyderabad, from 8 -12 August. The latest image is shown below.

Flooding on lower Indus River, 12 August 2010 (NASA MODIS image, combination of infrared and visible light)

Flooding on lower Indus River, 12 August 2010 (NASA MODIS image, combination of infrared and visible light)

Even when the final flood peak reaches the ocean and the Indus River returns to its banks all along its course, the human disaster will continue to unfold. 14 million people have been displaced by this flood – and those people may have lost everything they own. Beyond the personal losses, there has been devastation to infrastructure such as roads and bridges, complicating relief efforts and even making access to some areas nearly impossible. The Indus is the source of water for irrigation canals throughout Pakistan and damage to them is likely to be intense, especially near the river, so agricultural productivity will suffer even in areas that escaped inundation. Village wells will have been contaminated by floodwaters, so access to safe drinking water will be an issue for months.

That’s a lot for any nation to handle – 1 in 12 residents directly affected by the flood – but for Pakistan’s already fragile national government it will be an especially difficult challenge. As the flooding has unfolded, Pakistan’s government has appeared less equipped to provide immediate relief to flood victims than Islamist charities, which will probably increase their support as they fill empty stomachs and provide shelter. There are other aid groups working to ameliorate the suffering. Two of my favorites are MercyCorps, providing clean water, food, and clean up tools in the Swat Valley, and Medecins Sans Frontiers/Doctors Without Borders, which is providing sanitation kits and basic supplies in Kyber Pakhtunkhwa and Baluchistan. Dave Petley, who has worked in Pakistan, recommends FOCUS Humanitarian Assistance.

Anne's picks of the June literature: Fluvial Geomorphology and Landscape Evolution

ResearchBlogging.orgA post by Anne JeffersonHow do rivers erode bedrock streams, during big floods, and in the presence of groundwater? Laboratory and accidental experiments are providing some cool new insights.

Johnson, J., & Whipple, K. (2010). Evaluating the controls of shear stress, sediment supply, alluvial cover, and channel morphology on experimental bedrock incision rate Journal of Geophysical Research, 115 (F2) DOI: 10.1029/2009JF001335

Take a moment to contemplate the title of this paper…experimental bedrock incision rate….how do you measure something like bedrock incision in an experimental setting? how do you measure it in time scales than can be accomplished in the laboratory? Johnson and Whipple figured out how to do it – building a weak concrete streambed in a flume at the National Center for Earth-surface Dynamics and then conducting a series of experiments to isolate each of the variables. Their study is related to question of the role of loose sediment in controlling the rates of bedrock river erosion. When does sediment act as a “tool” for erosion by banging into the river bed and abrading it, and when does sediment act as a “cover” for the river bed, protecting it from just such abrasion? Do these two effects create a trade-off suggesting that at some optimal level of sediment abundance, erosion rates are maximized? Johnson and Whipple’s experiments showed that erosion rates increased linearly with sediment flux , but decreased linearly with the extent of sediment cover. They also demonstrated that the extent of sediment cover was function of the ratio of sediment flux to sediment transport capacity, although it was sensitive to local topographic roughness. Their experiments also showed some interesting patterns of how bed roughness develops from focused erosion in interconnected topographically low areas (e.g., @colo_kea’s great video of the Skagway River), but that this development was muted by variations in discharge and sediment flux.* Also note that Johnson, Whipple, and L. Sklar have another new paper out, contrasting rates of bedrock incision from snowmelt and flash floods in Utah’s Henry Mountains. That paper is in GSA Bulletin.

Lamb, M., & Fonstad, M. (2010). Rapid formation of a modern bedrock canyon by a single flood event Nature Geoscience, 3 (7), 477-481 DOI: 10.1038/ngeo894

In 2002, a dam overspill in Texas created a 7 m deep, 1 km long gorge in jointed bedrock and this article by Lamb and Fonstad examines the mechanics of gorge formation and the importance of plucking as erosional mechanism. Brian Romans (Clastic Detritus) has written a nice post on this article and how it links to ideas of uniformitarianism and Kyle House posted before and after photos at Pathological Geomorphology.

Pornprommin, A., & Izumi, N. (2010). Inception of stream incision by seepage erosion Journal of Geophysical Research, 115 (F2) DOI: 10.1029/2009JF001369

An experimental study in layered sediment showed that seepage-drive scarp retreat was a function of the discharge per unit area and “a diffusion-like function that describes the incision edge shapes.” That diffusion-like function was then related to the weight of the failure block and hydraulic pressure. This paper potentially has some insights for thinking about landscape evolution in groundwater-rich areas (like I tend to do) and for those interested in slope stability analyses.*

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=http://www.youtube.com/watch?v=J4eduMJU710]
Youtube video from dankleinsmith of the Hood River flooding at the Farmers Irrigation Headgates

and flood…

West Fork Hood River flood, November 2006 from http://elskablog.wordpress.com/2006/11

West Fork Hood River flood, November 2006 from http://elskablog.wordpress.com/2006/11/. For the same view during normal flows, take a look at my picture from April 2007: http://scienceblogs.com/highlyallochthonous/upload/2009/10/IMG_1108.JPG.

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

Hood River delta created in November 2006 (photo found at http://www.city-data.com/picfilesc/picc30876.php)

Hood River delta created in November 2006 (photo found at http://www.city-data.com/picfilesc/picc30876.php

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.

A selected few Eyjafjallajokull links

The eruption of Eyjafjallajokull (which means Island Mountain Glacier in Icelandic) started out in March as a relatively quiet and tourist-friendly Hawaiian style eruption. That petered out and then a few days later, the magma reemerged subglacially, producing the spectacular ash-producing phreato-magmatic eruption that has transfixed the world and stranded all would-be European air passengers. The Boston Globe’s Big Picture coverage has been as-usual spectacular. Check out these two photo sets (15 April, 19 April). NASA’s Earth Observatory has also been producing some nice images of the ash plume. Follow them here.

And few days into the eruption, the magmatic heat produced enough glacier melt not just to fuel the ash production but also to generate an outburst flood, as captured on video below:
[youtube=http://www.youtube.com/watch?v=9sryalI57oo&feature=player_embedded]

The video above includes a rather spectacular train of standing waves, which led to some debate amongst some friends and I over whether they represented sub- or super-critical flow. Fortunately, I have access to an expert on the subject, Gordon Grant, who weighed in thusly:

The standing waves represent what is best referred to as “trans-critical flow”, that is flow that oscillates around Froude No. = 1. We have real-time (though very small scale compared to what you’re seeing in the video) measurements from Grant (1997; available on the WPG website). Basically flow on the downstream portion of the wave is accelerating (Fr >1), while flow on the upstream portion is decelerating. The flow then oscillates between Fr > 1 and Fr < 1, maintaining overall flow at close to Fr ~ 1. Cross-sectionally averaged Froude Number tends to be slightly less, due to drag at the boundary. See paper for details.

The paper to which he refers is Grant, G.E. 1997. Critical flow constrains flow hydraulics in mobile-bed streams: a new hypothesis. Water Resources Research. 33: 349-358. PDF available here: http://www.fsl.orst.edu/wpg/pubs/criticalflow.pdf

For the best scientific coverage of the on-going Icelandic eruption, you absolutely can’t miss Erik Klemetti’s Eruptions blog. Erik is a volcanologist and has been doing an astounding job of keeping up with this (and all other volcanic activity). He also benefits from an active, engaged, and informed community of commenters to keep the rest of us up-to-the-minute on volcanic activity around the world.

My picks of the December literature

Cross-posted at Highly Allochthonous

I’m a few days behind on sharing my picks from December’s journals, but Chris has been doing such a stupendous job of sharing absolutely wonderful geology posts (and of deconstructing terrible science reporting), that I hardly feel guilty waiting until he’s occupied with travels before sneaking this post onto the blog.

Without further ado, here is the odd assortment of articles that hit my email box in December that I found most intriguing. They reflect a mixture of my past, present, and future research and teaching interests and should not be considered a reflection of anyone else’s tastes in science.

Burbey, T.J. (2010) Fracture characterization using Earth tide analysis, Journal of Hydrology, 380:237-246. doi:10.1016/j.jhydrol.2009.10.037

Tides are popping up all over in the geology literature these days, from the Slumgullion earthflow (atmospheric tides) to the San Andreas fault (earth tides). Here Burbey uses water-level fluctuations in fractured rock confined aquifers to quantify specific storage and secondary porosity. Fractured rock aquifers are notoriously tricky to understand, and this method gives hydrogeologists one more tool in their arsenal for understanding places like the Blue Ridge Mountains and the Piedmont. Since I’m getting interested in the fractured rocks in just those areas, this paper caught my eye.

Burnett, W.C., Peterson, R.N., Santos, I.R., and Hicks, R.W. (2010) Use of automated radon measurements for rapid assessment of groundwater flow into Florida streams Journal of Hydrology, 380:298-304. doi:10.1016/j.jhydrol.2009.11.005

Radon is a conservative tracer with concentrations several orders of magnitude higher in groundwater than surface water. That means that it can be used to evaluate the groundwater inputs into different stream reaches, though it is often used in conjunction with other tracers to get quantitative estimates. In this paper, Burnett and colleagues lay out a method for using radon as a sole tracer to quantify groundwater discharge. I’m looking around for tracers to separate overland flow, flow through the soil/saprolite, and groundwater from rock fractures, so this paper piqued my interest as radon is one candidate I’m learning more about.

Garcia-Castellanos, D., Estrada, F., Jiménez-Munt, I., Gorini, C., Fernàndez, M., Vergés, J. and De Vicente, R. 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature, 462, 778-781, doi:10.1038/nature08555.

5.6 million years ago the Mediterranean basin was nearly dry and highly saline in the midst of a period known as the Messinian salinity crisis, but 5.33 million years the Atlantic Ocean rapidly refilled the basin by overtopping and incising through the sill at the Straits of Gibraltar. How fast did that sea refill? How big was the peak discharge? And what did all that water do the straits itself? Those are the questions tackled in this paper, which combines borehole and seismic data with hydrodynamic and morphodynamic modeling. The story that Garcia-Castellanos and colleagues tell as a result of their work is truly astounding. The Atlantic Ocean overtopped the sill and slowly began to refill the Mediterranean, but as the sill eroded, discharge (and incision) increased exponentially until peak discharges on the order of 108m3/sec were reached and sea levels in the Mediterranean were increasing by up to 10 m per day.  While the beginning and the end of the flood may have stretched out for thousands of years, the modeling work suggests that the vast majority of water transfer and the incision of greater than 250 m deep canyons across the Straits of Gibraltar was done on a time scale of several months to two years. That peak discharge is ten times greater than that estimated for the Missoula Floods, themselves not trifling events, and there may have been profound paleoclimate repercussions from such a significant change in the region’s hydrological status.

Grimm, R. E., and S. L. Painter (2009), On the secular evolution of groundwater on Mars, Geophys. Res. Lett., 36, L24803, doi:10.1029/2009GL041018.

Grimm and Painter created a 2D pole-to-equator model of subsurface water and carbon dioxide transport, initiated the model by simulating sudden freezing, and then looked at the effects over geologic time scales (secular evolution). According to their abstract, their model predicts water to be found in different places on the Martian landscape than previous ideas had suggested. I guess we’ll just have to go look and see who is right.

Jiang, Xiao-Wei; Wan, Li; Wang, Xu-Sheng; Ge, Shemin; Liu, Jie Effect of exponential decay in hydraulic conductivity with depth on regional groundwater flow Geophys. Res. Lett., 36, L24402, doi:10.1029/2009GL041251.

In soils and in the Earth’s crust, hydraulic conductivity (K) generally decreases exponentially with depth. This phenomenon is the result of the compaction and compression of the overlying strata. In this paper, Jiang and colleagues examine the implications such decreases in K on local versus regional groundwater flow systems. They find that the more quickly K decreases, the less water makes into the deeper regional flow systems and local flow systems extend deeper into the subsurface. They suggest that when hydrogeologists try to interpret regional flow problems, that we need to bear in mind the effects of decreasing K on the systems.

Knight, D.B. and Davis, R.E. 2009. Contribution of tropical cyclones to extreme rainfall events in the southeastern United States. J. Geophys. Res., 114, D23102, doi:10.1029/2009JD012511.

Knight and Davis used 25 years of observational, wind-corrected, and reanalysis data for the southeastern Atlantic coastal US states and found that extreme precipitation from tropical storms and hurricanes (TCs) has increased over the study period.  They find that this increase in TC contribution to extreme precipitation is a function of increasing storm wetness and frequency, but not storm duration. If TCs are producing more precipitation, their flood hazards are also increasing, and flooding is already the leading cause of deaths associated with TCs.

Meade, R.H. and Moody, J.A. 2009. Causes for the decline of suspended-sediment discharge in the Mississippi River system, 1940-2007. Hydrological Processes. 24, 35-49. doi:10.1002/hyp.7477

Dams on the Missouri and Upper Mississippi Rivers have been blamed for trapping almost 2/3 of the sediment that used to reach the Lower Mississippi and Delta.  Here, Meade and Moody show that the dams are only trapping half of the missing sediment, while engineering practices such as bank revetments and meander cutoffs, combined with better erosion control practices in the drainage basin, probably account for the rest. Meade and Moody suggest that this river system, in the largest basin in North America, has been transformed from transport-limited to supply-limited, which is a pretty amazing fundamental shift in the behavior of the river and its ability to deliver sediments to the Gulf of Mexico. [Note that there’s another article in the same issue on “A quarter century of declining suspended sediment fluxes in the Mississippi River and the effect of the 1993 flood.” Both articles are in the public domain and not subject to US copyright laws, though there doesn’t seem to be an obvious way to take advantage of that from the Wiley website.]

Neumann, R.B.,  Ashfaque, A.N,  Badruzzaman, A. B. M.,  Ali, M.A.,  Shoemaker, J.K., and Harvey, C.F. 2010. Anthropogenic influences on groundwater arsenic concentrations in Bangladesh, Nature Geoscience 3, 46-52. doi:10.1038/ngeo685

The story of groundwater of southeast Asia’s deltas, where tens of millions of people live at risk of arsenic poisoning from their drinking water, is perhaps the most compelling contemporary scientific story of how geology, geomorphology, hydrology, and humans intertwine. It’s also an extremely complicated story, with arsenic-laden sediment from the Himalayas settling in the deltas , irrigated rice fields and ponds  changing the local groundwater flow patterns, and microbially mediated oxidation of organic carbon driving the geochemical release of the arsenic into the groundwater. This story has been being pieced together in many papers in the last several years, and in this paper Neumann et al. show that groundwater recharge from the ponds, but not the rice fields, draws the organic carbon into the shallow aquifer, and then groundwater flow modified by pumping brings the carbon to the depths with the greatest dissolved arsenic concentrations. Add some biogeochemistry data, isotope tracing of source waters, incubation experiments, and 3-D flow modeling, and this paper adds some important elements to our understanding of how this public health risk came to be – and how we might be able to mitigate the risks for the people who have little choice but to drink the water from their local wells. [Also note that the same issue of Nature Geosciences has another article on “arsenic relase from paddy soils during monsoon flooding” as well as an editorial, commentary, backstory, and news and views piece on the southeast Asia arsenic problem.]

Pritchard, D., G. G. Roberts, N. J. White, and C. N. Richardson (2009), Uplift histories from river profiles, Geophys. Res. Lett., 36, L24301, doi:10.1029/2009GL040928.

In rivers that have adjusted to their tectonic and climatic regimes, the long profile of a river is smooth and concave. The interesting places are where river profiles don’t look like that ideal. This paper interprets river longitudinal profiles as a way to understand the tectonic uplift history of the area, through a non-linear equation. They check their interpretation against an independently constrained uplift history for a river in Angola.

Stone, R. 2009. Peril in the Pamirs. Science 326(5960): 1614-1617. doi: 10.1126/science.326.5960.1614

Dave Petley at Dave’s Landslide Blog has the must-read summary of this article on the risks associated with the giant lake impounded by the world’s tallest landslide dam. This is seriously fascinating stuff. I already talked a bit about the Usoi Dam in my dam-break floods spiel in my Fluvial Processes class, and now I have more ammunition for this year’s crop of students. In the same issue of Science, Stone also summarizes some of the other water issues facing Central Asia.

Please note that I can’t read the full article of AGU publications (including WRR, JGR, and GRL) until July 2010 or the print issue arrives in my institution’s library. Summaries of those articles are based on the abstract only. UNC Charlotte also does not have access to Nature Geoscience.

Megafloods from Glacial Lake Missoula

[Cross-posted at Highly Allochthonous]

If I had a time machine and could go back to any point in geologic history, as supposed in this month’s Accretionary Wedge call, the event I’d most like to see is the repeated flooding of the Pacific Northwest at the end of the last Ice Age. These “Missoula floods” are among the largest floods in Earth history and they irrevocably changed the topography of Washington and Oregon. My time machine would be an aircraft capable of flying with the floodwaters as they raced from Montana to the Pacific Ocean, and my time machine would also have LIDAR capabilities for collecting pre-, syn-, and post-flood digital measurements of topography and water surfaces and allowing an unparalleled determination of flood magnitudes and erosive volumes.

Our story begins shortly after the Last Glacial Maximum, deep in the heart of the Bitterroot Mountains in western Montana (Figure 1), as melt water from the waning glaciers began to pour into Clark Fork River valley and its tributaries. Water in the Clark Fork ponded up behind an enormous ice dam from a lobe of the Cordilleran Ice Sheet, and reached a maximum depth of 600 meters as Glacial Lake Missoula. The lake contained more than 2000 cubic kilometers of water, more than the modern volume of Lake Erie and Lake Ontario combined. Pedestrians in the modern day town of Missoula might notice a strange horizontal striping to the hillsides surrounding town (Figure 2)…these are the traces of the shorelines of the ancient lake.

Figure 1. USGS map of the Pacific Northwest between ~12 and 19 thousand years ago.

Figure 1. USGS map of the Pacific Northwest between ~12 and 19 thousand years ago.

Figure 2. Traces of Glacial Lake Missoula in Missoula, Montana, February 2009. Photo by the author.

Figure 2. Traces of Glacial Lake Missoula in Missoula, Montana, February 2009. Photo by the author.

After 19,000 years ago, water in Glacial Lake Missoula ruptured the ice dam, and the collected water went rushing downstream at speeds reaching 100 km/hr. Peak discharge in the Spokane Valley has been estimated at 17 +/- 3 million cubic meters per second, and drainage of the lake lasted several days.

Today, the Clark Fork River drains into the upper Columbia River and then into the Pacific Ocean. At the time of the Missoula Floods, the upper Columbia was buried under glaciers and that route was blocked to the floodwaters from Glacial Lake Missoula, so instead they were forced to take an overland route, carving new channels into the fertile Palouse loess deposits and underlying 17 Ma Columbia River basalts of eastern Washington. These channels are spectacular (Figures 3 and 4), up to 182 m deep and 32 km wide, with dry waterfalls, scoured potholes, and streamlined islands. Granite boulders the size of small cars were carried by the flood from the Idaho batholith and deposited in central Washington. Even for geologists, it can be hard to appreciate the full extent of these floods from the ground, but satellite photos (Figure 5) show the huge aerial extent of the erosion, covering a region we now call the Channeled Scablands.

Figure 3. Dry Falls and Grand Coulee near Coulee City, Washington, June 2004. The falls are 122 m high and 5.6 km wide. Photo by the author.

Figure 3. Dry Falls and Grand Coulee near Coulee City, Washington, June 2004. The falls are 122 m high and 5.6 km wide. Photo by the author.

Figure 4. Grand  Coulee downstream of Dry Falls as viewed from Lenore Caves, June 2004. Photo by the author.

Figure 4. Grand Coulee downstream of Dry Falls as viewed from Lenore Caves, June 2004. Photo by the author.

Figure 5. Landsat view of the Channeled Scablands region. This image was taken on August 31, 1972 and shows about 34,250 square kilometers of eastern Washington. The dark green features are the channels, the light green is the wheat-farmed Palouse region and the modern Columbia River is at the top right. Image retrieved from a<a href=Several times along their way across Washington, the floodwaters found basins in which to spread out and slow down. The fertile, wine-growing soils of the Yakima and Walla Walla valleys are slackwater flood deposits of loess from the Palouse valley. In a few special places, we can see the thick layers of silt, separated by thin horizons of soil (Figure 6). The number of layers preserved in these places help constrain the number of floods, and ash layers in the soils help constrain their timing. The best estimate is that there were more than 40 late Pleistocene megafloods that crossed eastern Washington, though not all of them may have come from from Glacial Lake Missoula itself. The floods began after 19 thousand years ago, many occur after 15 thousand years ago, and some post-date a Mount Saint Helens eruption 13 thousand years ago. More than 25 floods had peak discharges exceeding 1 million cubic meters per second.

Figure 6. Missoula flood deposits in the Walla Walley valley, Washington, June 2004. Photo by the author.

Figure 6. Missoula flood deposits in the Walla Walley valley, Washington, June 2004. Photo by the author.

Where the Columbia River turns east and runs along the modern-day border of Oregon and Washington, the Missoula Floods were once again confined to a single channel, at a place called Wallula Gap. As immense as the Columbia River Gorge is (Figure 7), this channel was a major constriction on the flood, only able to transmit 20% of the peak discharge. The waters back up behind the gap, flooding the Pasco Basin and creating temporary Lake Lewis around the tri-cities region of Washington. Floodwaters in the gorge were more than 215 m deep, as evidenced by the flood deposits found on surrounding ridgelines. The town of Lyle, Washington, in the Columbia River is built on a giant eddy bar left by the floods (Figure 7).

Figure 6. Columbia River Gorge, looking upstream from Rowena Crest, Oregon, June 2004. Photo by the author.

Figure 6. Columbia River Gorge, looking upstream from Rowena Crest, Oregon, June 2004. Photo by the author.

Figure 7. Lyle, Washington, as viewed from Rowena Crest, Orego. Photo by the author in June 2004.

Figure 7. Lyle, Washington, as viewed from Rowena Crest, Orego. Photo by the author in June 2004.

When the floods reached Portland, Oregon, the waters filled the Willamette Valley and turned into a lake 100 m deep, ~50 km wide, and 175 km long. Today the lake bottom is seen in the flat topography and fertile soils of the valley. Icebergs, probably remnants of the glacier that dammed Lake Missoula, were carried this far, because the valley is pocked by glacial erratics (Figure 8), rocks that had fallen onto or into the ice, were rafted downstream, and left behind when the icebergs melted. These rocks are not rounded, so it is clear that they did not as material moved by the flood itself.

Figure 8. Glacial erratic on the edge of the Willamette Valley in Erratic Rock State Park, Oregon. Photo taken May 2008 by <a href=Slowly the water would drain from the Willamette Valley and upstream lakes and empty completely into the Pacific Ocean through the Columbia River. Vegetation would encroach on the new sediments, and soil would begin to develop. In the meantime, another Glacial Lake Missoula would be forming behind a new ice dam on the Clark Fork River, getting ready to repeat the process again. Presumably floods of varying magnitudes continued until the glaciers had retreated north of the river. The last floods probably occurred less than 13 thousand years ago.

Floods over, measurements completed, my time machine and I would return to the present, ready to fill in the details of the floods that shaped the topography, soils, and agriculture of the Pacific Northwest. Even without a time machine, there’s so much more I could tell you about the Missoula Floods, but I’ve already gone on long enough. Instead I’ll refer you to the numerous scientific papers on the subject beginning with those of J. Harlan Bretz in the 1920s. When Bretz proposed a megaflood as the source of the Channeled Scablands, it flew in the face of the ruling uniformitarianism paradigm and his work was not accepted for several decades. The casual audience may want to check out the Ice Age Floods Institute website, an on-line USGS publication on the The Channeled Scablands of Eastern Washington, the book Cataclysms on the Columbia, and, if you can find it airing sometime, the excellent Oregon Public Broadcasting documentary Ice Age Floods. If you live in or are traveling to the Pacific Northwest, also keep an eye out for sites along the newly-approved Ice Age Floods National Geologic Trail.

AGU Abstract Submitted: Secular Streamflow Trends in Watersheds Receiving Mixed Rain and Snow, Pacific Coast and Cascades Ranges

The following abstract was submitted for the Fall AGU meeting:

Secular Streamflow Trends in Watersheds Receiving Mixed Rain and Snow, Pacific Coast and Cascades Ranges

A. Jefferson, University of North Carolina at Charlotte

Much existing research has focused on detecting climate change effects on snowmelt-dominated watersheds, but in the Pacific Coast and Cascades ranges, precipitation falls as either rain or snow, depending on latitude, elevation, and season. Watersheds often straddle the snow line, with some areas dominantly receiving rain and higher elevations accumulating seasonal snowpacks. These snowpacks are near the 0°C threshold, making them sensitive to the effects of climate warming. Climate sensitivity of seasonal and event hydrographs from watersheds with mixed rain and snow has not been fully explored. This project investigates detectable climate change signals in long-term streamflow records in the Washington, Oregon, and northern California Coast and Cascades Ranges.

Watersheds with mean elevations above the seasonal snow line show significant increases in streamflow during January through March and decreases in the percent of annual flow during April through June, the historical snowmelt period. These changes were not detectable in watersheds with mean elevations below the seasonal snow line. There were no consistent trends in peakflow dates or volumes. The multiple drivers of peakflow occurrence make it unlikely that any changes in peakflow timing will be detectable for several decades. Results suggest that Coast Range hydrology has been minimally impacted by historical climate warming, but that Cascades Range watersheds are already experiencing altered hydrologic regimes.

Pending acceptance, the work will be presented in session H32 Spatial and Temporal Trends in Hydrometeorological Records as Indicators of Climate Variability and Change.