Stuff we linked to on Twitter last week.

Posted on April 10, 2011 by Chris Rowan

Stuff we linked to on Twitter last week

A post by Chris RowanA post by Anne JeffersonWe’ve been away for a couple of weeks, but now we return you to your normal linky service.

Volcanoes

Earthquakes

(Paleo)climate

Flooding in Minnesota

For those following the on-going Red River flooding, the river appears to have crested yesterday in Fargo at 38.75 feet, about 0.75 feet lower than had been predicted. However, it is raining in the area now, and river levels will remain high for some time. 38.75 feet is the fourth highest crest on record, but 3 feet lower than the record flooding of 2009. Of course, the Red River is not the only Midwestern River above flood stage. There is moderate to major flooding now on the Minnesota, Mississippi, and St. Croix Rivers now as well. The links below are from several of those floods.

Water

Environmental

General Geology

Planets

Interesting Miscellaney

Categories: links

Floodwaters rising on the Red River

Posted on April 7, 2011 by Anne Jefferson

A post by Anne JeffersonFargo, North Dakota is coming out of its 3rd snowiest winter since 1885. Snow continued to fall into late March, and daytime temperatures have only been above freezing for few weeks. At night, it’s still below freezing, though starting tomorrow night the forecast calls for above freezing minimum temperatures. Soils are already saturated, and more rain is possible this weekend.

In short, it is perfect flood weather for the Red River that runs along the Minnesota-North Dakota border and into Canada. This is a place with the perfect geography for extensive flooding, and a long history of big spring floods.

Checking the water level on a bridge between Fargo and Moorhead. Photo from Minnesota Public Radio.

Checking the water level on a bridge between Fargo and Moorhead. Photo from Minnesota Public Radio.

Every town along the Red River has been devastated by a flood more than once. So they’ve all got emergency response plans in place for weather just like this. For example, Moorhead (Minnesota, across the river from Fargo) has a nifty GIS feature that shows how each foot of flood water affects each city block.

Residents are already filling sand-bags to build temporary levees. But with year after year of flooding, and with successful sand bag efforts the last two years, some residents might be taking this year’s flood predictions in a somewhat complacent fashion. But looking at the National Weather Service’s North Central River Forecast Center projections, there’s plenty of reason for concern all along the Red River.

As of 9 am Central time on 7 April 2011, most of the US portion of the Red River is already above flood stage, but water levels will continue to rise almost everywhere for at least the next week.

Flood stages as of 9 am 7 April 2011. Screen grab from NCRFC.

Current flood levels along the Red River and nearby drainages, as of 9 am, Thursday 7 April 2011. Orange circles indicate minor flooding, red indicates moderate flooding, purple indicates major flooding. Screenshot from the North Central River Forecast Center, using data supplied by the USGS.

The flood wave will move downstream – from south to north. In Wahpeton, a crest is expected today, with a second – equally high if not higher – crest next week. There the flood crest is likely to fall a few feet short of record water levels set in 1997.

Between Wahpeton and Fargo, tributaries to the Red River are having major flooding as well – in part because of backwater effects from the main river. If the Red River is flooding, there’s no place for water flowing down the tributaries to go. Instead they back up, causing even more widespread flooding.

In Fargo (ND) and Moorhead (MN) – which have a combined population of 200,000 people – the flood will not crest until late Sunday. Right now, the National Weather Service is predicting a crest of 39.5 feet, which 1.3 feet short of the record flooding of 2009. However, there some chance that the river will crest at 41 feet, or even higher if there is precipitation in the next few days. Currently, 80% of the city is protected by sand bags and levees to a height of 41 feet, but those may need to go even higher.

NWS Flood Forecast for Fargo, North Dakota (7 April 2011)

NWS Flood Forecast for Fargo, North Dakota (7 April 2011)

Two weeks ago, the National Weather Service issued a longer-term flood forecast for the Red River at Fargo. At that time they considered it a 10-50% percent chance that the river would reach 40 to 44.3 feet by mid-April. They provided a probability of exceedence curve for their modeled projections of this year’s flood season against the historical record of flooding, as shown below. To understand this graph, it helps to look at a few specific points. Right now, the river is at 35.32 feet. Based on the outlook from two weeks ago, it was virtually inevitable that the river would reach this level, with a probability greater than 98%, as shown by the black triangles. In contrast, 35.32 feet is reached less than 5% of the years in the historical record for Fargo, as shown by the blue circles. The current projected crest of 39.5 feet was given about a 50% chance of being exceeded as of two weeks ago, yet it has only be reached twice (1997, 2009) in 111 years of record. Two weeks ago, the National Weather Service was saying that there was a 25% chance the river could go above 42 feet, which is higher than the top of the sand bag levees now being prepared.

NWS Chance of exceeding river levels on the Red River at Fargo, conditional simulation based on current conditions as of March 24, 2011

NC River Forecast Center's 90 model showing the Red River at Fargo's chances of exceeding certain water levels, relative to the historical record.

The short term forecasts, like the one two above, have better skill than long term forecasts like the immediately above, but the long term forecasts are vital for emergency managers, city officials, and riverside land-owners in making early plans for the flood. The reason they’ve got all the sand and sand bags on hand in places like Fargo is because they knew there was a good chance a really big flood was coming. They’ve been talking about it since January.

Downstream (north) of Fargo-Moorhead lies Grand Forks, with about 100,000 people in its metropolitan area. Grand Forks was swamped by the flood of 1997, but the current forecasted peak stage this year is about 3.5 feet lower, though the crest won’t reach Grand Forks until late next week. For now, they are watching the water levels and making their preparations. Downstream further, lies Winnipeg, Manitoba. The flood crest won’t reach there until late April, but already the river is 17 feet above normal winter stage, and only 5 feet below the 2009 flood peak. Needless to say, they too are sand-bagging.

But for the next few days, the action focuses on the Fargo-Moorhead area. You can check out the updated data and forecasts or you can watch the flood play out in Moorhead with a live webcam pointed at the downtown waterfront:
Watch live video from 702 Flood Cam – Moorhead on Justin.tv

Categories: by Anne, geohazards, hydrology

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

Posted on April 4, 2011 by Anne Jefferson

A post by Anne JeffersonCommunities 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 (up now here), 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.

Categories: by Anne, geohazards, hydrology

How to (and how not to) talk about earthquake hazards in the media

Posted on March 22, 2011 by Chris Rowan

A post by Chris RowanThere’s often a fine line to be walked when you’re asked to talk about future earthquake risks in the media. People are looking for what scientists can’t provide: firm predictions of where the next big earthquake is coming from, and when. Meanwhile scientists have to downplay the hiding-to-nothing that is the specific prediction game and communicate what we do know: the areas that are most at risk from large and damaging earthquakes, so that people and emergency planners are not taken by surprise when they inevitably occur – be it next month or next decade. It’s tricky: talk up the dangers too much, and you might spark an unneccesary panic. Don’t go far enough, and you store up trouble for the future in the form of an unaware and unprepared populace.

As an example of how to do it right, read this excellent article on the CNN website by USGS seismologist Susan Hough. You should read the whole thing, but here are some key quotes:

Pointing to any one corner of the Earth as the location of the next Big One is not a winning game. Take a map of the world’s most active plate boundaries and throw a dart; where it lands is as good a guess as any.

…the next Big One might not be within the lifetime of any individual alive today but is very likely to occur within the lifetime of many of the buildings being constructed today.

What we know for sure is that preparedness remains our best defense against devastating earthquakes.

It’s perhaps no surprise that this particular seismologist is so good at talking about earthquake risks and predictions: she is, after all, the author of an excellent and highly readable book on the subject. However, if you want demonstration of just how easily one can go astray in this particular field, Simon Winchester, the author of a number of popular geoscience books, provides one with this rather less good article in Newsweek. It’s unpromisingly titled ‘The Scariest Earthquake Is Yet to Come’, but since titles are the first casualty of the editor’s red pen, I’ll overlook that. But following a few paragraphs describing the March 11th Japanese earthquake and tsunami – the standard “we are all helpless against the caprice of Nature” angle – we get this:

For this event cannot be viewed in isolation. There was a horrifically destructive Pacific earthquake in New Zealand on Feb. 22, and an even more violent magnitude-8.8 event in Chile almost exactly a year before. All three phenomena involved more or less the same family of circum-Pacific fault lines and plate boundaries—and though there is still no hard scientific evidence to explain why, there is little doubt now that earthquakes do tend to occur in clusters: a significant event on one side of a major tectonic plate is often—not invariably, but often enough to be noticeable—followed some weeks or months later by another on the plate’s far side…Now there have been catastrophic events at three corners of the Pacific Plate—one in the northwest, on Friday; one in the southwest, last month; one in the southeast, last year.

That leaves just one corner unaffected—the northeast. And the fault line in the northeast of the Pacific Plate is the San Andreas Fault, underpinning the city of San Francisco.

This assertion is problematic in a number of ways. Firstly, putting the magnitude 6.3 Christchurch earthquake – even last year’s magnitude 7 – in the same class as the events in Chile and Japan is wrong. The Christchurch quake released thousands of times less energy than either of those events, and was only so damaging due to its close proximity to Christchurch.

Secondly, the idea that megaquakes may beget other megaquakes is still at the ‘interesting speculation’ stage. In other words, there is still no ‘hard scientific evidence’ that there’s an effect to explain at all. Even if there is, it’s considerably more complicated than ‘look to the opposite side of the plate!’: when you’re considering the far-field effect of megaquakes, the other side of the planet would be just as susceptible as the other side of the plate. The state of stress in the crust is a far more important factor than any particular relative location.

Thirdly, the San Andreas Fault is probably not physically capable of producing much more than a magnitude 8 earthquake. The reason subduction thrusts are the source of such large earthquakes is because of their shallow dip, which results in a large area of the fault being above the depth where rocks become too warm and weak to rupture brittlely in earthquakes. Because the San Andreas Fault is almost vertical, the area that can potentially rupture is smaller, and the upper limit on the size of earthquake it can produce is also smaller. An earthquake on the San Andreas, which is on land, also wouldn’t produce a tsunami, which was what caused the real damage in Japan.

This isn’t to say a magnitude 8 earthquake isn’t a very serious future hazard for California. But to argue that it would be more ‘scary’ than what we witnessed a couple of weeks ago is pushing it a bit. To argue that this horror is imminent is borderline irresponsible – there is no scientific basis for stating the risk of a ‘Big One’ in California is any greater than it was a month ago. The same is true of the arguably much more scary Cascadia subduction zone to the north – which can potentially produce a magnitude 9 earthquake, and will produce a tsnuami when it does so. We know that both of these faults will rupture at some point in the future, and people need to be aware of that. But claiming we’re in some period of extra-special risk right now is, to put it bluntly, just making stuff up.

In conclusion: Susan Hough: take a bow. Simon Winchester: don’t. In any sense of the word.

Categories: earthquakes, geohazards, public science, ranting

New at Erratics – the largest meteorite crater in the world

Posted on March 22, 2011 by Chris Rowan

One of my regrets from my time in South Africa was that I didn’t have the chance to go and visit the Vredefort dome – at 250-300 km wide, the world’s largest known impact crater, formed 2 billion years ago and still a visible scar on the landscape. The pseudotachylites – formed when rocks are stressed so much and so fast that they actually melt, cooling to form glass – are meant to be quite something.

Vredefort pseudotachylites. Source: http://www.vdome.co.za/

However, as Simon Wellings explains in his latest post at our Earth Science Erratics guest blog, the clinching evidence for the Vredefort dome being formed by an impact did not come from aerial photos, but from under the microscope. In thin sections of rocks from the centre of the structure, one can find signs of the growth of new minerals that can only be explained by the instantaneous removal of 9 km of overlying rock. 9 km!. That’s almost a third of the thickness of normal continental crust!

Check out Simon’s post for the full story. I need to go and un-blow my mind.

Remember: Earth Science Erratics welcomes contributions from anyone who is tempted to dip their toes into the geoblogging waters, for one post or several, or from new bloggers who want to promote their work through cross-posting. If you’re interested, please contact us.

Categories: links, planets, Proterozoic, structures