Flood prevention

Flooding is a natural process, an annually recurring player in the shaping and re-shaping of Earth’s surface.

Floods are really only a problem when they intersect human activity, so the most effective flood prevention program is one that removes humans from the path of potential floods and bans activities that exacerbate flooding.

It would seem obvious that no one should live in the path of potential flooding, on a river flood plain or a coast vulnerable to storm surge, but flood plains and coastal beaches are desirable real estate.

Engineering solutions like dikes and levees may help stem flooding in the short term, but geologists view coasts and rivers as parts of dynamic systems, and time has shown that alterations to one part of the system will cause a response in another part of the system, often unanticipated and unintended.

The Watershed protection and flood prevention Act, originally enacted in 1954 was designed to address these issues: http://www.nrcs.usda.gov/Programs/watershed/pl56631705.pdf

Predicting flood paths

A team of geologists and physicists are modeling flooding in the laboratory to help predict the size and the route of floods from overflowing rivers.

The team was prompted to study the problem in the aftermath of the August 18, 2008 flood of the Kosi River in northern India. This flood displaced more than a million people. The scientists noticed that after overflowing its banks, the flood waters followed older, abandoned channels.

By constructing a scale model of a river-delta system in the lab, the researchers were able to determine that the initial formation of a river channel is random, but once channels formed, flooding was restricted to the previously formed channels rather than cutting new ones.

The scientists compared river flooding to earthquakes: just as earthquakes occur along pre-existing faults, flooding occurs along pre-established channels.

From March 19, 2010 post at http://www.terradaily.com/reports/Catastrophic_Flooding_May_Be_More_Predictable_999.html

Based on current issue of Geophysical Research Letters, Reitz, Meredith, et al.

Photo credit: http://www.geo.uu.nl/fg/palaeogeography/researchprogram

Transgressions of a different sort

Before fossils were understood to be the actual remains of once-living plants and animals, they were regarded in some quarters as tricks of the devil, planted by the fallen one to test the fidelity of the faithful.

After all, fossils of animals that clearly lived in the ocean were found far from any modern ocean (like those fossil clams in Nebraska mentioned in an earlier post) and even on the tops of mountains.

At a time when the Earth and everything in it was understood to be unchanged from its original creation, fossils of marine invertebrates on mountain tops were problematic, and a divinely sent global flood was an explanation that conformed both to the observations and to the philosophical framework of the time.

We now understand the dynamic nature of the Earth and can explain these observations in terms of mechanisms like plate tectonics and glaciation.

Illustration: The last major transgression of the sea over North America formed a seaway that reached from the Gulf of Mexico to the Arctic during the Cretaceous Period (144-65 million years ago). From http://www.emporia.edu/earthsci/student/mcgee2/hipln005.jpg

What about Noah?

The greatest flood story ever told is recorded in religious texts, a story of world-wide inundation precipitated, if you will, by a God angry at his faithless people.

The purpose of the story is to teach a moral lesson, not a meteorological one, but many such stories are rooted in experience. The half-meter rise in sea-level that resulted from the discharge of water from Glacial Lake Agassiz doesn’t sound like much, but in low-lying coastal areas the effect could have moved shorelines inland up to 10 kilometers.

The resulting flood would have forced ancient peoples to flee their coastal settlements, and lacking any knowledge of the glacial lake responsible for the floodwaters, the people created a story that helped explain both how and why the flood occurred.

Source: Teller, J.T., and Leverington, D.W., Outbursts from Lake Agassiz and their possible impact on coastal environments. Environmental Catastrophes and recoveries in the Holocene, August 29-Sept. 2, 2002, Brunel University, Uxbridge, UK. Also mentioned in a 2002 piece in Science News by Sid Perkins.

Photo credit: http://www.maritimequest.com/misc_ships/noahs_ark_3000bc/noahs_ark_4.jpg

Flooding on a continental scale

Why are there fossils of marine animals found in the sedimentary rocks of Nebraska, (or any other place far from today’s oceans where marine fossils are found)?

These fossils are evidence that ancient seas once covered the continents, leaving in their wake, the sediments and fossils deposited in the sea. Throughout Earth’s history, sea-level has risen and fallen, in response to glacial periods and changes in the rate of plate tectonic movement.

During glacial periods, water evaporated from the oceans falls as snow and is tied up in ice sheets on land, causing sea-level to fall—a regression in geologic terms. Melting ice returns water to the oceans, causing sea-level rise, and water from the deep ocean basins surrounding the continents laps up over the continents in what geologists term a transgression.

Illustration: North America during the Ordovician Period (about 440 million years ago). State boundaries are lightly dotted in; the light blue represents sea water overlapping--transgressing--the continent from the deep ocean basin (dark blue) From http://jan.ucc.nau.edu/~rcb7/namO450.jpg

Future floods?

The Earth is much more heavily populated today than it was when the great Ice Age floods occurred. If floods of the magnitude that resulted from the draining of Lake Agassiz or carving the Scablands were to happen today, the impact on humanity would be catastrophic.

Fortunately, the risk of huge, glacially induced floods like those of the Pleistocene is negligible. The mega floods of the Pleistocene were associated with continental ice sheets. Today, the only remaining continental ice sheets are in Greenland and Antarctica, and neither land mass contains large ice-dammed lakes.

Iceland’s volcanism can produce sub-glacial lakes that could cause flooding there, but not on the scale of the megafloods of the Pleistocene.

That isn’t to say that Earth is free of flood hazards; if global warming were to melt the continental ice sheets sea-level would rise 215 feet or 65 meters* but that’s another story.

* http://www.usatoday.com/weather/resources/askjack/2004-11-21-melting-polar-ice_x.htm

Image: World map showing sea level if all current ice were to melt http://www.johnstonsarchive.net/spaceart/earthicefree.jpg

Lake Agassiz

During the last ice age, a 700-mile long by 200-mile wide lake centered on the province of Manitoba covered central Canada, dammed by glacial ice.

Named Glacial Lake Agassiz, for the famous Swiss scientist and later Harvard professor Louis Agassiz, who was among the first to recognize landforms that indicated that vast continental ice sheets once covered the continents, the lake drained catastrophically when the ice dam collapsed.

Water tied up in glaciers and in glacial lakes during the last ice age caused sea-level to drop more than 100 meters. At its maximum extent, Glacial Lake Agassiz was the largest body of fresh water on the planet, and held more water than all the worlds lakes today. When the climate warmed and the ice dam was breeched, the resulting flood raised global sea levels by half a meter.

More info: http://www.cloudnet.com/~edrbsass/agassiz.htm

Source: Perkins, Sid, Once Upon a Lake, Science News November 2, 2002 v. 162:p. 283

Illustration credit: Maximum extent of Glacial Lake Agassiz,

http://mrbdc.mnsu.edu/mnbasin/fact_sheets/valley_formation.html

Channeled Scablands

Geologists long recognized that something was odd about the landscape of eastern Washington state. The landscape is marked by dry canyons and deeply eroded basalt flows—the channeled scablands. In other areas, parallel ridges of undulating low hills cover the bedrock.

In the 1920s geologist J Harlen Bretz interpreted these features as the result of erosion and deposition from floods originating with the breach of glacially dammed lakes—the ripple-shaped hills (shown in the aerial view photo) were exactly that—giant ripples deposited by enormous outpouring of water from Glacial Lake Missoula.

The channeled scablands formed around 15,000 years ago after an ice dam holding back the waters of glacial Lake Missoula failed, sending 2,000 cubic kilometers of water rushing through the breach and reshaping the landscape in a matter of days.

Further info: http://www.nps.gov/history/history/online_books/geology/publications/inf/72-2/intro.htm

Photo credit: http://hypography.com/forums/earth-science/12606-missoula-floods-channeled-scablands-drumheller-channels.html

Before the Chunnel

Half a million years ago England and France shared a land connection that also formed the dam for a glacial lake that covered much of northern Europe.

The soft chalk bedrock dam was breached by the waters of the glacial lake between 450 and 180 thousand years ago, forming the Strait of Dover. As with the torrent that breached the Straits of Gibraltar and filled the Mediterranean, modeling indicates that waters rushed through the Dover Strait at rates up to 1 million cubic meters per second.

The evidence for this rush of water comes from sonar imaging of the submerged valley carved by the flood waters. Broad grooves up to 100 meters wide and 15 km long were carved into the valley floor. These features indicate high flow rates, and are associated with other mega floods.

Source: Birth of an Island: Megaflood severed Europe from Britain. SN July 21, 2007, v. 172, p. 35; based on Gupta, S., et al., 2007. Nature July 19

Illustration credit: http://islesproject.com/2009/01/12/450000bce-200000bce-the-origins-of-island-consciousness-the-torrent-that-created-the-english-channel/

Real flood geology

Imagine a dry basin where the Mediterranean now sits. 5.6 million years ago the Mediterranean was closed off from the Atlantic Ocean by tectonic uplift, and its waters gradually evaporated, leaving behind tell-tale deposits of evaporate minerals and creating what geologists call the Messinian salinity crisis.

About 5 million years ago Atlantic waters breached the barrier at the Strait of Gibraltar creating an enormous flood. Recent modeling indicated that water may have poured into the Mediterranean basin at a rate 1,000 times the flow of today’s Amazon River, filling the basin in less than two years—a geological blink of an eye.

Source: Daniel Garcia-CAstellanos, et al, December 10, 2009 Nature.

Illustration credit: http://records.viu.ca/~earles/messinian-crisis-apr03.htm