Lecture 6 - Flooding hazards and mitigation

Lecture outline:


Some examples and statistics

Flooding in Nebraska, some examples:

1951 Missouri Flood at Kansas City - source USGS - http://ks.water.usgs.gov/waterwatch/flood/fld51.photos.html#HDR04

This is a satellite image of the Missouri River as it is more typically. You can find Omaha in the lower center with the Platte River joining the Missouri just to the south. Compare this with the image below to get an idea of the extent of flooding. Image source: http://eros.usgs.gov/views-news?page=6

This satellite image showing a similar area as above shows the extent of flooding (dark blue areas). Note how the bend with Eppley airport and Carter Lake, appears to almost act like a dam. One can argue that flood control structures here made the flooding up stream worse. Image source: http://eros.usgs.gov/views-news?page=6 .

Examples of better known flood events in the Midwest

1965 South Platte, near Denver, Colorado:

1972 Rapid Creek, Rapid City, South Dakota:

2013 Flooding along Front Range area of Colorado:

Flooding in the U.S.
            USGS overview of historic floods in the U.S. - http://ks.water.usgs.gov/pubs/fact-sheets/fs.024-00.html
            Great USA flood of 1993 - http://www.nwrfc.noaa.gov/floods/papers/oh_2/great.htm

Take away: flood events are both common enough and damaging enough they are high risk events. The fact that people like to live along rivers and infrastructure, such as roads. often follows rivers creates larger numbers for the cost factor.


What events can cause floods?

Floods are polygenetic:

Primary challenge during flood events – where to put the water?


Control/management Structures/strategies

Great variety of practices:

Flood control structure on Brays Bayou near downtown Houston. Note the cement lined channel. This helps move water efficiently through this portion and hopefully prevents channel migration. The grassy slopes above are part of the marginal levee system. The path, like the Keystone trail does here in Omaha, provides recreational purposes and so the flood control structures can serve dual purpose.

The red arrow points to the height of the debris left by recent flooding along the river. The photos were taken in 2011. Because of enhanced runoff flood waters in situations like this can rise fairly fast.

Adjacent to the Brays Bayou channel you can see a storage basin with a cement spillway between the channel in the foreground and the basin in the background. When the waters reach the spillway height (which you will notice is a couple feet lower than the grassy levee height on either side) they spill over, and some of the flood water can be stored in this basin. A careful look shows a lower drain between the two, so that as the bayou flow goes back down the storage basin will naturally drain back into the channel, and there will be room for storing the next floods water. Constructed properly, storage basins can also have recreational or aesthetic value.

This is the levee that separates down town New Orleans from the Mississippi river to the right. Even at normal flow, the Mississippi river level is higher than the street level on the other side of the levee. New Orleans also has a pumping system that during rain storms pumps the water up and over the levee to help it on its way to the Gulf.

An extensive array of levees attempts to keep the New Orleans area safe for flooding. Here we are on top of a levee with wetlands to the left and development to the right. The gray 'doors' attached to the cement portion can shut and close off the levee here during times of flooding, but kept open otherwise to let people drive through during dryer times. Why not just have the road go up and over the levee? This structure could also allow purposeful release of the water here to keep the 'pressure' of the system elsewhere (without having to dynamite the levee).

Cost of control structures is highly context dependent - some examples for Omaha.

Dams on Missouri River - source USGS - http://www.cerc.usgs.gov/rss/visualize/


Effect of dam on fluvial dynamics.

 

This diagram attempts to capture some of the changes that occur once a dam is put in place on a river.

Changes water flow regime downstream (decreases variability - that's why it was built in the first place):

Changes water chemistry and temperature downstream:

A cross section through Kirby Lake in Wisconsin showing the vertical distribution of dissolved oxygen levels, which is one manifestation of lake stratification. In addition to seeing how they decrease with depth, this diagram shows how communication with the atmosphere is a crucial source of the oxygen (note the ice seal at top with the whole in the one place), and how an aerator influences the distribution. Also note that there is a lot of vertical exaggeration in this image and that at its deepest this lake, which is about a mile wide, is some 16 feet. Image source: USGS Fact Sheet FS-066-98, Hydrology, Water Quality, and Phosphorus Loading of Kirby Lake, Barron County, Wisconsin, http://wi.water.usgs.gov/pubs/FS-066-98/ .

Loss of water from reservoir basin due to ground seepage:

Loss of water due to evaporation:

Changes sediment load downstream (decreases it), serving as a sediment trap.

Map of how much sediment has accumulated in Lake Olathe, in Kansas. Notice how the up river end is filling in the most. This work was done by the USGS. Source: Sediment Deposition and Selected Water-Quality Characteristics in Cedar Lake and Lake Olathe, Northeast Kansas, 2000http://ks.water.usgs.gov/pubs/reports/wrir.02-4073.html .

Produces a shore line environment (a riparian margin):

Produces a lake environment, potentially increasing the biodiversity.

Changes load on the crust, potentially producing reservoir-induced seismicity (not common, but occurs).

Changes sediment geochemistry – USGS study on Selenium in Republican River - http://ks.water.usgs.gov/pubs/fact-sheets/fs.080-98.html


Dam safety/failures

Importance of spillway: allows excess flood waters to bypass dam safely (without erosion or undercutting);

Oroville dam in California where excess waters have left the concrete spillway and are actively eroding the slope below the dam in the spring of 2017. Over 188,000 people downstream were evacuated in case the dam ended up failing. Fortunately it did not. Image source and more information at https://en.wikipedia.org/wiki/2017_California_floods .

Dam within a dam -> the importance of the grout curtain.

Examples of failures are not as rare as we would like.

Breakout floods:

Atchafalaya - a different sort of challenge and control structure - keeping a river in place.

Flood videos