Rivers and flooding

From GeoClasses

Contents 1 Basics about water on Earth: 1.1 Hydrologic Cycle 2 Basics about Rivers: 2.1 how does all this work? 2.2 rivers are open systems (material moves in and out) 2.3 effect of a dam on the river: 2.4 Channel patterns (shapes in rivers) 2.4.1 1. braided channel 2.4.2 2. meandering channel 3 Flooding 3.1 1. upstream flood 3.2 2. downstream flood 3.3 Human Land Use has increased magnitude and frequency of floods 3.4 Flood Hazards 3.5 Human Solutions 3.6 Regulations on flooding

Basics about water on Earth:

Hydrologic Cycle

• includes atmosphere, rivers, lakes, oceans, groundwater, ice and snow
• moves by evaporation, precipitation, infiltration and runoff (= rivers)

• region grained by single river or river system is called drainage basin (or watershed)
  • this is where recharge occurs

Basics about Rivers:

• streams flow within channels

the Danube River between Hungary and Slovakia

Gambia River, a meandering river

Victoria Falls

• rivers flow down a slope until they reach a base level where they no longer erode into rock (usually sea level)

case study: Victoria Falls

• rivers flow fastest where the slope is steepest = in mountains (usually where they begin)
  • where flow is faster in channel, river carves a deep valley with steep sides; where flow is slower, river widens, is shallow and has a wide floodplain (see fig. 7.7)
  • erosion is greatest where flow is fastest

• in addition to water, rivers carry sediment (called the load)
  • 1. gravel and rocks, sand, dirt that rolls along bottom (called bed load)
  • 2. silt and clay floating in water column (called suspended load)
  • 3. dissolved load = dissolved chemicals carried in water
    • commonly HCO3-, SO4--, Ca++, Na+, Mg+
    • if [Ca++] + [Mg+] is high then water is "hard"

how does all this work?

stream profile

• Q = W*D*V
  • Q = discharge (volume moving per unit time - m3/s)
  • W = width of river - m
  • D = depth of river - m
  • V = velocity of water - m/s
• if Q is constant (no water is coming in or leaving), then:
  • if W*D (cross sectional area of river) decreases, V increases
  • if W*D increases, V decreases
    • works like a garden hose, if you block the flow with your finger (W*D decreases) then spray (V) increases
    • examples: where river flows into large still body of water (lake or ocean), it creates a delta because V decreases
  • for both alluvial fans and deltas, rivers split into distributary channels - not one main channel - and channels change position frequently especially during flooding (unpredictable)

rivers are open systems (material moves in and out)

• but remain roughly in dynamic equilibrium (sediment transported ~ load imposed (input from tributaries and runoff)
  • if change in amount of water or sediment in river then channel slope or cross sectional area changes = changes velocity = increases or decreases amount of sediment it can carry
  • ex: clear a forest for agriculture = more erosion into streams
    • excess sediment is dumped at first, which increases slope and makes river flow faster
    • then erosion increases to erode what was dumped, slope returns

effect of a dam on the river:

• upstream water fills the reservoir, slows flow, causes deposition
• downstream water coming out base of dam has no sediment, can hold more sediment = erosion of channel, slope decreases until equilibrium regained

Image and Text from NASA Visible Earth:

" Not more than 80 years ago the mighty Colorado River flowed unhindered from northern Colorado through Utah, the Grand Canyon, Arizona, and Mexico before pouring out into the Gulf of California. But as one can see in this image of the Colorado River Delta taken on September 8, 2000, by the Spaceborne Thermal Emission and Reflection Radiometer (ASTER), flying aboard the Terra spacecraft, irrigation and urban sprawl now prevent the river from reaching its final destination.

The Colorado River can be seen in dark blue at the topmost central part of this image. The river comes to an end just south of the multicolored patchwork of farmlands in the northwestern corner of the image and then fans out at the base of the Sierra de Juarez Mountains. A hundred years ago the river would have cut through this entire picture and plowed straight through to the Gulf of California, the mouth of which can be seen in solid blue at the lower righthand corner of the image. Nearly all the water that flows into the Colorado River is now siphoned off for use in crop irrigation and for residential use. In fact, roughly only 10 percent of all the water that flows into the Colorado makes it into Mexico and most of that is used by the Mexican people for farming.

The bluish purple river that appears to be flowing from the Gulf of California to the north is actually an inlet that formed in the bed of the Colorado River after it receded. The island at the entrance to the Gulf of California is the Isle Montague. The gray areas surrounding this inlet and the gulf itself are mud flats created by sediments once carried by the river. The Hoover Dam built in 1935 and the Glen Canyon dam built in 1956 now trap most of the river's sediments long before they find their way to the gulf.

As to the other features on the image, the flat yellow expanse to the east of the farms is the Gran Desirto. Between the farmland and the desert one can see a dark blue pool covered with patches of green. Known as Sienega de Santa Clara, this salt-water marsh formed by return irrigation is home to a huge population of birds, including the endangered Yuma Clapper Rail and the Southwestern Willow Flycatcher. The white patches to the southeast of this swampy area are salt packs that separate the marsh from the near lifeless salt lake extending east."

Channel patterns (shapes in rivers)

1. braided channel

• numerous bars and islands (gravel) that divide and reunite channel
• occur in coarse sediments with steep slopes, high velocity
• wide and shallow channels
• common in areas currently uplifting or where glacial melt waters bring coarse sediments

2. meandering channel

• large bends like a snake
• on the outside of the bend (cut bank), water moves faster = erosion
• on inside of bend, water moves slower = deposition (forms point bars)
• process continues to make more exaggerated S until cuts off a bend to make oxbow lakes
• river can jump bank to make a new meander and come back again within the floodplain

Flooding

• height of river at any time is called a stage
  • flood stage = high water that causes damage to property (a relative term)

1. upstream flood

• occurs in upper parts of drainage basin
• produced by excessive rainfall
• will not cause flooding when small streams join larger rivers

2. downstream flood

• ex: Mississippi River, 1993; Red River, 1997
• heavy rain on frozen ground so no infiltration
• flood generally covers a large area
• produced by long term storms so ground becomes saturated
• extra water from each tributary = large flood downstream - see fig. 7.18b

Human Land Use has increased magnitude and frequency of floods

• 1. more impervious cover (cement)
• 2. more storm sewers = drain runoff quicker than natural into streams
• 3. (fig 7.21) larger floods are less frequent - but size of flood in increased by greater development (impervious)
• 4. lag time is decreased (time between high rainfall and flood events) due to urbanization
  • less water infiltrates so very low flow in river (no groundwater input) during dry years = concentrates pollutants

From NASA Visible Earth:

"Scientists are using a major advance in satellite-based land surface mapping to create more accurate and detailed maps of our cities. These maps provide urban planners with a better understanding of city growth and how rainfall runoff over paved surfaces impact regional water quality.

These space-based maps of buildings and paved surfaces, such as roads and parking lots, which are impervious to water, can indicate where large amounts of storm water runs off. Concentrated runoff leads to erosion and elevated discharge of soil and chemicals into rivers, streams, and ground water.

Andrew Smith, a faculty research assistant at the Mid-Atlantic Regional Earth Science Applications Center produced a map of the Washington-Baltimore area that quantifies how much impervious surface there is across the entire region. Baltimore and the counties that border it have at least 20 percent, and up to 40 percent, impervious surface area, indicating that pollution from runoff could be a problem. The District of Columbia and surrounding watersheds in Virginia and Maryland have levels of impervious surfaces between 20 percent and 30 percent percent. Areas between and beyond the Baltimore-Washington corridor are more "green" with levels that range from 0 percent to 20 percent impervious surface areas.

The image above shows the extent of impervious surfaces in and around Washington and Baltimore. Red represents high concentrations of impervious surfaces. Blue represents moderate concentrations and green represents low concentrations of impervious surfaces. The base image was acquired by NASA's Landsat satellite, while the map of impervious surfaces was derived with data from both Landsat and Space Imaging's high-resolution IKONOS satellite."

Flood Hazards

• short term - caused immediately by water
• long term - erosion, loss of soil/vegetation, pollution of river, hunger/disease, electrical or gas line fires

Human Solutions

• 1. physical barriers
  • levees, floodwalls - built of earth or cement (cement ones built to release water at a slower rate later)
  • but people then feel safe and develop on floodplain which negates the benefit

• 2. channelization

• • straightening, widening, deepening, or lining channels
  • supposed to control floods, drain wetlands, control erosion
  • results in loss of habitat for plants and animals, cutting trees (eliminates shade on river and increases erosion), straightening changes flow pattern and eliminates breeding areas

• 3. channel restoration
  • cleaning up urban waste, allowing stream to flow freely, and returning trees to banks (for shade and decreasing erosion)
  • if lateral erosion of banks must be prevented, then can use riprap on outside bends

Regulations on flooding

• to receive federal flood insurance, community must follow regulations on use of floodplains
  • 1994 National Flood Insurance Reform Act - even more stringent rules
    • raising foundations about flood level
    • using floodwalls
    • waterproofing construction
    • improved drainage with pumps
  • in severe cases, US government has begun relocating people off floodplain (buying property)