Chapter 6 Flooding-Geology

Understand how cut banks and point bars are made.

-Cut banks: The fast-moving water erodes the riverbank on the outside of the bend to form a steep or near-vertical slope. -Point Bars: Slower water on the inside of a meander bend deposits sand and sometimes gravel.

What are the benefits of using channelization to control floods?

-Decreasing Flooding -Improving Navigation

What does the term "100 year flood" mean in terms of actual years (not what chance there is every year of having one of these)?

-on average every 100 years, you have a 1% chance that it will happen every year.

What were the two main causes of the flooding of New Orleans due to Hurricane Katrina?

1) Levees failed 2) Houses weren't build strong enough

What is the major difference between an upstream and a downstream flood?

The way each flood is caused is completely different. Upstream Floods are caused by intense rainfall of short duration over a relatively small area. Downstream Floods are caused by storms of long duration that saturate the solid and produce increased runoff.

Understand and be able to define the meaning of the terms drainage basin and watershed. (They are the same thing.)

is land surface and waters (rivers and streams) that drain into a particular body of water such as a river, a lake, a wetland, or an ocean. The Basin acts like a funnel.

What is a delta and where do they form?

-A triangular or irregularly shaped deposit, if it extends into a larger body of water - It forms at the mouth of a river.

What are the four processes that combine to cause river flooding?

-Amount and distribution of precipitation in drainage basin. -Rate at which the precipitation soaks into earth. -How quickly surface runoff reaches river. -Amount of moisture in the soil.

What is a floodplain and how is it made?

-Area bordering a river channel that has the potential to flood; it is made by flooding.

Understand the causes of flash floods and which are natural and which are human induced.

-Caused by intense rainfall of short duration over a relatively small area. -Both natural and human induced

Define discharge of a stream. How are the discharge (Q), water velocity (V), and area of the stream cross section (A) related? Be able to calculate that relationship.

-Discharge: is the volume of water moving through a cross section of a river per unit time. Cross section is the sideways view of the river that you would have if you took a huge knife and sliced across the valley from top to bottom at right angles to each riverbank. (figure 6.10; P.169). -Relationship: Discharge=Cross-Sectional Area * Velocity of Flow. Or Q=V*A

What are the drawbacks of using channelization to control floods?

-Drainage affects plants/animals -cutting trees=no shade destroys habitats -changes diversity of flow patterns for aquatic life -degrades the aesthic

What are the natural service functions of flooding?

-Fertile Lands -Periodic deposits of minerals enriches the soil for agriculture -Aquatic ecosystems -Floods clear rivers of debris(anything that shouldn't be in the water) and sweep in nutrients -Sediment supply -Periodic flooding builds up elevation; example: New Orleans

Define gradient and, using a labeled longitudinal profile, be able to calculate a simple one.

-Gradient is the slope of a river. It is shown on a longitudinal profile. Gradient is rise over run.

What are the headwaters of a stream/river? Define base level.

-Headwater of a stream/river is the steep at high elevation. Base level is lowest elevation of river.

Primary Effects of Floods

-Injury and loss of life -Damage caused by currents, debris and sediment to farms, homes, buildings, railroads, bridges, and roads. -Erosion and deposition of sediment related to loss of soil and vegetation

Upstream Flood

-Known as a Flash Flood -Caused by intense rainfall of short duration over a relatively small area. -Common in an environment with steep slopes or little vegetation and following breaks of dams, levees and ice jams. -More people who die during flash floods are in cars

how do human impact dynamic equilibrium of a stream?

-Land use changes can affect that equilibrium -Forest to farming creates more erosion and sediment -Sediment will build up the gradient of the stream -Stream will flow faster until it can carry greater amount of sediment -Increases magnitude and frequency of floods -Urban areas have impervious cover and greater storm sewers -Carries water to stream channels more quickly causes flashy discharge-rapid rise and fall of flood water -Bridges block debris creating dams and flash flooding

What is a natural levee and how is it made?

-Natural Levees are formed by the river in its process of flooding -A natural levee is formed by a deposit of sand or mud built up along, and sloping away from, either side of the flood plain of a river or stream. This is done by the action of the water itself. The process occurs slowly over a number of year.

What are the largest "pools" for water and what are some of the important "processes" that cycle water between the pools?

-Ocean is the largest. Transpiration and evaporation

Understand the linkages of flooding with other natural hazards.

-Primary effect of Hurricanes -Flooding due to Hurricane Sandy -Secondary effect of earthquakes and landslides -Fires -Produce shorts in electrical circuits and erode and break natural gas mains -Cause of coastal erosion

What is the dynamic equilibrium of a stream?

-Rivers generally maintain a dynamic equilibrium -Balance between gradient, cross sectional shape and flow velocity for sediment load -That is, increase or decrease in the amount of water or sediment received by a stream changes gradient or cross-sectional shape, changing the velocity.

Secondary Effects of Floods

-Short-term river pollution of rivers -Hunger and Disease -Homelessness

Which part of the sediment load that is carried by a stream becomes part of a natural levee?

-Silt and Clay are part of the sediment load that is carried by a stream which becomes part of a natural levee.

Where is a stream's overall energy the highest - on a slope, or on flat ground?

-Slope, gravity is being pulled down on it.

What is an alluvial fan and where do they form?

-The River commonly forms a fan-shaped deposit on land. -formed where a stream leaves a canyon and expands outward into Death Valley.

In what type of climate do most natural flash floods occur?

-They are generally produced by intense rainfall of short duration over a relatively small area.

What factor other than riverbank overflow is going to cause flooding in the near future in Bangladesh?

-They are not above sea level, every year they go up a little bit. Soon sea level will reach Bangladesh and flood them.

Why do we build human-made levees?

-We build them to keep water contained

In what two significant ways does urbanization increase the magnitude and frequency of floods?

-anything that is paved -channelization

Downstream Flood

-cover a wide area -usually produced by storms of long duration that saturate the solid and produce increased runoff -can be caused by combined runoff from thousands of tributary streams

How does flood hazard mapping contribute to reducing the effects of flooding on humans?

-helps us know where the 20 year flood and 100 year flood in planning construction

Define flood stage.

Elevation of water surface that is likely to cause damage to property