Flooding Flashcards
Rain that falls is collected in what?
Drainage basins.
The point where rainwater flows in opposite directions is the _____.
Drainage divide.
Stream gradient refers to what? How is grade measured?
Steepness.
Measured by the ratio of drop in elevation of a stream per unit of horizontal distance.
Changes in gradient result in what?
Deposition of heavier materials.
Streams all eventually reach a lake or ocean, the base level. What is a base level?
Level below which stream cannot erode.
What is the difference between graded and ungraded streams?
Ungraded: down cutting, smoothing irregular gradient. Rapids, waterfalls, flat segments.
Graded: static conditions, no erosion or deposition, close to base level.
What is the calculation for stream discharge (Q)? What does ‘Q’ mean?
Q = Velocity x area
Q = flow rate at any point in stream.
Q (discharge) varies with what four things?
Amount of rain/snow melt.
Size of drainage basin.
Input from groundwater.
Loss by evaporation (& irrigation).
What is stream load?
Total mass of material moved by a stream at one time.
What are the three types of stream load?
Bedload: materials that move along the stream bed (pebbles).
Suspended load: smaller particles suspended in the water (clay).
Dissolved load: ions in the water.
What is stream capacity? What is it greater than, and propotional to?
Total amount of sediment a stream can transport.
Capacity is greater than the load.
Capacity is proportional to discharge.
What is stream competency? What is it proportional to?
Maximum size of particles a stream can transport at one time.
Proportional to velocity and gradient.
List three features of a meandering stream.
Relatively low gradient.
Sweep from side to side.
Diverted by irregularities.
List four features of bedrock streams.
Fast moving.
High energy.
Steep mountainous areas.
Stream bed is rock.
Where do braided streams occur? Describe their discharge and banks.
Dry areas with episodic precipitation.
Highly variable discharge and easily eroded banks.
When the amount of water entering a stream exceeds the capacity of the channel, what occurs?
Overflow of banks, inundation of floodplain.
During flooding, what increases?
Velocity, discharge, competency, capacity.
When the level of the stream is the height of the river bank, this is known as _____.
Bankfull.
Grain size a stream can carry is proportional to _____.
Velocity.
As a stream spills over its floodplain it undergoes what channel transition?
Deep, fast to shallow, slow.
What material drops out once the stream over-banks? What does this build?
Heavier material.
Natural levees.
For the following recurrence intervals (in years), list the probabilities:
- 2
- 5
- 10
- 25
- 50
- 100
- 200
- 500
2: 50%
5: 20%
10: 10%
25: 4%
50: 2%
100: 1%
200: 0.5%
500: 0.2%
Urbanization results in what two things?
Decreases amount of permeable surface.
Water forced to run off rapidly, without time to soak in.
Why is it dangerous to drive a car in a flood?
Most cars weigh less than volume of water required to fill them.
