Flooding Flashcards
The basic role of streams and rivers (larger streams) within the Earth system
to drain water off the landscape and to transport sediment
When a bank’s ability to carry water is overwhelmed by the
sheer volume of water flowing off the landscape, what happens?
Flood
True or False: humans tend to concentrate their settlements along waterways.
True
Described as the volume of water moving through a channel over a given time interval, commonly measured in units such as cubic feet per second (ft3/s)
Stream Discharge
a process where water flows through stream channels
Discharge
discharge of groundwater into the surface environment; fairly continuous unlike the sporadic input of water to a stream and groundwater may travel anywhere from a few days to thousands of years before discharging into a stream channel
Groundwater baseflow
What processes allows water to return back to the atmosphere?
Evaporation and plant transpiration
They provide information about a river or stream by simply plotting the discharge versus time.
Stream hydrographs
The amount of time for water to move across the landscape and into channels; will vary depending on the distance between where the rain is falling and the particular channel where discharge is being measured.
Lag time
Continuous input of groundwater baseflow allows streams in many areas to keep flowing at some minimum level. This contribution of groundwater baseflow keeps streams from going dry between rain events, thus is critical in maintaining the health of stream ecosystems.
Baseflow conditions
High precipitation allows more water to infiltrate to the water table which causes the water table in humid regions to be higher than the streams channel, thereby forcing groundwater to flow into streams. Such streams are often referred as
Gaining streams
are a result of less deep infiltration in arid areas resulting in a water table that is below the level of most stream channels. Under these conditions, the water in the stream will flow into the groundwater system.
Losing streams
is a network of stream channels where merging tributaries (smaller of any two merging channels) form progressively larger streams
Drainage System
The term _____ is often applied to the larger stream that
serves as the principal channel within a drainage system
River
The upper portion of the drainage system
Headwaters
found in the lower part of the system where a river empties into an ocean, lake, or another river
Mouth
True or False: Headwater streams are generally large and relatively slow, moving and occupy narrow valleys, but then evolve
toward the mouth into gently flowing rivers that occupy wider valleys.
False, they are generally small and relatively fast.
Individual systems are separated from one another by a topographic high or crest in the landscape called
Drainage Divide
Also known as a watershed
Drainage basin
What is the largest river in the world in terms of discharge?
Amazon river (7.8 million cubic feet per second)
All of the water within this basin will drain down to a
stream then flow out of the basin
Drainage basin
True or false: drainage basins are categorized by their size.
False, size does not matter
are any smaller streams that feed larger streams within a
drainage basin
Tributaries
A method of classifying or ordering the hierarchy of natural channels
Strahler Stream Order
It reflects downstream trade off between discharge and slope in setting transport capacity (and thus ability to move sediment and incise rock).
Stream Longitudinal Profile
The level below which a river or stream cannot incise. It is the limiting level below which a stream cannot erode the land.
Base level
The ultimate (or absolute) base level for most streams
Global sea level
approximately equals the inverse of the source basin length
Drainage density
The formula for drainage density
D = L/A
D = Drainage Density
L = Total stream channel length
A = Area of the Drainage Basin
Steep terrain tends to be highly dissected and thus have
High drainage density
Gentle terrain tends to be less well dissected, with substantial hillslope convexities and has
Low drainage density
An example of high drainage density but small source areas
Mancos Shale Badlands, Utah
Rill field, Mt. Pinatubo, Philippines
common on gently sloping convex
Low drainage density, large source areas-
Types of drainage patterns
Dendritic
Parallel
Trellis
Rectangular
Radial
Annular
Multibasinal
Contorted
Most commonly formed on horizontally bedded and uniform sediments or on uniformly resistant crystalline rocks.
Dendritic drainage pattern
Usually develops on moderate to steep slopes, but also where regional structure, such as outcropping resistant rock bands, are elongated and parallel. All forms of transition can occur this type and dendritic and trellis pattern.
Parallel drainage pattern
Patterns most commonly on dipping or folded sedimentary or weakly metamorphosed sedimentary rocks; also areas of joints and faults which intersect with right angles and old sand dunes with parallel alignment
Trellis drainage pattern
Usually have a more or less perpendicular turns mainly
caused by criss-crossing fractures.
Rectangular drainage pattern
Occurs around domes or cones, and particularly common on
volcanic areas.
Radial drainage pattern
Patterns also develop around domes, where there exists alternating resistant and weak beds, so that the major channels cut through the strike and the low order streams follow the dip of the rocks.
Annular drainage pattern
Can occur in a variety of conditions where local hummocks and depressions inhibit a continuous channel network. Irregular glacial deposits or erosional hollows due to solution of underlying lithology, irregular thawing permafrost. Coastal dunes, delta plains, and wind eroded hollows
Multibasinal drainage pattern
Incised into rocks with complicated structural patterns. Associated with crystalline metamorphic rocks with a history of intense folding, jointing, intrusions, alterations, and faulting.
Contorted drainage pattern
Checkpoint: Types of streams can be divided and subdivided according to:
Whether they are water-holding or not, (perennial, intermittent, & ephemeral)
Linear form of the river, i.e. straight, meandering, & braiding
Cross-profile or “transverse” section of the river, i.e., a gently or steeply sloping channel form. They can be either smooth or irregular in shape
The length-profile or “longitudinal” section of the river, i.e., a graded or interrupted profile, with waterfalls and lakes
The genesis or evolution of the river caused by bedrock structure, main geologic structure and riverbed stability
are loose rock particles/sediments deposited on a stream
Alluvium
One of the key factors in a stream’s ability to erode the landscape is the
velocity of the water
When water enters a meander bend it is forced to (A)_________ on the inner part of the bend, but (B)_________ on the outer part.
(A) slow down, (B) speed up
are unstable overhang located at the outer bank which is produced by the velocity increase on the outer bank which subsequently enhances the ability of the water to cut (erode) into the bank.
Cutbanks
On the inner bank where velocity decreases, sediment tends to accumulate and form a deposit known as a
Point bar
What is the primary cause of erosion and undercutting on the outer bank of a bend in a stream channel, leading to the formation of an unstable overhang that eventually collapses?
Increased water velocity on the outer bank of the bend
is the vertical erosion of a river or stream channel, resulting in the deepening of the channel over time
Downcutting
Downcutting by streams is not performed by the water itself, but rather by the sediment that physically scrapes or wears away rock in a process called
Abrasion
These form during periods of high stream discharge when the water column develops a swirling motion called an eddy current.
Potholes
The velocity of a particular stream segment is controlled
by the steepness of the channel, called the
Stream gradient
used to describe the lowest level to which a stream can erode
Base level
is often referred to as ultimate base level
Sea level
forms when the ability of a river flow to cut downward is reduced by a resistance rock body, lake, or inland sea.
Temporary base level
The ability of running water to transport and deposit sediment is dependent on both the water velocity and the types of particles being transported.
Stream Transport and Deposition
describes the fraction of solid particles that is in a suspended state and moving at the same velocity as the water— suspended material is what makes streams appear muddy.
Suspended load
Consists of sediment particles that roll, bounce, or remain stationary on the streambed.
Bed load
The _____ particles to be removed from the bed load are the
smallest, least dense, and most angular.
A. First
B. Last
A. First
The process whereby water separates sediment grains based on their size, shape, and density is called
Hydraulic Sorting
The combination of hydraulic sorting and chemical
weathering eventually produces relatively pure deposits of ________________
Sand and Clay
are mound-shaped channel deposits consisting of sorted material ranging in size from boulders to coarse gravel to fine sand
Bars
Boulder deposits are generally found where?
near the headwaters of a drainage system
Crescent-shaped bars, called point bars, develop on the ______ of meander bends where water velocity decreases.
Inside
formed when a river enters a lake or ocean and splits into smaller channels and begins to deposit sediment due to a decrease in velocity
Delta
are large fan-shaped deposits that form where steep mountain streams empty out onto valley floors at the mouth of rivers
Alluvial fans
Streams also transport considerable amounts of dissolved ions (charged atoms) in what scientists refer to as
Dissolved load
The erosion that occurs along the outside of meander bends produces wider valleys over time, whereas deposition on the inner banks helps to build a flat plain on the valley floor called a
Natural floodplain
When rivers overflow their banks, the first area to be inundated is
what portion of the valley?
The flat portion of the valley.
What is the floodplain’s primary role?
to periodically store large volumes of water moving through a drainage basin
are pair of ridges that run parallel to the bank which are formed due to the deposition of sediment, commonly sand, at the edge of the bank
Natural Levees
are areas on the floodplain that are poorly drained and can remain wet long after a flood
Backswamps
are old floodplain left high and dry as river migrates
Stream terraces
True or False: Well-defined terraces often develop in response to relatively rapid changes in base level resulting from tectonic uplift or lowering of sea level.
True
Natural levees are typically found along river channels that have ___________ floodplains.
Well-defined floodplains
Meanders tend to grow as the flow erodes the banks, favoring
development of _________________.
Meandering Channels
Scientists use ________________ and __________ to compare floods in a quantitative manner.
Stream discharge and height
is the height at which a river begins to overflow its banks.
Flood Stage
computed by multiplying the cross-sectional area of a river by its water velocity
Discharge
Floods are particularly difficult to predict because they are
dependent on specific weather events
represents the frequency a particular discharge value can be expected to repeat itself. It tells us how often we can expect floods of a certain size.
recurrence interval
Another useful way of measuring flood frequency is _____________, which is simply the inverse or reciprocal of the recurrence interval (1/RI).
Percent probability
The reliability of recurrence intervals is dependent on the availability of __________________________.
historical discharge records
The potential for flooding in a given area naturally increases as the _________ and __________ of rainfall increases.
intensity and duration
The ability of the ground to absorb water is referred to as
infiltration capacity
are floods that have short duration with relatively high peak discharge; small streams and rivers tend to rapidly overflow their banks.
Flash Floods
can be defined as one where a river leaves its channel farther down in its drainage basin, flowing out onto its floodplain and inundating large areas of the valley floor. They are caused by regional accumulations of water higher up in the drainage basin.
Downstream floods
Land-Use Factors that Affect Flooding
- Removal of natural vegetation
- Destruction of Wetlands
- Construction Activity
- Urbanization
A phenomenon where excessive dislodged sediment is moved off the landscape into drainage systems which causes channels to become filled with sediment
Sediment pollution
Flood Mitigation Measures
Dams
Artificial Levees
Channelization
Retention Basins
Erosion Controls
Flood Proofing
Flood Plain Management
Education
