Stream and Flood Hazards Flashcards
What are the two basic role of streams and rivers (larger
streams) within the Earth system.
drain
water off the landscape and to transport
sediment.
A stream may overflow its banks when the its
ability to carry water is overwhelmed by the
sheer volume of water flowing off the
landscape and create what is known as a
_______.
flood
It is 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
It is a process where water flows through
stream channels.
Runoff
It is a process of water movement when precipitation reaching the land
surface moves downslope in thin sheets.
Overland Flow
These are two processes of water movement which allows water to return back
to the atmosphere.
Evaporation and plant transpiration (water
uptake by root systems and then released
through plant leaves)
It is described as the 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
_____________________ provide information about a
river or stream by simply plotting the discharge versus
time.
Stream hydrographs
It is 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, often called _________________. 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 stream.
______________ 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
A ____________ is a network of stream channels where merging tributaries (smaller of any two merging channels) form progressively larger streams.
drainage system
This 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 is called the ___________, whereas the ____________ is found in the lower part of the system where a river empties into an ocean, lake, or another river.
headwaters, mouth
True or False: Headwater streams are generally small and relatively fast
moving and occupy narrow valleys, but then evolve toward the mouth into gently flowing rivers that occupy
wider valleys.
True
Individual systems are separated from one another by a topographic high or crest in the landscape called a
_____________
drainage divide
Drainage divides are useful for mapping what hydrologists call a _____________ or watershed, which
represents the land area that collects water for an individual stream or river.
drainage basin
It is the largest river due to its average discharge at mouth of 7,750,000 ft³/sec.
Amazon
Among the top 10 largest rivers, this river has the highest annual sediment load (1,670,000,000 tons).
Ganges
Give some information about the Mindanao River Basin. (where it traverses, and what is the land area)
• 5 Provinces (Bukidnon, Maguindanao, North Cotabato, South Cotabato, Sultan Kudarat)
• 3 Regions: (X, XII, ARMM)
• 2 Cities: (Cotabato, and Koronadal)
• Land area: 21,503 km² (2nd largest river basin in the Philippines)
These 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 (the other one is Shreve)
__________________ reflects downstream trade off between discharge
and slope in setting transport capacity (and thus ability to move
sediment and incise rock).
Longitudinal profile
The level below which a river or stream cannot incise.
Base level
The __________________________ for most streams is global sea level.
ultimate (or absolute) base level
True or False: Steep terrain tends to be poorly
dissected and thus have a low
drainage density.
False (highly, high)
Formula for Drainage Density
Drainage density = total stream channel length / area of the drainage basin
True or False:Gentle terrain tends to be less
well dissected, with substantial
hillslope convexities and low
drainage density
True
Enumerate all the DRAINAGE PATTERNS
Dendritic
Parallel
Trellis
Rectangular
Radial
Annular
Multibasinal
Contorted
A drainage patter most commonly formed on horizontally bedded and uniform sediments or on uniformly resistant crystalline rocks.
Dendritic
A drainage pattern that 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 patterns that are most common 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
A drainage pattern which usually have a more or less perpendicular turns mainly caused by criss-crossing
fractures.
Rectangular
A drainage pattern that occurs around domes or cones, and particularly common on volcanic areas.
Radial
Drainage patterns that 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
A drainage pattern that 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
A drainage pattern that incised into rocks with complicated structural
patterns. Associated with crystalline metamorphic rocks with a history of intense folding, jointing, intrusions, alterations, and faulting.
Contorted
Types of stream according to whether they are water-holding or not.
• Perennial
• Intermittent
• Ephemeral
Types of river (large stream) according to its linear form.
• Straight
• Meandering
• Braided
It is the section of the river that is defined as either a gently or steeply sloping channel form. They can be
either smooth or irregular in shape
Cross-profile or “transverse”
It is the section of the river that is defined as either a graded or interrupted profile, with waterfalls and
lakes.
Length-profile or “longitudinal”
These 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
True or False: When water enters a meander bend it is forced to slow down on the inner part of the bend, but speeds up on the outer part.
True
___________ 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
Crescent-shaped bars, called _____________, develop on the inside of meander bends where water velocity decreases.
point bars
__________ are mound-shaped channel deposits consisting of sorted material ranging in size from boulders to coarse gravel to fine sand.
Bars
True or False: Sand-sized deposits are generally found near the headwaters of a drainage system, whereas boulders are more common toward the mouth.
False: Boulders - headwaters, Sand-sized or finer - toward the mouth
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
__________ form during periods of high stream discharge
when the water column develops a swirling motion
called an _________.
Potholes, eddy current
The velocity of a particular stream segment is controlled
by the steepness of the channel, called ___________—also referred to as grade.
stream
gradient
As a river flows toward the ocean it may
encounter a series of ___________________ that form
when its ability to cut downward is reduced by a
resistance rock body, lake, or inland sea.
temporary base levels
_____________ 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 process whereby water separates sediment grains based
on their size, shape, and density is called __________________.
hydraulic sorting
_________ are 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
Deltas
_____________ are large fan-shaped deposits that form where steep mountain streams empty out onto valley floors at the mouth of rivers
Alluvial fans
The result is a channel that is choked with sediment called a ____________, which migrates back and forth across the entrance to the valley, creating a characteristic fan-shaped deposit.
braided stream
Streams also transport considerable amounts of dissolved ions (charged atoms) in what scientists refer to as the ______________.
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
_______________ 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.
Back swamps
____________ are old floodplain left high and dry as river migrates. Because they lie above the new flood plain, they are less likely to be inundated during a flood. It is not surprising then that humans have long made use of these geologic features as safe locations for building settlements and developing
agriculture.
Stream terraces
It is the area across which the river is prone to move.
Channel migration zone
A _________ is when normally dry areas of
the land become inundated
flood
The most common way a flood occurs is when excessive amounts of ______________ cause discharge to increase to the point that a river channel is no longer able to contain its flow, hence it overflows its banks
overland flow
Scientists use ____________ and
_________ to compare floods in a
quantitative manner.
stream discharge, height
______________ is the height at which a river begins to overflow its banks.
Flood stage
Formula for Discharge
Discharge = cross-sectional area of a river × water velocity
Based on the maximum yearly discharge, a ____________________ can be calculated for each value, which represents the frequency a particular discharge value can be expected to repeat itself.
recurrence interval
Another useful way of measuring flood frequency is percent probability, which is simply the inverse or reciprocal of the recurrence interval (1/RI). For example, a 200-year flood, what is the probability of it taking place in any given year?
1/200 = 0.005 × 100% = 0.5%
_____________ tell us how often we can expect floods of a certain size, whereas ____________ indicate their chance of occurring.
Recurrence intervals, percent probabilities
Calculate the Recurrence Interval of the specific discharge of 60,700 ft³/s. Based on a 77-year historical discharge record, this specific discharge is ranked as 2nd (M=2).
Recurrence Interval = (N+1)/M
= (77 + 1)/2
= 39 years
Three factors that affect Flooding
Nature of Precipitation Event
Ground conditions
Vegetation Cover
The ability of the ground to absorb water, referred to as ____________, plays a critical role in flooding because water that is unable to infiltrate is generally forced to move as overland flow.
infiltration capacity
The actual rate at which water can infiltrate is determined by the ______ of the land surface, ____________________, and _______________ of the material.
slope, type of ground material, moisture content
• steeper slope = lesser infiltration and more overland flow
• gravel and sand-sized ground material = high infiltration
• low moisture content (dry soil) = high infiltration or water absorption
These are floods that have short duration with relatively high peak discharge; small streams and rivers tend to rapidly overflow
their banks. These floods generally affect only localized areas, they are particularly dangerous due to their
sudden nature, which means people have less time to evacuate.
Flash floods
Because small channels are more
abundant in the upper parts of a basin, flash floods are also referred to as ______________.
upstream floods
True or False: Flashflood can occur in downstream areas.
True: Flash flooding can occur in downstream areas along small tributaries that flow into large rivers
This type of flood 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. This flood is caused by regional accumulations of water higher up in the drainage basin.
Downstream flood
Land-Use Factors that Affect Flooding
Removal of natural vegetation
Destruction Wetlands
Construction Activity
Urbanization
It is described as where
excessive dislodged sediment is moved off the landscape into drainage systems which causes channels to become filled with sediment, thereby
reducing its capacity to carry water and subsequently increase the frequency and severity of flooding.
sediment pollution
Wetlands (swamps) are commonly found in
topographic depressions and adjacent to river channels, in which case they are called ________________.
riparian wetlands
Because of their ____________, wetlands have a great capacity to capture and store water moving across the landscape. The destruction of
wetlands has resulted in an increase in flooding as these areas no longer store water.
porous nature
Large pipes called _________ are typically used for small streams that flow intermittently. However, the
amount of discharge able to flow through this is limited by the diameter of the pipe. Thereby, causing upstream areas to become flooded as these are not large enough to handle the large volume of water during flood events.
culverts
True or False: Urbanization can cause increased flood heights, and it also leads to longer lag times between precipitation events and peak discharge
False: shorter lag time
(The amount of time for water from precipitation to move across
the landscape and into channels is reduced (hence shorter lag time) due to impermeable surfaces caused by urbanization (i.e. Obrero’s roads and parking lots composed of concrete and asphalt). Ephemeral Obrero River XD
Flood Mitigation Measures
Dams
Artificial Levees
Channelization
Retention Basins
Erosion Controls
Flood Proofing
Flood Plain Management
Education
These structures benefit
society by protecting it against floods and serving as an important
source of freshwater and electrical power.
Dams
(Heavy or prolonged rains sometimes cause dams to
reach its maximum capacity. When this occurs engineers may be
forced to release water at such a high rate that it causes
downstream flooding, which, ironically, is what the dam is
supposed to prevent). kaysa naman mag possible failure ug mag overflow ang dam no kung pasagdan lang na mapuno, mas lala nuon hehe
These structures are those built by humans for the purpose of keeping a river from overflowing its banks and inundating its floodplain. Most of these
are constructed of earthen materials, but large concrete panels called floodwalls are sometimes used in urban areas.
artificial levees
True or False: Artificial levees as a mitigating measure reduces the frequency of flooding but ironically increases the risk of flooding in upstream areas.
True
This involves straightening and deepening a stream channel so that its discharge capacity is increased.
Channelization
These temporarily stores some the excesses water in a
series of depressions which are constructed within the
tributary network.
Retention Basins
This mitigating measures is best for places in urbanized area specifically adjacent to parking lots and within residential housing
developments in order to capture overland flow from
paved surfaces.
Retention Basins
This involves practices that tend to keep soil particles in
place so as to minimize the amount of material able to
move downslope (ex. use of vegetation). The other
approach is to use some type of physical barrier to trap
sediment before it can enter the drainage network.
Erosion Controls
_____________ are a type of barrier in which
vegetated strips line the banks of stream channels,
trapping sediment before it can enter the drainage
network. These are generally required in areas
where the land has been cleared by logging or
agriculture.
Stream buffers
Another common barrier system employs
temporary ___________, which are made of a synthetic
fabric that is fine enough to trap sediment, but yet allows
some water to pass.
silt fences
The last type of barrier system involves the use of ____________, which are ponds constructed for the purpose of
trapping any sediment that makes its way into a
drainage system. However, heavy equipment must
periodically be brought in to dig up the accumulated
sediment and haul it away.
silt basins
It involves raising the building above the expected flood level
Flood Proofing
It involves providing federally subsidized flood insurance to
property owners. This is sometimes called floodplain zoning as it works in a similar manner to local zoning ordinances, where commercial, industrial, and residential buildings are restricted to certain areas or zones within a community.
Flood Plain Management
This is the very cost-effective means
of reducing the number of fatalities and property damage caused by flooding
Education
What type of Flood Susceptibility of this description: Area likely to experience flood heights of greater than 2 meters and/or flood duration of more than 3 days. These areas are immediately flooded during heavy rains of several hours; include landforms of topographic lows such as active river channels, abandoned river channels and area along river banks; also prone to flashfloods.
Very High Flood Susceptibility
What type of Flood Susceptibility of this description: Area likely to experience flood heights of greater 1 up to 2 meters and/or flood duration of more than 3 days. These areas are immediately flooded during heavy rains of several hours; include landforms of topographic lows such as active river channels, abandoned river channels and area along river banks; also prone to flashflood.
High Flood Susceptibility
What type of Flood Susceptibility of this description: Area likely to experience flood heights of greater 0.5 up to 1 meters and/or flood duration of 1 to 3 days. These areas are subject to widespread inundation during prolonged and extensive rainfall over extreme weather condition.
Moderate Flood Susceptibility
What type of Flood Susceptibility of this description: Areas likely to experience flood heights of 0.5 meter or less and/or duration of less than 1 day. These areas include low hills and gentle slopes. They also have sparse to moderate drainage density.
Low Flood Susceptibility
