Streams and Floods (Ch 10) Flashcards
Running Water
the most important geologic agent in eroding,
transporting and depositing sediment
Hydrologic Cycle
the movement and
interchange of water between the sea, air, and
land
evaporation
solar radiation provides energy
precipitation
rain or snow
transpiration
evaporation from plants
runoff
water flowing over land surface
infilitration
water soaking into the ground
Stream
a body of running water, confined to
a channel, that runs downhill under the
influence of gravity
headwaters
upper part of stream near its
source in the mountains
mouth
place where a stream enters sea,
lake or larger stream
channel
a long, narrow depression eroded
by a stream into rock or sediment
stream banks
sides of channel
streambed
bottom of the channel
floodplain
flat valley floor composed of
sediment deposited by the stream
drainage basin
the total area
drained by a stream and its tributaries
tributary
a small stream
flowing into a larger one
divide
ridge or high ground that
divides one drainage basin from
another
continental divide
separates
the streams that flow into the
Pacific from those that flow into
the Atlantic and Gulf of Mexico
drainage pattern
the arrangement, in map
view, of a stream and its tributaries; most tributaries join the main stream at an acute
angle, forming a V or Y pointing downstream
dendritic
drainage pattern resembling the branches
of a tree
radial pattern
streams diverge outward like the
spokes of a wheel, such as on conical mountains
rectangular pattern
tributaries have frequent 90°
bends and join other streams at right angles
trellis pattern
parallel streams with short tributaries
meeting at right angles
stream erosion
Controlled by flow velocity and discharge
stream velocity
controlled by stream
gradient (slope), channel shape and channel
roughness; maximum velocity near center
of channel
discharge
volume of water passing a
particular point in a stream over time (Q = W * D * V)
_______ stream velocities promote erosion
and transport of coarser sediments
higher; erosion of very small particles difficult due to
molecular binding forces
stream gradient
the downhill slope of the
streambed; typically measured in feet/mile in the U.S., and in
meters/kilometer elsewhere, and usually decreases downstream
channel shape and roughness
effect stream velocity due to drag, narrower, deeper channels allow faster flow, smoother channels allow faster flow, wider, shallower channels decrease flow speed, and rougher channels decrease flow speed
hydraulic action
ability to pick up and
move rock and sediment
solution
dissolving of rocks
abrasion
grinding away of stream channel
by the friction and impact of the sediment
load
potholes
are eroded into streambed by the
abrasive action of the sediment load in the
stream
bed load
large or heavy particles that travel
on the streambed
traction load
large particles that travel along
the streambed by rolling, sliding or dragging
saltation load
medium particles that travel by
bouncing along
suspended load
small/light sediment that
remains above the stream bottom by turbulent
flow for an indefinite period of time
dissolved load
dissolved ions produced by
chemical weathering of soluble minerals
upstream
bars
ridges of sediment deposited in the
middle or along the sides of a stream
braided streams
contain sediment
deposited as numerous bars around
which water flows in highly
interconnected rivulets
delta
body of sediment deposited at the mouth of a river when flow velocity decreases; surface marked by shifting distributary channels; shape of a delta depends on whether its wave-dominated, tidedominated, or stream-dominated
meandering streams
flow faster along
the outside of bends and more slowly along
the inside, depositing point bars on the
insides of the meanders
meander cutoffs
may form when a new,
shorter channel is cut through the narrow
neck of a meander (as during a flood)
floodplains
are broad strips of land built
up by sedimentation on either side of a
stream channel
floodplain sediments
are left behind as
flood waters slow and recede at the end of
flood events
natural levees
main channel has slightly raised banks
with respect to the floodplain
downcutting
process of deepening a
valley by erosion of the streambed
v-shaped valleys
typically form
from downcutting combined with
mass wasting and sheet erosion
base level
the level which streams cannot erode below
graded streams
have concave-up longitudinal profile, lack rapids and waterfalls, represent a balance between available sediment load and transport capacity
lateral erosion
widens stream valleys
by undercutting of stream banks and valley
walls as stream swings from side to side
across the valley floor
headward erosion
the slow uphill
growth of a valley above its original source
by gullying, mass wasting, and sheet erosion
stream terraces
step-like landforms
found above a stream and its floodplain; occurs when river rapidly cuts
downward into its own floodplain, represents relatively sudden change
from deposition to erosion, and can be caused by rapid uplift, drops
in base level, or climate changes
incised meanders
retain sinuous pattern as they cut vertically downward; may be produced by profound base level changes, as when rapid tectonic uplift occurs
alluvial fan
large, fan- or cone-shaped pile of sediment that
forms where stream velocity decreases as it emerges from a
narrow mountain canyon onto a flat plain; well-developed in desert regions, such as the southwestern U.S., and larger fans show grading from large sediments nearest the
mountains to finer sediments farther away
flooding
Overbank flow condition, discharge
greater than channel’s holding capacity
stage
The height of the water level in a river at a
given location at a given time
hydrograph
Graphic representation of a river’s
discharge over time
lag time
The amount of time between the
occurrence of peak rainfall and the onset of
flooding
flash flood
local, sudden floods of large
volume and short duration; typically triggered by heavy
thunderstorms
recurrence interval
R = (N + 1)/M
N as the number of years of record, M is the rank
of individual flow within the recorded years; The probability of a given magnitude flood
P = 1/R; Statistical probability versus reality
Probability: One 25-year flood, on average, once
every 25 years
Reality: Two 25-year floods within the same year
dams
designed to trap flood waters in
reservoirs upstream and release it
gradually over time
artificial levees
designed to increase
capacity of river channel and works well
until stream overtops levees, leading to
extremely rapid flooding and erosion
wise land-use planning
including
prevention of building within 100-year
floodplains, is most effective
factors affecting flood damage
Regional land-use changes, such as urban development, deforestation, soil erosion, etc. Land use on the floodplain Frequency and magnitude of flooding Lag time and duration of flooding Sediment load Effectiveness of forecasting, warning, and emergency management
adjustments to flood hazards
The structural approach: -Engineering barriers: Levee augmentation -Channelization -River-channel restoration Flood insurance Flood-proofing Floodplain regulation: Obtaining the most beneficial use of floodplains -Flood-hazard mapping -Floodplain zoning
