Module 9 Flashcards
rills and gullies
A rill is a shallow channel in some soil, created by the erosion of flowing water. Rills can generally be easily removed by tilling the soil. When rills get large enough that they cannot easily be removed, they’re known as gullies.
Vegetation makes soil more ______
porous
If infiltration capacity exceeds precipitation intensity you…
DON’T PRODUCE OVERLAND FLOW
-REVERSE IS ALSO TRUE
the greater the slope angle, the ______ the magnitude of the shear stress.
greater
Only in ______ flow does erosion occur
turbulent
shear stress is resisted by the ________ of the soil
shear strength
the presence of a vegetation cover and it’s associated root mat serves to increase the _______ of the soil
cohesion
when shear stress (τ) exerted by overland flow exceeds the shear strength (S) of the soil, erosion will occur and ______ begin to form
rills
Hydrological Cycle Diagram: Name main parts
- Precipitation
- Overland Flow
- Stream Runoff
- Underground Water flow
- Groundwater recharge
- Baseflow
- Evaporation
- Evapotranspiration
- Condensation
Overland Flow(2) and Ground Water Flow(2)
- Hortonian Overland Flow: describes the tendency of water to flow horizontally across land surfaces when rainfall has exceeded infiltration capacity and depression storage capacity.
- Saturated overland flow: When the soil is saturated and the depression storage filled, and rain continues to fall, the rainfall will immediately produce surface runoff.
- creates fresh water springs
Overland Flow(2) and Ground Water Flow(2)
- Hortonian Overland Flow: describes the tendency of water to flow horizontally across land surfaces when rainfall has exceeded infiltration capacity and depression storage capacity.
- Saturated overland flow: When the soil is saturated and the depression storage filled, and rain continues to fall, the rainfall will immediately produce surface runoff.
- creates fresh water springs
1.Throughflow:
is the lateral unsaturated flow of water in the soil zone, where a highly permeable geologic unit overlays a less permeable geologic unit, and which returns to the surface, as return flow, prior to entering a stream or groundwater.
2.Baseflow:
The portion of stream flow that is not runoff and results from seepage of water from the ground(groundwater) into a channel slowly over time. The primary source of running water in a stream
during dry weather.
freshet
spring thaw and all the freed up water
freshet
spring thaw and all the freed up water
-Snowmelt means LARGEST INFLUX/DISCHARGE OF WATER(freshet)
hydrograph
changes in discharge in a river over time, recorded by a stream gauge, are plotted as a hydrograph
hydrograph
changes in discharge in a river over time, recorded by a stream gauge, are plotted as a hydrograph
Y AXIS: Is generally discharge
X AXIS: Time period(this case annual)
-ONLY WORKS FOR NATURAL OPEN RIVERS WITHOUT DAMS
- 4 sources are rainfall, snow melt, glacier melt, and groundwater
- discharge is commonly greatest during the spring and early summer when snow and glacier melt is greateest, also rain during the autumn
3 Flow Regimes
1.Ephemeral streams - river channels are usually dry but may carry water during and immediately after rainfall events:
Hortonian overland flow contributes to flash floods
2.Intermittent streams - river channels are dry for part of the year, but carry water at other times: supported by base flow when the groundwater table is sufficiently high
3.Perennial streams - river channels carry water throughout the year
The first 2 flow regimes predominate in arid and semi-arid regions, and in landscapes underlain by karst.
We will focus our attention on perennial streams.
flow of water in river channels involves deformation(____)
strain
ratio between shear stress and rate of strain, _____, indicates how resistent a fluid is to deformation
viscosity
low viscosity fluids (e.g., water) will deform more readily in response to an applied stress than high viscosity fluids (e.g., debris flows)
2 types of stream flow conditions based on viscosity:
1.laminar flow - water molecules flow in straight flow paths
internal friction within the fluid dominates the resisting forces
shallow, slow-moving,colder (high viscosity) water
2.turbulent flow - water molecules move continuously in all directions within the fluid
deeper, rapidly-moving, warmer (low viscosity) water
stream flow in natural channels is almost invariably turbulent
Froude Number:
Fr = v/√dg
v = flow velocity (m/s) g = acceleration due to gravity (m/s2) d = depth of flow (m)
is used to distinguish between different types of turbulent flows
-dimensionaless numbers:no units
3 flow conditions
subcritical (tranquil) flow - Fr < 1
-deep, sluggish flows
critical flow - Fr = 1
supercritical (rapid) flow - Fr > 1
shallow, rapid flows
Riffles
river/stream areas with rapid flow which looks like ruffle chips
Flow Velocity represents a balance between _________ responsible for stream flow and the energy consumed by resistance to stream flow (viscosity, friction)
flow velocity represents a balance between potential energy responsible for stream flow and the energy consumed by resistance to stream flow (viscosity, friction)
viscosity is influenced by _____ and ______
water temperature and suspended sediment concentration
Thalweg
Thalweg is the deepest part of a river channel where the fastest and strongest river flow is concentrated.
River Stage= River ______
depth
MathematicalEquations for relationship between velocity, gradient, and channel dimensions:
Manning equation
v = R0.66 s0.5/n
v = mean flow velocity R = hydraulic radius of the channel s = channel gradient n = coefficients of friction
Manning equation is the most common for use in EIA
R= ratio of cross sectional channel/wetted perimeter
Cross sectional channel= Width x Depth
Wetted Perimeter= 2d(depth)+w(width)
Units=square meters and meters
Discharge equation:
Q = wdv
Q = discharge of water (m3/s) w = width of stream channel (m) d = depth of stream channel (m) v = flow velocity (m/s)
amount of erosion and deposition of sediment depends stream discharge (Q)
discharge is the volume of water flowing through a cross section of a river per unit time (velocity x cross-sectional area = Q)
life force
drag force
water flowing in an open channel exerts forces against particles in the bed and banks of the channel:
lift force created by the acceleration of flow over particle surfaces
drag force - shear stress exerted by the flowing water
fluid force
the resultant of these two forces(drag and lift), the fluid force, is responsible for detaching particles from the bed and banks of the channel and entraining them in the flow.
the magnitude of the fluid force must exceed the strength of the materials present in the bed and banks of the stream channel for erosion to occur.
the magnitude of the _____ force must exceed the strength of the materials present in the bed and banks of the stream channel for erosion to occur.
fluid
Hjulström diagram
a diagram presenting current velocity vs. particle diameter which shows when transportation, deposition, or erosion is occuring
Transport
Once particles have been entrained into the flow they are transported in various ways downstream: 3
traction - particles are dragged or rolled along the bed of the channel
saltation - particles bounce across the bed of the channel
suspension - particles are suspended by turbulence above the bed of the channel
Transport
Once particles have been entrained into the flow they are transported in various ways downstream: 4
traction - particles are dragged or rolled along the bed of the channel
saltation - particles bounce across the bed of the channel
suspension - particles are suspended by turbulence above the bed of the channel
Solution-some of these materials are generated in hillslope environments and delivered to the stream channel via overland flow, through flow and base flow.
Stream Load define and describe 3 components
Definition: The total quantity of materials transported by a stream channel at a given discharge.
bedload - materials transported via traction and saltation: generally coarse sand, gravel, cobbles, etc.
suspended load - materials transported via suspension: generally clay, silt and fine sand
solution load - materials transported in solution
stream capacity
the maximum amount of sediment of a particular size that a stream can transport for a given discharge.
Suspended load is _____ in the winter and _____in the summer
qs is low in winter when the drainage basins are frozen and covered by snow
qs is high during floods initiated by snow melt and precipitation events in the spring and summer
Stream Power
Ω = ρgQs ~ τv (expressed in Watts)
ρ = density of water g = acceleration due to gravity Q = stream discharge s = stream gradient τ = shear stress v = mean flow velocity
the concept of stream power (Ω) was introduced to analyze the entrainment and transport of bedload materials:
Stream Competence
particle that a stream can transport for a given discharge.
according to the concept of stream power, whenever available stream power is greater than the power required to transport the stream load, _______ ______ will occur and vice versa.
channel degradation
Sediment is deposited when the current in the stream channel is no longer capable of transporting particles any further:
reduction in stream capacity
reduction in stream competence
Deposition of sediment within river channels contributes to channel aggradation.
3 conditions which promote deposition
decrease in stream discharge – relates to power
increase in the size of sediment supplied to the channel – relates to competence
increase in the quantity of sediment supplied to the channel – relates to capacity
matterial being transported by rivers is called
alluvium
bankfull discharge
width and depth of alluvial channels
reach
particular segment of a river between specified points
A reach is a river segment that has similar characteristics that allow it to be conveniently considered as a single entity.
Streams carrying coarse-grained sediment loads (i.e., bedload) tend to exhibit wide, shallow channels with steep stream gradients formed in non-cohesive materials.
=_______ river/stream
braided
rivers carrying fine-grained sediment loads (i.e., suspended load) tend to exhibit narrow, deep channels with gentle stream gradients formed in cohesive materials.
=_______ river/stream
meandering
When you look at reaches focus on the morphology and configuration of the channels within a specific reach and consider the following:3
- the number of channels
- the sinuosity of channels
- the stability of the channels over a given period of interest
Reaches are divided by number of channels into single-channel or multiple-channel segments.
Channel sinuosity is defined as the ratio between thalweg (main channel) length and valley length in a reach.
Channel stability is considered over a given period of interest, usually related to a specific problem: cohesive vs. non-cohesive bed and banks materials.
Braided reaches
Braided reaches consist of multiple, straight, transient, shifting channels.
braided channels carry gravel as bedload; also substantial proportion of granules and sand as seasonally variable bedload and suspended load, respectively
these materials are transported over steep channel gradients
the stream load tends to form longitudinal bars, as well as lateral bars
braided channels develop in environments marked by large variations in discharge, both seasonal and diurnal (i.e., in front of glaciers)
channels are highly transitory, lasting for hours to days, and are short and straight.
these channels are essentially unconfined laterally (i.e., unstable), forming a braidplain
longitudinal bars are related to _____ streams and are generally made of ______
braided
gravel
-develops from the convergence, increase in stream power, and divergence, decrease in power
flow convergence initiates bed scour and sediment entrainment, creating a pool
downstream, flow divergence initiates deposition leading to bar construction
lateral bars are associated with ______ streams and are generally made of ______
braided
sandy
.generally form allong channel banks
Meandering Reaches
these channels migrate back and forth within a floodplain
flow velocity is higher on the outside of meander bens, causing erosion; cutbanks develop here
flow velocity is lower on the inside of meander bends, causing deposition; point bars develop here
floodplains are created during overbank flows
avulsion occurs when new channels break through existing channel banks during flooding
during avulsion, a river abandons a looping section of the old channel, which is left as a oxbow lake
cutbanks and points bars develop from
flow velocity is higher on the outside of meander bens, causing erosion; cutbanks develop here
low velocity is lower on the inside of meander bends, causing deposition; point bars develop here
chute channels
in higher energy rivers transporting sand and gravel as bedload, point bars may be cut by chute channels
channels are cut across point bar surface during floods
abandoned chute channels exhibit lag deposits of coarse sand and gravel
lateral accretion
lateral accretion associated with lateral migration of meanders and point bars and creation of meander scrolls
meander scrolls consist of arcuate ridges of sand separated by shallow depressions known as swales
Meander neck
Meander neck: strip of land between to alluvial curves, when they connect the land is breached
oxbow lakes form..
oxbow lakes form when erosion of each cutbank allows the thalweg channel to cut through the narrow “neck” of land that separates the cutbanks, isolating the former meander bend.
over time, oxbow lakes are filled by fine-grained sediments delivered by overbank flows (i.e., floods), and organic materials derived from a succession of vegetation; forms meander scar
Levees and Crevasse Splay
levees are ridges or embankments of fine-grained sediments built up along the sides of an overtopped channel through overbank deposition during flooding; vertical accretion on floodplain
if a natural levee is breached, a crevasse is formed and river water pours downslope through the levee, forming a lobate crevasse splay sand deposit on the floodplain
not all rivers construct levees—large amounts of silt and clay are required
-River Levees: composed largely from sand and as the river decelerates more, more find grained material like silt and clay
A flood begins when a stream exceeds ______ ______
bankfull discharge
-the discharge at which water first flows out of the channel
______ floods are commonly much more destructive and deadly
downstream
Fraser River Flood Downstream Movement example
-1972 flood on the fraser river in souther BC.
-As the flood crest migrated downstream, its peak discharge and duration increased
.an example of “rain-on-snow” flood
Ice Jams and river floods
.some of the largest winter and spring floods in Canada are caused by ice jams
-initially results in flooding upstream of ice dam; subsequent sudden release of water and ice blocks may cause serious damage downstream
initial freezing results in formation of border ice attached to banks of channel
Subsequently, the production of frazil, randomly oriented, needle-shaped crystals, occurs within flowing water
combine into large clusters to form pancake ice floating at water surface
may adhere to submerged boulders and aquatic vegetation to form anchor ice
coalescence of pancake ice, anchor ice and border ice obstructs water flow in channel initiating thermal ice jams (also called freeze-up jams)
Frazil=in _____
Anchor: in ______
Border ice=on _____
water
water bed
margins
dynamic ice flooding
breakup of ice cover, either during the spring freshet or as a result of a mid-winter thaw, can trigger dynamic ice jamming
the most severe dynamic ice jams result from differential rates of ice melting and breakup in upstream and downstream reaches
rapid upstream melting and breakup coupled with slower melting and breakup downstream increases greater flooding risk
Glacial Lake outburst floods
sudden draining of moraine/glacier ice – dammed lakes in alpine valleys
produce large, short-lived flood events
Flood advisories can be made based on(7)
outlooks of peak spring discharges can be made weeks in advance by using models that include snowpack depths, ground moisture, lake and river levels, temperature, and weather forecasts
flood advisories or warnings may be issued in anticipation of severe weather conditions
ice-jam flooding routinely occurs in spring; models derived from historical data can be used to assess peak water levels during river ice breakup
TORF
the exceedance probability remains the same even after the occurence of an event
T
in recurrence interval analysis, the probability of future flooding is not changed because of previous flood events
the belief of many people that a “100-year flood” can occur only once every 100 years is completely wrong
the exceedance probability of a “100-year flood” remains 1 per cent
Several factors affect the damage caused by floods: 8
land use on the floodplain amount and type of sediment deposited by floodwaters extent, height, and strength of dykes depth and velocity of floodwaters rate of rise and duration of flooding season of the flooding extent, height, and strength of dykes effectiveness of flood forecasting, warning, and evacuation
Minimizing Flood Hazards: Physical Hazards
levees or dykes are barriers built to keep flood waters contained
earthen levees, concrete floodwalls, reservoirs, floodways allow flood waters to bypass populated areas
e.g., the Red River Floodway in Winnipeg has prevented many major floods
physical barriers encourage development on the floodplains they are intended to protect
Floodplain Regulation
Floodplain Regulation
objective is to maximize the benefits that floodplains offer while minimizing flood damage and the cost of flood protection
one approach is to disallow new development that would lessen a river’s access to its floodplain
Flood Damage Reduction Plan
Flood Damage Reduction Program
established in 1975; operates under a series of federal-provincial and federal-territorial cost-sharing agreements
agree to not build, approve, or finance developments in flood-risk areas
will not provide flood disaster assistance for developments built after an area is designated to have a flood risk, except for flood-proofed structures
Encourage local authorities to zone on the basis of flood risk
