FS
fluvial systems
fluvial systems
Set of flashcards Details
| Flashcards | 88 |
|---|---|
| Language | Deutsch |
| Category | Nature Studies |
| Level | Primary School |
| Created / Updated | 27.01.2014 / 18.01.2020 |
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what is a fluvial system?
biotic/abiotic -> interact on different scales in space + time
driven by water + sediment
sensitivity to disturbance is low on basin scale, high on habitat scale
structure: long, lat, verical & time
production, transfer & deposition zone
upstream/ downstream controlled
very dynamic
basin + floodplain scale = not manageable
name the 6 geomorph concepts of landscape change
uniformity
threaholds
landscape evolution
complexity
self organized criticality SOC
optimality
uniformity concept
process laws remain the same for past, present and future
narure of processes does not change, frequency distr does not change
threshold concept
system only responds when treshold is exceeded
external or internal thresholds
examples of internal thresholds
critical slope
meander cutoff
concept of landscape evolution
uplift and erosion form landscape
river is in equilibrium of transport cap and sediment supply
after disturbance: re-establishing of equilibrium
two models: progressive change, episodic change
concept of landscape evolution
uplift and erosion form landscape
river is in equilibrium of transport cap and sediment supply
after disturbance: re-establishing of equilibrium
two models: progressive change, episodic change
important factors for landscape evolution concept
rates of U(t) and e(t)
their temp variability
intermitency (periodizität)
prob dist of large events
feedbacks
complexity concept
FS is complex because:
processes act together
many timescales involved
adjustment take long and overlap
external and internal forces adjust the system
-> complexity = predictable??
Self-organized criticality SOC
system organises itself in a critical state
in the critical state, all seizes of events may occur
predictability is only in a mean statistical way possible
critical state is: an attractor, globaly stable, locally instable
stat properties of SOC are very similar to natur
problems of SOC
does not work for all granular piles
optimality concept
based on efficiency of transporting sediment
local optimality: equal energy dissipation per unit channel
global optimality: minimal energy dissipation in total
-> very similar to natur
what is landscape connectivity
what is important
fully coupled, uncoupled, de-coupled
important: effectiveness, magnitude, direction of coupling
forms of landscape connectivity
lateral: hillslope-channels
longitudinal: upstream-downstream
vertical: river-aquifer
disconnectivity: buffers, barriers, blankets
forms of disconnectivity
and boosters
buffers: swamps, floodplains
barriers: steps, slugs
blankets: floodplains
boosters: gorges
assesment of connectivity
sigma(r) = P(r)/T(r) = erosion-deposition/ transport capacity
if sigma ->0 : detachment limited
if sigma ->inf: transp cap limited
example for hillslope coupling
seasonality of hillslides and floods
landslide occur in winter -> sediment delivery
floods occur in summer -> transport of sediment
timescales of landscape change
system responds to disturbence depending on its: state, connectivity, forcing
can be hyper- or undersensitive -> new or old equilibrium
reaction, response, relaxion time
reaction: disturbance - response of system
response: disturbance - new eq.
relaxion: time it takes to change to new eq
response = reaction + relaxion
they all depend on: state, connectivity and forcing of the FS
exters equation
use of sediment continnuity/mass balance to compute channel change as a function of changes in the sediment transp flux
(1-p0) dz/dt = -dqs/dx, with p0 = Vwater/Vtotal
used in erosion-deposition models
allows assessment of river straightening
example of exters eq
effect of straigntehing of a river:
upstream: degradation z-
downstream: aggregation z+
qs: eincreased in straightened section
calculation: (1-p0)(z_t+dt -z_t) dx = (qs,x - qs,x+dx)dt
short term effect: higher flow velocity
long term effect: aggregation & degradation
dyn eq. of water and sediment
conservation of mass
conservation of momentum
sediment continuity (exter)
+ q = q(v)
-> all eq interact
how to determine sediment transport? -> morphodynamics
log windprofile
vx/u* = 1/k*ln(z/z0)
derived from turbulent flows in open channels
resistance to flow
res to flow: overland >> channel
due to: low flow depth, raindrops, vegetation (inc surf roughn, ext laminar flows, negates raindrops)
darcy-weisbach friction
f=kt/Re= (k0+Aib)/Re (laminar)
f ≈ 0.223/Re0.25 (turbulent)
i = precipitation intensity
particle incipient motion
shields parameter is a critical value, where transportation e.g. incipient motion begins
shields parameter
tau. = hydrodyn. forces/rising forces = tau0*ds2 / gamma_s*ds3-gamma_m*ds3 = rho_m * u.2/(gamma_s-gamma_m)*ds
= (F_lift + F_drag + F_rising) / (F_wweight + F_buoyancy)
only dealing with single grains
shields parameter
tau. = hydrodyn. forces/rising forces = tau0*ds2 / gamma_s*ds3-gamma_m*ds3 = rho_m * u.2/(gamma_s-gamma_m)*ds
= (F_lift + F_drag + F_rising) / (F_wweight + F_buoyancy)
only dealing with single grains
problems of shields parameter
multiple grain seizes -> partial motion
grain interaction
slope variation
geometry
shields diagram
tau.c vs. ds or Re
treshold where motion/no motion
the different bedforms in rivers
lower regime (subcritical): plane bed → ripple pattern → dunes (moving downstream) → washedout dunes
Upper regime (supercritical): Plane bed with sediment → antidunes/standing wave (moving upstream) → antidunes/breaking wave → chutes and pools
Bedforms influence resistance to flow
Total roughness = grain + form + system
bedload, suspended load, total load
bedload: transported in contact with the stream bed
Suspended load: suspended in the water columb, includes washload
Total load: all transported sediment
how to estimate sediment transport
deterministic methods: based on empirical relations on basis of flume experiments and river data. (duboys, meyer-peter+müller, for total load: bagnold, brownlie)
probabilistic methods:...
channel morphologies
cascade, step-pool, plane-bed, poll-riffle, dune-ripple
sediment rating curves
Qs = a*q^b, b:2...3
Graph with unit sediment discharge vs. unit water discharge
Different curves based on different approaches to calculate sediment transport
sediment production
Soil erosion
Soil loss
Sediment yield
soil production: due to physical weathering: frost, clay hydration, roots
Due to chemical weathering: acid rain, organic acids, oxidation, hydrolysis
Soil erosion: e.g. sediment production, removal of soil particles from a POINT due to wind and water
Soil loss: soil movement off a particular AREA
Sediment yield: delivery of lossed soil to a POINT
differences between geological and modern erosion rates
geological: erosion under native vegetation, on longterm: matching the rate of soil production
Agricultural: much higher, natural balance is disturbed
Critical time to erode through soil profile: Tc = S/(E-P)
what influences erosion?
Types of erosion/ sediment prod?
rainfall increases, vegetation decreases erosion
Erosion due to: rainsplash, overland + hillslope erosion (sheet, rill, gully), mass inputs (landslides, mudflows)
USLE
universal soil loss equation: to quantify rill + interrill erosion
E = R K LS C P
E: estimated soil loss
R: rainfall + runoff erosivity indexes
K: soil erodibility index
LS: slope-length-steepnes index
C: crop management index
P: erosion control practice
main problem of USLE
no consideration of flow through the basin.
Estimation of loss/prod on fields possible, but not at points → revised USLE
