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Home , Sediment transport

Erosion and transport

Lecture content Skript: Ch. VIII

– rationale for understanding and modelling and sediment transport processes – surface erosion – mechanisms – interaction with climate, land cover and topography – annual scale surface erosion model – sediment transport in – mechanisms – measurements – sediment characterisation – condition for incipient motion – sediment transport equation

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 1 Erosion and sediment transport is driven • by hydrological processes in the watershed • by hydraulics in plays an important role with regard to • evolution of landscape • loss of agricultural • stability of beds • resources infrastructures (, …) • natural hazards • coastal processes ⤵

Brienzersee,)Hochwasser)2005) Hochwasser/Murgang) Copyright © Philip Owens 2002 – Erosion and Sediment Transport – Autumn Semester 2017 2 Examples of effects of erosion and sediment transport on water infrastructures • filling in of reservoirs – reduces the active volume of the reservoir intake – can put at risk the correct operation of the reservoir organs (e.g. intakes) multipurpose • river bed reservoir deposited = dead volume – due to sediment after a event – due to imbalance between sediment supply from upstream and flow energy

• scour in river beds and embankment erosion – undermines the stability of river cross sections • pumps and turbines ⤵ • water supply derivations

• ecosystems

• … Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 3 erosion potential in EU

http://epp.eurostat.ec.europa.eu/statistics_explained/index.php?title=File:Soil _erosion_by_water_(tonnes_per_ha_per_year),_2006,_EU- 27,_NUTS_3_.png&filetimestamp=20130425135806

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 4 potential in CH [Weisshaidinger & Leser, 2006]

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 5 Long-term effects of erosion and sediment transport Landscape scale evolution • temporal scales >100 years • large space scales (large river basins, areas)

① early stage – landscape dominated tectonic activities à , , ② youth stage – lakes and swamps drained, deepening of gorges, formation of valleys ③ early – river profile and first riparian wetlands formed, broader tributary valleys A – natural levees ④ maturity D – alluvial deposit – large , meandering river, L – abandoned floodplain formation also in tributary C – lateral rivers valleys P – floodplain ⑤ full maturity S – alluvial slopes Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 6 Watershed scale erosion and sed. transport processes watershed scale evolution hillslope and river as space continuum

Naiman et al. (Riparia, 2005)

Sediment storage Sediment production area: Sediment transfer area: both area: deposition erosion is dominant erosion and deposition take place dominates Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 7 Erosion and its forms (1/8): splash erosion

• space scale – point • time scale – short term, event

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 8 Erosion and its forms (2/8): surface erosion • space scale – area, hillslope, agricultural field – 1 ÷ 100 m2 • time scale – short term, event, related to overland flow • effects – erosion of agricultural soil

Copyright © Philip Owens 2002 – transport of phosphorus used in agriculture into water bodies

Typical values (t ha-1 yr-1):

• Bare soil 23 • Vineyard 20 • Maize 14

• Grassland 1 • Orchards 1 ©"Praxis"Unterrichtsfilm" • Forest <1 Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 9 Erosion and its forms (3/8): erosion

Erosion%prone%soils% • space scale – linear development on erosion prone soils % – 10 ÷ 100 m length, < 50 cm depth • time scale – short-term, event scale • effects – slope incision – early channel formation

h"p://www.fao.org/ag/agl/agll/photolib/photolib.jsp?lang=e&nav=next&photo=077<

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 10 Erosion and its forms (4/8): erosion

• space scale – linear development on rills – 10 ÷ 100 m length, > 50 cm depth • time scale – short-term, event scale • effects – channel incision – river network formation

h"p://www.fao.org/ag/agl/agll/photolib/photolib.jsp?lang=e&nav=next&photo=097=

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 11 Erosion and its forms (5/8): spatial continuity

surface erosion

gully erosion rill erosion

http://www.fao.org/ag/agl/agll/photolib/photolib.jsp?lang=e&nav=next&photo=055 Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 12 Erosion and its forms (6/8): natural hazards

Brienzersee, BE, Hochwasser 2005 Rutschung Hellbüchel, Lutzenberg, AR Hochwasser/Murgang Sept. 1st, 2002

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 13 Erosion and its forms (7/8): river cross section NB vertical scale amplification x10

8 water level [m]

6

4

2

0

0 50 100 m • space scale – longitudinal, lateral, vertical • time scale – short-term, event scale – long-term, depending on imbalance between sediment supply and transport capacity • effects – channel incision / aggradation; river lateral migration Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 14 Erosion and its forms (8/8): river longitudinal profile

• the longitudinal profile tends to equilibrium by eroding upstream and depositing river longitudinal profile downstream – the process is in equilibrium if sediment supply equals transport capacity -- supply lower à erosion, river bed incision -- tr. capacity higher à deposition river plan view • plan river course progressively migrating laterally downstream, meandering river cross-sections • long-term evolution, short term disturbances

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 15 Sediment production / mobilization mechanisms

RAINFALL INDUCED EROSION OVERLAND FLOW INDUCED EROSION

• raindrop velocity: 8 ÷ 10 m/s • flow velocity: 5 ÷ 10 cm/s • splash erosion: v ≈ 15 m/s • flow depth: O(h) ≈ cm • particle size “diameter”: O(d) ≈ mm • particle size “diameter”: O(d) ≈ mm +

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 16 Surface erosion: influence of soil, climate and land cover • the more arid the climate, the less vegetation coverage à A • the less arid the climate, the higher is erosion and sediment transport on bare ground àB • “optimal” condition for erosion is a limited vegetation coverage and a significant amount of rainfall

B effect of vegetation on ⬇ Q(t) bare soil A forest

proportion of ground cover sediment transport rate not covered by vegetation A⋅B

ARID HUMID t increasing rainfall

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 17 Hillslope erosion: influence of topography EROSION EROSION

slope length slope angle

• supply exceeds transport capacity • all forces controlled by gravity are ⤷”saturation” effect enhanced by increasing slope angle due to non linearities of their dependence on it.

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 18 Estimation of surface erosion through models

• direct measurement of surface erosion are complex à estimation through models • surface erosion is caused by ⤷ natural agents (rainfall, runoff, , ice, temperature fluctuations, …), and ⤷ anthropogenic influence (land use, practice, agriculture, …) • most models are developed for estimation of agricultural soil losses ⤷ event-based à process-based models (e.g. erosion = f(, overland flow depth) ⤷ estimation of erosion over long-term scales à empirical relationships

example: UNIVERSAL SOIL LOSS EQUATION (U.S.L.E) ⤵

A: annual erosion per unit area A = R ⋅ K ⋅ L ⋅S ⋅ C⋅ P [t⋅m-2⋅yr-1] climate soil geomorphological land use controls controls controls controls

• R: erosivity index à rainfall forcing • S: slope factor à slope • K: erodibility index à soil characteristics • C: crop factor à land use, vegetation • L: length factor à hillslope length • P: conservation factor à agricultural practice

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 19 USLE equation: erosivity index, R [Wischmeier and Smith, 1978; Renard et al., 1991]

• the erosivity index quantifies the energy applied by rainfall to detach sediment particles on the surface à effects of raindrop impact and amount and rate of runoff associated with rainfall • it is computed as cumulative effect of the contribution of individual events

N ⤷ R = R ⤶ ∑i=1( ev )i

where N is the number of event per year, Rev is the erosivity index of an individual event, defined as

Rev = E ⋅ I30 • E: [MJ⋅ha-1] • h : [mm] with I30 corresponding to the maximum 30’ rainfall intensity within the event Δt • i: [mm⋅h-1] E = total energy associated with the rainfall event, computed as • ε: [MJ⋅ha-1⋅mm-1] M E = ε ⋅h ∑ Δt=1 Δt

with hΔt = rainfall depth in each Δt, M is the number of rainfall intervals in one event and

−1 ⎪⎧ε = 0.119 + 0.0873log(i) i ≤ 76 mm h ⎨ −1 ⎩⎪ε = 0.283 i > 76 mm h Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 20 USLE equation: erodibility index, K [Wischmeier and Smith, 1978; Renard et al., 1991] • the erodibility index measures the resistance of sediment on the soil surface to detachment by water • it is defined as the amount of soil that would be eroded for

– a standard experimental plot – unitary erosivity index (R = 1) – and ineffective geomorphologic and land cover controls (L = S = C = P = 1)

⤷ A = R⋅ K ⋅ L ⋅S ⋅C ⋅ P = 1⋅ K ⋅1⋅1⋅1⋅1 à A = K

from experimental investigations K is formulated to be function of

– γ, % of /; α, % of silt/ à M = α(100-γ) – a, % of organic matter – b = f() à b=1 for very fine granular, b=4 for massive compacted – c = f(soil permeability)à c = 1 ÷ 6 = high ÷ low

⤷ K = 2.77 ⋅10−7 (12 − a)M 1.14 + 4.3⋅10−3 (b − 2) + 3.3⋅10−3 (c − 3) ⤶ Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 21 USLE equation: template to estimate K

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 22 Sediment transport

• sediment particles eroded on hillslopes are transported Naiman et al. (Riparia, 2005) throughout the river network by means of different mechanisms

• sediment transport (total load) can be defined by – type of movement (bedload and ) – source of sediment (washload and )

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 23 Sediment transport characterisation

TYPE OF TRANSPORT CHARACTERISTICS

Bed load, gB Coarse, large bed material (, cobbles, …) carried by the stream in a layer adjacent to the bottom of the river bed by jump and roll mechanisms

Suspended , gBS Bed material carried in close to the bed layer ( driven transport)

Total bed load, gBT gBT = gB + gBS

Suspended load, gS fine sediment material (e.g. sand), carried in suspension, migrating between bed and suspension, depending on stream velocity

Wash load, gL fine sediment material NOT present in the bed, eroded upstream on hillslopes, carried in suspension, silt and clay, 2.4⋅10-3 ÷ 3⋅10-1 mm

Total load, gT gT = gBT + gS + gL Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 24 Sediment transport and engineering problems

PROBLEM TYPE OF TRANSPORT

Time needed to fill diversion dams gB , gBT (depending on size)

Estimation of dam dead capacity gBT , or gT

Design of channel intakes gBS , or gS

Design of pumping plants gBS , or gBT

River engineering, restoration gBT

Water supply treatment plants gL

Scour studies gBT

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 25 Sediment transport – suspended load measurements

requirements to be fulfilled by instruments ⤷ flow velocity at the sampler = stream velocity ⤷ minimum disturbance on flow at sampling point ⤷ sampler intake orientated both vertical and horiz.

• depth integrating samplers (A, B): ⤷ sampler lowered to the and raised to surface at a constant rate à sample collected at a rate proportional to local velocity

• point integrating samplers (C): ⤷ sample time integrated at a given level

• instantaneous samplers (D, E): ⤷ tube lowered into streams and aligned to flow, tube ends closed after short time.

• single stage samplers (F): ⤷ semi-automatic sampling, multi-level, time integrated

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 26 Sediment transport – bed load measurements • slot trap (A): ⤷ suitable for small streams (removable collecting box)

• basket sampler (D): ⤷ simplest, best suited for coarse bedload, low sampling efficiency

• pan / tray samplers (B) ⤷ low stream velocities and rates of bedload, disturbance of the stream velocity (lowering of velocity and reduction of bedload)

• pressure difference device (C) ⤷ control of flow velocities to reduce decive induced disturbances

• acoustic devices ⤷ record the audible sound waves caused by bedload hitting a plate or moving in the stream Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 27 Sediment transport estimation

REQUIRED DATA •channel hydraulic geometry – river cross section – hydraulic radius – river bed slope – – bed roughness

•sediment characteristics D90

– specific weight of particles ~D60

– concentration of suspended material ~D35 ~D – granulometric curve and main diameters 20

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 28 Sediment transport – granulometric curve (1/2)

Example of distribution survey • representative sampling across river bed • material passed through sieves • standard sieve sizes ê • measurement of the fraction of sediment retained by a sieve opening

⤷ Di = “diameter” of the particles that have passed an opening level, with i = passed percentage in weight

à D90 diameter corresponding to the opening that retained 10% of the material, i.e. let 90% pass

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 29 Sediment transport – granulometric curve (2/2)

D90

~D60

~D35 ~D20

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 30 Sediment transport – incipient motion

• sediment transport occurs when (hydraulic) thresholds are exceeded, which determines incipient motion of particles • criteria of incipient motion are based on exceeding of

– a critical flow velocity at the river bed, Vb

– a critical shear stress, τ* à τ0 = γ ⋅ R⋅S > τ* where γ=ρg , R = hydraulic radius, and S = slope

CRITICAL FLOW VELOCITY

• V < Vcr à no motion V ≥ Vcr à incipient motion

• Vcr estimated from – tables – empirical relationships – equilibrium criteria D(γ s − γ ) ⤷ lift and roll mechanism à hydrodynamic forces balanced by weight components à V = 2g b, r k ⋅ γ where D = diameter, γs = sediment specific weight k = shape coefficient D(γ s − γ ) ⤷ mechanism à hydrodynamic forces against friction à Vb, d = 2g f where f is a friction coefficient k ⋅ γ

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 31 Sediment transport – shear stress induced motion

sediment transport occurs when (hydraulic) resisting forces are exceeded by mobilizing

hydrodynamic forces à threshold condition: τ0 > critical shear stress, τ*

hydrodynamic forces τ = = * resisting forces F + F − F = L D R FW − FB

τ* = f (shear stress, particle size and shape, weight) ⤵

2 2 τ0ds τ0 ρmu* τ* = 3 3 = = γ sds − γ mds (γ s − γ m )ds (γ s − γ m )ds

ê Shield’s diagram

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 32 Sediment transport – Shield’s diagram • valid for a homogeneous mixture of sediments

• τ* = f (sediment properties, ds; flow characteristics, Re*)

area dominated transition regime fully turbulent

by τ* = τ* (Re*) τ* ⋍ 0.06 s d ) m 0 γ τ MOTION − s γ ( = * τ

SILT SAND NO MOTION GRAVEL

u*ds Re* = Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 ν 33 Sediment transport – threshold conditions

• non homogeneous bed material ⤷ partial motion ⤷ paving/armoring of the bed (i.e. washing of fine material, depending on flow conditions

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 34 Sediment transport – estimation of sediment discharge sediment discharge estimation is carried out by semi-empirical formulas ⤷ derived from laboratory experiments ⤷ steady flow conditions ⤷ homogeneous material ⤷ generally bed-load only

MEYER-PETER’S EQUATION

• incipient motion à τ* = 0.047 (γs – γ) D50 = resisting force

⤷ τ* < 0.03 à no erosion

⤷ 0.03 ≤ τ* ≤ 0.047 à transition zone

⤷ τ* > 0.047 à erosion 8 3 2 • bed load sediment discharge: gB = (τ0 − τ* ) [kg/s] 1 2 ⎛ ρs − ρ⎞ ρ ⎜ ⎟ g ⎝ ρs ⎠

8 3 2 3 gB = 1 2 (τ0 − τ* ) [m /(m ⋅s)] ρ (ρs − ρ)g

3/2 where τ0 = ρ⋅g⋅R⋅S (ks / kr) with ks = Strickler’s coefficient 1/6 1/3 kr = 26 / D90 [m /s] coefficient accounting for roughness induced by sediment transport 3 ρs = 2’600÷2’700 kg/m Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 35 Erosion and sediment transport

Engineering Problems: ⤷ Estimate the sediment accumulation in a hydropower reservoir

Solution ⤷ Compute the (annual) sediment production on hillslopes (supply to river channels) ⤷ Compute the (annual) sediment transport in river channels

Methods A = R ⋅ K ⋅ L ⋅S ⋅ C⋅ P ⤷ USLE equation climate soil geomorphological land use controls controls controls controls

8 3 2 3 ⤷ Meyer-Peter’s formula gB = 1 2 (τ0 − τ* ) [m /(m ⋅s)] ρ (ρs − ρ)g

Hydrology – Erosion and Sediment Transport – Autumn Semester 2017 36