Erosion and sediment transport Lecture content Skript: Ch. VIII – rationale for understanding and modelling erosion and sediment transport processes – surface erosion – mechanisms – interaction with climate, land cover and topography – annual scale surface erosion model – sediment transport in streams – 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 stream hydraulics in rivers plays an important role with regard to • evolution of landscape • loss of agricultural soils • stability of river beds • water resources infrastructures (dams, …) • natural hazards • coastal processes ⤵ Brienzersee,)Hochwasser)2005) Hochwasser/Murgang) Copyright © Philip Owens 2002 Hydrology – 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 aggradation reservoir deposited sediments = dead volume – due to sediment deposition after a flood 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 Soil 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 Soil erosion 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 lakes, waterfalls, rapids ② youth stage – lakes and swamps drained, deepening of gorges, formation of tributary valleys ③ early maturity – river profile and first riparian wetlands formed, broader tributary valleys A – natural levees ④ maturity D – alluvial deposit – large floodplain, meandering river, L – abandoned meanders 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): rill erosion Erosion%prone%soils% • space scale – linear development on erosion prone soils Rills% – 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): gully 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 hydrograph ⬇ 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, wind, ice, temperature fluctuations, …), and ⤷ anthropogenic influence (land use, soil conservation practice, agriculture, …) • most models are developed for estimation of agricultural soil losses ⤷ event-based à process-based models (e.g. erosion = f(shear stress, 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