Geomorphology. considerations

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1 Environmental Flows Geomorphology considerations Dr Beevers, Heriot Watt University: Dr A Crosato UNESCO IHE

2 Hydraulics Summary Water components important for ecology etc (depth, velocity, innundation width etc) Change water components into flows Vice versa (flows into components) Use cross sections Can put these crosssections into models Floodplain l Area=m 2 Channel Floodplain l WL=m * Velocity=m/s = Q=m 3 /s Survey components (land topography and river channel bathymetery) Topogr raphy take from DEM (digital elevation model) Bathymetry take from river survey (boat or person)

of planform Sediment Why sediment & morphology is important and what

3 Introduction Introduction Important concepts Aquatic Habitats Morphodynamic processes and phenomena Discharge regimes Classifications of planform Sediment Why sediment & morphology is important and what components need to be considered in EFA Natural rivers transport flow and sediment

4 atural rivers are dynamic systems that change with time

5 Important concepts: Spatial scales Basin scale Reach scale (2) Corridor scale (3) Cross sectional scale (4) Depth scale (5) Process scale (6)

6 SPATIAL SCALE: BASIN TIME SCALE: THOUSANDS YEARS RIVER REACH CENTURIES RIVER CORRIDOR DECADES CROSS- SECTION YEARS WATER DEPTH MONTHS PROCESS DAYS

7 River corridor scale: features main channel width corridor width

8 Cross sectional scale: point bars and alternate bars

9 Cross sectional scale: multiple bars corridor width main channel width

10 Aquatic Habitats River aquatic habitats include the floodplains. They depend on: Flow (discharge variations, velocities) Morphology (bed topography, spatial variability, cover) Sediment (transport and substrate) Water quality (pollutants, turbidity, temperature)

11 Morphology: spatial variability

12 Substrate

13 Substrate t and morpholog gy: cover from predators dt and sheltering

14 IMPORTANCE OF BEN NTHOS: Benthos is the food resources for fish and other aquatic animals and live on the substrate IMPORTANCE OF SUBS STRATE: Substrate is of basic importance for benthos, for spawning grounds and fish larvae

15 Different species require different habitats depending on the stage of their life cycle: adult juvenile fry egg incubation reproduction

Due to the existence of specific biological calendars (species dependent) the requirements interms of substrate, flowvelocit ty, depth,cover vary with

16 Due to the existence of specific biological calendars (species dependent) the requirements interms of substrate, flowvelocit ty, depth,cover vary with the season. Biological calendar of the Grayling (Thymallus thymallus) dult uvenile ry ncubation eproduction JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

17 Morphodynamic phenomena Morphological phenomena: are the results of the processes at different spatial and temporal scales. Each has specific spatial scales basin scale (sediment yield, discharge generation) reach scale (riverbed aggradation and degradation, planform formation, river slope changes, avulsion) river corridor scale (floodplainrise rise, cut off forming, regular flooding, bend growth, channel migration) cross sectional scale (bar forming, pool forming, bank retreat, river widening or narrowing g) depth scale (bank erosion, scour forming, dune and antidune forming) process scale scale (ripple formin ng)

18 Morphodynamic phenomena Morphological phenomena: are the results of the processes at different spatial and temporal scales. Each has specific spatial scales basin scale (sediment yield, discharge generation) reach scale (riverbed aggradation and degradation, planform formation, river slope changes, avulsion) river corridor scale (floodplainrise rise, cut off forming, regular flooding, bend growth, channel migration) cross sectional scale (bar forming, pool forming, bank retreat, river widening or narrowing g) depth scale (bank erosion, scour forming, dune and antidune forming) process scale scale (ripple formin ng)

19 Morphological processes at the reach/corridor scale Sinuosity Bend growth Channel migration Planimetric changes Bend cut off/aggradation

20 Morphological phenomenon deposit formation at the cross sectional scale: large

21 orphological phenomenon at the depth scale: bank rosion

22 CHARACTERISTIC TEMPORAL SCALES MORPHOLOGICAL PHE ENOMENA Depend on the amounts of sediment to be moved to produce the changes and on the flow strength Example assuming the same discharge: Ripple forming: fast (minutes) Dune forming: rather fast (days) Pointbar forming: medium (mon nths to years) Pool forming: medium (months Planimetricchanges: i slow (tens to years) of years) Valley formation: very slow (hundred of years)

23 DISCHARGE CYCLE Most rivers display an annual discharge cycle, although there are large differences between years Fl low, m^3/s g y, Month Avg Stdev Congo River at Kinshasa Black curve shows average, red shows standard deviation for the years

24 Jamuna River (Bangladesh) 100, Discharge of the Jamuna River 80,000 Discharg ge (m³/s) 60,000 40,000 20, Year

25 IS THERE A FORMATIVE DISCHARGE? A single formative discharge does not exist: each type of geomorphological phenomenon has a different formative discharge For environmental lflows many values of the discharge should be considered: Low flows (sedimentation of fines) High flows (flushing) specifying AMOUNT, TIMING & DURATION

26 River Reach Classification Each reach type displays different features as a result of morphological processes SEA STEEP MOUNTAIN BRAIDED MEANDERING DELTA REACH REACH REACH

27 Factors governing the river planform formation Flow strength Sediment supply Sdi Sediment composition Bank erodibility Riparian vegetation Frequency of floods SEA TEEP OUNTAIN EACH BRAIDED MEANDERING DELT TA REACH REACH

28 MEANDERING vs BRAIDING Low flow strength Low sediment supply Fine sediment Low bank erodibility Dense riparian vegetation Low frequency of intense floods High flow strength Highsediment supply Coarse sediment Highbank erodibility Scarce riparian vegetation High frequency of intense floods

29 River Connectivity: 4 Dimensions Longitudinal Sediment movement Dams Lateral Connection to floodplain Flooding regime Vertical Interstatial spaces Temporal Connected in time, seasonal Dams

30 River Connectivity: 4 Dimensions Longitudinal onsider these in e-flow assessment Dams Sediment movement Dams Lateral Connection to floodplain Flooding regime Vertical Interstatial spaces Temporal Connected in time, seasonal

31 TYPE OF SEDIMENT: NON COHESIVE sand + gravel cobbles

32 TYPE OF SEDIMENT: COHESIVE silt and clay

33 Sediment classification based on grain size (A.G.U.) Clay: D < 4 μm Silt: 4 μm < D < 62 μm Fine sand: 62 μm < D < 250 μmμ Medium sand: 250 μm < D < 500 μm Coarse sand: 500 μm < D < 1000 μm Very coarse sand: 1000 μm < D < 2000 μm Gravel: 2 mm < D < 64 mm Cobbles: 64 mm < D < 256 mm Boulders: D > 256 mm FINES

34 GRANULOMETRIC CURVES

35 Sediment Transport CAPACITY LIMITED Classification based on origin bed-material load washload bedload suspended load Classification based on transport mechanism SUPPLY LIMITED Z related to flow strength th (flow velocity / stream power / shear stress / Shields parameter) SUSPENDED LOAD BED LOAD c

36 Transport Capacity Formulas Give the amount of sediment passing through a crossvelocity section as a function of flow Derived in the lab Capacity limited formulas: Engelund & Hansen, Meyer Peter & Müller, Van Rijn Power law: qs Not for wash load = b u mu Qw water Qs suspension load Qs bedload

37 Sediment and Morphological Modelling Couples sediment transport formulas to hydraulic engines Models cover allscales Basin reach cross section etc Nested modelling can offer strategic, basin wide, overview of morphological processes and response Catchment model for sediment yield 1D for sediment movement, channel migration (aggradation/deposition) 2D for detailled processes bank erosion, over bank processes 3D for detail e.g. reservoir sediments

38 Example: Mekong River, Reservoir Sedimentation Mekong River Commission Reservoir sedimentation Flushing options (Francis Fruchart CNR France) Further work: catchment connectivity

39 Example: Ganga River: Lateral Connectivity Present form GIS, Satellite, DEM analysis Future form Cross sections increase to Cross sections, increase to flow depth

40 Summary Important components for EFA: Habitats at various scales Processes at various scales Sediment types Prediction methods Case studies

41 Thanks for listening!

Case Study: Sediment increasing flood risk Approximately 3000properties across the city may be at risk of flooding Historic perception that the accumulation of sediment is a cause of flooding

42 Case Study: Sediment increasing flood risk Approximately 3000properties across the city may be at risk of flooding Historic perception that the accumulation of sediment is a cause of flooding in central Carlisle. Past flood risk management along the lower Caldew involved repeated dredging Tributary of the River Eden Catchment area of 275 km 2 Rainfall 2800 mm/year Flashy flow regime

43 Methodology Nested spatial approach Geomorphological Assessment of 12km of river 1D Long term Sediment Modelling of 4km of River Geomorphological Assessme nt Field Study Sediment Budget Sediment and Hydraulic modelling (ISIS and ISIS Sediment) Long term hydrological hd linflow dt data for sediment model dl Modelling to ascertain trends in bed movement

44 Geomorphological Assessment Reviewofprevious studies Historical trend analysis Archive record search Environmental data sets Geomorphological mapping Sediment budget construction

45 Sediment Budget 1 Erosion Deposition

46 Sediment Budget 2 ~5 year budget Erosion and Deposition normalised by reach length (metres) 6 5 metre volume (m 3 ) per Sediment Buckabank weir Erosion per metre Reach number Erosion = 13,731 m 3 Deposition =14 4,842 m Deposition per metre Carlisle

47 Modelling: Sediment & Hydraulics (Method) Model construction Existing ISIS model coupled to sediment module Bed samples collected from the river Validation and sensitivity assessment Long term hydrological inflow data Model simulations (Event based, 5 year and 12 year) Areas of erosion/deposition linked to geomorphological findings Identify accumulation sites for berm analysis Investigate impact to Standard of Protection through h Carlisle l

48 Modelling: Sediment & Hydraulics (Results)

49 Modelling: Sediment & Hydraulics (Results)

50 Conclusions Currently the River Caldew is in dynamic equilibrium Deposition upstream of Holdmhead Weir Erosion downstrem Sediment deposition solely does not exacerbate current flood risk through Carlisle Vegetation stabilises bars through the reach Vegetation does exacerbate flood risk Natural sediment exchange upstream of Carlisle should be allowed to continue

Strategies for managing sediment in dams. Iwona Conlan Consultant to IKMP, MRCS

Strategies for managing sediment in dams Iwona Conlan Consultant to IKMP, MRCS 1 Sediment trapping by dams Active storage capacity Dead storage coarse material (bed load) Fine materials (suspension) Francis