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Future floods: An exploration of a cross-disciplinary approach to flood risk forecasting (26-27 February 2015) Estimating flood discharges using boulders, Ping River System, Thailand Lim Han She Dept of Geography NUS Source:

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Floods in Chiang Mai city (Source: Boonserm Satrabhava,

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Ping River gauging station, P1 (94 yrs of data) Royal Irrigation Department The need for data – do we have enough?

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Why study boulders? Velocity and depth reconstructions from boulders -Represent the maximum competence of the stream during flood conditions -Moved during more extreme floods Source:

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The study of boulders is not new Costa (1983) Williams (1983) Baker (1984) Causes of large boulder transport: Drainage of ice-damned lakes High viscosity flows (debris flow, mudflows, lahars etc.) Failure/collapse of natural or man made dams High magnitude rainfall

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Boulders in Thai river Boulders (granite, orthoquartzite), Mae Taeng River

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(Source:

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Study site – Mae Taeng River

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Cross-section profile at Mae Taeng bridge (Royal Irrigation Department)

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Methodology Step 1 – measure boulder dimensions 15 boulders Average intermediate axis (5 largest boulders) = 2.16 ± 0.48 metres (Source:

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Source Theoretical equations Helley (1969) F D + F L =F R Empirical equations Colorado Front Range data US Bureau of Reclamation data Combination equations Average of 4 equations above Step 2- applying known equations to calculate velocity for boulder initiation (Costa, 1983)

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Manning ’ s Equation Note: Hydraulic radius = cross sectional area/ wetted perimeter Step 3 - Calculating flow depth

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Boulder size: 2.16 ± 0.48m Depth (m) Theoretical equations: Helley (1969) Force equation (fluid, drag, lift, friction) Empirical equations based on data from: Colorado Range, USA Bureau of Reclamation, USA Average of 4 methods Calculated average flow velocity and depth

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Comparison of average flow velocity Intermediate axis (m) This study Thailand, alluvial river methods Kesel (1985) Costa Rica, alluvial fans Indirect methods (??) Bradley & Mears (1980) Colorado, USA, alluvial deposits Hydraulic equations Costa (1983) Colorado, USA, alluvial deposits Average of 4 equation cited in Costa (1983) (Source of data: Elfström, 1987)

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Depth (m) Helley (1969)35.7 Force equation (fluid, drag, lift, friction) 7.6 Colorado Range, USA6.2 Bureau of Reclamation, USA 12.3 Average of methods7.6 Calculated water depth with current cross- section

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Flow discharge associated with boulder initiation Using Manning’s equation (n=0.035): Flow velocity (m/s) Discharge (m 3 /s) Stream power (W/m) Unit stream power (W/m 2 ) At bankful conditions Floodplain completely submerged At P1 gauging station: Peak discharge for 2005 and 2011 floods were 695 m 3 /s and m 3 /s.

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Findings 15 boulders measured Average intermediate axes: 2.16 ± 0.48 metres Range of estimated flow velocities for boulder initiation: 6.68 – m/s. These velocities gave calculated water levels that ranged from 1m below bankful flow to 28 metres above bankful flow (complete floodplain inundation). Using current day cross-section profile, calculated flow discharge associated with boulder movement ranges between m 3 /s to over 4000 m 3 /s.

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Future experiments…. OSL dating of boulders

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