Presentation on theme: "Future floods: An exploration of a cross-disciplinary approach to flood risk forecasting (26-27 February 2015) Estimating flood discharges using boulders,"— Presentation transcript:
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:
Floods in Chiang Mai city (Source: Boonserm Satrabhava,
Ping River gauging station, P1 (94 yrs of data) Royal Irrigation Department The need for data – do we have enough?
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:
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
Boulders in Thai river Boulders (granite, orthoquartzite), Mae Taeng River
Study site – Mae Taeng River
Cross-section profile at Mae Taeng bridge (Royal Irrigation Department)
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)
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
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)
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
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.
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.