Abstract
A flash flood on 28th September, 2012, rose to a peak discharge of 2357 m3 s−1 from zero within one hour in the ephemeral Nogalte channel in SE Spain. Channel morphology and sediment sizes were measured at existing monitored sites before and after the flood and peak flow hydraulics calculated from surveyed floodmarks and cross-sections. Maximum peak sediment fluxes were calculated as ~600 kg s−1 m−1, exceeding maximum published, measured dryland channel values by 10 times and common perennial stream fluxes by 100 times. These high fluxes fit the established simple bedload flux - shear stress relations for dryland channels very well, but now extended over a much wider data range. The high sediment fluxes are corroborated by deposits at >1 m height in a channel-side tank, with 90 mm diameter sediment carried in suspension, by transport of large blocks and by massive net aggradation as extensive, structureless channel bars. Very high sediment supply and rapid hydrograph rise and recession produced the conditions for these exceptional sediment dynamics. The results demonstrate the extreme sediment loads that may occur in dryland flash floods and have major implications for catchment and channel management.
Highlights
Flash floods in semi-arid areas can be very hazardous and damaging, arising from water flow and inundation and from the sediment dynamics and impacts of sediment movement
Sediment flux was calculated for each cross-section using MPM and Bagnold sediment transport equations (Table 1), for pre -flood morphology and d50 sediment sizes of 5, 10 and 20 mm
Additional evidence of the size of material transported during the event is from deposits in a large, 45 × 15 m area, water storage tank/reservoir located at the channel side in the confined, middle part of the course, between sites Nog[2] and NogMon (Fig. 1), and beyond a wall 1–1.3 m high above the channel bed, which acted as a sediment trap in the event (Fig. 4a)
Summary
Flash floods in semi-arid areas can be very hazardous and damaging, arising from water flow and inundation and from the sediment dynamics and impacts of sediment movement. Sediment flux was calculated for each cross-section using MPM and Bagnold sediment transport equations (Table 1), for pre -flood morphology and d50 sediment sizes of 5, 10 and 20 mm.
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