Showing papers on "Stream power published in 1995"

Bed Load Sediment Transport in an Ephemeral Stream and a Comparison with Seasonal and Perennial Counterparts

Abstract: Bed load sediment flux in an ephemeral channel, the Nahal Yatir, is shown to be a comparatively simple function of stream power and to reach levels that are several orders of magnitude higher than maxima measured at similar levels of stream power in perennial counterparts. Channel average submerged unit flux rates are recorded as high as 4.3 kg s−1 m−1, while at the center of the channel, the highest rate recorded is 6.5 kg s−1m−1. Transport efficiency is at least an order of magnitude higher than in other channels for which there are comparable data and, on average, as much as 400 times that of Oak Creek. These differences are explained by the fact that the bed of the Yatir is not armored. It is surmised that the unvegetated nature of this desert watershed provides ample supplies of sediment of all sizes and that this, together with the rapid recession of the flash flood hydrograph and the extended periods of no flow, discourages the development of an armor layer. The flux rates are not sediment supply-limited, as they are in perennial channels. Nahal Yatir and Oak Creek represent two ends of a spectrum, between which come seasonal and less well armored perennial streams. Transport efficiency is shown to vary considerably for each stream and from one stream to another, suggesting that it may not be possible to incorporate it easily into bed load equations in order to improve levels of prediction.

Particle size variations between bed load and bed material in natural gravel bed channels

Abstract: Particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled, natural gravel bed channels. The ratio (D*) of median particle size of bed material to the transport- and frequency-weighted mean of median bed load size decreases to unity with increasing drainage area, bank-full discharge, dimensionless stream power, and bed material sorting. In channels with high values of D*, significant volumes of fine bed load are transported during discharges that are less than bank-full, which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels. This indicates transport of fine bed load over a more stable substrate of coarser bed material. The apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size. Likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities. A simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low-order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour.

The nahal yatir bedload database: Sediment dynamics in a gravel-bed ephemeral stream

Abstract: The Nahal Yatir Bedload Transport Database arises from the Northern Negev River Sediment Monitoring Programme and represents the first body of bedload information to be collected during flash floods in desert gravel-bed streams. Bedload flux was established automatically with three slot-samplers of the Birkbeck type. This was complemented by hydraulic measurements that allow sediment transport to be rated against channel average shear stress and specific stream power. Bed material and bedload grain-size distributions are also provided. The database is given in both printed and electronic tabular format.

Morphological Impacts and Implications of a Trial Release on the Wingecarribee River, New South Wales

Abstract: A trial, monitored release on the Wingecarribee River over 27 days had a peak discharge below the Wingecarribee Dam of 1090 ML/d, which had a flow duration of 0.95 per cent. Morphological impacts on 29 km of channel below the dam included bank erosion, concentrated neck overflow, bed degradation and channel widening at recent cutoffs, sedimentation in weir pools and overbank flow, particularly at discharges greater than 800 ML/d. Prolonged operational releases at bankfull discharge (400–800 ML/d) will have the greatest morphologic impacts and should be avoided. Pulses which dissipate stream power over a broad, well-vegetated floodplain should be incorporated in the operational release policy, provided such pulses are phased to facilitate channel and vegetation recovery.