Zonation patterns of riparian vegetation have been sampled and described in mountain streams in two catchments in the Hottentots-Holland Mountains, Western Cape, South Africa. Six main vegetation types that differ in ...Zonation patterns of riparian vegetation have been sampled and described in mountain streams in two catchments in the Hottentots-Holland Mountains, Western Cape, South Africa. Six main vegetation types that differ in structure and species composition, are dominant along these river banks: Aquatic vegetation, Wetbanks, Palmiet, Scrub, Forest and Shrubland(Fynbos). The study aims to correlate the vegetation patterns to flooding patterns, in particular the inundation frequency and stream power. A problem arises: because these catchments are ungauged, like most mountain catchments, with the only weirs at the downstream end of the catchment. Discharge data at the weirs are extrapolated to the sites upstream by multiplication with a factor based on the size of the subcatchment that drains through a sample site. In this way, recurrence intervals for floods in mountain streams are derived. Discharges at sites are also calculated using bed roughness(Manning's n) and slope in straight sections with uniform flow conditions. Stream power is derived from the discharges calculated in this manner. The combination of stream power and recurrence intervals explains the occurrence of most vegetation types occurring on the banks, except for one type: Afromontane Forest. This type is probably more dependent on other factors, such as protection from fire and the depth of the groundwater table.展开更多
The flow resistance factors of non-submerged rigid vegetation in open channels were analyzed. The formulas of drag coefficient CD and equivalent Manning＇s roughness coefficient na were derived by analyzing the force ...The flow resistance factors of non-submerged rigid vegetation in open channels were analyzed. The formulas of drag coefficient CD and equivalent Manning＇s roughness coefficient na were derived by analyzing the force of the flow of non-submerged rigid vegetation in open channel. The flow characteristics and mechanism of non-submerged rigid vegetation in open channel were studied through flume experiments.展开更多
The characteristics of the roughness coefficient are very important for practical application. Some experiments are conducted to study the variation of Manning＇s n with flow depth, mean velocity, and density of veget...The characteristics of the roughness coefficient are very important for practical application. Some experiments are conducted to study the variation of Manning＇s n with flow depth, mean velocity, and density of vegetation,. An assumed velocity distribution to describe the vegetative flow of submerged vegetation is confirmed by experimental results. The measured velocities in this study seem to have little effect on the curve of n ~ h, and a new linear relationship between Manning＇s n and flow depths is observed clearly. According to the arguments that the flow resistance of densely unsubmerged vegetation is dominated by the resistance exerted on vegetations, the influence of the density of vegetation on Manning＇s n is estimated. On the basis of the velocity distribution, the n - h curve under submerged condition is theoretically obtained from the n - h curve under unsubmerged condition. These results are also well confirmed by experimental results and very significant for practical applications.展开更多
基金supported by funding from the National Research Foundation to C. Boucher and the VSB Funds (the Netherlands) to E. Siebenadditional funding from the Water Research Commission
文摘Zonation patterns of riparian vegetation have been sampled and described in mountain streams in two catchments in the Hottentots-Holland Mountains, Western Cape, South Africa. Six main vegetation types that differ in structure and species composition, are dominant along these river banks: Aquatic vegetation, Wetbanks, Palmiet, Scrub, Forest and Shrubland(Fynbos). The study aims to correlate the vegetation patterns to flooding patterns, in particular the inundation frequency and stream power. A problem arises: because these catchments are ungauged, like most mountain catchments, with the only weirs at the downstream end of the catchment. Discharge data at the weirs are extrapolated to the sites upstream by multiplication with a factor based on the size of the subcatchment that drains through a sample site. In this way, recurrence intervals for floods in mountain streams are derived. Discharges at sites are also calculated using bed roughness(Manning's n) and slope in straight sections with uniform flow conditions. Stream power is derived from the discharges calculated in this manner. The combination of stream power and recurrence intervals explains the occurrence of most vegetation types occurring on the banks, except for one type: Afromontane Forest. This type is probably more dependent on other factors, such as protection from fire and the depth of the groundwater table.
文摘The flow resistance factors of non-submerged rigid vegetation in open channels were analyzed. The formulas of drag coefficient CD and equivalent Manning＇s roughness coefficient na were derived by analyzing the force of the flow of non-submerged rigid vegetation in open channel. The flow characteristics and mechanism of non-submerged rigid vegetation in open channel were studied through flume experiments.
基金the National Natural Science Foundation of China (Grant No.50139029)the Chinese Offshore Investigation and Assessment 908 (PJ9:Study on effect of the Yangtze estuary flow field to the important sea project of shanghai).
文摘The characteristics of the roughness coefficient are very important for practical application. Some experiments are conducted to study the variation of Manning＇s n with flow depth, mean velocity, and density of vegetation,. An assumed velocity distribution to describe the vegetative flow of submerged vegetation is confirmed by experimental results. The measured velocities in this study seem to have little effect on the curve of n ~ h, and a new linear relationship between Manning＇s n and flow depths is observed clearly. According to the arguments that the flow resistance of densely unsubmerged vegetation is dominated by the resistance exerted on vegetations, the influence of the density of vegetation on Manning＇s n is estimated. On the basis of the velocity distribution, the n - h curve under submerged condition is theoretically obtained from the n - h curve under unsubmerged condition. These results are also well confirmed by experimental results and very significant for practical applications.
