This study evaluates the effects of floods and their hydraulic characteristics on invasive vegetation in the Ahuriri River floodplain, a braided gravel-bed river in the eastern high country of the South Island, New Zealand. Invasive vegetation alters the natural braiding tendency of rivers such as the Ahuriri and has severe negative impacts on threatened and endangered native wading birds and floodplain ecology. Aerial photographs for a 10-year period (1991 to 2000) that includes the largest flood on record in 1994 (recurrence interval of almost 50 years) based on a 46-year record were analysed to determine changes in aerial cover of water, bare substrate, and several different vegetation classes. This was complemented by hydraulic modelling using HEC-RAS software for a representative 1.1-km reach to evaluate flood hydraulic characteristics that can impact vegetation, including water inundation area, depth, velocity, and shear stress. The 1994 flood (570 m 3 /s) removed up to 25% of the invasive vegetation in the study reach, with preferential removal of lupin and less removal of willow and grassland. However, flood effects varied considerably with overall minor effects on total vegetation cover over the entire lower 21-km reach. The spatial distribution of vegetation cover changed considerably in response to this flood event, in a pattern resembling a shifting-mosaic steady state model. Modelling showed that a 600 m 3 /s (50-year) flood is likely to result in almost total inundation of the study reach floodplain and significant velocities and shear stresses on floodplain vegetation. Over the study reach linear models approximate the relationships between flood peak magnitude with average water area, depth, velocity, and shear stress. Depth and velocity do not increase as fast as flow area or shear stress, and differences between hydraulic characteristics for the 600 and 155 m 3 /s floods ranged from 37% for velocities to 179% for flow inundation areas. Much of the vegetation appeared to recover relatively quickly after flood events so that vegetation removal effects are short-lived. Although large floods can remove a considerable amount of invasive vegetation in some reaches of the river, natural flood events are unlikely to significantly reduce invasive vegetation cover throughout the river.
remarked that the Author's excellent description of the Main Drainage of Cairo was especially interesting to him, as he was acquainted with some of the troubles of drainage work in the East ; and the Paper was a valuable record of the way in which the Author had grappled wit;h the diaculties.It was not an easy Paper to discuss, as it was chiefly descriptive, but he thought one point that might usefully be dwelt upon was the statement that the sewers were laid a t gradients which would give a velocity of liquid of 3$ feet per second when running half-full. Incidentally he might remark that the only sewer for which the Author gave any data gave a velocity of 3 feet per second. As that was the main collector, possibly the Author had been satisfied i n t h a t case with a less velocity than 3$ feet per second. I n all Eastern countries the difliculty of keeping sewers clean was immense, chiefly on account of the habits of the natives and the large quantity of road-detritus, sand, etc., which they put into the sewers. When he was consulted in connection with the Bombay scheme, he found t h a t the whole of the sewers of the main drainage of that city were half-full of sand and silt, and it was extremely difficult to know what was the cause of it and what was to be done with it. According to calculations, the main sewers of Bombay would give a velocity of liquid between 3 and 35 feet per second. The main sewer was 8 feet by 5 feet 4 inches, ovoid, with a gradient of I in 2,200, and when it was running half-full the velocity should be 200 feet per minute, which, according to t,he theories and knowledge of drainage-schemes in Great Britain, should carry forward all the silt. He had found that the silt collected because the engines provided at the end of the main sewer were running at a fixed speed :tnd their suctions were not nearly low enough. Consequently, the main sewer of Bombay leading to the pumping-station was acting as R depositing-chamber and becoming filled with road-detritus which refused to move, or moved very slowly. That blocked the remainder of thc sewers, with w r y disastrous results.IIe lutd tlwcforc suggested to the
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