The effectiveness of a multi-row Tamarix windbreak in reducing aeolian erosion and sediment flux, Niatak area, Iran.

Windbreaks are barriers that are widely used to reduce wind speed and aeolian erosion. Most windbreak studies have been done in wind tunnels and generally used rigid objects rather than live plants and most of these studies report on the modification of the flow field and not on the effectiveness at reducing sediment transport. A series of experiments were conducted to monitor the effectiveness of a fourteen-rows Tamarix windbreak in the field and in particular to measure the reduction in sediment erosion and transport. Over the course of six dust storm events, with mean wind speed ranging from 9.4 to 18.2 ms-1, sediment flux and wind speed were measured at seven heights (z/h = 0.05, 0.1, 0.2, 0.4, 0.8, 1.4 and 1.8 where h is the mean height of the windbreak) at five locations upwind (x=-100 m), within (x = 100 and 256 m) and downwind (x = 448 and 560 m) of the windbreak. Largest reduction of sediment flux (a reduction of 50%) was observed at the end of the windbreak (x = 256 m). The decrease in silt and clay component is quite uniform but the relative concentration of sand (>100 µ) generally decreases within the windbreak and remained lower downwind of the windbreak compared to the upwind value in two of the three events for which sediment size data are available. The windbreak is effective in aeolian erosion control under a variety of dusty conditions and, in general, the potential of the windbreak is optimum due to its optimal porosity (39%) and structure (multiple-row design). Because Tamarix withstand harsh conditions, it is optimal to use it as a windbreak for wind and blown-sediment control in arid regions.

Natural and human controls on dune vegetation cover and disturbance.

Beaches and dunes are one of the most heavily used environments on Earth, with tourism and residential uses leading to ecosystem loss and dune degradation. Many coastal dune fields also host a range of economic activities such as farming, mining, and animal grazing, which can affect their evolution. The second half of the 20th century has seen an increase of dune vegetation cover in many dunes around the world, with climatic forcing often cited as a driver for this. However, identification of the relative contributions to landscape change due to climate vs. natural and/or artificial disturbances remains unclear. This poses a problem for managers seeking to maintain some 'desirable' landscape characteristics, because understanding the reasons for dune field change is essential prior to implementing interventions, as is differentiating what is natural from what is not. This study proposes a systematic approach to identifying dune disturbances and isolating them from the effect of climate. The approach assumes that it is possible to measure dune disturbances by comparing observed vegetation cover with that expected due to climate. A semi-quantitative procedure is proposed to explore the existence of disturbance, its significance, and the causes for it. The procedure can also be used in reverse to explore the effect of variables driving disturbance and the likely landscape trajectory if the driver is removed. The approach is tested with a case study of the Sefton dunes in NW England, a large dune field subject to multiple interventions and degrees of human impact. The discussion focuses on the importance of disturbance location and the range of variables involved in changes to vegetation cover at this and other locations. In natural dune fields, it is recommended as best practice to managers that artificial stressors and human-led disturbances are minimized to allow coastal dune systems to evolve naturally.