沉水植被降低水体浊度的机理研究

Water Flow with Submerged Vegetation and Its Effect on Water Turbidity

水体浊度的变化影响着水生态系统的健康,而沉水植被对降低水体浊度具有显著效果.为明确沉水植物对泥沙的作用机理,以苦草为试验用草,开展室内水槽试验.结果表明,苦草斑块降低了水流的输沙能力,根据试验资料估算,斑块区水流的输沙能力仅约为无草时的10%.利用象限分析法分析了实测的紊动流场,解释了植物对泥沙作用的物理过程.在苦草斑块区,植物冠顶以下区域(相对高度小于1.00)为下扫流态,紊动漩涡形成的下扫流将泥沙带入水底,而近底层的喷射流抵消了下扫流的能量,抑制下扫流对泥沙的冲击起动,并使下扫流挟带的泥沙顺利淤积;在植物冠顶以上区域除高密度(172株m2)工况为下扫流态外,中、低密度(86、43株m2)工况均为喷射流态,不利于泥沙沉降.由于泥沙主要存在于下层水体,因此沉水植被有利于减小水体浊度.

Water turbidity affects the health of aquatic ecosystems. Submerged vegetation has an apparent effect in reducing water turbidity. This study aimed to reveal the action mechanism of submerged vegetation on sedimentation, which is closely related to water turbidity. Eelgrass was used as the test material in an indoor flume experiment. The results show that the eelgrass patch reduced the sediment carrying capacity significantly. According to the experimental data, the sediment carrying capacity in the plant patch area was reduced to about 10% of the non-plant case. This paper analyzed the measured turbulent flow field by using quadrant analysis, and explained the action mechanism of submerged vegetation on the sedimentation. The turbulent flow pattern is a sweep event below the canopy （ relative height is less than 1.00）. The sweep event formed by turbulent eddies brings the sediment toward the bed. Near the bed the turbulent flow is an upward eject event which meets the downward sweep event and offsets the energy of the sweep event. This then restrains the impact action of the sweep flow on the bed material and also promotes sediment brought by the sweep flow to he deposited on the bed. Above the canopy, the turbulent flow is a sweep event for high plant density （ 172 plants/m^2 ）, while the turbulent flow is an ejection event for medium and low plant density （86,43 plants/m^2） which is not in favor of sedimentation. Submerged vegetation reduces water turbidity, Since sediment is mainly in the lower part of the water.