Distribution Characteristics Of Nutrients Around Environmental Restoration Groynes

In recent years,the excessive interference from human made the degradation of aquatic ecosystem.Therefore,restoration of the degraded water environment has become an urgent demand.Groyne,a traditional hydraulic structure,not only plays a crucial role in channel stabilization,bank protection and navigation enhancement in alluvial rivers,but also plays an important role in recovering aesthetics,amenity and ecological functions of rivers.It is considered as one of the promising method for river environment restoration.Although a lot of research has been conducted over the past several decades to clarify the hydraulics and morphodynamics around groynes,the mechanism associated with the environmental restoration functions is yet unknown.This study aims at clarifying the local bed topography,grain size distribution in the substrate,flow structure and distribution of typical nutrients phosphorus around the groynes through the analysis of water samples and sediment samples and the determination of the bed level and flow level around a group of groynes in Kokubu river,Kochi,Japan in 2016.Considering the importance of nutrients in linking the physical environment and ecology environment as well as in maintaining life activities of riverine biospecies,special attention has been paid to the spatial and temporal distribution characteristics of bioavailable phosphorus(BAP)which is mainly from agricultural drainage and plays a key role in the growth of aquatic plants.Based on the results,the relationships among the distribution of phosphorus concentrations,the bed topographies and flow fields are explored,and the mechanism of groynes in environmental restoration is discussed.Firstly,the bed topographic features and flow fields around typical groynes were analyzed.The results demonstrated that groynes largely influenced the flow structure,sediment transport and local bed topography.Due to the flow separation,the thalweg was shifted away from the river bank,resulting in flow acceleration in the mainstream and flow reductions in the groyne fields.At the heads of the groynes,the strong vortices,flow separations and local scour holes were formed.In the groyne fields,flow velocities are small and horizontal vortex systems of different sizes were observed,which lead to sediment deposition especially in the near-bank regions.The results showed that both shallow and deep water regions,and both rapid and slow flow conditions were formed around groynes.Therefore,the groynes could largely enhance the diversity of riverine habitats.Secondly,the spatial and temporal distribution characteristics of phosphorus were analyzed in the water and bed sediment samples around groynes.The results indicated that the particulate phosphorus(PP)was the main form of total phosphorus(TP)in water.The available particulate phosphorus(BPP)was the main form of BAP,while dissolved reactive phosphorus(DRP)accounted for a small proportion of BAP in water.The BAP content of sediment in the wake of the most downstream groyne was significantly higher than other regions.Moreover,the concentrations of four different forms of BAP were in a sequence as follows: algae available phosphorus(AAP)> NaHCO3 extraction phosphorus(Olsen-P)> water soluble phosphorus(WSP)> readily desorption phosphorus(RDP)in TP.The results indicate that determination of AAP and TP in the sediment is effective for the evaluation of BAP release and potential contribution to the growth of algae around groynes.The concentrations of these BAP content varied with the seasons.The concentrations of WSP and RDP in January were larger than August,and the AAP and Olsen-P contents in August were larger than January,which indicates that the growth of algae and other aquatic plants are closely related to the change of BAP in sediment.Finally,the characteristics of grain size distribution in the riverbed and its relation with the nutrients distribution were analyzed.It was found that sediment properties in the mainstream were completely different from those in the groyne fields.Fine sediment deposited along the bank of the channel with a relatively small medium grain size D50 and a large standard deviation ?g.On the other hand,as fine particles were washed away by the rapid flow in the proximity of the groyne heads,coarse grains dominated with a relatively large medium grain size D50 and a small standard deviation ?g.According to the results of phosphorus concentrations along different cross-sections,the phosphorus concentration in the water was decreasing from upstream to downstream and the peak was found near the agricultural drainage in the study area.The terrain about the negative side water of groyne changed dramatically.In this area,the local scour is serious and phosphorus concentration was higher than other sites.There was similar concentration between the downstream region and the groyne wake.The phosphorus contents in the riverbed increased from upstream to downstream due to the accumulation of fine sediment in the groynes wakes.The nutrients content in the head of groynes was less than other zones due to the serious erosion in this area.On the other hand,nutrients content in the root of groynes was higher than other points as nutrients were trapped by the lowvelocity flows.The phosphorus content about positive side water of groyne was higher than that in the negative side water of groyne for the serious erosion.There is a certain relationship between the content of nutrient and river bed level in the surface sediment with bed level reducing nutrients content decreasing.Above all,the distribution of phosphorus nutrients in water was greatly affected by the agricultural drainage.The distribution of phosphorus content in the sediment was greatly resulted by the distribution of the bed material and less affected by the agricultural drainage.