Variability of Darcian Flux in the Hyporheic Zone at a Natural Channel Bend

Shaofeng Xu, Jinxi Song, Weiwei Jiang, Guotao Zhang, Ming Wen, Junlong Zhang, Ying Xue
2017 Water  
Channel bends are one of the most important characteristic features of natural streams. These bends often create the conditions for a hyporheic zone, which has been recognized as a critical component of stream ecosystems. The streambed vertical hydraulic conductivity (Kv), vertical hydraulic gradient (VHG) and Darcian flux (DF) in the hyporheic zone were estimated at 61 locations along a channel bend of the Beiluo River during July 2015 and January 2016. All the streambed attributes showed
more » ... ributes showed great spatial variability along the channel bend. Both upward fluxes and downward fluxes occurred during the two test periods, most of studied stream sections were controlled by downwelling, indicating stream water discharge into the subsurface. The average downward flux was higher at the downstream side than at the upstream side of the channel bend, especially in July 2015. The distribution of streambed sediment grain size has a significant influence on the variability of Kv; high percentages of silt and clay sediments generally lead to low Kv values. Higher Kv at the depositional left bank at the upstream site shifted toward the erosional right bank at the downstream site, with Kv values positively correlated with the water depth. This study suggested that the variabilities of Kv and VHG were influenced by the stream geomorphology and that the distribution of Kv was inversely related, to a certain extent, to the distribution of VHG across the channel bend. Kv and VHG were found to have opposite effects on the DF, and the close relationship between Kv and DF indicated that the water fluxes were mainly controlled by Kv. the hydraulic gradient over the stream channel boundary stemming from geomorphic features, such as stream meanders, bars, dunes, step-pools, and in-stream structures [6-8]; a chalk stream overlain by Palaeogene deposits and superficial drift from the Quaternary [9]; or ambient groundwater discharge [10] . The channel bend is one of the characteristic features of all streams and favors the formation of hyporheic zones [11] . It is now understood that the interfacial flux of the stream water and streambed increases with sinuosity and that the meander apex experiences the largest flux [6]. These fluxes toward or away from sinuous streams and hyporheic zones have implications for biogeochemical and ecological processes along the fluvial corridor from the river to riparian zones [12] . Many measurement methods can be used to determine the velocity or flux of the surface water, aquifer and transition zones, including the Darcy equation, tracer tests, temperature gradient, and seepage meters, but these methods all have certain limitations due to different measurement scales and hydrogeological conditions; therefore, it is important to choose the method most appropriate to the study goal to characterize the interaction between rivers and their hyporheic zones [13] . The specific discharge between stream and streambed can be obtained using the Darcian flux calculations employing the hydraulic conductivity and hydraulic gradient. The variability of streambed hydraulic conductivity depends on sedimentary characteristics, especially the distribution of the sediment grain size, and is related to the erosional and depositional processes induced by varying stream flows and influenced by the stream morphology [14, 15] . Another parameter characterizing the sedimentary hydrogeological control of hyporheic water exchange is the porosity of sediments [16] . Generally, hydraulic conductivity increases with particle size, but this relation can be modified by changes in overall porosity [17] . The hydraulic gradient is also one of the important streambed attributes used to provide an estimate of the potential strength of a hydrological exchange and has a significant influence on stream infiltration and storage zone in the aquifer [18, 19] . The direction and magnitude of Darcian flux vary greatly in different locations due to changes in these two variables, hydraulic conductivity and gradient, spatially and temporally, induced by dynamic environments in the stream, and have been identified as the two main factors controlling water exchange between streams and the surrounding groundwater systems [20, 21] . According to various laboratory experiments, the hyporheic water exchange rate is proportional to the square of the stream water velocity and to the permeability of streambed sediments and inversely proportional to the porosity of the sediments and to the depth of the streambed [16]. However, under field experiments, the relationship of these characteristics is not well established, especially in the channel bend of a natural stream. The objective of this study is to determine the variability of Darcian flux, streambed hydraulic conductivity, and head gradient at a natural channel bend and further reveal the relationship among these three streambed attributes in the hyporheic zone. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted:
doi:10.3390/w9030170 fatcat:v2x7pni4fbedzcd4lyuhp2nxrm