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

Shaofeng Xu,Junlong Zhang,Jinxi Song,Guotao Zhang,Weiwei Jiang,Ying Xue,Ming Wen

doi:10.3390/w9030170

Abstract

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 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.

Highlights

The hyporheic zone, the transition region from a stream to the surrounding aquifer, acts as a physical, chemical, and biological filter and has been recognized as a critical component of stream ecosystems [1]
The water exchange between streams and their hyporheic zones, termed hyporheic water exchange, is influenced by variations in the hydraulic gradient over the stream channel boundary stemming from geomorphic features, such as stream meanders, bars, dunes, step-pools, and in-stream structures [6,7,8]; a chalk stream overlain by Palaeogene deposits and superficial drift from the Quaternary [9]; or ambient groundwater discharge [10]
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

Summary

Introduction

The hyporheic zone, the transition region from a stream to the surrounding aquifer, acts as a physical, chemical, and biological filter and has been recognized as a critical component of stream ecosystems [1]. The water exchange between streams and their hyporheic zones, termed hyporheic water exchange, is influenced by variations in the hydraulic gradient over the stream channel boundary stemming from geomorphic features, such as stream meanders, bars, dunes, step-pools, and in-stream structures [6,7,8]; a chalk stream overlain by Palaeogene deposits and superficial drift from the Quaternary [9]; or ambient groundwater discharge [10]. It is 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]

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