A three dimension hyporheic model of the River Bure: Understanding the nutrient dynamics and the role of streambed heterogeneity
Abstract
The hyporheic zone is often de fined as the zone where mixing of surface
water and groundwater occurs in shallow sediments beneath and adjacent
to rivers. This mixing contributes to create unique biogeochemical conditions
that may attenuate contaminants from either upstream surface water
or groundwater under gaining and losing conditions. Hyporheic exchange
results from di erences in the channel near-bed head as it varies in space
in response to interactions between surface flow and bed topography, with
the interaction with the water table playing an important role too. Reactions
of contaminants in groundwater also dependent on mixing between
surface and subsurface water, which occurs in this zone. Therefore, representation
of the pro le of upwelling and downwelling exchange between surface water and groundwater have important consequences for contaminant transport. The present work studies nitrogen fate within a restored reach of the River Bure, Norfolk, United Kingdom. To this end, we confront
numerical simulations of the hyporheic
ow and tracer transport with field
measurements of surface flow properties, nearby groundwater table and nitrogen
compound concentration. . We numerically model mixing between hyporheic flow paths induced by sediment, bedform, meanders on riverbed, and flow paths of adjacent upwelling of deeper groundwater. Results of the analysis indicate that despite the coarse topographical data and with limited
surface water hydraulic data it is possible to defi ne the spatial extent
of hyporheic exchange and potential mixing zones for contaminants as a
function of residence time. The proposed work has the potential to depict
high residence time zones and biogeochemical reactivity in homogeneous
and heterogeneous sediments. Furthermore, eldwork analysis shows that
in this site the hyporheic zone have a little e ffect on nutrient concentration.
In addition, hydraulic modeling results indicate that streambed discharge
signi cantly influences hyporheic exchange. Especially, the residence times
under average stream discharge conditions are higher than the ones under
the high discharge conditions mostly on the part that has ri e - pool morphology.
From hydraulic point of view, heterogeneous domain has higher
connectivity than the homogeneous ne sand subsurface set. Such that, subsurface
flow has tendency to flow through high hydraulic conductivity zones, which is de ned as tunneling eff ect, therefore, low conductivity zones have minor e ect on hyporheic flow. The predictions based on the DaO2 index proves that heterogeneous sediment formations have more aerobic potential,
however, prevailing anaerobic conditions occur mostly vicinity of low
hydraulic conductivity zones.
Authors
G okdemir, C a gr iCollections
- Theses [3822]