Hyporheic exchange of solutes and colloids with moving bed forms

Abstract

Abstract. Stream-subsurface exchange provides the opportunity for stream-borne substances to interact with streambed sediments in the subsurface hyporheic mixing zone. The downstream transport of both solutes and colloids can be substantially affected by this exchange, with significant implications for contaminant transport and stream ecology. Several previous studies have demonstrated that bed form-induced advective flows (pumping) and scour/deposition of bed sediments (turnover) will often be the dominant processes controlling local exchange with the streambed. A new model is presented for combined turnover and pumping exchange due to relatively fast-moving bed forms, i.e., when turnover dominates the exchange in the upper part of the bed where active bed sediment transport occurs. While turnover rapidly mixes the upper layer of the bed, advective pumping produces exchange with the deeper, unscoured region of the subsurface. The net exchange due to these processes was analyzed using fundamental hydraulic principles: the initial exchange was calculated using an existing geometric model for turnover, and then the later exchange was determined by analyzing the advective flow induced under the moving bed form field. The exchange of colloidal particles due to moving bed forms was also modeled by considering the further effects of particle settling and filtration in the subsurface. Experiments were conducted in a recirculating flume to evaluate solute (conservative Li +) and colloid (kaolinite) exchange with a sand bed. The solute and colloid exchange models performed well for fast-moving bed forms, but underpredicted the colloid exchanges observed with lower rates of bed sediment transport. For very slowly moving bed forms it was found that turnover could be completely neglected, and observed colloid exchanges were represented well by a pure pumping model. In the intermediate case where turnover and pumping rates are similar, water carried into the bed by turnover is immediately released by pumping, and vice versa. Thus, while this work further elucidated the basic processes controlling solute and colloid exchange with a bed covered by bed forms and provided a fundamental model for exchange due to fast-moving bed forms, exchange in the intermediate case where turnover and pumping tend to compete can only be bounded by current models.

River bedform inception by flow unsteadiness: A modal and nonmodal analysis

“…Research interest about these morphologies derives from their relevance in hydraulic engineering and environmental applications. River bedforms not only interfere with river navigation [1,2] and human infrastructure [3,4], they also induce differential pressure gradients that modify the flow field and consequently the overall hydraulic resistance, induce hyporheic fluxes [5,6], and affect underground flows through preferential patterns within ancient sedimentary deposits [7,8]. The present work focuses on dunes and antidunes (see Fig.…”

Transport of nonsorbing solutes in a streambed with periodic bedforms

“…As discussed above, the quantity and concentration of solute in the overlying water decrease as the solute is transferred into the bed. This trend has been featured in the model predictions based on equation (14). To better match the experimental data, we adjusted the hydraulic conductivity of …”

“…where v is the kinematic viscosity of water, and the values of the various model 14 constants are  = 5/9,  = 3/40, * = 9/100 and k…”

“…It should be noted that the overlying water is relatively well mixed as shown in our experiments and hence a uniform concentration was assumed there. To simulate the flume experiments, t C , varying with time as a result of the mass exchange between the overlying water and the bed, was calculated as follows: (14) where 0 C is the initial solute concentration of the overlying water, (14) represents the amount of NaCl stored in the bed.…”

“…The hyporheic zone plays an important role in pollutant and nutrient transport, energy/heat transfer, and carbon cycling within a river/stream system [10][11][12][13][14][15]. It is a dynamic zone that experiences active exchange of mass (solute) and energy (heat) with the overlying stream water [13,16].…”