Mixing and circulation at the confluence of two rivers entering a meandering reservoir

Abstract

[1] A field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows (Ebro and Segre) with distinct physical-chemical characteristics as they join at the upstream end of Ribarroja reservoir in northern Spain. Given the short average residence time of water in the reservoir, the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam. In winter, inflows have similar densities-Á/ 0 % O(10 À5 )-and their spatial distribution is largely determined by inertial forces, and in particular, by the discharge ratio. Downstream of the confluence, both rivers flow side by side and largely unmixed over long distances. In summer, with Á/ 0 of O(10 À3 ), the flow fields at the confluence are largely controlled by buoyancy forces. Atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms, leading to localized high mixing rates. Mixing, in general, though is weak at this time of the year. In fall and early winter, density differences are largely controlled by conductivity differences between the incoming flows. The warmer Ebro water, with larger thermal inertia, flows beneath the colder Segre water. The spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong, likely as a result of mixed water being denser than either of the incoming flows.

Mixing dynamics at the confluence of two large rivers undergoing weak density variations

“…Inflow temperatures were allowed to vary in time, with the hourly variations within any given day of the simulation following the observations collected on day 50 (2009). Wind forcing was weak at the time of the experiments [ Ramón et al ., ] and was assumed negligible in the model. The model was run until more than 99% of the tracer mass initially existing in the domain has left (approximately 9 days).…”

“…Mixing lengths between the W‐River and N‐River are of O(10 1 ) channel widths which are indicative of rapid mixing between the confluent streams, consistently with the field observations of Ramón et al . [] and the prior results on mixing in river confluences [e.g., Rhoads and Kenworthy , ; Rhoads and Sukhodolov , ; Lane et al ., , and references therein].…”

“…The selection of the values of R q of 0.65 and 1.53 was intended to include situations in which inflows from the W‐River are still dominant but R q is closer to 1, and the reversed situation, in which the N‐River dominates the flow. All discharge ratios are within the range of variation of R q observed in Ribarroja under weakly stratified conditions [e.g., Ramón et al ., ]. The simulated velocity fields for the different scenarios were used to drive transport simulations in which the W‐inflows were traced as they entered into the computational domain.…”

“…The values of u * and D were, in turn, estimated from field data ( U = 0.4 m s −1 and D = 10 m; Ramón et al . []). Preliminary simulations showed, however, that the actual value of K h can be ignored for advection of scalars, since changes in mixing rates of at most 4% occurred in response to decreases in K h from 10 −2 to 0 m 2 s −1 .…”

“…Inflow rates and densities at the upstream boundaries, in turn, were set to follow the field data of Ramón et al . []. Inflow rates from the W‐River were set to a constant value of 730 m 3 s −1 and were distributed uniformly in the inflow section.…”