Characterization of Hyporheic Exchange Drivers and Patterns Within a Low-Gradient, First-Order, River Conﬂuence During Low and High Flow

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Article Characterization of Hyporheic Exchange Drivers and Patterns within a Low-Gradient, First-Order, River Conﬂuence during Low and High Flow

Ivo Martone 1, Carlo Gualtieri 1,* and Theodore Endreny 2

1 Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, 80125 Naples, Italy; [email protected] 2 Department of Environmental Resources Engineering, College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210, USA; [email protected] * Correspondence: [email protected]

Received: 14 January 2020; Accepted: 26 February 2020; Published: 28 February 2020

Abstract: Confluences are nodes in riverine networks characterized by complex three-dimensional changes in flow hydrodynamics and riverbed morphology, and are valued for important ecological functions. This physical complexity is often investigated within the water column or riverbed, while few studies have focused on hyporheic fluxes, which is the mixing of surface water and groundwater across the riverbed. This study aims to understand how hyporheic flux across the riverbed is organized by confluence physical drivers. Field investigations were carried out at a low gradient, headwater confluence between Baltimore Brook and Cold Brook in Marcellus, New York, USA. The study measured channel bathymetry, hydraulic permeability, and vertical temperature profiles, as indicators of the hyporheic exchange due to temperature gradients. Confluence geometry, hydrodynamics, and morphodynamics were found to significantly affect hyporheic exchange rate and patterns. Local scale bed morphology, such as the confluence scour hole and minor topographic irregularities, influenced the distribution of bed pressure head and the related patterns of downwelling/upwelling. Furthermore, classical back-to-back bend planform and the related secondary circulation probably affected hyporheic exchange patterns around the confluence shear layer. Finally, even variations in the hydrological conditions played a role on hyporheic fluxes modifying confluence planform, and, in turn, flow circulation patterns.

Keywords: environmental hydraulics; river conﬂuence; conﬂuence hydrodynamics; hyporheic exchange; vertical hydraulic gradient; heat tracing; hydraulic conductivity

1. Introduction River confluences are nodes in fluvial systems where the combining flows converge and realign further downstream, generating complex hydraulic processes within a region called confluence hydrodynamic zone (CHZ). It is known that confluence geometry, the momentum flux ratio between the merging rivers, the level of concordance of riverbeds at confluence entrance, as well as density differences could cause significant vertical, lateral, and streamwise gradients in velocity forming several distinct hydrodynamic zones [1–5]. Confluences also have important ecological and morphological functions, related to water chemistry, riverine ecology, in-stream, and riparian vegetation [6], physical habitat, as well as biological activity (fish spawning, feeding, etc.), resulting in highly varied habitat for organisms [7–10]. The hyporheic exchange (HE) has been shown to significantly affect riverine systems [11]. This exchange is characterized by river waters entering the groundwater alluvium as downwelling fluxes

Water 2020, 12, 649; doi:10.3390/w12030649 www.mdpi.com/journal/water Water 2020, 12, 649 2 of 18 and then emerging into the stream as upwelling fluxes. The streambed volume where surface waters and groundwater mix is termed the hyporheic zone (HZ). Within a delineated hyporheic zone, the hyporheic exchange flux (HEF) is related to spatial variations of energy head, streambed hydraulic conductivity, and alluvium depth and lateral confinement [12,13]. Hence, important roles in the HE is played by topography [14], bed-forms [15], lateral meandering [16], and vertical step-pool sequences [15,17]. As energy head includes pressure, elevation and dynamic heads, their variations related to riverbed morphological features (e.g., bedforms, pool-riffle, and step-pool morphology, etc.) are important drivers of the hyporheic exchange as well as any bottom pressure and velocity fluctuations induced by turbulence [13]. Predicting hyporheic exchange fluxes (HEF) is challenging since lateral and horizontal hyporheic zone dimensions are dependent on water trajectories through the sediment, and vary based on different surface water delineation methods. Despite the large number of experimental, both in the laboratory in the field, and in numerical studies concerning HEF, a definitive method for experimentally detecting hyporheic exchange is still not available due to its three-dimensional characteristic [11]. Despite extensive investigation of hydrodynamics, morphodynamics, and mixing at river confluences in the last decades using field, laboratory and numerical methods [18–20], only two studies have focused on HEF in river confluences so far. Song et al. [21] and Cheng et al. [22] studied HEF about a river confluence, looking at the influence of hydraulic conductivity and river morphology on vertical hyporheic fluxes (VHF), which were investigated through a one-dimensional steady-state heat model, using temperature time series with resistance temperature detectors. The hyporheic patterns observed at the confluence were attributed to confluence hydrodynamics, which are typically attributed to confluence junction angle and the momentum ratio of discharge in the two tributaries [21]. The field data collected in these studies was for a short-time period, missing seasonal drivers that may reveal changes in the process and dynamics. This study aims to characterize the spatial and temporal organization of hyporheic exchange flux across the riverbed. The spatial organization will be examined within sections of the confluence hydrodynamic zone, noting variation in confluence planform, discharge, and bathymetry. The temporal organization will be examined across time, using changes in flow between a dry low flow season and wet high flow season.

2. Field Site The field study was carried out in Marcellus, New York in Onondaga County of the USA, in a pool-riffle sand bed confluence situated within a natural park called Baltimore Woods Nature Center. This is located on 74 hectares of land which includes fields, successional and mature forest, and many brooks and springs. The confluence under investigation is centered at 42.966704◦ latitude and 76.346998 longitude, where Baltimore Brook and Cold Brook join in the park (BB and CB, respectively − ◦ and BBCB the confluence, Figure1) and their watershed area is about 1.73 km 2. The Onondaga County soil survey [23] indicates that most soils in the study area are derived from glacial till (83%); the rest are derived from glacio-fluvial sediments such as outwash, kames, and terraces (8.9%), postglacial lake sediments (2.6%), recent alluvial sediments (4.3%), and recent organic deposits (1.1%). According to the survey, soil permeability ranges from less than 2 to more than 51 mm/h. Climate in this area is characterized as humid continental and is moderated somewhat by the Great Lakes, especially the most proximate of these lakes, Lake Ontario. Lake Ontario creates frequent cloudiness and “lake-effect” precipitation when relatively cool air passes over relatively warm lake waters. Precipitation from late October through late March can be in the form of local snow squalls that produce an average snowfall of 2768 mm/year. As there was no meteorological station in Marcellus, the data collected at Syracuse Hancock International Airport weather monitoring station, located approximately 25 km from the study site, were used. The 1055 mm/year average precipitation reported at the airport is relatively evenly distributed throughout the year, although precipitation is slightly less Water 2020, 12, 649 3 of 18 in the winter, when moisture-holding capacity of the air is lower. Average annual runoff in this area is about 483 mm.Water 2020, 12, 649 3 of 20

Figure 1. BBCBFigure conﬂuence 1. BBCB confluence study study site site with with red red arrows arrows showing showing flow ﬂowdirection direction and blue anddashed blue line dashed line showing theshowing location the of location the shear of the shear layer layer (picture (picture taken taken on on 1 1January January 2019). 2019).

The Onondaga County soil survey [23] indicates that most soils in the study area are derived In this study,from glacial two till field (83%); campaigns the rest are werederived conducted: from glacio-fluvial one sediments during the such fall as outwash, low flow kames, and snow-melt season fromand September terraces (8.9%), to December postglacial lake 2018 sediments (FS-BBCB1) (2.6%), andrecent the alluvial other sediments during (4.3%), a spring and highrecent flow season from Marchorganic to April deposits 2019 (1.1%). (FS-BBCB2). According to Thethe survey, site wassoil permeability chosen based ranges onfrom accessibility less than 2 to more and scale: the than 51 mm/h. Climate in this area is characterized as humid continental and is moderated confluent channelssomewhatwere by thesmall Great Lakes, enough especially to be the surveyed most proximate in detail of these for lakes, studying Lake Ontario. spatial Lake and temporal variability, relationOntario creates between frequent fluxes cloudiness and hydraulic and “lake-effect” conductivity, precipitation comparison when relatively with cool air a 1Dpasses heat transport analytical model,over relatively adopting warm compact lake waters. survey instruments (Table1). Precipitation from late October through late March can be in the form of local snow squalls that produce an average snowfall of 2768 mm/year. As there was no meteorological station in Marcellus, Tablethe data 1. colSummarylected at Syracuse of field campaignsHancock International carriedout Airport at Baltimore weather monitoring Woods Nature station Center., located approximately 25 km from the study site, were used. The 1055 mm/year average precipitation Temperature Campaignreported Bathymetry at the airport is relatively Granulometry evenly distributedKv throughout the year, although precipitationDate is slightly less in the winter, when moisture-holding capacityTime of the air Series is lower. Average annual runoff FS-BBCB1in this area is about X 483 mm. X X September–December 2018 In this study, two field campaigns were conducted: one during the fall low flow and snow-melt FS-BBCB2 X X March–April 2019 season from September to December 2018 (FS-BBCB1) and the other during a spring high flow season from March to April 2019 (FS-BBCB2). The site was chosen based on accessibility and scale: 3. Instrumentationthe confluent and channels Data were Post-Processing small enough to be surveyed in detail for studying spatial and temporal variability, relation between fluxes and hydraulic conductivity, comparison with a 1D heat transport The flowratesanalytical atmodel, the adopting BBCB compact confluence survey wasinstruments measured (Table 1). using the float method. Soil samples were collected3. Instrumentation in the confluence and Data area Post to-Processing identify sediment granulometry and hydraulic conductivity Kv . Vertical temperatureThe flowrates profile at the BBCB monitoring confluence was was measured deployed using within the float themeth confluenceod. Soil samples as were a suitable and accurate methodcollected to in monitor the confluence hyporheic area to identify flux across sediment the granulometry riverbed, usingand hydraulic heat asconductivity a natural Kv tracer. within a one-dimensionalVertical temperature (1D) model profile [21 ,24 monitoring–26]. The was details deployed of within this monitoring the confluence are as provideda suitable and in Section 3.2. accurate method to monitor hyporheic flux across the riverbed, using heat as a natural tracer within Other methods to infer hyporheic exchange, such as passive solute tracers [27] and seepage meters [28] a one-dimensional (1D) model [21,24–26]. The details of this monitoring are provided in Section 3.2. to determineOther the methods patterns to andinfer hyporheic the magnitude exchange, of such hyporheic as passive solute water tracers exchange, [27] and wereseepage not meters utilized in this field study due[28] to determine temperature the patterns profiles and providingthe magnitude better of hyporheic spatial water resolution exchange, withinwere not theutilized small in confluence. Data were collectedthis field studyduring due two to temperature field campaigns, profiles providing in the low-flow better spatial period resolution from within September the small to December

2018, and in the high-ﬂow period from March to April 2019 (Table1).

3.1. Riverbed Characteristics, Grain Size Analysis and Kv A total station Topcon GTS-250 was deployed to survey riverbed elevation data and Surfer 15 was used to process them for spatial analysis, using a natural neighbor as gridding method with a surveyed point density of 0.90/m2. Kennedy et al. [29] suggested a minimum point density of about 0.05 points/m2 to reduce the occurrence of error value of 10%. Field tests provided in situ vertical Water 2020, 12, 649 4 of 20

confluence. Data were collected during two field campaigns, in the low-flow period from September to December 2018, and in the high-flow period from March to April 2019 (Table 1).

Water 2020, 12, 649 Table 1. Summary of field campaigns carried out at Baltimore Woods Nature Center. 4 of 18

Campaign Bathymetry Granulometry Kv Temperature Time Series Date FS-BBCB1 X X X September–December 2018 hydraulic conductivityFS-BBCB2 Kv , following a simplifiedX approachX of that ofMarch Hvorslev–April 2019 [30 ] proposed by Chen [31] for the vertical component. A vertical standpipe in the stream channel was pressed into 3.1. Riverbed Characteristics, Grain Size Analysis and Kv the streambed and water is poured into the pipe to fill the rest of the pipe. Due to the difference of hydraulic headsA at total the station two endsTopcon of GTS the-250 sediment was deployed column to survey in the riverbed pipe, elevation water flows data and through Surfer 15 the sediment was used to process them for spatial analysis, using a natural neighbor as gridding method with a column andsurveyed the water point table density in of the 0.90/m pipe2. Kennedy falls. Statistical et al. [29] suggested analysis a minimum of Kv took point into density account of about two different methods [310.05] for points/m hydraulic2 to reduce head the readings. occurrence In of thiserror studyvalue of their 10%. Field results tests were provided averaged: in situ vertical hydraulic conductivity Kv, following a simplified approach of that of Hvorslev [30] proposed by Chen [31] for the vertical component. A vertical standpipe in the stream channel was pressed into Lv h1 the streambed and water is poured Kintov =the pipe to fill theln rest of the pipe. Due to the difference of (1) (t t ) · h2 hydraulic heads at the two ends of the sediment2 − 1 column in the pipe, water flows through the sediment column and the water table in the pipe falls. Statistical analysis of Kv took into account two where Lv is thedifferent thickness methods of[31] the for hydraulic measured head streambed readings. In this in study the pipe, their results h1 the were hydraulic averaged: head in the pipe measured at time t1, and h2 the hydraulic head in퐿푣 the pipeℎ1 measured at time t2. The advantage of 퐾푣 = ⋅ ln (1) field-based tests for Kv over laboratory-based permeameter(푡2 − 푡1) ℎ2 test for Kv may not give representative results sincewhere soil core Lv is samplingthe thickness is of a the disruptive measured streambed process. in Test the pipe, areas h1 werethe hydraulic located head in in tributaries the pipe upstream of the junctionmeasured and in at thetime CHZt1, and (Figureh2 the hydraulic2). Water head level in the waspipe assumedmeasured at totime be t2 constant. The advantage during of slug tests, field-based tests for Kv over laboratory-based permeameter test for Kv may not give representative when no precipitationresults since soil in the core watershed sampling is a and disruptive rise in process. river T stageest areas were were observed. located in tributaries Test readings were collected byupstream pouring of water the junction into and a transparent in the CHZ (Figure pipe 2 and). Water taking level was note assumed of successive to be constant negative during piezometric heads. Soil samplesslug tests, were when collected no precipitat ation BBCB in thesite watershed (Figure and2) rise and in each river one stage was were dried observed. and Test poured into a readings were collected by pouring water into a transparent pipe and taking note of successive rototap for shakingnegative piezometric and then heads. categorized. Soil samples Along were withcollected the at cumulativeBBCB site (Figure curve, 2) and coe eachffi cientone was of uniformity 0.5 η η = (d84/d16)dried, and and poured porosity into na rototap= 0.255 for (1shaking+ 0.83 and then) were categorized. estimated Along following with the cumulative Vukovićand curve, Soro [32]. η Three set of samplescoefficient of were uniformity extracted η = (d84 from/d16)0.5, BB, and CB,porosity and n within= 0.255 (1 the + 0.83 CHZ.) were estimated following Vuković and Soro [32]. Three set of samples were extracted from BB, CB, and within the CHZ.

Figure 2. RiverbedFigure 2. monitoringRiverbed monitoring for FS-BBCB2 for FS-BBCB2 showing showing hydraulic hydraulic conductivity conductivity test location test (black location X), (black X), soil sample testsoil samp locationsle test locations (red circle). (red circle).

3.2. Temperature Time Series and 1D Heat Transport Modeling

For the vertical temperature proﬁles [33], six 2-cm diameter PVC pipes encasing wooden dowels with temperature probes, called iButtons (iButtonLink, LLC., Whitewater, WI, USA), were used in BB, CB and CHZ areas (Figures3 and4). The PVC pipe sleeves and dowels were drilled to create inserts for the temperature probes at distances of either distances of 5 cm or 20 cm. The iButton DS1922L was used for this study: each sensor has an accuracy of +/ 0.5 C (from 10 C to +65 C) and resolution − ◦ − ◦ ◦ 0.0625 ◦C for 11 bits. To secure the iButtons inserted in the PVC encased dowels, they were sealed with a silicon glue and let to dry for 24 h, which is shown to allow for a clear temperature signal while preventing corrosion. Water 2020, 12, 649 5 of 20 Water 2020, 12, 649 5 of 20 3.2. Temperature Time Series and 1D Heat Transport Modeling 3.2. Temperature Time Series and 1D Heat Transport Modeling For the vertical temperature profiles [33], six 2-cm diameter PVC pipes encasing wooden For the vertical temperature profiles [33], six 2-cm diameter PVC pipes encasing wooden dowels with temperature probes, called iButtons (iButtonLink, LLC., Whitewater, WI, USA), were dowels with temperature probes, called iButtons (iButtonLink, LLC., Whitewater, WI, USA), were used in BB, CB and CHZ areas (Figures 3 and 4). The PVC pipe sleeves and dowels were drilled to used in BB, CB and CHZ areas (Figures 3 and 4). The PVC pipe sleeves and dowels were drilled to create inserts for the temperature probes at distances of either distances of 5 cm or 20 cm. The create inserts for the temperature probes at distances of either distances of 5 cm or 20 cm. The iButton DS1922L was used for this study: each sensor has an accuracy of +/− 0.5 °C (from −10 °C to iButton DS1922L was used for this study: each sensor has an accuracy of +/− 0.5 °C (from −10 °C to +65 °C) and resolution 0.0625 °C for 11 bits. To secure the iButtons inserted in the PVC encased Water 2020, 12,+65 649 °C) and resolution 0.0625 °C for 11 bits. To secure the iButtons inserted in the PVC encased 5 of 18 dowels, they were sealed with a silicon glue and let to dry for 24 h, which is shown to allow for a dowels, they were sealed with a silicon glue and let to dry for 24 h, which is shown to allow for a clear temperature signal while preventing corrosion. clear temperature signal while preventing corrosion.

Figure 3. Locations of temperature rods used for vertical hydraulic gradient tests, for FS-BBCB1 and Figure 3. LocationsFigure 3. Locations of temperature of temperature rods rods used used for for vertical hydraulic hydraulic gradient gradient tests, for tests,FS-BBCB1 for and FS-BBCB1 and the temperature rod setup (right). the temperaturethe temperature rod setup rod (right). setup (right).

Figure 4. Locations of temperature rods used for vertical hydraulic gradient tests, for FS-BBCB2 and Figure 4. LocationsFigure 4. Locations of temperature of temperature rods rods used used for for vertical hydraulic hydraulic gradient gradient tests, for tests,FS-BBCB2 for and FS-BBCB2 and the temperature rod setup (right). the temperaturethe temperature rod setup rod (right). setup (right). The temperature profile rods, PVC with encased dowel, were driven into the riverbed to a The temperature profile rods, PVC with encased dowel, were driven into the riverbed to a depth which left the top iButton temperature sensors at surface-subsurface interface, and the The temperaturedepth which profile left the rods, top iButton PVC temperaturewith encased sensors dowel, at surface were-subsurface driven into interface, the riverbed and the to a depth subsequent sensors within the riverbed: profilers in the fall low flow season had a 20 cm gap which left thesubsequent top iButton sensors temperature within the riverbed: sensors profilers at surface-subsurface in the fall low flow season interface, had a 20 and cm the gap subsequent between sensors, and profilers in the spring high flow season had a 10 cm gap between sensors. Flux between sensors, and profilers in the spring high flow season had a 10 cm gap between sensors. Flux sensors withinmagnitudes the riverbed: and directions profilers are referred in the between fall low streambed flow seasonand the deepest had a sensor: 20 cm center gap-of between-pair sensors, magnitudes and directions are referred between streambed and the deepest sensor: center-of-pair depth was considered 20 cm and 10 cm below the bed, in FS-BBCB1 and FS-BBCB2, respectively. and profilersdepth in the was spring considered high 20 flowcm and season 10 cm below had athe 10 bed cm, in gap FS-BBCB1 between and FS sensors.-BBCB2, respectively Flux magnitudes. and Temperature time series were recorded at a 5-min or 1-min interval. These rods were inserted into directions areTemperature referred betweentime series streambedwere recorded and at a the5-min deepest or 1-min sensor: interval. center-of-pairThese rods were inserted depth into was considered the riverbed and removed from it at the end of each observation, in September 18 (0918), October 20 cm and 10the cm riverbed below and the removed bed, from in FS-BBCB1 it at the endand of each FS-BBCB2, observation, respectively. in September 18 Temperature(0918), October time series were recorded at a 5-min or 1-min interval. These rods were inserted into the riverbed and removed from it at the end of each observation, in September 18 (0918), October (1018), and December (1218), and in March (0319). The small bed shear stresses, due to shallow flows across the low-gradient confluence, posed little risk of scouring around the temperature sensors. Temperature time series data were processed with the VFLUX 2 code which is distributed as an open source MATLAB toolbox [34,35]. Sediment and thermal parameter are listed in Table2, and every time series was resampled to the "lowest common denominator" to trim the series to the shortest time range that is common to all the input series and interpolates and resamples the input series to have the lowest sampling rate of all the input series. The time series were reduced to 12 samples per diurnal cycle to reduce noise and improve the Dynamic Harmonic Regression model [36] used by VFLUX 2 efficiency in the filtering process [34]. This produced a time-varying apparent amplitude and phase coefficients for the time series, extracting harmonic signals from dynamic environmental systems [27,37]. A non-stationary approach for diurnal signal is mandatory because streambed temperature fluctuates over time due to weather and seasonality, fluxes having different temporal scales. VFLUX 2 identifies a trend, the fundamental signal (ω1), and the first and second harmonics (ω2 and ω3) using an auto-regression (AR) frequency spectrum created with the Captain Toolbox [36]. Water 2020, 12, 649 6 of 18

Table 2. Input parameters of VFLUX2 code. β is dispersivity, Kcal thermal conductivity, CsCal and CwCal volumetric heat capacity of sediment and water, respectively.

Parameter Value Unit β 0.001 m Kcal 0.0045 cal/(s cm C) · ·◦ CsCal 0.5 cal/(cm3 C) ·◦ CwCal 1.0 cal/(cm3 C) ·◦

The VFLUX 2 model analyzes the temperature profile between iButton sensor pairs, based on a “window” for sensor-spacings, which was set to 1, meaning fluxes were estimated between sensor 1 and 2, and 2 and 3, each either separated by 10 or 20 cm. The hyporheic flux rate was based on the Hatch amplitude method to calculate the vertical fluxes [38], which is an analytical solution to the 1D heat transport equation [39]: 2 δT δ T Cw δT = κe q (2) δt δz2 − C δz where κe is effective thermal diffusivity, q is fluid flux, Cw is the volumetric heat capacity of the saturated streambed, and C is the volumetric heat capacity of the saturated sediment calculated as the mean of Cw and Cs, the volumetric heat capacity of the sediment grains, weighted by total porosity. The effective thermal diffusivity is defined as:

λ0 κe = + β v (3) ρc f where λ0 is the baseline thermal conductivity (absence of fluid flow), excluding the effects of dispersion, β is thermal dispersivity, and c and ρ are specific heat and density of the sediment-water system respectively and v f is the linear particle velocity (m/s). The second term in Equation (3) represents the increase in effective thermal diffusivity caused by hydrodynamic dispersion and it is often assumed to have little influence in models with modest fluid flow rates [38]. The method provided by Hatch et al. solves for the vertical water flux between two sensors as a function of amplitude differences between the sensors’ temperature signals:  r   + 2  C 2κe α ν  q =  lnAr +  (4) Cw  ∆z 2 

r 8πκ α = v4 + e (5) P where q is vertical fluid flux in the downward direction (m/s), Ar is the amplitude ratio (a measure of amplitude attenuation) between the lower sensor and the upper sensor (dimensionless), ∆z is the distance between the two sensors in the streambed (m), v is the velocity of the thermal front (m/s) and P is the period of the temperature signal (s). Equations (3) and (4) need to be solved iteratively (or by optimization) and depend on thermal sensitivity which, in turn, is estimated on empirical ranges. The analytical model, Equation (4) estimated fluxes magnitudes, and positive and negative values indicated downward and upward fluxes, respectively.

4. Results

4.1. Planform Geometry, Hydrodynamics, Bed Morphology, Grain Size Analysis, and Hydraulic Conductivity

4.1.1. Confluence Geometry and Hydrodynamics Characteristics BB and CB were two first-order streams where water level was predominantly shallow over this field study, constituted by low-gradient velocities and pressure variations (Table3). The planform Water 2020, 12, 649 7 of 18

geometry had a junction angle between the tributaries of approximately 80◦ in base-flow conditions, and a 40◦ angle during one high-flow event in December 2018 during liquid precipitation and snowmelt (Figure5), while back-to-back angle between tributary and confluence downstream channel were: 145◦ and 150◦ (CB and BB) in base-flow conditions, whereas 142◦ and 162◦, in high-flow conditions. Water levels, velocities, and flowrates at the BBCB confluence were measured only at the beginning of the study. However, these values could be considered as indicative of typical low flow conditions within an acceptable range of uncertainty because in those conditions USGS gauge downstream and in nearby rivers did not record any flow events greater than a 1.1-year frequency and indirect observations, including those from piezometer installations, showed no notable change in the water leveled. Larger velocities were observed from CB due to its bed morphology and steep riffle slope into the confluence, with a velocity ratio of BB to CB, VR = Ubb/Ucb = 0.42 and a momentum flux ratio, QR = ρQbbUbb/ρQcbUcb = 0.64. The location of the shear layer moved towards the right bank or BB side of the confluence due to the dominant CB flow velocity (see the dotted red line in Figure5). Downward fluxes from prior channel forming flows eroded the BB riverbed forming a large scour hole within the CHZ (see Figure6). Flow paths induced by bed topography in CB defined the length and width of a sand bar located downstream from the CHZ [12,40] and caused fine sand infiltration into the bed [24,45] which could cause lower Kv.

Table 3. Low ﬂow active channel width, depth, average velocity, discharge, Froude and Reynolds numbers in BB, CB, and in the CHZ. Data for BB and CB were collected 1.5 m upstream of the junction, and data for CHZ were collected 3 m downstream of the river junction.

Parameters BB CB CHZ

Widthavg (m) 2.20 1.00 3.09 Depthavg (m) 0.31 0.08 0.24 Uavg (m/s) 0.268 0.635 0.376 Fr (-) 0.155 0.735 0.248 Re (-) 81720 48387 88411 3 Qavg (m /s) 0.179 0.048 0.273 Water 2020, 12, 649 8 of 20

Figure 5. Pictures of the confluence flow during low-flow (top left, 17 March 2019) and high-flow Figure 5. Pictures of the confluence flow during low-flow (top left, 17 March 2019) and high-flow (top (top right, 2 December 2018) conditions, and 3D maps with the hydrodynamic features at a right, 2 Decemberconfluence 2018) (bottom conditions,). and 3D maps with the hydrodynamic features at a confluence (bottom).

Figure 6. Topography of watershed and riverbeds about the confluence zone, with black solid contour lines showing elevation above sea level (m). Black dotted lines delineate the active channel during low flow, and the red dotted ellipse delineates a scour hole.

4.1.2. Bed Morphology The confluence was characterized by low-gradient water surface slopes, with slightly higher slopes in the CB tributary in the 5 m upstream the junction. The area between BB and CB was abundant in outwash sediments and partially submerged during high-flow discharge event (i.e., in December after a sudden snow melt, Figure 5) complicating the localization of the BB and CB riverbanks (Figure 6). The deepest pool was found in BB, at a scour hole of approximately 40–50 cm wide, along the outer bank as common in pool-riffle rivers [41–43], while CB and the CHZ depths were mostly without pool and averaged 5–10 cm. Sediment scour was located in the BB pool and

Water 2020, 12, 649 8 of 20

Water 2020, 12, 649 Figure 5. Pictures of the confluence flow during low-flow (top left, 17 March 2019) and high-flow 8 of 18 (top right, 2 December 2018) conditions, and 3D maps with the hydrodynamic features at a confluence (bottom).

Figure 6. Topography of watershed and riverbeds about the confluence zone, with black solid Figure 6. Topographycontour lines showing of watershed elevation and above riverbeds sea level (m). about Black the dotted confluence lines delineate zone, the withactive blackchannel solid contour lines showingduring elevation low flow, above and the sea red leveldotted (m).ellipse Black delineates dotted a scour lines hole. delineate the active channel during low flow, and the red dotted ellipse delineates a scour hole. 4.1.2. Bed Morphology 4.1.2. Bed MorphologyThe confluence was characterized by low-gradient water surface slopes, with slightly higher slopes in the CB tributary in the 5 m upstream the junction. The area between BB and CB was The confluenceabundant in was outwash characterized sediments and by partially low-gradient submerged water during surface high-flow slopes, discharge with event slightly (i.e., in higher slopes in the CB tributaryDecember inafter the a 5 sudden m upstream snow melt, the Figure junction. 5) complicat Theing area the between localization BB of and the BB CB and was CB abundant in outwash sedimentsriverbanks and (Figure partially 6). The deepest submerged pool was during found in high-flow BB, at a scour discharge hole of approximately event (i.e., 40 in–50 December cm after a wide, along the outer bank as common in pool-riffle rivers [41–43], while CB and the CHZ depths sudden snowwere melt, mostly Figure without5) complicatingpool and averaged the 5– localization10 cm. Sediment of scour the BBwasand located CB in riverbanks the BB pool and (Figure 6). The deepest pool was found in BB, at a scour hole of approximately 40–50 cm wide, along the outer bank as common in pool-riffle rivers [41–43], while CB and the CHZ depths were mostly without pool and averaged 5–10 cm. Sediment scour was located in the BB pool and outer bank, and depositional areas were observed downstream the confluence on the left bank and CB flow side, with a high presence of pebbles and fine sand on the river bottom.

4.1.3. Hydraulic Conductivity and Grain Size Analysis Hydraulic conductivity tests result showed very low Kv values throughout the confluence. The values ranged from 0.09 to 0.005 m/d (Figure7). According to mean and median Kv in BB and CB and the CHZ, CHZ3 had the smallest hydraulic conductivity. That area was subjected to sand (0.075 mm < PZ < 2 mm) deposition during fall low flow season that might have decreased Kv values due to layering of streambed sediments [44]. In fact, laboratory analysis of substrate samples showed that grain size in the confluence bed was mostly composed of fine sand (0.063 mm < PZ < 0.2 mm). The average of cumulative percentages by weight is listed in Table4. The silt-clay fraction, instead, was larger in CHZ sediment sample while, on average, BB and CB showed lower values (Figure8).

Table 4. Sediment size distribution about the confluence, reporting the cumulative weight (%) for the two size fractions of sand and finer than sand, the particle diameter (mm) for 14%, 50%, and 84% finer than the diameter, coefficient of uniformity, and porosity.

Sample BB4 BB5 CB4 CB5 CHZ4 Cumulative weight (%) <0.053 mm 2.67 6.21 2.13 1.44 11.56 <2 mm 99.64 99.46 99.57 99.52 99.18 D14 (mm) 0.064 0.032 0.059 0.065 0.080 D50 (mm) 0.152 0.077 0.092 0.136 0.068 D84 (mm) 0.435 0.180 0.304 0.416 0.103 Coeﬃcient of uniformity (η) 2.607 2.368 2.270 2.530 1.135 Porosity (n) 0.406 0.404 0.415 0.408 0.439 Water 2020, 12, 649 9 of 20

outer bank, and depositional areas were observed downstream the confluence on the left bank and CB flow side, with a high presence of pebbles and fine sand on the river bottom.

4.1.3. Hydraulic Conductivity and Grain Size Analysis Hydraulic conductivity tests result showed very low Kv values throughout the confluence. The values ranged from 0.09 to 0.005 m/d (Figure 7). According to mean and median Kv in BB and CB and the CHZ, CHZ3 had the smallest hydraulic conductivity. That area was subjected to sand (0.075 mm < PZ < 2 mm) deposition during fall low flow season that might have decreased Kv values due to layering of streambed sediments [44]. In fact, laboratory analysis of substrate samples showed that grain size in the conﬂuence bed was mostly composed of fine sand (0.063 mm < PZ < 0.2 mm). The average of cumulative percentages by weight is listed in Table 4. The silt-clay fraction, instead, was larger in CHZ sediment sample while, on average, BB and CB showed lower values (Figure 8).

Table 4. Sediment size distribution about the confluence, reporting the cumulative weight (%) for the two size fractions of sand and finer than sand, the particle diameter (mm) for 14%, 50%, and 84% finer than the diameter, coefficient of uniformity, and porosity.

Sample BB4 BB5 CB4 CB5 CHZ4 Cumulative weight (%) <0.053 mm 2.67 6.21 2.13 1.44 11.56 <2 mm 99.64 99.46 99.57 99.52 99.18 D14 (mm) 0.064 0.032 0.059 0.065 0.080 D50 (mm) 0.152 0.077 0.092 0.136 0.068 D84 (mm) 0.435 0.180 0.304 0.416 0.103 Water 2020, 12, 649 Coefficient of uniformity (η) 2.607 2.368 2.270 2.530 1.135 9 of 18 Porosity (n) 0.406 0.404 0.415 0.408 0.439

WaterFigure 2020, 12 7., 649Hydraulic conductivity values, Kv, (m/day) in box-and-whisker graph (bottom).10 of 20 Figure 7. Hydraulic conductivity values, Kv, (m/day) in box-and-whisker graph (bottom).

Figure 8.FigureSoil grain8. Soil grain size distributionsize distribution within within thethe confluence,confluence, showing showing a fining a fining of sand of from sand upstream from upstream (BB and(BB CB and sites) CB tosites) downstream to downstream into into the the confluence confluence zone zone (CHZ). (CHZ).

4.2. Temperature4.2. Temperature Time SeriesTime Series and and VFLUX VFLUX Code Code Application Application TemperatureTemperature profiles profiles obtained obtained across across the the confluence confluence were were used used in in VFLUX VFLUX to get directions directions (noted (noted as positive values indicate downwelling flux), magnitudes and patterns of vertical hyporheic as positive values indicate downwelling flux), magnitudes and patterns of vertical hyporheic fluxes fluxes in multiple locations over several months. Temperature differences between the groundwater in multipleand surface locations water over in FS several-BBCB1 and months. FS-BBCB2 Temperature were less than di 2ff °Cerences and 1 °C, between respectively. the groundwater and surface water in FS-BBCB1 and FS-BBCB2 were less than 2 ◦C and 1 ◦C, respectively. 4.2.1. Fall Low Flow and Snow-Melt Season 4.2.1. Fall LowDuring Flow the and fall season, Snow-Melt variations Season in flux direction within the confluence were observed within Duringthe top the 20 fallcm of season, the riverbed variations by temperature in fluxdirection profile analysis. within The the probes confluence at greater were depths observed did not within register sufficient temperature variation to reveal as vertical hyporheic fluxes. The vertical hyporheic the top 20fluxes cm had of the seasonal riverbed trends. by In temperature late summer, profile from 16 analysis. to 23 September, The probes fluxes at ranged greater from depths +1.12 did not registermm/d sufficientay (downwelling) temperature to variation−2.61 mm/d toay, reveal with four as verticalof these upwelling hyporheic fluxes fluxes. in the The majority vertical of the hyporheic fluxes hadconfluence seasonal (0918A, trends. 0918B, In late 0918C summer,, and 0918E, from Figure 16 to 239). September,The single downwelling fluxes ranged flux from(0918D)+ 1.12was mm/day (downwelling)observed to at the2.61 scour mm hole/day, section with of four the confluence. of these upwelling fluxes in the majority of the confluence − (0918A, 0918B,In the 0918C, mid-fall and period, 0918E, from Figure 23 October9). The to single 6 November, downwelling there were flux three (0918D) distinct was patte observedrns of at the vertical hyporheic exchange, during which time the site received 55 mm of precipitation and river scour holestage section increased of the~6 cm confluence. or 30%. At the beginning of this wet period, from 23 to 25 October, maximum downwelling fluxes of 405 mm/day were observed around rods 1018A, 1018B, 1018C, and 1018F in the upstream section, and upwelling in the downstream section (Figure 10). In this period, the maximum daily upwelling fluxes gradually transitioned from −400 to −145 mm/day at rod 1018E, while upwelling remained steady at 1018D. Second, on 26 October, rod 1018E flux direction changed to moderately downwelling from a strong upwelling, and the upwelling at 1018D in the BB section of the confluence increased to a maximum of −140 mm/day. The third pattern emerged from 1 to 6 November when the upwelling hyporheic flux shifted further upstream along the BB side of the confluence to rods 1018A and the downwelling at rod 1018B ceased and became neutral (Figure 10). These changes in flux pattern suggest that BB transitioned to greater upwelling during the wet period, while downwelling flux in the CB section of the confluence was relatively steady.

Water 2020, 12, 649 10 of 18 Water 2020, 12, 649 11 of 20

Figure 9. VerticalFigure 9. hyporheic Vertical hyporheic fluxes fluxes (mm (mm/d/day)ay for) for September 2018, 2018, derived derived from temperature from temperature profiles. profiles. Downwelling and upwelling fluxes are represented by red and blue contours, respectively. The light Downwelling and upwelling fluxes are represented by red and blue contours, respectively. The light Waterblue 2020 ,contour 12, 649 represents an interpolated transitional zone. 11 of 20 blue contour represents an interpolated transitional zone.

In the mid-fall period, from 23 October to 6 November, there were three distinct patterns of vertical hyporheic exchange, during which time the site received 55 mm of precipitation and river stage increased ~6 cm or 30%. At the beginning of this wet period, from 23 to 25 October, maximum downwelling fluxes of 405 mm/day were observed around rods 1018A, 1018B, 1018C, and 1018F in the upstream section, and upwelling in the downstream section (Figure 10). In this period, the maximum daily upwelling fluxes gradually transitioned from 400 to 145 mm/day at rod 1018E, while upwelling − − remained steady at 1018D. Second, on 26 October, rod 1018E flux direction changed to moderately downwelling from a strong upwelling, and the upwelling at 1018D in the BB section of the confluence increased to a maximum of 140 mm/day. The third pattern emerged from 1 to 6 November when − the upwelling hyporheic flux shifted further upstream along the BB side of the confluence to rods 1018A and the downwelling at rod 1018B ceased and became neutral (Figure 10). These changes in flux pattern suggest thatFigure BB9. Vertical transitioned hyporheic fluxes to greater (mm/day)upwelling for September 2018, during derived the from wet temperature period, profiles. while downwelling Figure 10. Vertical hyporheic fluxes (mm/day) for 23 October to 6 November 2018, derived from flux in the CB sectionDownwelling of the and confluence upwelling fluxes was are represented relatively by red steady. and blue contours, respectively. The light temperature profiles. The light blue contour represents an interpolated transitional zone. blue contour represents an interpolated transitional zone.

In late fall during 2–4 December, after a month of little rainfall, the hyporheic fluxes reversed from the mid-fall pattern. The upwelling fluxes were organized along the CB upstream section of the confluence, while downwelling fluxes were observed along the downstream confluence section and into the upper BB section. In this period, the CB temperature rods (1218A and 1218B) registered upwelling fluxes, while rods 1218D and 1218E had strong downwelling fluxes (Figure 11). The flux rates ranged from 602 mm/day to −397 mm/day, reaching greater values than in the mid-fall period. As with the late summer period, but unlike the mid-fall with the rains, the December fluxes were steady values over the sampling period even though there was a steady decline in river stage.

Figure 10. VerticalFigure 10.hyporheic Vertical hyporheic ﬂuxes fluxes (mm (mm/d/day)ay) for for 23 23 October October to 6 to November 6 November 2018, derived 2018, from derived from temperaturete proﬁles.mperature Theprofiles. light The blue light contourblue contour represents represents an an interpolated interpolated transitional transitional zone. zone.

In late fall duringIn late fall 2–4 during December, 2–4 December, after after a a month month of of little little rainfall, rainfall, the hyporheic the hyporheic fluxes reversed fluxes reversed from the mid-fall pattern. The upwelling fluxes were organized along the CB upstream section of the from the mid-fallconfluence pattern., while downwelling The upwelling fluxes were fluxes observed were along organized the downstream along confluence the CB section upstream and section of the confluence,into while the upper downwelling BB section. In this fluxes period, were the observedCB temperature along rods the (1218A downstream and 1218B) registered confluence section and into theupwelling upper BB fluxes, section. while rods In this 1218D period, and 1218E the had CB strong temperature downwelling rods fluxes (1218A (Figure and11). The 1218B) flux registered rates ranged from 602 mm/day to −397 mm/day, reaching greater values than in the mid-fall period. upwelling fluxes,As with while the late rods summer 1218D period, and but 1218Eunlike the had mid strong-fall with downwelling the rains, the December fluxes fluxes (Figure wer 11e ). The flux rates rangedsteady from values 602 mm over/day the sampling to 397 period mm/ evenday, though reaching there greater was a steady values decline than in river in the stage. mid-fall period. As −

Water 2020, 12, 649 11 of 18 with the late summer period, but unlike the mid-fall with the rains, the December ﬂuxes were steady values overWater the sampling2020, 12, 649 period even though there was a steady decline in river stage.12 of 20

Water 2020, 12, 649 12 of 20

Figure 11. VerticalFigure 11. hyporheic Vertical hyporheic fluxes (mm fluxes/ day) (mm/d foray) December for December 2018, 2018, derived derived from from temperature temperature profiles. profiles. Grey contour represents an interpolated transitional zone. Confluence margins (grey dotted Grey contour represents an interpolated transitional zone. Confluence margins (grey dotted lines) refer lines) refer to relatively high-flow condition. to relatively high-flow condition. 4.2.2.Figure Spring 11.H ighVertical Flow hyporheicSeason fluxes (mm/day) for December 2018, derived from temperature 4.2.2. Spring Highprofiles. Flow Grey Season contour represents an interpolated transitional zone. Confluence margins (grey dotted Thelines) spring refer to season relatively brought high-flow changes condition. in river flow, and this was used to organize three periods of The springdistinct season patterns brought in hyporheic changes flux. The in changes river flow,in flow and were this attributed was usedto a rainfall to organize event between three periods of 294.2.2. and Spring 31 March High andFlow another Season period of rainfall between 3 and 7 April. At the end of the March distinct patternsrained,in downwelling hyporheic fluxes flux. were The observed changes downstream in flow of the were confluence attributed, with an to isolated a rainfall corner event of between The spring season brought changes in river flow, and this was used to organize three periods of 29 and 31 Marchupwelling and at another rods 0319E period and 0319F of rainfall(Figure 12). between 3 and 7 April. At the end of the March rained, distinct patterns in hyporheic flux. The changes in flow were attributed to a rainfall event between downwelling29 fluxes and 31 wereMarch observedand another downstream period of rainfall of the between confluence, 3 and 7 April. with At an the isolated end of the corner March of upwelling at rods 0319Erain anded, downwelling 0319F (Figure fluxes 12 were). observed downstream of the confluence, with an isolated corner of upwelling at rods 0319E and 0319F (Figure 12).

Figure 12. Vertical hyporheic fluxes from FS-BBCB2 (0319). Upper left figure shows HEF pattern on 31 March, upper right figure on 1 April and lower figure on 6 April. Downwelling and upwelling fluxes are represented by red and blue contours, respectively.

Figure 12. VerticalDuringFigure 12. hyporheicthe Vertical period hyporheic between fluxes fluxes from the from rains, FS-BBCB2 FS -BBCB2 on 1 April, (0319). (0319). theUpper Upper hyporheic left figure left fluxshows figure pattern HEF shows pattern shifted HEF on and pattern on 31 March, upper right figure on 1 April and lower figure on 6 April. Downwelling and upwelling 31 March,upwelling upper now right existed figure across on 1 most April of and the confluence, lower figure at all on rods 6 April. except Downwellingfor the rod 0319C and in the upwelling fluxes are represented by red and blue contours, respectively. fluxes areupper represented confluence bywhere red CB and entered blue. contours,The continuation respectively. of rains from 3 to 7 April resulted in a general return to the late March pattern of flux, with downwelling extending across most of the monitoring During the period between the rains, on 1 April, the hyporheic flux pattern shifted and During upwelling the period now between existed across the rains,most of on the 1 confluence, April, the at hyporheic all rods except flux for pattern the rod 0319C shifted in theand upwelling now existedupper across confluence most of where the confluence,CB entered. The at continuation all rods except of rains forfrom the 3 to rod7 April 0319C resulted in in the a general upper confluence return to the late March pattern of flux, with downwelling extending across most of the monitoring where CB entered. The continuation of rains from 3 to 7 April resulted in a general return to the late March pattern of flux, with downwelling extending across most of the monitoring section, and upwelling at rod 0319E in the downstream section along the CB region, as well as at rod 0319D near the confluence vertex. Water 2020, 12, 649 12 of 18

4.3. Discussion Regional to local drivers of hyporheic exchange can gather within the confluence hydrodynamic zone. At a regionalWater 2020, scale, 12, 649 the dominant drivers of hyporheic exchange flux patterns are13 the of 20 relative levels of river stagesection and, groundwaterand upwelling at rod [11 0319E–13,21 in, 22the], downstream while at localsection scale along the bed CB morphology, region, as well as soil at rod heterogeneity, and channel0319D velocity near the influence confluence hyporheic vertex. exchange [13]. In pool-riffle channels with moderate slope, such as BB and4.3. Discussion CB, hyporheic exchange is usually driven by the variability of the spatial distribution of channel velocityRegional and toresulting local drivers pressure of hyporheic head [40 exchange]. Downwelling can gather and within upwelling the confluence zones are usually located on rihydrodynamicffle head with zone. lower At a velocitiesregional scale, (higher the dominant pressure) drivers and of hyporheic riffle tail exchange with higher flux patterns velocities (lower pressure), respectivelyare the relative [25 levels]. Local of river topography stage and groundwater is a prominent [11–13,21,22], driver while of changesat local scale in bedhyporheic flux direction whenmorphology, bed depth soil heterogeneity, increases or and decreases channel velocity in the downstreaminfluence hyporheic direction, exchange and [13]. the In concave or pool-riffle channels with moderate slope, such as BB and CB, hyporheic exchange is usually driven convex natureby the water variability surface of the profiles spatial distribution can also of change channel hyporheicvelocity and resulting flux direction pressure head [45]. [40 On]. bedforms, past studiesDownwelling demonstrated and upwelling that close zones to are the usually detachment located on pointriffle head downstream with lower velocities of dune (higher crest and to the reattachmentpressure) point onand theriffle dune tail with stoss higher side velociti arees minimum (lower pressure), and respectively maximum [25 pressure]. Local topography regions, is respectively, a prominent driver of changes in hyporheic flux direction when bed depth increases or decreases in creating patternsthe downstream of upwelling direction, and and downwelling the concave or convex [46]. nature water surface profiles can also change This fieldhyporheic study flux found direction distinct [45]. On upwelling bedforms, past or studies downwelling demonstrated patterns that close of to hyporheic the detachment exchange flux and observedpoint its downstream variation of across dune crest eight and to months the reattachment from late point summer on the dune to stoss spring side are seasons. minimum During this and maximum pressure regions, respectively, creating patterns of upwelling and downwelling [46]. period, these patterns exhibited variability while hydrodynamic and hydrological condition were This field study found distinct upwelling or downwelling patterns of hyporheic exchange flux changing. However,and observed as its BB variation and CB across were eight ungauged months from streams, late summer daily to spring measurements seasons. During of discharge,this and water stageperiod, were these not available.patterns exhibited These variability data werewhile hydrodynamic collected at and some hydrological characteristic condition locationswere of the confluence (Figurechanging. 13 However,), such asas BB the and confluence CB were ungauged junction streams, (September daily measurements 2018 and Decemberof discharge, 2018),and the scour water stage were not available. These data were collected at some characteristic locations of the hole (Septemberconfluence 2018) (Figure and the13), shearsuch as layerthe confluence (October junction/November (September 2018 2018 and and March December/April 2018), 2019), the where the aforementionedscour patternshole (September of hyporheic 2018) and exchangethe shear layer observed (October/November in pool-ri 2018ffle channelsand March/Ap mightril 2019), be modified by the confluencewhere hydrodynamic the aforementioned and patterns morphodynamics of hyporheic exchange features. observed in pool-riffle channels might be modified by the confluence hydrodynamic and morphodynamics features.

Figure 13. VHFFigure map 13. VHF extents map extents from from FS-BBCB1 FS-BBCB1 and and FS-BBCB2.FS-BBCB2. The Thered polygon red polygon refers to the refers September to the September 2018 map; dark green, dark blue, and black recall December 2018, October 2018, and March 2019 2018 map; dark green, dark blue, and black recall December 2018, October 2018, and March 2019 maps, maps, respectively. The scour hole is individuated by the red ellipse. respectively. The scour hole is individuated by the red ellipse. 4.3.1. Effect of Scour Hole on the Hyporheic Fluxes 4.3.1. Effect of ScourIn September Hole on2018, the at Hyporheicthe confluence Fluxes entrance the flow from CB tended toward the BB bank In September(Figure 5 2018,left). At at that the time confluence the area of entrance measurement the was flow located from just CB downstream tended the toward junction the BB bank corner, where the flow from BB is featuring an abrupt step at the entrance of the scour hole (Figures (Figure5 left).9, 13, At 14 that and time15a). The the flow area over of a measurement step is a classic type was of locatedseparation just flow, downstream termed backward the facing junction corner, where the flow from BB is featuring an abrupt step at the entrance of the scour hole (Figure9, Figure 13, Figure 14, Figure 15a). The flow over a step is a classic type of separation flow, termed backward facing step flow (BFSF), which has been extensively investigated using both experimental [47] and numerical methods [48] in laminar and turbulent flows. It is well-known that in the BFSF downstream the step bottom pressure is going down which is followed by a rapid increase to get a maximum close to the reattachment point. While in laminar flow the location of the minimum/maximum pressure point is depending upon the step height-based Reynolds number [48], in a turbulent BFSF the point Water 2020, 12, 649 14 of 20

step Water flow 2020 (BFSF),, 12, 649 which has been extensively investigated using both experimental14 [4 of7 ]20 and numerical methods [48] in laminar and turbulent flows. It is well-known that in the BFSF Water 2020, 12, 649 13 of 18 downstreamstep flow the (BFSF), step bottom which haspressure been is extensively going down investigated which is usingfollowed both by experimental a rapid increase [47] andto get a maximumnumerical close methods to the [4 8]reattachment in laminar and point. turbulent While flow ins. It laminar is well -known flow the that locationin the BFSF of the minimum/maximumdownstream the step pressure bottom pointpressure is is dependi going downng upon which the is followed step heigh by ta- basedrapid increase Reynolds to get number a of minimummaximum and maximum close to the bed reattachment pressure is point. located While at inx / laminarHstep = flow3.0 and the locationx/Hstep of= 9,the respectively [48], in a turbulent BFSF the point of minimum and maximum bed pressure is located at x/Hstep = 3.0 minimum/maximum pressure point is depending upon the step height-based Reynolds number (Figure 16and)[ x47/H].step In = 9, our respectively case, as the(Figure step 16) height [47]. In is our approximately case, as the step 0.15 height m, theis approximately maximum pressure 0.15 m, should [48], in a turbulent BFSF the point of minimum and maximum bed pressure is located at x/Hstep = 3.0 be locatedtheabout maximum 0918D pressure point should (Figure be 15locateda). This about is 0918D consistent point with(Figure the 15a). observed This is consistent hyporheic with fluxes, the which and x/Hstep = 9, respectively (Figure 16) [47]. In our case, as the step height is approximately 0.15 m, were directedobservedthe upwardmaximum hyporheic pressure (upwellings) fluxes, should which be upstreamwere located directed about and 0918D upward immediately point (upwellings) (Figure 15a). downstream Thisupstream is consistent and of immediately thewith stepthe bordering the scourdownstream holeobserved and ofhyporheic downward the step fluxes, bordering (downwelling) which thewere scour directed hole around upward and 0918Ddownward (upwellings) point, (downwel upstream where ling)and flow immediately around reattachment 0918D and point, where flow reattachment and maximum pressure should be located. On the other hand, in the maximum pressuredownstream should of the be step located. bordering On the the scour other hole andhand, downward in the area (downwel of measurement,ling) around 0918D the flow from area point,of measurement, where flow reattachment the flow from and CB maximum is moving pressure over should a plane be bed,located. where On the flow other was hand, accelerating, in the CB is movingand upwellingarea over of measurement, a was plane observed. bed, the whereflow from flow CB is was moving accelerating, over a plane andbed, upwellingwhere flow was was accelerating, observed. and upwelling was observed.

Figure 14. TestFigure points 14. Test and points cross-sections and cross-sections in in FS-BBCB1 FS-BBCB1 and and FS- FS-BBCB2.BBCB2. Red, light Red, blue, light dark blue, blue, and dark blue, and Figureblack 14. pointsTest points refer to and September cross-sections 2018, December in FS-BBCB1 2018, Octoberand FS -2018,BBCB2. and Red, March light 2019, blue, respectively. dark blue, and black pointsblack referpoints torefer September to September 2018, 2018, December December 2018, October October 2018, 2018, and andMarch March 2019, respectively. 2019, respectively.

Figure 15. Longitudinal sections A–A’ and B–B’ from Figure 14, with red fill depicting downwelling Commented [m3]: Please add an explanation of (a)(b) in and blue fill depicting upwelling. Figure 17 caption. Figure 15.FigureLongitudinal 15. Longitudinal sections sections A (a–)A’ A–A’ and B– andB’ from (b )Figure B–B’ 14, from with Figurered fill depicting 14, with downwelling red ﬁll depicting Water 2020, 12, 649 15 of 20 Commented [m3]: Please add an explanation of (a)(b) in downwellingand blue and fill bluedepicting ﬁll depictingupwelling. upwelling. Figure 17 caption.

Figure 16.FigureLongitudinal 16. Longitudinal distribution distribution of of pressure pressure coefficient coeﬃcient at atthe thebottom bottom in a turbulent in a turbulent backward backward facing step flow [47]. The dimensionless abscissa x/Hstep = 0 indicates the location of the step. facing step ﬂow [47]. The dimensionless abscissa x/Hstep = 0 indicates the location of the step. It should be noted that the PVC rod piezometer installations across the CHZ indirectly showed It should be noted that the PVC rod piezometer installations across the CHZ indirectly showed no notable change in the bed morphology. Additionally, the location of the scour hole remained no notableunchanged change during in the the bed field morphology. study. It is expected Additionally, that changes the inlocation the location of the of scourthe scour hole hole remained occurring on a longer timescale could modify the hyporheic exchange at the confluence.

4.3.2. Effect on Variations in Confluence Geometry on the Hyporheic Fluxes In December 2018 a snowmelt event increased discharge and water level at the confluence and the junction angle/location as well as the circulation at the confluence was accordingly modified (Figure 5right). The confluence junction as well as the stagnation zone shifted upstream, while the location of the shear layer moved toward the CB bank (Figure 5right). The area of measurement in December 2018 was partially overlapping with that of September 2018, as it was located more upstream and it was only bordering the scour hole (Figures 11, 13, 14 and 15b). In December 2018, in that area, the flow partially moves over the location of the junction in low flow conditions (Figure 15b), where upwelling was observed, and partially over a stepped bed, where downwelling was measured. Comparatively, the area and the rate of downwelling increased from September to December 2018. This might be explained by the observed changes in the main flow patterns at the confluence entrance.

4.3.3. Effect of Secondary Flows on the Hyporheic Fluxes In October/November 2018 and March/April 2019 the area of measurement was located downstream of that in September and December 2018, in the shear layer region (Figure 13), where hydrodynamics is generally characterized by complex 3D patterns and helical flow cells are also often observed [40], although their presence, characteristics and origin need further investigations [19,49]. Following the back-to-back bend or meander analogy, these cells are expected to converge at the surface in the center of the channel and to diverge near the bed [1,40,41]. Furthermore, these helical cells are associated with downward and upward flow patterns in the water column which could have an impact even on the hyporheic exchange. Cheng et al. [22] observed at the confluence between Juehe River and Haohe River (junction angle, 110°) downwelling patterns in the area across the shear layer where helicoidal flow cells were located. They argued that the encounter and impact of the two tributaries created in that area a downward flow causing a downwelling hyporheic exchange. In the present study, it was not possible to confirm or not the presence of helical cells at the BBCB confluence. However, confluence planform (Figure 6) and the related bend-like flow patterns of the tributaries might suggest the presence of the above secondary circulation. In October/November 2018, downwelling/upwelling was observed on the CB/BB side of the area of measurement, respectively, but some variations were noted from October to November (Figure 10).

Water 2020, 12, 649 14 of 18 unchanged during the ﬁeld study. It is expected that changes in the location of the scour hole occurring on a longer timescale could modify the hyporheic exchange at the conﬂuence.

4.3.2. Effect on Variations in Confluence Geometry on the Hyporheic Fluxes In December 2018 a snowmelt event increased discharge and water level at the confluence and the junction angle/location as well as the circulation at the confluence was accordingly modified (Figure5 right). The confluence junction as well as the stagnation zone shifted upstream, while the location of the shear layer moved toward the CB bank (Figure5 right). The area of measurement in December 2018 was partially overlapping with that of September 2018, as it was located more upstream and it was only bordering the scour hole (Figures 11, 13, 14 and 15b). In December 2018, in that area, the flow partially moves over the location of the junction in low flow conditions (Figure 15b), where upwelling was observed, and partially over a stepped bed, where downwelling was measured. Comparatively, the area and the rate of downwelling increased from September to December 2018. This might be explained by the observed changes in the main flow patterns at the confluence entrance.

4.3.3. Effect of Secondary Flows on the Hyporheic Fluxes In October/November 2018 and March/April 2019 the area of measurement was located downstream of that in September and December 2018, in the shear layer region (Figure 13), where hydrodynamics is generally characterized by complex 3D patterns and helical flow cells are also often observed [40], although their presence, characteristics and origin need further investigations [19,49]. Following the back-to-back bend or meander analogy, these cells are expected to converge at the surface in the center of the channel and to diverge near the bed [1,40,41]. Furthermore, these helical cells are associated with downward and upward flow patterns in the water column which could have an impact even on the hyporheic exchange. Cheng et al. [22] observed at the confluence between Juehe River and Haohe River (junction angle, 110◦) downwelling patterns in the area across the shear layer where helicoidal flow cells were located. They argued that the encounter and impact of the two tributaries created in that area a downward flow causing a downwelling hyporheic exchange. In the present study, it was not possible to confirm or not the presence of helical cells at the BBCB confluence. However, confluence planform (Figure6) and the related bend-like flow patterns of the tributaries might suggest the presence of the above secondary circulation. In October/November 2018, downwelling/upwelling was observed on the CB/BB side of the area of measurement, respectively, but some variations were noted from October to November (Figure 10). In March/April, the distribution of the hyporheic fluxes was different, as almost only downwelling was measured on 31 March and 6 April, while on 1 April upwelling was predominant (Figure 12). To try to explain this strong variability, two cross-sections located in the measurement area October/November 2018 and March/April 2019 were considered (Figure 14) and the distribution of the hyporheic fluxes was plotted (Figure 17a,b). In October/November 2018 and March/April 2019 a downwelling region was observed across the shear layer about the scour hole (Figure 17a,b). This finding is consistent with the observations by Cheng at al. [22] and it could be related to the bed pressure distribution across the back-to-back bend at the confluence. At the end, the observed patterns in the distribution of hyporheic fluxes seem to be related to the hydrodynamics and morphodynamics characteristics about the confluence and their changes during the hydrological cycle. Given the role of confluence junction angle in influencing hyporheic exchange flux, patterns of hyporheic fluxes are expected to fluctuate with changes in flow depth if the junction angle changes with channel depth of water. Further, given the role of momentum flux ratio influencing hyporheic flux, differences in water characteristics that lead to changes in density, temperature, conductivity, and suspended sediment concentration would likely trigger changes in hyporheic exchange patterns [19]. Water 2020, 12, 649 16 of 20

In March/April, the distribution of the hyporheic fluxes was different, as almost only downwelling was measured on 31 March and 6 April, while on 1 April upwelling was predominant (Figure 12). To try to explain this strong variability, two cross-sections located in the measurement area October/November 2018 and March/April 2019 were considered (Figure 14) and the distribution of the hyporheic fluxes was plotted (Figure 17a,b). In October/November 2018 and March/April 2019 a downwelling region was observed across the shear layer about the scour hole (Figure 17a,b). This finding is consistent with the observations by Cheng at al. [22] and it could be related to the bed pressure distribution across the back-to-back bend at the confluence. At the end, the observed patterns in the distribution of hyporheic fluxes seem to be related to the hydrodynamics and Water 2020, 12,morphodynamics 649 characteristics about the confluence and their changes during the hydrological 15 of 18 cycle.

Figure 17. FigureLongitudinal 17. Longitudinal sections sections (a C)–C’ C–C’ and D and–D’ from (b) Figure D–D’ 14, fromwith red Figure fill depicting 14, withdownwelling, red fill depictingCommented [m4]: Please add an explanation of (a)(b) in and arrows showing secondary flow in the channel. downwelling, and arrows showing secondary flow in the channel. Figure 17 caption. 5. Conclusions Given the role of confluence junction angle in influencing hyporheic exchange flux, patterns of hyporheic fluxes are expected to fluctuate with changes in flow depth if the junction angle changes In the literaturewith channel about depth of the water. environmental Further, given the interfaces role of momentum [50] few flux studies ratio influencing are available hyporheic regarding the flux, differences in water characteristics that lead to changes in density, temperature, conductivity, hyporheic exchangeand suspended at riversediment confluences. concentration would This likely experimental trigger changes research in hyporheic characterized exchange patterns the spatial and temporal patterns[19]. of vertical hyporheic exchange flux about a small headwater confluence in the USA. The field data and its analysis were used to explain the observed patterns of hyporheic exchange 5. Conclusions by confluence hydrology, geometry, hydrodynamics, morphodynamics, grain size distribution, and In the literature about the environmental interfaces [50] few studies are available regarding the vertical hydraulic conductivity. Furthermore, the effect on the hyporheic exchange due to some classical hyporheic exchange at river confluences. This experimental research characterized the spatial and features of atemporal confluence patterns flow, of vertical such the hyporheic stagnation exchange zone, flux flow about deflection, a small headwater and contraction confluence in zone,the including the scour hole,USA. and The the field shear data and layer its analysis was investigated. were used to explain Finally, the toobserved expand pattern on thes of twohyporheic prior studies of hyporheic exchangeexchange by at confluence a confluence hydrology, under geometry, baseflow, hydrodynamics, this study morphodynamics, monitored across grain eight size months to distribution, and vertical hydraulic conductivity. Furthermore, the effect on the hyporheic exchange capture lowdue flow to some and highclassical flow features patterns. of a confluence Principal flow, outcomes such the stagnation found zone, within flow this deflection research, and are: contraction zone, including the scour hole, and the shear layer was investigated. Finally, to expand Soil samples showed that BBCB site was mostly sandy-gravel and hydraulic conductivity tests • on the two prior studies of hyporheic exchange at a confluence under baseflow, this study reportedmonitored very low across values, eight months suggesting to capture that low flowlocal and sediment high flow transportpatterns. Principal processes outcomes allocated fine sedimentsfound into within pores, this research reducing are: soil permeability. Confluence geometry, hydrodynamics, and morphodynamics were found to significantly affect • hyporheic exchange rate and patterns. In September and December 2018, local scale bed morphology, such as the confluence scour hole and minor topographic irregularities, influenced the distribution of bed pressure head and the related patterns of downwelling/upwelling. Variation in hydrological conditions during a high flow event in December 2018 survey were seen • to modify confluence geometry, such as junction angle and position, and, in turn, flow circulation patterns, shifting back the stagnation zone and relocating the shear layer. The hyporheic flux pattern in low flow conditions was modified where upwelling was mostly observed, and partially over a stepped bed, downwelling was measured. In October/November 2018 and March/April 2019, classical back-to-back bend planform and • the related secondary circulation might probably affect hyporheic exchange patterns around the confluence shear layer as already documented in the literature [22]. Seasonal hydrological condition should be taken into account. There was a visible pattern among • October 2018 and March 2019 temperature rods: in these two cases fluxes were not only driven by morphological or hydrodynamic conditions.

Follow-up work must focus on the relationship among soil permeability, flow momentum changes, and groundwater which are still under investigation for this complex study. A continuous monitoring of streams discharge, velocity, and water depth, as well as and vertical fluxes is advisable to reduce the uncertainty in the analysis of hyporheic dynamics within a complex riverine system such as Water 2020, 12, 649 16 of 18 a confluence and to highlight main factors such as seasonal and regional changes and drivers of surface-subsurface water interaction.

Author Contributions: All the authors equally participated to this study. I.M. and T.E. conducted the field surveys while C.G. participated to the post-processing analysis. I.M., C.G. and T.E. wrote the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The research was carried out during the PhD Project in “Civil System Engineering” of the first Author. He also shows his gratitude to SUNY for hosting, support and instrumentation and thanks Guy Swenson for assistance with all field campaigns. Collaboration with staff of the Environmental Resources Engineering Department of the SUNY-ESF is gratefully acknowledged. The second author acknowledges the financial support from the Exchange Program for Professors and Researchers of the University of Napoli Federico II for the year 2018. Conflicts of Interest: The authors declare no conflict of interest.

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