A hydrodynamic investigation of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redd selection at the riffle scale

High‐resolution velocity profile measurements were taken over a series of riffles on a gravel‐bed stream using a Pulse Coherent Acoustic Doppler Profiler (PCADP) to quantify the fluid structure of riffles and nests (redds) where brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) spawned. Velocity profiles were obtained on a highly discretized planometric scale ranging between 20 and 40 cm grid spacings, with vertical observations occurring every 1.6 cm. From the velocity profiles, between 15 000 and 38 000 velocity measurements were obtained over each riffle on any given day of measurement. Velocity profiles were converted to streamwise velocity magnitude, flow depth, Reynolds number, Froude number, shear stress, vertical velocity components and turbulent kinetic energy per unit area to evaluate the spatial structure of the riffles and the spatial structure of redds (pits and tailspills) relative to the surrounding riffle structure. Semi‐variograms were employed to evaluate the persistence of the fluid structure based upon the metrics evaluated. Results showed that discrete velocity observations poorly described the spatial structure of the flow system and poorly correlated with redd locations. Reynolds number analysis identified a relatively consistent fluid property for distances typically 2–3 times the longitudinal length of redds. Turbulent kinetic energy per unit area consistently identified common regions on all riffles studied that corresponded with the location selections for redds where flow was identified as essentially uni‐directional. Froude number was found to be insensitive in predicting the fluid spatial structure in wadeable flow depths and relating it to the fluid structure of redds. Results indicated that a series of metrics at varying spatial scales of turbulence may be necessary to understand the spatial complexity of redd selection. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparison of Direct and Indirect Boundary Shear Stress Measurements along Vegetated Streambanks

Estimates of boundary shear stress along vegetated streambanks are needed to predict streambank fluvial erosion. Because fluvial shear stress cannot be directly measured in the field, reliable estimation techniques using field instrumentation are needed. This study evaluated local bank shear stress estimation methods applicable to sloping, vegetated streambanks. Two reaches of a second order stream were modelled in a flume using a fixed‐bed Froude‐scale modelling technique. One reach was dominated by dense shrubs while the other reach was located in a mature forest. Direct measurements of local bank shear stress using a hot‐film anemometer were compared to estimates based on velocity measurements (logarithmic method, Reynolds stresses, and turbulent kinetic energy). For channels with no or widely spaced vegetation, the velocity‐based estimates underestimated the bank shear stress due to secondary flow contributions. For banks with dense vegetation, Reynolds stresses and turbulent kinetic energy estimates were statistically similar to direct measurements on average, but substantial error occurred when making point comparisons. Velocity‐based estimates generally over predicted bank stress in areas of high shear at the vegetation edge and underpredicted stress within dense vegetation. Ultimately, results suggest that none of tested techniques can be broadly applied to streambanks, and flow structure is critical in selecting the appropriate estimation technique. Copyright © 2016 John Wiley & Sons, Ltd.     

Flume simulations of salmon bioturbation effects on critical shear stress and bedload transport in rivers

Nest (redd) construction by female salmonids involves sequences of pit excavation and filling that winnow fines, loosen grains, and moves sediment downstream into a tailspill mound shaped like a dune. Prior research suggests that such bioturbation may destabilize streambeds by reducing friction between grains and converging flow that elevates shear stress on tailspills. Bed stability may alternatively be enhanced by form drag from redds that lowers basal shear stress, an effect that varies with the proportion of the bed that is occupied by redds (P). I used simulated redds and water‐worked (“unspawned”) beds in a laboratory flume to evaluate these competing influences on critical conditions and bedload transport in experiments with P = 0.11 (1 redd), 0.29 (2 redds), and 0.38 (3 redds). Results from competence (largest grain) and reference transport rate estimates of Shields stress indicate that particle entrainment inversely related to P. Bedload transport rates also increased as exponential functions of P and the boundary shear stress that exceeded critical conditions. Therefore, redd form drag did not overcome the destabilizing effects of redd construction. Instead, grain mobility and bedload transport increased with P because larger bed areas were composed of relatively loose grains and redd topography that experiences elevated shear stresses, as suggested in prior research. By winnowing fines and increasing bed surface mobility that exposes small particles in subsurface areas to flow, bioturbation by salmon can mitigate fine sedimentation of streambeds, which suggests an active role for salmon in restoring fish habitat in streams.     

Mesohabitat use by brown trout (Salmo trutta) in a small groundwater‐dominated stream

Groundwater‐dominated streams have particular flow regimes that commonly support populations of trout. Meso‐ and micro‐habitat surveys were carried out on a reach of the river Tern that drains a Triassic sandstone aquifer in the English West Midlands, to investigate brown trout (Salmo trutta) habitat use with varying flows. Mesohabitats were mapped over a range of summer and autumn flows and coupled with direct underwater observation (snorkelling) of fish locations together with point measurements of velocity and depth. The number of habitat types recorded was low and dominated by glides, runs, and backwaters. Brown trout showed a strong association with glides and runs with adults being more associated with runs and parr with glides. General habitat use curves showed brown trout to favour depths between 0.30 and 0.40 m and velocities below 0.40 m s^?1. A clear preference was shown for sand and gravel bed materials. However, the differentiation of hydraulic habitats was weak and there was no trend in mesohabitats or change in trout use of mesohabitats with discharge. The study raises limitations of the mesohabitat survey approach when linking fish ecology, flow and physical habitat in small streams with low flow variability and low habitat diversity. In these situations, other factors (especially cover features) appear to strongly influence brown trout distribution. Copyright © 2010 John Wiley & Sons, Ltd.     

Annual redd counts across two decades are indicators of changing migratory bull trout population sizes in two proximate Montana creeks

Redd counts are the most widely used method for bull trout (Salvelinus confluentus Suckley) population monitoring. Redd counts are an assumed indicator of population size (N), particularly for female spawners, but previous studies rarely considered the measurement uncertainty of N. The present study of two proximate, migratory bull trout populations across two decades examined statistical correlations between annual redd counts and concurrent N (predominantly fish ages 3–7) that included measurement uncertainty. Results showed that redd counts explained about one-third of the concurrent N variation and, in one case, portended N 7 years later. But changing spawning distributions across years, as likewise shown here and as a probable effect of ongoing climate change, can confound the use of fixed index reaches for redd counts. Instead, abundance monitoring based on redd counts should be over a bull trout population's or metapopulation's entire spatial extent and have counts in all potential spawning areas. Such total-annual redd counts may be the most cost-effective and precise means of bull trout population monitoring.     

Intergranular flow velocity through salmonid redds: sensitivity to fines infiltration from low intensity sediment transport events

This paper presents results from a novel technique allowing continuous monitoring through multiple storm events of interstitial flow in salmonid redds. Previous studies have shown that long‐term increases in fine sediment inputs into rivers can silt up spawning beds, reduce intergravel flow and threaten egg survival. Not enough is known, however, about the temporal and spatial scales of the physical processes affecting spawning habitat. The short‐term sensitivity of intergravel flow through salmon nests to low‐intensity sediment transport events has not been documented. Furthermore, it is unclear if the egg pocket flow vital to incubation is principally controlled by the hydraulic conductivity of the redd patch or by that, generally lower, of the ambient riffle substrate. The purpose of this study was to determine if individual runoff events could affect intergravel flow in salmon nests and to investigate the sensitivity of interstitial flow to the fines content and conductivity of the redd patch. During the summer and autumn of 2001, a new intergravel velocity sensor based on the hot wire principle made it possible to continuously monitor, over five months, interstitial velocities in artificial redds in four tributaries of the Cascapedia River, Quebec. Fifteen low and moderate intensity runoff events (up to 50% bankfull) were monitored. Data were obtained for each storm on suspended sediment transport as well as sand infiltration rates in sediment collectors emplaced in redd zones. It was found that redd interstitial velocities were reduced whenever a runoff event deposited more than 7 kg/m² of sands in infiltration traps. In addition, redd interstitial velocities were reduced four out of the five times that the event‐integrated suspended sediment dose exceeded 7 mg l^?1 day (dose is defined as the area under the concentration time curve). In the study conditions, where ambient riffle sediment has relatively moderate permeability and localized groundwater upwelling is negligible, our data suggest that significant intergravel flow (0.1–0.6 mm/s) can be triggered through 2 m long redd patches, in response to the redd‐scale water surface gradient and the relatively higher conductivity of the redd patch, after spawner activity. Copyright © 2005 John Wiley & Sons, Ltd.     

Redds of brown trout in the deep channels of a hydroelectric power station

Natural spawning redds of brown trout were surveyed for the first time in 1?6‐m deep urban channels of a hydroelectric power station (HEP) between two lakes in Finland, by scuba diving and wading. Before damming a century ago, thousands of lake migrating trout spawners ascended through the rapids to upstream spawning areas. Between 2015 and 2018, 11 trout redds were observed at depths of 250?340 cm; these may be the deepest reported in running waters. Nine redds were found at depths of 75?230 cm. The nearest redds were situated only 55 m upstream and 110 m downstream from the HEP inflow and outflow, respectively. Channel substrate and trout redds were dominated by particle size classes 32?64, 64?128, and 16?32 mm. The risk of diving in the area increased due to the high amount of litter on the riverbed. Observations of wild trout parr by local anglers provide evidence that natural spawning produces some fish in the HEP channels.     

Large wood aids spawning Chinook salmon (Oncorhynchus tshawytscha) in marginal habitat on a regulated river in California

To determine whether large wood (LW, ≥1‐m length, ≥10‐cm diameter) plays a role in Chinook salmon (Oncorhynchus tshawytscha) redd (i.e. egg nest) placements in a regulated, Mediterranean‐climate, medium‐sized river (where channel width is less than the upper quartile of length of potential instream wood pieces), characteristics of 527 large wood pieces, locations of 650 redds, and mesohabitat delineations (riffle, run, glide, pool) were collected during a spawning season along a 7.7 km reach directly below Camanche Dam on the Mokelumne River, CA. LW was regularly distributed across the study reach an average 70 LW pieces km^‐1. Some LW clustering was evident at islands and meander bends. Spawners built 85% of redds within one average channel width (31 m) of LW. Spawners utilized LW within a 10 m radius 36% of the time in the upper 3 km rehabilitated reach, and 44% of the time in the lower 4.7 km marginal habitat reach. A greater percentage of LW was utilized in riffles in the upper 3 km reach where 90% of redds were built, while a larger percentage of spawners used LW in riffles in the lower 4.7 km reach. LW‐redd interactions occurred at greater rates than by random chance alone in the lower 4.7 km reach, which implies that LW aids spawning in marginal habitats. River managers and salmonid spawning habitat rehabilitation (SHR) projects should take LW additions into consideration as an important component of river rehabilitation. Copyright © 2010 John Wiley & Sons, Ltd.     

Abiotic Characterization of Brown Trout (Salmo trutta f. fario) and Rainbow Trout (Oncorhynchus mykiss) Spawning Redds Affected by Small Hydropower Plants — Case Studies from Austria

Salmonid rivers in Austria are considerably regulated by small hydropower facilities, resulting in potential declines of the spawning habitats of salmonids. To assess the restrictions and possible quality of hydropower‐influenced river sections for salmonid, spawning redd densities of brown trout and rainbow trout were monitored in two rivers in 2014 and 2015. The results showed spawning close to small hydropower facilities for both investigated species — with similarities in redd characteristics like pit and tail length. Differences occurred concerning the distance of redd construction to the next shore. Brown trout spawn close to the banks in comparison to rainbow trout which use the entire active channel width. In addition to the preference of brown trout for certain cover types, it turned out that the presence of high quality spawning gravel in the river is the dictating abiotic variable (probably bottleneck) in the control of salmonid populations even for river reaches impacted by small hydropower plants. Moreover, the assessments of spawning redd densities enabled a discussion of different opportunities for spawning habitat enhancement of salmonids in river sections regulated by small hydropower facilities. Here, in conclusion, it was found that the fill‐up of the backwater sites by transported sediments or the structural modification (e.g. boulder placement) in the tail of the backwater could improve the spawning situation in a sustainable way. Copyright © 2016 John Wiley & Sons, Ltd.     

GRAVEL AUGMENTATION INCREASES SPAWNING UTILIZATION BY ANADROMOUS SALMONIDS: A CASE STUDY FROM CALIFORNIA,USA

Anadromous salmonid diversity and abundance worldwide have been adversely impacted by anthropogenic forces, and millions of dollars are spent each year on stream habitat restoration and enhancement. However, there is a paucity of data comparing site use by salmonids before and after enhancement implementation, and few studies examine the specific environmental conditions that determine whether salmonids utilize an enhanced site. This study examines the use of gravel augmentation to improve spawning site utilization by Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss) on the Lower American River, California, USA. Spawning increased across all augmentation sites for both species, although there were species‐specific and year‐specific differences in the degree to which a site was utilized and in the spatial distribution of redds in relation to substrate size, habitat features and other redds. There were also differences in redd architecture across sites that were related to differences in gravel size. This study illustrates that gravel augmentation projects can enhance spawning habitat for salmonids where spawning beds have degraded but that species‐specific and site‐specific attributes and gravel size can influence the relative effectiveness of a project. Copyright © 2013 John Wiley & Sons, Ltd.     

ASSESSING THE IMPACTS OF RIVER REGULATION ON NATIVE BULL TROUT (SALVELINUS CONFLUENTUS) AND WESTSLOPE CUTTHROAT TROUT (ONCORHYNCHUS CLARKII LEWISI) HABITATS IN THE UPPER FLATHEAD RIVER,MONTANA, USA

Hungry Horse Dam on the South Fork Flathead River, Montana, USA, has modified the natural flow regimen for power generation, flood risk management and flow augmentation for anadromous fish recovery in the Columbia River. Concern over the detrimental effects of dam operations on native resident fishes prompted research to quantify the impacts of alternative flow management strategies on threatened bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi) habitats. Seasonal and life‐stage specific habitat suitability criteria were combined with a two‐dimensional hydrodynamic habitat model to assess discharge effects on usable habitats. Telemetry data used to construct seasonal habitat suitability curves revealed that subadult (fish that emigrated from natal streams to the river system) bull trout move to shallow, low‐velocity shoreline areas at night, which are most sensitive to flow fluctuations. Habitat time series analyses comparing the natural flow regimen (predam, 1929–1952) with five postdam flow management strategies (1953–2008) show that the natural flow conditions optimize the critical bull trout habitats and that the current strategy best resembles the natural flow conditions of all postdam periods. Late summer flow augmentation for anadromous fish recovery, however, produces higher discharges than predam conditions, which reduces the availability of usable habitat during this critical growing season. Our results suggest that past flow management policies that created sporadic streamflow fluctuations were likely detrimental to resident salmonids and that natural flow management strategies will likely improve the chances of protecting key ecosystem processes and help to maintain and restore threatened bull trout and westslope cutthroat trout populations in the upper Columbia River Basin. Copyright © 2011 John Wiley & Sons, Ltd.     

The spatial arrangement of Neritina virginea (Gastropoda: Neritidae) during upstream migration in a split‐channel reach

This paper relates differences in flow hydraulics between a main channel (MC) and a side channel (SC) of a river to patterns of upstream migration by Neritina virginea (Neritidae: Gastropoda), a dominant diadromous snail in streams of Puerto Rico (Greater Antilles). Near‐bed water velocity, snail density and shell size were measured on a weekly basis between August and December 2000 along cross‐sections in a main channel (MC) and an adjacent channel (SC) under a bridge crossing of the Río Mameyes of Northeastern Puerto Rico. Near‐bed velocity and water depth were used to compute Reynolds (Re) and Froude (Fr) numbers, and to classify flows within each channel. During base flow conditions (<2 m³ s⁻¹), flow was chaotic and supercritical (Fr > 1) in the MC, and non‐chaotic and subcritical (Fr < 1) in the SC. Higher mean densities (>100 ind m⁻²) of relatively small snails (mean ± s.d., 6.3 ± 2.8 mm) were consistently recorded in the MC. Conversely, the SC had lower mean densities (<20 ind m⁻²) and significantly larger snails (7.6 ± 2.4 mm). Within the MC, migratory groups preferred near‐bed velocities > 0.8 m s⁻¹. Within the SC, they preferred the channel thalweg and depths > 30 cm. The spatial arrangement that was observed between and within the channels may be related to food resources, predation pressure or biomechanics. Characteristics of preferred upstream migration pathways of N. virginea must be accounted when building road crossings in coastal streams with diadromous fauna. Published in 2007 by John Wiley & Sons, Ltd.     

Quantifying the behavioral response of spawning chum salmon to elevated discharges from Bonneville Dam,Columbia river,USA

Chum salmon Oncorhynchus keta that spawn in main‐stem habitats below Bonneville Dam on the Columbia River, USA, are periodically subjected to elevated discharges that may alter spawning behaviour. We investigated behavioural responses of spawning chum salmon to increased water velocities associated with experimental increases in tailwater elevation using acoustic telemetry and a dual‐frequency identification sonar. Chum salmon primarily remained near their redds at base tailwater elevations (3.5 m above mean sea level), but displayed different movement and behavioural responses as elevations were increased to either 4.1 or 4.7 m for 8‐h periods. When velocities remained suitable (<0.8 m s^?1) during elevated‐tailwater tests, female chum salmon remained near their redds but exhibited reduced digging activity as water velocities increased. However, when velocities exceeded 0.8 m s^?1, the females that remained on their redds exhibited increased swimming activity and digging virtually ceased. Female and male chum salmon that left their redds when velocities became unsuitable moved mean distances ranging from 32 to 58 m to occupy suitable velocities, but returned to their redds after tailwaters returned to base levels. Spawning events (i.e. egg deposition) were observed for five of nine pairs of chum salmon following tests indicating any disruptions to normal behaviour caused by elevated tailwaters were likely temporary. We believe a chum salmon's decision to either remain on, or leave, its redd during periods of unsuitably high water velocities reflects time invested in the redd and the associated energetic costs it is willing to incur. Published in 2009 by John Wiley & Sons, Ltd.     

Effects of dam removal on brook trout in a Wisconsin stream

Dams create barriers to fish migration and dispersal in drainage basins, and the removal of dams is often viewed as a means of increasing habitat availability and restoring migratory routes of several fish species. However, these barriers can also isolate and protect native taxa from aggressive downstream invaders. We examined fish community composition two years prior to and two years after the removal of a pair of low‐head dams from Boulder Creek, Wisconsin, U.S.A. in 2003 to determine if removal of these potential barriers affected the resident population of native brook trout (Salvelinus fontinalis). Despite the presence of other taxa in the downstream reaches, and in other similar streams adjacent to the Boulder Creek (including the brown trout, Salmo trutta), no new species had colonized the Boulder Creek in the two years following dam removal. The adults catch per unit effort (CPUE) was lower and the young‐of‐the‐year catch per unit effort (YOY CPUE) was higher in 2005 than in 2001 in all reaches, but the magnitude of these changes was substantially larger in the two dam‐affected sample reaches relative to an upstream reference reach, indicating a localized effect of the removal. Total length of the adults and the YOY and the adult body condition did not vary between years or among reaches. Thus, despite changes in numbers of adults and the YOYs in some sections of the stream, the lack of new fish species invading Boulder Creek and the limited extent of population change in brook trout indicate that dam removal had a minor effect on these native salmonids in the first two years of the post‐removal. Copyright © 2007 John Wiley & Sons, Ltd.     

Scour depths at sites selected for spawning by brown trout (Salmo trutta) along a longitudinal gradient of a North American mountain river

In the Intermountain West, USA, fry of fall‐spawning brown trout (Salmo trutta) are susceptible to scour‐related mortality because they are still in the gravel during spring snowmelt run‐off events. The goal of our research was to understand patterns of gravel scour on the Logan River, Utah, in relation to brown trout spawning and whether mobility of spawning gravels could explain the absence of brown trout from higher elevations. We collected data to characterize local entrainment potential at spawning gravels longitudinally on the Logan River during 2009 and 2010 spring flood events. We used scour chains to measure scour depth at spawning locations, and we also examined the position of redds in channel cross sections in relation to the centre line. The flood magnitude in both years approximated the 2‐year flood magnitude, but the flood in 2009 was much longer in duration. Scour at 27% of scour chain locations exceeded the estimated median upper limit of developing fry in 2009, whereas scour at 0% of locations exceeded this depth in 2010. Brown trout spawned in locations with similar entrainment potential at both mid and high elevations, which contributed to a lack of trend in scour depth with elevation. In addition, the majority of areas chosen for spawning were channel margins. The relationship between local entrainment potential at spawning gravels and scour depth was similar for the mid‐elevation canyon zone with medium brown trout density and the high‐elevation noncanyon zone with low brown trout density. In a low‐elevation backwater zone containing high densities of brown trout, scour was high despite low levels of entrainment potential. Overall, findings suggest that spawning gravel scour is not limiting brown trout abundance at high elevations in this system given shallow scour depths overall and a general lack of increase in scour depth with increasing elevation/distance upstream.     

The influence of coarse particle mobility on scour depth in salmonid spawning habitat

This study examined the influence of flow hydraulics and coarse particle mobility on bed scour adjacent to coho salmon (Oncorhynchus kisutch) redds in a coastal California watershed for a bankfull flood. It was theorized that coarse particle mobility (i.e., mobility of particles larger than the median bed particle size, D₅₀) exerts a strong control on bed scour depth. Maximum scour depth at the study sites was found to be negatively correlated with flow shear stress, which is dissimilar to findings from previous scour studies in spawning reaches. This resulted from a relatively similar coarse particle size (D₈₄) for all study sites and a negative relationship between shear stress and coarse particle exposure to flow (or the D₈₄/D₅₀ ratio), which together caused sites with low shear stress to have a high degree of localized coarse particle mobility and an associated high maximum scour depth. This study provides new insights into the vulnerability of spawning reaches with low flow energy to redd scour and highlights the need to consider the mobility of coarse particle sizes explicitly when examining the dominant controls on redd scour.     

Investigating the turbulent flow characteristics in an open channel with staggered vegetation patches

In the present study, flow around circular and staggered vegetation patches was investigated numerically. For turbulence modelling, the Reynolds‐averaged Navier–Stokes technique and Reynolds stress model were adopted. The numerical model was validated with the experimental data using varying vegetation density and flow velocities. The simulated results of mean stream‐wise velocities were in close agreement with the experimental results. The results show that the mean stream‐wise velocity in the downstream regions of vegetation patches were reduced, whereas the velocity in the free stream regions were increased. The influence of neighbouring and staggered vegetation patches on the flow was observed. The vegetation patches with larger nondimensional flow blockage (aD = 2.3, where a is the frontal area per volume of patches, and D is the diameter of vegetation patches) offered more turbulence when compared to the patches with a smaller flow blockage (aD = 1.2). Larger turbulence in the form of kinetic energy and turbulent intensity was recorded within the vegetation as well as the regions directly behind the patches. Negative Reynolds stresses were observed at the top of submerged vegetation. The turbulence characteristics peaked at the top of vegetation, that is, z/h = 1.0 (where z is the flow depth, and h is the vegetation height), which may be migrated vertically as the frontal area of the vegetation patch is increased. This high frontal area also increased stream‐wise velocity above the vegetation, leading to an increased variation in turbulence around the vegetation canopy.     

Effect of sinusoidal corrugated bed on hydraulic jump characteristics

Hydraulic jump characteristics were studied experimentally over six corrugated beds with varying wave steepness, which had corrugation and Froude numbers in the range of 0.286–0.625 and 3.8–8.6, respectively. The effects of wave height and length of corrugation on the basic jump characteristics, including free surface location, velocity, shear stress distribution and energy dissipation, were studied for a range of Froude numbers. The dimensionless hydraulic parameters were found to be a function of the Froude number. The results showed that the tailwater depth and the length of the jump on corrugated beds are smaller than those of the corresponding jump on a smooth bed. The analysis of velocity profiles at different sections in the jump showed that the velocity profiles were similar to those of a simple plane wall jet. The normalized boundary layer thickness δ/b was equal to 0.57 for jumps on a corrugated bed, compared to 0.16 for the simple wall jet. The analysis and comparison of the bed shear force and shear stress coefficient showed that shear stress on a corrugated bed is about 10 times that of a smooth bed. The results of this study are in good agreement with previous results and showed that corrugated beds can be used to dissipate the excess hydraulic jump energy in stilling basins.     

Study of Flow Over Side Weirs Under Supercritical Conditions

The present study focuses on the application of momentum principle to the analysis of spatially varied flow under supercritical conditions. Experimental studies were conducted on rectangular side weirs of different lengths and sill heights fitted in the test section of a rectangular aluminium channel that was built in a tilting flume. Measurements of discharges in the main channel and through the side weir were done separately. A pitot tube with direction finder was used to determine the velocities and angle of spill flow with the side weir. Depths of flow were measured both in longitudinal and transverse directions at regular intervals and their profiles were studied. Experiments were conducted with different test plates and flow conditions in the main channel. Coefficients of discharge were computed using momentum principle for different Froude numbers (between 1.5 and 3). The variation of discharge coefficient of the side weir as a function of Froude number was found to exhibit a non-linear relationship. Discharges over side weirs were computed using the computed coefficients of discharge for different Froude numbers and it was verified with the observed discharges. Coefficients of discharge were also computed using energy principle for different Froude numbers. Chi-square test was done between observed discharges over side weirs and discharges computed using momentum and energy principles, it was found that momentum principle is fitting better. Variation of the ratios of longitudinal components of velocity vector of spill flow to the mean velocity of the main channel flow at upstream end of the side weir with Froude number was found to exhibit a non-linear relationship. Variation of the discharge ratios of spill flow and main channel discharges with Froude number was also studied.     

Hydraulic and turbulence structure of triangular labyrinth weir‐pool fishways

Detailed laboratory experiments were conducted to analyze and evaluate a new weir‐pool fishway design. The new design consisted of a series of one‐cycle, triangular labyrinth weirs that formed a weir‐pool fish passage. Hydraulic characteristics of flow over the proposed fishway were examined for three pool lengths and a wide range of discharges. It was found that the new design had superior discharge capacity over classical weir‐pool fishways in plunging flow regime. Spatial and point analyses of measured velocity data were carried out to understand turbulence structure and distribution of turbulent flow. The power dissipation per unit volume was calculated for different tests, and it varied nonlinearly with discharge. Based on the limiting criteria for energy dissipation rates of different fish species, the maximum operating discharge for different slopes was extracted for the proposed design. The model‐to‐prototype scaling analysis was performed based on the recommended scaling factor of three for prototype. It was found that the approach velocity of prototype increased by 12%, whereas the total discharge increased by more than five times. The contour plots of time‐averaged velocity indicated that the proposed fishways offered diversified flow fields without exceeding the maximum velocity above the standards recommended for weir‐pool fishways. To study the size, direction, and periodicity of turbulent eddies, magnitude of the most energetic eddy and dominant frequencies were extracted using power spectrum analysis. Quadrant analysis consistently showed that Reynolds shear stresses in ejection and sweep events became weaker as the pool length increased. The small region of strong ejection and sweep events evidenced a good resting place for fish migration, whereas the generated turbulence was large enough to attract fish species to use the passage.