Step-pool Streams Research Articles

Flow resistance evaluation based on three morphological patterns in step-pool streams

Flow resistance evaluation based on three morphological patterns in step-pool streams

The Morphological Evolution of a Step–Pool Stream after an Exceptional Flood and Subsequent Ordinary Flow Conditions

Mountain streams are frequently characterized by step–pool morphology that provides stability and energy dissipation to the channel network. Large flooding events can overturn the equilibrium of the step–pool condition by altering the entire configuration. This work focuses on the impact of the “Vaia” storm (27–30 October 2018) on a step–pool mountain stream (Rio Cordon, Northeast Italy) and on its evolution after two years of ordinary flow conditions. To achieve the aims, this work uses both remote sensing data (LiDAR and UAV) and direct field measurements (i.e., longitudinal profiles and grain sizes distributions) performed pre-event, post-event, and 2 years later (current conditions). The results show a significant widening (width +81%, area +68%) and the creation of a new avulsion after the storm and a substantial change between the number of units (51 in the pre-event, 22 post-event, and 51 in the current conditions) and characteristics of step–pool sequences between pre- and post-conditions. Furthermore, it proves the ongoing processes of morphological stabilization since the current step–pool sequences parameters are heading back to the pre-event values. Such results suggest clear susceptibility of step–pool to exceptional events and fast recovery of such setting during barely two years of ordinary flow conditions.

The influence of different morphological units on the turbulent flow characteristics in step-pool mountain streams

The morphology of step-pools is often implemented for ecological restoration and for the creation of close-to-nature fish passes. Step-pools display spatio-temporal variations in bed and flow characteristics due to meso-scale units such as step, tread, base of step, and pool. Exclusive research on the effects of bed variations in step-pools on the flow dynamics is limited. Here, we conducted laboratory experiments on a physical model downscaled from a field site in the Western Ghats, Kerala, India. The results of Kruskal–Wallis ANOVA show significant differences in the velocity and turbulent intensities for the morphological units. A regression equation of the form Power-Allometric1 has been proposed to relate the normalized turbulent kinetic energy with the velocity magnitude. The present study also estimated the range of Reynolds shear stress and energy dissipation factor existent in the step-pool systems. The normalized values of Reynolds shear stress in the x–z plane ranged from − 19.477 to 13.729, and energy dissipation factors obtained for the three step-pool systems are 321, 207, and 123 W/m3; both the results reveal insufficient pool volume for adequate energy dissipation. The study concludes that while designing close-to-nature step-pool fish passes, pool dimensions should be finalized with respect to the target aquatic species.

Dissipative scaling of step-pool features

This paper focuses on the dissipative similarity of step-pool units at rill, flume and stream scale. This investigation is carried out using recent advances in open channel flow resistance, applications of close-range photogrammetry to rill erosion, available published data on step-pool features in flumes and streams and a new dataset of measurements in fixed bed step-pool rills. A theoretically-based equation for calculating the Darcy-Weisbach friction factor obtained by integration of a power velocity profile is presented. The scale factor Γ of this power velocity profile, which is included in the flow resistance equation, was previously calibrated (Eq. 10) for mobile bed rills with step-pool units. At first, in this investigation an additional test of this flow resistance equation is developed using measurements carried out in fixed bed rills with step-pool features. In particular, the proposed Γ function is tested using the measurements carried out in 63 fixed bed rill reaches incised on a 24 and 26% sloped plot. This test shows that the effect of sediment transport on the flow resistance law can be considered negligible as compared to the effect of the form-induced dissipative mechanisms due to the presence of step-pools structures. Then, using measurements carried out in 61 flume experimental runs and 109 reaches of step-pool streams, this investigation demonstrates that the Γ function for flume and stream conditions can be obtained scaling that (Eq. 10) determined for step-pool rills. The Darcy-Weisbach friction factor values measured in flumes and streams are, on average, higher than those related to rills. Finally, the application of the theoretical equation for calculating the Darcy-Weisbach friction factor for flume and stream conditions requires Γ, estimated by Eq. (10), to be multiplied by a specific scale factor.

Interacting geomorphic and ecological response of step-pool streams after wildfire

AbstractDetailed study of the response of step-pool mountain channels to wildfire is rare despite increasing vulnerability of these systems. This paper reports the initial biogeomorphic response of step-pool streams to storms following the 2012 Waldo Canyon Fire, Colorado, USA. Field surveys, benthic macroinvertebrate sampling, and light detection and ranging (LiDAR) terrestrial laser scanning through 2014 generated a substantial data set comparing changes within seven channels burned by a range of severity with three unburned reference reaches. Results showed that wildfire increased the susceptibility of step-pool channels to destabilize according to the severity of burn. Whereas the step-pool morphology (step height H, step length L, and H/L) remained relatively unchanged in unburned and low-severity burned channels—even through an extreme rainfall event with recurrence intervals up to 500–1000 years—study sites affected by high burn severity altered significantly following the first comparatively minor storms. Study channels burned by moderate severity exhibited variable responses ranging from minor to substantial morphological changes, suggesting that additional factors, including slope, local topography, and proximity to other highly disturbed sites may play explanatory roles. The ecological response corroborated the geomorphic findings, with data for benthic macroinvertebrates varying according to burn severity along with the changing post-fire step-pool morphology. Ordination analysis enabled data synthesis and showed that precipitation intensity (r = 0.59) and the severity of burn (r = 0.60) together influenced the interacting morphologic and ecological responses after the Waldo Canyon Fire. Variations in H (r = 0.72), L (r = 0.56), and thalweg elevation (r = 0.75) correlated the most with the types of benthic macroinvertebrates present. Following the Waldo Canyon Fire, the highest rainfall intensities coincidentally occurred over study sites in high-severity burn areas. Although disentangling the effects of rainfall intensity and burn severity proved difficult given this scenario, the interacting and complementary roles of burn severity and rainfall intensity likely led to the greatest morphologic and ecological impacts at those sites. Findings offer insights for the post-fire management of step-pool streams that characterize frequently burned mountain areas. As wildfires grow in frequency and magnitude under warming regimes, management becomes increasingly important for anticipating future changes in burned river landscapes, mitigating potential floods and hazards, and promoting sustainable river ecosystems.

Dissipative analogies of step-pool features: From rills to mountain streams

In this paper the dissipative similarity of step-pool units at different spatial scales ranging from rills to streams is analyzed. This investigation benefits from the latest theoretical advances in open channel flow resistance, high-resolution topography from close-range photogrammetry applied to rill erosion and the availability of published data from literature on step-pool streams.At first, the integration of a power velocity distribution allowed to obtain a theoretically-based expression of Darcy-Weisbach friction factor, in which Γ function and δ exponent of the velocity profile are included.Then this theoretically-deduced flow resistance relationship is calibrated and tested by flow measurements carried out in rill reaches where the step-pool units occurred. In particular, the proposed Γ function is calibrated using the measurements corresponding to 88 rill reaches shaped on a plot having a slope equal to 14, 22 and 24% and the calibrated equation is also positively tested with measurements carried out in 48 rill reaches shaped on a 26% sloping plot. For the rill flow the developed analysis stated that the friction factor is characterized by estimate errors which are less than or equal to ±15% for 86% of cases and less than or equal to ±10% for 70% of cases.Using measurements of flow velocity, water depth, width and bed slope measurements carried out in 109 reaches of step-pool streams, this investigation demonstrates that the theoretical flow resistance equation can be applicable to step-pool streams carrying out a specific calibration of the Γ function. The comparison between the Darcy-Weisbach friction factor values measured in streams and step-pool rills demonstrates that in the stream features the friction factor values are, on average, higher than those related to rills with step-pool sequences. In conclusion, the comparison between rills and streams with step-pool units highlights that the same theoretical flow resistance equation can be applied even if a scale effect between the two features is detected.

Field Study of Flow Resistance in Step-pool Streams (Case study of Dizin River)

A series of steps and pools are ubiquitous bed forms in mountain stream channels, occurring where gradients exceed 2% and materials are in the gravel to boulder size range. Flow resistance, reflected by roughness elements, appears to be an important control on bed load transport rates and mean flow velocity. To estimate flow resistance some morphological features and velocity were measured in the step-pool channel of Dizin River, located in Karaj River watershed in Iran. Topographic surveys and bed sediment sampling were made in low flow condition while three-dimensional velocity measurements were made in low, medium and high flow conditions. Gradient variations are in the range of 7% to 14%. As flow resistance is a function of geometric, bed material size, longitudinal slope and hydraulic radius, dimensional analysis was conducted to develop a non-dimensional relationship for flow resistance in step-pool reaches. Thereafter, it was calibrated for the measured data set of Dizin river and validated for Rio Cordon data set. Comparable results of validation with a river located in a different environment may suggest that flow resistance features in semi arid and humid streams may have similar effects on non-dimensional resistance coefficient

A reduced-complexity model for sediment transport and step-pool morphology

Abstract. A new particle-based reduced-complexity model to simulate sediment transport and channel morphology in steep streams in presented. The model CAST (Cellular Automaton Sediment Transport) contains phenomenological parameterizations, deterministic or stochastic, of sediment supply, bed load transport, and particle entrainment and deposition in a cellular-automaton space with uniform grain size. The model reproduces a realistic bed morphology and typical fluctuations in transport rates observed in steep channels. Particle hop distances, from entrainment to deposition, are well fitted by exponential distributions, in agreement with field data. The effect of stochasticity in both the entrainment and the input rate is shown. A stochastic parameterization of the entrainment is essential to create and maintain a realistic channel morphology, while the intermittent transport of grains in CAST shreds the input signal and its stochastic variability. A jamming routine has been added to CAST to simulate the grain–grain and grain–bed interactions that lead to particle jamming and step formation in a step-pool stream. The results show that jamming is effective in generating steps in unsteady conditions. Steps are created during high-flow periods and they survive during low flows only in sediment-starved conditions, in agreement with the jammed-state hypothesis of Church and Zimmermann (2007). Reduced-complexity models like CAST give new insights into the dynamics of complex phenomena such as sediment transport and bedform stability and are a useful complement to fully physically based models to test research hypotheses.

Reply to comment by Keith Richardson on “Flow regimes, bed morphology, and flow resistance in self‐formed step‐pool channels”

Reply to comment by Keith Richardson on “Flow regimes, bed morphology, and flow resistance in self‐formed step‐pool channels”

Linking Theory and Practice for Restoration of Step-Pool Streams

Step-pools sequences are increasingly used to restore stream channels. This increase corresponds to significant advances in theory for step-pools in recent years. The need for step-pools in stream restoration arises as urban development encroaches into steep terrain in response to population pressures, as stream channels in lower-gradient areas require stabilization due to hydrological alterations associated with land-use changes, and as step-pools are recognized for their potential to enhance stream habitats. Despite an increasingly voluminous literature and great demand for restoration using step-pool sequences, however, the link between theory and practice is limited. In this article, we present four unique cases of stream restoration using step-pools, including the evolution of the approaches, the project designs, and adjustments in the system following restoration. Baxter Creek in El Cerrito, California demonstrates an early application of artificial step-pools in which natural adjustments occurred toward geomorphic stability and ecological improvement. Restoration of East Alamo Creek in a large residential development near San Ramon, California illustrates an example of step-pools increasingly used in locations where such a channel form would not naturally occur. Construction of a step-pool channel in Karnowsky Creek within the Siuslaw National Forest, Oregon overcame constraints posed by access and the type and availability of materials; the placement of logs allowed natural scouring below steps. Dry Canyon Creek on the property of the Mountains Restoration Trust in Calabasas, California afforded a somewhat experimental approach to designing step-pools, allowing observation and learning in the future. These cases demonstrate how theories and relationships developed for step-pool sequences over the past two decades have been applied in real-world settings. The lessons from these examples enable us to develop considerations useful for deriving an appropriate course of design, approval, and construction of artificial step-pool systems. They also raise additional fundamental questions concerning appropriate strategies for restoration of step-pool streams. Outstanding challenges are highlighted as opportunities for continuing theoretical work.

Dissipative analogies between a schematic macro‐roughness arrangement and step–pool morphology

AbstractWe investigate flow resistance developed by macro‐roughness represented by pebbles positioned on a granular layer according to a regularly spaced stripe pattern on steep bed slopes. Flume experiments under various geometrical and hydraulic conditions are carried out and interpreted by means of a theoretical model. Results show that flow resistance reaches a maximum and is due mainly to form drag when the spacing between macro‐roughness stripes is about 10 times the average macro‐roughness height. A statistical analysis based on various field observations of step–pool geometry underlines that this spacing appears to be one of the most frequent occurring in step–pool bed morphology sequences. Comparison between the present results and flow resistance evaluated for step–pools reproduced in the laboratory and observed in the field suggests that step–pool streams are characterized by a bed geometry able to develop the maximum flow resistance. Finally, a criterion is obtained to estimate flow resistance developed by natural step–pool streams when a formative flow discharge occurs based on geometric quantities only. Copyright © 2007 John Wiley & Sons, Ltd.

Field-derived relationships for flow velocity and resistance in high-gradient streams

We measured velocity and channel geometry in 10 reaches (bed gradient = 0.08–0.21) of a predominantly step-pool channel, the Rio Cordon, Italy, over a range of discharges (3–80% of the bankfull discharge). The resulting data were used to compute flow resistance. At-a-station hydraulic geometry relations indicate that in most reaches, the exponent describing the rate of velocity increases with discharge was between 0.48 and 0.6, which is within the range of published values for pool-riffle channels. The Rio Cordon data are also combined with published hydraulics data from step-pool streams to explore non-dimensional relationships between velocity and flow resistance and factors including unit discharge, channel gradient, and step geometry. Multiple regression analysis of this combined field dataset indicated that dimensionless unit discharge (q∗) is the most important independent variable overall in explaining variations in velocity and flow resistance, followed by channel slope and the ratio of step height to step length. Empirical equations are provided both for dimensionless velocity and flow resistance, but prediction of the former variable appears more reliable.

Sediment mobility and bedload transport conditions in an alpine stream

AbstractThe mobility conditions of bedload transport in an alpine high‐gradient step–pool stream (Rio Cordon) are analysed. Since 1986, a device system at the downstream end section of the stream has been operating in order to monitor the water discharge, suspended sediment and bedload transport. Sediment distribution of bedload transported by various floods has been analysed, and equal‐mobility evidence is recognized only for the high‐magnitude flows ever recorded (RI > 50 years). The thresholds for size‐selective and equal‐mobility transport conditions are identified and quantified by using both data provided by the fractional transport rate and by length displacements of marked particles. Size‐selective bedload transport seems to dominate when the critical shear stress of the size fractions τci considered is exceeded, whereas the equal‐mobility condition is approached as levels of excess shear stress become higher (τeqi = 1·45τci). Copyright © 2006 John Wiley & Sons, Ltd.

When does bedload transport begin in steep boulder‐bed streams?

AbstractThe paper presents the results of field measurements of critical conditions for bedload motion conducted in the Rio Cordon, a steep boulder‐bed stream in the Italian Alps, under conditions of high Reynolds numbers and low relative submergence poorly explored before. Two methods have been used to determine threshold of motion: the displacement of marked clasts and the flow competence approach, which uses the largest grain size diameter transported by each flood event.The high values of $\tau_{\rm {c}i}^{*}$ confirm the great relevance of non‐bedload effective shear stresses in step–pool streams given by the additional form drag associated with this morphology. Relative submergence effects on the dimensionless critical shear stress $\tau_{\rm {c}i}^{*}$ have been quantified by considering the relative submergence ratio Rh/D84, and the major effect of relative size on the mobility of each particle in steep, widely graded bed mixtures has been evaluated. Finally, the dimensionless critical unit discharge $q_{\rm {c}i}^{*}$ has also been adopted in the regression equations as the critical hydraulic parameter, because it may represent an easier parameter to use than the critical shear stress for steep, rough mountain rivers. Copyright © 2006 John Wiley & Sons, Ltd.

Dimensions of standing waves at steps in mountain rivers

Oscillations of the water surface are common features of flows around critical conditions (Fr ∼ 1) and of both natural and structure‐induced undular hydraulic jumps. The paper describes experimental tests and field data on the dimensions of standing waves created by artificial drops in steep gravel bed rivers. The purpose of the research is to analyze what parameters affect the phenomenon, to provide semiempirical formulations for wave dimensions, and to assess whether previous models for undular hydraulic jumps and antidunes are reasonably predictive. The dimensionless wavelength and wave amplitude were correlated to the Froude number at the sill. The first wavelength appears linearly related to the Froude parameter, whereas the amplitude of the first wave shows a less defined behavior, possibly indicating that a maximum value occurs for sill Froude numbers around 1.6–1.7. The corresponding wave steepness (i.e., wave amplitude/length) results between 0.15–0.16, and this range also characterizes the point at which the dimensionless trough depth below the sill achieves a local maximum, probably related to hydrodynamic conditions just before wave breaking. The transition between roller and undular jumps at drops is then argued to likely represent a critical stage in step pool streams, and the hydraulic and morphological implications of this finding are also discussed, with particular emphasis on step stability and step wavelength.

Toward a theory for step pools in stream channels

Over the past 10 to 15 years there has been a surge in interest in step-pool streams in fluvial geomorphological research. This reflects recognition of the geomorphic significance of step pools in high-gradient channels, and a growing interest in rivers in mountainous regions in general. This paper synthesizes the recent literature on step pools, with particular reference to theory construction and progress made over the past decade and a half. Four groups of contributions are identified. The first focuses on understanding morphological relations in order to seek causal mechanisms. The second emphasizes processes that result from the step-pool configuration. The third is a group of integrative studies that seek broader explanations for step-pool sequences. The fourth and growing body of literature applies what has been learned about step pools toward managing and restoring steep channels. We conclude by highlighting some future possible research directions toward gaining a complete explanation for step pools in stream channels.

Morphological effects of local scouring in step–pool streams

AbstractStep–pool morphology characterizes many high‐gradient streams in a variety of natural settings, but formative processes and evolutionary dynamics are still poorly understood. In this paper, natural step–pool geometry is compared with steep alluvial channels where grade‐control structures such as check‐dams and bed sills make the stream profile resemble a natural stepped stream. Along these channels, local scouring due to falling jets forms plunge pools under each structure, analogous to natural steps determining the formation of pools. In order to test the hypothesis that natural pools are analogous to pools formed below grade‐control works with respect to their dimensions, shape and formative dynamics, 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern Italian Alps. Pools below grade‐control works featured a transitional zone between the scour hole and the downstream sloping bed, marked by a depositional berm. When geometric parameters such as maximum pool depth, length and step–berm distances are normalized to the jet virtual energy, no statistically significant differences were detected between natural and artificial systems. These results lend support to an upstream‐forced cascade model for step–pool formation, where the energy of falling jets controls the geometry of the pools, and is therefore regarded as the most important scaling‐independent variable. Copyright © 2005 John Wiley & Sons, Ltd.

Magnitude‐frequency analysis of bed load data in an Alpine boulder bed stream

The purpose of this study is to investigate the connection between bed load, channel processes, and sediment sources in mountain basins using data from the Rio Cordon basin (northeastern Italian Alps). The main channel is a steep, boulder bed, step pool stream, and bed load volumes are measured at a special facility where particles >20 mm are trapped. Results from a combined frequency analysis of peak water discharges and total bed load volumes based on 17 years of field data are presented, focusing on discrepancies between recurrence intervals of peak discharge and bed load volume for each event. A cause of major disturbance is a high‐magnitude, low‐recurrence event that occurred in 1994. Dimensional and nondimensional bed load intensity‐duration curves are also reported, which emphasize differences between short‐ and long‐duration events. The relationship flow–bed load rates (investigated using a total of 180 data) shows no breakpoints, and two different curves are evident, representing pre‐1994 and post‐1994 floods, respectively. The analysis demonstrates both the control exerted by sediment availability on bed load transport rates and the persisting long‐term impact of major floods on mountain streams.

Geomorphic Influences on Large Wood Dam Loadings, Particulate Organic Matter and Dissolved Organic Carbon in an Old-Growth Northern Hardwood Watershed

ABSTRACT We quantified large wood loadings and seasonal concentrations of particulate organic matter (POM) and dissolved organic carbon (DOC) in three different geomorphic zones (each with unique hydrogeomorphic characteristics) of a pristine, old-growth northern hardwood watershed. The highest large wood dam loadings were in the high-gradient, bedrock controlled geomorphic type characterized by a cascading and step pool stream channel (19 dams km−1), while the lowest loadings were in the low-gradient, bedrock controlled geomorphic type (5 dams km−1). POM and DOC concentrations varied seasonally and were generally significantly correlated with discharge before and following leaf-off (mid to late September). The highest concentrations of POM were in the wetland dominated, low-gradient, bedrock controlled geomorphic type (3.74 mg L−1), while the highest DOC concentrations were in the high gradient, bedrock controlled geomorphic type following leaf-off (14.55 mg L−1). Our results suggest that organic matter dynamics vary both spatially and temporally in a watershed in response to changes in hydrogeomorphic conditions, and the processing of different types of organic matter may be intimately linked, resulting in the significant differences in how organic matter is processed and utilized in stream ecosystems flowing through forested watersheds.

Local scouring and morphological adjustments in steep channels with check-dam sequences

This paper describes bed profile and grain size distribution adjustments in a mountain river (Maso di Spinelle River, Italian Alps) stabilized by a sequence of boulder check-dams. The control works were originally designed to simulate the geometry of natural step-pool channels, where tumbling flow is the dominant hydraulic regime. Local scouring downstream of 29 drop structures is analysed through the use of nondimensional parameters where maximum scour depth and scour length are normalised to the drop height. Prior laboratory data reveal a pattern similar to field scours, where complex interactions occur between drop height, critical flow depth, and step spacing. The linkage between scour length and depth is also discussed. There seems to be a maximum step height for impinging jets that is approximately twice the drop height; this maximum may explain the upper limit of the steepness factor found in high-gradient step-pool streams. If such a maximum upper limit is confirmed by further studies, this may aid designs of foundation heights for transverse control works in steep channels.