Morphology of step-pools in a wilderness headwater stream: The importance of standardizing geomorphic measurements

The morphology of step-pools and cascades reflect the geological and climatic factors affecting channels in mountain watersheds. This study uses longitudinal and cross-section surveys to describe a headwater stream in the Boston Mountains of the Ozarks Plateau region in Arkansas and develop morphological relationships for comparisons with other regions. In the Bowers Hollow Creek watershed (3.5 km²), located within the boundaries of the Upper Buffalo Wilderness Area, step height and wavelength relationships are generally similar to those reported from other regions. Step-pool reaches were widely distributed in a discontinuous manner throughout the watershed. Average values of the sampled reaches are: reach slopes, 0.105 m/m; width, 6.10 m; crest particle sizes, 440 mm; step heights, 0.87 m; and step wavelengths, 6.62 m. The mean step steepness for the watershed was 0.13, whereas the mean length of a reach step to height ratio was 9:1. A comparison of morphological definitions found that the values of step height and steepness can vary by > 30% according to how step parameters are defined. Step height is particularly sensitive; thus, comparison of step height-based relationships from published data requires great care.     

西南山区河流阶梯-深潭系统的生态学作用

山区河流发育的阶梯—深潭系统具有显著的生态学作用。阶梯—深潭系统增大水流阻力和河床抗冲刷力,稳定了河床和岸坡。大卵石堆积成阶梯,细颗粒泥沙在深潭河段的缓流滞流区沉积下来形成淤泥层,形成适宜多种生物的栖息地。选择小江支流——深沟、蒋家沟和小白泥沟,以及四川九寨沟和金沙江进行野外实验、取样分析。结果发现阶梯—深潭系统较发育的深沟和九寨沟底栖动物密度高达552个/m2,生物量高达5.96 g/m2。而邻近的小白泥沟和蒋家沟底栖动物密度仅0.75个/m2,生物量不到0.006 g/m2。考虑河流不同部位底质、水深、流速等特性,提出了生物栖息地多样性及其计算方法,研究发现大型无脊椎动物的生物多样性随栖息地多样性增加而增加。利用阶梯—深潭系统治理山区河流,既能保持河道稳定,又能维持较高的生物多样性,保持健康的河流生态系统。     

Geomorphic response of a headwater channel to augmented flow

Since 1974, flow releases from Long Draw Reservoir have increased annual peak flows on La Poudre Pass Creek, Colorado, from ~ 5.6 m³/s to > 8.4 m³/s. The creek drains 61 km² and channel morphology varies from step-pool to pool-riffle. Comparison of five channel reaches along the creek to channel reaches along neighboring rivers without flow regulation indicates that channel width has increased by as much as a factor of three along La Poudre Pass Creek. Width-to-depth ratio has also increased, the bed material in step-pool channel reaches has coarsened, and residual pool volumes have increased in pool-riffle channel reaches. Pool-riffle channel reaches have undergone the greatest change in response to flow augmentation. Although discharge has increased consistently for all five channel reaches, morphologic response varies in relation to gradient and channel morphology, making it impractical to precisely predict a priori the magnitude of channel response to flow augmentation.     

DEVELOPMENT OF STEP-POOL SEQUENCE AND ITS EFFECTS IN RESISTANCE AND STREAM BED STABILITY

Experiments were conducted and field investigations were performed to study the development of step-pool sequence and its effects on resistance to the flow and stream bed stability. Step-pool sequence develops in incised channels as a result of streambed erosion, which is compared with sand dunes and armor layer of the role in resistance and streambed protection. The tight interlocking of particles in steps gives them an inherent stability which only extreme floods are likely to disturb. That stability suggests that step-pools are a valid equilibrium form, especially when coupled with their apparent regularity form and their role in satisfying the extreme condition of resistance maximization. The development degree of step-pools, SP, is proportional to the streambed slope. If the incoming sediment load is equal to or more than the sediment-carrying capacity of the flow, there is no bed erosion and thence there are no step-pools. If the flow depth increases and is over the step-height the resistance caused by the step-pool sequence will be greatly reduced. The rote of energy dissipation by step-pools is a function of SP. The higher is SP,, the larger is the rate of energy dissipation. The step-pool sequence increases the resistance and flow depth, reduces the shear stress of the flow and protects the streambed from erosion. Moreover, step-pool sequence provides ecologically sound habitats for aquatic bio-community as well.     

城市泥石流沟的治理启示——以深沟为例

为研究典型工程措施在泥石流沟治理中的作用及影响,结合野外调查及历史资料,对云南东川城区的深沟泥石流治理进行总结：深沟在一系列坝系群建设和多年植树造林的治理下,发育了一定规模的阶梯一深潭系统,使沟道阻力明显增强（经测河床结构强度5p达0．3以上）,消减了泥石流能量,并保护河床不被侵蚀下切,岸坡稳定;而另一方面林草生长能提...     

STEP-POOL MORPHOLOGY IN HIGH-GRADIENT STREAMS

l 1NTRODUCTIONGravel bed rivers are found in man parts of the worid, mpically in moUntainous regions with highgradients and seasonally high flows. These rivers are imPotalt in contrlling flood waters from sPringrunoff in regions such as the Pacific Northwest, Where heaVy snowfall can be followed by equally heaVyranfall. The combination of high stream gradient and high discharge causes significant erosion of thebed and bank of the strCam, in some cases moving large boulders with ease.It…     

Equal-mobility bed load transport in a small, step-pool channel in the Ouachita Mountains

Equal-mobility transport (EMT) of bed load is more evident than size-selective transport during near-bankfull flow events in a small, step-pool channel in the Ouachita Mountains of central Arkansas. Bed load transport modes were studied by simulating five separate runoff events with peak discharges between 0.25 and 1.34 m³/s (1.0- to 1.6-year recurrence intervals) in a natural channel using controlled releases from a storage tank. EMT occurrence was investigated using four different bed load relationships suggested by previous research. With each of these approaches, the relationship of a given bed load characteristic (D_max, distribution percentile, displacement distance and skewness) to some independent factor (τ_c*, τ and grain size) was assessed to determine which transport mode was evident. Regression models derived using combinations of these four relationships with different datasets provide seven separate tests. Five of the seven tests indicate that EMT occurred or was predominant. Several reasons may explain the apparent contradictory results, but the confounding effects of changes in the structural arrangements of bed material prior to or during the events seem particularly important.     

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.     

Morphology and controlling factors of step-pools in rills based on field investigation in the Yuanmou Dry-hot Valley,China

Rill erosion is well developed in Yuanmou Dry-hot Valley. The appearance of step-pools in rills (SPRs) is an early stage of erosion and necessary condition for rill development. We measured length, width, depth of plunge pools, and length, height, and slope of step walls. We observed the developmental environment of every SPR in the field. Through research on relationships among SPR parameters, morphological characteristics, and control factors, we conclude that (1) correlations of step slope with pool length and width, and of step length with pool width are not evident (p > 0.1); correlations of pool length with step height and length are evident (p < 0.1); correlations of step slope with pool depth, and of step height with pool width are significant (p < 0.05); and correlations of pool depth with step length and height are highly significant (p < 0.01). Plunge-pool length is affected by soil properties and step height, but not by step gradient. (2) SPR formation shows a coupling effect of a single factor or many factors, such as terrain conditions, vegetation, confluence area, soil properties, and boundary conditions, and the shapes and scales of SPRs notably change with differences of the main control factors.     

Field measurements of three-dimensional hydraulics in a step-pool channel

We investigated the effects of morphologic position and discharge on flow structure in a steep (0.10 m/m) mountain channel by collecting three-dimensional measurements of time-averaged and turbulent velocity components with a SonTek FlowTracker Handheld ADV (acoustic Doppler velocimeter) on a 30-m reach of a step-pool channel in the Colorado Rockies. Velocity profiles were measured at morphologic positions characteristic of steep channels (above steps, step lips, base of steps, pools, cascades, runs), and at five different discharges. A marked three-dimensionality of flow structure was documented in East St. Louis Creek. Velocities in the streamwise component were the largest contributors to overall velocity vector magnitudes; cross-stream and vertical components contributed averages of 20% and 15%, respectively, to overall vector magnitudes. Turbulence intensities were especially multi-dimensional, however, with large contributions to turbulent kinetic energy from the vertical component of velocity. Analysis of variance indicated that discharge and morphologic position significantly affected mean streamwise velocities, with substantially higher velocities upstream from steps than in pools. Discharge and morphology effects on cross-stream and vertical velocity components, however, were not significant. Discharge and morphologic position also significantly affected turbulence intensities for all flow components, with the greatest turbulence intensities occurring in pools and at high discharges. These results illustrate the strong discharge-dependence of hydraulics in step-pool channels, where relative submergence of bedforms changes rapidly with discharge, and the substantial spatial variation in hydraulics created by step-pool sequences.