Vertical Scour Research Articles

Pipeline Scour Rates Prediction-Based Model Utilizing a Multilayer Perceptron-Colliding Body Algorithm

In this research, the advanced multilayer perceptron (MLP) models are utilized to predict the free rate of expansion that usually occurs around the pipeline (PL) because of waves. The MLP model was structured by integrating it with three optimization algorithms: particle swarm optimization (PSO), whale algorithm (WA), and colliding bodies’ optimization (CBO). The sediment size, wave characteristics, and PL geometry were used as the inputs for the applied models. Moreover, the scour rate, vertical scour rate along the pipeline, and scour rate at both right and left sides of the pipeline were predicted as the model outputs. Results of the three suggested models, MLP-CBO, MLP-WA, and MLP-PSO, for both testing and training sessions were assessed based on different statistical indices. The results indicated that the MLP-CBO model performed better in comparison to the MLP-PSO, MLP-WA, regression, and empirical models. The MLP-CBO can be used as a powerful soft-computing model for predictions.

The sedimentology of river confluences

AbstractChannel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.

GMDH-GEP to predict free span expansion rates below pipelines under waves

ABSTRACTIn this research, group method of data handling (GMDH) as a one of the self-organized approaches is utilized to predict three-dimensional free span expansion rates around pipeline due to waves. The GMDH network is developed using gene-expression programming (GEP) algorithm. In this way, GEP was performed in each neuron of GMDH instead of polynomial quadratic neuron. Effective parameters on the three-dimensional scour rates include sediment size, pipeline geometry, and wave characteristics upstream of pipeline. Four-dimensionless parameters are considered as input variables by means of dimensional analysis technique. Furthermore, scour rates along the pipeline, vertical scour rate, and additionally scour rates in the left and right of pipeline are determined as output parameters. Results of the proposed GMDH-GEP models for the training stages and testing ones are evaluated using various statistical indices. Performances of the GMDH-GEP models are compared with artificial neural network (ANN), GEP, GMDH, and traditional equations-based regression models. Moreover, sensitivity analysis and parametric study are conducted to perceive influences of different input parameters on the three-dimensional scour rates.

Jet scour

This paper discusses scour without bed protection, such as propeller scour, two-dimensional horizontal, three-dimensional horizontal scour generated by free jets downstream of gates and outlets, two-dimensional vertical and three-dimensional vertical scour. The flow velocities in two- and three-dimensional propeller and free jets are briefly summarised where special attention is paid to the Dutch and German approaches for calculating the near-bed load caused by propellers. The time scale of propeller scour is approximated by applying the Breusers' time-dependent scour equation. For the maximum scour depth in the equilibrium phase Newton's second law is used.

Quantifying the potential for flow to remove the emergent aquatic macrophyte Sparganium erectum from the margins of low-energy rivers

The emergent macrophyte species Sparganium erectum occurs commonly at the margins of low- to medium- energy river systems across the northern temperate zone. It is considered as an invasive species along low-energy water courses in many parts of the US and Australia. The life-cycle and biomechanical properties of this species make it very well adapted to such environments, allowing rapid growth and sediment trapping, such that encroachment into the channel occurs as the growing season progresses. The widespread growth of species such as S. erectum is therefore of particular concern, when considering the flood risk potential of many rivers. As such, the conditions required for survival or uprooting and scouring of this plant are of interest, as are the times of the year and processes by which these plants spread to increase the size of current stands, and to form new stands. It is known that S. erectum reproduces by several vegetative methods including rhizome growth, dispersal of detached rhizomes, and relocation of entire plants. However, the mechanisms and flow conditions necessary for uprooting or scouring of entire plants, and the separation of fragments of this species, at different times of the year, are largely unknown. The aim of this paper is to model the uprooting resistance of S. erectum plants as reported by Liffen et al. (in press), and to investigate the manner by which this species is adapted to proliferate in low-energy, low-gradient streams. The results presented here show that Monte Carlo simulations using the RipRoot root-reinforcement model can be used to accurately model plant pullout forces, rhizome interconnectivity and length changes for S. erectum plants throughout the growing season. Analysis presented here also suggests that plant uprooting forces are several orders of magnitude larger than potential drag forces that could act on the S. erectum plants at the River Blackwater site modeled, and even at sites with much higher channel slopes. This result suggests that the ability of these plants to thrive in low-energy rivers, but not in higher-energy river environments, is less related to driving forces causing drag on the plants, and more related to the energy conditions controlling erosion and deposition of the fine substrate materials these plants thrive in. The critical shear stress of the fine within-vegetation material was shown here to only be exceeded by the average boundary shear stress within the vegetation, during winter months when above-ground biomass and thus Manning's n values were at their lowest. For example, during March and April average boundary shear stress was predicted to exceed critical boundary shear stress for 6% of the time. Erodibility measurements from jet-tests conducted at the River Blackwater fieldsite suggested that this excess in boundary shear stress could result in potential vertical scour of up to 0.09 m in both March and April. During the majority of the growing season sediment trapping rather than erosion dominated, with enough deposition occurring over the summer to protect all but the shallowest, weakest and least interconnected rhizomes and plants from being scoured in the winter months. The balance between erosion and deposition within stands of S. erectum in these low-energy environments therefore allows for the maintenance of established stands of vegetation, whilst still allowing for scouring of weaker S. erectum plants that can establish previously un-colonized channel margins further downstream.

SCOUR BELOW AN OVERFALL: PART I. INVESTIGATION

Scour below an overfall contributes to headcut instability and gully advance. Thirteen largescale scour tests wereconducted by preforming an overfall in a compacted cohesive soil bed. Horizontally compacted soil layers were placed usingtypical embankment construction methods. Water flowing over a horizontal approach was allowed to plunge over an overfalland impinge on the compacted soil bed. Different combinations of soil moisture and soil density were examined for two similartailwater or backwater conditions. For a constant flow rate, the vertical scour was measured downstream of the gully overfall,and typical erosion behavior is discussed. The rate of vertical scour was observed as the plunge pool deepened.<br><br>The vertical scour rate increased as the moisture content at the time of compaction decreased. The scour rate was alsoobserved to decrease as the dry density of the fill material increased. For the soil conditions tested, the vertical scour rateincreased as the unconfined compressive strength increased. The stressstrain modulus and the vertical scour rate alsodisplayed a direct relationship. The layered soil placement methods, typical of embankment construction, can have aninfluence on the observed vertical scour rates. These fieldscale erosion tests were conducted to enhance our understandingof scour processes. This scour investigation focuses on the test description, observed results, and fundamental relationships,while a companion article addresses prediction methods.

A geomorphologic and hydrologic evaluation of an extraordinary flood discharge estimate: Bronco Creek, Arizona

The Bronco Creek, Arizona flood of August 19, 1971, was an extraordinary and rare flash flood. The published peak discharge estimate of 2080 m3 s−1 makes it virtually the world's largest known rainfall‐generated flood to come from a 50‐km2 basin. Previous workers have suggested that the peak discharge was overestimated based on its unlikely hydrological and hydraulic implications. In this study we reevaluate the Bronco Creek flood discharge by modeling peak discharges using relict high‐water marks in bedrock canyon reaches near the mouths of the three major subbasins of the watershed. The new estimated peak discharge of 750 to 850 m3 s−1 is based on summing the subbasin estimates and correcting for omitted drainage area. Qualitative and quantitative analyses of site‐specific and regional hydrological, meteorological, and geomorphological information support a lower peak discharge. We assessed preflood and postflood channel characteristics at the site of the original estimate using historical aerial photographs. Our analysis indicates that much of the discrepancy between the estimates may be explained by significant morphologic changes in the channel caused by the flood. The original estimate was made in a wide, high‐gradient alluvial channel where the flood changed the channel pattern from braided with vegetated bars to straight, wide, and devoid of vegetation. It is also likely that net vertical scour occurred in the channel. Although our new estimate is much lower than the original estimate, it is consistent with the dominant trend in regional flood magnitude‐drainage area relationships. By integrating hydraulic modeling of peak discharges in stable channel reaches with assessment of ancillary hydrological, meteorological, and geomorphological information, this study presents a viable approach to assessing the accuracy of extreme flood estimates in general. It also highlights potential uncertainties in estimating extreme flood discharges in steep, alluvial channels.