A. N. Thanos Papanicolaou

Influence of Collective Boulder Array on the Surrounding Time-averaged and Turbulent Flow Fields

Arrays of large immobile boulders, which are often encountered in steep mountain streams, affect the timing and magnitude of sediment transport events through their interactions with the approach flow. Despite their importance in the quantification of the bedload rate, the collective influence of a boulder array on the approach time-averaged and turbulent flow field has to date been overlooked. The overarching objective is, thus, to assess the collective effects of a boulder array on the time-averaged and turbulent flow fields surrounding an individual boulder within the array, placing particular emphasis on highlighting the bed shear stress spatial variability. The objective of this study is pursued by resolving and comparing the time-averaged and turbulent flow fields developing around a boulder, with and without an array of isolated boulders being present. The results show that the effects of an individual boulder on the time-averaged streamwise velocity and turbulence intensity were limited to the boulder’s immediate vicinity in the streamwise (x/dc < 2–3) and vertical (z/dc < 1) directions. Outside of the boulder’s immediate vicinity, the time-averaged streamwise velocity was found to be globally decelerated. This global deceleration was attributed to the form drag generated collectively by the boulder array. More importantly, the boulder array reduced the applied shear stress exerted on the individual boulders found within the array, by absorbing a portion of the total applied shear. Furthermore, the array was found to have a “homogenizing” effect on the near-bed turbulence thus significantly reducing the turbulence intensity in the near-bed region. The findings of this study suggest that the collective boulder array bears a portion of the total applied bed shear stress as form drag, hence reducing the available bed shear stress for transporting incoming mobile sediment. Thus, the effects of the boulder array should not be ignored in sediment transport predictions. These effects are encapsulated in this study by Equation (6).

The effects of protruding rock boulders in regulating sediment intrusion within the hyporheic zone of mountain streams

Excessive sedimentation in mountain stream ecosystems is a critical environmental problem due to the clogging of streambeds by sediment particles within the hyporheic zone, with detrimental effects on fish spawning habitat. In this research, the effects of an array of boulders in regulating the intrusion of incoming sand within a gravel substrate were evaluated by performing detailed experiments in a laboratory flume. A unique experimental setup and two different sampling techniques were utilized for measuring the infiltrated sand within the gravel bed under two bed shear stress conditions (moderate vs. high). For comparison purposes, experiments were performed without and with the presence of partially submerged to the flow (protruding) boulders, which is typical for the average flow conditions found in mountain streams. Results indicated that sand infiltrated primarily in the upper part of the gravel bed creating a surface seal which hindered the penetration of sand particles deeper into the bed. An exponential decrease of the amount of the infiltrated sand within the hyporheic zone was observed in all experiments regardless of the presence of boulders. However, the presence of boulders promoted sediment intrusion of sand particles especially for the moderate applied bed shear stress condition, since the total amount of the infiltrated sand was found to be on average 44% greater when boulders were present. The findings from this study can provide additional insight regarding the role of boulders on promoting downwelling of flow and sediment within the gravel substrate with potential effects on fish habitat.

Improved streambank countermeasures: the Des Moines River (USA) case study

Abstract In the Midwestern USA, bank erosion is a common hazard due to the high erodibility of the bank soils. In this paper, an improved methodology aimed at identifying the optimal countermeasures to control bank erosion was developed and applied in two sites of the Des Moines River (USA). In situ flow measurements, bed bathymetry and soil properties were collected for providing boundary conditions and parameters of the two-dimensional, depth-averaged hydrodynamic finite element surface water modeling system (FESWMS) model. The model was used to compare the hydraulic performances of four streambank countermeasures: riprap lining (referred to as S1); a series of barbs (S2); alternating barbs and spurs (S3); and the combination of barbs with lining (S4). A key feature of FESWMS was its ability to simulate the wetting/drying conditions of mesh elements, which allowed the simulations of unsubmerged or partially submerged structures for different hydraulic conditions. This research showed that the combination of alternating barbs and spurs (S3) was the only design which provided protection during overbank flows at a competitive cost compared to the other designs analysed. The uniqueness of this methodology is found in the coupling of field measurements and theoretical approaches for depth-averaged velocity profiles to calibrate and validate a hydrodynamic model; and in the proposed design to protect streambanks from severe erosion.

Understanding mass fluvial erosion along a bank profile: using PEEP technology for quantifying retreat lengths and identifying event timing

This study provides fundamental examination of mass fluvial erosion along a stream bank by identifying event timing, quantifying retreat lengths, and providing ranges of incipient shear stress for hydraulically driven erosion. Mass fluvial erosion is defined here as the detachment of thin soil layers or conglomerates from the bank face under higher hydraulic shear stresses relative to surface fluvial erosion, or the entrainment of individual grains or aggregates under lower hydraulic shear stresses. We explore the relationship between the two regimes in a representative, US Midwestern stream with semi-cohesive bank soils, namely Clear Creek, IA. Photo-Electronic Erosion Pins (PEEPs) provide, for the first time, in situ measurements of mass fluvial erosion retreat lengths during a season. The PEEPs were installed at identical locations where surface fluvial erosion measurements exist for identifying the transition point between the two regimes. This transition is postulated to occur when the applied shear stress surpasses a second threshold, namely the critical shear stress for mass fluvial erosion. We hypothesize that the regimes are intricately related and surface fluvial erosion can facilitate mass fluvial erosion. Selective entrainment of unbound/exposed, mostly silt-sized particles at low shear stresses over sand-sized sediment can armor the bank surface, limiting the removal of the underlying soil. The armoring here is enhanced by cementation from the presence of optimal levels of sand and clay. Select studies show that fluvial erosion strength can increase several-fold when appropriate amounts of sand and clay are mixed and cement together. Hence, soil layers or conglomerates are entrained with higher flows. The critical shear stress for mass fluvial erosion was found to be an order of magnitude higher than that of surface fluvial erosion, and proceeded with higher (approximately 2–4 times) erodibility. The results were well represented by a mechanistic detachment model that captures the two regimes. Copyright

Role of Structure Submergence on Scour Evolution in Gravel Bed Rivers: Application to Slope-Crested Structures

AbstractThis paper, written to celebrate the 60th anniversary of the Journal of Hydraulic Engineering, examines scour evolution around sloped-crest structures, known as barbs. The results extend th...

Dawn of a New Era

I have not been the editor-in-chief for very long, but for over the past year, one of the most pleasant surprises has been the high quality of the manuscripts that have been received covering many thematic areas focused on the fundamentals of hydraulic engineering and waterways, sediment transport, watershed sedimentation, computational hydraulics, and sensor technology. These papers that integrate science across several of the traditional disciplines in the Journal are welcomed. But, submissions of new manuscripts that present innovative concepts and approaches are also encouraged. Because hydraulic infrastructure is vital to maintaining the nation’s economy, understanding its resilience, as well as improving its design and performance, it is the centerpiece of the work of a hydraulic engineer. The Journal must be the leading interdisciplinary and multidisciplinary journal for the hydraulics community by integrating field, laboratory, and numerical investigations that support the effective design of the critical infrastructure. This is an area that I would like to grow, so I am challenging readers to advance the state-of-the-art and report the findings in the Journal. As part of the natural evolution of the community’s Journal to address new thematic areas that researchers will face in the years ahead, a new international team has been established. In the past year, 13 new associate editors (AEs) have begun a 4-year term of service: (1) James Fox, University of Kentucky; (2) Philippe Frey, IRSTEA Grenoble; (3) Mohamed Ghidaoui, Hong Kong University of Science and Technology; (4) Abdul Khan, Clemson University; (5) Eddy Langendoen, USDA–Agricultural Research Service (ARS); (6) Xiaofeng Liu, Penn State; (7) Gregory Pasternack, University of California, Davis; (8) Jose Rodriguez, University of Newcastle; (9) Luca Solari, University di Firenze; (10) Thorsten Stoesser, Cardiff University; (11) Kyle Strom, Virginia Tech; (12) Blake Tullis, Utah State University; and (13) Daniel Wren, USDA–ARS. This new team of AEs brings a high level of expertise, as well as a lot of energy. But as the new crop of AEs is welcomed, we have not forgotten those who have contributed their time and efforts over the years but are currently stepping down. First and foremost, we would like to pay tribute to the distinguished service of Professor Terry Sturm, the previous editor-in-chief of the Journal. Many in the community recognize and appreciate the effort and commitment that Professor Sturm has put towards the Journal, and I am asking that the entire community thank Professor Sturm, once more, for excellent work. I would also like to thank the following professors and former AEs for their service to the Journal, as well as their efforts and dedication to the engineering community at large: (1) Panos Diplas with Lehigh University, (2) Ana Maria da Silva of Queen’s University, and (3) Andrea Marion from University of Padua. The editorial board sincerely wishes them continued success for the future. With the expanded team of AEs, it is our hope to spend more time communicating with the authors and reviewers. The review process is so critical to the success of the Journal, as well as serving the academic and practitioner communities. The mission of the editor and the AEs is not only to render publishing decisions, but also to help the authors with expressing their writing and research clearly and succinctly. As former Editor-in-Chief James Liggett said, “The value of a referee lies far less in the yes or no judgment of the overall paper than in the service rendered by pointing out flaws that would bother or mislead most other readers.” In that sense, I must recognize now, and will continue to recognize, the contributions of the conscientious and very helpful reviewers. The quality of the reviews received is invigorating and it is part of the success story of achieving the highest impact factor (1.75) of all ASCE journals and one of the highest in the area of water resources and engineering. The quality of the Journal in general depends on the quality of the reviews as much as on the quality of the submitted paper. With this in mind, the Journal implemented 10 important changes to the submission process and the handling of papers, as follows: 1. The authors should follow more closely the formatting guidelines provided in the authors’ guide available on the Journal’s website. This is an important step for sustaining high quality and, at the same time, helping the reviewers as they go through the manuscript. Manuscripts that are not in accordance with the formatting guidelines will be returned back to the authors without review through Editorial Manager. 2. Authors are encouraged to provide names of knowledgeable and experienced referees during the submission process. This will expand the editorial board’s refereeing database and alleviate some of the pressure that current referees are experiencing with the multiple reviews they are asked to provide. 3. Authors are also encouraged to provide the most updated and recent references on their topic, usually from the last 10 or 15 years. The fact of the matter is that several papers submitted to the journal have very few references to other Journal publications relevant to their topic of interest. This must be changed! There is a wealth of knowledge in the ASCE Library. 4. Authors must pay special attention to the discussion section of the papers. Too often, discussion sections turn into simple description sections of result trends (e.g., they are too high, too low, and so on). Authors should please refrain from merely describing the results. Instead, use this section to offer more insight into the results and concisely convey the emphasis of the research reported in the paper. For example, explain why is this particular result too high, or too low, and so on. 5. Revisions must be completed within 45 days of receiving the comments. If the authors are unable to meet this deadline, then they are encouraged to indicate the original manuscript number in their letter to the editor so that the paper can make it back to the originally assigned AE. This will help expedite the reviewing process. 6. Authors who submit papers to the Journal will be encouraged to share their data either through university repository, thirdparty site, such as Dryad or Figshare, by request, and so on. 7. The Journal already enjoys the submission of several highquality academic manuscripts; however, the Journal also encourages receiving papers from so-called practical engineers. The focus of these practical papers can be on everyday problems in engineering, the development of a new technique, or a new understanding of site-specific problem areas.

Bank Stability Analysis for Fluvial Erosion and Mass Failure

The central objective of this study was to highlight the differences in magnitude between mechanical and fluvial streambank erosional strength with the purpose of developing a more comprehensive bank stability analysis. Mechanical erosion and ultimately failure signifies the general movement or collapse of large soil blocks due to geotechnical instability and is the upper limit of streambank erosion. Conversely, fluvial erosion is the detachment of individual particles or aggregates due to the shearing action of flow and is the lower limit of streambank erosion. A total of 24 streambank samples from a representative stream in the U.S. Midwest (i.e., Clear Creek, IA) with semi-cohesive soils were analyzed in terms of both mechanical and fluvial erosional strength. Mechanical strength was measured using a direct shear device and ranged from 400 to 6,600 Pa. Fluvial erosional strength was measured using a conduit flume, which applied a shearing force to the sample and had values between 1.28 and 2.37 Pa. Thus, mechanical strength was two to three orders of magnitude larger than fluvial erosional strength, which suggests that identifying the different modes of streambank erosion (e.g., mechanical or fluvial) during a hydrograph is needed to provide better design specifications for bank stabilization practices.