Gessler, Daniel

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Article, Published VersionGessler, DanielAdvances in physical modeling of mobile bed systems atAlden during the past 15 yearsHydrolinkVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/109312Vorgeschlagene Zitierweise/Suggested citation:Gessler, Daniel (2016): Advances in physical modeling of mobile bed systems at Aldenduring the past 15 years. In: Hydrolink 2016/1. Madrid: International Association for Hydro-Environment Engineering and Research (IAHR). S. 10-11. https://iahr.oss-accelerate.aliyuncs.com/library/HydroLink/HydroLink2016_01_Hydraulic_Labs.pdf.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.10 hydrolink number 1/2016To help predict the effect of channel modifica-tions on sediment transport and deposition,engineers have long used scaled physicalmodels. If used properly, modeling can providevery helpful information for resolving sedimentproblems at riverine water intakes. During thepast 15 years, there have been significantadvances in physical modeling of sedimenttransport at Alden, further reducing the risk ofbuilding expensive solutions that may not work.This article considers advances in four areas:low density materials available for modelingsediment, instrumentation available for physicalmodels, computer models that are coupled withphysical models, and field data collection.Scaling and Sediment SelectionIn a physical model, the particle size scales withthe model length scale; in a 1:100 scale model,the sediment should be 100 times smaller thanin the prototype. This presents challenges whenmodeling sand bed rivers where the modelsediment must have a diameter on the order ofmicrons. When impractically small modelsediment is required, based on scaling criteria,coarser sediment can be used, but with a lowersediment density.  Historically, mobile bed models at Alden andelsewhere were constructed with coal beds.Coal has a specific gravity of about 1.4 wheresand has a specific gravity of about 2.65.However, coal particles tended to be coarserAlternative Model SedimentA wide range of approximately sphericallyshaped particles with specific gravities as low as1.05 and particle sizes as small as 60 to 100microns have become available in the past 10 to15 years. Polymers and other materials can bemade to a specific density. Through grinding, it ispossible to obtain a defined grain size distri-bution. Physical models can now be constructedwith less or no tilting and distortion, reducingscaling problems. The use of lightweightsediment can introduce operational challengeswith surface tension and getting the sediment tosink; however, these are readily addressed withsurfactants that reduce surface tension withoutchanging other properties of the water.Lightweight sediments should be used whenappropriate to minimize scale effects from modeldistortion. Figure 1 shows a sediment bed beinginstalled on an underlying fixed bed model.  InstrumentationOne of the most significant advances in modelinstrumentation has been with the use of laserscanners such as the one on the yellow tripod inFigure 2. Historically, after a model test, the staticwater level in the model was slowly lowered toestablish contour lines at the water/bed interface.The contour lines were surveyed and a contourmap of the beginning and ending riverbed condi-tions were produced. The laser scanners now inuse at Alden can automatically measure thehorizontal and vertical position of millions ofADVANCES IN PHYSICAL MODELINGOF MOBILE BED SYSTEMS AT ALDENDURING THE PAST 15 YEARSBY DANIEL GESSLERFigure 1 - Lightweight sediment being installed in a physical model Figure 2 - Sub-millimeter scanner in use on a model to determine scour anddeposition. Scanner is on the yellow tripod and the river bed is bluethan sand and resulted in physical modelsdominated by bed load. Further, these modelsfrequently required tilting (enhanced slope ordistorted) to obtain any sediment transport atall. Tilted models have significant scalingproblems. Einstein (1967) and Gessler (1971)both demonstrated the following limitations todistorted models:1. Distorted models can only be designed forone discharge2. The hydraulic time scale differs from thesediment time scale, by a large amount(factor of 10 or more) for only a small (2:1)distortion3. The suspended concentration in the modeldiffers significantly from the prototype,compromising model results4. Local scour and deposition cannot bemodeled, because the angle of repose in themodel does not have the distortion of themodelEinstein and Chien (1956), Einstein (1967) andGessler (1971) indicate that distorted modelsare a compromise which should be avoidedwhen possible. The cost of a distorted modelcan be significantly less than that of an undis-torted model and testing schedules can alsobe shorter. As such they have become popularwith some engineers. However, results arepurely qualitative and in some instances haveresulted in the construction of sedimentationcounter measures that exacerbated (ratherthan mitigated) sedimentation problems.H Y D R A U L I C  LABORATORIES   S P E C I A L  11hydrolink number 1/2016termining three dimensional flow patterns andeddies. Three dimensional numerical tools arenow used to compliment physical modeling ef-forts. While three dimensional models have sig-nificant limitations for predicting changes in bedelevation associated with sediment scour ordeposition, the models are extremely valuablefor visualizing flow patterns that may be difficultto measure directly in a physical model. The ad-ditional cost of a parallel numerical modeling ef-fort is typically offset in a savings in the physicalmodel and additional insight that is gained aboutthe physical model. Every proposed physicalmodel study at Alden now considers the poten-tial benefits of a parallel numerical modelingstudy. Figure 3 shows CFD results that are su-perimposed on the physical model in a hybridCFD – physical model study.Field DataThe fourth pillar of physical modeling is fielddata. The most significant advances in field datacollection have been the ability to measure watervelocity and bathymetry. Historically, point veloc-ity measurements were made to determine thetime averaged velocity at a single point. Measur-ing water velocity at a range of depths and loca-tions in a large river was time consuming andfrequently cost prohibitive. The development ofAcoustic Doppler Current Profilers (ADCP) hasrevolutionized velocity measurement. Widelyused now for over 20 years, the method meas-ures the instantaneous water velocity from amoving boat and is able to acquire both the hori-zontal and vertical velocity profile across thewidth of the river. ADCP velocity data has signifi-cant spatial fluctuations and multiple passescombined with statistical methods must be usedto analyze the data.  Bathymetric data vastly improved with the use ofDifferential Global Positioning Systems (DGPS)during the past 15 years. When combined withecho sounders and shipboard navigation sys-tems, thousands of linear feet of bathymetricdata can now be collected and processed in asingle day with a previously unachievable preci-sion. Figure 4 shows a pontoon boat equippedwith modern bathymetric survey and ADCP ve-locity measurement equipment.SummaryTechnology has revolutionized physical modelingof mobile bed rivers in the past 15 years. Majorimprovements have been realized in the materi-als used for simulating the river sediment andlaser scanners used for measuring a model riverbed to an accuracy of 1mm or better. Combiningphysical models with three dimensional numeri-cal models can help visualize the flow patternsresponsible for the depositional patterns noted inphysical models. Field velocity measurementswere revolutionized by ADCP systems andDGPS-based surveying systems have improvedthe bathymetry used in models. nReferencesa. Einstein, H.A. , Chien, N., (1956) Similarity of Distorted RiverModels with Movable Beds, Transactions of the ASCE, Vol 121, No1, January 1956b. Einstein, H.A., (1967) Discussion on Movable Bed Model for RiverStructure Design, Journal of the Waterways and Harbors Division,ASCE, Vol 93, February 1967c. Gessler, J. (1971), Chapter 21, Modeling of Fluvial Processes,River Mechanics, Volume II, H.W. Shen, Editor, Library ofCongress, 70-168730points on the riverbed, creating a point cloud thatdefines model surface. The scanners have sub-millimeter accuracy (e.g. the Trimble FX 3Dscanner) and are now used in conjunction withdata processing software to define changes inriver bed elevation to less than 1 mm modelscale. The scanner can automatically scan all ofthe surrounding environment to a distance of 50meters or more depending on the scanner.Numerical ModelsOne of the least intuitive but equally significantadvances in physical modeling is numericalmodeling.  One limitation of physical models hasbeen the time consuming effort of accurately de-IAHRFigure 3 - Computational Fluid Dynamics (CFD) model results superimposedon a physical model for a combined CFD – physical model studyFigure 4 - One of Alden’s boats for field measurement equipped with DGPSnavigation, bathymetry and velocity measurement equipmentDr. Gessler is a registered professional engineer and has over 25 years of experiencein numerical and physical modeling and generally oversees the numerical and physi-cal hydraulic modeling activities at Alden. He is responsible for hydraulic modeling using computational fluid dynamic (CFD)models and one and two dimensional hydraulic models, including sediment transport models. In addition to working onnumerical models, he provides technical expertise on physical models involving sedi-ment transport. Dr. Gessler also managesAlden’s Colorado office, and strives to provide engineering recommendations andvaluable engineering options for the projectsin which Alden is involved. He is a Vice President and prior to joining Alden, heworked as a Research Scientist and AssistantProfessor at Colorado State University.About AldenAlden (Alden Research Laboratory, Inc.) was founded in 1894, and is an acclaimed leader in solving flow-related engineering and environmental problems. The firm has 95 employees and over 150 000 sq feet ofindoor lab space on a 32 acre campus in Holden, MA, and an additional 25 000 square feet of laboratoryspace in Redmond, WA. Alden provides engineering, physical and computational flow modeling along withenvironmental and flow meter calibration services. For more information, please visit www.aldenlab.com H Y D R A U L I C  LABORATORIES   S P E C I A L  

Advances in physical modeling of mobile bed systems at Alden during the past 15 years

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Advances in physical modeling of mobile bed systems at Alden during the past 15 years

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