Numerical Simulation of Scour Depth Variation Around Vertical Wall Abutments

Transcription

1 World Journal of Reearch and Review (WJRR) ISSN: , Volume-5, Iue-6, December 2017 Page Numerical Simulation of Scour Depth Variation Around Vertical Wall Abutment Evangelia Farirotou, Nikolao Xafouli Abtract The HEC-RAS one dimenional, free-urface numerical model wa ued to imulate bed formation in alluvial channel. Baed on the hydraulic equation all the hydrodynamic parameter were firtly imulated and then a ediment tranport imulation wa performed. The model can compute the ediment tranport uing a hydrodynamic implification, the quai-unteady flow aumption, which approximate a continuou hydrograph with a erie of dicrete teady flow profile. The current reearch work aim to invetigate cour depth variation in the region of vertical wall abutment. Numerical computation of cour depth variation around a vertical wall abutment are verified by comparing them with available meaurement. Moreover the tudy report experimental reult of cour depth variation along the center flow line of a channel in the vicinity of a vertical wall abutment. The experimental meaurement of bed variation for ix different inflow dicharge and three different abutment width, after a pecified flow duration, are graphically preented and can be ued by other reearcher in order to validate their imulation. Comparion between computed reult and available meaurement of cour depth i atifactory along the center flow line a the numerical imulation i one dimenional. The one dimenional model give an accurate etimation of the maximum cour depth near the obtruction and it i applicable for a afe deign criteria for the aement of bridge vulnerability which i very important in river engineering work. Index Term Bridge Abutment, Numerical Simulation, Bed Level Variation, Scour Depth. I. INTRODUCTION Eroion, tranportation and depoition of olid particle have haped the preent landcape of our world and can caue evere engineering and environmental problem. Local cour at bridge pier and abutment are the mot common caue of bridge failure during catatrophic flood. Local cour around pier and abutment involve removal of material from around the obtruction and it i caued by an acceleration of flow and reulting vortice. The flow obtructed by an abutment accelerate and form a vortex tarting at the uptream end of the abutment and running along the toe of the contruction. The flow pattern and the mechanim of local couring around bridge abutment i very complex and many fluvial bridge uffered evere damage or collaped during heavy flood. River tructure are expoed to both contraction and local cour around bridge pier and abutment and eroion phenomena are urely one of the mot important factor in bridge vulnerability and have been Evangelia D. Farirotou, Department of Civil Engineering T.E., Technological Educational Intitute of Thealy, Laria, Greece Nikolao G. Xafouli, Department of Civil Engineering T.E., Technological Educational Intitute of Thealy, Laria, Greece reported by variou invetigator. From the engineering view point, the accurate quantitative etimation of local cour proce around hydraulic tructure i neceary for the prevention of evere environmental problem and for a afe river deign. Extended reearch ha been conducted to determine the depth and location of the cour hole that develop from the vortex that occur at the abutment, and numerou abutment cour equation have been developed to predict thi cour depth. Therefore, for the computational imulation of actual river problem, it i neceary to conider unteady flow, bottom topography, edimentation etc. The three-dimenional hydrodynamic model coupled with a proper ediment tranport equation are expected to give more accurate reult than other model. However, the emi-three-dimenional or two-dimenional or one dimenional model are very popular. Their popularity i due, not only on their implicity (relatively to the three dimenional model), but alo on the atifactory approximation in river and canal. Numerou publication are reported for one-dimenional bed deformation in alluvial channel. Among them Bhallamudi and Chaudhry [1] developed an explicit numerical model to imulate one - dimenional cour and depoition. Klonidi and Souli [7] and Farirotou, Souli and Dermii [3] developed one dimenional model for bed morphology imulation. Biglary and Sturm [2] developed a two dimenional hydrodynamic model for the flow around a bridge abutment in order to compute the maximum velocity near the uptream corner of the abutment face. They ued a finite volume difference cheme with a taggered grid. Richardon and Panchang [8] imulated with a three dimenional hydrodynamic model the flow occurring at the bae of a cylindrical bridge pier. The reult were compared with experimental obervation. Kaem and Chaudhry [6] compared a two-dimenional fully coupled model with a emi-coupled one uing the Beam and Warming alternative-direction implicit cheme to calculate bed variation in alluvial channel. Farirotou, Souli and Dermii [4] preented an explicit, integral, two-dimenional, finite volume, numerical cheme to imulate bed morphology variation in alluvial channel. Farirotou and Xafouli [5] reported an experimental invetigation on the effect of variou parameter on local cour depth around vertical wall abutment. The objective of thi reearch work i to invetigate bed formation in alluvial channel a well a in region around abutment. For thi purpoe a one dimenional numerical model which combine hydrodynamic and ediment tranport equation wa applied. Numerical computation of cour depth variation around a vertical wall abutment were verified by comparing them with available meaurement. A laboratory experimental procedure wa etablihed to imulate local cour around vertical-wall abutment in 25

2 Numerical Simulation of Scour Depth Variation Around Vertical Wall Abutment uniform ediment under clear water cour condition. Comparion of cour depth variation along the center flow line of the channel, in the region of the abutment, for different flow dicharge and abutment width how that the one-dimenional imulation model appear to give a reaonable etimation of the maximum local cour depth. II. COMPUTATIONAL ANALYSIS A. Hydraulic and ediment tranport model The Hydrologic Engineering Center River Analyi Sytem (HEC-RAS [9]) numerical model wa ued in order to perform a one dimenional ediment tranport and bed level variation analyi. The HEC-RAS model can compute the ediment tranport uing a hydrodynamic implification, the quai-unteady flow aumption, which approximate a continuou hydrograph with a erie of dicrete teady flow profile. For each record in the flow erie, flow remain contant over a pecified time window for tranport. It i obviouly that before performing any ediment tranport numerical imulation the river hydraulic are already determined. B. Governing flow equation Water urface profile are computed from one cro ection to the next by olving the energy equation to a body of water encloed by two cro ection at location 1 and 2 a: 3 a2v2 a1v 1 h2 Z2 h1 Z1 h e (1) 2g 2g where h i the water depth at cro ection, Z i the elevation of the main channel invert, V i the average velocity, a i velocity weighting coefficient, g i the gravitational acceleration and h e i the energy head lo. The energy head lo between two cro ection i compried of friction loe and contraction and expanion loe a: h LS e f a2 V2 a1v 1 C (2) 2g 2g where L i the ditance between cro ection 1 and 2 along the direction of the flow, S f i the friction lope between two cro ection, which i computed uing Manning equation a: 1 2 / 3 1/ 2 Q AR S f (3) n where Q i the flow dicharge, n i the Manning roughne coefficient, A i the flow area, R i the hydraulic radiu (area/wetted perimeter) and C i expanion or contraction lo coefficient. The program aume that a contraction i occurring whenever the velocity head downtream i greater than the velocity head uptream. When the change in river cro ection i mall and the flow i ubcritical, the contraction and expanion coefficient are equal to 0.1 and 0.3, repectively. In more abrupt change, uch a thee occurring at bridge, the ued value are 0.3 and 0.5, repectively. The HEC-RAS ediment model olve the ediment continuity equation, known a the Exner equation, a: z Q ( 1 p) B (4) t x where B i the channel width, z i the channel elevation, λ p i the active layer poroity, t i the time, x i the ditance along the flow direction and Q i the ediment dicharge. Equation (4) tate that the change of ediment volume in a control volume (aggradation or degradation) i equal to the difference between the inflowing an outflowing ediment load. The ediment continuity equation i olved by computed a ediment tranport capacity through the control volume aociated with each cro ection. If ediment tranport capacity i greater than upply there i a ediment deficit and bed eroion. If upply exceed tranport capacity there i a ediment urplu and material depoition [9]. C. Sediment tranport empirical relation Since ediment tranport i enitive to many hydraulic and ediment variable there are different ediment tranport empirical equation available in the literature. The ediment dicharge may be predicted by one of the following bed-load formulae available in the HEC-RAS model [9]: a. Acker and White (1973), b. Engelund and Hanen (1976) c. Lauren-Copeland (1996), d. Meyer-Peter and Muller (1968), e. Toffaleti (1968), f. Yang (1984) and g. Wilcock (2001) The tranported ediment load computed by the different equation can vary by order of magnitude, depending on the project material and hydrodynamic. III. SCOUR PROCEDURE AROUND A BRIDGE ABUTMENT The local cour variation around a vertical wall abutment in uniform ediment under clear water cour condition i numerically imulated. The one-dimenional numerical reult are compared with available experimental meaurement oberved by Farirotou and Xafouli [5] in a laboratory flume. In the laboratory of Flooding Protection Hydraulic Work & Water Reource Management of the Department of Civil Engineering T.E. of the Technological Educational Intitute of Thealy a primatic open channel, of rectangular cro-ection, of 0.078m wide and 6.0mm long wa ued. Three different ize of vertical wall abutment model were ued, with abutment width, b, tranvere to the flow, equal to 0.036m, 0.048m and 0.051m. The experimental inflow dicharge were equal to m 3 /, m 3 /, m 3 /, m 3 /, m 3 / and m 3 /. After the required time duration period i reached, the bed elevation meaurement data were recorded uing a water-level gauge moving in longitudinal and in tranvere direction of the flume introducing an optical error of ~0.1mm. Meaurement data were obtained along three different flow line preented in Fig. 1: 26

3 World Journal of Reearch and Review (WJRR) ISSN: , Volume-5, Iue-6, December 2017 Page a) the flow line located at a contant ditance of 0.005m to the treamwie face of the abutment parallel to the flow direction ( flow line 1), b) the flow line located at a contant ditance of 0.005m to the uptream tranvere face of the abutment tranvere to the flow direction ( flow line 2) and c) the center flow line of the channel located at an equal ditance between the olid wall ( flow line 3). All meaurement along the hypothetical flow line 1 and 3 were obtained at ucceive ditance of 0.02m while the meaurement along the flow line 2 were taken at ucceive ditance of 0.01m apart. The canned bed variation region wa included in an area extending 0.06 m uptream to the abutment to 0.06 m downtream to it, while in the tranvere direction the length i extended along the total channel width (Fig. 1) [5]. where S i the channel bottom lope. The particle fall diameter i computed a: 2 d 69.07d d if d 0,026m (7) f d d otherwie (8) f Exner 5 orting method wa ued to compute active layer thickne and vertical bed layer tracking aumption and fall velocity of ediment wa computed uing the available in the model Ruby algorithm [9]. Thoe empirical relation were proved to be more reliable and the choice wa made after everal numerical tet experimentation and comparion with experimental reult. Figure 1. Meaurement data location The ued grid for the one-dimenional numerical computation had a patial tep, Δx, equal to 0.01m uptream, downtream of the abutment and in the abutment region. The Manning roughne coefficient, n, wa etimated a 0.02 and the initial bed lope wa equal to 0.0. The bed material conited of gravel which had a mean diameter of 0.002m. Sediment tranport numerical imulation wa baed on quai-unteady hydraulic which approximate a flow hydrograph by a erie of teady flow profile aociated with correponding flow duration. At the uptream cro ection of the channel each experimental inflow dicharge accompanied by a time duration, over which the flow i contant, wa pecified a an uptream hydraulic boundary condition. At the downtream cro ection the lope of the energy grade line wa ued and the model determine a downtream depth for each dicharge, olving Manning equation. Moreover, the equilibrium load wa pecified, at the uptream cro ection, a a ediment boundary condition. Thi mean that at the uptream cro ection the ediment tranport capacity i computed, for each time tep, and i ued a the ediment inflow. Since load i et equal to capacity, there will be no aggradation or degradation at thi cro ection. The ediment dicharge wa predicted, in the one dimenional numerical model, by Engelund and Hanen bed-load formula a [9]: Q d f 2 2 o wv B (5) g 1 w w d f 3 IV. COMPARATIVE STUDY OF NUMERICAL SIMULATIONS WITH EXPERIMENTAL MEASUREMENTS Comparion between numerical imulation reult and experimental meaurement of bed level variation in the region of the vertical wall abutment of b=0.036m are hown in Fig. 2-7, along the center flow line (flow line 3, equal ditance between upper and lower wall), at different inflow dicharge, after t=30min. Figure 8-13 and preent the characteritic comparion between numerical prediction and meaurement of the bed elevation, along the center flow line, with abutment width b=0.048m and b=0.051m, repectively. In all figure experimental meaurement along the flow line 1 are alo preented in order to provide the maximum cour depth at the abutment for each geometry and hydraulic condition. Figure 2. Comparion between computed and meaured cour depth, at Q=0.0004m 3 / and b=0.036m, after t=30min Figure 3. Comparion between computed and meaured cour depth, at Q=0.0005m 3 / and b=0.036m after t=30min where γ w i the pecific weight of the water, i the pecific gravity of ediment, V i the average velocity, d f i the particle fall diameter, B i the channel width and τ ο i the bed level hear tre given by: o w RS (6) Figure 4. Comparion between computed and meaured cour depth, at Q=0.0006m 3 / and b=0.036m after t=30min 27

4 Numerical Simulation of Scour Depth Variation Around Vertical Wall Abutment Figure 5. Comparion between computed and meaured cour depth, at Q=0.0007m 3 / and b=0.036m after t=30min Figure 11. Comparion between computed and meaured cour depth, at Q=0.0007m 3 / and b=0.048m after t=30min. Figure 6. Comparion between computed and meaured cour depth, at Q=0.0008m 3 / and b=0.036m after t=30min Figure 12. Comparion between computed and meaured cour depth, at Q=0.0008m 3 / and b=0.048m after t=30min Figure 7. Comparion between computed and meaured cour depth, at Q= m 3 / and b=0.036m after t=30min Figure 13. Comparion between computed and meaured cour depth, at Q= m 3 / and b=0.048m after t=30min Figure 8. Comparion between computed and meaured cour depth, at Q=0.0004m 3 / and b=0.048m after t=30min Figure 14. Comparion between computed and meaured cour depth, at Q=0.0004m 3 / and b=0.051m after t=30min. Figure 9. Comparion between computed and meaured cour depth, at Q=0.0005m 3 / and b=0.048m after t=30min Figure 15. Comparion between computed and meaured cour depth, at Q=0.0005m 3 / and b=0.051m after t=30min. Figure 10. Comparion between computed and meaured cour depth, at Q=0.0006m 3 / and b=0.048m after t=30min Figure 16. Comparion between computed and meaured cour depth, at Q=0.0006m 3 / and b=0.051m after t=30min. 28

5 World Journal of Reearch and Review (WJRR) ISSN: , Volume-5, Iue-6, December 2017 Page adequately compared to the correponding experimental meaurement. Figure 17. Comparion between computed and meaured cour depth, at Q=0.0007m 3 / and b=0.051m after t=30min. Figure 18. Comparion between computed and meaured cour depth, at Q=0.0008m 3 / and b=0.051m after t=30min. Figure 19. Comparion between computed and meaured cour depth, at Q= m 3 / and b=0.051m after t=30min. Meaured cour depth in the uptream to the abutment region are under predicted by the numerical imulation. In general, good agreement between meaured and computed bed eroion i obtained in the region and downtream of the abutment. The comparative performance between computed cour depth and meaurement along the center flow line (flow line 3) give atifactory reult. Difference between prediction and meaurement in region immediately uptream to the abutment are uually expected a in the flow high vicou effect co-exiting with downward velocitie in the cloe to the leading edge region of the abutment. It i in thee region that high vicou effect combined with three-dimenional flow retrict a one dimenional model to adequately predict the flow behavior. Thu, the ue of a vicou three-dimenional model to imulate the flow i recommended. Experiment how that along the uptream to the abutment region and cloe to the vertical ide of the abutment, eroion increae and a cour hole develop. The maximum cour depth occur uptream of the abutment, at the uptream corner of the contruction. At the downtream ide of the abutment, the previouly eroded material i depoited there and the cour depth i relatively mall. Thi proce i well numerically predicted by the one-dimenional hydrodynamic and ediment tranport numerical model. The maximum cour depth at the uptream edge of the abutment, i influenced by the abutment width and by inflow dicharge and increae with increae in the abutment width, normal to the flow direction and with inflow dicharge increment. The computed maximum cour depth at the uptream edge of the abutment, for each geometry and inflow dicharge, i V. CONCLUSION The one dimenional, quai-unteady, hydrodynamic and ediment tranport HEC-RAS numerical model wa applied to imulate bed level variation in the region of vertical wall abutment. Experimental meaurement of cour depth were obtained at variou location near the abutment, for different inflow dicharge and abutment width. Comparion between computed reult and available meaurement of cour depth i relatively atifactory along the center flow line a the numerical imulation i one dimenional. The HEC-RAS model give an accurate and reliable etimation of the maximum cour depth near the obtruction and it i applicable for a afe deign criteria for the aement of bridge vulnerability. However, wherever poible, a two or three dimenional numerical model i neceary in order to give an improved validation methodology for complex river geometrie with different contruction. REFERENCES [1] S. Bhallamudi and M. Chaudhry, Numerical modeling of aggradation and degradation in alluvial channel, Journal of Hydraulic Engineering, ASCE, 117, 1991, [2] B. Biglari and T. Sturm, Numerical modeling of flow around bridge abutment in compound channel, Journal of Hydraulic Engineering, ASCE, 124(2), 1998, [3] E. Farirotou, J. Souli and V. Dermii, A numerical algorithm for bed morphology equation, International. Conference on European River Development, Budapet, Hungary, [4] E. Farirotou, J. Souli and V. Dermii, A numerical method for 2-D bed morphology calculation, International Journal of Computational Fluid Dynamic, vol 16(3), 2002, pp [5] E. Farirotou and N. Xafouli, An experimental tudy of local cour depth around bridge abutment, International Journal of New Technology and Reearch, vol 3(9), 2017, pp [6] A. Kaem and M. Chaudhry, Comparion of coupled and emicoupled numerical model for alluvial channel, Journal of Hydraulic Engineering, ASCE 124, 1998, [7] A. Klonidi and J. Souli, An implicit numerical cheme for bed morphology calculation, VII International Conference on Computer Method and Experimental Meaurement, Rhode, Greece, 1997, [8] J. Richardon and V. Panchang, Three-dimenional imulation of cour-inducing flow at bridge pier, Journal of Hydraulic Engineering, ASCE, 124(5), 1998, [9] US Army Corp of Engineer, Hydrologic Engineering Center HEC RAS, River Analyi Sytem, Verion 4.1,

6 Numerical Simulation of Scour Depth Variation Around Vertical Wall Abutment Evangelia D. Farirotou i Aociate Profeor at the Department of Civil Engineering, laboratory of Flooding Protection Hydraulic Work & Water Reource Management Technology at the Technological Educational Intitute of Thealy. Her cientific area i in Hydrodynamic and Flood Prevention Work. She ha a diploma in Civil Engineering, Ph.D. at the Aritotle Univerity of Thealoniki, Department of Civil Engineering in the Diviion of Hydraulic & Environmental Engineering and Mater of Science in Hydraulic Engineering. She i director of the Mater of Science in Advanced Environmental Management Technologie in Engineering Work of her Department. She teache Hydraulic, Regulation of River and Stream-Flood Prevention Work, Hydrodynamic Work-Dam, Computer Aided Application on Hydraulic Work, Environmental Management of Natural River and Flood Protection and Sutainable Development. She i active in reearch in hydraulic of open channel, river bed morphology and ediment tranport, degradation, aggradation and cour imulation in alluvial channel and river flood protection work. She ha publihed a number of paper in international peer-reviewed journal and conference. Moreover, he ha participated in the reearch project titled: Water Reource Management of coatal Agricultural Environment-Reilience of climate change impact (Agroclima) and Invetigation of the impact of the climate change on river delta. Cae tudy: the delta of river Pinio (Thealy, Greece). Nikolao G. Xafouli Civil Infratructure Engineer M.Sc. in Advanced Environmental Management Technologie in Engineering Work. Reearch aitant in the laboratory of Flooding Protection Hydraulic Work & Water Reource Management Technology in the Department of Civil Engineering T.E of Laria (Technological Educational Intitute of Thealy). Reearch interet: hydraulic of open channel, ediment tranport in natural river, river flood protection work, hydrology and water reource management. 30