A PHYSICAL MODEL STUDY OF SCOURING EFFECTS ON UPSTREAM/DOWNSTREAM OF THE BRIDGE

Transcription

1 A PHYSICA MODE STUDY OF SCOURING EFFECTS ON UPSTREAM/DOWNSTREAM OF THE BRIDGE JIHN-SUNG AI Hydrotec Researc Institute, National Taiwan University Taipei, 1617, Taiwan HO-CHENG IEN National Center for Hig-Performance Computing No. 7 R&D 6t Rd., Science-Based Industrial Park, Hsincu, Taiwan JINN-CHUANG YANG Civil Engineering Dept., National Ciao Tung University Hsincu, Taiwan Dredging in an alluvial cannel to increase te cross-section area of te cannel can reduce te flooding risk. However, it may also create te potential of te scouring problem, especially around ydraulic structures suc as bridge piers. Tis study focuses on te river reac at Yue-mei-tan Bridge over te Pu-zi River in soutern Taiwan. A pysical model was constructed to investigate te scouring effects around bridge due to te excavation of te sediment deposits upstream and downstream of te bridge. Te geometric scale ratios of prototype to pysical model were determined to be 4 in vertical direction and 1 in orizontal direction. 1 Introduction In Taiwan, te geologic formations are relatively young and unstable. Frequent eartquake and typoon events cause drastic variations in river flow as well as sediment yield brougt from waterseds. As a result, deposition and scour in river cannels occur quite frequently and significantly to affect te cannel stabilization, especially wit te existing of ydraulic structures suc as bridge pier, abutment, groyne, etc. In te deposition reaces, sediment deposition may reduce te cannel cross-sectional area and decrease flow capacity, and amper flood protection function to endanger traffic safety on te bridge. To understand te scouring effects on upstream/downstream of te bridge in te present study, a pysical model is constructed to investigate te scouring effects around bridge piers due to te excavation of te sediment deposits upstream and downstream of te bridge. Te study reac in te Pu-zi River is located upstream and downstream of te Yuemei-tan Bridge. Te Pu-zi River as a natural drainage area of 427 km 2 wit average slope of 1/53. It flows from te mountain area to te alluvial plain and runs troug te Jianan Plain in wic is famous for its agriculture products. Cities are developed along te Pu-zi River wit Dense population. Tere are 26 bridges crossing te river. Te 1

2 2 mean annual rainfall is 1,4 mm. However, te average slope of te study reac is 1/2,4, wic represents te mild slope on te Jianan Plain. 2 Model Scaling Te geometric scale ratios of prototype to model were determined to be 4 in vertical direction and 1 in orizontal direction due to te limitation of te construction space. In oter words, te pysical model as a distorted model as te geometry scales of 1 and 4 in vertical and orizontal directions. is denoted as te ratio of prototype to model. Te subscripts and denote orizontal lengt dimension and vertical lengt dimension, respectively. Te pysical model must satisfy te similarity of geometric, kinematic and dynamic conditions, and it sould enable to simulate closely te penomena wic take place in te prototype. In te open cannel flow, most of te cases satisfying te dynamic condition by Froude number similarity are sufficient. According to 1 / 2 te geometric scale constraints; terefore, te flow velocity scale is u 6. 32, te 1/ 2 flow discarge scale is Q u A 25, 298, and te time scale is t / , Te subscripts u, Q, A and t denote velocity dimension, discarge dimension, crosssectional area dimension and time dimension, respectively. For model scaling in te movable bed pysical model, parameters describing te sediment transport beaviors are usually adopted to acieve dynamic similarity in an alluvial system wit te Froude number similarity. According te study reported by te Water Resources Agency, it was found tat wit te Yang s sediment transport formula te numerical model could simulate te longitudinal bed variations troug te period from 1975 to 1996 quite well. In te Yang s sediment transport formula, te unit stream power (VS/w) is te key parameter, in wic V is te average velocity of te cross section, S is energy slope, and w is fall velocity of te sediment particle. According to te grain size data of bed materials in te study reac of te Pu-zi River, te sediment grain size is fair uniform and te d 5 in te samples is about.3 mm. Te fall velocity can be related to f(d 5 ). To satisfy te sediment transport dynamic condition by similarity, we can obtain te following relationsip in Equation (1) based on te parameter: unit stream power. 3 / 2 1 (1) D s V S For sediment concentration C in te water column, it is assumed tat te C in te model is te same as tat in te prototype. Tus, te ratio of sediment transport rate q s per unit cannel widt is obtained as: 2 3 / (2) Q s Q C According to Equation (1), te ratio of model to prototype for sediment size can be derived to be 3 / / 2 1 (4) (1) 2. 53, and te ratio of sediment D s 2 3 / 2 transport rate is 3 / (4) (1) 25, 298. Based on te ratio of te grain Q s size, relatively uniform sand wit d 5 =.12 mm sould be used in te model. Q

3 3 3 Experimental Setup and Results Based on te constraints in te geometric scale ratios of prototype to model, te model construction is set to be 4 in vertical direction and 1 in orizontal direction. Te cross section selected at te upstream end as te inflow boundary sould be relatively straigt and uniform for water and sediment supply control purposes. Te river reac near Section 55 located before te river bend is relatively straigt, and te patterns of deposition and erosion at Section 55 ad been quite stable from 1996 to 22. Te downstream boundary is set at Section 48 at wic locates before anoter river bend emerges. Te layout of te pysical model is sketced in Figure 1. Water & sediment supply R e s e r v io r H e a d w a te r ta n k Y u e -m e i- ta n B r id g e S e d im e n t s e ttlin g b a s in S e c. 4 8 S e c. 4 9 T a il w a te r c o n tr o l S e c. 5 S e c. 5 1 S e c. 5 2 S e c. 5 4 S e c. 5 5 S e c. 5 3 Figure 1. Te layout of te pysical model Te movable bed pysical model is constructed to study te scouring on upstream and downstream of te Yue-mei-tan Bridge at te Nansijiao Hydraulic Field Station of Water Resource Agency, Taipei. Te main testing cases erein are modeling te effects on te excavation of sediment deposits under te design flood event for te 1-yr return period flood. Before eac experiment, te sand wit d 5 =.12 mm is carefully paved along te study reac by matcing te topograpical data from field survey obtained in 22. Te sediment supply released at te upstream boundary (Section 55) is obtained based on te sediment transport rate calculated by te NSTARTS model. Te relationsip between te sediment transport rate and discarge is plotted in Figure 2. Te discarge ydrograp for te design 1-yr return period flood is cited from te report by te Water Resources Planning Institute (22) and sown in Figure 3(a). Using Equation (2) and Figure 3(a), te sediment transport rate in te pysical model can be calculated as sown in Figure 3(b). Te ydrograps of unsteady flow presented in Figure 3(a) are given stepwise at te eadwater tank and controlled by a sluice gate, wic needs to be calibrated accurately.

4 4 Qs(kg/s) Q s.665q Q(m 3 /s) Figure 2. Te regressed relation between Q and Q s 25 (a)q(t) 7 (b)qs(t) Q(cms) Qs(kg/s) Time(r) Time(r) Figure 3. Hydrograps of (a) prototype discarge (b) sediment transport rate Te survey of bed elevation contour lines between Section 51 and Section 49 is plotted using fine solid lines in Figure 4. Various proportions for te reduction of te cross sectional area are investigated and modeled by te NSTARTS model. It is found tat removing about 2% of sediment deposits along te left bank between Section 51 and Section 49 is beneficial for reducing te scour dept downstream and te flood stage upstream from te bridge. However, te NSTARTS model may simulate te quasi-twodimensional (quasi-2d) beaviors of scouring as well as deposition in lateral direction, but it still is a one-dimensional (1D) model. In order to obtain te 3D flow and sediment movement near te piers locally, te experimental results from te pysical model are essentially important.

5 To increase te cross-sectional area of te Yue-mei-tan Bridge, excavation of sediment deposits between Section 51 and Section 49 is designed as plotted using tick solid lines in Figure 4. Under te 1-yr return period design flood wit excavated cross section sown in Figure 5, te ydrograps of discarge and sediment supply are imposed at te upstream of te pysical model. Tere are two testing cases including excavation wit/witout gabions using rock-and-wire baskets along te cross section to protect te piers. In te case witout gabion protection around te piers, te measured bed elevation contour presenting degradation in te river reac between Section 51 and Section 49 is plotted in Figure 6. Te result sows tat scouring is severe around te first four piers. It may comprise te general scour as well as contraction scour. Due to te bend effect of te river reac between Section 51 and 52, te scour depts adjacent to te bridge piers between Pier 2 and Pier 4 are relatively deeper. However, te scour dept downstream of te bridge is not larger tan tat before excavation. Te deeper scour ole may occur because water drop troug te lower part of te gabions during te low flow. Similarly, te case wit gabion protection results in a scour ole downstream of te bridge wic is smaller tan tat before excavation, because it only goes troug one flood event. 5 Yue-mei-tan Bridge Figure 4. Bed elevation contour lines before (fine solid lines) and after (tink solid lines) excavation bed variation after excavation wit gabion gison protection witout gabion gison protection Abutment Ele.(m) Distance(m) Figure 5. Te bed variations at te cross section downstream side of te bridge

6 6 Yue-mei-tan bridge (a)w itout gabion protection Yue-mei-tan Bridge (b)wit gabion protection 4 Concluding Remarks Figure 6. Te effects of scouring (a) witout (b) wit gabion protection Te present study focuses on te scouring problem in te river reac near te Yue-meitan Bridge over te Pu-zi River in soutern Taiwan. Te geometric scale ratios of prototype to pysical model are set to be 4 in vertical direction and 1 in orizontal direction. Te pysical model is constructed to investigate te scouring effects around bridge due to te excavation of te sediment deposits upstream and downstream of te bridge. Te result sows tat scouring is significant around te first four piers. It may comprise te general scour as well as contraction scour. It sows tat using rock-and-wire basket gabions along te cross section to protect te piers is necessary in tis case. Acknowledgments Te autors appreciate te Water Resources Planning Institute, Water Resources Agency, Taiwan, for providing grants and data in tis study. References ee H.Y, Hsie, H.M. (23). Numerical Simulations of Scour and Deposition in a Cannel Network. International Journal of Sediment Researc, Vol. 18, No. 1, pp Water Resources Planning Institute (22). Scouring Effects around Bridge Sites due to te Excavation of Sediment Deposition in te Upstream and Downstream River Reac. Water Resources Agency, Tecnical Report (in Cinese). Yang, C.T. (1996). Sediment Transport Teory and Practice, McGraw-Hill, New York.