A STUDY OF LOCAL SCOUR AT BRIDGE PIERS OF EL-MINIA

Transcription

1 A STUDY OF LOCAL SCOUR AT BRIDGE PIERS OF EL-MINIA Dr. Gamal A. Sallam 1 and Dr. Medhat Aziz 2 ABSTRACT Bridges are critical structures that require a substantial investment to construct and serve an important role in economic development. For a developing country such as Egypt, there is need to protect these assets by continuous monitoring, maintenance, proposing any required repair works and execute them with a very high quality control. El-Minia Bridge was constructed in It consists of 15 vents each of m wide, except the navigation vent, which is m wide. Recently, there have been some difficulties regarding navigation underneath the bridge. Since some submerged islands are formed near the navigational vent, some deposition near the navigational vent of the bridge has raised the bed level. This has resulted in an insufficient depth of water for navigation through the navigational vent. In addition, river bed has changed around bridge piers. Nile Research Institute (NRI) has conducted the analysis for this problem to avoid any damages that could occur to the bridge. Nile Research Institute research team were sent for collecting the field data for the site of El-Minia Bridge Km upstream of El-Roda site along the Nile River, in which pier size, sediment size and mean approach flow velocities are recorded. A mathematical model was used to study the measured scour hole values and to predict to predict the future maximum expected scour hole. That was conducted to evaluate the causes and consequences of scour holes on the bridge piers of El-Minia. In addition, some methods and tools for minimize the effect of scour holes on the bridge piers were discussed. KEYWORDS: Nile River Bridges, Local scour, Bridge local scour. Researcher, Nile Research Institute, Egypt Assoc. Prof., Nile Research Institute, Egypt

2 1. INTRODUCTION El-Minia bridge constructed in 1987 to join El-Minia City with the west main highway (2) (Egypt - Aswan) and with the main new highway (Cairo/ Bani Suif/ El-Minia) East of the River Nile which was constructed lately through the main Governorate plan to support the network roads and to decrease the traffic density on the Upper Egypt highway. The new El- Minia Bridge will create a new civilization on the East of River Nile, in addition to gain the best use for stone quarries in this area. The total length of El-Minia Bridge is about 1803 m, it consists of 15 vents, width of each is 40 m. While the navigational vents, the biggest is about 62 m and on each side 2 vents with a width of about 50 m and the maximum navigational height is 13 m from the highest water level of River Nile. The total road width on the bridge is 21 m, the bridge foundations are penetrated piles with a length of about 20 m, all these piles are supported on sand or stone soils. Yet every year more additions are made to the list of bridges that have failed because of scour of the stream bed around the piers and abutments. Considering the overall complexity of field conditions, it is not surprising that there is no generally accepted principles for the prediction of scour around bridge piers and abutments have evolved from field experience alone, (Emmett M. Laursen and Arthur Toch, 1956). The alignment, cross section, discharge, and slope of a stream must all be correlated with the scour phenomenon, and this in turn must be correlated with the characteristics of the bed material ranging from clays and fine silts to gravels and boulders. Finally, the effect of the shape of the obstruction itself - the pier or abutment - must be assessed. The equilibrium of any river under natural flows is affected by any interference. Some examples of the interfering structures are bridges, groins and hydraulic structures. Study the effect of any of these structures before and after construction is an important factor to avoid any unexpected results. One of the Nile Research Institute tasks is to monitor, study, and analyze these effects and to propose the suitable measures to be taken to protect these structures and avoid major changes to the river, (Ahmed Fahmy, 2002). During this research local scour caused by bridges is

3 analyzed, a case study, El-Minia Bridge, is considered for this analysis. Figure (1) shows El-Minia Bridge along the River Nile. Figure (1) El-Minia bridge location along the River Nile 2. SCOUR PHENOMENON Scour is the removal of bed and bank material as a result of the erosive action of flowing water. The three components of total scour are general scour, contraction scour, and local scour. The contraction scour is the scour that results from the acceleration of the flow due to a contraction, such as a bridge. This type of scour also occurs in areas where revetments are placed such that they reduce the overall width of the stream segment. Contraction scour is generally limited to the length of the contraction, and perhaps a short distance up and downstream, whereas general scour tends to occur over longer reaches. The local scour is the scour that occurs at a pier, abutment, erosion control device, or other structure obstructing the flow. These obstructions cause flow acceleration and create vortexes that remove the surrounding sediments. Generally, depths of local scour are much larger than general or contraction scour depths, often by a factor of ten. Local scour can affect the

4 stability of structures such as riprap revetments and lead to failures if measures are not taken to address the scour. Factors that affect local scour include: 1. width of the obstruction 2. length of the obstruction 3. depth of flow 4. velocity of the approach flow 5. size of the bed material 6. angle of the approach flow (angle of attack) 7. shape of the obstruction 8. bed configuration 9. debris 3. FIELD INVESTIGATIONS A field trip has been carried out to El-Minia Bridge; the bridge is located in El-Minia Governorate, kilometer along the River Nile south of Cairo. This field trip was conducted to observe, measure, and monitor the different factors affecting bridge local scour. This field trip has included the water levels at the site, bridge pier dimensions and geometry. Figure (2) shows one of the El-Minia Bridge piers as an example. A hydrographic survey of the bed nearby and around bridge piers was performed along the riverbed covering 1.00 Km (about 500 meter upstream and 500 meter downstream the bridge), and 21 surveyed cross sections. Velocity measurements were taken at five locations, two cross-sections upstream the bridge, two cross-sections downstream bridge and at the centerline of bridge. Samples of bed material were taken at the same crosssections. Table (1) shows surface bed grain size analysis at the bridge site. It is noted from the grain size distribution that most of the bed samples are medium sand with D50 ranging from 0.25 to 0.50 mm (medium sand is ranging from 0.2 to 0.60 mm). Downstream cross sections have some coarse sand ratio. It has to be noticed also that this is typical configuration for the Nile River bed, and the existence of fine sand indicates the possibility of further scour. Figure (3) shows the cross section changes at the bridge for years 1982 and It is observed the major cross section changes due to bridge existence. It can be noticed that a maximum local scour depth of about 4.60 m has occurred during the period 1982 to 2001.

5 Figure (2) One of the El-Minia bridge piers Table (1) Surface bed grain size analysis at the bridge site X-S UPSTREAM THE BRIDGE CENTER LINE OF BRIDGE DOWNSTREAM THE BRIDGE KM. from Aswan High Dam Point 1 East LOCATION D 50 (mm) D mean (mm) SOIL TYPE Medium sand Point Medium sand Point Medium sand Point Medium sand Point Medium sand West Point Medium sand Point Medium sand Point Medium to East coarse sand Point Medium sand Point Medium to fine sand Point Medium to coarse sand Point Medium to fine sand

6 A study of local scour at bridge piers of El-Minia bridge Bed level (m) Distance (m) Figure (3) Cross section changes at the bridge (1982 and 2002) 4. COMPUTING SCOUR Because of complexity of evaluating flow pattern around piers and shear forces generated by flow pattern, most of the estimates of scour depth have been obtained by experimental work. An example of these estimates is the Colorado State University (CSU) equation for predicting maximum pier scour depths, the equation is (ASCE, 1975): Y s /Y 1 = 2.0 K 1 K 2 K 3 K 4 (A/Y 1 ) 0.65 Fr for round nose piers aligned with the flow. where: Y s = Scour depth, m Y 1 = Flow depth directly upstream of the pier, m K 1, K 2, K 3, and K 4 = Correction factors for pier nose, angle of attack, bed condition, and armoring by bed material size respectively. A = Pier width, m

7 L = Length of pier, m Fr 1 = Froude Number directly upstream of the pier. Y s 2.4 times the pier width (A) for Fr 0.8 Y s 3.0 times the pier width (A) for Fr > COMPUTER MODEL A Computer Model: The computer model HEC-RAS Model was used during this analysis to evaluate local scour around El-Minia Bridge piers. This model is developed by the US Army Corps of Engineers (US Army Corps of Engineers, 2001). It is a one-dimensional model able to simulate steady, unsteady and sediment transport for movable boundary conditions. It has a special module to compute bridge piers and abutment local scour. To be able to simulate the actual flow conditions, the whole reach (Reach 4 - from downstream Assiut Barrage to upstream Delta Barrage) has to be modeled and analyzed. Reach 4: It extends downstream Assiut Barrage to upstream Delta Barrage with a total length of about km. It is the longest reach all over the Nile River reaches. The surveyed cross sections for this reach in both years 1982, 1997 and 2002 were used for this analysis. Figure (4) shows the location of Reach 4 along the River Nile, and Table (2) shows the cross section locations along the reach used during this study.

8 Reach 4 Figure (4) Location of reach 4 along the River Nile Model Calibration Process: The actual water level readings for gauging stations along the fourth reach were used for the calibration analysis. Two actual flow conditions were considered for the calibration analysis; 170 and 350 million cubic meters per day. Figure (6) shows the calibration process for the flow of 170 m.m 3 /day and Figure (7) shows the calibration process for the flow of 350 m.m 3 /day. From these two figures, we can conclude that there is a close agreement between measured and predicted water levels. This indicates that the selected roughness values are suitable for the simulation process.

9 Table (2) Cross Section Locations Number Cross Section Km from Aswan Number Cross Section Km from Aswan INPUT DATA The data that are provided to the program is as follows: 1. Bed material grain size distribution, Table (1). 2. Reach 4 cross sections 5 Km a part, Table (2). 3. Water level for reach 4 gauging stations. 4. Bridge geometry pier dimensions, Figure (2) shows one of these piers, where the pier width is 1.50 meter, and the pier length is meter. 7. LOCAL SCOUR SIMULATION Figure (5) shows the model output for the simulated flow conditions. The simulated flow, considered during this analysis, is 180 m.m 3 /day. This flow is considered to simulate flow conditions causing local scour around the bridge piers. This flow is almost the largest flow discharge passed along this reach during the past period ( ) and it is close to the

10 calibration discharges. The computed water levels along the reach are shown in this figure. 60 reach 4 brdg Plan: Plan 06 4 Legend Elevation (ft) EG PF 2 WS PF 2 Crit PF 2 Ground Main Channel Distance (ft) Figure (5) The model output for the computed water level

11 Model Calibration (170 m.m3/day Level (m) Km (from Aswan) Predicted Measured Figure (6) The calibration results for 170 m.m 3 /day Model Calibration (350 m.m3/day) Level (m) Km (from Aswan) Predicted Measured Figure (7) The calibration results for 350 m.m 3 /day

12 9. MODEL RESULTS The model results represent the local scour occurred due to passing a discharge of 180 m.m 3 /day along El-Minia Bridge. Bed material gradations were included for this analysis to compute the local scour. The CSU equation is used by the model for local scour computations. The K terms are taken as follows: K 1 correction factor for pier nose is taken as 1.00 K 2 angle of attack of flow 90.0 K 3 correction factor for bed conditions 1.1 (clear water scour) K 4 armoring effect coefficient 0.7 The computed scour represents both the contraction and the piers scour occurred for this flow condition. The abutment local scour is not applicable for this case because there were no interfering abutments to the flow. Table (3) shows the results of the local scour for the bridge piers. A maximum value of 4.9 meters was computed for pier scour. It has to be mentioned that the value obtained from comparing the cross section ( ) is about 4.60 m of local scour and degradation for the bridge cross section. Figure (3), shows cross section change at the bridge ( ). Also, Table (3) shows the computed local scour for 180 m.m 3 /day and a maximum flow of 350 m.m 3 /day. The differences between the measured and the computed local scour depths could be attributed to the variation of bed grain size distribution and model simplification but the model gives a good indication for the local scour values. However, the maximum computed expected local scour is 5.60 m, which should be taken into consideration during future programs. Table (3) Computed local scour for a flow of 180 m.m 3 /day and 350 m.m 3 /day Piers Local scour of 180 m. m 3 / day Local scour of 350 m. m 3 / day All 4.9 meter 5.6 meter

13 10. CONCLUSION AND RECOMMENDATIONS - Local scour monitoring is very important to avoid major damages that may occur. - The monitoring of El-Minia bridge local scour shows a local scour depth of about 4.60 m since its construction up till now. - The mathematical simulation of the bridge local scour shows a local scour of a depth 4.90 m. that means the difference between the measured scour and calculated scour about 6.5 %. - The HEC-RAS model is a good tool for predicting local scour for bridge piers. Hence, it could be used to determine the type of bridge pier foundations before constructing. - The expected local scour of passing a discharge of 350 m.m 3 /day is about 5.60 m. - Since the local scour was found 4.60 m during the last 10 years and the bridge pier foundations can tolerate this scour. It is recommended to continue local scour monitoring every 5 years minimum. REFERENCES Ahmed Fahmy, A study of local scour around El-Minia Bridge, Report No 341, Nile Research Institute, National Water Research Center, Egypt (2002). ASCE Manuals and Reports on Engineering Practice, Sedimentation Engineering, ASCE, No.54, Bruce W. Melville, Live-Bed Sour at Bridge Piers, J., Hyd. Div., ASCE, Vol. 110, No. 9, (1984). Emmett M. Laursen and Arthur Toch, Scour around Bridge Piers and Abutments, Iowa Institute of Hydraulic Research, State University of Iowa, (1956).

14 US Army Corps of Engineers, HEC-RAS River Analysis System Computer Model, The Hydrologic Center, Davis, California, 2001.