General Flood Resistant Bridge Design Guidelines

General Flood Resistant Bridge Design

Guidelines

FLOOD RESISTANT BRIDGE DESIGN IN

PAPUA NEW GUINEA

Gibson Ali HolembaResearch Student

Graduate School of EngineeringHokkaido University

May 1, 2023 2

Presentation Outline1. Introduction2. Bridge Abutment 3. Bridge Superstructure 4. Bridge Pier5. Bridge Foundation6. Design of Flow7. Design for Structural Stability8. Design of Afflux9. Estimating Scour 10. Scour Protection Measures11. Aggradation and Degradation 12. Specific Design Considerations13. Conclusion

Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University

May 1, 2023 3

Introduction • This study is undertaken to address the ever-increasing flood damaged bridges in Papua New Guinea. Bridge damage by flooding is so frequent that unless a research is carried to provide some solutions, it will continue to affect the livelihood of people and cost the government unbudgeted expenditures in emergency restoration works. Most restoration works undertaken are very expensive without proper justification of the cost with no or less engineering guidelines on long-term improvement of the failed structures.

• The research question that has guided this study was “How can we improve flood damage bridges in Papua New Guinea?” This is a big question and this research alone cannot answer the question.

• This presentation will highlight some of the general flood resistant bridge design guidelines undertaken by some researchers and State Agencies in improving flood affected bridges around the world.


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Bridge Abutment• Available equations do not satisfactorily predict scour depths for abutments. It is recommended in this study that concrete sandbag riprap or guide banks must be considered for abutment protection. Correctly designed and constructed, the suggested protective measures can negate the need to compute abutment scour.

• Relief openings, guide banks and river training works must be used, where necessary, to minimize the effects of adverse flow conditions at abutments.

• Scour at spill-through abutments is about half of that for vertical wall abutments, however, consideration must be given to the loss of spill-through embankment material due to scour.


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Bridge Superstructure• Bridge superstructure soffit levels must be positioned above the general level of the approach roadways wherever practicable. In the event of overtopping of approach embankments this provides for a reduction of any hydraulic forces acting on the bridge. This is particularly important for bridges over rivers or streams carrying large amounts of debris, which could clog the waterway of the bridge.

• Bridge superstructures must be securely anchored to the substructure if the deck will become buoyant, or floating debris is probable. Where overtopping is likely, the superstructure cross-section must be shaped to minimize resistance to the flow.


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Bridge Pier• Bridge pier foundations on floodplains must be positioned at the

same depth as the pier foundations in the stream channel if there is any likelihood that the channel will shift its location onto the floodplain over the life of the bridge.

• Piers must be aligned as far as is practical, in the direction of flood and tidal flows. Assess the hydraulic advantages of different pier shapes, particularly where there are complex flow patterns during floods and use the most appropriate pier shape.

• Streamline pier shapes to decrease scour and minimize potential for the build-up of debris.

• Evaluate the hazard from debris build-up when considering the use of multiple pile bents in stream channels. Where debris build-up is a problem, the bent must be designed as though it were a solid pier for the purposes of scour estimation. Consider the use of other pier types where clogging of the waterway area could be a major problem.


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Bridge Foundation• Different bed materials scour at different rates. Thus, investigation must be conducted on the riverbed material and design considerations must be undertaken to prevent scouring action on riverbed material near the foundation.

• Bridge foundation analysis must be carried out on the basis that all stream-bed material within the scour prism above the total scour depth will have been removed and is not available for bearing or lateral support.

• Spread Footing Foundation on stabilized fill material shall be ensured that the abutment foundation footing is below the calculated scour depth.

• Ensure that the bottom of the abutment and retaining wall footing is at least 2m below the present streambed level.

• Ensure that circular slip-failure of the soil foundation do not occur.


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Bridge Foundation continue…• For pile designs subject to scour, consideration shall be given to using a lesser number of long piles to develop bearing resistance, as compared to a greater number of shorter piles.

• Place the top of the pile cap at a depth, below existing riverbed level and equal to the estimated general scour depth to minimize obstruction to flood flows and its resulting local scour.


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Design of Flow• Calculations must be based on a range of flood return periods of up to 200 years in order to assess which events produce the worst effects from considering different flow velocities and depths.

• In this study the range will be from Q20 – Q200. The reason for this is that in many rivers, velocities can be high when flows are just within the banks, and scour can be worse than the higher flooding discharge rates.

• Bridge located on a local road can be designed for Q20 design discharge that is able to safely mitigate Q100 flood. Bridges on on important economic highways must be designed to withstand Q100 floods and Q200 be used as a safe design check.


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Design for Structural Stability• In order to satisfy that the structure is adequate to resist against the hydraulic action of flooding water, the structural design must be carried out in this order:

a) Calculate the total potential scour depth and check that the structural design is adequate with that depth of scour.

b) Incorporate appropriate scour protection measures in the design such as the Groins, Riprap, Levees and Sheet Piling.

c) Calculate the load on the structure and its foundations and check for structural adequacy.


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Design of Afflux• Afflux is the increase in water level upstream of a bridge over that which

would have occurred if the structure was absent. • For a given cross-sectional area of an opening, the greater the wetted

perimeter, the greater is the afflux. Therefore, it must be considered at the planning stage that a smaller number of large openings are preferable to a larger number of small openings.

• There are number of methods available for calculating afflux. The most widely used is the US Bureau of Public Roads (USBPR) method. This is applicable to bridges with vertical piers and horizontal soffits. (Δh = kHref + Hu – Hd)

• To control the afflux at a bridge crossing, particularly where long embankments cross the flood plain are required, it is necessary to provide additional flood openings. In simple cases, methods of calculating afflux such as the USBPR method can be used to determine the length of openings required. The calculations will indicate the overall length of openings required in achieving a certain afflux but the appropriate location for these openings will depend upon the local geometry.


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Estimating Scour• The estimation of scour effects may involve the following:1. Obtain all relevant data from the bridge site2. Select critical return periods and calculate design discharge. 3. Draw cross-sections at the proposed bridge site showing proposed

foundation depths. Additional cross-sections must be taken in the neighborhood of the bridge site, e.g. within approximately 5m river widths upstream and downstream of the bridge site. These areas shall be inspected for signs of scour or irregularities, which might influence flow conditions or bed levels at the bridge site.

4. Decide whether long-term bed level variation such as progressive degradation is allowable.

5. Calculate design water levels and velocities. Establish or estimate direction of flow trajectories in relation to alignment of bridge piers - flow trajectories may be significantly different at various flood conditions than at normal flow conditions.


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Estimating Scour continue…6. Calculate hydraulic parameters such as Froude Number

(Fr) and floodplain or main channel discharge split.7. Calculate general scour depths. Redistribute general scour

to the most critical bed profile, taking into account the layout of the bridge crossing and its foundation details.

8. Compare the measurements of bed level at the bridge site with the calculated bed levels.

9. Calculate local scour at each potentially vulnerable foundation, including abutments. Superimpose local scour upon general scour, assuming the top width of local scour holes, measured from the pier face, to be within approximately 1.0 to 2.8 times the local scour depth.

10.Interpret scour depths in the light of potential effects upon the structural strength and stability of the foundations.


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Scour Protection Measures• It may not be economical to design the elements of a bridge to

withstand the maximum possible scour. However, an alternative is to carry out scour protection works to prevent or reduce scour of the bed and banks.

• Concrete sandbags to be placed along the bridge abutment and the riverbank to prevent scouring at bridge abutments and control bank erosion. Ideally, the concrete sandbag will be abrasion resistant and of sufficient weight to prevent it from being moved by the flow. The size or weight of the concrete sandbag must be designed to be roughly proportional to the sixth power of the flow velocity.

• Gabion and grouted mattresses must be placed locally around piers and abutments or across the full width of the invert to increase the resistance of the riverbed to scour. The riverbed should be pre-excavated so that the mattresses lie below bed level. Mattresses can also be used to protect the riverbanks.

• Increase the size of the waterway opening at the downstream end and channel improvements are some methods to resist flood.


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Aggradation & Degradation • Progressive degradation results from modification of the stable regime conditions to which a river has become adjusted. This may be as a result of alterations to water or sediment flows in the river. The result of progressive degradation at a bridge site will be a lowering of bed level, which may place the foundations at risk.

• Degradation will normally increase the risk to bridge structures from scour, however, in some cases, aggradation may occur - this will cause increased water levels but will probably reduce the risk from scour.

• If the channel is expected to degrade, then the estimation of long-term bed elevation will be used to calculate general and local scour depth levels.


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Specific Design Considerations• Freeboard must be such that bridge soffit levels at flood spans are 600mm above the design flood level or maximum known flood level on minor watercourses in order to allow floating debris to pass freely through the structure. In determining the freeboard, allowance shall be made for afflux.

• Unless, otherwise specified, the design checks must be carried out both at the ultimate limit state (ULS) and the serviceability limit state (SIS) using the applicable combination rules and the partial factors of safety.


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Specific Design Considerations cont…

• Hydrodynamic forces from the action of flowing water past the submerged parts of a bridge can act in addition to hydrostatic forces. The Indian Road Congress (IRC) and American Association of State Highway and Transportation Officials (AASHTO), recommend the following equation for the hydrodynamic flow pressure P (kN/m2), P = 0.51KU2 to determine the Hydrostatic Pressure.

• Debris forces - the type of debris occurring in a river depends upon the size and characteristics of the river and the area through which it flows. An investigation must be carried out to determine the type and size of floating debris to be expected at the bridge site.

• A minimum allowance must be made for a debris collision force equivalent to that exerted by a three (3) ton log travelling at the stream velocity calculated for the peak design event and arrested within distances of 150mm for slender column type piers and 75mm for massive, non-yielding type piers.


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Conclusion • What is now common in Papua New Guinea is we tend to have Reactive Approach to undertake the failing infrastructure maintenance in the country then Proactive Approach to plan and maintain the infrastructure in a more structured way. This research is undertaken in a way to help decision makers to relook on how best to address the problem in a simple innovative engineering approach that is more cost efficient and applicable in the context of PNG.

• This study is undertaken to improve the road construction industry in PNG to develop better resilient methods to improve basic infrastructure that affects the livelihood of people significantly.

• There is always a better and simple way in addressing everyday engineering challenges however, we think too big that non-complex solutions becomes so simple to be accepted.


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References 1. Scottish Design Manual for Roads and Bridges, The Design of Highway Bridges for Hydraulic Action, Volume

1, Section3, Part 6, (1994). 2. I. Zevgolis and P. Bourdeau, Mechanically Stabilized Earth Wall Abutments for Bridge Support, (2007), U.S

DOT FHWA, Washington DC, USA.3. US Army Corps of Engineers, Sandbagging Techniques, (2004), Portland, USA.4. U.S DOT FHWA, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes,

Volume 1, (2009), Washington DC, USA.5. WisDOT, Bridge Manual, Chapter 12, (2016), Wisconsin, USA. 6. Sham et al, Foundation Design Methodology for the Padma Main Bridge, (2010), EACOM Asia Company Ltd,

Shatin, New Territory, Hong Kong.7. U.S DOT FHWA, Evaluating Scour at Bridges, 5 th Edition, (2012), Washington DC, USA.8. E. Snell and A Smith, The Design of Flood Resisting Bridge Abutments and approach Embankments, (2012). 9. European Commission JRC, Seminar on Bridge Design with Eurocodes, (2012).10. K. Johnson, Abutments, (2012), MinDOT, Minnesota, USA. 11. U.S DOT FHWA, Hydraulic Design of Bridges, (2012), Washington DC, USA.12. Piellca et al, Flood Damage in the United States 1926 – 2000: A Reanalysis of National Weather Service

Estimates, (2012). 13. Department of Main Roads, Bridges and Retaining Walls (Chapter 22), (2006), Queensland, Australia. 14. NSW Transport Roads and Maritime Services, Country Bridge Solutions, Edition 2, Volume 1.2, (2016),

Sydney, Australia.15. PNG Mirror News, Fuel Crisis Looms in Highlands, (3/10/2016), Papua New Guinea.16. Department of Works, Bridge Inspection Manual, Volume 2, (2005), Papua New Guinea. 17. Markham Culverts Ltd, Tensar Geogrid and Geotextile Guide, (2016), Lae, Papua New Guinea.


General Flood Resistant Bridge Design Guidelines

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