This document provides guidelines for designing flood resistant bridges in Papua New Guinea. It discusses design considerations for bridge abutments, superstructures, piers, and foundations to withstand flooding and scour. It also covers estimating scour depths, designing for structural stability, afflux, and providing scour protection. The goal is to develop low-cost, practical solutions to improve flood resilience of bridges and reduce damage.

1. General Flood Resistant
Bridge Design Guidelines
FLOOD RESISTANT
BRIDGE DESIGN IN
PAPUA NEW GUINEA
Gibson Ali Holemba
Research Student
Graduate School of Engineering
Hokkaido University

2. Presentation Outline
1. Introduction
2. Bridge Abutment
3. Bridge Superstructure
4. Bridge Pier
5. Bridge Foundation
6. Design of Flow
7. Design for Structural Stability
8. Design of Afflux
9. Estimating Scour
10. Scour Protection Measures
11. Aggradation and Degradation
12. Specific Design Considerations
13. Conclusion
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 2

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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 3

4. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 4

5. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 5

6. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 6

7. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 7

8. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 8

9. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 9

10. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 10

11. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 11

12. Estimating Scour
• The estimation of scour effects may involve the following:
1. Obtain all relevant data from the bridge site
2. 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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13. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 13

14. 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.
5 February 2017
Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido
University 14

15. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 15

16. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 16

17. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 17

18. 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 18

19. 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, 5th 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.
5 February 2017 Flood Resistant Bridge Design in PNG - Gibson A Holemba, Graduate School of Engineering, Hokkaido University 19