Failure Analysis and Design Improvement Proposal for Flood- damaged Bridges in Papua New Guinea Gibson Ali HOLEMBA Candidate for the Degree of Master of Engineering Supervisor: Professor Takashi MATSUMOTO Division of Engineering and Policy for Sustainable Environment Introduction people as a by-product of the water cycle process. The only way out to reduce or control and provide a The climatic effect of flood against the road infrastructure sustainable solution is an innovative way of engineering such as a bridge is so prevalent that it requires deeper and technology and better flood mitigation planning and engineering and technological intervention to address this control works. Fig. 1 shows 5.0m of road approach and ever-present phenomenon. Papua New Guinea has been bridge abutment of Pine Tops Bridge damaged by the experiencing frequent bridge failures and collapses due to flood in April of 2017. flooding rivers in the recent past. According to the internal records from Papua New Guinea Department of Works, it has shown that over Two Hundred and Eighty (285) bridges, fords (causeways) and major culverts were damaged by flood action alone in the last Five years. That is at a rate of 57 bridges in a year and this result is very staggering. Bridge damages have been observed to be mainly at the bridge foundations. More specifically, the flooding waters erode the bridge abutments, scour the bridge piers and weaken the bridge’s resistance against the flood loads and eventually destroy the bridge. In addition, it is also Figure 1. Flood-damaged Pine Tops Bridge, Wau attested that riverbank and road approach embankment Highway, Morobe Province, Papua New Guinea. erosion by flooding rivers have been one of the leading causes of bridge failures in Papua New Guinea, according Field Investigation and Data Collection to this study. The bridge inspections were carried out in three Field investigations were carried out in Papua New provinces of Papua New Guinea, namely; Morobe Guinea at twenty-one flood-damaged and affected bridge Province, Madang Province and New Ireland Province. In sites. The bridges that were investigated were all Morobe Province, five (5) number of bridges were constructed over natural river crossings in three investigated, three (3) bridges along Wau Highway, one distinctive provinces in the country. The field (1) bridge along Highlands Highway and one (1) bridge investigation works gathered field data such as, river along Ramu Highway. In Madang Province, eight (8) channel width, bridge dimensions, river cross-sections, bridges were inspected and all were along the Ramu flow depth, scour depth, flow angle, clearance height Highway section of Madang Province between (freeboard), debris and log sizes. Inspections were carried Pompaquato Bridge and Usino Junction. Moreover, in out on both superstructure and substructure with damages New Ireland Province, eight bridges were inspected, three the floods have caused on the bridge. (3) bridges along Boluminski Highway, two (2) bridges The river cross-section was measured manually by along Lanzarote Road and three (3) bridges along West using a measuring tape. The width of the main river Coast Road. All in all, twenty-one (21) bridges were channel was measured from the top of the east bank to the inspected in three different provinces along six key socio- top of the west bank in three different locations upstream, economic roads that support the livelihood of people in at bridge and downstream. A 10m of offset distance was Papua New Guinea. taken from the centerline of the bridge both upstream and Davies (2016), reported that Papua New Guinea is downstream from the bridge. In addition, the average vulnerable to both inland and coastal flooding. The river channel depth was measured with survey stuff at 3m intervals across the main channel in accordance with the country has suffered from severe coastal flooding in 2008 respective channel widths. in which as many as 75,000 people were displaced from The general information of these twenty-one inspected eight (8) different provinces. In 2016, around 10,000 bridges is provided in Table 1. These bridges have fallen people were affected by flooding in West New Britain victim to flooding sustaining major structural damages Province while thirty-five (35) houses, bridges, roads and while several bridges were destroyed by flood as agricultural farms were damaged across both provinces of discussed in the following chapters. Most of the studied Gulf and Southern Highlands [2]. rivers had trapezoidal channels while few were Rain and its effect of flooding are a natural rectangular open channels, especially those that have non- phenomenon and are here to stay whether we like it or not. erodible bank slopes. Fig. 2 shows the river channel cross- Flooding will continue to affect the livelihood of the sectional profile as measured in this study. Table 1. General Information of Inspected Bridges in Papua New Guinea Q100 Peak Flow Bridge Bridge Bridge Structure Catchment Design Velocity @ No. Bridge Name Length Width Span Type Size Discharge Q100 (m) (m) (No) (km2) (m3/s) (m/s) 1 Asas Bridge 40.0 3.72 1 Bailey Truss 11.39 64.71 0.97 2 Aumea Bridge 56.0 3.40 2 Bailey Truss 68.63 193.53 1.28 3 Bora Bridge 48.7 4.34 1 Bailey Truss 211.00 345.95 2.90 4 Cedar Bridge 35.7 7.50 3 Beam/Slab 812.43 627.91 2.64 5 Daulom Bridge 36.6 3.15 1 Bailey Truss 224.09 606.49 3.00 6 Himutu Bridge 30.84 3.22 1 Bailey Truss 41.40 199.90 1.57 7 Iruan Bridge 124.97 5.20 3 Bailey Truss 90.43 348.67 1.20 8 Kalili Bridge 21.3 3.14 1 Bailey Truss 20.00 185.98 1.31 9 Kesuai Bridge 73.5 3.55 2 Bailey Truss 56.31 171.53 0.79 10 Labur Bridge 21.5 3.15 1 Bailey Truss 0.07 4.07 1.44 11 Marakalang Bridge 37.0 3.40 1 Bailey Truss 17.20 169.63 1.35 12 Mea Bridge 146.3 3.64 3 Bailey Truss 35.22 128.83 0.71 13 Menia Bridge 45.7 4.40 1 Bailey Truss 40.29 126.00 0.73 14 Pine Tops Bridge 27.4 3.75 3 Beam/Slab 531.62 624.50 5.68 15 Punam Bridge 35.2 8.49 8 cells Arch Culvert 9.50 81.44 0.91 16 Rumu Bridge 30.1 7.38 3 Beam/Slab 325.46 401.69 4.91 17 Sausi Bridge 137.2 4.40 4 Bailey Truss 78.43 209.95 0.53 18 Surinam Bridge 49.5 4.20 3 Warren Truss 280.57 411.62 2.59 19 Wara Pita Bridge 33.0 3.10 6 cells Arch Culvert 11.47 91.37 1.85 20 Waterbung Bridge 36.8 4.07 3 Beam/Slab 91.79 114.29 1.14 21 Yakumbu Bridge 46.3 4.72 1 Bailey Truss 21.56 95.50 0.80 Figure 2. River Channel Cross-section. Figure 3. An example of Google Earth Pro® image of a plotted catchment area. Watershed and River Morphology The catchment areas were automatically calculated by Watershed is the land area or ridge that separates surface the software and were used for flood design estimations. run-offs from precipitations such as rainfall into different The accuracy of the calculations is limited to the accuracy river basins, lakes or ocean. The river morphology is the of the software used and as such the data used in this study study of river shapes or forms with respect to time. It is is for this purpose only and should not be used for design referred to as fluvial morphology in the study of purposes. It is highly recommended that adequate hydrology and hydraulics. investigation must be carried out using the topographic River morphology assessments were undertaken by contour maps or the hydrographic charts when visual inspection within the bridge periphery while undertaking design for these studied bridges for upstream and downstream environment were studied with permanent works. the use of drone survey. Mavic DJI Pro® drone was used to undertake the aerial survey by taking photographs Bridge Failure Assessment Method along the stream length with short video recordings of the river flow characteristics. From the field investigations, bridge failure analysis was The aerial photographs were taken at 200m-500m undertaken to correctly identify the resultant factors spacing both upstream and downstream. The catchment associated with a flood that caused the damages. The main area of the river from the bridge site was estimated using cause of the bridge damage was flooding, however, other the Google Earth Pro© software. The catchment size was factors such as scouring, bank erosion, debris impact, and determined by plotting the lines along the ridge dividing blockages were the root causes of the bridge damages the watershed as shown in Fig. 3. made possible by flooding rivers. Most of these phenomena are associated with flooding and as such a bridge failure analysis was required. LEGEND: Flood Level 1 = Main Failure Cause Level 2 = Structural Failure Cause Level 3 = Principle Failure Cause Level 1 Level 4 = Root Failure Cause Which part of the bridge was damaged by flood? Level 2 Substructure Failure Superstructure Failure What are the Principle Causes of Bridge Failure? Contraction Hydrodynamic Level 3 Local Scour Debris & Logs Hydrodynamic Scour Loads Sedimentation Overtopping Accumulation Loads Vertical Road What are the Main Foundation Blocked Deck Foundation Approach Root Causes of Structure Failure Bed Aggradation Waterway Displacement Failure Damage Bridge Failure? Damage Level 4 Ancillary Lateral Joint Reduced Direct Impact Structure Foundation Embankment Movements Bank Erosion Freedboard Damage Damage Failure Failure Figure 4.