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Increasing complexity of engineering design requirements for bridge hydraulic structures on constricted seasonal rivers. A case study of 

Kaabong Bridge in Karamoja, Uganda

Presented by: Eng. Dr. Seith MugumeManaging Director, Centre for Infrastructure Consulting (CIC) Ltd &

Lecturer, Department of Civil & Environmental Engineering, Makerere University, Kampala

Presentation Outline

• Introduction• Key changes in design requirements• Hydrological studies• Hydraulic modelling and design• Case study: Kaabong Bridge • Discussion & Conclusions

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Introduction

• State-of-the art design of hydraulic structures such as bridges and drainage systems is changing! Climate change; Urbanization; Upstream Catchment Degradation; Emerging threats have limited guidance in existing design codes &

guidelines• Presentation discusses the changes in complexity of hydrological

studies and hydraulic design of a bridge structure constructed on constricted seasonal river in Karamoja, Uganda

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Introduction

• The state of the art of design of hydraulic structures ischanging rapidly (North 2016)

• Increasing need for specialization in planning, design,materials selection, construction & risk assessment

• Emerging threats: Climate change => More frequent extreme rainfall , Urbanization => Increase in percentage imperviousness

(PIMP), reclamation of urban wetlands Upstream Catchment Degradation => Sediments

• Minimal or no guidance from existing design codes andguidelines

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Hydrological studies – key changes

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- Use of hard copy topographic maps- Manual catchment delineation methods- Use of Rational Method for Estimation of Q

- Digital Elevation Models & Satellite Imagery- ArcGIS based catchment delineation- Physically-based rainfall run-off models for Estimation of Q- Climate Change Impacts on rainfall extremes

Previous approach

State-of-the art

Hydraulic Design – key changes

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- Use of the Continuity Equation, Q = AV and- Manning’s EquationV = (1/n) R2/3S1/2

- Steady State Design in MS Excel for design of a typical section- Model build and Simulations in HECRAS Software- Consideration of Sediment Transport Aspects- Scour Computations - River Protection Works integrated in Bridge Design

Previous approach

State-of-the art

Advances in hydrological & hydraulic modelling

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Main steps entailed in model build of an urban catchment in SWMM (Mugume 2015)

1D Modelling 1D-2D Modelling 3D Modelling Multi Core/Parallel Computing

Case Study: Kaabong bridge, Uganda

Existing bridge section

Case study: Kaabong Bridge

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Key challenges considered in design Old and narrow existing bridge Frequent river flooding at the bridge site Flood duration: 3 – 4 hrsMaximum observed flood depths: 1 m above

the bridge deck level Constricted cross section at bridge crossing point Heavily silted river bed River morphology Course shifting northwards

Design Scope

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Geotechnical Investigations

Topographic Surveys

Hydrological Modelling & Hydraulic Design

Structural Analysis & Design

Topographic Surveys

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Detailed topographic plan layout and sections of the bridge site

Geo referencing of natural and manmade features

Installation of permanent beacons

Surveying of river channel cross sections upstream and downstream of the existing bridge.

Geotechnical Investigations

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Trial Pits & DCP

tests => Access

roads

Drilling of boreholes

=> Proposed bridge

site

SPT tests

Bearing Capacity

Computations

(Tezerghi & Peck

1967)

Hydrological Studies ‐ Catchment

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Un gauged Catchment, 296 km2

Seasonal River, 31.9 km upstream ofbridges

Predominant land uses Cultivated land (105.8 km2) Pastures and grassland (139 km2) Mountainous woodland (47.9 km2)

Narrow bridge/hydraulic opening High rate of sediment deposition=> Shifting/widening of the river bed

immediately upstream of theexisting bridge.

Hydrological Studies – Extreme Rainfall Analysis

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Climate Change Factor of 1.2 applied to the 2 yr 24 hour rainfall

Effects of spatial rainfall variations within the catchment => Areal reduction factor of 0.790 applied to the point rainfall.

Rainfall Return Period, T 10 25 50 100 200

Flood Factor 1.65 1.80 2.25 2.50 2.75

Index ‘n’ 0.96 0.96 0.96 0.96 0.96

24 hr Rainfall Depth (mm) 85.41 93.17 116.46 129.40 142.34

Hydrological Studies – Catchment Delineation

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Hydrological Studies – Flood Volume Estimation

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Hydrological Studies – Flood Volume Estimation

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Method/Approach Q 100 yr(m3/s)

Q 200 yr(m3/s)

US Soil Conservation Service (SCS) Method

443.0 516.0

TRRL Method 378.0 418.0

Hydrological Studies – Hydraulic Modelling

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Schematic Representation of old bridge location and corresponding River Channel in HECRAS Hydraulic Modelling System

Hydrological Studies – Hydraulic Modelling

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Modelled water surface profile across upstream part of the old bridge

Hydrological Studies – Hydraulic Modelling

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Modelled water surface profile – New bridgeAdopted bridge section => 2 x 20 m Span, 18o Skew, Free board = 1.5 m

Hydrological Studies – Bridge Scour Computations

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• Contraction Scour• Live bed contraction Scour (3.4 m)• Clear water contraction Scour (4.2m)• Local (Pier) Scour (5.8 m)

Adopted Foundation Depth=> 7.5 m

Conclusions

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• Complexity of hydrological studies & hydraulic design for a case study bridge site described;

• Consulting Engineer increasingly required to use of computer modelling software => to account emerging and complex design aspects such as climate change, sediment transport, scour, global structural analysis etc.

• Performance of river & bridge sections designed using steady state conditions (manual calculations) should be evaluated under unsteady flow conditions using open source software.

• Most software such as HEC-HMS, HECRAS etc. are freely available (Open Source)

Thank you for attending.

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