Draft Engineering Report for Bridge Design.pdf

7/30/2019 Draft Engineering Report for Bridge Design.pdf

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Ministry of Works & Transport June 2010Engineering Report Detailed Design of Semiliki,Kaguta, Karujumba, Kabaale and Kanyamateke

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Engineering Report

TABLE OF CONTENTS

ITEM PageTABLE OF CONTENTS i

LIST OF TABLES ii

LIST OF FIGURES ii

LIST OF ANNEXES iii

LIST OF ACRONYMS AND ABBREVIATIONS iv

1 EXECUTIVE SUMMARY ............................................................................................................................. 1

1.1 INTRODUCTION .......................................................................................................................................... 11.2 SCOPE OF PROPOSED WORKS .................................................................................................................... 1

2 INTRODUCTION ........................................................................................................................................... 5

2.1 BACKGROUND ........................................................................................................................................... 52.2 FIELD SURVEYS ......................................................................................................................................... 5

3 HYDROLOGICAL INVESTIGATIONS AND ANALYSIS...................................................................... 6

3.1 INTRODUCTION .......................................................................................................................................... 63.2 DESCRIPTION OF THE BRIDGE SITES........................................................................................................... 63.3 RETURN PERIOD......................................................................................................................................... 73.4 FLOOD ANALYSIS METHODS ...................................................................................................................... 73.5 THE FLOOD FREQUENCY ANALYSIS METHODOLOGY ................................................................................. 83.6 THE TRRL MODEL METHODOLOGY .......................................................................................................... 93.7 FLOOD ESTIMATES................................................................................................................................... 103.8 HYDRAULIC DESIGN CRITERIA AND THE HEC-RASRIVERANALYSIS SYSTEM.................................... 103.9 THEORETICAL BASIS FOR THE HYDRAULIC ANALYSIS............................................................................. 113.10 COMPUTATION PROCEDURE .................................................................................................................... 133.11 BRIDGE MODELING GUIDELINES .............................................................................................................. 143.12 CULVERT MODELING GUIDELINES ........................................................................................................... 163.13 ANALYSIS METHODOLGY........................................................................................................................ 173.14 KABAALE BRIDGE ................................................................................................................................... 193.15 KAGUTA BRIDGE ..................................................................................................................................... 293.16 SEMILIKI BRIDGE..................................................................................................................................... 393.17 KARIJUMBA BRIDGE................................................................................................................................ 493.18 KANYAMATEKE BRIDGE ......................................................................................................................... 60

4 TOPOGRAPHIC SURVEYS .................................................................................................................. ..... 68

4.1 INTRODUCTION ........................................................................................................................................ 684.2 FIELD SURVEYING ................................................................................................................................... 684.3 PROCESSING THE FINAL DRAWINGS........................................................................................................ 684.4 OUTPUTS.................................................................................................................................................. 69

5 GEOTECHNICAL AND MATERIALS INVESTIGATIONS ................................................................ 85

5.1 OBJECTIVES ............................................................................................................................................. 855.2 SITE LOCATIONS...................................................................................................................................... 855.3 SCOPE OF WORK...................................................................................................................................... 855.4 FIELD INVESTIGATIONS AND LABORATORY TESTS................................................................................. 855.5 CHALLENGES ........................................................................................................................................... 865.6 LOGGINGS................................................................................................................................................ 875.7 CONSTRUCTION MATERIALS ................................................................................................................... 955.8 LABORATORY TESTING ........................................................................................................................... 96


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5.9 INTERPRETATION OF THE LABORATORY TESTS RESULTS AND RECOMMENDATIONS........................... 975.10 EVALUATION OF THE BEARING CAPACITIES ........................................................................................... 97TESTRESULTS ............................................................................................................ .................................... 100

6 STRUCTURAL DESIGNS ......................................................................................................................... 106

6.1 DEFINITIONS AND BRIDGE COMPONENTS ............................................................................................. 1066.2 HIGHWAY BRIDGE DEAD LOADS (RDM,REF:5.1.2) ........................................................................ ... 106

6.3 HIGHWAY BRIDGE LIVE LOADS (RDM,REF:6.2.1) .......................................................................... ... 1076.4 MINIMUM EARTHQUAKE FORCES FORSTRUCTURES ............................................................................ 108

7 APPROACH ROADS ................................................................................................................................. 110

7.1 DESIGN CRITERIA .................................................................................................................................. 110

8 ENVIRONMENTAL AND SOCIAL IMPACT ASSESSMENT ........................................................... 112

8.1 PROJECT OBJECTIVES ............................................................................................................................ 1128.2 BASELINE CONDITIONS ......................................................................................................................... 1128.3 RELEVANT LEGISLATION RELATED TO BRIDGES .................................................................................. 1148.4 EVALUATION OF POTENTIAL ENVIRONMENTAL IMPACT ...................................................................... 1158.5 PROPOSED MITIGATION MEASURES...................................................................................................... 1168.6 IMPLEMENTATION PROCEDURE ............................................................................................................. 1178.7 ENVIRONMENTAL MANAGEMENT AND MONITORING PLAN ................................................................. 118

8.8 CONCLUSIONS AND

RECOMMENDATIONS

............................................................................................. 1189 PRICING OF WORKS ......................................................................................... ...................................... 119

9.1 PRICING OF BILLS OF QUANTITIES ........................................................................................................ 119

LIST OF TABLES

TABLE 1:LOCATIONS OF THE 5 PROPOSED BRIDGE CROSSINGS .............................................................. 6TABLE 2:FLOOD FLOW ESTIMATES AT THE GAUGING SITE FOR THE CANDIDATE DISTRIBUTIONS ....... 24TABLE 3:DESIGN FLOWS AT KABAALE BRIDGE SITE ............................................................................ 25TABLE 4:DESIGN STORMS FOR DIFFERENT RETURN PERIODS FORKABAALE SITE ............................... 25TABLE 5:DESIGN FLOODS FOR THE KABAALE BRIDGE SITE BEFORE ADJUSTING FOR STORAGE .......... 26TABLE 6:DESIGN FLOODS FOR THE KABAALE BRIDGE SITE AFTER ADJUSTING FOR STORAGE ............ 26TABLE 7:FLOW CONDITIONS AROUND BRIDGE SITE ............................................................................. 27TABLE 8:FLOOD FLOW ESTIMATES AT THE GAUGING SITE FOR THE CANDIDATE DISTRIBUTIONS ....... 35TABLE 9:DESIGN FLOWS AT KAGUTA BRIDGE SITE .............................................................................. 36TABLE 10:DESIGN STORMS FOR DIFFERENT RETURN PERIODS FORKAGUTA SITE ............................... 36TABLE 11:DESIGN FLOODS FOR THE KAGUTA BRIDGE SITE BEFORE ADJUSTING FOR STORAGE .......... 37TABLE 12:DESIGN FLOODS FOR THE KAGUTA BRIDGE SITE AFTER ADJUSTING FOR STORAGE ............ 37TABLE 13:FLOW CONDITIONS AROUND BRIDGE SITE ........................................................................... 38TABLE 14:FLOOD FLOW ESTIMATES AT THE GAUGING SITE FOR THE CANDIDATE DISTRIBUTIONS ..... 43TABLE 15:DESIGN FLOWS AT SEMILIKI BRIDGE SITE ........................................................................... 44TABLE 16:DESIGN STORMS FOR DIFFERENT RETURN PERIODS ............................................................. 44TABLE 17:DESIGN FLOODS FOR THE SEMILIKI BRIDGE SITE BEFORE ADJUSTING FOR STORAGE.......... 45TABLE 18:DESIGN FLOODS FOR THE SEMILIKI BRIDGE SITE AFTER ADJUSTING FOR STORAGE ............ 45TABLE 19:FLOW CONDITIONS AROUND BRIDGE SITE ........................................................................... 47

TABLE 20:FLOOD FLOW ESTIMATES AT THE GAUGING SITE FOR THE CANDIDATE DISTRIBUTIONS ..... 55TABLE 21:DESIGN FLOWS AT KARUJUMBA BRIDGE SITE ..................................................................... 56TABLE 22:DESIGN STORMS FOR DIFFERENT RETURN PERIODS FORKARUJUMBA SITE ........................ 56TABLE 23:DESIGN FLOODS FOR THE KARUJUMBA BRIDGE SITE BEFORE ADJUSTING FOR STORAGE.... 57TABLE 24:DESIGN FLOODS FOR THE KARUJUMBA BRIDGE SITE AFTER ADJUSTING FOR STORAGE...... 57TABLE 25:FLOW CONDITIONS AROUND BRIDGE SITE ........................................................................... 58TABLE 26:DESIGN STORMS FOR DIFFERENT RETURN PERIODS FORKANYAMATEKE SITE ................... 64


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TABLE 27:DESIGN FLOODS FOR THE KANYAMATEKE BRIDGE SITE BEFORE ADJUSTING FOR STORAGE....................................................................................................................................................... 65

TABLE 28:DESIGN FLOODS FOR THE KANYAMATEKE BRIDGE SITE AFTER ADJUSTING FOR STORAGE. 65TABLE 29:FLOW CONDITIONS AROUND BRIDGE SITE ........................................................................... 67

LIST OF FIGURES

FIGURE 1:MAP OF UGANDA SHOWING THE LOCATIONS OF THE PROPOSED BRIDGE CROSSINGS ............ 7FIGURE 2:CROSS SECTION LOCATIONS AT BRIDGE ............................................................................... 15FIGURE 3:TYPICAL CULVERT CROSSING (RIGHT: ENERGY AND HYDRAULIC GRADE LINE FOR A FULL

FLOWING CULVERT) ....................................................................................................................... 16FIGURE 4:KABAALE BRIDGE SITE AND CATCHMENT........................................................................... 19FIGURE 5:LANDSCAPE TYPE IN THE R.MAYANJA CATCHMENT ........................................................... 20FIGURE 6:R.MAYANJA CATCHMENT GEOLOGY .................................................................................. 21FIGURE 7:MONTHLY RAINFALL AND EVAPORATION VARIATION (SOURCE:HYDROCLIMATIC STUDY

(2001)) ........................................................................................................................................... 22FIGURE 8:FLOW DATA FORRIVERMAYANJA....................................................................................... 23FIGURE 9:ANNUAL MAXIMUM FLOWS FORR.MAYANJA ..................................................................... 23FIGURE 10:FITS FOR VARIOUS DISTRIBUTIONS TO R.MAYANJA DATA.CLOCKWISE STARTING FROM

THE UPPER LEFT CORNER ARE FITS FORNORMAL,LOGNORMAL,EXTREME VALUE AND WEIBULLDISTRIBUTIONS RESPECTIVELY...................................................................................................... 24

FIGURE 11:BRIDGE CONFIGURATION SHOWING THE 50-YEAR FLOOD LEVEL ...................................... 27FIGURE 12:SCOUR CONDITIONS FOR THE 100-YEAR FLOOD CONDITIONS ............................................ 28FIGURE 13:KAGUTA BRIDGE SITE AND ITS CATCHMENT ...................................................................... 29FIGURE 14:LANDSCAPE TYPES IN THE R.ASWA CATCHMENT.............................................................. 30FIGURE 15:LAND-USE TYPES IN R.ASWA CATCHMENT ....................................................................... 31FIGURE 16:R.ASWA CATCHMENT GEOLOGY........................................................................................ 32FIGURE 17:MONTHLY RAINFALL AND EVAPORATION VARIATION (SOURCE:HYDROCLIMATIC STUDY

(2001)) ........................................................................................................................................... 33FIGURE 18:FLOW DATA FORRIVERASWA AT PURANGA ..................................................................... 34FIGURE 19:ANNUAL MAXIMUM FLOWS FORR.MAYANJA ................................................................... 34FIGURE 20:FITS FOR VARIOUS DISTRIBUTIONS TO R.MAYANJA DATA.CLOCKWISE STARTING FROM


FIGURE 21:BRIDGE CONFIGURATION SHOWING THE 50-YEAR FLOOD LEVEL ...................................... 38FIGURE 22:SCOUR CONDITIONS FOR THE 100-YEAR FLOOD CONDITIONS ............................................ 39FIGURE 23:SEMILIKI BRIDGE SITE AND CATCHMENT .......................................................................... 40FIGURE 24:MONTHLY RAINFALL AND EVAPORATION VARIATION (SOURCE:HYDROCLIMATIC STUDY

(2001)) ........................................................................................................................................... 41FIGURE 25:FLOW DATA FORRIVERSEMILIKI AT BWERAMULE ........................................................... 42FIGURE 26:ANNUAL MAXIMUM FLOWS FORR.SEMILIKI AT BWERAMULE ......................................... 42FIGURE 27:FITS FOR VARIOUS DISTRIBUTIONS TO R.SEMILIKI DATA.CLOCKWISE STARTING FROM


FIGURE 28:BRIDGE CONFIGURATION SHOWING THE 50-YEAR FLOOD LEVEL ...................................... 46FIGURE 29:PROFILE ALONG CHANNEL CENTRELINE SHOWING THE PASSAGE OF THE 50 YEAR FLOOD 47FIGURE 30:SCOUR CONDITIONS FOR THE 100-YEAR FLOOD CONDITIONS ............................................ 48FIGURE 31:KARUJUMBA BRIDGE SITE AND CATCHMENT .................................................................... 49FIGURE 32:LANDSCAPE TYPES IN THE R.NYAMUGASANI CATCHMENT .............................................. 50FIGURE 33:LAND-USE TYPES IN R.NYAMUGASANI CATCHMENT ........................................................ 51FIGURE 34:R.NYAMUGASANI CATCHMENT GEOLOGY ........................................................................ 52


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FIGURE 35:MONTHLY RAINFALL AND EVAPORATION VARIATION FORZONE MW(SOURCE:HYDROCLIMATIC STUDY (2001))................................................................................................... 53

FIGURE 36:FLOW DATA FORRIVERNYAMUGASANI AT KATWE-CONGO ROAD ................................. 54FIGURE 37:ANNUAL MAXIMUM FLOWS FORR.NYAMUGASANI........................................................... 54FIGURE 38:FITS FOR VARIOUS DISTRIBUTIONS TO R.NYAMUGASANI DATA.CLOCKWISE STARTING

FROM THE UPPER LEFT CORNER ARE FITS FORNORMAL,LOGNORMAL,EXTREME VALUE ANDWEIBULL DISTRIBUTIONS RESPECTIVELY...................................................................................... 55

FIGURE 39:BRIDGE CONFIGURATION SHOWING THE 50-YEAR FLOOD LEVEL ...................................... 58FIGURE 40:SCOUR CONDITIONS FOR THE 100-YEAR FLOOD CONDITIONS ............................................ 59FIGURE 41:KANYAMATEKE BRIDGE SITE AND CATCHMENT ............................................................... 60FIGURE 42:LANDSCAPE TYPES IN THE R.KANYAMATEKE CATCHMENT ............................................. 61FIGURE 43:LAND-USE TYPES IN R.KANYAMATEKE CATCHMENT ....................................................... 62FIGURE 44:R.KANYAMATEKE CATCHMENT GEOLOGY ....................................................................... 63FIGURE 45:MONTHLY RAINFALL AND EVAPORATION VARIATION FORZONE MW(SOURCE:

HYDROCLIMATIC STUDY (2001))................................................................................................... 64FIGURE 46:BRIDGE CONFIGURATION SHOWING THE 50-YEAR FLOOD LEVEL ...................................... 66FIGURE 47:SCOUR CONDITIONS FOR THE 100-YEAR FLOOD CONDITIONS ............................................ 67FIGURE 48:TOPOGRAPHIC MAP OF SEMILIKI BRIDGE SITE .................................................................. 70FIGURE 49:LONGITUDINAL PROFILE ALONG CENTRE-LINE OF THE PROPOSED ROAD ACROSS SEMILIKI

BRIDGE .......................................................................................................................................... 71FIGURE 50:TOPOGRAPHIC MAP OF KAGUTA BRIDGE SITE .................................................................. 73FIGURE 51:LONGITUDINAL PROFILE ALONG CENTRE-LINE OF THE PROPOSED ROAD ACROSS KAGUTA

BRIDGE .......................................................................................................................................... 74FIGURE 52:TOPOGRAPHIC MAP OF KARUJUMBA BRIDGE SITE ............................................................ 76FIGURE 53:LONGITUDINAL PROFILE ALONG CENTRE-LINE OF THE PROPOSED ROAD ACROSS

KARUJUMBA BRIDGE..................................................................................................................... 77FIGURE 54:TOPOGRAPHIC MAP OF KABAALE BRIDGE SITE................................................................. 79FIGURE 55:LONGITUDINAL PROFILE ALONG CENTRE-LINE OF THE PROPOSED ROAD ACROSS

KABAALE BRIDGE ......................................................................................................................... 80FIGURE 56:TOPOGRAPHIC MAP OF KANYAMATEKE BRIDGE SITE ....................................................... 82FIGURE 57:LONGITUDINAL PROFILE ALONG CENTRE-LINE OF THE PROPOSED ROAD ACROSS

KANYAMATEKE BRIDGE................................................................................................................ 83

LIST OF ANNEXES AND ABBREVIATIONS

ANNEX A1: HYDROLOGICAL DATA ANALYSIS RESULTS FOR KABAALE BRIDGE

ANNEX A2: HYDROLOGICAL DATA ANALYSIS RESULTS FOR KAGUTA BRIDGE

ANNEX A3: HYDROLOGICAL DATA ANALYSIS RESULTS FOR SEMILIKI BRIDGE

ANNEX A4: HYDROLOGICAL DATA ANALYSIS RESULTS FOR KARUJUMBA BRIDGE

ANNEX A5: HYDROLOGICAL DATA ANALYSIS RESULTS FOR KANYAMATEKE BRIDGE

GOU Government of UgandaMoWT Ministry of Works and Transportkm Kilometrem Metre

m3

Cubic metre% Per cent


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1 EXECUTIVE SUMMARY

1.1 Introduction

The Government of Uganda (GoU), represented by the Ministry of Works and Transport(MoWT) intends to improve five (5) strategic bridges along the road network in the

country to standards required to cope with the present and anticipated growth, with thesole aim of achieving the targets of Poverty Eradication Action Plan (PEAP). The MoWThas therefore embarked on addressing the bottlenecks on classified and feeder roadsnetwork by removing major impediments to effective and efficient movement of goods,services and people.

1.2 Scope of Proposed Works

The Consultants scope of works included, but not limited to, the following:

design and tender documentation of Semiliki Bridge and approach roads,

design and tender documentation of Kaguta Bridge and approach roads,

design and tender documentation of Karujumba Bridge and approach roads,

design and tender documentation of Kabaale Bridge and approach roads,

design and tender documentation of Kanyamateke Bridge and approach roads,

Environmental and Social Impact Assessments of the Project.

1.2.1 Hydrological Investigations and AnalysisThe assignment involved carrying out drainage investigations to assess the requirements for

the approach roads and for purposes of determining suitable types and sizes of five bridges.

1.2.2 Topographic SurveysThe assignment involved carrying out topographic surveys to provide data that wouldsubsequently be used during the design the five strategic bridges. Using Leica T180 TotalStation, land-surveying techniques were used to capture the location and elevation offeatures and spot heights at each bridge site.

The following details were targeted while collecting data:

a) Spot heights for enabling accurate representation of the terrain;

b) Centre-line of the existing road, estimated by measuring the width of the road;

c) The center-line of the water channel/river also estimated by measurement of the

channel/river width;d) Heights along the banks of the water channel/river;

e) Changes in terrain features such as break lines in the general slope;

f) Location of trial pits for geotechnical investigations or soil samples, and;

g) Trees and other vegetation.


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1.2.3 Geotechnical and Materials Investigations

The investigations were aimed at determining the geotechnical properties of the soils wherethe bridges are to be constructed and the existing subsurface condition to enable theEngineer to determine the bearing capacities of the soils hence design the foundations. Theinvestigations involved identifying suitable sources of construction materials. The samples

collected from the trial pits from the bridge sites were taken for laboratory testing andanalysis at the Ministry of Works and Transport, Central Materials Laboratory, Kireka.

1.2.4 Structural Designs

For all bridges, the loads to be considered are the permanent loads, with the appropriateprimary live loads, together with those due to wind and temperature range and difference, aswell as temporary erection loads; during erection.

Design loads are selected and applied in such a way that the most adverse total effect iscaused in the element or structure under consideration.

The design of foundations is based on the principles set out in CP 2004.

1.2.5 Designs of Approach Roads

All the bridge approaches are designed to the Ministry of Works and Transport Class BGravel standards with the following parameters Ref: Ministrys Roads Design Manual:

Table 1.2.5.1a: Bridge Approaches (Road Design) Class

DesignClass

Capacity[pcu x1,000/day]

Road-waywidth[m]

MaximumDesign speedKph

Mountainous

B Gravel 2 6 8.6 50

Table 1.2.5.1b: Road Design Class (continued)

Design classRight of Way

width [m]Road waywidth [m]

Carriage wayShoulder

width[m]

Width[m]

Lanewidth[m]

No. oflane

B Gravel 30 8.6 5.6 2.8 2 2 x 1.5


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Table1.2.5.2: Geometric Design Parameters for Design Standard B Gravel

Design Element Unit Flat Rolling Mountainous

Design Speed km/h 80 60 50

Min. Stopping Sight Distance m 115 75 60

Min. Passing Sight Distance m 545 410 345

Min. Horizontal Curve Radius m 240 130 85Max. Gradient (desirable) % 4 6 9

Max. Gradient (absolute) % 6 8 11

Minimum Gradient in cut % 0.5 0.5 0.5

Maximum Super elevation % 7 7 7

Crest Vertical Curve stopping Kmin 32 14 9

Crest Vertical Curve passing Kmin 310 176 126

Sag Vertical Curve stopping Kmin 25 15 11

Normal Cross fall % 4 4 4

Shoulder Cross fall % 4 4 4

Right of Way m 30 30 30

It is observed that all bridges have big spans of ranging from 35m to 110m.

1.2.6 Environmental and Social Impact AssessmentThe purpose of the environmental impact assessment was to identify potential significantenvironmental impacts, including impacts on the ecological and socioeconomiccomponents of the environment. The findings of the Environmental Impact Assessmentwill contribute to the accountable decision making with regard to the upgrading of thebridges and approach roads, and ensure that the necessary mechanisms are put inplace to effectively manage the potential impacts. The objectives of the impact studywere: to identify and evaluate the environmental impacts of upgrading the bridges and

approach roads on the biophysical (ecological and physical) and socio-economiccharacteristics, during construction and operation;

to provide the basis for environmentally sound decision-making in which all reasonablealternatives are examined;

to undertake a comprehensive public participation exercise whereby interested andaffected parties (I&APs) are identified and given the opportunity to comment on theproposed project;

to identify and describe procedures and activities that will enhance the positive impactsand avoid or mitigate the negative environmental impacts;

to address medium to long term management and monitoring during all phases of theroad project (site preparation, construction, operation and maintenance) by implementingan environmental management plan.

The environmental study included:

Scoping and public consultation, Description of the proposed project, Description of the affected environment (ecological, physical and socioeconomic),

Environmental impact identification and evaluation, Environmental Impact Management Plan.

The Environmental Impact Assessment was carried out in compliance with the UgandaEnvironmental guidelines on bridge and road construction.


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The cost estimates for all the works is given below.

SUMMARY OF COST ESTIMATES FOR THE WORKS

SUMMARY BILL ALL BRIDGES Amount

SIMILIKI BRIDGE 3,284,143,800

KAGUTA BRIDGE 1,370,516,175

KARUJUMBA BRIDGE941,412,675

KABAALE BRIDGE 2,705,425,800

KANYAMATEKE BRIDGE 960,396,675

TOTAL 9,261,895,125


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2 INTRODUCTION

2.1 Background

The Government of Uganda (GoU), represented by the Ministry of Works and Transport(MoWT) intends to improve five (5) strategic bridges along the road network in the

country to standards required to cope with the present and anticipated growth, with thesole aim of achieving the targets of Poverty Eradication Action Plan (PEAP). The MoWThas therefore embarked on addressing the bottlenecks on classified and feeder roadsnetwork by removing major impediments to effective and efficient movement of goods,services and people.

Detailed designs have been completed, and this is the Design Report.

2.2 Field Surveys

Field surveys were carried out and all relevant site plans and the vertical and horizontalalignments for all proposed bridges and approach roads were produced.

Geotechnical investigations for ground conditions were carried out to determine:

the bearing capacity of the soils as an input to the structural design,

water table depth as a general input into the design,

construction materials required for the structures, river training and the approachroads, and

construction materials required for bridge approaches embankment fills and gravelpavement and wearing course.

Topographic surveys were carried out to establish data and basic drawings for thedetailed designs, including location, elevation, foundation levels and hydraulicdimensioning of the structures and river-training protective measures for thewatercourses.

Hydrological field investigations were carried out to determine hydraulic effectiveness ofthe proposed dimensions of all the structures, including freeboard.


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3 HYDROLOGICAL INVESTIGATIONS AND ANALYSIS

3.1 Introduction

Under the terms of reference of the consultancy services for detailed design of 5strategic bridges around Uganda, the hydrologist was required to carry out drainageinvestigations to assess the requirements for the approach roads for purposes of

determining suitable types and sizes of the bridge crossings.

This report reviews the procedure that was adopted in the hydrologic analysis andhydraulic design of the bridges; assessment of the data collected during the field study,analysis the hydrological characteristics, presentation of the alternative bridgeconfigurations and recommendations of the best designs.

3.2 Descr iption of the bridge sites

The proposed bridge crossings are located in several districts in northern, central andwestern Uganda (Figure 1, Table 1). The terrain varies considerably with elevations ranging from 628m aslat Semiliki to 1,781 m asl at Kanyamateke. The river widths also vary considerably fromabout 12 m for Karujumba to more than 80 m at Kabaale.

During the field visits it was noted that the river flow depths were below flood levels atvirtually all the sites. This is not surprising given that the timing of early Octobercoincided with the end of the dry season just before the start of the short rains for mostparts of Uganda. However, early approximations were that the water depths variedbetween 1.2 m for Kanyamateke to over 4 m for Semiliki.

The predominant land uses included light forest cover and thickets with agriculture andlivestock rearing practiced in some places. The main crops include cassava andmatooke, coffee, sun flower, cocoa etc.

Table 1: Locations of the 5 proposed bridge crossings

Site River District Elev.(m asl)

CatchmentArea (km2)

Sub-county

Kabaale Mayanja Kiboga/Nakaseske

1058 4,568 Kyankwanzi (Kiboga),Ngoma (Nakaseke)

Kaguta Aswa Lira 1000 4,667 Okwang, Ogur

Semiliki (atRwebisengo)

Semiliki Bundibugyo 628 33,165 Rwebisengo(Bundibugyo) andBuguma (DR Congo)

Karujumba Nyamugasani Kasese 1109 242 Kyondo and Kyarumba

Kanyamateke Kanyamateke Kisoro 1781 738 Busanza


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Figure 1: Map of Uganda showing the locations of the proposed bridge crossings

3.3 Return period

The Road Design Manual (1994) recommends that design return periods for structuresin rural areas be selected as follows:

Minor structures - 10-25 years Major bridges and culverts - 25-50 years

The factors considered in selecting the design return period include construction costand level of acceptable risk to life and property and design life of project (physical life oreconomic life). In this study, the 50 year return period flood has been used in sizing ofthe structures. The 100 year flood has been used to check if overtopping conditionsoccur. The 100 year flood event is also used to evaluate the bridge foundation against

scour.

3.4 Flood analysis methods

By definition flood flows are rare events and data availability is a major issue.Sometimes, the data is completely unavailable (in ungauged sites) or where flow dataare available, extreme flood conditions may be such that no flow measurements can betaken and estimates have to be made (i.e. extrapolation of rating curves). Careful


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consideration of the available data is important before selecting the analysis method. Inorder of preference, Watkins and Fiddes (1984) recommend the following methods forestimating design floods:

a) Methods based on analyzing flow data i.e. Extreme value analysis, Floodtransposition, Slope-area method, Bank full flows

b) Regional flood formulae like envelope curvesc) Rainfall runoff models i.e. the rational method, unit hydrograph techniques and

synthetic hydrographd) Hybrid methods based on a regionalization of rainfall runoff models i.e. the

ORSTOM method (developed in West Africa), TRRL method (based on 14catchments in Kenya and Uganda), the SCS curve number method and thegeneralized tropical flood model.

The choice between these methods depends on whether the detailed shape of the floodor the probable maximum flood is needed and on availability of the reliable flow recordsat the design site or nearby sites, whether on the same river or some other catchment. Italso depends on availability of suitable data.

In the current assignment, 4 of the 5 rivers have measured discharge records. These areR. Mayanja, R. Aswa, R. Semiliki and R. Nyamugasani. For the gauged rivers, statisticalanalysis using flood frequency estimation was carried out in deriving the design floodmagnitudes.

Where measured flow data is not available, the Road Design Manual (1993) appears tofavour use of the SCS curve number method in cases where measured flow data is notavailable. However, the TRRL East African model has been found to provide morereliable estimates for small catchments especially in areas where the gauging network isvery sparse. The following advantages of the method make it suitable for applying in thestudy area.

a) It was experimentally derived and tested using measurements of rainfall andrunoff 14 representative catchments in Kenya and Uganda for 4 years and isspecifically tailored for use in flood estimation for highways bridges and culverts.

b) The methodology for development of the model made extensive use of reliablerainfall records for over 867 stations available in the archives of the East AfricanMeteorological Department with a record length of 10-40 years. Depth-durationdata were obtained for stations in Kenya, Tanzania and Uganda (Busia, Kasese,Wadelai, Matuga, Atumatak, Entebbe, Gulu, Kampala, Jinja, Mbarara, Tororo,and Fort Portal).

c) It incorporates both unit hydrograph approaches and regionalization techniques.d) It was designed to provide estimates of peak discharges at recurrence intervals

of 5-25 and up to an upper limit of 50-100 years for small catchments of up to200 km2.

e) Areal reduction factors for East African rain gauge networks as well as variationsin vegetation are also incorporated in the model

3.5 The flood frequency analysis methodology

The methodology used for estimating the design flood for different recurrence intervalsusing statistical analysis of extremes was as follows:


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a) From a record of daily historical flows, the annual maximum values (themaximum daily flow for each year) were selected

b) From a number of candidate statistical distributions the distribution that best fitsthe annual maximum flows was selected. Four candidate distributions wereselected for the current study namely: Normal, Lognormal, Extreme Value and

Weibull distributionsc) The parameters of the distribution were estimated and the growth curve derivedd) The flood flows corresponding to the set return periods at the gauging stations

were estimatede) A suitable factor to convert mean daily flows into peak flow values was applied.

The factor takes into account the shape of the flood hydrograph and depends on,among others, the catchment size, time taken to route the flow through channelsand available storage (in lakes and swamps). The factor can vary between 1 and2.5 for large catchment. For smaller catchments, a much higher factor may beneeded

f) Where the gauging station location is different from the bridge site (which isnormally the case), the flows were transferred to the bridge site using the Flood

Transposition method. In this method, it is assumed that the catchmentcharacteristics for the catchment contributing the two (gauging station and bridgesite) do not vary considerably and the flood generation mechanisms are similar.In this case, the flows at the two points are proportional to the areas of theircatchments. Therefore, the flow at the bridge site is simply estimated as the flowat the gauging site multiplied by the ratio of the two areas.

3.6 The TRRL model methodo logy

The steps involved in estimating the design flood for different recurrence intervals usingthe TRRL Model were derived from Watkins and Fiddes (1984) in as follows:

a) The catchment upstream of each bridge site was generated using an SRTM90digital elevation model (DEM) of the area

b) Catchment area (A), land slope and channel slope were measured from the mapc) From site inspection the catchment type was established and the surface cover

flow time (TS) was computed using equation 7.27 and Table 7.16d) Soil type was determined by both geo-technical investigations and available soil

maps the soil permeability class and slope class were established using Table7.10 and 7.11 and, entering these into Table 7.12 or 7.13, the basic runoffcoefficient (CS) was determined.

e) The land use factor (CL) and catchment wetness factor (CW) were determinedfrom Tables 7.14 and 7.15

f) The runoff coefficient (CA)was computed using equation 7.22g) The base time (TB) was computed from equation 7.29h) The Kampala Equation (equation 4.11) was used to estimate the areal reduction

factor to take into account that tropical catchments rarely receive rainfalluniformly over the entire catchment.

i) The design storm rainfall (P) for each recurrence interval, to be allowed for duringbase time was then computed.

j) The average flow )(Q during base time was calculated from


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B

A

T

PACQ

360

(Equation 1)

k) The design peak )(Q was the computed from

QFQ (Equation 2)l) Where and appropriate value of F is taken from Table 7.17

3.7 Flood estimates

In cases where both methods (frequency analysis and TRRL method) were used, themethod that gave higher estimates was selected for use in hydraulic design. Thisapproach was used for the 4 bridge sites that had measured flow data, namely Kabaale(R. Mayanja), Kaguta (R. Aswa), Semiliki (R. Semiliki) and Karujumba (R.Nyamugasani). The flood estimates for Kanyamateke (R. Kanyamateke) were based ononly the TRRL method, since there was no suitable gauging nearby on which to basestatistical analysis.

3.8 Hydraulic Design Criteria and the HEC-RAS River Analys is System

Flow analysis and bridge design were carried out using the HEC-RAS River AnalysisSystem developed by the US Army Corps of Engineers Hydrologic Engineering Centre.The software has been widely used in different countries for hydraulic analysis anddesign of hydraulic structures including bridges and culverts. It consists of a graphicaluser interface, analysis components, data preparation, storage and managementcapabilities, graphics and reporting facilities.

The HEC-RAS system contains four 1-dimensional river analysis components for:a) Steady flow water surface profile computationsb) Unsteady flow simulationc) Movable boundary sediment transport computationsd) Water quality computationse) Hydraulic design features that can be invoked once the basic water surface

computations have been carried out

Program capabilities

HEC-RAS is designed to perform one-dimensional hydraulic calculations for a fullnetwork of natural and constructed channels. For the current assignment, use was madeof the steady flow water surface profile component. The following features of the steadyflow component make it particularly suitable for the assignment.

a) The steady flow water surface profiles component is intended for calculatingwater surface profiles for steady gradually varied flow. The system can handle afull network of channels, a dendritic system or a single river reach. The steadyflow component is capable of modeling subcritical, supercritical, and mixed flowregime water surface profiles.

b) The basic computational procedure is based on the solution of the onedimensional energy equation. Energy losses are evaluated by friction (Manningsequation) and contraction/expansion (coefficient multiplied by change in velocity


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head). The momentum equation is utilized in situations where the water surfaceprofile is rapidly varied. These situations include mixed flow regime calculations(i.e. hydraulic jumps), hydraulics of bridges, and evaluating profiles at riverconfluences (stream junctions).

c) The effects of various obstructions such as bridges, culverts, dams, weirs, andother structures in the flood plain may be considered in the computations. Also

capabilities are available within the system for assessing the change in watersurface profiles due to channel modifications etc.

Special features of the steady flow component include: multiple plan analyses; multipleprofile computations; multiple bridge and/or culvert opening analysis; bridge scouranalysis; split flow optimization; and stable channel design and analysis.

3.9 Theoretical basis for the hydraulic analysis

The theoretical framework for the flow calculations is founded on long establishedprinciples of fluid dynamics including mass, energy and momentum conservation(Featherstone and Nalluri, 1995; Brunneret al., 2001). A number of implicit assumptionsare made in the steady flow analysis component of the software including;

a) Flow is steadyb) Flow is gradually varied (except at hydraulic structures such as bridges, culverts,

and weirs. At these locations, where the flow can be rapidly varied, themomentum equation or other empirical equations are used instead)

c) Flow is one dimensional (i.e. velocity components is directions other than thedirection of flow are not accounted for)

d) Rivers have small slopes, say less than 1:10

Below is a review of some of the key issues of interest.

Equations for the basic profi le calculations

In the HEC-RAS system, water surface profiles are computed from one cross-section tothe next by solving the Energy equation with an iterative procedure called the standardstep method. The energy equation is written as follows

ehg

VZY

g

VZY

22

2

1111

2

2222

(3)

WhereY1, Y2 = depth of water at cross sectionsZ1, Z2 = elevation of the main channel invertsV1, V2 = average velocities (total discharge/total flow area)

21,

= velocity weighting coefficientsg = gravitational accelerationhe = energy head loss

the energy head loss (he) between two cross sections is comprised of friction losses andcontraction or expansion losses. The equation for the energy loss is as follows.


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g

V

g

VCSLh fe

22

2

11

2

22 (4)

Where: L = discharge weighted reach length

fS = representative friction slope between two sections

C = expansion or loss coefficientThe distance weighted reach length, L, L is calculated as

robchlob

robrobchchloblob

QQQ

QLQLQLL

(5)

Where robchlob LLL ,, = cross section reach lengths specified for flow in the left

overbank, main channel, and right overbank respectively

robchlob QQQ = arithmetic average of the flows between sections for the left

overbank, main channel, and right overbank respectively

Cross section subdivision for conveyance calculations

The determination of the total conveyance and the velocity coefficient for a cross sectionrequires that flow be subdivided into units for which the velocity is uniformly distributed.The approach used in HEC-RAS is to subdivide the flow into the overbank areas usingthe input cross section n-value break points (location where the Mannings n-valueschange) as the basis for subdivision. Conveyance is then calculated within eachsubdivision from the following form of Mannings equation based on SI units

n

ARK

KSQ f32

21

(6)

Where: K = conveyance for the subdivisionn = Mannings roughness coefficient for the subdivisionA = flow area for subdivisionR = hydraulic radius for subdivision (area/wetted perimeter)

The program then sums up all the incremental conveyances in the overbanks to obtainthe conveyance for the left and right overbank. The main channel is normally computedas a single conveyance element. The total conveyance for the cross section is obtainedby summing the three subdivision conveyances (left, channel and right).

Composite Mannings n for the channel

Flow in the main channel is not subdivided, except when the roughness coefficient ischanged within the channel area. HEC-RAS tests the applicability of subdivision ofroughness within the main channel portion of a cross section, and if it is applicable, theprogram will compute a single composite main channel n value.


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Evaluation of Mean Kinetic Energy Head

Because the HEC-RAS software is a one dimensional water surface profiles program,only a single water surface and therefore a single mean energy are computed at eachcross section. For a given water surface elevation, the mean energy is obtained by

computing a flow weighting energy from the three subsections of a cross sections (leftoverbank, main channel, and right overbank).

Friction loss evaluation

Friction loss is evaluated in HEC-RAS as a product of fS and L (equation 6.2) where

fS is the representative friction slope for a reach and L is is defined by equation 6.3. The

friction slope (slope of the energy grade line) at each cross section is computed fromMannings equation as follows:

2

K

Q

Sf (7)

Alternative expressions for the representative friction slope used in HEC-RAS areexplained in (Brunneret al., 2001) and include:

a) Average conveyance equationb) Average friction slope equationc) Geometric mean friction slope equationd) Harmonic mean friction slop equation

Contraction and expansion loss evaluation

Contraction and expansion losses in HEC-RAS are evaluated by the following equation:

g

V

g

VChce

22

2

22

2

11 (8)

Where: C = contraction or expansion coefficient

The program assumes that a contraction is occurring whenever the velocity headdownstream is greater than the velocity head upstream and vice versa. Typical C valuesare available in standard textbooks and manuals on Hydraulics.

3.10 Computation procedure

The unknown water surface elevation at a cross section is determined by an iterativesolution of equations 6.1 and 6.2 as follows:

a) Assume a water surface elevation (WS2) at the upstream cross section (ordownstream cross section if a supercritical profile is being computed)

b) Based on the assumed water surface elevation, determine the correspondingtotal conveyance and velocity head


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c) With values from step 2, compute fS

and solve equation 6.2 for eh

d) With values from steps 2 and 3, solve equation 6.1 for WS2.e) Compare the computed value of WS2 with the value assumed in step 1; repeat

steps 1 through 5 until the values agree within 0.003m, or a user definedtolerance.

3.11 Bridge modeling guidelines

HEC-RAS computes energy loses caused by structures such as bridges and culverts inthree parts:

a) One part consists of losses that occur in the reach immediately downstream fromthe structure, where expansion of the flow generally takes place

b) The second part consists of losses at the structure itself, which can be modeledwith several different methods.

c) The third part consists of losses that occur in the reach immediately upstream ofthe structure, where the flow is generally contracting to get through the opening.

Cross section locations

The bridge routines utilize four user defined cross sections in the computations of energylosses due to the structure (numbered 1, 2, 3 and 4 in Figure 2). During the hydrauliccomputations the program automatically formulates two additional cross sections insidethe bridge. Whenever the user is performing water surface computations through thebridge, additional cross sections should always be included both downstream andupstream of the bridge to prevent any user-entered boundary conditions from affectingthe hydraulic results through the bridge.


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Figure 2: Cross section locations at bridge

Contraction and expansion losses

Losses due to contraction and expansion of flow between cross sections are determinedduring the standard step profile calculations. Mannings equation is used to calculatefriction losses, and all other losses are described in terms of a coefficient times theabsolute value of the change in velocity between adjacent cross sections. When thevelocity head increases in the downstream direction, a contraction coefficient is used;and when the velocity head decrease, an expansion coefficient is used.

Hydraulic computations through the bridge

The bridge routines in HEC-RAS allow the modeller to analyse a bridges with severaldifferent methods without changing the bridge geometry. The bridge routines have theability to model low flow (class A, B, and C) when the bridge opening operates as anopen channel. The routines can also model high flows which are flows that come intocontact with the maximum low chord of the bridge deck. The energy equation is mainlyused in both cases though other alternative equations like momentum balance, Yarnell


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equation in case of low flows or the pressure and weir flow method in case of high flows.In cases of combination flows (when low flows and high flows occur) and iterativeprocedure is used to determine the amount of each flow and the appropriate equationsapplied.

Selecting a b ridge modelling approach

The choice of the modelling approach depends significantly of the type of flow (low orhigh) and local conditions like level of obstruction by the piers, predominant type oflosses level of obstruction by the bridge deck, whether the bridge is submerged or notetc. other factors include the bridge skew to the flow direction, and presence of multiplebridge openings at a cross section.

3.12 Culvert modeling guidelines

Because of the similarity between flow in bridges and culverts, culverts are modeled in asimilar manner to bridges. Figure 3 shows a typical box culvert crossing and illustratesthe similarities between culvert and bridge crossings. The selection of lay out cross

sections, the use of ineffective areas of flow, the selection of loss coefficients and mostother aspects of bridge analysis apply to culverts as well. The most common types ofculvert crossings includes circular, box (rectangular), arch, box arch, low profile arch,high profile arch, elliptical and semi-circular. Flow conditions at the entrance and exit ofthe culverts are defined by the contraction and expansion coefficients which are uniqueto each culvert type. The head losses are computed by multiplying this coefficient by theabsolute head difference between two cross sections (one upstream and the otherdownstream of the culvert section).

Figure 3: Typical culvert crossing (right: energy and hydraulic grade line for a fullflowing culvert)


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3.13 Analysis Methodolgy

The following methodology was adopted for the hydrological analysis and hydraulicdesign of the bridges;

Data collection and inventory of the existing structures:

Review of the existing design and assessment reports. Field studies to obtain site datalike location, type, geometrics and condition. Detailed data concerning flow conditions(discharge) and river cross-sections is useful in the hydraulic design of the crossings.Flow data (where available), rainfall, etc was also be collected from the respectiveagencies. This involved collection of historical flood data including high water marks,river cross-sections (upstream, downstream and at bridge site), existing activities andmanmade features in the flood plain. Evidence of bridge overtopping and scour wasalso collected. Use was made of existing reports, the MoWT Road Design Manual,maps, drawings and such other documents. Field visits were also carried out for on-siteassessments of the sections.

Hydrological analysis

Hydrological analysis involved determination of discharges with different return periodsfor each site on the basis of which performance of alternative bridge designs wereevaluated. Determination of the discharges was based on the following procedure

a) Estimation, for each site, of catchment area, rainfall, catchment slope, flowvelocities, cross-sectional area, roughnesses (in river and flood plain) etc. Toobtain this information, use was made of existing reports, topographical maps

(1:50000 scale) and digital datasets using GIS techniques.b) Estimation of the flood discharges corresponding to specific return periods (QT)

which included Q10, Q25, Q50, and Q100. The bridges were designed to passthe 50 year flood (Q50). The proposed bridge designs were then crosscheckedagainst failure resulting from the 100-year flood (Q100). For the rivers that hadflow gauging stations flood estimation was based on both the TRRL method andflood frequency analysis using measured data.

c) Using the survey data the elevations corresponding to each of the abovedischarges were computed

Analysis o f alternati ves

This was an iterative process involving an evaluation of the alternative designs, selectionof the most appropriate and refinement to suit conditions on ground. The HEC-RASsoftware used to carry out hydraulic analysis of alternatives. For each site, selection wasmade on the basis of assessing the suitability of the design for given conditions offlooding. In particular, issues of concern included;


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a) Availability of acceptable freeboard under flood conditionsb) Assessment of the backwater effects vis--vis flooding damagec) Comparisons with historical occurrence of flooding

Bridge types

The bridge material will depend on the considerations of the structural and materialsengineers. However, for purposes of the hydraulic design, it is assumed that the bridgeswill be made of reinforced concrete reinforced concrete decks and rounded piers. Theother consideration was the Mabey Bridge system (www.mabebridge.co.uk) which aremade of steel trusses. However, based on information available at the design stage, theprefabricated members are single lane. To get the necessary two lanes, we would haveto lay two Mabey bridges side-by-side which would be too expensive.

Evaluation and selection

The selection of a best alternative was accomplished by comparison of the studyresults and considerations to acceptable limitations and controls. Best alternative meansthe bridge configuration that meets all or most of the following criteria.

a) Backwater will not significantly increase flood damage to property upstream ofthe crossing.

b) Velocities through the structure(s) will not damage the highway facility or undulyincrease damages to adjacent property.

c) Existing flow distribution is maintained to the extent practicable.d) Level of traffic service is compatible with that commonly expected of the class of

highway and projected traffic volumes.e) Minimal disruption of ecosystems and values unique to the floodplain andstream.

f) Cost for construction, maintenance and operation, including probable repair andreconstruction, and potential liabilities are affordable.

g) Pier and abutment location, spacing, and orientation are such to minimize flowdisruption, debris collection and scour.

h) Proposal is consistent with the intent of the standards and criteria of the Ministryof Works guidelines.

Documentation o f design

All information pertinent to the selection of the "best" alternate was documented asfollows:

a) A report including all computations (design floods, scour, sizing, etc)b) Sketch of proposed structure(s) and roadway grade in plan and profile showing

crown grade elevation, super structure, limits and elevations of any channel


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modifications as well as a table or performance curve forming a depiction of thenatural and post-design water surface elevations at the upstream section for thedesign flood.

3.14 Kabaale Bridge

Introduction

The Kabaale site is located on River Mayanja at GPS location 378534E and 124008N(Figure 4). It is located at the border between the sub-counties of Lwebisanja in Kibogadistrict and Ngoma in Nakaseke district. The site is located at an elevation of 1058m aslwhile the catchment area is 4,658 km2. At the site, the river exits from a swamp andenters another swamp downstream with a total clear length of about 500m. The width ofthe river at the proposed site is about 80m but the flood plain extends over 250 m atleast. The water is quite clear with no evidence of sediment transportation. However,evidence of bank erosion during flooding exists.

Figure 4: Kabaale Bridge Site and Catchment


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Catchment characteristics

Landscape

The upstream of the catchment is characterised by a rolling terrain with numerous hillsdrained by wide valleys (Figure 5). The areas close to the bridge site are generally flatand swampy. The elevation varies between 1040 masl and 1600 masl. The land slopesare generally low, varying between 7% in the upper reach and 4% in the lower reach.The average slope is 5.6% while the channel slope is 0.04%.

Figure 5: Landscape type in the R. Mayanja catchment

Land cover

The land cover in the basin consists of a combination of open shrubs with herbaceousand sparsely distributed trees. Small scale agriculture is the dominant activity in theupstream areas while livestock rearing is the dominant activity in the lower reaches. The


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river flood plains are dominated by permanent papyrus swamps which provide extensivestorage of flood water thereby providing some attenuation of the peak flows.

Geology and Soils

The upper reach of the R. Mayanja catchment is mainly made up of undifferentiatedbasement system gneisses (Figure 6). The lower reach is made up of unconsolidatedmaterial which is eroded from the upstream areas and deposited due to reduction inchannel slopes. The soils range from sandy to sandy loams. The valleys are filled withclayey mixtures.

Figure 6: R. Mayanja Catchment Geology

Climate

The area falls within climatic zone L according to the Uganda Hydroclimatic Study(2001). The zone receives an average of 1270 mm of rainfall which is principally spreadover 2 rainy seasons: The long rains of March to May and the short rains of Septemberto November (Figure 7). During the dry months, evaporation can be very high (in theorder of 5 times the rainfall).


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Figure 7: Monthly Rainfall and Evaporation Variation (source: Hydroclimatic Study

(2001))

Flood estimation results

Both flood frequency analysis and the TRRL method were used for flood estimation asdetailed below.

Flood frequency analysis

The data used for frequency analysis was obtained for gauge number 83218 onKapeeka Kakunga road (Figure 4). Figure 8 shows the daily flow data for the gaugewhile Figure 9 shows the extract of annual maximum daily flows. The annual maximum

flows range from 25 m

3

/s in 2000 to 43 m

3

/s in 2006. The extensive swamp storageattenuates this flood magnitude quite significantly. Therefore, when compared with thecatchment area, the flood magnitudes are quite low. For 8 years out of the 9 years ofrecord the annual maximum flows occur in October and November during the secondrainy season that lasts from September to December though it can sometimes extend toJanuary.


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1998 2000 2001 2002 2004 2005 20060

10

20

30

40

50

60

Flow

(m3/s)

River Mayanja (83218)

Figure 8: Flow data for River Mayanja

1998 1999 2000 2001 2002 2003 2004 2005 20060

5

10

15

20

25

30

35

40

45

Annual

MaximumF

low

(m3/s)

River Mayanja (83218)

Figure 9: Annual maximum flows for R. Mayanja

The fits for the various distributions are not particularly good (Figure 10). There is someclustering of the annual maximum flows. This is probably because the data length isrelatively short (13 years). Therefore, the strategy adopted for estimating the design

flows was to carry computations using all candidate distributions and select the one withthe higher flow estimates for each return period.


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24 26 28 30 32 34 36 38 40 420

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cumulativeprobability

Annual maximum flow (m3/s)

Observed data

Normal distribution

95% confidence bounds

24 26 28 30 32 34 36 38 40 420

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1



Observed data

Lognormal distribution


24 26 28 30 32 34 36 38 40 420

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cu

mulativeprobability


Observed data

Extreme value distribution


24 26 28 30 32 34 36 38 40 420

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cu

mulativeprobability


Observed data

Weibull distribution


Figure 10: Fits for various dist ributions to R. Mayanja data. Clockwise startingfrom the upper left corner are fits for Normal, Lognormal, Extreme value and

Weibull dis tributions respectively

Estimates for the lognormal distribution are higher than those for other distributions(Table 2). The lognormal distribution has, therefore, been selected for estimating theflood flows at Kabaale bridge site.

Table 2: Flood flow estimates at the gauging s ite for the candidate distribut ions

T (Years) QT (m3/s) for each of the candidate distributions

Normal Lognormal Extreme Value Weibul l

10 134.9 139.1 129.5 131.0

20 143.8 152.7 134.2 137.4

50 154.0 169.5 139.1 144.3

100 160.7 181.7 142.1 148.7

Estimates of the design flood at the bridge site were made using the flood transpositionmethod. The ratio of the area of the bridge site catchment (area = 4,658 km2) to thegauging site catchment (area = 2,297 km2) was computed as 1.99. The estimates of thedesign flood at Kabaale bridge site are shown in table 3.


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Table 3: Design flows at Kabaale bridge site

T (Years) QT (m3/s)

Flow Gauge Kabaale Bridge

10 139.1 276.7

20 152.7 303.6

50 169.5 337.0

100 181.7 361.3

TRRL method

In the TRRL method, use was made of results from direct analysis of observed intensitydata from various parts of East Africa to derive the design rainfall storms. The methodinvolved initially setting the runoff coefficient (percentage of rainfall that is converted to

runoff) using factors like land use, catchment slope class, soil class, surface cover andcatchment wetness factor. The hydrograph base time was then estimated fromcatchment area and slope class. The base time can be thought of as being made of 3components viz. the storm duration, time taken for the surface runoff to drain into thestream system, and the flow time down the stream and river system to the bridge site.

The mean 24 hour rainfall (also called the 2-year, 24 hour rainfall) was estimated from astorm rainfall map of East Africa and found to be between 60 and 70 mm in most parts ofUganda. Factors of 1.49, 1.74, 1.95 and 2.2 were then applied to derive the designstorm having return period of 10, 25, 50 and 100 years respectively. Table 4 shows thecomputed design storms for the site.

Table 4: Design s torms for di fferent return periods for Kabaale site

T (years) Design storm (mm)

10 114.8

25 129.5

50 154.0

100 175.0

Each of the above design rainfall were adjusted by applying 2 factors:

a) An area reduction factor to take into account the variability of rainfall in spaceb) A rainfall ratio to take into account the movement of the design storm in time.

The average and peak flow during base time for each return period was computed usingequations 1 and 2 given above and applying a peak factor of 2.5 which applies for humidregions.


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Table 5 shows the design floods that were obtained for each bridge site using the TRRLmodel. The bank-full flows were computed using Mannings equation of friction flow tocheck whether occurrence of the design floods would cause the rivers to bust theirbanks. From the values of the bank-full flows it is clear that the design floods can becarried within the river banks.

Table 5: Design floods for the Kabaale bridge site before adjusting for storage

T (years) Peak flood (m3/s)

10 277.4

25 312.9

50 372.1

100 422.9

The above design floods should be adjusted to take into account the effect of storage

within the basin and catchment shape. The river basin has extensive swamps thatprovide considerable storage. The peak flows are therefore greatly attenuated. About20% of the basin is filled with papyrus swamps. This was estimated to result in a 15%attenuation of the peak flows and the flow values were adjusted accordingly. The lengthof the catchment is 121km while the width is 49km giving a ratio of 2.5 which is withinthe range of 2-6 and is assumed in the derivation of the TRRL approach and thereforeno further adjustments were carried out. Table 6 shows the final estimates of flood flows.

Table 6: Design floods for the Kabaale bridge site after adjusting for storage

T (years) Adjusted Peak Flood

(m3/s)10 249.7

25 281.6

50 334.9

100 380.6

Bridge Design Results

The proposed bridge is a multiple span bridge with vertical abutments and 45 degree

wing-walls (Figure 11). The end spans (between abutments and first pier from eachbank) are 10 m wide. There are 6 internal spans (pier to pier) each 15m wide. The totalbridge width is 110m while the effective flow area is 300 m2. Each of the 7 piers is of theround-nose type and is 0.5m wide. The bridge deck high chord is 1052.9 m high, whilethe deck low chord is 1052.0 m. The invert level at lowest point of the river is 1048.5 mgiving a clear height of 3.5m. The 50-year flood level is 1051.1 m giving a free board of0.9 m.


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0 50 100 150 200 250 300 350

1048

1049

1050

1051

1052

1053

KabaleWithBridge Plan: Plan 01 1/10/2010

Station (m)

Elevation(m)

Legend

EG 50-year

WS 50-year

Crit 50-year

Ground

Bank Sta

.075 .03 .075

Figure 11: Bridge configuration showing the 50-year flood level

The flow velocity at the bridge site is about 1.65m/s and the Froude number is 0.32(Table 7). Therefore, flow through the bridge is subcritical.

Table 7: Flow conditions around bridge site

Assessment of the scouring conditions around the abutments using the 100 year floodindicates a maximum scour hole depth of 1.8 m and 1.4 m for the left and rightabutments respectively (Figure 12). There is, therefore, a need to protect the abutmentsagainst scour with riprap. Pier scour is negligible.


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0 50 100 150 200 250 300 3501048

1049

1050

1051

1052

1053

Bridge Scour RS = 36.8

Station (m)

Elevation(m)

Legend

WS 100-year

Ground

Bank Sta

Contr Scour

Total Scour

Figure 12: Scour conditions for the 100-year flood conditions


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3.15 Kaguta Bridge

Introduction

The Kaguta bridge site is located on River Aswa in Lira district at GPS location 501047Eand 275998N (Figure 13). At this section, the river serves as the border between the

sub-county of Orit, Erute County, Lira district and the sub-county of Amoyai, OtukeCounty, Pader district. The site is at an elevation of 997 asl and the catchment area is4,667 km2. The site is located at a 90degree bend within a gorge of about 3.5 m depth.

The width of the river at the proposed site is about 20m but the flood plain extends over200 m at least. The water carries sediment that is eroded from the upstream areas inKaramoja. There is evidence of bank erosion during flooding.

Figure 13: Kaguta bridge site and its catchment

Landscape

The upstream of the catchment (area of Labwor in Kotido district) is characterised by arelatively hilly terrain with wide valleys (Figure 14). The areas close to the bridge site aremade up of shallow gorges with flood plains that may extend 200 m on either side of theriver. The elevation varies between 997 masl and 1868 masl with a mean of 1088 masl.


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The slopes vary between 8% in the upper reaches and 1.9% in the lower reaches. Theaverage slope is 2.5% while the channel slope is 0.034%.

Figure 14: Landscape types in the R. Aswa catchment

Land-use

Woodlands, pasture lands and grasslands are dominant in the upstream areas.Subsistence agriculture is dominant in the mid to lower reaches. The main crops growninclude maize, sunflower, sorghum, millet. The flood plains tend to be bushy withsomewhat dense tree cover.


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Figure 15: Land-use types in R. Aswa catchment

Geology and SoilsThe catchment is mainly made up of a combination of granitoid, undifferentiated andunconsolidated sediments as well as basement system gneisses (Figure 16). The soilsrange from sandy to sandy loams. The valleys are filled with gravely soils and clayeymixtures.


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Figure 16: R. Aswa catchment geology

ClimateThe area falls within climatic zone I according to the Uganda Hydroclimatic Study (2001).The zone receives an average of 1340 mm of rainfall which falls in one rainy seasonfrom April to mid November (Figure 17). The dry season is from November to March.January is the driest month and evaporation can be 10 times the rainfall received.


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Figure 17: Monthly rainfall and evaporation variation (source: Hydroclimatic study(2001))

Flood estimation results

Both flood frequency analysis and the TRRL method were used for flood estimation asdetailed below.

Flood frequency analysis

Flow data was obtained for gauge number 86201 at Puranga. Figure 18 shows the dailyflow data for R. Aswa at the gauging station while Figure 19 shows the extract of annualmaximum flows. The annual maximum flows range from 26 m3/s in 1965 to 208 m3/s in1970. The flows are mainly driven by flush flooding in the headwater areas of Labwoh,Jie and Bokora but also the rainy season in the area from April to October. The annualmaximum series is dominated by flows in the months of August and September whichcoincides with the peak of the rainy season.


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1952 1954 1957 1960 1963 1965 1968 1971 1974 1976 19790

50

100

150

200

250

Flow

(m3/s)

River Aswa I (86201)

Figure 18: Flow data for River Aswa at Puranga

1949 1954 1960 1965 1971 1976 19820

50

100

150

200

250

AnnualMaximumF

low

(m3/s)

River Aswa I (86201)

Figure 19: Annual maximum flows for R. Mayanja

The fits for the various distributions are not particularly good (Figure 10). There is someclustering of the annual maximum flows but the fits are generally good.


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40 60 80 100 120 140 160 180 200 2200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


Observed data

Normal distribution


40 60 80 100 120 140 160 180 200 2200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


Observed data

Lognormal distribution


40 60 80 100 120 140 160 180 200 2200

0.1

0.2

0.3

0.4

0.5

0.6

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0.8

0.9

1

Cu

mulativeprobability

Observed data

Extreme value distribution


40 60 80 100 120 140 160 180 200 2200

0.1

0.2

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0.4

0.5

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0.9

1

Cu

mulativeprobability

Observed data

Weibull distribution


Figure 20: Fits for various dist ributions to R. Mayanja data. Clockwise startingfrom the upper left corner are fits for Normal, Lognormal, Extreme value and

Weibull dis tributions respectively

The Weibull and lognormal distributions fit the measured data better than the otherdistributions (Table 8). The normal distribution estimates are higher and seem moreconsistent with estimates using the TRRL method. The lognormal distribution has beenused to make the flood estimates for Kaguta bridge site.

Table 8: Flood flow estimates at the gauging s ite for the candidate distribut ions

T (Years) QT (m3/s) for each of the candidate distributions

Normal Lognormal Extreme Value Weibul l

10 196.4 209.9 203.0 199.0

20 218.8 257.6 219.9 225.9

50 244.0 324.5 237.0 256.8

100 260.8 378.4 247.5 277.8

Estimates of the design flood at the bridge site were made using the flood transpositionmethod. The bridge site commands a catchment area of 4,667 km2 while the gauge sitecommands and area of 5,002 km2. Therefore, the ratio of the area of the bridge sitecatchment to the gauging site catchment was computed as 0.93. The estimates of thedesign flood at Kaguta bridge site are shown in Table 9.


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Table 9: Design f lows at Kaguta bridge site

T (Years) QT (m3/s)

Flow Gauge Kaguta Bridge

10 209.9 195.8

20 257.6 240.450 324.5 302.8

100 378.4 353.1

TRRL method

In the TRRL method, use was made of results from direct analysis of observed intensitydata from various parts of East Africa to derive the design rainfall storms. The methodinvolved initially setting the runoff coefficient (percentage of rainfall that is converted torunoff) using factors like land use, catchment slope class, soil class, surface cover andcatchment wetness factor. The hydrograph base time was then estimated from

catchment area and slope class. The base time can be thought of as being made of 3components viz. the storm duration, time taken for the surface runoff to drain into thestream system, and the flow time down the stream and river system to the bridge site.The mean 24 hour rainfall (also called the 2-year, 24 hour rainfall) was estimated from astorm rainfall map of East Africa and found to be between 60 and 70 mm in most parts ofUganda. Factors of 1.49, 1.74, 1.95 and 2.2 were then applied to derive the designstorm having return period of 10, 25, 50 and 100 years respectively. Table 10 shows thecomputed design storms for the four catchments.

Table 10: Design storms for dif ferent return periods for Kaguta site

T (years) Design storm (mm)

10 114.8

25 129.5

50 154.0

100 175.0

Each of the above design rainfall were adjusted by applying 2 factors

a) An area reduction factor to take into account the variability of rainfall in spaceb) A rainfall ratio to take into account the movement of the design storm in time.

The average and peak flow during base time for each return period was computed usingequations 1 and 2 given above and applying a peak factor of 2.5 which applies for humidregions.Table 11 shows the design floods that were obtained for each bridge site using theTRRL model. The bank-full flows were computed using Mannings equation of frictionflow to check whether occurrence of the design floods would cause the rivers to bust


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their banks. From the values of the bank-full flows it is clear that the design floods canbe carried within the river banks.

Table 11: Design floods for the Kaguta bridge site before adjusting for s torage

T (years) Peak flood (m3/s)

10 246.0

25 277.5

50 330.0

100 375.0

The above design floods should be adjusted to take into account the effect of storagewithin the basin and catchment shape. The river basin has limited swamps that provideminimal storage. The peak flows are therefore greatly attenuated. About 5% of the basinis filled with papyrus swamps. This is estimated to result in a 5% attenuation of the peak

flows and the flow values were adjusted accordingly. The length of the catchment is 123km while the width is 42 km giving a ratio of 2.5 which is within the range of 2-6 that isassumed in the derivation of the TRRL approach and therefore no further adjustmentswere carried out. Table 12 shows the final estimates of flood flows

Table 12: Design floods for the Kaguta bridge site after adjusting for storage

T (years) Adjusted Peak Flood(m3/s)

10 233.7

25 263.6

50 313.5

100 356.3

Bridge Design Results

The proposed bridge is a 3 span bridge with vertical abutments (Figure 21). The endspans (between abutments and first pier from each bank) are 10 m wide. The internalspan (pier to pier) is 10 m wide. The total bridge width is 25 m while the effective flowarea is 135 m2. Each of the 2 piers is of the round-nose type and is 0.5m wide. Thebridge deck high chord is 1001.5m high, while the deck low chord is 1000.6m. The invert

level at lowest level of the river is 993.7m giving a clear height of 5.9m. The 50-yearflood level is 998.5m giving a free board of 2.1 m.


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0 20 40 60 80 100 120 140 160993

994

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996

997

998

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1000

1001

Kaguta RC Bridge Plan: Plan 05 3/2/2010

Station (m)

Elevation(m)

Legend

EG 50yr

WS 5

Draft Engineering Report for Bridge Design.pdf

Documents

Transcript of Draft Engineering Report for Bridge Design.pdf