Lugogo/Ngoma Bridge

Hydrological Assessment and Hydraulic design report

1

Table of Contents

List of Figures ................................................................................................................................ 3

List of Tables .................................................................................................................................. 5

1.0 Background. ....................................................................................................................... 6

1.1 Introduction ................................................................................................................................. 6

1.2 Scope of Services ........................................................................................................................ 6

1.3 Description of the bridge site ....................................................................................................... 7

2.0 Analysis Methodology ........................................................................................................ 9

2.1 Hydrologic assessment ................................................................................................................ 9

2.1.1 Data collection and inventory of the existing structures, if any ............................................................. 9 2.1.2 Flood analysis methods .......................................................................................................................... 9

2.1.3 The flood frequency analysis methodology ......................................................................................... 10

2.2 Hydraulic Design ....................................................................................................................... 11

2.2.1 Introduction.......................................................................................................................................... 11 2.2.2 Bridge types considered ....................................................................................................................... 11 2.2.3 Evaluation and selection ...................................................................................................................... 12

2.2.4 Documentation of design ..................................................................................................................... 13

3.0 Hydrological assessment .................................................................................................. 14

3.1 Catchment characteristics .......................................................................................................... 14

3.1.1 Landscape and Drainage ...................................................................................................................... 14 3.1.2 Land cover ........................................................................................................................................... 14

3.1.3 Geology and soils ................................................................................................................................ 14 3.1.4 Climate ................................................................................................................................................. 19

3.2 River flow derivation ................................................................................................................. 19

3.2.1 Approach to flow derivation ................................................................................................................ 19 3.2.2 Lugogo flow characteristics ................................................................................................................. 20 3.2.3 Annual maximum flow series .............................................................................................................. 21

3.2.4 Distribution fitting ............................................................................................................................... 22 3.2.5 Peak flows ............................................................................................................................................ 23

4.0 Hydraulic design .............................................................................................................. 24

4.1 Proposed bridge configurations ................................................................................................. 24

4.2 Bridge design results ................................................................................................................. 25

4.3 Assessment of scour .................................................................................................................. 29

4.4 Bridge deck elevation ................................................................................................................ 32

4.5 Bridge deck drainage ................................................................................................................. 33

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4.5.1 Estimation of deck peak flows ............................................................................................................. 33

4.5.2 Sizing of the deck kerb inlets ............................................................................................................... 33

APPENDIX A: HYDRAULIC ANALYSIS METHODOLOGY ................................................. 35

Hydraulic Design Criteria ....................................................................................................................... 35

The HEC-RAS River Analysis System .............................................................................................................. 35 Theoretical basis for the hydraulic analysis ....................................................................................................... 36

Computation procedure ...................................................................................................................................... 39 Bridge modelling guidelines .............................................................................................................................. 39

Culvert design approach guidelines ........................................................................................................ 41

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LIST OF FIGURES

Figure 1: Location map of the proposed Lugogo Bridge .............................................................................. 7

Figure 2: Boats that a currently used for river crossing ................................................................................ 8

Figure 3: view of papyrus plants ................................................................................................................... 8

Figure 5: Landscape and drainage .............................................................................................................. 15

Figure 6: Land cover types ......................................................................................................................... 16

Figure 7: Catchment geology ...................................................................................................................... 17

Figure 8: Soil types ..................................................................................................................................... 18

Figure 9: Monthly rainfall and evaporation variation (source: Hydroclimatic study (2001)) ..................... 19

Figure 10: Daily flow variation ................................................................................................................... 20

Figure 11: Mean monthly flow variation .................................................................................................... 21

Figure 12: Annual maximum flows for R. Lugogo .................................................................................... 22

Figure 13: Fit for lognormal distribution to R. Lugogo data. ..................................................................... 22

Figure 14: Peak flow variation with return period for lognormal distribution ............................................ 23

Figure 15: Option 1 - Concrete bridge configuration showing the 100-year flood level ............................ 26

Figure 16: Option 2- Composite bridge configuration showing the 100-year flood level .......................... 26

Figure 16: Option 3 - post conditioned, prestressed bridge configuration showing the 100-year flood level

.................................................................................................................................................................... 27

Figure 16: Option 4 - minimal constriction bridge configuration showing the 100-year flood level ......... 27

Figure 17: Scour conditions for Option 1- concrete bridge for the 100-year flood conditions ................... 30

Figure 18: Scour conditions for the Option 2 - composite bridge for the 100-year flood conditions ......... 31

Figure 18: Scour conditions for the Option 3 - post tensioned prestressed bridge for the 100-year flood

conditions .................................................................................................................................................... 31

Figure 18: Scour conditions for the Option 2 - minimal constriction bridge for the 100-year flood

conditions .................................................................................................................................................... 32

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Figure 19: Cross section locations at bridge ............................................................................................... 40

Figure 20: Typical culvert crossing (right: energy and hydraulic grade line for a full flowing culvert) .... 42

5

LIST OF TABLES

Table 1: Flow statistics for R. Mayanja and R. Lugogo (m3/s) .................................................................. 20

Table 2: Flood flow estimates at the gauging site for the candidate distributions ...................................... 23

Table 3: Bridge configurations ................................................................................................................... 25

Table 4: Comparison of flow conditions for the two bridge options .......................................................... 25

Table 5: Flow conditions for Option 1 - concrete bridge for 100 year flood .............................................. 27

Table 6: Flow conditions for Option 2 - composite bridge for 100 year flood ........................................... 28

Table 6: Flow conditions for Option 3 - post conditioned, prestressed bridge for 100 year flood ............. 28

Table 6: Flow conditions for Option 4 - minimal constriction bridge for 100 year flood .......................... 29

Table 7: Scour assessment .......................................................................................................................... 30

Table 8: Computation of deck soffit elevation ............................................................................................ 32

Table 9: Computation of design floods for bridge deck ............................................................................. 33

Table 10: Kerb inlet sizing .......................................................................................................................... 34

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1.0 BACKGROUND.

1.1 Introduction

The proposed Lugogo Bridge will be located along the Ngoma - Kasozi - Kyamukonda Road,

7.1 Kms from Kasozi . This is currently an earth road that links the Ngoma Trading Centre and

Kyamukonda Trading Centre via Kasozi Trading Centre with the swamp section between Ngoma

and Kasozi currently being crossed by canoe (Figure 1).

The client, Uganda National Roads Authority, procured the services of MBW Consulting to

carry out Feasibility Study, Detailed Design and Tender Documentation of the proposed Lugogo

Bridge on Ngoma - Kasozi - Kyamukonda Road. Under the terms of reference of the consultancy

services, the hydrologist is required to carry out drainage investigations to carry out hydrological

assessments and hydraulic sizing of the bridge. This report reviews the procedure that was

adopted in the hydrologic analysis and hydraulic design of the bridges, assesses the data

collected during the field study, analyses the hydrological characteristics, presents the alternative

bridge configurations and recommends the best designs.

The objective of the drainage design was to undertake document review, as well as field

investigations to enable the Consultant to carry out a hydrological and hydraulic assessment for

the proposed Lugogo bridge and approach roads. The hydraulic conditions have been carefully

checked against the results of the drainage field investigations and discharge/velocity

calculations. Possible scour effects were evaluated and taken into consideration when selecting

the type and size of bridge structure.

This design report is based on recommendations of the Uganda Road Design Manual, especially

Vol. 2: Drainage Design Manual, supplemented by other internationally acceptable standards

concerning hydraulic design of bridges.

1.2 Scope of Services

The Consultants scope of services as per the Terms of Reference and our proposal includes:

1) To carry out Feasibility study in the project area to identify the best location for the

bridge;

2) To carry out Preliminary design considering the findings from the geotechnical

investigations, hydrological investigations and topographic surveys;

3) To carry out Detailed Design of the superstructure and substructure considering the

recommended loadings for bridges;

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4) To prepare Tender Documentation.

Figure 1: Location map of the proposed Lugogo Bridge

1.3 Description of the bridge site

The proposed bridge crossing is marks the boundary between Nakasongola and Nakaseke

districts connecting trading centres of Kyamukonda and Ngoma, respectively (Figure 1). The

crossing is composed of a swamp section that is at least 1060 m with a middle faster flowing

section that is 300 m wide (Figure 2). The river is relatively shallow at about 1.3-1.7 m deep and

carries considerable floating plants like papyrus (Figure 3). River Lugogo is one of the tributaries

of River Kafu and drains a catchment of about 2,722 km2. The river flows in a north-westerly

direction for a distance of over 90 km, starting near Bombo Town to the confluence with R.

Kafu.

8

Figure 2: Boats that a currently used for river crossing

Figure 3: view of papyrus plants

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2.0 ANALYSIS METHODOLOGY

The following methodology was adopted for the hydrological analysis and hydraulic design of

the bridge;

2.1 Hydrologic assessment

2.1.1 Data collection and inventory of the existing structures, if any

This involved field studies as well as review of the existing designs and assessment reports. Field

studies were carried out to obtain site data like 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 were also collected from the

respective agencies. Collection of historical flood data including high water marks, river cross-

sections (upstream, downstream and at bridge site), existing activities and manmade features in

the flood plain. Evidence of bridge overtopping and scour was also be collected. Use was made

of existing reports, the MoW road design manual, maps, drawings and such other documents.

Field visits were also carried out for on-site assessments of the sections.

2.1.2 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 data are available, extreme

flood conditions may be such that no flow measurements can be taken and estimates have to be

made (i.e. extrapolation of rating curves). Careful consideration of the available data is important

before selecting the analysis method. In order of preference, Watkins and Fiddes (1984)

recommends the following methods for estimating design floods:

1) Methods based on analyzing flow data i.e. Extreme value analysis, Flood

transposition, Slope-area method, Bank full flows

2) Regional flood formulae like envelope curves

3) Rainfall runoff models i.e. the rational method, unit hydrograph techniques and

synthetic hydrograph

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

ORSTOM method (developed in West Africa), TRRL method (based on 14

catchments in Kenya and Uganda), the SCS curve number method and the generalized

tropical flood model.

The choice between these methods depends on whether the detailed shape of the flood or the

probable maximum flood is needed and on availability of the reliable flow records at the design

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site or nearby sites, whether on the same river or some other catchment. It also depends on

availability of suitable data.

While River Lugogo does not have a long term flow gauge, the nearby River Mayanja has one. It

was noted that the two rivers have significant similarities including drainage patterns, land-use

and land cover. They have similar climate conditions, extensive swamp storage and flow in a

north-westerly direction from densely populated areas of central Uganda through more rural

terrain. In addition, their catchment areas are characteristics like slope and elevation ranges are

also quite similar. Therefore a flow transposition technique was adopted for determining the flow

of River Lugogo. In this method, it is assumed that the catchment characteristics for the

catchment contributing the two (gauging station and bridge site) do not vary considerably and the

flood generation mechanisms are similar. In this case, the flows at the two sites are proportional

to the areas of their catchments. Therefore, the flow at the bridge site was simply estimated as

the flow at the gauging site multiplied by the ratio of the two areas.

2.1.3 The flood frequency analysis methodology

The methodology used for estimating the design flood for different recurrence intervals using

statistical analysis of extremes was as follows:

a) From a record of daily historical flows, the annual maximum values (the maximum daily

flow for each year) were selected

b) From a number of candidate statistical distributions the distribution that best fits the

annual maximum flows was selected. Four candidate distributions were selected for the

current study namely: Normal, Lognormal, Extreme Value and Weibull distributions. The

lognormal distribution was used for estimating the flood quantiles as it has shown better

performance for Ugandan rivers in the past.

c) The parameters of the lognormal distribution were estimated and the growth curve derived

d) The flood flows corresponding to the set return periods at the gauging stations were

estimated

e) 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 channels and available storage (in

lakes and swamps). The factor can vary between 1 and 2.5 for large catchments. For

smaller catchments, a much higher factor may be needed

Hydrological analysis involved determination of discharges with different return periods for each

site on the basis of which performance of alternative bridge designs were evaluated.

Determination of the discharges was based on the following procedure

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a) Estimation of catchment area, rainfall, catchment slope, river flow, cross-sectional area,

roughnesses (in river and flood plain) etc. To obtain 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 Q2, Q50, Q100, Q200, and Q500.

c) Based on recommendations in the Road Design Manual (2010), Vol 2: Drainage Design

Manual, the bridge was designed to pass the 100 year flood (Q100). The proposed bridge

design was then crosschecked against failure resulting from the 200-year flood (Q200)

while scour conditions were estimated for the 500 year flood (Q500).

d) Using the survey data the elevations corresponding to each of the above discharges were

computed

2.2 Hydraulic Design

2.2.1 Introduction

This was an iterative process involving an evaluation of the alternative designs, selection of the

most appropriate and refinement to suit conditions on ground. The HEC-RAS software used to

carry out hydraulic analysis of alternatives. Details of the HEC-RAS analysis procedure are

shown in Appendix A. Selection was made on the basis of assessing the suitability of the design

for given conditions of flooding.

2.2.2 Bridge types considered

The bridge material will depend on the considerations of the structural and materials engineers.

However, for purposes of the hydraulic design, it was assumed that the bridge will be made of a

suspended slab supported on concrete piers between the end embankments. Two slab options

were considered

1) A reinforced concrete deck (Option 1):- The deck also serves as the road surface. In this

case, the bridge opening will be 150 m wide and the maximum pier span was restricted to

a maximum of 12.5 m because of the considerable self-weight of concrete.

2) Composite bridge (Option 2): Including steel girders to span the piers with a concrete

slab to act as the road surface. The reduced weight of steel girders will allow for larger

pier spans of up to 20 m. Increased spacing between piers will result in less flow

disruption. The bridge opening will be 150 m wide.

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3) Post-tensioned prestressed concrete bridge (Option 3): In this cases longer spacing

between piers of up to 30 m can be achieved with the benefit of reduced flow disruption

and fewer numbers of piers. The bridge opening will be 150 m wide.

4) Wider opening with minimal flow constriction (Option 4): This option will be similar to

Option 3 but with a bridge opening of 600 m width. This option will result in reduced

impacts on ecosystem biodiversity between the upstream and downstream because a

wider opening will maintain better mobility of flora and fauna.

To reduce the impacts of high levels of constriction and also provide for some additional flows,

Options 1 to 3 will also include provision of relief culverts on either side of the bridge.

2.2.3 Evaluation and selection

The approach to hydraulic comparison in detailed in Appendix A (Hydraulic Analysis

Methodology). The selection of a best alternative was accomplished by comparison of the

study results and considerations to acceptable limitations and controls. Best alternative means the

bridge configuration that meets all or most of the following criteria.

1) Backwater will not significantly increase flood damage to property upstream of the

crossing. Backwater and/or increases over existing condition up to 0.5 m during the

passage of the 100-year flood, if practicable

2) Velocities through the structure(s) will not damage the highway facility or unduly

increase damages to adjacent property.

3) Existing flow distribution is maintained to the extent practicable.

4) Level of traffic service is compatible with that commonly expected of the class of

highway and projected traffic volumes.

5) Pier spacing, orientation, and abutment are to be designed to minimize flow

disruption and potential scour.

6) Selection of foundation design and/or scour countermeasures to avoid failure by

scour. The design for the bridge foundation scour was made considering the 100 year

flood magnitude which generates the maximum scour depth. The resulting design was

then checked using a superflood that is 1.7 times the magnitude of the 1% event (i.e.

the 500 year flood).

7) Freeboard at structure(s) designed to pass anticipated debris. The MoWT Drainage

Design Manual requires that the minimum freeboard should be 250 mm. However,

the river carried significant amounts of floating plants and these would have to be

safely transmitted during flooding. Therefore the freeboard for the proposed bridge

was set to 1000 mm. However, a minimum clearance below the bridge of 1500 mm

13

was made to allow for passage of small boats during normal high flow conditions (i.e.

the 2-year flood events)

8) Minimal disruption of ecosystems and values unique to the floodplain and stream.

9) Cost for construction, maintenance and operation, including probable repair and

reconstruction, and potential liabilities are affordable.

10) Pier and abutment location, spacing, and orientation are such to minimize flow

disruption, debris collection and scour.

11) Proposal is consistent with the intent of the standards and criteria of the Ministry of

Works guidelines.

2.2.4 Documentation of design

All information pertinent to the selection of the "best" alternate was documented in a report

including all computations (design floods, scour, sizing, etc).

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3.0 HYDROLOGICAL ASSESSMENT

3.1 Catchment characteristics

3.1.1 Landscape and Drainage

The catchment upstream of the bridge site has a total area of 2722 km2 (Figure 4). The

catchment has an elongated shape with a length of 114 km (in the north-westerly direction) and a

maximum width of 32 km.

The upstream of the catchment is characterised by a rolling terrain with numerous hills drained

by wide valleys (Figure 5). The areas close to the bridge site are generally flat and swampy. The

elevation varies between 1051 masl and 1334 masl. The land slopes are generally low, varying

between 6% in the upper reach and 2% in the lower reach. The average slope is 3.5% while the

channel slope is 0.07%.

3.1.2 Land cover

The upper reach consists mainly of subsistence farmlands while the middle and lower reach is

dominated by woodland. and grasslands (Figure 5). Small scale agriculture is the dominant

activity in the upstream areas while livestock rearing is the dominant activity in the lower

reaches. The river flood plains are dominated by permanent papyrus swamps which provide

extensive storage of flood water thereby providing some attenuation of the peak flows.

3.1.3 Geology and soils

Most of the catchment is mainly made up of undifferentiated basement system gneisses (Figure

6). The upper reach have some indurate mudstone (argillite). The lower reach is made up of

unconsolidated material which is eroded from the upstream areas and deposited due to reduction

in channel slopes. The soils range from clayey mixtures to sandy loams. The valleys are filled

with clayey mixtures (Figure 7).

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Figure 4: Landscape and drainage

16

Figure 5: Land cover types

17

Figure 6: Catchment geology

18

Figure 7: Soil types

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3.1.4 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 spread over 2 rainy

seasons: The long rains of March to May and the short rains of September to November (Figure

8). During the dry months, evaporation can be very high (in the order of 5 times the rainfall).

Figure 8: Monthly rainfall and evaporation variation (source: Hydroclimatic study (2001))

3.2 River flow derivation

3.2.1 Approach to flow derivation

The daily flows for R. Lugogo were derived from flows in R. Mayanja using the flow

transposition method described in Section 2.1.2. The flow gauge on River Mayanja is located at

Kapeeka-Kakungu Road. The gauge has 14 years of record covering the period 1997-2010. The

catchment area of R. Mayanja upstream of the flow gauge is 2297 km2. The catchment area of R.

Lugogo upstream of the bridge site is 2722 km2 or 1.19 times the area cachment area of the R.

Mayanja gauge. Therefore, assuming that the catchment characteristics for the catchment

contributing the two (gauging station and bridge site) do not vary considerably and the that the

flood generation mechanisms are similar, the flow at the Lugogo bridge site was estimated as

1.19 times that at the R. Mayanja Gauge. Table 1 shows a comparison of the flow statistics at the

two sites.

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Table 1: Flow statistics for R. Mayanja and R. Lugogo (m3/s)

Statistic R. Mayanja R. Lugogo

Mean 8.95 10.61

Median 4.73 5.61

Standard Deviation 9.4 11.15

Minimum 1.2 1.42

Maximum 52.86 62.47

3.2.2 Lugogo flow characteristics

The historical flow varied between 1.4 m3/s and 62.5 m

3/s with a mean daily flow of 10.61 m

3/s

(Table 1 and Figure 9). The extensive swamp storage attenuates this flood magnitude quite

significantly. The flow is negatively skewed with a median flow of 5.6 m3/s.

Figure 9: Daily flow variation

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Figure 10: Mean monthly flow variation

The monthly flow variation has two low flow periods; one in July with a mean flow of 4.2 m3/s

and a second in March with a mean flow of 5.2 m3/s (Figure 10). There are also two high flow

periods; one period is in May with a mean flow of 14.2 m3/s and a second high peak is in

November with a mean flow of 23.7 m3/s.

3.2.3 Annual maximum flow series

Figure 11 shows the extract of annual maximum daily flows. The annual maximum flows range

from 12.2 m3/s in 2000 to 62.6 m

3/s in 2006. The extensive swamp storage attenuates this flood

magnitude quite significantly. Therefore, when compared with the catchment area, the flood

magnitudes are quite low. For 11 years out of the 13 years of record the annual maximum flows

occur in October and November during the second rainy season that lasts from September to

December though it can sometimes extend to January.

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Figure 11: Annual maximum flows for R. Lugogo

3.2.4 Distribution fitting

The lognormal distribution tends to be the most robust for flood frequency analysis in areas of

central Uganda and was selected for the proposed bridge site. The fit for the annual maximum

data to a lognormal distribution is generally acceptable (Figure 12). There is some clustering of

the annual maximum flows around the middle of the plot. This is probably because the data

length is relatively short (13 years) but this was not expected to grossly affect the results.

Figure 12: Fit for lognormal distribution to R. Lugogo data.

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3.2.5 Peak flows

Estimates for the lognormal distribution are shown in Table 2 while the variation of peak flow

with return period is shown in Figure 13. The analysis was based on daily flows which average

out the sub-daily variations. A peak flow factor is therefore required to convert the daily peak

flows (column 2 of Table 2) to the design peak flow (column 4). The peak flow factor varies

between 1 and 2.5 depending on the peakedness of the flood hydrograph. As explained above,

swamp storage attenuates peak flows in R. Lugogo and a peak flow factor of 1.75 was selected

Table 2: Flood flow estimates at the gauging site for the candidate distributions

Return

period, T

(years)

Lognormal

peak flow

(m3/s)

Peak

flow

factor

Design flow,

QT (m3/s)

2 37.8 1.75 66.1

25 78.6 1.75 137.6

50 89.3 1.75 156.2

100 100.1 1.75 175.1

200 111.1 1.75 194.4

500 126.1 1.75 220.6

Figure 13: Peak flow variation with return period for lognormal distribution

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4.0 HYDRAULIC DESIGN

4.1 Proposed bridge configurations

The bridge has been designed as a suspended slab supported on concrete piers between the end

abutments. The bridge will have an opening of 150 m between the abutments. As explained in

Section 2.2.2, 4 types of bridge configurations have been evaluated.

1) Option 1 - will be a reinforced concrete deck with piers spaced at 12.5 m centres. The

bridge opening will be 150 m wide. The piers will have rounded shapes for smooth

hydraulic inlet and outlet conditions. The end spans (between abutments and first pier

from each bank) will be also be 12.5 m wide. Four relief culverts are provided with 2 on

either side of the bridge cantered at 100 m intervals. Each relief culvert will have a span

of 3.5 m and rise of 1.5 m.

2) Option 2 - will be of composite type including steel girders to span the piers with a

concrete slab to act as the road surface. The bridge opening will be 150 m wide. The

internal piers will be spaced at 20 m centres and will also have rounded end shapes. The

end spans (between abutments and first pier from each bank) will be 15 m wide. Four

relief culverts are provided with 2 on either side of the bridge cantered at 100 m intervals.

Each relief culvert will have a span of 3.5 m and rise of 1.5 m.

3) Option 3 - will be a post-tensioned prestressed concrete bridge which will allow for

longer spacing between piers of up to 30 m. The bridge opening will be 150 m wide. The

end spans (between abutments and first pier from each bank) will be 30 m wide. Four

relief culverts are provided with 2 on either side of the bridge cantered at 100 m intervals.

Each relief culvert will have a span of 3.5 m and rise of 1.5 m.

4) Option 4 - will be similar to Option 3 but with a bridge opening of 600 m width for

reduced flow constriction. The wider opening will result in fewer impacts on ecosystems

in the area.

Table 3 summarises the different bridge configurations. The free board has been set to a

minimum of 2000 mm which is higher than the minimum of 250 mm set in the Drainage Design

Manual. The higher value will allow for easy transmission of the considerable floating material

in the river, especially during flooding.

Assessment of the performance of the two bridge options was carried out by considering the

maximum backwater recommendations and the scour conditions. The drainage design manual

sets the backwater to a maximum of 0.5 m where possible. Given the low ground slopes of in the

25

vicinity of the bridge, the maximum backwater was set to 0.35 for the 100 year flow but was

crosschecked against the 500 year flood to ensure that it doesn't exceed the 0.5 m limit.

Table 3: Bridge configurations

Bridge

Option

Abutment

chainages Pier spacing Relief culverts chainages

Left Right Middle Abutment to pier No 1 No 2 No 3 No 4

Option 1 945 1095 12.5 12.5 745 845 1195 1295

Option 2 945 1095 20 15 745 845 1195 1295

Option 3 945 1095 30 30 745 845 1195 1295

Option 4 730 1330 30 30 - - - -

4.2 Bridge design results

The water levels for the proposed bridge options are shown in Figure 14 to Figure 17 while

Table 5 to Table 8 show the backwater computations. All bridge options satisfy the backwater

requirements for the 100 year and 500 year flood magnitudes. Option 4 will result in the lowest

backwater increases while option 1 will result in the highest increase (Table 4). For Options 1-3,

the downstream velocities will be 1.69 m/s, 1.67 m/s and 1.66 m/s respectively which is higher

than the recommended maximum of 1.2 m/s in the clay and peat that underlies the river.

Therefore bed protection will be required. It is proposed that a layer of riprap of diameter 450

mm be placed on the channel bed for a distance of at least 15 m on the upstream side and 30 m

on the downstream side of the bridge. For Option 4, the downstream velocity will be 0.58 m/s

which is less than the recommended maximum of 1.2 m/s in the clay and peat that underlies the

river. Therefore, no bed protection is required for Option 4.

Table 4: Comparison of flow conditions for the two bridge options

Design Option 100 year flood 500 year flood

WS elevation backwater WS

elevation

backwater

Without bridge 1052.39 1052.47

Option 1 - concrete bridge 1052.58 0.19 1052.7 0.23

Option 2 - composite bridge 1052.57 0.18 1052.69 0.22

Option 3 - post-tensioned and

prestressed bridge

1052.56 0.17 1052.68 0.21

Option 4 - minimal constriction 1052.4 0.01 1052.47 0

26

Figure 14: Option 1 - Concrete bridge configuration showing the 100-year flood level

Figure 15: Option 2- Composite bridge configuration showing the 100-year flood level

800 1000 1200 14001051

1052

1053

1054

1055

lugogo_new Plan: Option1Concrete 7/13/2015

Station (m)

Ele

vation

(m

)

Legend

EG 100 yrs

WS 100 yrs

Crit 100 yrs

Ground

Bank Sta

.07 .04

800 1000 1200 1400

1052

1053

1054

1055

lugogo_new Plan: option2_composite 7/13/2015

Station (m)

Ele

vation

(m

)

Legend

EG 100 yrs

WS 100 yrs

Crit 100 yrs

Ground

Bank Sta

.07

.04 .07

27

Figure 16: Option 3 - post conditioned, prestressed bridge configuration showing the 100-year flood level

Figure 17: Option 4 - minimal constriction bridge configuration showing the 100-year flood level

Table 5: Flow conditions for Option 1 - concrete bridge for 100 year flood

800 1000 1200 1400

1052

1053

1054

1055

lugogo_new Plan: option3_posttensioned 7/13/2015

Station (m)

Ele

vation

(m

)

Legend

EG 100 yrs

WS 100 yrs

Crit 100 yrs

Ground

Bank Sta

.07

.04

600 800 1000 1200 1400

1051

1052

1053

1054

1055

lugogo_new Plan: option4_minimal_constriction 7/13/2015

Station (m)

Ele

vation

(m

)

Legend

EG 100 yrs

WS 100 yrs

Crit 100 yrs

Ground

Bank Sta

.07 .04 .07

28

Table 6: Flow conditions for Option 2 - composite bridge for 100 year flood

Table 7: Flow conditions for Option 3 - post conditioned, prestressed bridge for 100 year flood

29

Table 8: Flow conditions for Option 4 - minimal constriction bridge for 100 year flood

4.3 Assessment of scour

Results of the assessment of the scour conditions around the abutments using the 500 year flood

for both bridge options are shown in Table 9 while the individual results are shown in Figure 18

for the concrete bridge and Figure 19 for the composite bridge. Maximum scour will be

experienced around the abutments, especially the left abutment on Ngoma side. However, the

30

pier scour is also quite considerable. Protection of the piers and abutments against scour will be

required. Contraction scour is also considerable for Options 1-3.

Table 9: Scour assessment

Scour condition Option 1 -

Concrete

bridge

Option 2 -

Composite

bridge

Option 3 - post-

tensioned,

prestressed

bridge

Option 4 -

minima

constriction

Contraction scour 1.01 0.99 0.97 0

Pier scour 0.69 0.7 0.7 0.78

Left abutment scour 3.47 3.47 3.44 2.14

right abutment scour 3.29 3.26 3.23 1.14

Combined scour

Pier scour + Contraction scour 1.7 1.69 1.67 0.78

Left abutment scour + Contraction scour 4.48 4.46 4.41 2.14

Right abutment scour + Contraction scour 4.3 4.25 4.2 1.14

Figure 18: Scour conditions for Option 1- concrete bridge for the 100-year flood conditions

900 1000 1100 1200

1049

1050

1051

1052

1053

1054

1055

Bridge Scour RS = 10050

Station (m)

Ele

vation

(m

)

Legend

WS 500 yrs

Ground

Bank Sta

Contr Scour

Total Scour

31

Figure 19: Scour conditions for the Option 2 - composite bridge for the 100-year flood conditions

Figure 20: Scour conditions for the Option 3 - post tensioned prestressed bridge for the 100-year flood conditions

800 900 1000 1100 1200 1300

1049

1050

1051

1052

1053

1054

1055

Bridge Scour RS = 10050

Station (m)

Ele

vation

(m

)

Legend

WS 500 yrs

Ground

Bank Sta

Contr Scour

Total Scour

800 900 1000 1100 12001049

1050

1051

1052

1053

1054

1055

Bridge Scour RS = 10050

Station (m)

Ele

vation

(m

)

Legend

WS 500 yrs

Ground

Bank Sta

Contr Scour

Total Scour

32

Figure 21: Scour conditions for the Option 2 - minimal constriction bridge for the 100-year flood conditions

4.4 Bridge deck elevation

The bridge deck elevation was set as the elevation that meets the requirements for freeboard (2.0

m under the design flood condition - 100 years to allow for debris passage) and the allowance for

passage of small boats (1.5 m under the typical flood condition - 2 years). Table 10 shows the

minimum bridge soffit elevation.

Table 10: Computation of deck soffit elevation

Bridge type

Debris passage Boat passage

Proposed

minimum

soffit

level (m)

100-year

Flood

Water

Surface

Elevation

(m)

Required

Freeboard

(m)

Required

Soffit

level (m)

2-year

Flood

Water

Surface

Elevation

(m)

Required

allowance

for boat

passage

(m)

Required

Soffit

level (m)

Option 1 - concrete

bridge 1052.58 2.00 1054.58 1052.21 1.50 1053.71 1054.58

Option 2 - composite

bridge 1052.57 2.00 1054.57 1052.2 1.50 1053.7 1054.57

Option 3 - post-

tensioned and

prestressed bridge 1052.56 2.00 1054.56 1052.2 1.50 1053.7 1054.56

Option 4 - minimal

constriction 1052.4 2.00 1054.4 1052.13 1.50 1053.63 1054.4

600 800 1000 1200 1400

1050

1051

1052

1053

1054

1055

Bridge Scour RS = 10050

Station (m)

Ele

vation

(m

)

Legend

WS 500 yrs

Ground

Bank Sta

Contr Scour

Total Scour

33

4.5 Bridge deck drainage

The design of the drainage for the bridge deck was aimed at quick removal of water from the

bridge and reduction of the risk of hydroplaning.

4.5.1 Estimation of deck peak flows

Table 11 shows the computation of the peak flows for the bridge deck. The sizing of kerb inlets

was based on the 10-year peak flows.

Table 11: Computation of design floods for bridge deck

Parameter Description Abrev. Return period (n)

2 yrs 5 yrs 10 yrs 25 yrs

Area km2 A 0.005 0.005 0.005 0.005

Catchment slope Average Sr 3.0% 3.0% 3.0% 3.0%

Slope class Table 7.11 S 3 3 3 3

Surface cover flow time Swamp-filled valley (Table

7.16)

Ts (hr) 0 0 0 0

Soil class Fairly permeable (Table 7.10) I 3 3 3 3

Basic runoff coeficient Table 7.12 Cs 41% 41% 41% 41%

Land use factor Dense vegetation - Table 7.14 CL 1.5 1.5 1.5 1.5

Catchment wetness factor Dry zone, perennial streams -

Table 7.15

Cw 0.75 0.75 0.75 0.75

Percentage of runoff Equation 7.22 Ca 46% 46% 46% 46%

Base time Equation 7.29 TB 0.2 0.2 0.2 0.2

2yr, 24 hr rainfall millimeters - Figure 3.6 65 65 65 65

10:2 year ratio Table 3.6 1.64 1.64 1.64 1.64

Return period 2 5 10 25

n:2 year ratio Figure 3.11 0.67 0.86 1.00 1.20

Constant b Table 4.6 b 0.3 0.3 0.3 0.3

Constant n Table 4.5 n 0.95 0.95 0.95 0.95

Area reduction factor Equation 4.11 ARF 0.99 0.99 0.99 0.99

Rainfall ratio Equation 4.3 RR 0.35 0.35 0.35 0.35

n-yr 24-hr storm depth 15.0 19.3 22.6 26.9

Average flow during base

time

Equation 7.31 0.04 0.05 0.06 0.07

Peak factor Humid zone - Table 7.17 2.5 2.5 2.5 2.5

n-yr peak flow m3/s 0.10 0.12 0.15 0.17

4.5.2 Sizing of the deck kerb inlets

Rectangular kerb inlets of width 300 mm and depth 120 mm were considered. Table 12 shows

the sizing of the kerb inlets for all 4 bridge options.

34

Table 12: Kerb inlet sizing

Parameter Options 1-3 Option 4

Bridge length (m) 150 600

Inlet type Rectangular Rectangular

Inlet width (mm) 300 300

Inlet height (mm) 120 120

Flow condition Weir Weir

Weir coefficient 1.7 1.7

Weir condition No

depression

No

depression

Inlet discharge (m3/s) 0.018 0.018

Total deck flow (m3/s) - 10 year

flood

0.15 0.17

Required number of kerb inlets 10 10

Inlet spacing on either side of deck

(m)

7.5 30.0

35

APPENDIX A: HYDRAULIC ANALYSIS METHODOLOGY

Hydraulic Design Criteria

The HEC-RAS River Analysis System

Introduction

Flow analysis and bridge design were carried out using the HEC-RAS River Analysis System

developed by the US Army Corps of Engineers Hydrologic Engineering Centre. The software

has been widely used in different countries for hydraulic analysis and design of hydraulic

structures including bridges and culverts. It consists of a graphical user interface, analysis

components, data preparation, storage and management capabilities, graphics and reporting

facilities.

The HEC-RAS system contains four 1-dimensional river analysis components for:

Steady flow water surface profile computations

Unsteady flow simulation

Movable boundary sediment transport computations

Water quality computations

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 full network of

natural and constructed channels. For the current assignment, use was made of the steady flow

water surface profile component. The following features of the steady flow component make it

particularly suitable for the assignment.

The steady flow water surface profiles component is intended for calculating water

surface profiles for steady gradually varied flow. The system can handle a full

network of channels, a dendritic system or a single river reach. The steady flow

component is capable of modeling subcritical, supercritical, and mixed flow regime

water surface profiles.

The basic computational procedure is based on the solution of the one dimensional

energy equation. Energy losses are evaluated by friction (Mannings equation) and

contraction/expansion (coefficient multiplied by change in velocity head). The

momentum equation is utilized in situations where the water surface profile is rapidly

36

varied. These situations include mixed flow regime calculations (i.e. hydraulic

jumps), hydraulics of bridges, and evaluating profiles at river confluences (stream

junctions).

The effects of various obstructions such as bridges, culverts, dams, weirs, and other

structures in the flood plain may be considered in the computations. Also capabilities

are available within the system for assessing the change in water surface profiles due

to channel modifications etc.

Special features of the steady flow component include: multiple plan analyses; multiple profile

computations; multiple bridge and/or culvert opening analysis; bridge scour analysis; split flow

optimization; and stable channel design and analysis.

Theoretical basis for the hydraulic analysis

The theoretical framework for the flow calculations is founded on long established principles of

fluid dynamics including mass, energy and momentum conservation (Featherstone and Nalluri,

1995; Brunner et al., 2001). A number of implicit assumptions are made in the steady flow

analysis component of the software including;

Flow is steady

Flow is gradually varied (except at hydraulic structures such as bridges, culverts, and

weirs. At these locations, where the flow can be rapidly varied, the momentum

equation or other empirical equations are used instead)

Flow is one dimensional (i.e. velocity components is directions other than the

direction of flow are not accounted for)

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 profile calculations

In the HEC-RAS system, water surface profiles are computed from one cross-section to the next

by solving the Energy equation with an iterative procedure called the standard step method. The

energy equation is written as follows

eh

g

VZY

g

VZY

22

2

1111

2

2222

(1)

Where

Y1, Y2 = depth of water at cross sections

37

Z1, Z2 = elevation of the main channel inverts

V1, V2 = average velocities (total discharge/total flow area)

21, = velocity weighting coefficients

g = gravitational acceleration

he = energy head loss

The energy head loss (he) between two cross sections is comprised of friction losses and

contraction or expansion losses. The equation for the energy loss is as follows.

g

V

g

VCSLh fe

22

2

11

2

22

(2)

Where: L = discharge weighted reach length

fS = representative friction slope between two sections

C = expansion or loss coefficient

The distance weighted reach length, L, L is calculated as

robchlob

robrobchchloblob

QQQ

QLQLQLL

(3)

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 section

requires 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 using the input cross

section n-value break points (location where the Mannings n-values change) as the basis for

subdivision. Conveyance is then calculated within each subdivision from the following form of

Mannings equation based on SI units

38

n

ARK

KSQ f32

21

(4)

Where: K = conveyance for the subdivision

n = Mannings roughness coefficient for the subdivision

A = flow area for subdivision

R = hydraulic radius for subdivision (area/wetted perimeter)

The program then sums up all the incremental conveyances in the overbanks to obtain the

conveyance for the left and right overbank. The main channel is normally computed as a single

conveyance element. The total conveyance for the cross section is obtained by 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 is changed

within the channel area. HEC-RAS tests the applicability of subdivision of roughness within the

main channel portion of a cross section, and if it is applicable, the program will compute a single

composite main channel n value.

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 each cross 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 (left overbank, main channel, and right

overbank).

Friction loss evaluation

Friction loss is evaluated in HEC-RAS as a product of fS and L where fS is the representative

friction slope for a reach and L is defined the equation below. The friction slope (slope of the

energy grade line) at each cross section is computed from Mannings equation as follows:

2

K

QS f

(5)

39

Alternative expressions for the representative friction slope used in HEC-RAS are explained in

(Brunner et al., 2001) and include

Average conveyance equation

Average friction slope equation

Geometric mean friction slope equation

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

(6)

Where: C = contraction or expansion coefficient

The program assumes that a contraction is occurring whenever the velocity head downstream is

greater than the velocity head upstream and vice versa. Typical C values are available in standard

textbooks and manuals on Hydraulics.

Computation procedure

The unknown water surface elevation at a cross section is determined by an iterative solution of

the equations as follows

Assume a water surface elevation (WS2) at the upstream cross section (or

downstream cross section if a supercritical profile is being computed)

Based on the assumed water surface elevation, determine the corresponding total

conveyance and velocity head

With values from step 2, compute fS and solve equation 6 for eh

With values from steps 2 and 3, solve equation 5 for WS2.

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 defined tolerance.

Bridge modelling guidelines

HEC-RAS computes energy loses caused by structures such as bridges and culverts in three parts

One part consists of losses that occur in the reach immediately downstream from the

structure, where expansion of the flow generally takes place

40

The second part consists of losses at the structure itself, which can be modeled with

several different methods.

The third part consists of losses that occur in the reach immediately upstream of the

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 energy losses

due to the structure (numbered 1, 2, 3 and 4 in Figure 22). During the hydraulic computations the

program automatically formulates two additional cross sections inside the bridge. Whenever the

user is performing water surface computations through the bridge, additional cross sections

should always be included both downstream and upstream of the bridge to prevent any user-

entered boundary conditions from affecting the hydraulic results through the bridge.

Figure 22: Cross section locations at bridge

Contraction and expansion losses

Losses due to contraction and expansion of flow between cross sections are determined during

the standard step profile calculations. Mannings equation is used to calculate friction losses, and

all other losses are described in terms of a coefficient times the absolute value of the change in

velocity between adjacent cross sections. When the velocity head increases in the downstream

direction, a contraction coefficient is used; and when the velocity head decrease, an expansion

coefficient is used.

41

Hydraulic computations through the bridge

The bridge routines in HEC-RAS allow the modeler to analyse a bridges with several different

methods without changing the bridge geometry. The bridge routines have the ability to model

low flow (class A, B, and C) when the bridge opening operates as an open channel. The routines

can also model high flows which are flows that come into contact with the maximum low chord

of the bridge deck. The energy equation is mainly used in both cases though other alternative

equations like momentum balance, Yarnell 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 iterative procedure is used to determine the amount of each flow and the

appropriate equations applied.

Selecting a bridge modeling approach

The choice of the modeling approach depends significantly of the type of flow (low or high) and

local conditions like level of obstruction by the piers, predominant type of losses level of

obstruction by the bridge deck, whether the bridge is submerged or not etc. other factors include

the bridge skew to the flow direction, and presence of multiple bridge openings at a cross

section.

Culvert design approach guidelines

Because of the similarity between flow in bridges and culverts, culverts are modeled in a similar

manner to bridges. Figure 23 shows a typical box culvert crossing and illustrates the 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 most other aspects of bridge

analysis apply to culverts as well. The most common types of culvert 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 of the culverts are defined by the contraction and

expansion coefficients which are unique to each culvert type. The head losses are computed by

multiplying this coefficient by the absolute head difference between two cross sections (one

upstream and the other downstream of the culvert section).

42

Figure 23: Typical culvert crossing (right: energy and hydraulic grade line for a full flowing culvert)