LNE Route 2013/14 Clienting Programme

Detailed Scour Assessment Report

Bridge: NOC - 17N&L at 8 miles 0792 yards

Priority Category - High

Priority Score - 2

Priority Rating - 16.66 (Pier 4)

LH Abutment - 14.35

Pier 1 - 16.22

Pier 2 - 16.22

Pier 3 - 16.55

Pier 4 - 16.66

Pier 5 - 16.22

Pier 6 - 16.22

RH Abutment - 14.35

October 2013

Network Rail – LNE Route

George Stephenson House

Toft Green

YORK

YO1 6JT

NOC - 17N

JBA Office

JBA Consulting The Old School House St. Joseph’s Street Tadcaster North Yorkshire LS24 9HA UK

JBA Project Manager / Project Number

Matthew Kendall / 2013s7403

Revision History

Revision Ref / Date Issued Amendments Issued to

1.0 / October 2013 - Peter Barry

1.1/ November 2013 Minor Amendments Peter Barry

Contract

This report describes work commissioned by Network Rail. Network Rail’s representative for the contract was Peter Barry. Matthew Lee, Matthew Kendall and Richard Buck of JBA Consulting carried out this work.

Prepared by .................................................. Matthew Lee, HNC BSc

Engineer

Reviewed by ................................................. Matthew Kendall, HNC BEng

Senior Engineer

Approved by .................................................. Richard Buck BEng CEng MICE MCIWEM

Director

Purpose

This document has been prepared solely as a Detailed Scour Assessment for Network Rail LNE Route. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

Copyright

© Jeremy Benn Associates Limited 2013

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Contents 1. Introduction .......................................................................................................... 1

1.1 Background ............................................................................................................ 1

1.2 About this report .................................................................................................... 1

2. Data gathering ...................................................................................................... 2

2.1 Site location and access ........................................................................................ 2

2.2 Asset data .............................................................................................................. 2

2.3 Structure description .............................................................................................. 2

2.4 Previous Analyses ................................................................................................. 2

2.5 Underwater Examinations...................................................................................... 2

2.6 Coring Information ................................................................................................. 3

2.7 Archive Information ................................................................................................ 3

3. Method .................................................................................................................. 4

3.1 Design flood, water levels and velocities ............................................................... 4

3.2 Depth of scour ....................................................................................................... 4

3.3 Priority category ..................................................................................................... 5

4. Results .................................................................................................................. 6

4.1 Design floods, water levels and velocities ............................................................. 6

4.2 Depth of scour ....................................................................................................... 6

4.3 Priority category ..................................................................................................... 7

4.4 Superstructure ....................................................................................................... 8

5. Conclusions and recommendations .................................................................. 9

5.1 Conclusions ........................................................................................................... 9

5.2 Recommendations ................................................................................................. 9

5.1 Flood Closure Marker ............................................................................................ 10

References......................................................................................................................... I

A. Appendix A - Figures ........................................................................................... II

B. Appendix B – Coring Report .............................................................................. VI

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List of Tables

Table 2-1: Asset data ...................................................................................................... 2

Table 4-1: Catchment characteristics ........................................................................... 6

Table 4-2: Design floods, water levels and velocities ................................................. 6

Table 4-3: Scour and foundation depths ...................................................................... 6

Table 4-4: Priority category ........................................................................................... 7

Table 5-1: Priority categories and recommended actions ......................................... 9

List of Figures

Figure 1: Location plan (Scale 1:200,000) .................................................................... II

Figure 2: Location plan (Scale 1:5,000) ........................................................................ II

Figure 3: Photographs ................................................................................................... III

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1. Introduction

1.1 Background

As part of the innovative Network Rail LNE Clienting Delivery Route, a programme of detailed scour assessments in accordance with the Specification NR/SP/CIV/080 Management of Existing Bridges and Culverts, Issue 1, April 2004, and Company Standard NR/CS/CIV/032 Managing Existing Structures, Issue 1, April 2004, has been developed.

As part of this programme, Network Rail LNE Route has let a contract for the inspection and assessment of a number of Underbridges to Jeremy Benn Associates Limited of which the current report forms part.

1.2 About this report

NOC-17N was assessed in September 2011 using the EX2502 Stage 1 Scour Assessment Procedure. The findings of the site survey and calculation showed the structure to have a Priority Rating of 15.39, i.e. a structure of Medium Priority for scour.

The purpose of this assessment is to undertake a detailed analysis of scour potential using a combination of computational modelling and further site investigation, if required, to estimate scour depths. A study to determine likely flows, water levels and velocities at the structure for given return periods has been undertaken to establish the risk (if any) posed by floodwaters on the structure and line. The results of these analyses have then been used to calculate the Priority Score using the EX2502/BSIS Assessment Procedure.

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2. Data gathering

2.1 Site location and access

This structure is located 500m north of Ulleskelf, North Yorkshire.

Parking is available on Churchfenton Road 500m south of the structure. The structure can be accessed easily by walking along a footpath through the fields that leads underneath the structure.

2.2 Asset data

Asset data is given in Table 2-1 below.

Table 2-1: Asset data

Structure Name: RIVER WHARFE, ULLESKELF

ELR: NOC

Mileage: 8.08

Local Route: LNE

OS NGR: SE 520 405

OS 1:50,000 Sheet: 105

Watercourse Name: River Wharfe

Watercourse Type: Main River

Agency Region: North East

Agency Area: Yorkshire

Existing Flood Marker: None Present

2.3 Structure description

NOC-17N&L consists of two identical structures standing side by side with 2m separation. Each structure consists of 6 red brick piers supporting concrete and steel decks. The abutments and piers 1, 2, 5 and 6 are located on the floodplain. Piers 3 and 4 stand on the edges of the watercourse and the deck between them spans 18m. Piers 3 and 4 are fitted with rounded concrete break waters.

The River Wharfe is considered to be tidal at this point and the gradient is flat. The bed material is comprised of silts and sands. The channel is straight through the structure and there is no angle of attack. The flow velocity was low on the day of inspection and the flow regime glide.

2.4 Previous Analyses

JBA consulting completed a Stage 1 Scour Assessment in September 2011. The structure returned a Medium Priority Rating of 15.39.

2.5 Underwater Examinations

An underwater examination was undertaken in February 2008 by Bridgeway Consulting Ltd on behalf of Amey Consulting. This underwater examination identified that "the structure is in overall fair condition".

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2.6 Coring Information

This structure was cored as part of this assessment by Henderson Thomas Associates Ltd. The results of which have been included in this assessment. A copy of this coring report can be found in Appendix B - Coring Report, at the back of this report.

Position Depth (m) Level (mAOD) Recovered Material

Pier 3 1.75 0.13 Clay

Pier 4 1.50 0.34 Clay

A copy of this coring report can be found in Appendix B - Coring Report, at the back of this report.

For Pier 3 of the structure, coring was undertaken from 0.79m below river bed level, the core team drilled at an inclined angle of 18 degrees for 2.44m. This corresponds to a vertical length of 2.32m and a foundation depth of 1.75m below river bed level, at 2x pier width from core location in accordance CIRIA guidance (2002).

For Pier 4 of the structure, coring was undertaken from 0.31m below river bed level, the core team drilled at an inclined angle of 15 degrees for 2.41m. This corresponds to a vertical length of 2.33m and a foundation depth of 1.50m below river bed level, at 2x pier width from core location in accordance CIRIA guidance (2002).

2.7 Archive Information

No archive information has been found for this structure

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3. Method

3.1 Design flood, water levels and velocities

The design flood is the flood that a structure should withstand without suffering damage, the probability of which is normally expressed by the return period, the average period between events of a similar magnitude. For T-year return period flood, the lifetime probability of exceedance r during a design life N is given by

rT

n= − −1 1

1( )

(3.1)

where

r = lifetime probability of exceedance (%)

T = return period of event (years)

N = design life (years)

The design flood for 10 to 200-year return periods were estimated using the Flood Estimation Handbook (Reed, et al, 1999) and in-house Flood Estimation Software (FES) Version 2.0 (JBA Consulting, 2007). The median annual flood (QMED) was estimated from catchment characteristics then scaled up to the design peak flows using a pooled growth curve. The effects of climate change are not considered. The findings are given in Section 4.1.

The design water levels and velocities were estimated using a simple one-dimensional hydraulic model of the watercourse in the vicinity of the structure. The model software was HEC-RAS model (Version 4.1.0, US Army Corps of Engineers, 2010) and the model geometry comprised four cross-sections, two upstream and two downstream of the bridge, based on measurements taken during a topographic survey. The modelled water levels and flow velocities are given in Section 4.1. It should be noted that greater confidence in the results could be obtained using more detailed topographical survey data of the watercourse channel and floodplain.

3.2 Depth of scour

The depth of scour at a structure is the sum of the general scour (also known as contraction scour) and local scour. The depth of general scour dg at the bridge was estimated using NR Scour (JBA Consulting, 1997), which is based on the Bridge scour information system (BSIS) (Bullen and Partners, 1990) and Hydraulic aspects of bridges: assessment of the risk of scour (EX2502) (HR Wallingford, 1993).

The depth of local scour at piers ds was estimated using the method in the Manual on scour at bridges and other hydraulic structures (MAY ET AL, 2002, pp. 70-82), with a safety factor to allow for the probability of scour depth being exceeded during a flood event, and a debris factor to allow for an increase in effective width due to debris accumulation. This method was adopted by JBA Consulting in March 2012 and replaces the method used in previous scour assessments (Melville & Sutherland, 1988). Benefits include a wide range of pier shape factors, user-friendly formulae for alignment, depth and velocity factors. The method was applied using an in-house spreadsheet Local scour at bridges v1.23.xls (JBA Consulting, 2012).

( )debrisFvelocitydepthshapeangles FSFFFFDd .....= (3.1)

where

ds = depth of local scour (m)

D = width of pier or abutment (m)

Fangle = alignment factor (piers only) (-)

Fshape = shape factor (-)

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Fdepth = flow depth factor for piers (-)

Fvelocity = velocity factor (-)

SF = safety factor (-)

Fdebris = debris factor (-)

The depth of local scour at an abutment is given by (Melville, 1995), from Bridge scour (Melville and Coleman, 2000). Local scour depth is related to the discharge intercepted by the embankment rather than the length of the embankment itself. This is an envelope equation and no safety factor is required.

GsyDs KKKKy ... θ= (3-2)

where

KyD = flow depth-foundation width factor (-)

Ks = foundation shape factor (-)

Kθ = foundation alignment factor (-)

KG = approach channel geometry factor (-)

For all calculations it was assumed that there was no scour protection and that the bed material was unchanged with depth.

3.3 Priority category

The preliminary priority rating (PR) was calculated for each bridge element after EX2502 (HR Wallingford, 1993). The PR is a function of the ratio between scour depth and bridge foundation depth and indicates the risk of bridge failure due to scour (Equation 3.3).

+=

f

t

d

dPPR ln15

(3.3)

where

PPR = priority rating (range 10.0-21.0)

dt = depth of total scour below bed level (m)

df = depth of foundation below bed level (m)

If the load bearing material is rock, a satisfactory invert or scour protection is present, then the final priority rating PR is

Priority rating = 10 (Low priority) (3.4)

For all other cases, PR is

Priority rating = PPR + TR + LBM (3.5)

where

PPR = Preliminary priority rating (-)

TR = correction factor for river type (-)

LBM = correction factor for load-bearing material (-)

Finally, a priority score, priority category and recommended action were assigned to each bridge element in accordance with EX2502 (HR Wallingford, 1993) (Table 5-1).

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4. Results

4.1 Design floods, water levels and velocities

The catchment area and catchment characteristics for the watercourse at the structure were identified using the Flood Estimation Handbook (FEH) CD-ROM Version 3.0 (Centre for Ecology and Hydrology, Wallingford, 2010) (Table 4-1 below).

Table 4-1: Catchment characteristics

Catchment Area (DTM AREA) (km2) 892.74

Baseflow Index (BFI-HOST) (-) 0.45

Standard Annual Rainfall (SAAR) (mm) 1089.00

Urban Index (URBEXT) (-) 0.02 Source: Flood Estimation Handbook (FEH) CD-ROM Version 3.0, 2010.

The design floods, water levels and flow velocities calculated in accordance with Section 3.1 are given in Table 4-2 below. The 200-year return period flood was used as the design flood as this is the design standard for scour protection schemes.

Table 4-2: Design floods, water levels and velocities

Return period 2- year

5- year

10- year

25- year

50- year

100- year

150- year

200-year

Design flood (m³/s) 212.34

269.15 309.09 365.27

412.41 464.73 498.13 523.19

Modelled water level (mAOD)

7.57

8.24 8.66 9.20 9.67 10.14 10.41 10.66

Average channel velocity (m/s)

2.05

2.19 2.27 2.39 2.48 2.81 2.87 2.87

4.2 Depth of scour

The scour and foundation depths are given in Table 4-3 below.

Table 4-3: Scour and foundation depths

Nr Description General scour, dg (m)

Local scour, ds (m)

Total scour, dt (m)

Foundation depth, df (m)

1 LH Abutment 0.00 1.00 1.00 1.00**

2 Pier 1 0.00 6.48 6.48 1.00**

3 Pier 2 0.00 6.48 6.48 1.00**

4 Pier 3 2.00 13.77 15.77 1.75

5 Pier 4 0.98 14.06 15.04 1.50

6 Pier 5 0.00 6.48 6.48 1.00**

7 Pier 6 0.00 6.48 6.48 1.00**

8 RH Abutment 0.00 1.00 1.00 1.00**

** Assumed foundation depth.

NOC - 17N 7

4.3 Priority category

The priority rating (PR) is given in Table 4-4 below.

Table 4-4: Priority category

Nr Description Priority rating, PR (-)

Priority score

(-)

Priority category (-)

1 LH Abutment 14.35 4 Medium

2 Pier 1 16.22 2 High

3 Pier 2 16.22 2 High

4 Pier 3 16.55 2 High

5 Pier 4 16.66 2 High

6 Pier 5 16.22 2 High

7 Pier 6 16.22 2 High

8 RH Abutment 14.35 4 Medium

NOC - 17N 8

4.4 Superstructure

The bridge soffit is 9.34mAOD and design flood water level is 10.66mAOD. During the design flood, the bridge has a freeboard of -1.32m and flood water levels are likely to reach the deck or cause track flooding.

However, the risk of flooding or surcharging may increase in the future due to blockage at the structure, accumulations of sediment, climate change or changes within the catchment area.

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5. Conclusions and recommendations

5.1 Conclusions

5.1.1 Priority Rating

For a known foundation depth of 1.50m for Pier 4, this bridge falls within the High priority category, with a priority score of 2 and a final priority rating of 16.66.

The increase in priority score from the previous Stage 1 Scour Assessment which was carried out in September 2011 returning a score of 15.39, is due to an increase in detail and information which is required in order to carry out the assessment. As a result of this, a topographical survey and hydraulic model has been carried out.

The detailed hydraulic model which was carried out for this structure shows the watercourse to have extremely high velocities during the 200-year flood event. These high velocities, combined with a flat, hydraulically inefficient profile and increased width of piers following a topographical survey, has led to the increase in score to reach a priority score of 2 and a final priority rating of 16.66.

5.1.2 Superstructure

For the design flood, the bridge has a freeboard of -1.32m and there is a high risk of surcharging of the bridge or inundation of the track.

5.2 Recommendations

5.2.1 Flood warning procedure

The EX2502 procedure for High priority bridges should be adopted for this bridge, in accordance with NR/SP/CIV/080 (Table 5-1). This can be achieved either by the construction of physical scour protection or by the introduction of monitoring/line closures based on flood warnings issued by the Environment Agency.

Table 5-1: Priority categories and recommended actions

Priority rating Priority score

Priority category

Recommended Action

17.00 - 21.00 Priority 1 High Detailed Hydraulic Assessment. If the High Priority is confirmed then the structure is to be included in the flood warning plan or physical protection works to be constructed. Whilst at High Priority the structure should have yearly underwater exams. The priority should be reviewed following protection works, a flood event or underwater exam.

16.00 – 16.99 Priority 2

15.00 – 15.99 Priority 3 Medium Repeat of EX2502 assessment and underwater exams (where necessary) every 3 years. The priority should be reviewed following protection works, a flood event or underwater exam.

14.00 – 14.99 Priority 4

13.00 – 13.99 Priority 5 Low Repeat of EX2502 assessment every 6 years and underwater exams (where necessary) every 3 years. The priority should be reviewed following protection works, a flood event or underwater exam.

10.00 – 12.99 Priority 6

5.2.2 Foundations

Coring is not considered necessary at this structure.

5.2.3 Routine examinations

In addition, regular inspections for scour and flood damage, particularly following a major flood, should be carried out and the bridge reassessed using the EX2502 calculation procedure at a minimum interval of every 3 years.

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Furthermore, it is also recommended that underwater examinations of the structure be undertaken every 1 year dependent on the results of first examination.

The priority should be reviewed following protection works, a flood event or underwater examination.

5.2.4 Remedial works

It is recommended that scour protection measures be installed, to protect the structure and ensure that it is no longer considered high risk.

5.2.5 Superstructure

We consider it necessary to include this structure in the current route flood plan for water pressure.

5.1 Flood Closure Marker

Using hydraulic modelling, it is possible to determine when a bridge becomes 'high' risk by incrementally increasing or reducing the magnitude of design flood events until a high risk score (PR 16.00) is triggered.

Due to the nature of this structure and based on foundation depths, this structure is considered to be at high risk of scour during low flows. Therefore it is recommended that scour protection measures be installed, to protect the structure and ensure that it is no longer considered high risk.

It should be noted that this level is not based on detailed hydraulic modelling.

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References

BULLEN AND PARTNERS (1994) Bridge scour information system

HIGHWAYS AGENCY (1994) The design of highway bridges for hydraulic action, Advice Note BA59/94.

HIGHWAYS AGENCY (1998) Assessment of Scour at Highway Bridges, Revision D, Advice Note.

HR WALLINGFORD (1993) Hydraulic Aspects of Bridges - Assessment of the Risk of Scour, Report EX2502, HR Wallingford, Wallingford. This is the Preliminary Scour Assessment Procedure developed for British Rail which provides an indicative assessment of scour at a structure based on a site visit, measurement of key structure dimensions and a simple assessment of river geomorphology.

JBA CONSULTING (1997) NR Scour. Replaces the former BSIS (Bridge Scour Information System). A database developed for British Rail Engineering Technical Support Group, Kings Cross, NR Scour is essentially a computerised version of the EX2502 scour assessment procedure and contains information on: ELR, Bridge Name, Mileage, Watercourse crossed, Bridge and stream type, Details of diving and coring surveys, Details of EX2502 scour assessments.

MAY, R.W.P., ACKERS, J.C. and KIRBY, A.M. (2002) Manual on scour at bridges and other hydraulic structures, Report C551, Construction Industry Research and Information Association (CIRIA), London.

MELVILLE B.W. & SUTHERLAND, A.J. (1988) Design Method for Local Scour at Bridge Piers, In: Journal of Hydraulic Engineering, Vol.114(10), 1988, ASCE.

NETWORK RAIL (2004) Management of Existing Bridges and Culverts, Issue 1, Specification NR/SP/CIV/080.

NETWORK RAIL (2004) Managing Existing Structures, Issue 1, Company Standard NR/CS/CIV/032.

REED D. et al (1999) Flood Estimation Handbook, 5 Volumes. CEH Institute of Hydrology, Wallingford.

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A. Appendix A - Figures

Figure 1: Location plan (Scale 1:200,000)

Contains Ordnance Survey data © Crown copyright and database right 2013

Figure 2: Location plan (Scale 1:5,000)

Contains Ordnance Survey data © Crown copyright and database right 2013

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Figure 3: Photographs

Photograph 1 Upstream Face

Photograph 2 View Upstream

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Figure 3: Photographs

Photograph 3 Existing flood marker

Photograph 4 View between structures

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Figure 3: Photographs

Photograph 5 View Downstream

Photograph 6 Downstream Face

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B. Appendix B – Coring Report

CLIENT: STRUCTURE :

TEST AREA LOCATION :

REMARKS :

DATE TESTED:

TEST AREA REF:

FILE NO : L/1082/R1/13/WDTN.T.S

JBA CONSULTING

Core 1 - Right abutment

( mm )

( mm

)

AN

GLE

D L

EN

GT

HO

F C

OR

E =

m

m

POSITION OF BOTTOM OF CORE ABOVE WATER LEVEL = mm

DIST FROM U/S END OF BRIDGE - 620 mm

DATUM POINT TO WATER LEVEL = mm

CORE CONSISITS OF -

CORED INTO - CLAY

Core 2 - Left abutment

( mm )

( mm

)

AN

GLE

D L

EN

GT

HO

F C

OR

E =

m

m

POSITION OF BOTTOM OF CORE ABOVE WATER LEVEL = mm

DIST FROM U/S END OF BRIDGE = mm

DATUM POINT TO WATER LEVEL = mm

CORE CONSISITS OF -

CORED INTO - CLAY

NOC / 17N

165

640

560

175

24

10

24

40

30

6730

300 mm brickwork and 2110 mm concrete

6987

3060

70

15/8/2013

Datum Point = soffit / underside of beam River bed is made up of stone and silt

UPSTREAM ELEVATIONOF BRIDGE/RIVER BED

17660 mm

6730 mmC2

250 mm brickwork and 2190 mm concrete

4530mmMax depth = 200mm

500mmDEEP 1250mm

DEEP 380mmDEEP

C1

Plate 1 – U/S elevation Plate 2 – View to C1 Plate 3 – Angle of C1

Plate 4 – Extracted Core 1 Plate 5 – View to C2 Plate 6 – Angle of C2

Plate 7 – Extracted Core 2

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