River Tyne recovery studies: Volume 4 Recommendations for modelling and flood resilience

Transcription

1 River Tyne recovery studies: Volume 4 Recommendations for modelling and flood resilience FINAL Report January 2017 Environment Agency Tyneside House Skinnerburn Road NEWCASTLE UPON TYNE NE4 7AR

2 JBA Project Manager Vicky Shackle JBA Consulting Suite E6 Milburn House Newcastle upon Tyne NE1 1PE Revision History Revision Ref / Date Issued Amendments Issued to V1.0 December 2016 Draft for review Amanda McKevitt V2.0 January 2017 As comments provided by Amanda McKevitt EA Contract This report describes work commissioned by Amanda McKevitt, on behalf of the Environment Agnecy, by a letter dated 6 June 2016, under commission reference ENV600044R. The Environment Agency s representative for the contract was Amanda McKevitt. Rosie Hampson and Kate Bradbrook of JBA Consulting carried out this work. Prepared by... Vicky Shackle BSc PhD MCIWEM C.WEM Principal Analyst Reviewed by... Kate Bradbrook MA PhD CEng FCIWEM C.WEM Technical Director Purpose This document has been prepared as a Final Report for the Environment Agency. 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. JBA Consulting has no liability regarding the use of this report except to the Environment Agency i

3 Copyright Jeremy Benn Associates Limited 2017 Carbon Footprint A printed copy of the main text in this document will result in a carbon footprint of 132g if 100% postconsumer recycled paper is used and 132g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex. JBA is aiming to reduce its per capita carbon emissions. ii

4 Executive Summary This is Volume 4 of reporting for the River Tyne Recovery Study. A full catchment description, and an overview of Storm Desmond rainfall, are given in Volume 1, interim hydrological report. Volume 2 reviews and updates inflows to the hydraulic models used in the Tyne catchment. Volume 3 looks at the volume of the December 2015 event hydrographs. This volume considers a range of modelling and flood risk issues which have been identified as requiring attention. For each issue we have made recommendations about how it could be addressed. iii

5 Contents 13 Modelling issues Overview Riding Mill Haydon Bridge Warden Corbridge Ovingham Hexham Summary of modelling issues Flood resilience issues Overview Riding Mill Haydon Bridge Warden Ovingham and Prudhoe Hexham Summary of flood resilience issues v

6 List of Figures Figure 13.1: Riding Mill location, flood zones, and properties flooded in December Figure 13.2: Model representation of the railway embankment at Riding Mill... 2 Figure 13.3: Haydon Bridge location, flood zones, and properties flooded in December Figure 13.4: Reported flow paths at Lipwood, December Figure 13.5: Langley Burn location, flood zones, and properties flooded in December Figure 13.6: Haydon Bridge gauge datum information and channel cross section... 6 Figure 13.7: Warden location, flood zones, and properties flooded in December Figure 13.9: Model long section at Corbridge Figure 13.10: Ovingham location, flood zones, and properties flooded in December Figure 13.11: Hexham location, flood zones, and properties flooded in December Figure 13.12: 2D model domain and wrack mark elevations, Hexham Figure 14.1: Existing FWAs at Haydon Bridge Figure 14.2: Existing FWAs at Warden Figure 14.3: Existing FWAs at Ovingham and Prudhoe Figure 14.4: Existing FWAs at Hexham List of Tables Table 14.1: Current flood warning thresholds, Haydon Bridge gauge Table 14.2: Current flood warning thresholds, Warden gauge Table 14.3: Current flood warning thresholds, Ovingham gauge Table 14.4: Current flood warning thresholds, Hexham vi

7 Abbreviations 1D... One Dimensional (modelling) 2D... Two Dimensional (modelling) AIMS... Asset Information Management System (a database_ CEH... Centre for Ecology and Hydrology FAS... Flood Alleviation Scheme FEH... Flood Estimation Handbook HEC-RAS... Hydrologic Engineering Center River Analysis System (developed by the US Army) ISIS... Hydrology and hydraulic modelling software JFLOW... 2-D hydraulic modelling package developed by JBA LIDAR... Light Detection and Ranging (ground survey method) NFCDD... National Flood and Coastal Defence Database QMED... Median Annual Flood (with return period 2 years) ReFH... Revitalised Flood Hydrograph method TUFLOW... Two-dimensional Unsteady FLOW (a hydraulic model) vii

8 13 Modelling issues 13.1 Overview In this chapter we examine modelling issues identified by the Environment Agency, and propose recommendations for improving the modelling Riding Mill At Riding Mill the following modelling issues have been identified: (1) the modelled flood level heights could potentially be lower than historic evidence shows. The EA require a review of the need for updated modelling here to take into account backing up from the River Tyne, and provide an opinion on the approach needed to model the interaction between the March Burn (also known as Riding Mill Burn) and the River Tyne. (2) properties outside Flood Zone 2 flooded to significant depths in the winter floods of How can the mapping/modelling be improved here? Is the railway embankment represented in the existing model and if so how, for example is the model boundary cut off at the railway embankment? What additional information is needed to extend the model boundary further up the March Burn? Response and recommendations March/Riding Mill Burn is a right bank tributary of the Tyne with a relatively steep and responsive catchment; backwater from the Tyne can affect its water levels for some distance upstream ( m, as detailed in (1), below). The burn flows through the settlement of Riding Mill, and joins the River Tyne some distance downstream. The existing hydraulic model of Riding Mill Burn is a 1D steady state HEC-RAS model, developed in The inflows and downstream boundary have been updated for this study, but the model has not yet been re-run nor flood outlines re-mapped. Figure 13.1 shows the location, model extent and flood information for Riding Mill. Figure 13.1: Riding Mill location, flood zones, and properties flooded in December

9 (1) Interaction between March Burn and the Tyne When the water level in the Tyne is high, it pushes up the March Burn. The hydraulic model uses a fixed downstream boundary of 21.0mAOD, originally equivalent to the 5 year flow in the Tyne as calculated in the 2009 study. Using the standard backwater equation 1 it is possible to calculate the upstream distance on the Burn which is affected by raised water levels in the Tyne. Although we do not know the exact depth of water at the March Burn/Tyne confluence in December 2015, we can make an estimate; downstream at Riding Mill gauge the peak level recorded on 6 December 2015 was 5.99m stage. Assuming that the water depth at the March Burn confluence was between 3m and 6m, the backwater length is approximately m. A backwater length of 240m does not reach the railway embankment, whereas 480m from the confluence will reach the A695 road bridge, meaning that in December 2015 the water level in the Tyne potentially caused direct flooding to properties downstream of that bridge (see Figure 13.1). If the March Burn was also in flood, discharge may have been slowed by the backwater, but it would be necessary to run the model under a range of scenarios to prove this. It is also known that issues were encountered with the Northumbrian Water Pumping Station situated on the southern side of the railway embankment and also that surface water flooding occurred. The modelled flood levels being lower than those observed could indicate a need to review the model hydrology or recalibrate the model. However it would be sensible to run the model with the updated inflows from this study first. In summary, there is potential for interaction between the March Burn and the Tyne to worsen flood risk in Riding Mill. Surface water flooding was also reported during Storm Desmond and issues identified with the surface water pumping station situated on the southern side of the railway embankment. The need for an integrated model should be discussed. Before extending the model we recommend running it with the updated inflows and all available evidence from the December floods to check the calibration. (2) Improving the model At Riding Mill the railway embankment is represented in the model as a bridge with a high deck, blocking flow except through the arch (Figure 13.2). The model extends downstream of the embankment for a sufficient distance to allow backwater effect from the Tyne to be accounted for. Figure 13.2: Model representation of the railway embankment at Riding Mill RIDING MILL Plan: Legend Ground Ineff Bank Sta Elevation (m) Station (m) We recommend running the March Burn model with the updated flows and downstream boundary derived for this study, and attempting to calibrate the model to the December 2015 flood events. As there is no flow gauge on March Burn the flow during these events is a large source of uncertainty; however it may be possible to also use rainfall data as a guide to the likely flow. 1 Backwater length = 0.7*water depth*channel slope 2

10 It does not seem necessary to extend the model upstream on the March Burn. In a 1D model, the water levels on the floodplain are taken from the nearest river section. In Riding Mill, it is possible that, the flooding pathway for the properties shown in Figure 13.1 is overland flow from further upstream on the river which then ponds against the road/railway embankments. In this case, the flood levels could be higher than at the nearest river section. In order to model this, we recommend that the model is converted to an unsteady 1D/2D model Haydon Bridge At Haydon Bridge the following modelling issues have been identified: (1) How can the existing model be improved at Lipwood, as it does not represent the current flooding mechanism? (2) What is required to accurately represent the confluence with the Langley Burn and the backing up effect? (3) The South Tyne model at Haydon Bridge has been updated with the post-flood bed profile in order to assess the impact of the gravels, and the 2015 event has been run. As part of this work it has come to light that the gauge data read approximately 0.9m higher than the modelled data, suggesting a potential issue with the gauge datum. On further inspection, the GPS gauge zero is documented as mAOD whereas the WISKI datum is documented as mAOD. How can this issue be resolved for updating the model? Response and recommendations Haydon Bridge is primarily at risk of flooding from the River South Tyne. The 2D VDS modelling study (2013) did not consider flooding from Honeycrook Burn (upstream extent of the study area), Langley Burn (downstream edge of Haydon Bridge) or Crossley Burn (downstream of Haydon Bridge), or from any other minor tributaries. Figure 13.3 shows the location, model extent and flood information for Haydon Bridge. Figure 13.3: Haydon Bridge location, flood zones, and properties flooded in December

11 (1) Improving the model at Lipwood Properties at Middle Lipwood and Lipwood Well flooded in December A local resident reported the following flood mechanism (see also Figure 13.4): - water coming out of bank at the railway bridge, with some returning to the river immediately upstream of the railway bridge - the majority of out of bank water flooding the field between the A69 and railway embankment. - water exiting southwards through the [west] railway arch, which is small and causes water to back up against the railway embankment. - when water in the northern field reaches a critical level it overspills onto the A69 at the [Middle Lipwood] gates and flows across the road into properties. - water then appears to follow the low farm access track to flood Lipwood Well from behind, and along the A69 to flood from the front - water then exits the A69 through the [east] farm access beneath the railway opposite Lipwood Well. Figure 13.4: Reported flow paths at Lipwood, December 2015 This flooding mechanism is not reproduced in the model for flows similar to that recorded in December Rather than accumulating in the field upstream of the railway embankment and overtopping onto the A69 (as observed), in the model water preferentially flows southwards through the westernmost culvert under the railway line. At the upstream bend modelled river levels are not sufficient for water to get out of bank and overtop onto the A69. A resident in Lipwood has suggested the gravel island on the upstream bend was a cause of flooding; to assess this possibility, topographic survey of the gravel island has been commissioned and we recommend that model geometry is updated with this. Sensitivity of channel water levels to changes in the gravel island dimensions and channel roughness can then be tested. There are some unknown aspects of the December 2015 flood event which are likely to be contributing to the discrepancy between the modelled and observed flood mechanisms. These are how accurately the access culverts are represented, whether either was blocked during the event, 4

12 and whether the model is sensitive to channel roughness and bed levels where there is a gravel island upstream of the railway bridge. The 2006 version of the Haydon Bridge model demonstrated that a significant increase in channel roughness was necessary to raise water levels sufficiently to overtop onto the road. We suspect the west culvert underneath the railway line may have been blocked and subsequently caused water to back up and overflow onto the main road. If the west culvert was not blocked, observed flow routes would be more like those modelled, where water passes through the culvert rather than building up on the northern side of the railway embankment. The east culvert has a gate across it and may have been blocked in addition to, or instead of, the west culvert, causing water to back up along the A69 to flood property. There are no reports of blockage having occurred in these locations to corroborate or eliminate this. We recommend carrying out sensitivity testing of the model to investigate the possible impacts of re-modelling or blocking the two culverts underneath the railway line. Sensitivity to channel roughness and bed levels around the gravel island upstream of the railway bridge should also be investigated. Running the model with the lateral inflows separated from the main hydrograph would help to calibrate it accurately. The catchment to the upstream extent of the model includes Honeycrook Burn therefore we see no need to include a separate model inflow for that Burn. (2) Modelling the confluence with the Langley Burn Langley Burn, which joins the South Tyne on the south bank at Haydon Bridge, is currently represented as a 1D HEC-RAS model with a constant level applied at its downstream boundary to represent levels in the South Tyne. This model was developed in 2006 and the accompanying report stated there was no known flooding as a result of water levels in the burn. The model has been run with two sets of downstream boundary conditions: firstly using levels on the Tyne for the same return period as on the Langley Burn (i.e. 100 year Langley Burn flow with 100 year Tyne level). Secondly using a level on the Tyne which represents 60% of the peak flow for the return period, based on relating Tp and LAG on the two watercourses (i.e. 100 year Langley Burn flow with level on Tyne calculated from 60% of 100 year peak flow). Langley Burn is not represented in the updated 2D model of the South Tyne, and model inflows or downstream boundary have not been updated for this study. However flows in the Tyne at Haydon Bridge, 1.1km downstream, have been updated and could be applied to the Langley Burn downstream boundary. Figure 13.5 shows the Langley Burn/South Tyne confluence. Figure 13.5: Langley Burn location, flood zones, and properties flooded in December

13 The EA Impact Report (December 2015) stated that surface water flooding accumulated behind the Tyne right bank defences upstream of the Langley Burn confluence. The defences were not observed to overtop in this location. The outfall flap on the evacuation pipe into the Langley Burn was open so water from both the South Tyne and Langley Burn was allowed to back up the pipe and contribute to water pooling in the car park near Martins Close. Water from Langley Burn was observed to seep through a stone wall into the garden of 12 Martins Close, then through another wall into the car park. Existing modelling demonstrates the potential for the South Tyne to overtop defences upstream of the Langley Burn confluence at the 75 year event, and for flood water to flow across the floodplain into the Langley Burn. Defences downstream of the confluence overtop from the South Tyne at the 50 year event. These over-topping frequencies are for design events. To improve representation of the interaction between the South Tyne and the Langley Burn, we propose incorporating the existing 1D Langley Burn model into the existing 1D-2D South Tyne model and linking the Langley Burn to the floodplain. (3) Resolve gauge datum issue We believe that the datum values supplied refer to Brigwood level gauge (23026), at grid reference NY84119, Datum values supplied from GPS and WISKI differ by 0.71m, being 60.33mAOD and 61.03mAOD respectively. The supplied datums are plotted this against the model cross-section (node Sout01_1808) at the gauge location, from the 2013 South Tyne model in Figure A cross section from the LIDAR data is also shown for comparison. Figure 13.6: Brigwood gauge (23026) datum information and channel cross section The available evidence about the true elevation of the Bridgwood gauge zero is inconclusive. Survey used to build the model suggests the lowest point in the channel is 58.9mAOD, which is 1.43m below the GPS datum value of 60.33mAOD. However a gauge zero need not necessarily be at the lowest point of the channel. 6

14 We strongly recommend that a new survey of the Brigwood gauge board is carried out to determine the level of gauge zero Warden At Warden the following modelling issues have been identified: (1) Historic evidence suggests that the defence standard of protection on the south bank may be higher than suggested by the model. Identify what bank/defence data is used in the existing model in comparison to the historic evidence, and suggest recommendations for improvements. (2) Examine the existing model results and flood mechanisms compared to the winter flood historic information. Is additional survey upstream of the flood defences required? Review the asset data used in the model, especially at the Paper Mill. Response and recommendations Warden village is 9km downstream of Haydon Bridge, just upstream of the confluence of the South Tyne with the North Tyne. Flooding is primarily from the South Tyne. Figure 13.7 shows the location, model extent and flood information for Warden. Figure 13.7: Warden location, flood zones, and properties flooded in December 2015 (1) South bank/defence data used in the existing model in comparison to the historic evidence On the north bank of the South Tyne at Warden, bank crest elevations were updated based on the most recent available AIMS data (2007) and As Built drawings (2008) for this project, and checked against channel survey (August 2016). No changes were made to the original model on the south bank, where NFCDD defence crest levels are used. Only one post-event survey point exists on the south bank, at Riverside Cottage, and is approximately 0.7m higher than the defence crest in this location (41.46mAOD compared to a defence crest of 40.74mAOD), suggesting that flood protection is indeed better in reality than as modelled. There is no post-event survey around West Boat Cottages, although these are known to have flooded during Storm Desmond; no wrack evidence was present at the time of post-event survey (December 2015 and January 2016). Some further survey is currently in progress for this reach. We recommend re-surveying defence and bank crests along the south bank in the area around Riverside Cottage and incorporating this new survey into the existing ISIS-TUFLOW model. 7

15 (2) Is additional survey upstream of the flood defences required? Defences on the north bank of the South Tyne have been updated using the most recent available AIMS data and As Built drawings for this project. The upstream limit of the defence, near the paper mill, does not seem to tie into high ground and the baseline model shows water escaping onto the road between the paper mill and the farm. This was not observed in December 2015 and it is possible that there is some high ground not picked up in the TUFLOW model grid that prevents this occurring. Comparing the defence crest level to the TUFLOW grid shows a difference of 0.73m in the elevations of the defence and the ground (Table 13.1). Table 13.1: Defence and ground levels near Paper Mill, Warden AIMS crest level 42.61mAOD Ground levels from LiDAR in TUFLOW grid 41.88mAOD We recommend additional survey along the north bank of the South Tyne from upstream of the paper mill to the upstream extent of the formal defences, to confirm ground heights. This should be followed by updating the model geometry and re-running the model to test how well the observed flood mechanisms are represented by the model. Re-calibration of the model is recommended to accurately match the observed water surface upstream of the road bridge. A model sensitivity test carried out in November 2016 looked at what would happen if the defence was tied into the high ground, preventing flow along the paper mill road. Results showed that flooding would commence first at Riverside Cottage, and reach Bankside Inn well before any floodwater from upstream is in the vicinity. This concurs better with resident s observations in December Corbridge At Corbridge the following modelling issues have been identified (1) Existing modelled flood levels appear to be lower than historic evidence suggests. We know properties have flooded more frequently than the model suggests at Well Bank. Review these levels in line with the flood defence levels, surveyed flood levels and flood reports. (2) Review how Corbridge bridge is represented in the existing model, and whether this is adequate or if changes need to be made. Response and recommendations Corbridge lies largely on the north (left) bank of the Tyne, with the railway station and a small cluster of properties on the right bank. On the right bank the Dilston Haughs defence embankment ties into high ground at the bridge with AEP 1.3% (75 years), and downstream of the bridge a floodwall with AEP 1.3% (75 years) protects The Stanners and Little Croft. This ties into an embankment near the Cricket Club Pavillion also with AEP 1.3% which continues downstream to the railway embankment. There are no defences on the left (north) bank at Wellbank. Flood zones and properties flooded in December 2015 are shown in Figure

16 Figure 13.8: Corbridge location, flood zones, and properties flooded in December 2015 (1) Review modelled and observed flood levels Figure 13.9 shows the modelled elevation of the north bank of the River Tyne in the Well Bank area, together with post-flood survey points, modelled water levels and properties affected during Storm Desmond. The Tyne model at Corbridge was built from survey taken in July Upstream of Well Bank (XS 5007) 1m resolution LiDAR flown in March 2012 shows the bank to be 1.2m lower than the 2005 survey, but at Well Bank and downstream the modelled bank crests appear broadly reasonable when compared to 1m and 2m resolution LiDAR (Figure 13.9). Elevation data are not available for this reach in AIMS; as the banks are well vegetated LiDAR data may be somewhat uncertain. We recommend undertaking check survey to confirm the height of the river bank in this reach. 9

17 Figure 13.9: Model long section at Corbridge Figure 13.9 shows that flood levels in September 2008 and January 2005, as well as December 2015, exceed the property thresholds of at least four properties at Wellbank, Corbridge. We know that four Wellbank properties did flood in January 2005 (5.25 m stage), but none in November 2015 (4.64m stage, the same level as September 2008, when we believe no Wellbank properties flooded). Earlier in this study (Model inflows report) model inflows to the main Tyne model were revised upwards for events larger than the 10-year return period compared to previous inflows. Consequently, modelled levels at Well Bank are now higher for a given return period than previously, and the model therefore predicts that properties here flood more frequently. The water level at which Wellbank properties flood is at approximately the updated 25 year return period. Assuming for now that the flood threshold for Wellbank is 4.64m (November 2015 and September 2008 observed level), this level has been reached four times in the 18 year record at Corbridge, giving an exceedance frequency of 0.22%. The 25 year exceedance frequency is 0.04%. There is therefore potentially a discrepancy between the modelled and observed flood frequencies. However, two of the four exceedances are in 2015 (15 November and 5 December), and the statistical method assumes that these are independent flood events; whether this is really true is the subject of current hydrological debate, as the very heavy rainfall observed in November 2015 will have created catchment conditions conducive to producing severe floods for many following weeks. Further investigation of the discrepancy between the modelled and observed flood frequencies would be useful, to ensure that the model hydrology is representative. (2) How Corbridge Bridge is modelled Overall, representation of Corbridge Bridge is adequate in the existing model. To improve model stability we have revised floodplain representation on the south (right) bank so that above the 100 year event, flow is conveyed more stably downstream when the bank is overtopped. If retaining the model in 1D, we recommend no further changes. There may be some benefit to representing the floodplain in 2D to capture floodplain flow routes which cannot be represented in 1D, for example flow routes between buildings. Using a 2D model to assess whether the gravel island at Corbridge affects water levels at properties would require starting the model build afresh, with 2D representation of both the channel and the floodplain; this is a significant undertaking. Using the 1D model initially to gain understanding of how important the gravel island is would be sensible. 10

18 13.6 Ovingham At Ovingham the following modelling issues have been identified: Outline what would be required to include the Whittle Burn flows in the existing River Tyne model. What would need to be considered to produce a joint Whittle Burn/ River Tyne model and produce model flood extents? Response and recommendations Ovingham village lies on the north (left) bank of the Tyne, at the confluence of the Whittle Burn with the Tyne. There are no raised defences protecting Ovingham. Whittle Burn is not modelled at present, and the only information about water levels there can be gleaned from extending cross sections in the 1D reach of the main Tyne. This is far from ideal, as there may be interaction between the two watercourses including a backwater effect. Figure shows the location, model extent and flood information for Ovingham. Figure 13.10: Ovingham location, flood zones, and properties flooded in December 2015 Since the 1D Tyne model was built there have been significant advances in both hydraulic modelling and computing, and it would be straightforward to add the Whittle Burn to the Tyne model, and also to implement a 2D domain in the (currently 1D) HEC-RAS model. As there is now capability in HEC-RAS to model in 2D, we see no need to convert the model to a different software. Once calibrated against the flood event data, both actions would greatly improve understanding of flood risk in Ovingham. To produce a joint model and flood extents would require the following steps: Obtain survey cross sections for Whittle Burn and add to Tyne model Define 2D domain, and implement in HEC-RAS model Derive inflows and hydrograph for Whittle Burn - this will require consideration of the outflow from Whittle Dene reservoir and phasing with the main Tyne peak Calibrate and validate model against flood extent evidence from December 2015 and other large events, such as January 2016 on the Whittle Burn Explore whether an integrated model, which could include surface water flooding, would be appropriate in Ovingham (EA are currently working with Northumbrian Water and Northumberland County Council on this issue) 11

19 13.7 Hexham Run model, attempting to reproduce December 2015 flood extent. When calibration satisfactory, produce flood outlines At Hexham the following modelling issues have been identified (1) We have additional data collected from the winter floods at Bridge End and Eggers. Please use this data for calibration/validation/reviewing model performance. During the event there was significant overtopping of high ground. (2) It has also been reported that there may have been flow paths near Eggers which had not previously been included as part of flood mapping potentially culverts/pipes (3) Consider what is required to accurately represent the confluence of the Tyne with the Cockshaw Burn, and the backing up effect. (4) Also consider whether there is any scope for improving modelling in the Tyne Mills area which suffered flooding in December Response and recommendations Hexham is on the main River Tyne about a kilometre downstream of the North Tyne-South Tyne confluence. A flood embankment on the left bank, tied into high ground, protects Bridge End Industrial Estate with AEP 1% (100 years). Tyne Mills Industrial Estate on the right bank is undefended from the Tyne. Figure shows the location and flood information for Hexham. Figure 13.11: Hexham location, flood zones, and properties flooded in December 2015 (1) Using additional data When the Hexham model was updated in 2012 to ESTRY-TUFLOW the left bank representation was retained and the right bank linked to a 2D domain. The left bank floodplain, which includes Bridge End and Eggers, is currently not modelled in 2D (Figure 13.12). 12

20 Figure 13.12: 2D model domain and wrack mark elevations, Hexham Wrack marks collected on the left bank in and near Bridge End industrial estate were at elevations between 33.98mAOD and 34.35mAOD, shown in Figure Figure 13.13: Long section, Tyne left bank at Hexham Results from the D HEC-RAS hydraulic model gave the 100 year water level as 34.10mAOD at cross-section Now that model inflows have been updated for this study the 100 year water level at cross-section 5809 is 33.88mAOD, which reflects the fact that the right bank is now modelled 13

21 in 2D, and inflows have been updated for this study. The 100 year level of 33.88mAOD broadly agrees with the wrack marks at mAOD, deposited by an event with estimated return period of 150 years. Interestingly, the water level at the overtopping point matches the 100 year water surface, and then downstream modelled levels appear too high (Figure 13.13). One explanation for this is that most of the left bank floodplain is not represented in the model; channel cross-section 5809 only extends to 130m from the bank. This limits the modelled flow which actually leaves the channel because flow routes such as roads are not represented. In reality we know that considerable flow did leave the channel and spread out via roads and floodplain flow routes between buildings, which would have lowered the overall water level. In summary, the additional data collected on the left bank at Hexham broadly agree with other evidence that the December 2015 event had a return period in excess of 100 years (i.e. the wrack marks are higher than the modelled 100 year water level). To further validate the model, we recommend it is run with peak flows and hydrograph which represent the December 2015 event, and the calibration adjusted to replicate the observed water levels. If necessary, the left bank could be represented by a 2D domain. (2) Unmodelled flow paths Surface water flooding is not captured in the current fluvial model. There are a number of surface water drains along Ferry Road which may have been overwhelmed during the December 2015 event, however the capacity of the drains and culverts in the area is not currently understood, and sewer network maps would need to be studied. Presently the left bank floodplain of the Tyne at Hexham (Ferry Road and Eggers area) is not modelled explicitly in a 2D domain, meaning that flow paths between buildings are not captured. To improve flood outline mapping on the left bank at Hexham, we recommend that the model is extended to include sewer network information and a 2D representation of the left bank. (3) Cockshaw Burn and Tyne confluence Cockshaw and Halgut Burns have small, steep and flashy catchments with a mix of rural and urban land uses, and both have historically caused property flooding in Hexham. A comprehensive FAS, completed in 2007, now protects most property to a high SOP; this consists of a flood diversion channel for Cockshaw Burn, and upstream storage on Halgut Burn. The 2012 ESTRY-TUFLOW model contains four 1D networks covering Cockshaw Burn, Halgut Burn, the flood diversion tunnel and the River Tyne from the A69 road bridge to 500m downstream of Broomhaugh Island. Three separate 2D domains, all with 2m grid size, represent over-bank flow (Figure 13.12). In the ESTRY-TUFLOW model Cockshaw Burn is linked to the River Tyne and includes the flapped outfall on the diversion channel, which facilitates modelling of any flow backing up Cockshaw Burn as a result of high levels in the Tyne. The existing representation of the interaction between the Cockshaw Burn and the River Tyne is deemed appropriate. Greater uncertainty lies in the timing of peak flows on the burns relative to the Tyne. In the existing model, critical storm durations for each watercourse are applied in the model and the inflow hydrographs phased so the Tyne is at a low, rising flow equivalent to QMED when peak flows occur on the burns. No sensitivity testing has been carried out relating to phasing the peaks so they occur simultaneously or applying a catchment-wide critical storm. (4) Additional comment regarding Tyne Mills In the December 2015 event, the Tyne Mills area on the right bank was flooded. This area is included in the existing 2D domain, with a grid resolution of 2m which is sufficient to capture floodplain flow routes between buildings. The predominant risk here is from the Tyne rather than the unnamed watercourse which runs through the industrial estate. The unnamed watercourse is not explicitly modelled, but appears as a depression in the 2D grid which allows water to propagate up from the Tyne; when channel capacity is exceeded, flooding occurs from the Tyne. There is no known control such as a flapped valve to prevent water backing up from the Tyne. We believe the current representation of the watercourse is adequate, and it does not need to be modelled explicitly. A surveyed cross-section of the unnamed watercourse could be used to check geometry of the 2D grid. 14

22 13.8 Summary of modelling issues Riding Mill We recommend running the model with the updated inflows and downstream boundary derived for this study, and all available evidence from the December floods to check the calibration. In addition we recommend that an integrated fluvial-surface water model is considered. If a satisfactory 1D calibration cannot be obtained, we recommend the model is converted to a linked 1D-2D model. Haydon Bridge We recommend carrying out sensitivity testing of the model to investigate the possible impacts of re-modelling or blocking the two culverts underneath the railway line, also to channel roughness and bed levels around the gravel island upstream of the railway bridge. We propose incorporating the existing 1D Langley Burn model into the existing 1D-2D South Tyne model, to improve representation of the interaction between the South Tyne and the Langley Burn. We recommend conducting topographic survey to confirm the gauge datum and resolve the discrepancy with model data. Warden We recommend re-surveying defence and bank crests along the south bank near Riverside Cottage and incorporating this new survey into the existing ISIS-TUFLOW model. We recommend additional survey along the north bank of the South Tyne from upstream of the paper mill to the upstream extent of the formal defences, to confirm ground heights, followed by updating the model geometry and re-calibrating the model. Corbridge We recommend undertaking check survey to confirm the height of river banks in the Wellbank reach. Further investigation of the discrepancy between the modelled and observed flood frequencies would be useful, to ensure that the model hydrology is representative. Conversion of the 1D HEC-RAS model to a linked 1D-2D HEC-RAS model would facilitate modelling floodplain flow routes which are not captured in a 1D-only model. If understanding the impact of the gravel island on water levels is considered essential, we recommend a preliminary investigation using the 1D model, to determine whether building a full-channel plus floodplain 2D model is justified. Ovingham We recommend surveying Whittle Burn and adding it to the existing HEC-RAS model, then converting the model to HEC-RAS 1D-2D. If ongoing consultation between the EA and partners determines a need for surface water modelling in Ovingham, an integrated model, rather than a HEC-RAS 1D-2D model, should be used instead. Hexham Additional data collected on the left bank at Hexham broadly agrees with other evidence that the December 2015 event had a return period in excess of 100 years. The model should be extended to include sewer network information and/or a 2D representation of the left bank if representation of surface water flow routes is required. The existing representation of the interaction between the Cockshaw Burn and the River Tyne is deemed appropriate, but sensitivity testing relating to phasing the peaks so they occur simultaneously, or applying a catchment-wide critical storm, could be carried out. The existing representation of the Tyne Mills area is deemed appropriate for assessing flood risk from the River Tyne here. A surveyed cross-section could be used to check the existing model geometry of the unnamed watercourse. 15

23 14 Flood resilience issues 14.1 Overview In this chapter we examine flood resilience issues identified by the Environment Agency, and propose recommendations for improving flood warning and forecasting Riding Mill At Riding Mill the following flood resilience issue has been identified: Five properties flooded during the Storm Desmond event, possibly due to River Tyne flood flows backing up the March Burn. What would be required to determine whether an appropriate flood warning threshold can be set from the Riding Mill gauge on the River Tyne, based on this flooding mechanism? Response and recommendations At present there are no Flood Warning Area polygons at Riding Mill. Our brief assessment of the backwater caused by the Tyne in March Burn (Section 13.2) suggests that this does not extend far enough up March Burn to directly flood properties in Riding Mill. However it may influence the discharge of the March Burn to cause secondary flooding. As recommended in Section 13.2, the March Burn model should be run with the updated inflows and all available evidence from the December floods to check the calibration. If the model results indicate that interaction between March Burn and the Tyne can cause flooding in Riding Mill, then flood outlines can be mapped and appropriate warning areas delineated. To set warning thresholds at the Riding Mill (River Tyne) gauge we would need the following: Surveyed property thresholds, or LIDAR and address layer All available information about flood onset in the December 2015 events All available information about other large events in the Tyne which did or did not cause flooding in Riding Mill Level record from Riding Mill gauge The steps outlined above would allow flood warnings to be issued based on water levels in the River Tyne only. If modelling showed the March Burn to pose a flood risk independently of levels in the Tyne, a new gauge would need to be installed on the March Burn itself to enable warnings based on levels in the Burn Haydon Bridge At Haydon Bridge the following flood resilience issues have been identified: (1) The onset of flooding at the Joiners Shop, Temple Houses seems to occur before flood warning thresholds are met, while other properties in Haydon Bridge had not flooded when result thresholds were exceeded. This suggests some properties may not be in the most appropriate warning area. Provide recommendations on what a full review would entail and what additional information is required. (2) Flood warning result thresholds were met during the event, however the timing of flooding to properties in different areas suggested that a review of the flood warning areas and thresholds is required. Provide recommendations on what this review could entail. Response and recommendations Flood Warning Areas and warning thresholds have been updated at Haydon Bridge following the December 2015 event. Flood Warning Areas at Haydon Bridge are shown in Figure 14.1, and warning thresholds set at Haydon Bridge gauge (23004) in Table

24 Figure 14.1: Existing FWAs at Haydon Bridge Table 14.1: Current flood warning thresholds, Haydon Bridge gauge FWA Properties at risk RES FW, m stage 121FWF FWF FWF (1) and (2) Review existing flood warning provision Flood Warning Areas and thresholds have been adjusted by the Environment Agency using information collected following Storm Desmond. A full review of the existing flood warning provision at Haydon Bridge, however, would need to include the following steps: Collate as much information as possible about the time of flooding onset in December 2015, and other large flood events. Timing information is key to setting accurate warning thresholds as it allows observed flooding to be related to the gauged hydrograph. Postevent information collected so far from the Haydon Bridge community includes some timing information, but not from all properties; we recommend a review of existing information followed by a targeted search for additional information (e.g. further property visits or letters) once the flood mechanisms are better understood. Check model calibration to ensure modelled flood mechanisms corroborate those observed in December 2015, and other large events. This may involve running the model with flow and hydrograph shape which represent those recorded during the event. A preliminary model run has been completed with observed flows at Haydon Bridge gauge input to the upstream extent of the model; however floodplain storage and lack of lateral inflows meant the modelled hydrograph does not match the observed at Haydon Bridge, therefore further calibration is required before using the model results directly for this purpose. Assuming model calibration is acceptable, run hydraulic model with updated inflows for design events 17

25 Map flood risk at modelled return periods (currently 20, 75, 100 and 1,000 years) Review flood warning areas against new flood outlines to ensure risk to properties is split appropriately. Consider properties flooded in December 2015 and other large events to ensure polygons are appropriate. Review warning thresholds, ensuring that properties flooded in December 2015 would receive an appropriate warning in a repeat of that event, and that RES FW is higher than previous large events which did not result in flooding Incorporate Langley Burn into the 1D-2D Haydon Bridge model to determine risk to property from it; if modelling showed flood risk from the Burn alone, install a gauge on the Burn to use as a basis for issuing flood warnings. Develop a forecasting model for Brigwood gauge; this gauge is intended to be used for flood warning purposes in future rather than Haydon Bridge gauge, in line with the Consistent Thresholds approach adopted by the Environment Agency. To carry out a full review of flood warning for Haydon Bridge, the following additional data would be required: 14.4 Warden Property threshold survey (which has been obtained for most affected properties, and work to collect threshold survey at remaining properties is ongoing), or LIDAR which could be used with address points to determine thresholds. At Warden the following flood resilience issues have been identified During Storm Desmond the Warden warning was issued at 14:10, and over-topping of the floodwall adjacent to Riverside Cottage occurred at approximately 15:45. It was discovered that properties situated on the south side of the watercourse began flooding before the warning was issued (Coastley Burn Foot, Ferryman s Cottage, Baddox Farm from 12:00 onwards). The event has highlighted that the current warning service needs to be improved. Based on the data validation information can you suggest what would be needed to split the existing warning areas and how thresholds could be determined for these. Response and recommendations Flood Warning Areas and thresholds have been adjusted by the Environment Agency using information collected following Storm Desmond. Flood Warning Areas at Warden are shown in Figure 14.2, and warning thresholds set at Warden level gauge (23025) in Table

26 Figure 14.2: Existing FWAs at Warden Table 14.2: Current flood warning thresholds, Warden gauge FWA Properties at risk RES FW, m stage 121FWF FWF Properties on the south bank of the Tyne have been separated into a new Flood Warning Area with a new lower threshold based on data collected following Storm Desmond. Flooding at Coastley Burn Foot, which is reported to have commenced at approximately 1130 (am) is most likely to have come from Darden Burn rather than the South Tyne, although may have been secondary flooding caused by high water in the South Tyne. However Darden Burn is not represented explicitly in the South Tyne 2D model. Other properties on the south bank which flooded at approximately 1200 (am) may be assumed to have flooded from the South Tyne. At 1200 the level at Warden gauge was 5.1m, therefore 0.4m lower than the current Results Threshold. A full review of the existing flood warning provision at Warden would need to include the following steps: Confirm whether flooding in December 2015 at Coastley Burnfoot originated from the South Tyne or Darden Burn. Check model calibration to ensure modelled flood mechanisms corroborate those observed in December Model calibration has been undertaken as part of this study and makes recommendations for checking ground levels in a small number of isolated locations (see Volume 2, Appendix G for full details). Following these checks, it may be necessary to update the model geometry and re-run the model. Represent Darden Burn in the 2D model if flooding to property is known to originate from the Darden Burn. Assuming model calibration is acceptable, run hydraulic model with updated inflows for design events Map flood risk at modelled return periods (currently 20, 75, 100 and 1,000 year) 19

27 Review flood warning areas against new flood outlines to ensure risk to properties is split appropriately. Consider properties flooded in December 2015 to ensure polygons are appropriate. Review warning thresholds, ensuring that properties flooded in December 2015 would receive an appropriate warning in a repeat of that event, and that RES FW is higher than previous large events which did not result in flooding To carry out a full review of flood warning for Warden, the following additional data would be required: Property threshold survey (which has been obtained for most affected properties, and work to collect threshold survey at remaining properties is ongoing), or LIDAR which could be used with address points to determine thresholds For best efficiency, we suggest that flood warning reviews for Haydon Bridge and Warden are carried out together Ovingham and Prudhoe At Ovingham and Prudhoe the following flood resilience issues have been identified We currently provide a Flood Warning Service at Ovingham (121FWF305 River Tyne at Ovingham, and 121FWF306 River Tyne at Bywell and Low Prudhoe). Provide recommendations on how best to split the Flood Warning Area polygons for both warnings, as there is some confusion within the community over which warning area covers which property. Response and recommendations Current Flood Warning Areas at Ovingham and Prudhoe are shown in Figure 14.3, and the warning thresholds in Table Figure 14.3: Existing FWAs at Ovingham and Prudhoe 20

28 Table 14.3: Current flood warning thresholds, Bywell gauge FWA Properties RES FW, m stage 121FWF FWF In Ovingham village flooding is reported to have begun slightly before 1900 at 1 Burnside Close, although it is not known if this was actually the first property to flood. The level at that time at Ovingham gauge is not known, as the gauge stopped recording at 1400 (5.39m). Time of first flooding in December 2015, compared to other events at this location, is likely to have been brought forward by factors such as surface water flooding (failure of pumping station contributed to this), Whittle Burn flow, and afflux caused by the Ovingham Bridge scaffolding. As the scaffolding was temporary its effects would not be taken into account when setting flood warning thresholds. Since December 2015 an interim flood warning review has been carried out by the Environment Agency. Ovingham is now in a separate Flood Warning Area (121FWF305), and properties in Bywell and Low Prudhoe have been combined (121FWF306). We recommend that a more in-depth flood warning review be carried out for Ovingham and Prudhoe if and when an improved hydraulic model becomes available. Any future model should be extended upstream to Bywell gauge to allow appropriate thresholds to be determined Hexham At Hexham the following flood resilience issues have been identified: Properties within the Tyne Green area of Hexham were flooded from the River Tyne during Storm Desmond, however they did not receive a flood warning as their warning area is only based on flooding from the Cockshaw Burn (which was not in flood during the event). The risk from both watercourses needs to be defined and an appropriate flood warning area and threshold needs to be set for properties that are at risk from the Tyne and Cockshaw Burn. Can you recommend what would be required to do this? Response and recommendations Current Flood Warning Areas for the Tyne at Hexham are shown in Figure 14.4, and warning thresholds set at Hexham gauge (23020) in Table

29 Figure 14.4: Existing FWAs at Hexham Table 14.4: Current flood warning thresholds, Hexham FWA Properties RES FW, maod 121FWF Currently properties within FZ2 at Tyne Green are only warned about possible flooding from Cockshaw and Halgut Burns. There is residual risk from the River Tyne, but the existing flood warning areas do not include Tyne Green. Flood warning improvements were carried out in 2010, since when the hydraulic model and flood outlines have been updated. The current model has been used to map flood risk, and the resulting outlines could be used to define FWAs and set warning thresholds. The following steps would be needed: Check model calibration to ensure modelled flood mechanisms corroborate those observed in December This may involve running the model with flow and hydrograph shape which represent those recorded during the event Assuming model calibration is acceptable, define new flood warning areas for flood risk from the Tyne, based on mapped outlines. If necessary, split existing FWAs so that properties at risk from both the Tyne and Cockshaw Burn are separated from those at risk from a single watercourse. Using property threshold survey (or LIDAR and address points) define First Property To Flood in each FWA. If FWA is at risk from both the Tyne and Cockshaw Burn, FPTF for each watercourse may be different. Considering average observed rate of rise on the Tyne, set warning thresholds. Ensure that properties flooded in December 2015 would receive an appropriate warning in a repeat of that event, and that RES FW is higher than previous large events which did not result in flooding. 22

30 14.7 Summary of flood resilience recommendations Riding Mill We recommend the March Burn model be run with the updated inflows and all available evidence from the December floods to check the calibration. If the model results indicate interaction between March Burn and the Tyne, flood outlines can be mapped and appropriate warning areas delineated. Haydon Bridge We recommend a full flood warning review (and search for additional information) which focuses on establishing the timing of flooding onset at as many properties as possible. Check that model calibration accurately reproduces the December 2015 event, and remap flood risk. Incorporate Langley Burn into the hydraulic model, and develop a forecast plus thresholds for Brigwood gauge. Warden We recommend a full flood warning review which establishes the source(s) of flooding for the affected properties. Ensure the model calibration accurately reproduces the observed flood mechanisms. Incorporate Darden Burn in the hydraulic model, and remap flood risk. Ovingham and Prudhoe Once a new hydraulic model has been developed following discussion between the EA and partners about modelling surface water flood risk, a full flood warning review is recommended. Hexham Ensure the model calibration accurately reproduces observed flood mechanisms. Use the 2D model and updated outlines to define new FWAs for risk from the Tyne (left and right banks) and set warning thresholds. 23

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