Hodder Water Treatment Works - Ribble Valley

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Hodder Water TreatmentWorksFlood Risk Assessment21 August 2019

Mott MacDonaldFloor 31 Whitehall RiversideLeeds LS1 4BNUnited KingdomT 44 (0)113 394 6700mottmac.comHodder Water TreatmentWorksFlood Risk Assessment21 August 2019Mott MacDonald Limited. Registered inEngland and Wales no. 1243967.Registered office: Mott MacDonald House,8-10 Sydenham Road, Croydon CR0 2EE,United Kingdom

Mott MacDonald Hodder Water Treatment WorksFlood Risk AssessmentIssue and Revision tionA21/08/19E HaleD ChickE WrenFirst IssueDocument reference: 80040117-01-MMB-HODDE-NA-97-RP-I-0007Information class: StandardThis document is issued for the party which commissioned it and for specific purposes connected with the abovecaptioned project only. It should not be relied upon by any other party or used for any other purpose.We accept no responsibility for the consequences of this document being relied upon by any other party, or beingused for any other purpose, or containing any error or omission which is due to an error or omission in data suppliedto us by other parties.This document contains confidential information and proprietary intellectual property. It should not be shown to otherparties without consent from us and from the party which commissioned it.This r epo rt h as b een pre par ed s olely fo r us e by the par ty which co mmissio ned i t (t he ‘Clien t’) in conn ectio n with t he c aptio ned pr oject. I t sho uld not b e us ed f or a ny o the r pu rpos e. No pe rson oth er t han the Clie nt o r a ny pa rty w ho h as ex pres sly ag ree d te rms of r elianc e with us (t he ‘Re cipien t(s)’ ) m ay r ely on the cont ent, i nfo rma tion or a ny views exp resse d in t he rep ort. W e acc ept no d uty o f ca re, resp onsibility or lia bility to any oth er recipie nt of this docu men t. T his r epo rt is c onfid ential and cont ains p rop riet ary in tellect ual p rop erty .No re pres enta tion, w ar ranty or und ert aking, exp ress or i mplied, is m ade and no resp onsibility or li ability is accept ed by us to a ny p arty othe r th an t he Clie nt o r an y Recipi ent( s), as to the accu racy or co mpl eten ess of the info rma tion c ontai ned in this re port . F or t he a voida nce of d oubt this repo rt d oes not i n any way p urp ort to incl ude any l egal, i nsu ranc e or fina ncial a dvice or o pinio n.We disclaim all a nd a ny liability whet her arisi ng in tort or cont ract or othe rwise w hich it might oth erwise hav e to any par ty ot her tha n the Client or t he R ecipien t(s) , in resp ect of this rep ort, or any in for matio n at trib uted to it.We acce pt no res ponsi bility fo r a ny er ro r or omissi on in the re port which is due to an e rro r o r o mission i n d ata, i nfor mati on o r sta tem ents suppli ed t o us by ot her pa rties in cludin g th e client (‘Dat a’). W e hav e n ot ind epe nde ntly ve rified such Data and hav e ass ume d it t o be accu rat e, co mplet e, r eliable an d cu rre nt as of t he d ate of suc h inf orm ation .For ecasts pre sent ed in this d ocu ment wer e p repa red usin g Dat a an d th e re po rt is d epe nde nt o r bas ed on Dat a. I nevita bly, so me of th e ass um ptions use d to devel op t he fo rec asts will n ot b e re alised and un anticip ated eve nts a nd cir cums tanc es m ay occ ur. C onse que ntly Mott MacDo nald doe s no t gu ara ntee or w ar rant the conclu sions c ont ained in th e r epo rt as the re are lik ely to be differ enc es be twee n the for ecast s an d th e act ual r esults and thos e diff ere nces may be mat erial. W hile we consid er t hat the i nfor mati on a nd opinio ns giv en in this r epo rt a re s o und all par ties must rely o n th eir own skill a nd ju dge me nt whe n m aking use of it.Under no ci rcu mstan ces may t his re por t or any extr act o r su mm ary t he reof be used i n co nnecti on wit h any pu blic or priv ate s ecuriti es of ferin g incl uding any rela ted me mor and um or p rosp ectus for any secu rities offe ring or st ock ex chan ge listi ng o r a nno unce ment .80040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 2019

Mott MacDonald Hodder Water Treatment WorksFlood Risk AssessmentContentsExecutive summary11Introduction21.11.21.32222Site information42.12.2444455662.32.42.53Site mapExisting infrastructure2.2.1Existing watercourses2.2.2Stocks Reservoir2.2.3Existing water mains and drainage infrastructureEvidence of historical floodingSite topographyExisting ground conditionsAssessment of flood risk to the ope of assessmentProposed developmentFluvial flooding3.1.1Fluvial Flood Map3.1.2Site Specific Fluvial Flood RiskSurface water flooding3.2.1Surface water flood map3.2.2Influence of climate changeGroundwater floodingFlooding from sewers and drainsFailure of infrastructureStocks ReservoirWater treatment works infrastructureAccess and egress7778111112131313131414Assessment of flood risk as a result of the l flooding4.1.1Pass forward flows from reservoir4.1.2Loss of floodplain storageSurface water flooding4.2.1Management of surface water flood risk during constructionGroundwater floodingFlooding from sewers and drainsReservoir flood risk80040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 2019

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessment4. flood riskDownstream flood risk1616Application of the National Planning Policy Framework175.117171718National Planning Policy5.1.1National Planning Policy Framework5.1.2The Sequential Test5.1.3Exception TestConclusions196.16.26.3191920Flood risk to the proposed developmentFlood risk resulting from the proposed developmentCompliance with National Planning PolicyAppendices21A.Stocks Reservoir Flood 0007 21 August 2019

Mott MacDonald Hodder Water Treatment WorksFlood Risk AssessmentExecutive summaryMott MacDonald has been appointed by Mott MacDonald Bentley (MMB) to undertake a floodrisk assessment (FRA) on behalf of United Utilities at Hodder Water Treatment Works (WTW)(the site), located to the north east of Slaidburn in the Forest of Bowland.The site is bounded to the north by Stocks Reservoir, which supplies water for treatment atHodder WTW. The River Hodder flows adjacent to the site on the south eastern boundary.The proposed development comprises installation of a new building containing rapid gravityfilters (RGFs) which are required to ensure a consistent supply of quality drinking water. Inaddition, works to raise the crest level of the reservoir spillway weir will be undertaken toincrease the capacity of the reservoir, helping to safeguard water supply to the north west ofEngland in times of drought.The proposed development is not considered to be at risk of flooding from fluvial, surface water,groundwater or sewer flooding. The treatment works themselves pose a risk shouldinfrastructure become blocked or malfunction, however, this is considered to be managedthrough ongoing operation and maintenance of the treatment works by trained operatives. Dueto the location of the site immediately downstream of Stocks Reservoir, the site will beinundated should the reservoir embankment breach or fail. However, this is not considered to bea significant source of flood risk owing to regulations under the Reservoirs Act 1975 forinspection and maintenance of Category A reservoirs and the requirement to safely pass flowsup to the PMF (approximately equivalent to a 1:10,000-year flood) over the spillway.Similarly, although works to increase the capacity of the reservoir by raising the spillway levelmay result in a greater extent of flooding downstream should the embankment breach, thelikelihood of this occurring is very low. Raising the spillway crest level will not result in floodingof any additional receptors. Therefore, no significant change to reservoir flood risk is anticipatedas a result of this scheme. In addition, the proposed development will not increase flood riskelsewhere from fluvial, surface water, groundwater or sewer flooding. The increased capacity ofthe reservoir will contribute to greater attenuation of fluvial flood flows downstream.The proposed development meets the Exception Test to allow the development of essentialinfrastructure in Flood Zone 3. Therefore, it is considered that the proposed development iscompliant with the National Planning Policy Framework (NPPF).80040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 20191

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessment1 Introduction1.1BackgroundMott MacDonald has been appointed by Mott MacDonald Bentley (MMB) to undertake a floodrisk assessment (FRA) on behalf of United Utilities at Hodder Water Treatment Works (WTW),located in the Hodder Valley to the north east of Slaidburn, Forest of Bowland, Lancashire. TheNational Grid Reference for the Site is SD718545.United Utilities are undertaking works at Hodder WTW (hereafter referred to as the site) toupgrade the treatment process to improve water quality and increase reservoir capacity.1.2Scope of assessmentThe site is partially situated within Flood Zones 2 and 3. Therefore, a FRA must accompany anyplanning application to ensure the renovations do not lead to an increase in risk of floodingeither at the site or downstream.This FRA has been carried out in accordance with the National Planning Policy Framework(NPPF)1 and associated Planning Practice Guidance (PPG)2 to assess the risk of flooding to thesite from all sources, and the possible impact of the development on flood risk elsewhere. Thescale and nature of the FRA is considered appropriate for the development.Information presented within this report is dependent upon the accuracy and reliability of thesupplied information, correspondence, and data available to Mott MacDonald, at the time of theassessment. Any party developing detailed design should not rely on assumptions made in thisreport but should satisfy themselves in that regard.Mott MacDonald has followed accepted procedure in providing the services but, given theresidual risk associated with any prediction and the variability that can be experienced in floodconditions, Mott MacDonald takes no liability for and gives no warranty against actual flooding ofany property or the consequences of flooding in relation to the performance of the service. Thisreport has been prepared for the purposes of supporting a planning application only. MottMacDonald accepts no responsibility or liability for this document to any party other than bywhom it was commissioned.1.3Proposed developmentThe proposed development comprises installation of a new building containing rapid gravityfilters (RGFs) which are required to ensure a consistent supply of quality drinking water. Watertreatment works which need to remain operational in times of flood are classed as “essentialinfrastructure” in the PPG. Therefore, the proposed development is also considered to beessential infrastructure.In addition, permitted development rights have been granted for works to raise the crest level ofthe reservoir spillway weir. This will increase the capacity of the reservoir, helping to safeguardwater supply to the north west of England in times of drought. While these works are permitted1Ministry of Housing, Communities & Local Government (2019) National Planning Policy Framework. Accessed 16/08/2019. Availablefrom: l-planning-policy-framework--22Ministry of Housing, Communities & Local Government (2019) Planning Practice Guidance. Accessed 16/08/2019. Available -RP-I-0007 21 August 20192

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessmentdevelopment, Ribble Valley Borough Council (RVBC) have requested their inclusion in the FRA.These works are considered to be “water compatible” under the PPG.The design life of the combined works is taken to be 100 years for the purpose of thisassessment. Individual elements may have shorter lifespans and require replacement over thelifespan of the overall works.80040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 20193

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessment2 Site information2.1Site mapThe layout and location of the site is shown in Figure 2.1.Figure 2.1: Site locationSource: Mott MacDonald, 20192.22.2.1Existing infrastructureExisting watercoursesThe site is bounded to the south east by the River Hodder, which flows within an engineeredchannel for approximately 200m before returning to a natural channel profile. To the west,Phynis Beck flows south east and joins the River Hodder at the southern corner of the site.2.2.2Stocks ReservoirStocks Reservoir is a Category A reservoir located immediately upstream of the site. Thespillway weir is located at the south east corner of the reservoir. The spillway discharges to theRiver Hodder approximately 240m downstream of the weir, adjacent to the site. A compensationflow is maintained to the River Hodder via a culvert through the reservoir embankment. There is80040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 20194

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessmenta daily flow gauge downstream of the spillway which indicates a mean flow of 0.583m 3/s3. It isassumed that the compensation flow is roughly equivalent to this.Figure 2.2 shows how flows are routed through the reservoir to the River Hodder.Figure 2.2: Flow routing through Stocks ReservoirSource: Mott MacDonald, 20192.2.3Existing water mains and drainage infrastructureUtilities records for the site indicate the presence of several surface water sewers as well asland drains. Flows are either captured by the treatment process on site or discharged to theRiver Hodder.In addition, United Utilities hold a consent to discharge up to 50l/s trade effluent into the RiverHodder, consisting of settled filter backwash effluent during planned downtime and in anemergency. The location of the discharge point at SD 71675 54226 is shown in Figure of historical floodingThe reach of the River Hodder extending between the site and Slaidburn (2.3km downstream) isnot a designated Main River and therefore the Environment Agency do not hold a record of pastflood events on this reach of the river.Anecdotal evidence from the site operatives indicates that no flooding has occurred at the sitewithin the last 10 years.3National River Flow Archive. 71002 – Hodder at Stocks Reservoir. Accessed 16/08/2019. Available 7100280040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 20195

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessment2.4Site topographyThe site is slopes downwards from the northern corner to the south east, where the site isbordered by the River Hodder. Elevations on site range from approximately 195mAOD toapproximately 140mAOD, as indicated in Figure 2.3.The proposed development is located at an approximate elevation of 168mAOD.Figure 2.3: Site topographySource: Mott MacDonald, 2019. Elevation data derived from Esri World Elevation Terrain data, source: Airbus, USGS, NGA, NASA,CGIAR, NLS, OS, NMA, Geodatastyrelsen, GSA, GSI and the GIS User Community2.5Existing ground conditionsAccording to the British Geological Survey (BGS) mapping4, the site is underlain by acombination of limestone and mudstone overlain by superficial till deposits. Records indicatethat much of the site has been artificially raised with made ground of variable composition.4British Geological Society (2019) GeoIndex Onshore. Accessed 16/08/2019. Available 0040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 20196

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessment3 Assessment of flood risk to thedevelopment3.13.1.1Fluvial floodingFluvial Flood MapThe Environment Agency publishes floodplain extents for all significant watercourses throughoutEngland. These extents, displayed on the Risk of Flooding from Rivers and Sea flood map5, areavailable to the public via the internet and are the primary source of publicly available flood riskinformation. The Environment Agency also provides the Flood Map for Planning which displaysthe Flood Zones (Figure 3.1).Figure 3.1: Fluvial Flood Map for PlanningSource: Mott MacDonald, 2019. Contains Environment Agency data Crown copyright and database rights 2018 OS 100024198.Contains public sector information licensed under the Open Government License v3.0.5Environment Agency. 2017 Risk of Flooding from Rivers and Sea dataset. Available under Open Government License v003.80040117-01-MMB-HODDE-NA-97-RP-I-0007 21 August 20197

Mott MacDonald Hodder Water Treatment WorksFlood Risk Assessment8Table 3.1 provides definitions of the Flood Zones as stated in the PPG 6. It should be noted thatthe flood extents given on the Flood Map are only indicative and do not necessarily account forany man-made structures such as railway embankments, roads, or flood defences.Table 3.1: Flood zonesFlood ZoneDefinitionZone 1 Low ProbabilityLand having a less than 1 in 1,000 annual probability of river or sea flooding.(Shown as ‘clear’ on the Flood Map – all land outside Zones 2 and 3)Zone 2 Medium Probability Land having between a 1 in 100 and 1 in 1,000 annual probability of riverflooding; or land having between a 1 in 200 and 1 in 1,000 annual probabilityof sea flooding. (Land shown in light blue on the Flood Map)Zone 3a High ProbabilityLand having a 1 in 100 or greater annual probability of river flooding; or Landhaving a 1 in 200 or greater annual probability of sea flooding.Zone 3b The FunctionalFloodplainThis zone comprises land where water has to flow or be stored in times offlood. (Not separately distinguished from Zone 3a on the Flood Map)Source: Ministry of Housing, Communities and Local Government (2014) Planning Practice GuidanceThe reservoir embankment and land at the eastern boundary of the site are within Flood Zone 2and Flood Zone 3, indicating the site is in areas of ‘medium’ (1% - 0.1% AEP) and ‘high’ ( 1%AEP) flood risk respectively.Consultation with the Environment Agency has identified that the Flood Zones in this locationare based on the 2004 National Generalised Modelling (NGM). The NGM is intended to beindicative, providing basic flood risk information for areas which had not been extensivelymodelled. The NGM does not represent flow routed through reservoirs or any representationspillways / overflows, and so the Flood Zones do not represent the true flood risk at the site.3.1.2Site Specific Fluvial Flood Risk3.1.2.1Influence of climate changeThe Environment Agency7 advise the higher central and upper end allowances for essentialinfrastructure in Flood Zone 2 and upper end allowance in Flood Zone 3a.Table 3.2: Peak river flow allowances for the North West River Basin DistrictAllowance Category2020s (2015-2039)2050s (2040-2069)2080s (2070-2115)Upper end20%35%70%Higher central20%30%35%Central15%25%30%Source: Environment Agency. 2019. -climate-change-allowances6Department for Communities and Local Government. 2012. Technical Guidance to the National Planning Policy Framework.Paragraph 5, Table 1: Flood Zones. [Online] Available ment/uploads/system/uploads/attachment data/file/6000/2115548.pdf[Accessed 17/06/2019]7Environment Agency (2019) Flood risk assessments: climate change allowances. Accessed 16/08/2019. Available DE-NA-97-RP-I-0007 21 August 2019

Mott MacDonald Hodder Water Treatment WorksFlood Risk AssessmentTherefore, all considerations of fluvial flood ris

risk assessment (FRA) on behalf of United Utilities at Hodder Water Treatment Works (WTW) (the site), located to the north east of Slaidburn in the Forest of Bowland. The site is bounded to the north by Stocks Reservoir, which supplies water for treatment at Hodder WTW. The River Hodder

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