Failures of Betts Hydro Flood Assessment

Background To This Article

The Rossett Focus Group and local residents have continuing concerns regarding the Castle Green Homes Development in Rossett. We are alarmed by the proposals to raise site levels and to introduce bunds around the existing site. This article has been produced as a result of the multiple concerns being expressed by local residents who live adjacent to both parcels of land and their general view is that Betts Hydro have totally failed to understand how and whence the Rossett flooding emanates. There is great concern that flooding to local properties will be drastically increased by Betts proposals, and the down-stream flooding already being experienced in Darland & Lavister will be made much worse by the  plan to export flood exceedance into the Darland School ditch. We have been in direct contact with Betts Hydro regarding historical information and they have paid as much heed to our communication as they have paid to NRW’s comments on the FCA (Flood Consequences Assessment) and Hydraulic modelling.

The current FCA is Betts Hydro fifth attempt at producing a solution to the flooding issues for Castle Green Homes. Bellis Brothers the former owners of the land previously commissioned Waterco back in 2018 to produce an FCA, but following their infiltration tests on the fields they clearly failed to provide a suitable SuDs solution.

This article refers to the Reserved Matters Planning Applications P/2021/0110 & P/2021/0111 for Residential Development of Two Parcels of Land Located North & South of Lane Farm Rossett Road Rossett Wrexham LL12 0DS. Outline Planning permission was given on appeal to P/2018/0560. There are 16 Conditions (Reserved Matters) placed on this planning permission by the Welsh Government Inspector and the Minister.

Increased Flooding Risk

The proposal to increase the topographic levels adjacent to the properties in Moss Green, Trevalyn Way, Trevalyn Hall View and Darland View will result in flooding to those properties if the Flood Flow Route is that predicted in Figure 5 (see below) of Betts Hydro FCA (see Hydraulic Modelling Assessment – Appendix I) . The existing topographic level of a property in Trevalyn Way is 13.09mAOD. The Castle Green site level is to be raised at a point running alongside this property and the other properties in Moss Green to at least 13.50mAOD.

At present should the River Alyn flood from the direction being suggested by Betts, the flood water can flow in the locations as suggested in Betts Hydro Figure 5 below but if the topographic (site) level is raised by the levels recommended by Betts the Bungalows in Trevalyn Way and Moss Green will be flooded, e.g., a typical Trevalyn Way bungalow has a Finished Floor Level of 13.24mAOD and can expect internal flooding to a depth of 260mm before the Castle Green raised site levels experience any flooding runoff from the predicted direction.

In addition to the bungalows referred to above, there will be a greater risk of flooding to the lower even number properties in Trevalyn Way and those odd numbered properties back towards Eaton Close as the topographic levels of the gardens are lower than the increased site levels being proposed by Betts Hydro. This worsening of the flood risk situation on the properties adjacent to the Southern Parcel of Land is completely unacceptable and contravenes the provisions of TAN15 Section 8.3 by potentially worsening the existing situation. 

Hydraulic Modelling Assessment (Appendix I) – Figure 5

Hydraulic Modelling Failures

It is quite clear from even the limited hydraulic modelling work undertaken by Betts that the whole area from the proposed development site back to the Alyn Road Bridge B5445 is at risk from flooding. The modelling undertaken by Betts does not include any constraint of the River Dee flood waters on the River Alyn upstream of Cooks Bridge. In the last two major flooding events 1976 and 2000 the River Dee was at an extremely high level resulting in flooding of Trevalyn Meadows from Almere to a considerable distance above Cooks Bridge towards Trevalyn.

In addition, no recognition has been given in the Hydraulic Modelling to the influence of the River Dee on the flood direction from the Farm Track south of the Development Site into Harwoods Lane. In the last major river flooding event in Nov 2000 the runoff from the development site was from the southern parcel of land into Trevalyn Way and Moss Green. In the January 2021 storm event the runoff was from the northern parcel of land into Trevalyn Hall View and Darland View with flooding experienced all around Darland View with actual internal flooding to a property in Darland View. 

Betts Hydro Appendix D gives rise to question the adequacy or reliability of any of the Hydraulic Modelling and the scenarios put forward in support of the Castle Green Development Plans. Not one of the suggested scenarios listed in Appendix D reconciles with the EA Evidence Photograph No 6 (see copy below) presented to the UDP Inquiry in 2002. In the EA photograph the River Alyn flooding quite clearly enters Alyn Drive Rossett from the adjacent land not from Station Road & Alyn Road Bridge with a known flood line in the photograph of 14.53m AOD by the far lamp column on the right. 

EA Evidence Photo 6

The photograph above depicts the real event of flooding mid-afternoon on the 6th November 2000. The modelling proffered by Betts Hydro supposedly includes climate change but the predicted flood water levels do not reach, in any scenario, the flood levels recorded in this 2000 photograph which was entered as EA evidence to the UDP Inquiry.

River Dee and River Alyn Combined Effects

The hydraulic modelling  has not included any blockage at Cooks Bridge (Hydraulic modelling node Alr02d). History has shown that with the River Dee breaking its banks and flooding Trevalyn Meadows the flood levels of the Dee and the Alyn have combined at a point up stream of Cooks Bridge. This fact was confirmed at the 2002 UDP Inquiry by Mr David Goodson the EA Officer responsible for this stretch of River Alyn at that time. At no point does the FCA recognise the NRW Development Advice Map which shows the C2 floodplain actually abutting the southern parcel boundary in Harwoods Lane. 

In view of the modelling failing to reach the areas depicted above which were flooded in 2000 no reliance can be placed on any of the flooding predictions made by Betts Hydro in their FCA or in their modelling Appendices.

In view of the perceived inadequacies of the hydraulic modelling it is considered that the FCA does not properly inform the flood risks being presented by the Castle Green Development and must result in the proposed drainage strategy failing to satisfy the Discharge of Condition 7 of the Outline Planning Consent granted by the Welsh Minister.  

Unsubstantiated Assumptions

In the base data a steady state River Flow has been assumed in the modelling  above and below Cooks Bridge but this is far from the case as the flood water levels of the River Dee on Trevalyn Meadows have under extreme flooding, impeded any steady state outfall conditions on that stretch of River and places further doubts as to the Hydraulic Modelling suitability or accuracy. There have been so many unsubstantiated assumptions in the base data of the hydraulic modelling between Pont-y-Capel and Cooks Bridge that its conclusions on flood risk appear meaningless as they continue to defy comparison with reality.

This omission to carry out risk assessment of any blockages at Cooks Bridge is a failure to accord with the provisions of TAN15(2004) Paragraph A1.17.17 and should require further hydraulic modelling to be undertaken using the known flood outlines in Alyn Drive and Harwoods Lane as an absolute minimum. 

Earthwork Bunds Presents More Risk

The proposals to build earthwork bunds to protect the Northern Parcel of land will place existing properties adjacent to the development site at further risk of flooding as well as some of the new proposed adjacent terraced houses. The imposition of  bunds to the NW and E boundaries as shown in Betts Figure 5 above together with the raising of site levels along the remainder of the NW boundary will effectively create a bund with the inherent problems of restricting surface water flow on or off the site as presently occurs.

The whole essence of Betts Hydro proposal appears to be to raise the site levels above the present ground water levels to facilitate some form of SUDs ground infiltration. It is noted that the earlier infiltration tests undertaken by Waterco have been repeated in part and again there are substantial areas of the site that will not support infiltration drainage.

Sustainable Drainage (SuDs)

In paragraph 5.6.3 of Betts FCA reference is made to encountering of groundwater during infiltration testing and has the remarks that “Should groundwater be encountered during any future construction phase(s) then suitable land drainage will need to be installed to intercept groundwater flows”.  The suggested provision of suitable land drainage is unlikely to prove successful as the  southern parcel of land apparently has a system of land drainage in the areas of poor test infiltration but this has not prevented the flooding pictured during recent major storm events.

The FCA fails to acknowledge the existence of an Aquifer on the site and the required SPZ1 protection required. Proposals to raise the ground levels to artificially high levels to create some degree of infiltration should be rejected as this process is totally in contravention of WCBC Policy EC12 and EC13 and the provisions of TAN15(2004) Paragraph A1.17.13.

Landscape and Layout Errors

At no time has it been acknowledged by Betts Hydro or Castle Green Homes that the LVIA (Landscape and Visual Appraisal) produced by Ark Company Landscape Architects for Bellis Brothers, that was approved on appeal (Document APP76), limits the ridge height of all dwellings on the new development to 7.30m above topographic level to ridge.

The whole suite of houses and flats are indicated on average to be well above 8.40m from FFL (Finished Floor Level) to ridge thus the proposals to raise a number of properties by over 1.50m will compound the problem further.  The average increase in site level to FFL is 660mm, in addition to impeding flood water flow it will create a situation of significant overshadowing and overlooking when compared to the approved LVIA of 7.30m to ridge.  

The plans being used by Betts Hydro obviously follow closely those created by Castle Green Homes but the plans do not accord with the determined access plans approved by the Inspector and Welsh Minister, and the housing layout should be amended to accord with those plans and should result in a revision of the drainage layout and possible re-siting of infiltration ponds into other areas. This failure to accord with the determined plans has been communicated to the Planning Case Officer and the Developers.

History of Flooding

The Betts Hydro proposals continue to extol the virtues of discharging any exceedance from both parts of the development site into the distant ditch in the grounds of Darland High School. The existing ditch has a history of flooding to substantial areas of Darland and Lavister and the proposal to discharge exceedance from 132 dwellings into an existing flood risk area is totally unsatisfactory.

The proposal to construct simple earthwork bunds in an attempt to stem the exceedance flow or flooding to neighbouring properties is also unacceptable. The outline planning consent was given following claims by the speculator’s agent that the community claims of flooding were mischievous and that no risks from flooding existed; the recent FCA by Betts Hydro has failed to uphold these claims, it acknowledges that the development is at risk of flooding and has proposed unreasonable site level changes in mitigation bearing in mind the inadequate modelling indicated above.

Suitable SuDs Scheme Not Forthcoming

The Speculator was advised by WCBC Flood Officer prior to appeal that failure to provide a suitable SUDs scheme may result in the proposed development being changed or reduced to meet SAB requirements. To date no suitable SUDs scheme has been tabled by Betts for Castle Green Homes and the planning consent is conditional on an approved SUDs scheme. Local residents have always contended that the development site is not sustainable or deliverable on drainage & flooding grounds but this has always been dismissed by the Speculators Agent.

To date nothing that Betts Hydro has produced has altered local perceptions and evidence that the development site results in runoff flooding to the surrounding area and will if built out make the situation worse. Under the circumstances it would not be appropriate to discharge Conditions or Reserved Matters as a suitable SUDs scheme has not been approved or proposals demonstrated that long term flood risks have been properly addressed.

Flooding Evidence Ignored

In the FCA it is clear that inadequate research was undertaken regarding the flooding incidents in Rossett since 1976 leading to wrong assumptions on where and when pluvial & fluvial flooding occurs. Had a proper internet search been undertaken by this company the Rossett Focus Group (RFG) website would have shown the level of flooding that has been evident over the last 45 years.

There have been two major flood events in 1976 and 2000 with a number of lesser but important river flooding events in the 1980s, 1990s and 2020s. A video link to the 2000 event is and was provided on the website. Examination of the flooding photo gallery should have alerted the company to the inherent flood risks in the area. Any close examination of the photographs show that the company have continued with a dismissive attitude towards pluvial and fluvial flood risks and have deliberately cropped Development Advice Maps to conceal directions of flood risk or have deliberately underplayed the flooding that occurred during the January 2021 storms as evidenced by Appendix G compared with the photographic details displayed on the RFG website.

Summary of Failures

In conclusion, the aspects and comments on the FCA and drainage strategy documents have considerable discrepancies between the text and the graphical content, diagrams or figures.

The proposals do not alleviate local concerns that the flooding risks both on and off site have been properly or adequately addressed.

The overall proposal to regrade the site from Harwoods Lane down to Darland View is both unacceptable and irresponsible as it will place existing residents at greater risk of flooding, it will impede natural water flows to and from the land.

The wisdom and permanency of earth bunds to impede or to attempt to contain exceedance in both the short and longer term must be questioned as there can be no guarantee in the proposals that existing properties will not be flooded along with a number of the new houses.

The issue of the lifetime expectancy of the earth bunds and how these can be guaranteed to survive intact for the anticipated lifetime of some 80 years for the development must be considered along with the issues of climate change.

Hydraulic modelling has always included a number of assumptions regarding inflow rates, run-off  and flow between gauging points etc. but this set of modelling parameters has more unknowns than knowns and downstream estimates are all guesswork as there is no downstream monitoring below the Alyn Road Bridge or the Castle Green Development site.

Confidence that the FCA properly informs on the flooding risks to the Castle Green Development site(s) has not been demonstrated by the latest version and that the Drainage Strategy built upon a deficient FCA should be rejected.

With all the above unsatisfactory issues the Betts Hydro FCA and Drainage Strategy fails to satisfy both the national and local policies on flood risk and SUDs and will result in increased flooding risks in the surrounding areas.

We place this here as Wrexham Council hide all objection documents from the public, which means no residents can see what other residents are objecting about and how many objections that are being presented, also there is no means to check that your objection has actually been read and placed on file.

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