Cotswold Water Cycle Study

METHODOLOGY

General approach

The Water Cycle Study to accompany the Local Plan Partial Update (LPPU) is now complete. Where applicable, the new WCS will draw on this, however the LPPU WCS was prepared over several years, so substantial sections would require revision, in particular because we would anticipate needing to assess the impact of development at a greater volume and number of settlements than the LPPU WCS.

A draft will be provided to the Council, and to key stakeholders (Thames Water, Severn Trent Water, Wessex Water, Bristol Water and the Environment Agency). One round of comments has been allowed for.

Growth scenario

A growth forecast will be prepared, to include the latest commitments and completions, and your proposed allocations. We anticipate we will need further information from neighbouring authorities where infrastructure is shared across boundaries. From an initial view of the study area this would need updated growth information from:

• Cheltenham
• Tewkesbury
• Gloucester, Cheltenham and Tewkesbury Joint Core Strategy
• South Gloucestershire
• Stratford-on-Avon
• Stroud
• Swindon
• Vale of White Horse
• West Oxfordshire
• Wiltshire
• Wychavon

In the interests of time, we recommend that CDC initiate this process in advance of the project starting in order to avoid delays in obtaining data.

You have advised that the Council will be moving towards a preferred growth option when the study commences. If it is the case that there remain optional approaches which the Council are still assessing, we can include up to four growth scenarios within our assessment. We will adapt our growth spreadsheet so these scenarios can be turned on and off in order to generate the appropriate water demand forecasts we need in the various assessments.

Water resources

The final WRMPs will be reviewed and compared to the overall housing need. Should there be a discrepancy (which is likely given the recent NPPF changes) this will be discussed with the water company and their comments sought on whether the additional growth could be accommodated within their existing plans or whether there are any constraints on timing of growth until additional water resources can be developed.

The analysis of a water efficiency standard for CDC will be further developed including the evidence to support a tighter target if required.

Once the growth scenario is prepared, we will provide it to you for review and approval prior to proceeding with the WCS assessments.

Water supply

We will provide details of potential allocations to the water supply companies and request that they provide comments / RAG assessment of these sites. These will be collated and followed up where appropriate to understand any additional infrastructure required, constraints or implications for the phasing of sites.

Wastewater network

As in the water supply assessment we will request RAG assessments from the sewerage undertakers, alongside information contained in their Drainage and Wastewater Management Plan (DWMP).

Storm overflows

The LPPU WCS assessed storm overflow performance using data from 2020 to 2022. We will update the storm overflow assessment with the latest available data (up to 2024) and Environment Agency methodology (published in 2025) which now requires 5 years of data to be used where available.

Wastewater treatment

We will assess current and future headroom at each wastewater treatment works (WwTW). Where this assessment shows a WwTW is close to, or exceeding its permit limit, comments will be sought from the water company on whether there are any constraints to providing upgrades, or land that requires safeguarding in the Local Plan. This will be supported by information from the Drainage and Wastewater management Plan (DWMP).

Water quality and environmental impact

The EA's SIMCAT water quality modelling tool will be used to assess the impact of growth in the Local Plan period on water quality. A baseline scenario will be created using the existing model updated to the latest WwTW flow data and planned improvements at WwTWs. Future scenarios will then be created using the new growth forecast to simulate the predicted wastewater discharges at the end of the plan period. The following tests will then be carried out:

• Could the development cause a greater than 10% deterioration in water quality? This objective is to ensure that all the environmental capacity is not taken up by one stage of development and there is sufficient capacity for future growth.
• Could the development cause a deterioration in WFD class of any element assessed? This is a requirement of the Water Framework Directive to prevent a deterioration in class of individual contaminants.
• Could the development alone prevent the receiving watercourse from reaching Good Ecological Status (GES) or Potential? Is GES possible with current technology or is GES technically possible after development with any potential WwTW upgrades.

Cotswold District is covered by two EA SIMCAT models (Thames and Severn). If the Council has multiple growth scenarios to be tested, this, in combination with climate change testing, would increase the volume of work significantly if we were to test every option. We therefore propose that we test the preferred option and create a "worst case scenario" combining several of the scenarios.

Should the water quality modelling indicate an issue that cannot be resolved by improving treatment processes, we may recommend that further water quality modelling is carried out in order to understand the limit for this catchment. An indicative fee for additional model runs is provided.

We will undertake a screening exercise to identify protected sites (SACs, SPAs, SSSIs and Ramsar sites) that could be impacted by a deterioration in water quality. The water quality modelling results will be used to determine the predicted water quality in the waterbodies adjacent to these protected sites. These results can be used to inform the appropriate ass…