Under the Water Framework Directive (WFD) regulations, only 16% of rivers in England are classed as achieving good or high ecological status. In comparison, this value is 29% in Northern Ireland, 43% in Wales, and 57% in Scotland. The protection of waterbodies is therefore an essential consideration for local authorities allocating future development within their district/borough.
Further to this, communities are increasingly concerned about the impacts sewage discharges, development, and nutrient enrichment is having on the condition of their local rivers. Therefore, decisions about growth must be supported by strong and transparent evidence detailing the capacity of the water environment and the impact growth may have upon it. These factors are driving the increased requirements for Water Cycle Studies as a core part of the evidence base for Local Plans.
Assessing Infrastructural Capacity
In many catchments not achieving good status, a contributing factor is the quality and volume of effluent discharged from sewage treatment works (STWs). New development increases the volume of wastewater to be collected, conveyed, and treated. Where the sewer network or treatment works are already close to their permit limits, development growth can cause breaches of these permits unless upgrades are planned and delivered.
A headroom assessment reviews the average effluent flow from a STW against the permitted effluent flow to identify the current available capacity. A future flow, considering all new proposed developments within the catchment of the STW, is also calculated and compared to the current permitted flow. Where the future flow is greater than the permitted flow, this suggests infrastructure upgrades, development phasing, or permit changes are required to avoid detrimental impacts to the receiving waterbody.
This assessment relates spatial planning decisions to actual infrastructure capacities, helping local authorities, water companies, and regulators understand where investments are needed to allow development to proceed sustainably.
SIMCAT Water Quality Modelling
Even where a STW has sufficient capacity to treat additional flows, it is also important to assess the capacity of the receiving waterbody to absorb the resulting effluent without deterioration or prevention of it meeting its WFD objectives.
SIMCAT (SIMulation of CATchments) is a catchment-scale river water quality model used to assess how changes in flow and pollutant loading affect downstream water quality indices. It represents the catchment through river reaches, confluences, and point and diffuse inputs. It is commonly used alongside SAGIS (Source Apportionment Geographical Information System) which provides a spatial representation of the catchment model using ArcGIS Pro and shows how different sectors contribute to water quality pressures.
As a stochastic model, SIMCAT can represent the variability of flows and water quality indicators using statistical distributions and repeated simulations. It applies a Monte Carlo approach, where many ‘shots’ are completed during each model run representing different combinations of possible flows, discharges, and pollutant concentrations. By repeating the process many times, SIMCAT can build up a statistical picture of the likely range of outcomes. This is particularly useful for regulatory or planning studies as environmental standards are often expressed as annual average or percentile-based limits.
The calculations of water quality determinant concentrations within SIMCAT are based on established mass-balance and process equations. It is able to represent the dilution and mixing of substances along the length of the model and can represent the effects of decay for determinants such as biological oxygen demand (BOD), ammonia, and other nutrients.
An important stage of the modelling process is calibration. The model can be adjusted to match simulated flows and concentrations at specific points with observed data from flow gauges, monitoring points, or discharge locations. This can be done manually by the user or automatically using SIMCAT’s gap-filling tool. The calibration process ensures the model provides a reasonable representation of real-world catchment conditions, allowing future scenarios to be tested.
Assessing Environmental Capacity
Scenario testing for Water Cycle Studies compares the baseline scenario with one or more future scenarios based on the future flows at STWs estimated through the headroom assessment. Two future scenarios are typically run, one using the current quality limits at the STW and another using the technically achievable limits (TALs) through infrastructure upgrades. The results for specific water quality determinants within the receiving waterbody are compared to the WFD environmental quality standards to determine whether the proposed growth can be accommodated without deterioration or whether mitigation measures are required to protect the receiving water environment.
The distinction between the two future scenarios is valuable for local planning authorities as it separates issues that can be addressed through wastewater infrastructure upgrades, from issues that require broader catchment management measures like development phasing or specific Local Plan policies.
Future Pressures
The 2024 changes to the standard method for calculating housing targets, mean many local authorities are planning for significant growth in their district/borough. At the same time, water companies have major investment requirements due to aging networks and treatment assets, with regulators and communities increasingly concerned about their environmental outcomes, accountability, and transparency.
The existing pressures are likely to be intensified by climate change. More frequent and intense rainfall events will increase the volume of water entering combined sewers raising the risk of spills from storm overflows. Conversely, drought conditions will lower the dilution available for treated discharges increasing the loading on the water environment.
Water Cycle Studies
By reviewing the interaction between development growth, water supply, wastewater treatment capacity, flood risk, drainage, and the environmental capacity of waterbodies, Water Cycle Studies provide a holistic assessment of the issues facing the water environment, identifying current and future capacity. This highlights constraints to growth, and requirements for infrastructure upgrades or policy interventions.
WHS support local authorities by producing Water Cycle Studies to form an integral part of the evidence base behind the spatial and policy decisions made within their Local Plans. By providing a platform for discussions between local authorities, water companies, regulators, and stakeholders, Water Cycle Studies help ensure development growth is planned to protect the whole water environment supporting long-term resilience.