Risk Based Catchment Screening

Risk based catchment screening is a process completed at the outset of developing a Drainage and Wastewater Management Plan (DWMP). It's used to identify which sewer catchments are likely to be most vulnerable to future changes, such as climate change or new development, so effort can be focused accordingly.

Introduction

The objective of the Drainage and Wastewater Management Plans (DWMPs) is to understand how future changes across the river basin catchment could affect drainage and wastewater management systems and bring about negative impacts on people and the environment. This will then enable investments to be identified and planned to mitigate these impacts in the short, medium or long term.

We own and operate 365 wastewater treatment works (WTWs), more than 3,100 wastewater pumping stations and over 40,000 km of sewers across 381 sewer catchments in our operating area covering Kent, Sussex, Hampshire and the Isle of Wight. Each sewer catchment consists of a complex system of pipes, pumps, values and treatment works to recycle water so it can be safely discharged back to the environment. There are fewer WTWs than sewer catchments as some catchments are shared with neighbouring water companies and some catchments are not connected to WTWs.

Due to the large number of sewer catchments, a screening process is completed at the outset of developing a DWMP to identify which of these 381 sewer catchments are likely to be most vulnerable to future changes, such as climate change or new development. Those catchments flagged in the screen process will need further investigation during the development of the DWMP. This process is called the Risk Based Catchment Screening (RBCS).

 

Screening Process

The RBCS uses existing, readily available data to identify where there is a potential risk or vulnerability in the sewer catchment to future changes. This enables effort to be focused on these catchments during the development of the DWMP. The DWMP will then seek to understand these risks in more detail and why they are likely to occur, or seek to obtain further data where the catchment was flagged purely as a result of no readily available data.

The RBCS involves the assessment of each sewer catchment against 17 indicators set out in guidance published by Water UK. Water companies can add additional indicators to ensure that other important issues are highlighted at this early stage in the development of the DWMPs. We have included an additional metric on customer complaints as this provides a flag for catchments with ongoing or outstanding concerns.

The 18 risk-based catchment screening indicators to be used in the RBCS for our DWMPs are summarised in Table 1 below.

 

Table 1: RBCS Indicators of risk in sewer catchments
No. Indicator Description Criteria for the indicator to flag as a concern and needing further investigation in the BRAVA stage
1 Catchment characterisation This provides a mechanism to understand the vulnerability of the sewer catchment (L3) to sewer flooding as a result of an extreme wet weather event (defined as a 1-in-50-year storm event). Catchment vulnerability score = 1 to 5 (where 5 is the most vulnerable or sensitive to an extreme wet weather event). Catchment vulnerability score = 4 or 5

(i.e. the most vulnerable or sensitive to a one in 50 year storm)
2 Intermittent discharges impact upon bathing or shellfish waters This is a mechanism to understand the significance of any impact of water company operations on bathing or shellfish waters. Exceeding the permitted number of spills in each bathing water season, or per annum for shellfish waters.
3 Continuous or intermittent discharges impact upon other sensitive receiving waters (Part A) This mechanism is to understand the significance of any impact of water company operations on sensitive receiving waters not addressed by other indicators. ‘Remedy’ on Natural England’s Designated Sites system (associated with freshwater pollution discharges or freshwater drainage).
4 Continuous or intermittent discharges impact upon other sensitive receiving waters (Part B) A mechanism to understand the significance of any impact of water company operations on sensitive receiving waters not addressed by other indicators. ‘Threat’ on Natural England’s Designated Sites system (associated with water pollution).
5 Storm Overflow Assessment Framework (SOAF) This considers the current / potential future activity to identify and address high spilling storm overflows. Are spill frequency investigation triggers likely to be crossed within next five years?
6 Capacity Assessment Framework (CAF) The measure provides an indication of capacity constraints in the sewer network. There are accepted issues around the confidence in outputs from the Initial CAF model which does not include for surface water inputs. When categorised as 4 or 5 (due to performance, in full or part, within the catchment) will progress to the next stage of the process.
7 Internal sewer flooding This is a common performance commitment by water companies to reduce flooding inside customer properties. It is a historical measure that records the number of internal flooding incidents per year, and it is indicative of capacity constraints within the sewer network. The number of incidents is more than one in total over the last three years (and other specific criteria depending upon size of sewer catchment).
8 External sewer flooding This is a common performance commitment by water companies to reduce flooding within the external curtilage of customer properties. It is a historical measure that records the number of external flooding incidents per year, and is indicative of sewer capacity constraints. The number of incidents is more than 10 in total over the last three years (and other specific criteria depending upon size of sewer catchment).
9 Pollution incidents (categories 1, 2 and 3) This is a historical measure that identifies incidents of unexpected release of contaminants that have resulted in environmental damage. Categorised in accordance with the 2017 definition in the Environmental Performance Assessment (EPA). For any of the previous three years data, a category 1 or 2 incident has occurred (see guidance for further details).
10 WTW quality compliance This is a historical measure relating to the performance of the wastewater treatment works (WTWs). In any of the previous three years, the WTW discharge has been confirmed as failing and was included as such in the calculation of overall permit compliance.
11 WTW dry weather flow compliance (DWF) This is a historical measure of compliance with DWF permits at WTWs. Has the Q90 of the measured yearly flows exceeded the DWF permit condition on two consecutive years in the last five years? Or is the works at risk of exceeding its flow permit conditions?
12 Storm overflows A measure that focuses on using available data to examine permit risks that have not been captured by other indicators (e.g. pass forward flow conditions). Is there evidence to indicate that over the last three years any overflow is not operating in accordance with permit conditions?
13 Risks from interdependencies between Risk Management Authority (RMA) drainage systems A mechanism to understand risk posed by interdependencies / interactions between other RMA drainage systems in the catchment. Where it is considered that significant risks arise from interaction with other RMA drainage systems / receiving waterbodies.
14 Planned residential new development A measure to understand the risks from forecast residential population growth in the sewer catchment. Planned residential development is greater than thresholds set out in the guidance.
15 Water Industry National Environment Programme (WINEP) The WINEP sets out the actions that water companies need to complete to meet their environmental obligations. Where there are specific WINEP drivers it is considered necessary that a long-term approach to managing the issues is developed. Known WINEP drivers impacting the specific Level 3 catchment.
16 Sewer collapses This is a historical measure that identifies risks to the integrity of the sewer system. Sewer collapses are more than two per year in any of the preceding three years (see specific criteria in guidance).
17 Sewer blockages This is a historical measure that records obstructions in a sewer (that require clearing) which causes a reportable problem (not caused by hydraulic overload), such as flooding or discharge to a watercourse, unusable sanitation, surcharged sewers or odour. If the number of blockages (normalised by sewer length) in any of the preceding three years is greater than the company average.
18 Customer complaints A measure of the number of customer complaints received in the catchment as an indication of ongoing or outstanding issues that are yet to be adequately resolved. Number of customer complaints.
How are the results used?

The results of the assessment of the sewer catchments against these 18 criteria are used to determine if a sewer catchment progresses onwards to the Baseline Risk and Vulnerability Assessment (BRAVA) stage of the DWMP.

Indicators have been classified into two tiers to support the scoring of which catchments proceed to the risk assessment stage. All indicators are ‘tier 1’, except for indicators 1, 4 and 18 above which are classed as ‘tier 2’.

The Water UK guidance sets out the rules for determining whether a sewer catchment passes through to the BRAVA assessment, as follows:

  1. If two or more indicators are flagged (excluding sewer collapses and blockages) then a BRAVA is required to identify whether and to what extent changes in future inputs impact on planning objectives
  2. If one indicator is flagged (again, excluding sewer collapses and blockages) then a BRAVA is required, if the indicator causing the single breach is tier 1.
  3. If only the sewer collapses and/or blockages indicators are flagged then at present this is to be treated as if no indicators are flagged
  4. If no indicators are flagged, this implies that there is no current evidence to suggest that the sewer catchment (L3) is likely to be vulnerable to changes in the future.

The results of the RBCS for each of the river basin catchments can be found on our website.

Prepared by Southern Water based on the DWMP Guidance published by Water UK November 2018.

August 2020

 
 
 
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