CSO Management


What is required? What do we propose? Why you might want to do more than compliance?



The first element of CSO management is regulatory compliance. The first review cycle for the Water Framework Directive has produced new requirements for combined sewer overflow pollution prevention.

For England, the Environment Agency has produced the matrix opposite to define the Event Duration Monitoring (EDM) requirements for a site.

In Wales, following environmental sensitivity criteria, the standard requirement is to monitor Combined Sewer Overflows at 15 min intervals.

CSO Monitoring Data


How CSO data is collected and used


The data journey shows how the information is gathered, processed and used.

Basic systems provide a date & time stamp of whether the CSO is spilling or not. Alarms can be triggered upon a basic event and/or upon certain criteria set at the site such as high level alarm or event duration.

More sophisticated systems monitor data such as levels at a fixed frequency, to produce a trended data set. Understanding trends allows a better understanding of what is happening at the site and provides more useful data for hydraulic modelling.

When determining what equipment to use to generate the data from the site there are 5 determining factors:

  1. Signal availability
  2. Frequency of data transmission
  3. Reliability
  4. Accuracy
  5. Cost

Because the more data points you have the better your information system will be cost becomes a determining factor for many decisions. Cost not just in terms of equipment but also installation and commissioning and then ongoing operations cost and flexibility to modify and upgrade when necessary.


Selecting the appropriate solution reduces monitoring costs considerably.


The CSO solutions summary matrix provides a rough guide to the most suitable choices for particular situations.

There is a considerable cost penalty for mains powered solutions if the power is not already available on site.

(click links below for detail)
Short Description
2min data, mains power Using mains power to monitor at 2 minute intervals
2min data, battery power Monitoring at 2 minute intervals using battery power because mains power not practical or too costly
CSO Monitoring 15min data, mains power Using mains power to monitor at 15 minute intervals because there is no requirement for greater frequency
CSO Monitoring 15min data, battery, easy site access Battery power to monitor at 15 minute intervals
CSO Monitoring 15min data, battery, difficult site access Extended battery power at 15 minute intervals to minimise visits to site for battery replacement
CSO Monitoring 24hr data, battery, easy site access Battery power to collect data daily for lowest cost solution
CSO Monitoring 24hr data, battery, difficult site access Extended battery power to collect data daily and minimise visits to site for battery replacement
Catchment modelling
Accurate Data is at the heart of hydraulic modelling.


Why you might want to go beyond regulatory compliance


With the uncertainty around weather patterns and the increasing frequency of flooding events there is growing pressure to invest heavily to mitigate the effects of floods.

The problem is to accurately model the hydrology of a catchment to determine what investments will have the greatest impact for the lowest total expenditure.

Accurate data from as many significant points as possible is at the heart of good modelling. However, adding data points means investment which has to fight with other funding requirements. Justifying this investment can be based around three areas:

  1. Reducing monitoring system costs: less total cost both capex and running costs is achieved in a variety of ways.
  2. Data accuracy: often reduces the cost of catchment investment decisions by ensuring the correct sizing and mix of hard investments and SuDS tools
  3. Data points can in the future build towards a dynamic system, reacting in real time to weather events.


Click on one of the boxes below