Stormwater is a critical component of urban water management that must be properly managed. If not drained accordingly, it can also cause pollution and health risks, along with flooding. Surface water flooding is an increasing problem in the UK, due to growing urbanisation, climate change and irregular localised intense rainfall. This makes it a more frequent and critical threat to homes, businesses, infrastructure, and people.
Many drainage systems rely on combined sewers to convey sewage and stormwater away from urban sources towards treatment plants. Alongside controlling the volume of water that would accumulate on urban surfaces (reducing flood risk), another main function of combined collection systems is to reduce pollution levels of stormwater. Water quality issues from stormwater have been extensively acknowledged. Therefore, it is beneficial to convey stormwater away from urban surfaces and treat the water as necessary. Yet, there is one drawback to this arrangement. Combined systems have not been designed to cope with both sewage and stormwater during high precipitation periods. During times of dry weather, the collection system will only carry effluent. When it rains, sewer flow amalgamates with stormwater. As such, stormwater will dominate the pipe flow, even in relatively light precipitation. In fact, in heavy storms, runoff produced by extreme rainfall could be up to one hundred times greater than the mean dry weather flow of domestic wastewater. This signifies that it cannot always be possible to convey the entire combined flow to treatment.
To avoid flooding in urban regions, combined sewer overflow may discharge untreated sewer flow into a receiving water as a safety measure. This has become a hot topic in the media over the last few years. As such, there is a growing public unease stemming from widely reported incidents of sewage contamination affecting bathing waters, with particular concern directed at the use of storm overflows.
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Flood models
Preventing exposures and eliminating the microbial hazards using several strategies is key when trying to protect people from contaminated floodwater. An important part of contemporary flood management is the use of numerical models that can simulate hydrodynamics, sediments and water quality in combined sewers and flows across urban surfaces. This means that wastewater and stormwater can be simulated together in the same modelling framework.
Using scenario analysis (the process of simulating and evaluating different possible future conditions or events to understand how a flood system might respond) provides insights into the impacts of sewer infrastructure, land-use changes, and climate adaptation strategies. This approach contributes to evaluating a region's resilience to flooding.
However, using numerical models has its challenges due to the complex nature of simulating hydrodynamics and the interaction between surface and pipe flows. Another large barrier in these applications is acquiring accurate data to calibrate and validate numerical models. Some examples of data paucity include flood depths and extents, inundation times, flow velocities, topography, known pipe flows and depths and pipe physical characteristics. However, novel data collection methods exist.
Data collection methods
In situ remote sensors are able to collect real-time data from various field instruments and transmit it to a central system for analysis and decision-making. Remote sensors typically gather data on:
- Rainfall intensity and duration
- River water levels
- Flow rates in pipes or channels
- Groundwater levels
- Water quality parameters (e.g. turbidity, pH, contaminants)
The accuracy of any flood model is therefore directly related to the possession of high-quality data. For the sake of discussion, one could argue that some data is better than no data. However, due to the harsh and often unpredictable conditions in which in situ remote sensors are deployed, data quality can be compromised. Therefore, scrutiny and validation of the collected data are essential before it is used in analysis or decision-making. Remember, poor data inputs equal poor model output!
The use of novel experimental facilities and water laboratories within academic institutions to generate high-resolution datasets is becoming increasingly popular, supporting more detailed and accurate hydrological research and modelling, as data is collected in controlled, clean environments.
For example, Co-UDlabs offers a collaborative, multidisciplinary research platform that brings together stakeholders, academics, and innovators in the urban drainage sector. It enables joint development of project ideas and provides access to advanced research facilities to support the testing, refinement, and demonstration of those concepts, ultimately fostering a connected and innovative European urban drainage community.
Continue reading... the full article is here in Issue 9 of Flood Industry magazine.