Sewage Spill Explorer

Aim

The aim of the map below is to provide an interactive visualisation of the sewage spills by England’s water companies in 2023. The idea is that the user chooses a location along any English river and the interactive map then calculates how this location was affected by spill events in 2023.

To achieve this, a so-called upstream trace analysis is performed. That is, starting from the chosen location on the map, the analysis identifies all water bodies that lie upstream from the current location (i.e., its catchment area). Based on this upstream trace analysis, all spill events that contributed to the current location are then identified.

The map uses the sewage spill data released by the Environment Agency on 27 March 2024.

The map

Once the app has loaded, it will display a map with spill sites overlaid onto England. Green circles indicate sites that spilled for less than 30 minutes in 2023. Brown-gold circles are sites that spilled for more than 30 minutes.

You can then zoom in to a location of your choice and click on a river. The location of the click will be indicated by a blue pin marker. From that location, the upstream trace analysis will be performed (for very large catchment areas, this can take a while!). It is a good strategy to zoom in quite far initially to increase the probability of actually clicking on a river.

Alternatively, you can click on the “Locate me” button (the crosshairs icon). If you do so, a green pin marker will be added to the map to indicate your current location and the map will automatically zoom in on this location.1

The upstream traces will be shown on the map as an overlay of blue lines. It might be necessary to zoom out to see the whole catchment area. Any spill sites associated with the current catchment area will be shown as red circles.

Once the blue lines are displayed, clicking on the blue pin marker will open a popup with a summary of spill events that affected the current location. “Total duration of spills” represents the total length of spills in this location’s catchment area.2

Due to the limitations described below, a click on a river might not always result in an upstream trace being generated. In this case, it often helps to click on a nearby river location.

Limitations

Please note the following limitations of the Sewage Spill Explorer:

  • The analysis is based on a digital elevation model. That is, it uses elevations (measured from space) to predict where rivers should be. In general, these predictions correspond well with actual river locations, but this is not always the case (in particular in cities and areas that are quite flat). Note that there might also be gaps in the river network; that is, cases where a water body should be connected to another one, but isn’t. Also note that such a model can’t predict the locations of canals as these won’t usually follow the natural topography.
  • The present analysis focuses on inland water bodies. The map also displays spills at beaches and out at sea, but there is of course no upstream trace analysis that can be meaningfully performed for these sites.
  • The data are retrospective. To my knowledge, only Thames Water publishes a map showing spills in close to real time.
  • The data are based on spill durations, not volume. It’s a bit like asking someone how much they had to drink and in response they tell you how much time they spent at the pub. This is clearly suboptimal, but monitoring the volume is technically more challenging (and costly) and not currently enforced by law, so water companies tend not to do it.
  • The spill data provided by the Environment Agency are likely unreliable. Analyses by Peter Hammond who works with Windrush Against Sewage Pollution suggest that the data may include both missed spills (false negatives) and spills that did not actually occur (false positives).
  • The data only show sewage spills by water companies. Other contaminations of water bodies are not included (see this Rivers Trust map for a comprehensive overview of the Consented Discharges to Controlled Waters or this Guardian article about spills by dairy farms).

Alternatives

Please also note these other maps:

Methodology

The Sewage Spill Explorer map is based on a rather impressive dataset published by Amatulli et al. (2022). The Hydrography90m dataset, among other things, includes river segments, river nodes and sub-catchment areas for the whole world. For the Sewage Spill Explorer, the data were cropped to England and Wales using QGIS.

After cropping, further pre-processing was completed in R. igraph was used to generate a graph of the river connections. This graph underlies the upstream trace analysis. The map app itself was implemented using Shiny.

Reference

Amatulli, G., Garcia Marquez, J., Sethi, T., Kiesel, J., Grigoropoulou, A., Üblacker, M. M., Shen, L. Q., & Domisch, S. (2022). Hydrography90m: A new high-resolution global hydrographic dataset. Earth System Science Data, 14(10), 4525–4550. https://doi.org/10.5194/essd-14-4525-2022

Footnotes

  1. Note that your location (i.e., the location of the green pin marker) is not sent to the server running the Shiny app. Determining your location happens locally in your browser. Out of necessity, the location where you click on the map itself (i.e., the location of the blue pin marker) is sent to the server. This is the information the Shiny app needs to generate the upstream trace. Whether or not this is close to a location where you actually are, I do not know.↩︎

  2. Note that this can be more than one year. To give an illustrative example, if two sites contributing to a location were to spill sewage for all of the year, the total duration would be two years. The reason for this seemingly paradoxical result is of course that in reality the spills overlapped. However, the data provided by the water companies only include the total hours of spills at a given site and not when these happened and thus do not allow for a more fine-grained analysis of the time-course of spills across the year.↩︎