Since December 2019, Derby Bus Station has experienced frequent winter flooding, suspected to be linked to the installation of a flood barrier at Riverside Gardens. Geo2 Remediation Ltd appointed H Fraser Consulting Ltd to provide hydrogeological advice to investigate the issue. The flooding was believed to be caused by tidal locking at the bus station’s outfall, with drainage water backing up when the River Derwent’s water levels rise above the outfall.
- Case Study: Flooding at Derby Bus Station
- Client: Geo2 Remediation Ltd (Geo2)
- Hydrogeological Consultant: H Fraser Consulting Ltd (HFCL)
- Location: Derby Bus Station
- Service provided: Hydrogeological Advice and Flooding Investigation
Flooding problem overview
Flooding occurred 18 times since December 2019, particularly when the river flow exceeded 75 m³/second. Notably, during the winters of 2019-2020 and 2020-2021, river flow levels were higher than in the past 20 years. However, flooding did not occur before 2019 at similar flow levels, suggesting a change in conditions.
Approach and methodology
HFCL conducted an updated technical assessment, including:
| Flooding analysis: | Identified causes of flooding: |
| Flooding dates were plotted against river flow levels. | Culverted Markeaton Brook: The culvert under the bus station discharges to the Mill Fleam and River Derwent. The Riverside Gardens flood barrier, built in 2019, may have blocked historic waterways, leading to flooding. |
| HFCL held a site meeting on 22 November 2022 and discussed the situation with a representative from Our City Our River (OCOR) on 7 December 2022. | Bus station drainage issues: The station’s drainage system could also be contributing to flooding. |
| Culvert backing up: High river levels could cause water to back up in the culvert, exacerbating flooding. |
Findings
- Flooding linked to river flow:
Flooding was consistently reported when river flow exceeded 75 m³/second, but this threshold did not cause flooding before 2019, suggesting a change in conditions since the flood barrier installation. - Potential causes:
- The Riverside Gardens flood barrier might have blocked waterways, causing flooding.
- Backing up of the culvert and the bus station drainage system during high river levels may be contributing to the issue.
Recommendations
Flood event monitoring should continue at the bus station, recording the duration and depth of each occurrence. If flooding recurs, HFCL should be contacted immediately to conduct a site visit and measure water levels at key locations. Additionally, a drainage survey should be carried out to confirm whether a functional flap valve exists at the outflow into the River Derwent. A detailed diagram of the bus station’s stormwater drainage system should be produced, and if not already completed, a CCTV survey should be undertaken to assess the discharge pipe from the attenuation tanks and the entire drainage system.
Conclusion
H Fraser Consulting’s investigation revealed multiple possible causes for the flooding at Derby Bus Station, including the impact of the Riverside Gardens flood barrier and drainage system issues. Recommendations for further investigation and monitoring were made to pinpoint the root causes and mitigate future flooding events.
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