A special guest blog post on PHD research, which was partly based in the Upper Calder Valley. NFM is oft-under-proven, and suffers from a lack of funding as a result. Studies that do exist often focus on ‘lowering river levels’. NFM has a positive effect in many other areas, including green infrastructure benefits like biodiversity, air quality, and mental health.

In this study, Charlie focuses on proving an additional flood alleviation benefit: the improvement of surface drainage performance downstream, which can mitigate nuisance flooding, including combined sewer overflow, in urban areas.

Guest blog post by Charlie Ferguson

Background

While the multiple benefits accrued from Natural Flood Management (NFM) are widely recognised, there remain questions about interventions’ ability to mitigate fluvial flood risk downstream. The need for ‘building the evidence base’ remains as true as ever.

However, when evaluating NFM’s impact on out-of-bank flow, we are inevitably evaluating interventions’ ability to influence catchment response during the most extreme rainfall events. While undoubtedly very important, this frame of reference means there has been little consideration of how the same NFM interventions could be impacting downstream flows during less severe (and much more frequent!) events. 

So, over the last few years, we have been conducting EPSRC-funded research to better understand the ability of upstream NFM interventions to mitigate risk of surface flood risk in downstream urban areas during more frequent storms. 

Urban surface drainage uses underground pipes to transfer water (under gravity) to the nearest urban watercourse. In the UK systems are typically designed to cope with storms up to a 5 year event, but performance can be severely hampered by circumstantial failure. An acknowledged (if not often formally identified) example of this is the drowning of drainage outfalls by the elevated river local watercourse. By trapping water in the drainage system, there is greater risk of drains being surcharged and surface flooding on the road above.

Figure 1 gives four different ‘states’ that can occur for surface drainage outfalls. In state 1a and 1b, the level in the urban watercourse is too low to impact discharge from the outfall. In state 2a, the outfall is inundated but there is no discharge from the outfall and so the surface drainage is not impacted. In state 2b, flow from the outfall is blocked by the receiving watercourse which can lead to surcharging of the drainage system.

Figure 1: Different flow states in an urban watercourse with a flapped surface drainage outfall

So the research focused on whether upstream NFM solutions could moderate water levels in urban watercourses and promote free discharge from urban drainage systems. 

Method

To do this, a coupled modelling approach was developed using Dynamic TOPMODEL (a semi-distributed hydrological model), HEC-RAS (a hydraulic model for open channel flow) and Infoworks ICM (an industry standard urban drainage model). 

This model was applied to three catchments – the Bin Brook in Cambridgeshire, the River Asker in Dorset and the River Calder above Todmorden (whose results will be discussed here). 

Local authorities believe that high water levels in the River Calder can inundate surface drainage outfalls and exacerbate surface flooding along the nearby Burnley Road in Todmorden. The modelling approach was built for the contributing area (see Figure below). Two hypothetical interventions were evaluated – tree planting and in-channel woody debris (interventions seen elsewhere around the Calderdale).  Together they hypothesise the impact of significant changes in land management practices in the upper Calderdale. 

Results

The results suggest that the interventions create modest benefits for the downstream drainage system in parts of Todmorden. Under frequent storm events (e.g., a 1 in 10 year storm), the inundation of low lying outfalls is completely removed. As storm severity increases (and surface flooding becomes an issue), impact from upstream NFM attenuation on outfall inundation durations diminishes significantly. However, the delay in rural response created by the modelled NFM allows more water to escape surface water pipe systems, increasing the effective capacity of networks and reducing surface flooding. For instance, outfall inundation during an estimated 20 year event is delayed by 30 minutes which results in up to 25% reduction in surface flood volumes. 

This observation on the impact of timing led to another piece of work evaluating the same question, but under variable storm tracks (i.e., the path of storms across a catchment). The results shows that Todmorden’s surface drainage is influenced by the degree of synchronisation between urban rainfall and heightened levels in the receiving watercourse. In particular storms from the north and west lead to synchronisation of urban rainfall and outfall inundations, which hinders drainage performance. Conversely south and east storms result in the urban rainfall passing through the drainage system before outfalls are impeded.  In all cases where inundation of outfalls occurred, upstream NFM had a beneficial impact (albeit modest).

Figure 2: Calder Catchment with hypothetical NFM scenario and overlaid variable rainfall grid,  showing first three time steps of eight different storm directions

Conclusions

Overall, the research has shown that NFM interventions in upper rural catchments can contribute to water level management strategies by moderating downstream water levels and promoting free discharge at surface drainage outfalls in downstream urban areas. For catchments where drainage outfall inundation is an issue, there is a “window” of events (defined by storm track and severity) where NFM could improve surface drainage performance and mitigate nuisance flooding in urban areas. The benefits are modest but could help engage local stakeholder and community support for wider catchment-based mitigations.

For more details, please get in touch (Charlie.Ferguson@cranfield.ac.uk) or see the formal research output from this project:

Ferguson, C. and Fenner, R.A. (2020a) The potential for natural flood management to maintain free discharge at urban drainage outfalls.  Journal of Flood Risk Management; 13 (3). https://doi.org/10.1111/jfr3.12617

Ferguson, C. and Fenner, R.A. (2020 b) The impact of Natural Flood Management on the performance of surface drainage systems: A case study in the Calder Valley.  Journal of Hydrology; 590 (125354).  https://doi.org/10.1016/j.jhydrol.2020.125354

Ferguson, C. and Fenner, R.A. (2020c) Evaluating the effectiveness of catchment-scale approaches in mitigating urban surface water flooding.  Royal Society Philosophical Transactions A; 378 (2168).   doi.org/10.1098/rsta.2019.0199 

Ferguson, C and Fenner, R.A. (2021) How natural flood management helps downstream urban drainage in various storm direction. Proceedings of Institution Civil Engineers Journal of Water Management. https://doi.org/10.1680/jwama.19.00057