It’s the not knowing that kills you. Monitoring the benefits of Natural Flood Management
Two of our previous blogs explained what Natural Flood Management is and how it works. But how do we know that Natural Flood Management (NFM) is having the desired effects? These include reducing flood risk to properties downstream, as well as providing multiple benefits such as water quality improvement, reduced soil erosion, biodiversity and habitat creation. There are many examples of studies which have tried to quantify the impact of NFM, with an extensive literature review of such studies carried out by the EA.
These studies use a combination of computer modelling, field studies and observations to try to quantify the impact of NFM on the environment, but more information is still needed. This blog will talk about the importance of monitoring NFM, the challenges and how we propose to monitor our NFM.
The Importance of Monitoring
Whilst the concept of Natural Flood Management has been around for some time now, we are still in the early stages of its implementation on the ground. There is a large amount of work going on across the country to monitor NFM, yet there is still much to learn above how NFM features behave in a range of conditions – even at the feature scale. When we move towards the catchment scale to try to quantify the impact on flood risk downstream, the existing evidence base becomes smaller still.
Unlike traditional hard-engineered flood defences, which can be designed and modelled in detail to predict specific volumes of flood attenuation for a particular rainfall event, NFM does not lend itself to such accurate predictions. When installing NFM we want to work with conditions at the site or local scale and no two areas of land or sections of watercourse will be the same. Therefore, neither will any two field bunds, or any two Large Woody Structures (LWS). NFM features, such as LWS, can be highly dynamic over time. Therefore, there is a lot of knowledge to be gained by monitoring this change over time.
The growing volume of research on NFM is being used to inform:
- How best to design NFM features to maximise their effectiveness;
- How this effectiveness can change over time and for different storm events and catchment typologies;
- How to maintain NFM;
- The cost-benefit of NFM.
By carrying out our own monitoring we hope to contribute towards answering these questions, helping us to take a science-based approach to our future work. This will allow us to maximise the benefits to people and wildlife and help us best use available funding.
Challenges of Monitoring
A key challenge our project – and many other projects – face when trying to determine the impact of NFM is the lack of baseline flow data within the catchment. Baseline data gives us an idea of how the catchment responds to storm events, before we put in NFM. With this information we can then compare how the catchment reacts to similar storms with NFM installed. From there, we can start to draw conclusions about the benefit the NFM actually has to communities at risk of flooding.
However, the extent of environmental monitoring is declining worldwide and the UK is no exception. The result is that very limited baseline data on river flows is available. The short timescale of most funded projects limits the potential to develop a new, large-scale monitoring network to boost this baseline data.
A key gap in the knowledge base for NFM is how impacts at the local scale propagate downstream (Dadson, SJ et al. 2017). It is intuitive that slowing runoff from hillsides and flow within small watercourses and storing it temporarily in the headwaters of a catchment will help to delay and reduce the movement of the flood peak downstream to where vulnerable properties lie. However, the extent and magnitude of this benefit are less clear. As the size of a catchment increases, the complexity of processes determining flood risk increases and the impact of small-scale measures decreases.
To carry out large catchment-scale monitoring would be very costly and time-intensive and therefore not currently realistic.
However, by carrying out the monitoring outlined below we hope to notably increase the extent of monitoring within the Medway catchment. The data captured will inform future design, optimise site selection and inform continued maintenance of the features.
So what approaches are we using to monitor the NFM features we plan to install?
A key priority sub-catchment within the Medway is the Alder Stream. Within the catchment, a number of properties are at risk of flooding, with a concentration of properties in the town of Five Oak Green. With regards to baseline data, the nearest existing flow gauge is located over 15 km downstream and drains a catchment over 1200 km2 in area. For perspective, the Alder Stream at Five Oak Green drains a catchment of only 8 km2.
To gain a better understanding of the flow regime within the Alder Stream a flow meter has been installed within the main channel upstream of Five Oak Green. By also installing monitoring stations recording water level in the headwaters of the catchment we hope to better understand how the catchment reacts to extreme rainfall events, as well as the movement of the flood wave along tributaries and then downstream along the main channel. This will help inform the location of NFM features beyond the FRAMES project. Recording the downstream flow, combined with monitoring storage at the NFM feature-scale will give us an idea of what percentage of the flow peak we have been able to attenuate.
Where small earth bunds are built to intercept and store excessive runoff from hillsides, monitoring of water levels will be carried out to help us understand how quickly they fill up and drain, and the volumes of water stored. This shows how effective the bund is at providing flood attenuation during storm events.
By recording the water level upstream and downstream of our Large Woody Structures we can develop an understanding of how these measures store and slow flow. We plan to continue monitoring the NFM measures for many years to come. This will give us information on how effective the NFM is for different magnitude storm events and also how this effectiveness changes over time as the structure evolves.
The build-up of sediment behind the LWS will also be monitored to tell us how much sediment we are trapping and preventing from moving downstream. This is important in watercourses impacted by high volumes of fine sediment which can have detrimental effects on the river habitat. Excessive sediment blocking light, carrying pollutants such as phosphates, cementing and smothering gravels can negatively impact invertebrates and fish. In high volumes, sediment can also reduce conveyance. Whilst storing sediment can have benefits, it may also mean that the effective storage behind the LWS is reduced over time. Monitoring this information on the long term effectiveness of the LWS will be very important in informing cost-benefit analysis of the work.
We have teamed up with King’s College London and AmbioTEK, a Community Interest Company, to monitor the first of our LWS at Bedgebury. Through another EU funded project: NAIAD (Nature Insurance Value: Assessment and Demonstration) the partnership is working to develop and install low-cost environmental sensors called FreeStations, with the overall aim of increasing the number of sensors worldwide. FreeStation sensors have been installed upstream and downstream of a LWD in Bedgebury to quantify the volume of flood storage upstream. Live data can be viewed at: http://www.policysupport.org/smart/smartriver/medway
Time-lapse cameras give us an idea of what is actually happening when a rainfall event occurs and levels within watercourses rise. They are an important visual tool to help landowners and communities better understand what NFM is and how it functions.
Biodiversity and Habitat Creation
For all of our NFM sites we are carrying out baseline ecology surveys to firstly identify what is currently there so we don’t adversely impact it through our work. Follow up surveys in the months and years following installation will allow us to determine any benefits to wildlife. This will demonstrate the very important multiple benefits of NFM beyond flood mitigation.
It can take quite a while for meaningful data to be gathered from a monitoring scheme. To see how the NFM features perform requires heavy rainfall and high flows within the Medway catchment. Benefits to biodiversity from the work may also take many years to be realised. We’ll provide updates on our results over the next few years. In the meantime we’ll be keeping an eye on the weather!