Hydromorphological, hydraulic and ecological effects of engineered woody dams: a Natural Flood Management technique
The term Natural Flood Management (NFM) describes a range of techniques that seek to reduce flood hazards by restoring innate hydrological and morphological processes, features and characteristics while sustaining or enhancing environmental co-benefits such as biodiversity, increased soil and water quality and carbon sequestration. NFM is increasing in popularity, and many new NFM projects are proposed across the UK. However, there are still relatively few studies evaluating the effectiveness of NFM measures in the field, their success over complex and wide catchments or their ability to provide additional environmental co-benefits.
The aim of this project is to determine if engineered woody dams can effectively reduce flood hazards by influencing flood peaks at a catchment scale, and modify flow velocities, sediment fluxes and habitat at the local scale thus yielding benefits in terms of biodiversity and functionality of the riverine macroinvertebrate communities. This work is being carried out in the Evenlode catchment (Oxfordshire), where a 5-year project (2017-2021) has been developed by Thames water.
- The effects of NFM on stream biodiversity: are woody dams beneficial to riverine invertebrate communities?
It is known that natural woody dams are beneficial to river biodiversity by supplying a higher diversity of habitats via the differentiation of slow and fast-flowing water, an increase of the riffle-pool sequences and local refugia and the enhancement of local food sources. Engineered woody dams developed through NFM projects are similarly suggested to provide high co-benefits in terms of habitat creation and restoration of degraded reaches. However, little evidence exists about these co-benefits and their effects on the riverine communities and the related ecosystem services.
To test this, I am analysing macroinvertebrate samples that I have collected from 50 sites in the Littlestock Brook catchment (sub catchment of the Evenlode) over two sampling seasons. I am also employing, at the same sites, a cotton strip assay for measuring organic matter decomposition, to see if woody dam construction increases this important ecological function.
- The effects of NFM on water-flow regulation at the catchment scale: can woody dam decrease and delay the peak flow?
The presence of woody dams in a stream increases the hydraulic roughness and drag forces which in turn reduce water velocities and increase the water level upstream of the woody dam. Woody dams are therefore predicted to reduce the likelihood of flooding downstream by increasing the temporary storage of water locally and decreasing the velocity of the stream flow. This slowing and storage of water is thought to delay the movement of peak flows through the catchment network following heavy rainfall. While it is known that the hydraulic effects of woody dams on flow resistance do result in changes in stream discharge, and that these can affect the magnitude, celerity and timing of peak flow downstream, there is still a large knowledge gap relating to the effects of woody dams on big flood events and in large catchments (>100 km2).
To test this, I have developed a hydrological model of the Evenlode catchment and I am currently testing it with data collected in the field.
- The effects of NFM at different scales: how are woody dams impacting water and sediment fluxes at the catchment and reach scales?
In order to predict the impact of engineered woody dams within a stream network, or to scale up the potential impact of woody dams to wider catchments, models of hydraulic and hydrologic processes are widely used. Hydrologic models aim to simulate the flow and storage of water of a specific area or watershed. Hydraulic models, on the other hand, aim to simulate the movement of water into open channels dealing with the physical properties of water. Both approaches are important for understanding the impact of woody dams on catchment level water flow regulation, but there is a trade-off in these models between complexity and spatial scale; the fine detail of woody dams on channel processes are too computationally demanding to calculate at large spatial scales, while models that can represent hydrology at the catchment scale are necessarily too simple to capture the important local effects of small features such as woody dams. As a result, the models do not satisfactorily capture both of the processes. Given that engineered woody dams are built and have an impact on sediment and water fluxes at the microscale impacting instream processes but are often used in the scope of a catchment-based approaches to reduce flood risk with nature-based structures and solution, an integrated assessment is needed.
To understand the impact of woody dams at the local scale I am developing an hydraulic model for a sub-catchment of the Evenlode. To do an integrated assessment of the woody dam effects at different scales I am modelling using a cascade approach in which the spatial complexity of the problem is reduced by first applying a semi-distributed hydrological model to simulate hillslope runoff into the channel network, before a reach-level 1D model and then 2D channel-level model are deployed, each building on the outputs of the former.
- Macroinvertebrate response to NFM measure: can woody dams modify the riverine habitats thus yielding a potential for a change in the macroinvertebrate community structure?
The environmental co-benefits of NFM projects on stream biodiversity and ecosystem services are difficult to foresee and, therefore, incorporate into predictive models. The scope of my final chapter is to predict potential taxa occurrence and community structure with habitat changes on the basis of the simulated abiotic parameters with the modelling. An integrated modelling approach will simulate the impact of one NFM measure (engineered woody dam) on flow velocities, sediment fluxes and thus the effects of these on the riverine habitats. These simulations will be used to understand the impact on macroinvertebrate communities and develop a modelling framework that could be applied in the scope of understanding the variety of effects of NFM projects on the stream biota. A Driver-Pressure-State-Impact-(Response) (DPSIR) concept will be utilised to depict the complex cause-effect chain of hydro-morphological changes on macroinvertebrate habitats in the Littlestock Brook catchment (sub-catchment of the Evenlode).