Natural Flood Management

23July2021

Source2Sea in the Stour (UK) and Authie (France) catchments

This Interreg project covered the Stour catchment in Kent and the Authie catchment in northern France, which together cover an area of approximately 2400km². Viridian first sourced all the open-source data needed to run HydroloGIS in both these territories, then modelled the most efficient and effective solutions to a variety of local problems. These included:

Flooding of local towns and agriculture;
Areas liable to accumulate water and cause pluvial flooding;
Diffuse nitrate and phosphate pollution in the rivers;
Soil being washed into the rivers to cause siltation; and
Drought and reduced baseflow to the rivers during periods of low rain

: Stour Management catchment

The Kent Stour management catchment covers approximately 1100 km² on the eastern coastal side of Kent. The Stour catchment is low elevation (below 200m) and primarily consists of pasture and grassland, with areas of coastal and inland marsh. The Great Stour and Little Stour form the main rivers in this catchment, flowing from west to east; with other rivers flowing directly to the sea.

The Authie catchment covers approximately 1200 km² on the northern coast of France. The Authie catchment is low elevation (below 200m) and primarily consists of pasture and arable land, with areas of woodland and some few areas of marshland. In contrast to the Kent Stour catchment, there are no significant settlements within the Authie catchment, with smaller settlement areas being distributed throughout the catchment. The Authie is the main river in this catchment flowing from east to northwest.

: Authie catchment

The client wished all these problems to be solved through natural catchment management, so that they would benefit wildlife and be robust to climate change. Viridian therefore modelling the most effective interventions from the following options:

Woodland planting;
Creation of meadows (semi-natural grassland);
Planting of hedgerows;
Increasing soil infiltration;
Construction of reservoirs or ponds;
Construction of leaky dams or woody debris dams;
Construction of mid-field swales and bunds;
Creating or re-instating wetlands; and
Reconnecting flood plains

: Natural depressions for reservoir potential

This map shows where natural depressions in the Stour landscape have been determined through digital elevation model analysis. These provide a set of opportunities for locating reservoirs or improving groundwater infiltration.

This map shows the current relative ability of a portion of the Authie catchment to deliver a combination of ecosystem service benefits (erosion control, diffuse pollution reduction and flood mitigation). The hydrologic treatment of the landscape is convolved with factors such as vegetative cover, peak monthly rainfall, soil, etc, to establish how water flows across the landscape. Where Viridian scores the landscape poorly indicates where habitat planting will have a greater impact in mitigating flow, and hence flooding. One can see individual fields in this map due to the availability of open spatial crops data, allowing HydroloGIS to make finer tuned prioritisations across the landscape.

: Service provision in Authie

: Woodland priority planting against flow paths

Since Viridian’s HydroloGIS tool is equipped to exclude solutions, we provided a set of data layers that showed where woodland/hedgerow features would be best situated to intercept landscape flows to reduce downstream flooding. In this map, the deeper green features are higher priority planting areas. One can see from the overlaid flow paths that woodland features are optimised to intercept more significant flow paths to reduce the overall flooding to the conurbations around Dover.

In addition to determining the relative ability of the landscape to deliver ecosystem services, HydroloGIS determines where planting new habitat will maximally increase those ecosystem services using a ranked system (here, deeper colours represent higher ranks). In doing so, the decision work of determining what to do where is given an intuitive priority ordering, enabling people to know where habitat planting will best reduce the impacts of flooding, soil loss, diffuse pollution, and combinations thereof.

: Optimised solutions for a basket of ecosystem services

: Infiltration opportunites

Using the principle that holding water on the surface in areas of deep, well functioning soils will encourage a high degree of percolation, the HydroloGIS flood prioritisation scores were filtered to only include those areas above permeable geology. Slowing overland flows through nature based solutions in these areas, thus offers a guide to where opportunities to improve soil infiltration are best investigated for additional ecosystem service benefit.

Using our regular eyes, as well as our AI’s, Viridian gets a little more insight into the situation on the ground, and thus reveals opportunities that are sensible ideas for natural flood management which may not be within the remit of optimised habitat planting. In this map, we illustrate where there are opportunities for floodplain reconnection and leaky dams along the riparian system

: Floodplain reconnection and leaky dam opportunities

3October2018

National Ecosystem Assessment of Montserrat

Case Study, National Ecosystem Assessment, Natural Flood Management

We recently undertook a National Ecosystem Assessment of Montserrat on behalf of the JNCC. This involved a data acquisition exercise, including interviews with various governmental departments on Montserrat, liaising with NGOs and community groups, literature reviews and searching international databases for relevant datasets. We then created natural capital categories, mapped out assets and modelled the provision of ecosystem services.

The island of Montserrat is a British Overseas Territory, located in the Leeward Islands of the Caribbean. This broad habitat map was developed by combining Environmental Systems’ Earth Observation data for terrestrial areas, with Waitt Institute data for marine areas. The map helps identify the key provisioning and cultural services from the island’s ecosystems.

We first collated all the data and analysed it for usability and reliability including formats, verification, ownership/curation, update frequency, age, duplications, omissions, inter-relation of data and general management issues. The data was often ‘dirty’, so where problems could not be resolved through background research alone – such as a complete lack of metadata making it impossible to date datasets – we projected the data within GIS to compare and contrast features, and so understand which were most appropriate to the project. We then combined the relevant datasets into categories and created a natural capital baseline for Montserrat. This in turn allowed its economic contribution to the island’s GDP to be calculated. We advised the government on data management and created a template for the forward use of the data, including how to update and add to the datasets in the future to track changes in natural capital and improve on the economic calculations. Finally, we ensured the Montserrat government were able to continue with the data management protocols and GIS manipulation of the project outputs.

An interesting part of our involvement was to model how rainfall would flow across the island to understand how the landscape is currently functioning to mitigate storm water impacts. The Viridian model was robust enough to produce useful outputs, even though there was extremely limited input data. The calculations of current land function would allow the next phase of modelling, looking at how to manage the land more effectively to reduce erosion, minimise storm water damage to infrastructure and protect the populace from flooding.

Whilst river network data was present for the island of Montserrat, it was found that it hadn’t been well maintained, with data versioning causing confusion and the river network data itself being disjointed. For proper evaluation of the islands flooding profile, Viridian calculation the flow accumulation across the whole island from first principles, so that a coherent picture of land and river flow could be established.

This map shows the amount of flood mitigation service that each land unit within Montserrat provides relative to every other part of the island. Where data for the island was coarse (or even non-present!), the Viridian platform capitalises on its statistical ranking methodology to deliver GIS data and mapping. By appropriately leveraging the data, decision making can still proceed based on the best interpretation of all presently available information, with further insight into the geographic data/survey needs of the region.

3September2017

Three Sub-catchments in the Upper Aire

Case Study, Natural Flood Management

The Yorkshire Dales National Park Authority asked Viridian to analyse certain sub-catchments of the Upper Aire to enhance the landscape’s ability to mitigate flooding and understand natural flood management (NFM) options. Viridian created a variety of maps to illustrate the results, as well as derived data layer to feed directly into the Authority’s geographic information system (GIS).

The Upper Aire catchment covers approximately 370 km² within Yorkshire Dales National Park. The specific areas that Viridian was asked to look at are on the West side of the catchment, labelled here as Gargrave, Earby, and Glusburn (which cover 84, 56, and 41 km², respectively). These sub-catchments primarily consist of pasture and natural grassland, whilst higher elevation areas transition to heath, moorland and peatland.

As these sub-catchments coincide with the catchment edge, it was straightforward to process them through the Viridian system since the upstream sources are contained within each sub-catchment. The flow paths were calculated for these landscapes and evaluated for their current ability to mitigate flooding. Those areas found to be performing poorly were evaluated against different habitat types to find out what land use would maximise the ability of the landscape to slow the flow of water into the river system, and thus reduce the risk of the river bursting its banks further downstream.

This map shows the current flood mitigation ability of the landscapes within the Gargrave, Earby, and Glusburn sub-catchments. The hydraulic treatment of the landscape is convolved with factors such as vegetative cover, peak monthly rainfall, soil, etc, to establish how water flows across the landscape. Where Viridian scores the landscape poorly indicates where habitat planting will have a greater impact in mitigating flow, and hence flooding.

This map shows the current flood mitigation ability within the Glusburn sub-catchment. The calculations are performed on a 5m pixel scale allowing high resolution of flow and landscape features to be recognisable in the output maps. Here we see the southern river branches are relatively well provisioned; this is mostly due to peatland and moorland in the southern reaches of this sub-catchment.

This map shows the most effective 20% of solutions to improve local flooding problems within the Earby area of interest. The solutions entail mainly creating wetlands: either rewetting peat, creating leaky dams, or forming ponds/swales, depending on local circumstances. Wetland is often selected for flood mitigation as it is effective at slowing flows across the landscape, reducing flow volumes downstream whilst offering a lower risk of synchronising peak flood heights than would planting trees.

1September2017

Windrush: Catchment-Scale Planning

Case Study, Natural Flood Management

The Berks Bucks and Oxon Wildlife Trust (BBOWT) asked Viridian to model the entire Windrush catchment for a basket of ecosystem services. As a wildlife trust, they were particularly interested in better understanding where to focus their efforts on the ground. They are using Viridian’s analyses to assist with planning, inform stakeholder engagement and influence policy formation.

The Windrush flows through Oxfordshire and Gloucestershire, surrounded by a lowland catchment of approximately 750 km². The river drains towards the lowland in the southeast, flowing toward Oxford. The surrounding land is principally occupied by pastures and non-irrigated arable land interspersed with woodland and urbanised land.

Viridian used largely open source data – such as rainfall, land use and geology – to rank every 5m cell across the catchment for how well it was currently providing flooding, diffuse pollution, erosion and groundwater services. The model then assessed entire flow paths to understand where making interventions would most improve the provision of these services. Finally, the model analysed which habitat was best to create at each location.

This map describes the current provision of flood mitigation services provided by the landscape. This is analysed along entire flow paths, and includes the effects of topography, geology and vegetation (amongst others). It shows how well the landscape is working to resist the flow of water into the river system, offset by the precipitation the landscape receives.

This map shows the most effective new habitats to create in the most efficient locations to reduce flooding in the catchment. The darkest colours show the best of any possible interventions in the catchment; the paler colours show the subsequent ‘next best’ places to plant. In this catchment, creating swales, ponds and leaky dams are the best options to reduce the magnitude of the downstream river flow, and should be generally placed upstream, in the north west of the catchment. Reducing the flow reaching the river network in these locations, means the cumulative flow reaching downstream areas is reduced the most for the least land-use change by area.

This map is a cutout of a northern portion of the Windrush catchment, showing solutions that offer the ‘best bang for buck’ in reducing soil adsorbed, diffuse pollution (such as phosphates) entering the river. This demonstrates how the Viridian system can be reconfigured to optimise reduction of other river pollutants by calculating how pollutants are transported across the landscape. Creating natural grassland along riparian zones (river banks) is the best solution here, but planting riparian woodland would also be fairly effective, should these be more desirable for external reasons (such as grant availability).

6May2017

Twite in the Wessenden Valley

Case Study, Natural Flood Management

Viridian were asked to identify the best Natural Flood Management options in the Wessenden valley. A complicating feature was the presence of twite: a protected species that only likes to nest in areas away from trees and within access to feeding grounds. It was also important to consider budgetary constraints, since both future modelling and implementation would need to offer best value for stretched resources.

Wessenden Valley is located in West Yorkshire and chiefly consists of peatland and moors in the upland, and grassland and grazing in the lowland. The Wessenden river catchment drains towards the north-east. We see that twite nest along the two main tributaries that join at Marsden, where the river catchment drains.

Having identified the best open-source datasets to use, such as climate, erosivity, and land use, Viridian blended in data on twite locations and habitats produced by local NGOs. These were processed through the flow-modelling and habitat interaction algorithms to understand how the landscape was currently interacting with flood waters, and how best to improve such interactions.

Factoring in the twite nesting and feeding areas within the Viridian framework is quite straight-forward. The Viridian platform works by prioritisation through ranking each pixel (5-by-5 metre area) of land. Where a pixel has ranked highly for intervention, but falls too close to twite habitat, it is simply excluded from the ranking scheme and the next best ranked pixel area takes its place. In this way, Viridian allows quick recalculation of plans when external factors prevent mitigation measures from being taken across the whole catchment.

The outputs were maps showing the best new habitats to create and the best places to create them to most reduce flooding for least investment. The maps here are examples: easy to understand and showing exactly what should be done where.

Where to plant/create habitat, such as wetland (using actions such as leaky debris dams) and woodland, to most reduce flooding in the valley. This solution was calculated allowing a relatively conservative ranking threshold, which correlates to a leaner investment. The new woodland is sited mainly on existing grassland areas in the north; the wetland partially involves grip blocking to re-wet peat.

As the prioritisation is done by a ranking mechanism (ensuring results are very robust) it is possible to explore the next best areas for service provision. This demonstrates how it is possible to create an optimised ecosystem service solution when some areas become excluded from calculation. Here we see the woodland extended in the upper and central regions, whilst upland peatland re-wetting becomes the next best option in the uplands around the rest of the catchment.

This map shows prioritisation for the best locations for habitat creation to provide a basket of ecosystem services: erosion control, flood mitigation, diffuse pollution reduction and groundwater recharge (baseflow to streams). Here the emphasis is much more on re-wetting peat and wetland creation upstream from where the river drains out of the catchment, demonstrating the multiple benefits of peatland restoration over woodland planting.

6May2017

NFM and Co-benefits at Fillongley

Case Study, Natural Flood Management

A previous study for the County Council had identified some traditional mitigation options, including creating engineered holding ponds in unidentified locations upstream of the village. Viridian were commissioned to identify the best Natural Flood Management (NFM) measures in the catchment to reduce flood impact to the village, as well as look at secondary benefits around water flows. A scheduled ancient monument close to the village had to be protected from any NFM measures.

The Bourne Brook and Didgeley Brook form a ‘flashy’ catchment, where heavy rainfall leads to rapid run-off and floods the village of Fillongley. The landscape is dominated by pasture and arable land, with woodland to the south and west of the village. The Fillongley catchment drains from south to north.

Viridian took a few days to research what had already been done in the area and model accordingly. One aspect of interest to the client was to identify the best locations for pond creation in the valley, with a view to minimising their engineering requirements and so cost of creation.

The outputs were maps showing the best new habitats to create and the best places to create them for least investment. This was done for flooding alone and then for a combined solution, where the combined solution balanced flood mitigation with planting for erosion control, groundwater recharge, and pollutant retention (both water soluble and soil adsorbed). The maps here showing exactly what should be done where, down to the nearest 5 metres, highlighting areas that deliver the most benefit.

This map shows the optimal areas for natural flood mitigation for the catchment, plotted with deeper coloured shades corresponding to higher priority areas. One can see that in this area, wetland is the highly preferred intervention, either creating leaky dams, or forming ponds/swales, depending on local circumstances. Wetland is often selected for flood mitigation as it is effective at slowing flows across the landscape, reducing flow volumes downstream whilst offering a lower risk of synchronising peak flood heights than would planting trees.

This map shows the optimal areas to plant habitat for a basket of ecosystem services (see above). The map has been plotted with different colours representing the different habitat choices and with deeper shades of colour corresponding to higher priority areas. One can see that existing green infrastructure, such as the woodlands to the south and west, lowers the priority for land use change in the immediate area (i.e., there are far fewer highlighted areas between the existing woodland and stream) as they already provide a good degree of ecosystem service to the current landscape.