Flood resilience and river management

Moderator of this session: Kees van der Lugt (World Waternet)

This session will include the speakers: Annemargreet de Leeuw (Deltares), Bas Reedijk (BAM) and Liduin Bos-Burgering (Coastar)

Take a look at the abstracts and speakers profiles below: 

Flood resilient landscapes: area-specific solutions with added value
for society and safety

Presenting author: Annemargreet de Leeuw

Press this link to see the recording.

The Netherlands is not finished; there are large spatial tasks and transitions ahead, such as climate change, making agriculture and the energy sector more sustainable, restoring biodiversity, but also, for example, the task of housing construction and ensuring sufficient clean (drinking) water. In the latest IPCC report (August 2021), we see that climate change is accelerating: faster rise in sea level, more intense precipitation and more extreme periods of heat and drought. In short, it is time to roll up our sleeves to keep the Dutch Delta safe and livable.

A complex dilemma arises here: a degree of uncertainty about complex system changes, and at the same time the task of working out the intended transitions within the existing, limited context of laws and regulations, current policy and social acceptance. In short, the task of keeping the Dutch delta liveable and safe demands a systematic future study of adjustments to and in the spatial organisation of the water (safety) system.

In the coming decades regional water authorities together with regional governments and local partners will reinforce flood defences in many places in the Netherlands. How can we together ensure that we make the right decisions and do not regret these investments in 10-20 years’ time? This is a relevant question, especially given the complex societal challenges (climate adaptation, housing, nature development, energy and agricultural transition). Current society, in terms of norms and values, will also change. This calls for insight into future society: which values will we find important and can we accommodate them?

That is why, together with the sector, we are developing the concept of flood resilient landscapes. The underlying principle is to create social added value while promoting or at least maintaining flood risk management and climate change, given (future) spatial and societal developments. The necessary knowledge development takes place through real-world practical cases. We use both forecasting (using adaptive delta management) and back casting methods (e.g. Reframing Process) to develop perspectives on action; on the one hand based on uncertainties around e.g. climate changes, and on the other hand based on certainties and trend for the future society, including the issues that are important for the design by that time.

The flood resilient landscapes concept offers the prospect of keeping the Netherlands safe beyond 2100 at socially acceptable costs and with public support now and in the future, and paves the way towards implementation throughout the international deltas.

Flexible groynes made of Xstream blocks are the future of river management

Presenting author: Bas Reedijk

Press this link to see the recording.

Groynes have been applied in rivers for more than 100 years. Traditionally, groynes are fixed structures built from several materials such as sand, rock, pitched stone, fascine mattresses, geotextiles and wooden elements. Main purpose of groynes is to fix the layout of a river and to maintain the depth in the central part of the river for navigation. The groynes guide the river flow to the center of the river and the increased velocities scour the river bed. The groynes also reduce the velocities along the river bank, protecting the banks from erosion.

The existing groynes are impermeable structures which have a large scour protection around the groyne heads. Near the head of these groynes large scour holes are formed due to flow concentration and turbulence. Sand eroded from these scour holes can form local sand waves in the shipping channel (in Dutch kribvlammen). These local sand waves form an obstruction for the inland shipping

Due to climate change the hydraulic boundary conditions for rivers are changing. There is more extreme rainfall, and extreme river discharges increase. The floods which previously occurred only in winter now also occur in summer. Rivers adapt to climate change and also adapt to manmade changes such as shortening the river length by cutting off river bends. Therefore, in the Netherlands river beds are deepening. Because of increasing river flow and changing bed levels, damage occurs at river groynes such as outflanking at the root of the groyne. As the existing groynes have multiple layers and transitions they cannot be easily adapted to the increasing boundary conditions due to climate change.

As groynes are located at right angles to the river flow, they form an obstruction to the river flow at extreme discharges and lead to increased water levels in the river, which are dangerous for water safety. Therefore, at many locations in the Netherlands groynes are being modified by lowering the crest height to increase the discharge capacity. Due to the many layers and transitions in traditional groynes, this is difficult to do. In addition, inland water transport on Dutch rivers is increasing. Inland ships increase in size and have bigger engines. The waves and currents caused by these ships lead to erosion of the bank and outflanking of the groynes, therefore traditional groynes require continuous maintenance.

A concept has been developed by BAM called “Flexible Groynes”. Flexible Groynes are built from only 1 material, called Xstream blocks. Xstream blocks are concrete X-shaped blocks of 35 cm height and 28 kg weight. As they interlock, they can be placed on steep slopes of up to 1:1. The porosity of a Flexible Groyne with Xstream blocks is 60%. So, only 40% of the structure is concrete. Low strength concrete with low CO2 footprint binders can be used. Flexible Groynes are only made of Xstream blocks. So there are no transitions, no geotextiles no rock or wood. The CO2 footprint of a Flexible Groyne is less than half that of a traditional groyne because of the large reduction in material quantities.

The Flexible Groyne adapts to changing river bed levels. If required because of climate change or other reasons, it is very easy to shorten, lengthen or change the height of the groyne. Xstream blocks can be placed in bulk, by dumping or using a crane with a big grab which places 20 blocks at a time.

Because of the porosity of the Flexible Groyne, wave energy of ship waves is absorbed. Since no scour protection with fascine mattresses is required, sand can move more easily around the groyne head and unwanted local deep scour is reduced. Flexible groynes have a high porosity and can therefore form a habitat for river life. During a pilot in the IJssel river in the Netherlands, 3 pilot groynes have been built and monitored in cooperation with the partners (Rijkswaterstaat, Van den Herik, Deltares, BTE, Meteoor and Aquatic Drones) and provided by the leerruimte Self Supporting River Systems. More projects with Xstream blocks are being planned.

Blue-green roofs with forecast-based operation to reduce the impact of weather extremes in cities 

Presenting author: Liduin Bos-Burgering

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Research shows that climate change will increase the intensity and frequency of extreme summer precipitation events as well as heatwaves, over the coming decades. Green roofs are generally seen as an effective climate adaptation measure, but they are also criticized for having a limited water retention capacity during high-intensity rainfall events. Therefore, the city of Amsterdam has started a project (RESILIO, https://resilio.amsterdam/en/smart-blue-green-roofs) to investigate a new innovation in this field: smart blue-green roofs. These roofs have an extra water retention layer underneath the green roof, which increases storage capacity during extreme rainfall and functions as a capillary irrigation system for the plant layer on hot and dry summer days. The smart valve on the roof can be opened when extreme precipitation is forecast to create enough capacity to capture and retain the incoming rainfall. However, it is yet unknown if the forecast accuracy is high enough to provide added value, as false alarms in the forecast can also lead to a reduction of water availability, and therefore a reduction in evaporative cooling and plant health.  

To test the magnitude of this trade-off, we evaluate the performance of ensemble precipitation forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) to trigger drainage from blue-green roofs. We simulate blue-green roofs in a hydrological model on 28 locations over the Netherlands, using hourly meteorological observations over the last 7 years and different probabilities of the ECMWF ensemble forecasts to open the smart valve and trigger drainage.  

We show that this forecast-based drainage creates enough buffer capacity to capture 70-97% of extreme rainfall (>20mm/h). Moreover, evaporation rates relative to potential evaporation on hot summer days is high (around 70%) , and much higher than from traditional green roofs (around 30%). This serves to underscore the higher capacity of blue-green roofs to reduce heat stress.   

These results show that blue-green roofs are much more effective in reducing pluvial flood risk and heat stress than conventional green roofs, and that relatively low-resolution ECWMF forecasts can further increase their effectiveness. Initial results of a suitability analysis to upscale this solution in Amsterdam show that approximately 14kmof roof surface is potentially suitable for blue-green roof application, which is on average 10% of the area of urban catchments. This underscores the contribution that blue-green roofs could make to help cities adapt to the weather extremes of the future.  

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