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Problem statement

The Dutch have a complex, inescapable relationship with water, which makes them experts in water management. Water has played a huge part in shaping the culture and institutions of this country.

The local water supply and management solutions are quite advanced yet still facing bottlenecks and exploring areas for improvement. The most important balance lies between saline and fresh water resources.

Climate change and certain events such as the drought period of 2018 have raised the issue of salinity monitoring. The Ministry of Infrastructure and Water management wants to explore more efficient ways of surface water salinity monitoring. This is important for water supply for drinking water, agriculture, industries, as well as the ecology.

Salt intrusion

In areas where fresh water rivers discharge into the marine environment, a transition zone between fresh-water and saline water is created. Seawater weighs on average 2 to 3% more than fresh water. In areas where both fresh and salt water are present, fresh water floats on top of the heavier salt water, which forms the so-called salt wedge. This salinity process is inhibited by the mixing of fresh and salt water and counter pressure from the fresh water inflow, gradually reducing the salinity upstream. The extent and development of salt intrusion depends on factors such as the amount of (fresh) water the river discharges, the shape of the estuary and the sea level. Because these factors can vary in terms of time and location, salt intrusion process may occur over a short distance, or very gradually over tens of kilometres.

In the Netherlands, salt intrusion is one of the natural processes that must be taken into account in (daily) water management. In the north of the country, the Lake IJsselmeer was created in 1932 by the construction of a 30 km long tidal closure dam, called the “Afsluitdijk”, separating the Wadden Sea from the former northern Rhine estuary, called the “Zuiderzee”. Initially, Lake IJsselmeer was

a large brackish water lake that gradually freshened as relatively fresh river water flushed the former estuary. This created an abrupt transition from fresh water in the Lake IJssel to saline water in the Waddenzee. Saline intrusion near the Afsluitdijk (mainly at the locks) was experienced, in particular in 2018 when Rhine discharge into the Lake IJssel was relatively low.

Knowledge gaps

While the processes involved in salt/fresh water exchange in estuaries and locks are generally well known, their interaction in specific locations (e.g. the exchange of salt/fresh water between river outflow and longshore current flows as a function of the tide, or the interaction between the passage of ships and density current flows at locks) is not. Monitoring of salinity in surface water is important to understand these processes. Up to now, salinity monitoring is mainly reliant on point measurements (for example at salinity measurement buoys) which restricts the number of data points. Recent developments have shown opportunities to monitor salinity from space, for example using data collected by the Aquarius satellite. This is however restricted to the top 2 cm of the water column and has a horizontal resolution of 150km. This project aims at combining different data sources (point observations with satellite data) to better understand salt water intrusion in fresh surface water bodies.

Area of focus

The Afsluitdijk is a major dam and causeway in the Netherlands which is a fundamental part of the larger Zuiderzee Works, damming off the Zuiderzee, a salt water inlet of the North Sea, and turning it into the fresh water lake of the IJsselmeer. The dam serves as a sea barrier to protect the inland against flooding.

This preferred area for exploration has encountered 2 main problem points: low flow from the river and saline waters entering the lake through the locks of the Afsluitdijk.

Vulnerabilities in The Netherlands - Future projections of salinization lakes IJsselmeer and Markermeer

Lake IJsselmeer and Lake Markermeer form the largest artificial freshwater system in both the Netherlands and northwestern Europe and are fed primarily by the Rhine River. Both lakes are an important source of freshwater for drinking water production, agriculture and industry in the Netherlands.

The main inflows of chloride into Lake IJsselmeer include the IJssel River (11% of the Rhine discharge), drainage from the surrounding polders and seepage from the Wadden Sea through the tidal closure dam. The main inputs of water and chloride to Lake Markermeer are exchanged with Lake Ijsselmeer (predominantly Rhine River water), rainfall and polder drainage.

The expected rise in salt intrusion will increasingly result in unusable inlet points for agriculture and drinking water.

In 2018 there was an especially dry period that had unexpected consequences.

Due to the prolonged drought, the lock gates had to stay closed and the De Aanleider pumping station pumped freshwater from the Amsterdam Rhine Canal into the Leidsche Rijn, towards the west. This had long lasting effects on industries and on drinking water providers.

The most striking vulnerability: salinization of the IJsselmeer and the Amsterdam-Rhine Canal, significantly decreased groundwater levels on the high sandy soils. The drought of 2018 not only offers opportunities to learn in operational terms for the next extreme drought period, but it also provides new insights for the preferred strategy for freshwater. Combining the existing point measuring systems with high resolution satellite data can provide new ways to monitor and, respectively, alert stakeholders in a timely manner about changes in salinity.

Challenge

How can we help water managers to better understand and monitor salt water intrusion in the IJsselmeer by combining satellite data with existing point monitoring. Results should ultimately include an alert (early warning) tool. This should consider the changes in weather and water quality including their potential consequences instead of relying solely on in-situ data.

Sub-questions

• How can we help water managers to map salt water intrusion and discern vertical differences in salinity in the water column (salt water under fresh water)?
• How can we help stakeholders of the Ijsselmeer to manage the risks of increased salinity (alerts and predictions) by using high resolution satellite data when the sluice gates are opened?

This challenge affects key stakeholders such as water-quality and water-management organizations, drinking water companies, water boards, policy makers, the agriculture sector, nature organizations, workers at ships and dikes and local citizens.

Read the article here

Criteria

• Improves monitoring by combining current point measurements … with satellite data (The Netherlands collects high resolution satellite data)
• Incorporates an alert procedure/system to notify key stakeholders or can distribute notification messages
• The solution maps out the spreading of saline water and following developments or cascading effects
• Scalable (procedure that can be applied to other areas with sea water intrusion [examples: Hollandse IJssel,  IJssel, Brielse meer, etc. for farming and drinking water])
• Allows for a future upgrade which can give a salinity forecast based on weather forecasts
• Takes into consideration places with deeper/shallower waters (bathymetry data)
• Takes into account that the frequency of these events is unknown: drought periods or accidents (they can be rare, unexpected or unnoticed initially)
• Can incorporate historical insights (thinking about the changes observed and working towards predictions [use previous scenarios from 2018 as an edge case])
• Promising areas to explore: mapping tools, heatmaps, algorithms, [excluding the need of dashboard due to the current availability of other dashboards and due to the fact that real time updates are perhaps too detailed and unnecessary]

Sources for open data

• Map of Water information in the Netherlands (https://waterinfo.rws.nl/#!/kaart/zouten/)
• Rijkswaterstaat (https://www.rijkswaterstaat.nl/zakelijk/open-data)
• Satellite data Nederland (https://www.satellietdataportaal.nl/?base=brtachtergrondkaart&res=0.5&datemin=27-09-2020&loc=51.37178%2C8.217773%2C6z)
• Bathymetrie Nederland (https://data.overheid.nl/en/dataset/14776-bathymetrie-nederland---kust)
• Sentinel Playground (https://www.sentinel-hub.com/explore/sentinelplayground/) https://nationaalgeoregister.nl/ Open geo data
• Fresh water supply and water quality report in the Netherlands