Making room for rivers : quantification of benefits from a flood risk perspective

Since 1996, the Netherlands has adopted a flood risk management policy based on making more room for the rivers. Currently, the focus in flood risk management is being adapted again, in view of increasing societal vulnerability and foreseeable effects of climate change. In this context, the choice between making more room for the river and/or strengthening embankments is again of major concern. This calls for further quantification of the effectiveness, costs and benefits of all possible measures. Making room for rivers was originally advocated by referring to a reduction of the probability of failure of the embankments and a reduction of the consequences in case of breaching. These arguments still apply, but the effect of making more room for rivers on flood risk has never been properly quantified yet. In this paper we identify three potential risk reducing effects, and quantify their effect for along the Rhine and Meuse Rivers in the Netherlands. We show that lowering the flood levels means smaller flood probabilities, that larger floodplain surface area significantly influences the relationship between discharge and flood level, and that lower flood levels in the river translate into smaller flooding depths and/or flood extent, and thus reduce the consequences of flooding.


Introduction
In the 1990s, the Netherlands experienced two major ÀRRGV ZLWKLQ D IHZ \HDUV 'HFHPEHU DQG -DQXDU\ 7KH ÀRRGV WULJJHUHG ERWK D UDSLG UHLQIRUFHPHQW RI the existing embankments and a policy change with UHVSHFW WR GHDOLQJ ZLWK ULYHU ÀRRGV, i.e. by giving room to the river.Reinforcement of the embankments was implemented after many years of opposition and debate about whether or not and how to reinforce the embankments, as earlier reinforcements had caused large impacts to highly valued natural and cultural landscape features.Giving room to the river avoids the need to strengthen the embankments, or at least decreases the magnitude of the required reinforcement.The policy change ± or transition ± from strengthening of existing embankments to giving more room to the rivers was thus partly inspired by a re-valuation of natural and cultural heritage.
Other arguments for a room for the river approach UHODWHG WR VXVWDLQDEOH ÀRRG ULVN PDQDJHPHQW &RQWLQXHG UDLVLQJ RI WKH HPEDQNPHQWV OHDGV WR KLJKHU ÀRRG ZDWHU levels in the river.In case of failure of the embankments this will result in larger water depths and increased consequences in terms of economic damage or casualties.In response to an increase in the design flood discharges, a programme to make more room for the rivers was therefore implemented between 2006 and 2015.
In 2010 the Delta programme started.The aim of the Delta programme was to develop long-term strategies to minimise flood risk and to ensure sufficient supply of fresh water, while accounting for climate change and socio-economic developments [1].The Delta Programme consists of nine sub-programmes.Three sub-programmes cover the national level, namely fresh water supply, spatial planning and flood risk management.The latter involves, among other things, an updating of the protection standards as these largely date from the 1960s.As the economic value of the protected land behind the embankments has increased, it was decided to re-valuate these standards.The outcome of the evaluation was that, especially along the major rivers, the protection standards must be raised considerably [2,3].The updated protection standards are shown in Figure 1.
The other six sub-programmes of the Delta Programme looked at specific regions.One of those regional sub-programmes was the Delta Programme on Rivers (DPR).The aim of this sub-programme was to develop a long-term strategy to ensure adequate flood risk management in the areas that can be flooded from the major rivers in the Netherlands, while taking into account the updated protection standards, climate change and socio-economic developments.The preferred strategy that was developed by DPR consisted of a combination of dike reinforcement and making more room for the river [e.g.Meanwhile, new knowledge on the failure mechanisms seepage and macro-stability revealed that the actual failure probability of the embankments (especially along the major rivers) is much larger than expected.This new knowledge and the raised protection standards both require taking measures.Additional measures are also required to cope with the expected increase in extreme river discharge caused by climate change.
The measures that are required to meet the new protection standards, while accounting for climate change, are such that a strategy consisting of only making more room for the river is not possible.The required lowering of the water level would simply be too much.This was the main reason for DPR to propose a strategy that consists of reinforcement of existing embankments in combination with measures that give room to the river [5].However, a strategy that consists of a combination of both measures is more expensive than a strategy that only consists of strengthening embankments.This has rekindled the debate about the present policy.Again, the choice between making room for the river and/or levee strengthening is of major concern.A sound and evidenceinformed decision on the implementation of dike reinforcement and room for the river can only be taken when the effectiveness, costs and benefits of these measures are thoroughly understood.
Making room for rivers was originally advocated by referring to: 1.The higher the flood levels in the river, the larger the loading on the defences, the larger the probability of insufficient strength (which might cause moving, slumping, sliding or piping); and 2. The higher the flood levels in the river, the larger the flooding depths, the larger the consequences.
These arguments still stand, but the effect of making room for rivers on flood risk has never been properly quantified yet.When looking at the impact of making room for the rivers on flood risk, three potential risk reducing effects can be identified:     The additional flood risk reduction achieved by implementing the room for the river measures that were proposed in the preferred strategy of DPR, on top of what the reduction of flooding probability already achieves, is shown in Table 1.The computed risk reduction varies significantly between the different rivers.As no room for the river measures were proposed for the Nederrijn-Lek River, the additional risk reduction for this river is zero.The largest risk reduction was computed for the upstream part of the Meuse River.This is partly due to the large number of measures proposed, which lower the flood levels by much more than 0.5 m, at several locations even up to 2 m (see the upper graph in Figure 5) But also, the protection standards for this river reach are much lower, mainly in the order of 1:300 per year.For comparison: protection standards along the downstream part of the Meuse River, and along the Waal and Nederrijn-Lek Rivers are predominantly 1:10,000 to 1:30,000 per year (see Figure 1).This implies that flood probabilities are up to 100 times smaller along these rivers than along the upstream part of the Meuse River.Finally, this river stretch includes large unprotected areas that benefit considerably from lower flood levels and reduced flood frequencies.The unprotected floodplain areas along the other rivers are small and mainly used as pasture.

Figure 1 .
Figure 1.Updated protection standards of the main levees in the Netherlands

Figure 2 .
Figure 2. Increase in water level with increasing river discharge in a river with a narrow and one with a wide cross section.