Water Management

River bed management in the Nieuwe

A.M. Ruijmschoot1,2, M.S. Krol1, C.M. Dohmen-Janssen1, J.S. Ribberink1, R.H. Buijsrogge1, G.W.R. Gerrits2 1 University of Twente, Faculty of Civil Engineering, Postbus 217, Enschede; ([email protected]) 2 Royal Haskoning, Barbarossastraat 35, 6522 DK Nijmegen

Abstract Goal & method study Integrated solutions to the problems posed by The findings of this study consist of robust the inheritance of contaminated sediments in alternative measures for sediment-remediation the Dutch lowland-rivers are available, yet (clean-up) and management of the river bed expensive. In this study, measures are tested until 2037, with the influence of climate change for the . Due to a trend of taken into account. The applied method is clean sedimentation which will cover the shown in Fig. 2. contaminated layer, dredging the top layer (50 cm) of contaminated areas will minimize the possibility that erosion of contaminated sediments can occur in future. Even when combined with maintenance dredging of the shipping channel, currently contaminated layers will stay in place. This is not the case if the shipping channel is deepened further than the maintenance depth. Climate change does Figure 2: Research method does not affect these conclusions. Objectives alternatives Introduction Long-term policy defined in national and For years the large volumes of severely European laws provide a basis on which the contaminated sediment deposits in the objectives for river bed management lowland-rivers have been a practically alternatives are designed for 30 years: insolvable problem for Dutch water managers. Guaranteed depth for shipping is 4,95 m; no Now that the quality of sediment from maintenance dredging during 30 years upstream has improved to acceptable No severely contaminated sediments in: chemical levels, a solution to this problem can morphologically active top layer of 50 cm be implemented. Treatment of contaminated locations that show large-scale erosion in long sediment is still extremely expensive, as is term simulations deposition elsewhere. This makes storage of No rise of normative water levels due to these deeper layers of sediment in their original measures location preferable, as long as they do not pose any human or environmental threat. This SOBEK-RE model study researches possibilities to integrate a An existing one-dimensional morphological solution to this problem with other river- model of the - delta is applied functions such as shipping and safety. The (Mol, 2004). Lack of detailed maintenance- study area is the Nieuwe Merwede, a branch dredging data complicates validation. of the Rhine river (Fig. 1).

Development bed level shipping channel 30 years

1 1

0 0

-1 -1 Nieuwe Merwede Hollandsch Diep

-2 -2 2007-01-01 2017-01-01 2027-01-01 2037-01-01 -3 -3 Lowest accepted water level (OLW) Maintanance level shipping channel Change over 30 years (legend 2nd y-axis) -4 -4

-5 -5

Bed level shipping channel (m) -6 -6 Changeof bel level shipping channel (m)

-7 -7

-8 -8 955,0 960,0 965,0 970,0 975,0 980,0 985,0 Rhine kilometers Figure 3: Morphological development Nieuwe Merwede in

30 years Figure 1: Two-dimensional map of bed levels in Nieuwe Merwede. The shallow sandbanks (brown) are heavily The model is accepted on basis of long term contaminated. Green and blue areas comply with the simulations (Figure 3), showing that, a demanded shipping-depth morphological equilibrium has almost been achieved (match to reality).

Proceedings NCR-days 2007 - 60 - Water Management

Sensitivity climate change Results The average bed level is hardly sensitive to Each of five (A,..,E) alternative strategies is extreme climate change (KNMI, 2006). Levels implemented in SOBEK-RE (as in purple line decline due to more frequent and intense Fig. 5) and tested on 4 criteria: shipping, extreme discharge events (+/- 20 cm) and rise environment/ecology, security and cost. due to sea-level rise (+/- 15 cm). These effects Alternatives A and C concentrate on fulfilling almost even each other out, as shown in shipping demands, whereas Alternative B aims Figure 4. The difference is smaller than the at maximising the environment/ecology score. model-uncertainty (Van Deursen, 2002 & De Alternatives D and E consist of integrated Wit et al., 2007). measures in which all goals are combined. An example of measures is shown for the most successful Alternative strategy D in Fig. 5. The summary of results for all alternative measures is shown in Table 1.

Development bed level shipping channel 30 years Alternative strategy D: Integrated Management 2 2

1 1 Sensitivity average bed levels Nieuwe Merwede to climate change

0 0 1

-1 -1

-2 -2 Boven Merwede Nieuwe Merwede Hollandsch Diep 0,5 Boven Merwede Nieuwe Merwede Hollandsch Diep -3 -3

-4 -4

-5 -5 0

-6 -6

-7 -7 Bed level shipping channel (m) shipping channel level Bed Change bed level shipping channel (m)

Bed levelBed shipping lane (m) -0,5 Influence 35 cm sea-level rise in 30 years -8 2007 after measures D 2017-01-01 -8 Influence more frequent and intense extreme discharge events 2027-01-01 2037-01-01 Accumulated influence -9 Lowest accepted water level (OLW) Maintenance level shipping channel -9 Measures strategy D (legend 2nd y-axis) Change over 30 years (legend 2nd y-axis) -1 -10 -10 955,0 960,0 965,0 970,0 975,0 980,0 985,0 955,0 960,0 965,0 970,0 975,0 980,0 985,0 Rhine kilometer Rhine kilometer Figure 4: Sensitivity of bed levels to climate change Figure 5: Measures and morphological development for alternative strategy D: “Integrated Management”

Table 1: summary results all alternative strategies Alternative strategy Ship-ping Environ-ment/ Secu-rity Cost ++ = best ecology 1 = best Index = 100 -- = worst ++ = best 6 = worst (Average cost) -- = worst Do nothing -- 0 6 0 A: Shipping 0 -- 4 41 B: Complete remediation -- ++ 2 132 C: Shipping with sediment trap + -- 5 34 D: Integrated management + 0 1 137 E: Integrated management with cover -- + 3 156

Conclusions References • Successful integrated management is Mol, A.C.S., 2004. Morfologische Effecten Ruimte voor de Rivier in het Benedenrivierengebied, Enschede: technically viable, yet expensive Universiteit Twente, Vakgroep Waterbeheer • Dredging once in 30 years is impossible; Van Deursen, W., 2002. Klimaatveranderingen in de maintenance dredging is indispensable Stroomgebieden van Rijn en Maas, Modelstudies met • A most effective management strategy will Rhineflow-3 en Meuseflow-2, : Carthago Consultancy contain: KNMI, 2006. Klimaat in de 21e eeuw, vier scenario’s voor o Nautical dredging no lower than Nederland, De Bilt: Koninklijk Nederlands maintenance level Meteorologisch Instituut Monitoring of bed level shipping channel, De Wit, M., Buiteveld, H., Van Deursen, W., 2007. o Klimaatverandering en de afvoer van Rijn en Maas, combined with maintenance dredging Arnhem: RIZA WRR. RIZA memo: WRR/2007-006 when necessary* o Dredging top layer (50 cm) of contaminated sediments, combined with natural or artificial cover

*Confirmation needed through further research

- 61 - Proceedings NCR-days 2007