Assessment of future stratification induced by opening of Haringvliet sluices

Assessment of future stratification induced by opening of Haringvliet sluices

Binsma, Jouke (TU Delft Civil Engineering and Geosciences)

Pietrzak, J.D. (mentor)
Katsman, C.A. (graduation committee)
Aarninkhof, S.G.J. (graduation committee)

Delft University of Technology

DELTA21

2021-03-19

Key points:- Pre-closure salinity intrusion into Haringvliet-Hollands Diep is known to have reached Biesbosch National Park at flood tide. Post-Delta21 salinity intrusion is projected to be less extensive than pre-closure. Maximum intrusion is estimated to reach the westernmost Moerdijk ports mainly due to diminished tidal flow at the estuary mouth.- Freshwater intake in the Haringvliet is projected to be compromised for the majority of the year whereas freshwater intake in Hollands Diep is compromised during prolonged drought.- Salinity outwash from the Haringvliet on ebb tide is projected to be poor under both drought and normal conditions due to widespread salinity diffusion in lateral and vertical directions on flood tide. The Delta21 framework, in which this study is positioned, aims at increased flood safety and ecological restoration of the Haringvliet, a former estuary in the Dutch southwestern delta that has been closed as part of the Delta works. To achieve this, Delta21 proposes to reopen the Haringvliet freshwater basin to tidal effects in an attempt to restore its estuarine character. Implementation of Delta21 introduces the risk of compromising agricultural and industrial activities around the Haringvliet-Hollands Diep by reintroducing salinity and tidal movement in the basin. The central problem lies in quantifying the extent and stability of the expected periodic salt intrusion post-Delta21. This has been done by projecting a geographical bandwidth of salinity intrusion patterns depending on the severity of SLR (2020-2100) and low-to-mean Rhine-Meuse discharges (in 2100 CE) using a numerical model. Mitigation strategies are then recommended based on these numerical results. The OSR-HV model (owner: Port of Rotterdam) is used to run predictive scenarios of salinity intrusion for the lower reaches of the Rhine-Meuse basin. OSR-HV runs in TRIWAQ (Rijkswaterstaat), which is 3D numerical modelling software that employs coupled hydrodynamics and constituent transport thereby resolving salinity transport. An upper-end critical scenario consists of a Rhine (Lobith) discharge averaging 1000 m3/s for 31 days combined with 85cm SLR in 2100. This resulted in an intrusion pattern reaching the westernmost port of Moerdijk in Hollands Diep. Results show that the basin geometry, possibly nudged by Coriolis deflection, initially causes a preferential path of salinity intrusion along the southern bank of the Haringvliet. Lateral and vertical mixing is extensive in the western part of Haringvliet which is thought to be a combined effect of weakened tidal flow at the estuary mouth and robust and erratic geometry of the basin. Further up-estuary, the historical flood-ebb tidal channel structures is the main transporter of salinity. The Haringvliet shows distinctly different estuarine behaviour compared to the neighbouring Rotterdam Waterways where stratification is more stable, causing less up-estuary diffusion. The relative robustness of the Haringvliet and mild freshwater forcing give rise to extensive 3D mixing which subsequently limits the maximum horizontal excursion of salinity. Salinity intrusion into the Old Meuse is observed to aggravate upon opening of the Haringvliet sluices due to flow reversal in Spui which connects Haringvliet with the Rotterdam Waterways. This effect can even result in salt intrusion from the Rotterdam Waterways via Old Meuse and Spui back into the Haringvliet, causing a secondary spike in salinity on ebb tide there. Finally, height-limitation of the Haringvliet sluice gates is somewhat effective in countering horizontal excursion of salinity but runaway diffusion in the Haringvliet results in similar salinity profiles compared to full opening of the sluice gates. A shipping channel that crosses the current Haringvliet front delta is included in Delta21. This deep feature promotes advection of salt through the Haringvliet sluices. Subsequent diffusion patterns cause for poor washout of salinity from the basin upon ebb tide. Limiting the depth of this channel is recommended if salt intrusion is to be reduced. Full opening of the Haringvliet sluices furthermore causes approx. 0.50m lowering of Mean Low Water (MLW) near Moerdijk which affects busy shipping routes between Rotterdam-Moerdijk-Scheldt. Partial reduction of the conveying area of the Haringvliet sluices may be used to suppress the tidal wave penetration into Haringvliet-Hollands Diep. Significant gain in ecological value is likely post-Delta21 due to addition of approx. 1900 ha of intertidal areas and a 40 km salinity gradient (excl. front delta). Ample recommendations on further research have been made in this exploratory study. It is recommended to further study the effects of Delta21 interventions on macro hydrodynamics of the Dutch coastal shelf. The interconnected nature of the region requires a larger modelling domain to prevent the occurrence of non-physical effects obtained from the current numerical schematization. Furthermore, hydrological relations and bathymetry were generated from 2020 data. It is therefore recommended to precede the assessment of salinity intrusion into the Haringvliet with numerical projections on change to these environmental factors. Lastly, significant gain in accuracy may be obtained from applying spatially varying temperature and wind to the domain to better replicate baroclinic flows and turbulent mixing.

Haringvliet
Salt Intrusion
DELTA21
Climate projections
Stratification

http://resolver.tudelft.nl/uuid:79cd9834-63a7-40f4-a43c-05d005c2f48f

51.832729, 4.049717

Student theses

master thesis

© 2021 Jouke Binsma