Assessment of future stratification induced by opening of Haringvliet sluices

Abstract

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.