Modelling the salinity response to fresh water flow and variable tidal amplitude in an - PowerPoint Presentation

1. Modelling the salinity response to fresh water flow and variable tidal amplitude in an idealized river delta EGU 28/04/2021Session : sinking, SHRINKING and SALTIER DELTASConstantinos Matsoukis Supervisors: Nicoletta Leonardi University of Liverpool Laurent Amoudry National Oceanography Centre Lucy Bricheno National Oceanography Centre

2. IntroductionSALINITY DISTRIBUTION IN A COASTAL SYSTEM DEPENDS ON FRESHWATER FLOW,WIND AND TIDAL FORCINGLANDWARD SALT TRANSPORT IS DRIVEN BY EXCHANGE FLOW AND TIDAL DISPERSIONTHE TIDES PLAY A CRITICAL ROLE AND THEY CAN AFFECT EXCHANGE FLOW AND STRATIFICATIONSEA LEVEL RISE WILL CAUSE CHANGES TO TIDAL AMPLITUDE THAT IN RESPONSE COULD DECREASE OR INCREASEWhat IS the impact of DIFFERENT amplitudes on the salinity FIELD OF AN EXISTING DELTA CHANNELS NETWORK?

3. An idealized delta configuration is built in DELFT3D The bathymetry (figure b) is the output of a morphological simulation - complex channels network (figure a), 8 bed slope IMPOSED at X=10kmSalinity simulationsConstant bathymetry to isolate the tidal influence from any morphological effects Upstream fresh water flow (figure c) at the river boundary – annual symmetric hydrograph based on data from the Po Delta (scaled down)Solar tide implemented at the offshore boundary  3D MODEL SETUPABC

4. 5 simulations – a common hydrograph and various amplitudes Tidal range determined based on data from real deltasEstuary number equal between real and idealized cases: river discharge , tidal period, tidal prism TIDAL FORCINGScenarioEstuary numberScaled Model Tidal Amplitude (m)ClassificationData Source1n/a0River Dominatedn/a20.490.02River DominatedPo Delta, Italy30.0730.15Micro-tideIrrawaddy, Myanmar40.0180.56Meso-tideYangtze, China50.0091.1Macro-tideGanges-Brahmaputra (Bangladesh)

5. SPATIAL SALINITY DISTRIBUTION – SEASONAL VARIABILITYRIVER DOMINATED CASESWith TideA very low tidal level LEAVES ALMOST UNAFFECTED THE spatial salinity distributionThe flow remains unidirectional and similar salinity patterns developAsymmetric salinity distribution at periods of low flows (Figure A and C)Most probable cause is the bathymetry (The model assumes no ambient crossflows and Coriolis force IS neglected )Flow increase leads to symmetric salinity distribution (Figure b and d)No TideSalinity and flow vectors at the top layer on the day of minimum flow Salinity and flow vectors at the top layer on the day of maximum flow

6. SPATIAL SALINITY DISTRIBUTION – SEASONAL VARIABILITYTIDALLY INFLUENCED CASESMicroTideMesoTideMacroTideTidal amplitude increase enhances the flow in the delta overriding the bathymetry effects (Fig. E,G,I)Tidal amplitude increase results in more symmetric distributions but constrains low water areas (Fig. F,H,I)Top layer – minimum flow- flood tide Top layer -maximum flow – flood tide Mean velocity in the delta for each amplitude

7. VERTICAL SALINITY DISTRIBUTION Salinity maps indicate the development of an offshore buoyant plume in the tide-influenced cases The plume transforms progressively from a surface (figure A and D) to bottom advected type (figure C and F) as the amplitude increasesThe plume breaks where the bottom slope deepens sharplyThe plume in the mesotide scenario (figure B and E) spreads through the entire vertical column at the start but detaches from the bed further downstreamThe depth of plume detachment is shorter at the flood tide (figure B) due to tide-induced mixing 3 hours after Low Water 3 hours after High Water ABC

8. Freshwater Volume decreases with the amplitude increase at high water times (figure a) Freshwater volume is higher in the tidal cases than the river dominated ones at Low water times (figure b)tide-induced mixing helps to increase freshwater volume in downstream areas far from the river inlet while in the absence of tides they remain partially mixed or stratifiedFRESHWATER VOLUME IN THE DELTALow Water High Water   = seawater salinity, grid cell salinity, grid cell volume  

9. THE DELTA CHANNELS ARE CLASSIFIED BY THEIR STREAM ORDER (SO) (STRAHLER-HORTON METHOD – figure a) PREVIOUS work FOR RIVER DOMINATED CASES SHOWED that Salinity increases AS the so decreases (Matsoukis et al. 2021)The relationship remains valid as long as the spatial salinity distribution is symmetric (Figure B) – BATHYMETRY EFFECT The tide does not affect this relationship (Figure C)CORRELATIONS OF SALINITY WITH DELTA’S GEOMETRYNo TideMacroTideB

10. Investigation of ALTERNATIVE METHODS TO CLASSIFY THE DELTA CHANNELSThe Width function (Kirkby 1976) counts the number of channel links within a certain distance (e.g. every 1.1km in figure a)The Method appears to be complementary to the stream order lateral averages of salinity decrease with the increase of the width as long as the spatial salinity distribution is not symmetric (Figure B) CORRELATIONS OF SALINITY WITH DELTA’S GEOMETRYNo TideMacrotide

11. DURATION OF FRESHWATER CONDITIONS IN THE DELTAFreshwater if salinity < 2psu (MONISMITH ET AL.2002)Number of days that AVERAGED PER SO salinity remains below 2psu decreases downstreamIN THE TIDAL CASES THE Duration of freshwater conditions decreases as the TA increases Microtide (ta = 0.15m) case keeps the delta fresh for longer periods and is the only case where the most distant channels (SO1) become fresh for a certain period

12. CORRELATION OF SALINITY WITH RIVER DISCHARGEMatsoukis et al. (2021) detected the existence of a negative and exponential correlation between radial averages of salinity and river discharge - resembles solutions of the 1D advection-diffusion equationDifferent features develop between low and high flows/amplitudesAs long as the spatial distribution is asymmetric, a double exponential equation fits bestResembles the solution of the 1D diffusion equation in the case of a wall not allowing the flow behind of it (Fischer et al.1979)  

13. CONCLUSIONS The tide enhances the flow in the delta resulting in more symmetric salinity distributions (agrees with similar plume studies e.g. Isobe 2005;Lee & Valle-Levinson 2013)STRONG FLOW AND HIGH AMPLITUDES OVERRIDE BATHYMETRIC EFFECTS THAT CAUSES deviations TO THE FORM OF PREVIOUSLY SUGGESTED CORRELATIONS (E.G. SALINITY-RIVER DISCHARGE)TA INCREASE TENDS GENERALLY to DECREASE FRESHWATER AREAS AND VOLUMESmedium amplitudes EXHIBIT SEVERAL positive effects such as Increase of the freshwater volume through tide-induced mixing Longer periods with fresh water in the delta Spreading Fresh water in longer distances from the river source

14. THANK YOUAny questions?