OXYGEN DEFICIENCIES IN THE SCHELDE AND ELBE

Transcription

1 OXYGEN DEFICIENCIES IN THE SCHELDE AND ELBE ESTUARY: SAME DIFFICULTIES, DIFFERENT CAUSES Geerts L 1, Soetaert K 2, Maris T 1, Wolfstein K 3, Meire P 1 (lindsay.geerts@uantwerpen.be) 1) Ecosystem Management Research Group (ECOBE), University of Antwerp, Belgium 2) Royal Netherlands Institute for Sea Research (NIOZ), the Netherlands, Korringawaeg 7, 4401 NT Yerseke, the Netherlands 3) Hamburg Port Authority (HPA), Neuer Wandrahm 4, Hamburg, Germany

2 Hypoxia in estuaries Estuaries concentrate water from large surface areas in small waterbodies: mostly naturally eutrophic Further man-induced eutrophication ( excess nutrients ) Secondary production, increased organic matter Oxygen deficiencies (2 mg/l 5mg/) First solution = water treatment Estuaries = last opportunity to manage water quality problems 2

3 Hypoxia in the Schelde and Elbe estuary Schelde Elbe 3

4 Biochemical oxygen demand Schelde and Elbe Schelde Elbe 4

5 5 Why does the oxygen minimum zone in the Elbe persist, while in the Schelde oxygen conditions improved despite water treatment efforts in both estuaries?

6 Tributary input Schelde Elbe FW input upper 54 % of total discharge FW input upper 89 % of total discharge 6

7 janv.- juil.-04 janv.- juil.-05 janv.- juil.-06 janv.- juil.-07 janv.- juil.-08 janv.- juil.-09 (m³/s) Freshwater discharge 3000 Elbe >>> Schelde!!! ELBE SCHELDE

8 Morphology: area Similar in estuarine length Final similar cross-sectional area Differences in first 100 km 8

9 Morphology: width Clear differences In the Elbe abrupt increase In the Schelde more gradual widening 9

10 Morphology: avg depth Avg depth increase in the upstream fw part in the Elbe Contrary to the Schelde, the Elbe depth decreases towards the sea, resulting in a similar cross section 10

11 Aim of study How do these differences between Schelde and Elbe, explain the difference in functioning between them? Tributaries Discharge Morphology To answer this question two oxygen models were developed 11

12 Oxygen model 1-dimensional 2 state variables: O2, BOD 100 boxes 4 parameters 1. BOD decay (d -1 ) 2. Piston (m/s) 3. Temperature coëfficient (Q10 ) 4. Dispersion coëfficient (E) (m²/s) 12

13 Oxygen model: reactions Transport Tidally avg advection dispersion equation C t = 1 A Reactions Q C x BOD = BODdecay BOD TempRate O2 = BODdecay BOD TempRate + OxygenFlux OxygenFlux = piston TempRate = exp D T A x 10 (O2sat O2) log (Q10) A E C x 13

14 Oxygen model: calibration Schelde Elbe 14

15 Effect of tributaries - Schelde Schelde 15

16 Effect of tributaries - Elbe In the Elbe no effect of tributaries along the estuarine gradient Discharge upper high relative to discharge from tributaries Elbe 16

17 Effect of tributaries - Elbe In the Elbe no effect of tributaries along the estuarine gradient Elbe Discharge upper high relative to discharge from tributaries 0,9 Tide Elbe (0 km) 0,8 0,7 0,6 BOD 0,5 input also 0,4 decreases 0,3 0,2 0,1 0 y = -5E-05x + 2,1149 R² = 0,

18 Effect of tributaries - Elbe In the Elbe no effect of tributaries along the estuarine gradient Discharge upper high relative to discharge from tributaries However, upper BOD input also decreases Effect of morphology? Elbe 18

19 Effect of morphology - Elbe Elbe 19

20 Width (m) Depth (m) Effect of morphology in the Elbe? 2 scenarios tested with the oxygen model Abrupt widening Deepening Ref Ref 20

21 Width (m) Depth (m) Effect of morphology in the Elbe? 2 scenarios tested with the oxygen model 1) Gradual widening Deepening Ref Scen 3A Ref 21

22 Width (m) Depth (m) Effect of morphology in the Elbe? 2 scenarios tested with the oxygen model 1) Gradual widening ) Un-deepening Ref Scen 3A Ref Scen 3B 22

23 Effect of morphology in the Elbe? Sag can be indeed simulated between 30 and 100 km coinciding with both deepening and abrupt widening Dissolved oxygen (mmol/m³) Reference 23

24 Effect of morphology in the Elbe? Largest effects from abrupt widening to gradual widening Sag moves downstream Dissolved oxygen (mmol/m³) Reference Gradual widening 24

25 Effect of morphology in the Elbe? Largest effects from abrupt widening to gradual widening Sag moves downstream Undeepening lifts oxyygen sag slightly Dissolved oxygen (mmol/m³) Reference Gradual widening Undeepening 25

26 Effect of morphology in the Elbe? Largest effects from abrupt widening to gradual widening Sag moves downstream Undeepening lifts oxyygen sag slightly Seemes to be related to residence time jump related to high discharge and morphology Dissolved oxygen (mmol/m³) Reference Gradual widening Undeepening 26

27 Days Effect of residence time in the Elbe! preomz OMZ postomz 0 Ref Gradual widening Deepening summer 27

28 Conclusion Rupel and Bovenschelde cause a surplus input of organic matter to be processed in the Schelde estuary; with improvement of the waterquality in the tributaries, the water quality in the Schelde improves Combination of hydrology and morphology defines the difference in functioning by residence time and explains why water quality treatment efforts in the Elbe estuary might be less effective than in the Schelde estuary 28

29 Thank you for your attention! Questions?