WORKSHOP #2 Modelling and sediment transfers. Bucharest 07 October 2015

Transcription

1 STUDIU DE FEZABILITATE PENTRU SOLUŢII TEHNICE ALTERNATIVE/COMPLEMENTARE PRIVIND LUCRĂRILE CE SE VOR EXECUTA ÎN PUNCTUL CRITIC 01 BALA DIN CADRUL PROIECTULUI ÎMBUNĂTĂŢIREA CONDIŢIILOR DE NAVIGAŢIE PE DUNĂRE ÎNTRE CĂLĂRAŞI ŞI BRĂILA, KM 375 KM 175 WORKSHOP #2 Modelling and sediment transfers Bucharest 1

2 A B C Understanding of the river evolution in Critical Point 1 Bala Recommendations for numerical modelling Choice of the reference state to consider 2

3 Chapter A Understanding of the river evolution in Critical Point 1 Bala 1. Sediment supply 2. Plan-form evolution 3. Longitudinal profile and local shape of the channel 4. Conceptual scheme for Critical Point 1 Bala evolution 3

4 A1. Sediment supply A I Understanding of the river evolution in Critical Point 1 Bala Sediment transportation processes 4

5 A1. Sediment supply A I Understanding of the river evolution in Critical Point 1 Bala Coarse sediments bed-load [after Schwarz, 2008] 5

6 A1. Sediment supply A I Understanding of the river evolution in Critical Point 1 Bala Total suspended sediment (washload & sand) [after Bondar, 2002] Decreased post 1970 s Iron Gates, tributaries damming, flood plain dykes 6

7 A1. Sediment supply A I Understanding of the river evolution in Critical Point 1 Bala Bed-material (sand) in graded suspension [after Bondar, 2002] Maintained post 1970 s sediment sources downstream Iron Gates 7

8 A1. Sediment supply A I Understanding of the river evolution in Critical Point 1 Bala Synthesis of sediment supply: indicative for current situation Morphological impact Clay-silt transported as uniform suspension (washload): 11.0 Mt/year [indirect] Sand transported as graded suspension: 3.3 Mt/year 1st Sand transported as bed-load: 0.6 Mt/year 2nd Gravel transported as bed-load: 0.0 Mt/year Corresponding total sediment load: 14.9 Mt/year 8

9 A I Understanding of the river evolution in Critical Point 1 Bala A2. Plan-form evolution 1920 [AFDJ, 2013] Bala is narrow and highly meandering 9

10 A I Understanding of the river evolution in Critical Point 1 Bala A2. Plan-form evolution 1950 [AFDJ, 2013] Bala width increased flood events dykes 10

11 A I Understanding of the river evolution in Critical Point 1 Bala A2. Plan-form evolution 1965 [AFDJ, 2013] Puiu Turcului island development 11

12 A I Understanding of the river evolution in Critical Point 1 Bala A2. Plan-form evolution 1977 Iron Gates: 1972, 1984 bed incision Closure of Braţul Turcescu 12

13 A I Understanding of the river evolution in Critical Point 1 Bala A2. Plan-form evolution 2003 Constriction of Bala inlet 13

14 A I Understanding of the river evolution in Critical Point 1 Bala A2. Plan-form evolution 2005 Puiu Turcului island huge development 14

15 A I Understanding of the river evolution in Critical Point 1 Bala A2. Plan-form evolution 2014 Bala inlet full constriction, bottom sill 15

16 A I Understanding of the river evolution in Critical Point 1 Bala A3. Longitudinal profile & local shape of the channel Bala-Borcea shortcut Most attractive branch Slope incision Once engaged, self-sustained Bala-Borcea 87.5 km 15% 103 km Lower Old Danube Source: Google Earth 16

17 A I Understanding of the river evolution in Critical Point 1 Bala A3. Longitudinal profile & local shape of the channel Summer 2011 [INCDPM, 2011] Local effects: Parjoaia rock scour Bala inlet incision Parjoaia rock 17

18 A I Understanding of the river evolution in Critical Point 1 Bala A3. Longitudinal profile & local shape of the channel May 2014 [INCDPM, 2014], post guiding wall construction Flow concentration Connection transverse channel Extra flow diversion to Bala Erosion pit downstream the sill Bala branch Lower Old Danube Transverse deeper channel Caragheorghe sand bar Parjoaia rock 18

19 A I Understanding of the river evolution in Critical Point 1 Bala A4. Conceptual scheme for CP1 Bala evolution 1. Global incision process, due to the decrease of sediment supply: flood control started before the 20 th century tributaries damming became important in the last 50 years Iron Gates I & II speeds up after the 1980 s 2. Stream concentration by dykes along the floodplains, and then construction of groynes to constrain Bala inlet: increase of the velocities stressed by low roughness of the structures series of major floods in the s local aggravation of bed incision self-sustained shortcut mechanism unbalanced situation: incision in Bala branch, Caragheorghe sand bar in Lower Old Danube sill construction is challenging: phasing, downstream protection, scour pit 3. Parjoaia rock influence unburied by global incision process local scour becoming more prominent flow deflection to the left side enforcement for Bala branch connection between local scour and Bala inlet incision transverse channel worsening 4. Bottom sill sorting effect (hypothesis to be confirmed by measurements) clear surface water flows toward Bala branch (graded suspension) less sediment-concentrated bottom water is taken by Bala branch incision worsening 19

20 Chapter B Recommendations for numerical modelling 1. Characteristics and spatial extent of the model 2. Sediment modelling 3. Short-term and long-term simulations 4. Fish migration analysis 5. Habitats analysis 6. Discharge distribution 7. Navigation analysis 8. Floodplain drought analysis 20

21 B I Recommendations for numerical modelling B1. Characteristics and spatial extent of the model Looped network Discharge distribution evolution common downstream boundary condition as a reference Structured mesh For local precision, a smaller, more refined model is an option Unstructured mesh! Coupling concerns: Delft3D model Rsim-3D/iSed Different calibration parameters Downstream boundary condition for river bed 21

22 B2. Sediment modelling B I Recommendations for numerical modelling Main phenomenon: sand transported as graded suspension Total load formula or suspension formula Washload only for results interpretation Initial bed sediments: sand [Bondar, 2002] or [INCDPM, 2011] Sediments characteristics at the upper boundary condition: sand [Bondar, 2002] Solid discharge at the upper boundary condition: Yearly average input [Bondar, 2002] Or local rating curve Qs=f(Q) if available [Habersack, 2015]? Calibration of the solid transport formula: Long-term trend in bed level development at some place (e.g. Silistra: no average evolution or 2 cm/year) 23

23 B3. Short-term and long-term simulations B I Recommendations for numerical modelling Long-term simulations Looking for the morphological steady state; instead, 10~20 years period Seasonal variability of the inputs: yearly pattern Post-processing of the important parameters: seasonal envelope (=min-max span) over the steady state (last year simulated) Short-term simulations Post-processing: seasonal envelope over the first year after construction General post-processing of the important parameters 3 envelopes: short-term (first year), long-term (last year), absolute (whole period) 24

24 B4. Fish migration analysis B I Recommendations for numerical modelling Local map for difficult areas (sill crest) Local cells size: max 10 m length, max 1 m height Map of the bottom cells (sturgeons are bottom swimmers) Important parameters: max stream velocity [1] < 1.0 m/s [2] m/s [3] m/s [4] > 1.7 m/s Criteria: Analysis: existence of a path with no over-speed [4] to overcome the obstacle Analysis: the average colour of this path indicates its difficulty 25

25 B5. Habitats analysis B I Recommendations for numerical modelling Evolution of the conditions for known habitats Important parameters: Morphological trends Water depths Bottom velocities Analysis by fish specialists Sturgeons habitats: feeding ground wintering site. N 26

26 B6. Discharge distribution B I Recommendations for numerical modelling Evolution of the flow diversion to Bala branch Important parameters: Seasonal envelope of the discharge in each reach Proportion of discharge relatively to Upper Old Danube 27

27 B7. Navigation analysis B I Recommendations for numerical modelling Evolution of the flow diversion to Bala branch Important parameters (for each reach): Longitudinal profile of the highest bottom level in the fairway Longitudinal profile of the water level Possible fairway width: analysis by Inland Waterway specialist Cross section Water level Fairway Highest bottom level 28

28 B8. Floodplain drought analysis B I Recommendations for numerical modelling Evolution of water table levels in Bala and Lower Borcea branches Estimated by the water level in the neighbouring reach Important parameters (for each reach): Longitudinal profile of the water level 29

29 Chapter C Choice of the reference state to consider 1. Goal of the reference state 2. Reference = initial state 3. Reference = natural state with evolution 4. Reference = initial state with evolution 5. Reference = previous initial state with evolution 30

30 C1. Goal of the reference state C I Choice of the reference state to consider Impact analysis Impact = Future situation Reference state Short-term and long-term impacts What s for? Choice of an alternative solution Impact of the solution Matching with the objectives (target state) Environmental Impact Analysis: legal requirement Unsteady reference state may lead to biased impact Anthropic pressure estimation Out of scope (but high scientific interest) 31

31 C2. Reference = initial state C I Choice of the reference state to consider Initial state is sill construction with a crest at 0.0 mbsc Definition according to the Consortium contract Post 2015 state? Data is not yet available + Corresponds to AFDJ s specifications and usual EIA requirement Unsteady reference state The impact signification is not clear: reference doing-nothing situation Risk if the Authorities reject the proposed alternative project for too high impact: the doing-nothing situation is unknown Important parameters Reference state Short-term impact Long-term impact Alternative situation Time 32

32 C I Choice of the reference state to consider C3. Reference = natural state with evolution Natural state: no, of few, anthropic perturbation 1965 without the groynes but with the dykes: prior to major sediment retention Sediment supply: nowadays? + Meaningful from an ecosystem point of view + Comparison between two possible evolutions of the system Very different situation to compare: interpretation? Computation capacity? No data for the reference state Not included in the Consortium contract Important parameters Reference state Short-term impact Long-term impact Alternative situation Time 33

33 C4. Reference = initial state with evolution C I Choice of the reference state to consider Natural state: no, of few, anthropic perturbation Post 2015 state? Data is not yet available + Correction of the initial state unsteadiness bias + Comparison between two possible evolutions of the system No data (yet) for the reference state: schedule? Not included in the Consortium contract Important parameters Reference state Short-term impact Long-term impact Alternative situation Time 34

34 C I Choice of the reference state to consider C5. Reference = previous initial state with evolution Previous initial state: before the guiding wall and the bottom sill 2011 state + Correction of the initial state unsteadiness bias + Comparison between two possible evolutions of the system + Same reference state as the previous project (to correct): comparison is possible Not included in the Consortium contract Important parameters Reference state Short-term impact Long-term impact Alternative situation Time 35

35 Thank You 36