Danube Floodrisk THE STAKEHOLDERS IN THE LOWER DANUBE … · SECURITY DEGREE CONCEPT Main steps: a)...

THE STAKEHOLDERS IN THE LOWER DANUBE AND THE RESIDUAL RISK

17-18 April 2012 Bucharest

Radu DROBOT1, Aurelian Florentin DRAGHIA1, Maria CHEVERESAN1, Mary-Jeanne ADLER2, Sorin RANDASU3

1 Technical University of Civil Engineering Bucharest

2 National Institute of Hydrology and Water Management 3 National Administration Romanian Waters

SECURITY DEGREE CONCEPT

• The residual risk is based on social and economical acceptability, which in case of security degree concept is related to a medium or low probability of the hazard.

• The design flood in Romania is established according to the following standards:

STAS 4068/1-82;

STAS 4068/2-87;

STAS 4273/83


Main steps:

a) Type of hydraulic structures (permanent,

temporary, main structures, secondary structures)

b) Category of hydraulic structures (STAS 4273/83)

c) Classes of importance (STAS 4273/83)

d) Probability of exceedance (STAS 4068/2-87)

SECURITY DEGREE CONCEPT a) Type of hydraulic structures

b) Category of hydraulic structures (STAS 4273/83)

Dykes

Importance of the flooded

area

Category of the hydraulic

structures

> 10,000 inhabitants 2*

2,000 – 10,000 inhabitants 3

< 2,000 inhabitants 4

*These hydraulic structures can be included in the immediate upper category, based on flooding studies and cost-benefit analyses


CLASSES of IMPORTANCE (STAS 4273/83)

Type of hydraulic structures Category of the hydraulic structures

According to

the duration

of operation

According to

its

functionality

1 2 3 4

Classes of importance of the

hydraulic structures

Permanent

Main

structures

I II III IV

Secondary

structures

III III IV IV

Temporary

Main

structures

III III IV IV

Secondary

structures

IV IV IV V


Probability of exceedance of the maximum discharge

(STAS 4068/2-86)

Classes of

Importance

(STAS 4273-83)

Normal conditions Special conditions

(only for dams)

Probability of exceedance of the maximum discharge %

I 0.1 0.01

II 1 0.1

III 2 0.5

IV 5 1*

V 10 3*


• Most of the dykes along the Danube river are designed at 1% probability of exceedance of the maximum discharge.

• A freeboard of 1-1,5 m from the design level 1% to the crest of the dykes provides a safety guard against the medium floods, increasing the security level.

• If temporary measures like dykes heightening are taken during the floods, floods having higher return periods can pass between the dykes without overtopping.

• In such conditions, the residual risk is supposed to be lower than 1%.


• This general perception is true when flooding is due only to dyke overtopping.

• Still, the dykes can be destroyed by many other mechanisms: wave overtopping, erosion of the outer slope, instability of the inner or outer slope or internal erosion (piping).

• The exceeding probability of failure by these mechanisms could be higher than that of the maximum discharge of the design flood.


• Generally, the residual risk is much higher than the norm values, due to the lack of dykes maintenance.

• In these cases, the dykes degradation is visible and can be corrected at least locally.

• In other cases, the internal structure of the dykes or the foundation present weak areas due to internal erosion processes or the occurrence of high permeable layers in the dyke foundation.


• The dykes on the Danube were not designed with drainage system in order to maintain as low as possible the infiltration curve.

• In some years the flood duration on the river exceeds 2-3 months. Due to the high levels of such floods the infiltration curve can reach the downstream slope of the dyke and the danger of dyke failure becomes present.

• The situation becomes worse in case of permeable layers under the dyke, conducting the water directly in the protected areas.

Danube 2006 flood

(Mihailovici et al, 2006)

Annual probability of exceedance of the breaches development

• 1100 km, which makes almost impossible a high reliability of the dykes on such a long stretch. • The geotechnical characteristics of the dykes and the foundation vary along the Danube river and could be considered as random variables. • The probability of dyke failure by internal erosion of the dyke or its foundation was roughly considered equal to the exceedance probability of the 2006 flood


• The probability of exceedance of the flood 1% during 100 years is: •The probability of exceedance of 7 breaches development was considered equal with the probability of exceedance of the flood 1% during 100 years, being thus 63%.


• The probability of exceedance of 1 breach formation in 100 years is: •The annual probability of exceedance of one breach formation is thus:


• The probability of exceedance of 2 breaches formation in 100 years is: •The the annual probability of exceedance of 2 breaches formation is:


• The annual probability of exceedance of breaches formation for the Romanian stretch of the Danube river is in the range (1% ....2.73%)

Number of

breaches

Probability of exceedance

of breaches formation in

100 years

Annual probability of

exceedance of breaches

formation

1 93.7 % 2.73 %

2 87.8 % 2.08 %

3 82.2 % 1.71 %

4 77.1 % 1.46 %

5 72.2 % 1.27 %

6 67.7 % 1.12 %

7 63.4 % 1.00 %


• The population is protected 99% against overtopping, but it is not protected against flooding at the same standard level due to other mechanisms of breaches formation • The situation becomes worse for floods having higher return periods, but which still do not lead to dyke overtopping. In fact, due to the freeboard over the 1% water level, a higher flood can pass between dykes.

Infiltration curve through the dyke

• The location of the future breaches can hardly be anticipated in case of a flood similar to 2006 flood. They can occur in new places or at the same locations as in 2006 because beside the dyke reconstruction no special measures have been taken to improve the dyke foundation. •In order to identify the conditions which lead to dyke breach formation, an analysis of the infiltration curve evolution through the dyke was undertaken.


• Typical cross-section of a Danube dyke and its foundation:

Infiltration curve through the dyke •The lithology in the Danube floodplain: - Silty clay from the ground surface to 2.50 m - Fine sand from 2.50 m to 4.00 m - Sandy silty clay from 4.00 m to 5.00 m - Medium uniform sand the next 8.40 m.

Infiltration curve through the dyke •The material characteristics

Infiltration curve through the dyke •The boundary conditions

•The material characteristics


• Scenario 1

47 days (contour lines)

47 days (hydraulic gradients)


• Scenario 2

Discretization of the typical cross-section


• Scenario 2


20 days (free surface)


• Scenario 2


47 days (hydraulic gradients)


• After 47 days the hydraulic gradients at the exit equals 0.8, being greater than the critical values. •At this hydraulic gradient the smallest particles of the dyke body are evacuated outside and preferential infiltration ways are developed. •This is the beginning of an evolutionary process, leading to the voids and caverns formation in the dyke body, which is the start of the breach development.


• Similar scenarios can be imagined by considering as the weak zone the dyke foundation. •It is the case when the dykes were put in operation in areas of former meanders or branches of the river. •Fine or even coarse sand covered by a thin layer of silty clay are found in such cases in the foundation. •The water circulation through these layers is activated during long lasting floods.

Conclusion

• The floodplain along the Danube river is protected by 1100 km of dykes, designed in most of the cases at 1% probability of exceedance of the maximum discharge. • The residual risk is much higher (till 2.73%), due to breach formation during long lasting floods close to a probability of exceedance of 1% of the maximum discharge.

Conclusion

•Two mechanisms are at the origin of breach formation: - The advancement of the infiltration curve through the dyke during long duration floods, followed by the seepage on the inner slope - The occurrence of highly permeable layers in the dyke foundation, located especially where the dykes are crossing old meanders or arms of the river, which were cut or drained in the past.

Conclusion

• In order to minimise the residual risk, some technical and organisational measures can be taken as it follows: -Identification of the weak zones of the dykes during floods (Thermacam). -Population should be learned to recognize and to report any weak dyke areas. - Rapid Intervention Centres should be placed along the lower Danube dykes.

Conclusion

• The areas with a weak foundation can hardly be located by in-situ measurements . • On the contrary, the former arms can be identified using Lidar or areal photogrametry. •The areas where in 2006 artesian spring occurred in the floodplain are susceptible to have the same behaviour for any new flood of the same magnitude; new weak areas can be activated for long duration floods.

Conclusion

• If settlements are located in the proximity of the former Danube branches, the only solution to protect them in such cases is to put in operation a second line of defence (a second line of dykes).

Heightening the dykes (ANAR)

Stabilization works (ANAR)

Thank you for your attention !

Danube Floodrisk THE STAKEHOLDERS IN THE LOWER DANUBE … · SECURITY DEGREE CONCEPT Main steps: a)...

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