3-1_2. ywp 2012 igneva

Risk Assessment Methodology for Risk Assessment Methodology for Hydraulic Overloading of Urban Drainage Hydraulic Overloading of Urban Drainage

Networks and Flooding of Urban AreasNetworks and Flooding of Urban Areas

T. IGNEVA-DANOVAT. IGNEVA-DANOVA

University of Architecture, Civil Engineering and GeodesySofia, Bulgaria

CASE STUDYCASE STUDY

• The town is situated in the middle north part of Bulgaria • The climate in the region is typical continental – cold winters, springs

with intensive rainfalls and hot summers • The town is built on relatively steep terrain with considerable

displacement between its upper and lower parts • The drainage area is approximately 75 ha • The urban area is drained by combined sewer system with total length

of nearly 8 km • The total number of the population is approximately 4 000 people • Existing sewer pipes and joints are in relatively good technical

condition • Existing combined sewer system after rehabilitation is planned to be

exploited as storm system

RAINFALL INTENSITY AND RETURN RAINFALL INTENSITY AND RETURN PERIODPERIOD

• Definitions for the term “intensive rainfall” • Inability to accept universal border of intensity and duration of

these rains • Standards for modelling hyetographs - “intensive rainfalls” are

accepted to be these with intensity more than 30 l/s.ha (0,18 mm/min) regardless to their duration.

• For the area of the town intensive rains with duration of 5 minutes (independently from its intensity) are 30 cases per a year and these with duration from 10 to 17 minutes – 17 cases per year. The number of heavy rains with duration of 30 minutes is 5 to 6.

• The probability of occurring heavy rains with 60 minutes duration is about 60-70% per year

• Return period P is considered in accordance with type of sewer system

• The existing combined sewer network is designed for 2 years return period

• About 67% of the total drainage area is taken by yards • On the territory of the two districts there are no underground

warehouses, stores and business buildings. • Eventual flooding with waste waters in basements would not

cause so many damages in comparison with flooding in town centre

• During the past 20 years no actual damages due to hydraulic overloading of sewer network were recorded

• The design return period of the storm water system is diminished to 1 year

RAINFALL INTENSITY AND RETURN RAINFALL INTENSITY AND RETURN PERIODPERIOD

HYDRAULIC MODELLINGHYDRAULIC MODELLING

• Exiting urban drainage network was performed by means of MOUSE software, based on the available electronic cadastre

• Imperviousness is precisely defined in accordance with surface type and information from the cadastre. The mean percentage of imperviousness in the considered districts is 33%

• New storm water collectors • The influence of these seven newly designed collectors on the

conveyance of the existing network is examined through the computer model

• Two of the sewer overflows are planned to be used as dividing chambers and the excess storm water is going to be discharged to the river. The third sewer overflow will be reconstructed as a manhole with no direct discharge to the river


• The hydraulic model comprises of:• 241 manholes• 236 pipes• 67 catchments with total drainage area of 50,3 ha• 2 outlets• 2 dividing chambers• The total length of the modelled network is 7740 m


8667200.0 8667400.0 8667600.0 8667800.0 8668000.0 8668200.0 8668400.0 8668600.0 8668800.0 8669000.0 8669200.0 8669400.0 8669600.0 8669800.0[m]

4660400.0

4660500.0

4660600.0

4660700.0

4660800.0

4660900.0

4661000.0

4661100.0

4661200.0

4661300.0

4661400.0

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4661700.0

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4662000.0

[m] Diameters

Legend:Diameters of pipes:--- 0,3 – 0,5 m--- 0,5 – 0,6 m--- 0,6 – 0,8 m--- 0,8 – 1,2 m

Δ - outlet - weir - manhole

outlet

weir

manhole

pipe

COMPUTER SIMULATIONSCOMPUTER SIMULATIONS

Design rainfall• Return period P=1 year• Rainfall duration 30 minutes• Rainfall intensity 100 l/s.ha


• Depth of cover over sewer pipes lower than the minimum admissible• Sewer pipes with steep slope followed by pipes with flat slope. Reducing

of velocity is precondition for backflow and pressure conditions in sewer pipes.

• Higher velocity in secondary sewer collectors at the point of attachment to main sewer than in the main collector. This non-compliance with design criteria leads to backflows at three certain sewer sections.

• Pressurized sewer pipes - total length is 455 m or 6,2 % of the whole sewer network.

• No surface flooding occurs during the simulated event. • Despite the mentioned above design shortcomings, the existing sewer

network possesses relatively good hydraulic capacity and can be exploited without serious problems.

• Pressurized regime in storm water sewers is not so dangerous because there is no potential threat of basements’ flooding.

5.11.2009 г. 12:30:10


Methodology for Risk Assessment of Methodology for Risk Assessment of Hydraulic Overloading and Flooding of Urban Hydraulic Overloading and Flooding of Urban

Drainage System Based on the Fuzzy Set Drainage System Based on the Fuzzy Set ApproachApproach

Fuzzy Set Theory

• Zadeh – 1965

• Application in physically controlled systems, different engineering problems, statistics, medicine, biology

• No information for application of this theory in risk assessment for hydraulic overloading of urban drainage networks is available for author’s best knowledge



1. Area of absolutely safety (normal performance of the sewer pipe) ►

2. Area of decreasing safety (increasing risk – the sewer pipe is pressurized) ►

3. Area of absolute risk (the water level is above the terrain -

flooding) ►

4. Area of external load to the sewer pipe (rain with a definite return period – P, at witch in this example the water level rises up to 2 m above the sewer pipe invert) ► ►

► ► ►



1. External Load of the system - LIts membership function µµL is changing in the interval (0,1) at

maximum µµL = 1 at a head/depth of 2 m, for this example ►

2. Response of the system – RµµR – increasing the pipe depth from D to terrain surface, the

membership function is changing from 1 to 0 linearly ►



Reliability measure

Reliability of the system Re =

___

LRM

hM

hM

dmm

dmm

)(

)(

_

_

0



Dependence between Risk and Reliability

Re + Ri = 1 Ri = 1 - Re



Risk of overloading and flooding of the sewer network depending on return period P

Risk of hydraulic overloading and flooding

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

Manhole ID

Ris

k

P=2 years

P=5 years

P=10 years

P=20 years

P=30 years

P=40 years

Methodology for Risk Assessment of Hydraulic Methodology for Risk Assessment of Hydraulic Overloading and Flooding of Urban Drainage System Overloading and Flooding of Urban Drainage System

Based on the Fuzzy Set ApproachBased on the Fuzzy Set Approach

8667200.0 8667400.0 8667600.0 8667800.0 8668000.0 8668200.0 8668400.0 8668600.0 8668800.0 8669000.0 8669200.0 8669400.0 8669600.0 8669800.0[m]

4660400.0

4660500.0

4660600.0

4660700.0

4660800.0

4660900.0

4661000.0

4661100.0

4661200.0

4661300.0

4661400.0

4661500.0

4661600.0

4661700.0

4661800.0

4661900.0

4662000.0

[m] DiametersMethodology for Risk Assessment of Hydraulic Methodology for Risk Assessment of Hydraulic

Overloading and Flooding of Urban Drainage System Overloading and Flooding of Urban Drainage System

Based on the Fuzzy Set ApproachBased on the Fuzzy Set Approach Map of risk - spatial distribution of reliability over the territory of the

town of Novi Iskar before and after the reconstruction

Legend:

Ri 0 – 0,25 Ri 0,25 – 0,5 Ri 0,5-0,75 Ri 0,75 - 1

CONCLUSIONS

Risk/reliability of hydraulic overloading of the urban Risk/reliability of hydraulic overloading of the urban drainage networks can be adequately assessed only by drainage networks can be adequately assessed only by

applying the appropriate approaches, models and softwareapplying the appropriate approaches, models and software

In our view the most appropriate approach for assessment In our view the most appropriate approach for assessment of the hydraulic overloading/flooding of the sewer network of the hydraulic overloading/flooding of the sewer network

should be one, based on the Fuzzy Set Theoryshould be one, based on the Fuzzy Set Theory

The proper assessment of the hydraulic capacity of sewer The proper assessment of the hydraulic capacity of sewer networks includes not only simulations with design rainfall, networks includes not only simulations with design rainfall, but also giving quantitative assessment of risk for different but also giving quantitative assessment of risk for different rain eventsrain events

THANK YOU FOR YOUR ATTENTION!THANK YOU FOR YOUR ATTENTION!

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3-1_2. ywp 2012 igneva