3-1_2. ywp 2012 igneva
TRANSCRIPT
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
HYDRAULIC MODELLINGHYDRAULIC MODELLING
• 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
HYDRAULIC MODELLINGHYDRAULIC MODELLING
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] 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
COMPUTER SIMULATIONSCOMPUTER SIMULATIONS
• 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
COMPUTER SIMULATIONSCOMPUTER SIMULATIONS
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
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
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) ► ►
► ► ►
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
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 ►
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
Reliability measure
Reliability of the system Re =
___
LRM
hM
hM
dmm
dmm
)(
)(
_
_
0
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
Dependence between Risk and Reliability
Re + Ri = 1 Ri = 1 - Re
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
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!