2.4 urban stormwater flow idf and design storms

19-9-2013

Challenge the future

Delft University of Technology

CT4491 Lecture. IDF and Design Storms for urban drainage systems design Marie-claire ten Veldhuis

2 CIE4491 Lecture. IDF curves and design storms

Use of rainfall data

in urban drainage system design

and analysis

Two approaches:

Stationary/steady state analysis:

constant rainfall intensity, stationary flow

Dynamic analysis:

variable rainfall intensity, non-stationary flow

3 CIE4491 Lecture. IDF curves and design storms

Rainfall data in urban drainage

system design and analysis

How to compose or choose a representative

rainfall intensity/event from rainfall time-series?

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Rainfall data in urban drainage

system design and analysis

for Design:

How to choose rainfall characteristics, representative of

a pre-defined protection level, over a system’s lifetime?

5 CIE4491 Lecture. IDF curves and design storms

Rainfall data in urban drainage

system design and analysis

for Analysis:

How to find rainfall intensities characteristic of the

conditions we want to check performance for?

6 CIE 4491. Lecture IDF curves and design storms | xx

Stationary rainfall intensity for

stormwater design, IDF-curves

7 CIE 4491. Lecture IDF curves and design storms | xx

Storm water system design and

analysis

What method (stationary/dynamic) to apply when?

What rainfall intensity/intensities to use for analysis?

8 CIE 4491. Lecture IDF curves and design storms | xx

Storm water system design and

analysis

Examples of design/analysis situations:

Stationary .. Or..

.. Dynamic?

Design of small sewer system in Delft

Design of small sewer system in Jakarta

Analysis of small, existing sewer system

Analysis of large, complex existing sewer system

Design of large sewer system in Jakarta

9 CIE 4491. Lecture IDF curves and design storms | xx

Storm water system design and

analysis

Examples of design/analysis situations:

Stationary .. Or..

.. Dynamic?

NB:

Complexity of flow network

Comparison to real-life situation

Computer resources

Spatial rainfall variability

10 CIE 4491. Lecture IDF curves and design storms | xx

Storm water system design and

analysis

Design assignment, part 1: stationary conditions

Application of Rational Method

11 CIE4491 Lecture. IDF curves and design storms

Finding a representative IDF curve

Example of Intensity-Duration-Frequency curves

Horizontal axis: aggregation times

Vertical axis: corresponding rainfall intensities

Each curve: different return periods (T), derived from statistics

12 CIE 4491. Lecture IDF curves and design storms | xx

Finding a representative IDF-curve

Design assignment: 3 IDF-curves provided for 3 different climates: NW-Europe, Mediterranean, Tropics

13 CIE 4491. Lecture IDF curves and design storms | xx

Finding a representative IDF-curve

Design assignment: 3 IDF-curves provided for 3 different climates: NW-Europe, Mediterranean, Tropics

Make a motivated choice for 1 of the curves to use in your design:

Identify and report which curves stands for which climate

Choose and motivate use of 1 curve (think for instance of climate change)

14 CIE4491 Lecture. IDF curves and design storms

Spatial variation, annual rainfall

Average annual

rainfall depth

in the Netherlands

1971-2000

min: 700 mm/yr;

max: 950 mm/yr

Interpolated rain gauge data, period 1971-2000, Courtesy: KNMI

15 CIE4491 Lecture. IDF curves and design storms

Spatial variation daily rainfall

Return period T=20 years

Radar data, period 1998-2008

Daily rainfall depth in the

Netherlands for T=20 yrs

max: 100 mm/day

Yearly rainfall sums and

daily extremes - different

spatial pattern

16 CIE4491 Lecture. IDF curves and design storms

Rainfall curves used in the

Netherlands

Statistical curves: Braak (1933)

Rainfall data of different stations during 1899 t/m 1931: 33 years Van de Herik en Kooistra (1973)

Time series based on 5-minute rainfall data collected over 12 years: 1928, 1933 and 1951 – 1960

Buishand en Velds (1988) Time series of rainfall data collected over 1906 - 1977: 72 jaar

17 CIE4491 Lecture. IDF curves and design storms

Example: Buishand and Velds (KNMI report 1980) Curves are being updated for climate change effects (KNMI, 2004 a.o.)

18 CIE4491 Lecture. IDF curves and design storms

Rainfall data in storm water system

design and analysis

Stationary conditions: representative of real-life conditions?

Why use stationary conditions and IDF-curves?

Quickscan required dimensions new system

Quickscan capacity limits of existing system

Manual design: where there is no computer

NO

19 CIE4491 Lecture. IDF curves and design storms

Rainfall data in storm water system

design and analysis

Stationary conditions: representative of real-life conditions?

Why use stationary conditions and IDF-curves?

Manual design: where there is no computer

Some areas of the world

19th and 20th century, up to ±1990

NO

20 CIE4491 Lecture. IDF curves and design storms

Rainfall data in storm water system

design and analysis

Stationary conditions: representative of real-life conditions?

Why use stationary conditions and IDF-curves?

Manual design: where there is no computer

where there is a lack of data to build a proper model (many

areas worldwide, incl Europe!)

NO

21 CIE 4491. Lecture IDF curves and design storms | xx

Dynamic rainfall intensity for

stormwater design, design

storms

22 CIE4491 Lecture. IDF curves and design storms

Rainfall data in urban drainage

design and analysis

If dynamic calculation is reasonable: use dynamic rainfall

conditions

What rainfall characteristics to choose?

Maximum intensity of a rain event (mm/h)

Total volume of a rain event (mm)

Duration of a rain event (h)

Variation in intensities, high versus low

What is critical for the system we want to design/analyse?

23 CIE4491 Lecture. IDF curves and design storms

What is critical for the system we want to design/analyse?

Depends on characteristics of the catchment: dimensions,

imperviousness, slope

24 CIE4491 Lecture. IDF curves and design storms

Example synthetic standard design

storm T=2 years (NL: “Bui 08”) Rain

fall

inte

nsi

ty (

l/s/

ha)

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Max intensity, at end of event 110l/s/ha

(39.6mm/h), during 10 minutes

Total volume 19.8 mm

Duration 1 hour

25 CIE4491 Lecture. IDF curves and design storms

Synthetic storm T=2 jaar (e.g Belgium)

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T=2 years: Rainfall volume 48.09 mm Max rainfall intensity: 53 mm/h

26 CIE4491 Lecture. IDF curves and design storms

Can you explain why different design storms have been chosen

for BE and NL?

What do you expect to find when you apply the BE T=2yr design

storm to a system designed according to NL T=2yr storm ?

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Max intensity 39.6mm/h during 10 minutes

Total volume 19.8 mm

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T=2 years: Rainfall volume 48.09 mm Max rainfall intensity: 53 mm/h

27 CIE4491 Lecture. IDF curves and design storms

Use of rainfall data in urban

drainage design

Multiple event: Historical: rainfall measurements

e.g. in the Netherlands: time series of KNMI De Bilt, 15 minute time step: • 10 year series: 1955-1964 • 25 year series: 1955-1979

Mainly used for analysis of annual pollution from cso’s Because (why not for flooding analysis?):

Synthetic rainfall series

28 CIE4491 Lecture. IDF curves and design storms

Rainfall input for urban drainage

design

To summarise: Stationary design

- IDF curves, fixed design rainfall intensity

Dynamic design, single event: - Design storm

Multiple event/rainfall series

- Historical series e.g. in the Netherlands: time series of KNMI De Bilt, 10 or 25 yrs

- Synthetic rainfall series

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