Climate change and Urban Vulnerability in AfricaAssessing vulnerability of urban systems,

population and goods in relation to natural and man-made disasters in Africa

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Course on Hazard, Risk and (Bayesian) Multi-risk assessement

Napoli 10/24/2011 – 11/11/2011

Module 5: Case study: from climatic data to flooding risk assessment - Application for informal settlements

Prof. F. De Paola, prof. F. Jalayer, ing. R. De Risi

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MOLFETTA

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BASIN MAIN CHARACTERISTICS

Area S [Kmq]

Length of main channel [Km]

Medium Slope

Drainage density DD

Average height

ii

i

i i

m

P

l

l

S

lDD i i

A= 11.02 km2

L= 7.9 km

P= 3.05 %

Hm= 77.77 mslm

tot

iiim S

SHH

_

The runoff curve number CN is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess.The curve number method was developed by the USDA- Natural Resources Conservation Service, which was formerly called Soil Conservation Service or SCS. The number CN is still popularly known in the literature as a "SCS runoff curve number".

The runoff curve number is based on the drainage basin characteristics (hydrological soil group, land use, land management) and hydrological condition. References, such as from USDA, indicate the runoff curve number for characteristic land cover descriptions and a hydrologic soil group

CN METHOD

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CN fundamental relationship:

CN METHOD

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SIP

IPP

a

anet

2

where:

- Pnet

: accumulated runoff depth (mm)

- P : accumulative rainfall depth (mm)

- Ia : initial abstraction (mm), or the amount of water before runoff, such as infiltration, and rainfall interception by vegetation

- S : potential maximum soil moisture retention after runoff begins (mm).

Assuming Ia = 0.2S

SP

SPPnet

8.0

2.0 2

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CN METHOD

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CN has a range from 30 to 100.Lower numbers (permeable soils with high infiltration rates) indicate low runoff potential.Larger numbers are for increasing runoff potential.

The runoff curve number, CN, is then related to potential maximum soil moisture retention, S, by:

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Curve Number based on hydrological soil groups:

Group A is composed of soils with very high infiltration rate and low runoff potential

Group B soils with high infiltration rate when thoughly wetted and moderate runoff potential

Group C soils with low infiltration rate and moderately high runoff potential

Group D soils with low infiltration rate even when saturated and high runoff potential

HYDROLOGIC SOIL GROUP

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A Very permeable Medium to high permeability

B Moderately permeable Medium to low permeability

C poorly permeable low-permeability

D raincoats

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Class A B C D1 Non-irrigated arable

Non-irrigated trees Agricultural areas with

significant areas of natural

Non-irrigated arable land

Non-irrigated vineyards Temporary crops

associated with permanent crops

Minor non-irrigated fruits

62 71 78 81

2 Urban Urban Areaas 92 92 92 92

3 Wet Zone Residential Areas 77 85 90 92

4 Water bodies Basins 100 100 100 100

5 mixed farming trees irrigated irrigated arable watering lawns

Herbage from the open-loop spring summer

Summer vegetable crop cycle autumn / spring

Horticultural crops in spring-summer cycle

Minor irrigation orchards and orchards

irrigated olive groves Cropping systems and

particle complex irrigated vineyards Cropping systems and

particle complex

72 81 88 91

6 Non-irrigated meadows Non-irrigated meadows 30 58 71 787 Natural Wooded Areas 45 66 77 83

 

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LAND USE MAP

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STAGIRR IRRIGUO IRR Area (km2) CNII  

NI arboreal NI non-irrigated 5.56 62 344.72

NA urban non-agricultural 1.45 92 133.4

NI arable NI non-irrigated 0.12 62 7.44

NI meadows NI non-irrigated 0.09 30 2.7

NA urban non-agricultural 0.15 92 13.8

NA urban non-agricultural 0.52 92 47.84

NI mixed farming non-irrigated 0.12 72 8.64

NI arboreal NI non-irrigated 0.08 62 4.96

NI arboreal NI non-irrigated 1.99 62 123.38

NI mixed farming non-irrigated 0.77 72 55.44

NI arboreal NI non-irrigated 0.08 62 4.96

NA urban non-agricultural 0.09 92 8.28

      11.02   755.56

        CNII 68.56261

Average on the area

 

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CN

Peak hydrograph (Mockus)

7.0

5.0

8.0

91000

342.0

CNs

Lt l

lca ttt 5.0

tl: catchment lag time (time between the hydrograph

centroid and the net rainfall centroid)

L [km]; s[%]

ab tt 67.2

ap t

AVQ

208.0

V [mm]

A [km2]

ta [h]19/04/2023 12F. De Paola

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0

2

4

6

8

10

12

0.00 5.00 10.00 15.00

t (h)

T=10

T=50

T=100

15.3exp

5.3

aap t

t

t

tQQ

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T Duration Volume Peak hydrograph

years (h) (m3) (m3/s)

10 15 60138 2.80

30 15 118072 5.50

50 15 173555 8.08

100 15 242078 11.27

200 15 314302 14.64

500 15 427252 19.90

Depth-averaged shallow waterequations on land surface

INUNDATION MODEL

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TWO - DIMENTIONAL MODEL

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T = 30 YEARS

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T = 200 YEARS

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T = 500 YEARS