Integrated Flood Management Kampala by Victor Jetten

Integrated flood Management Kampala
1D/2Dmodelling and scale issues
Victor Jetten,
Richard Sliuzas, Janneke Ettema
Shuaib Luaise (Makerere University)
DISASTER RISK MANAGEMENT @ ITC
Somewhat based on: IPCC SREX report, 2012
INTEGRATED FLOOD MANAGEMENT KAMPALA
1. City-wide assessment of flood risks & climate
change impacts
2. Detailed flood risk assessment in flood ‘hotspot’
3. Develop a strategy and action plan for improved and
integrated flood management.
Partners:
UN-HABITAT Cities and Climate Change Initiative (CCCI)
Counterpart: KCCA – Kampala Capital City Authorities
Makerere University, Hydroc Consultants, Local NGO’s
MONTHLY FLASH FLOODS
DRIVER 1: ONE OF THE FASTEST GROWING CITIES IN AFRICA
2010
Modeled density 2020
DRIVER 2: CLIMATE CHANGE
Temperature: significantly warmer by a few degrees in 2090. Very
little change up to 2030. Impacts unknown: drought, lake Victoria
water balance, local weather systems?
Rainfall
there is
already
a large
variability
Extremes: possibly larger proportion of rainfall in large events
(statistically not significant, but best to be prepared!)
“Inconvenient truth”: cities are not “climate proof” now, used often
as an excuse for management/planning problems
Tyndall Centre for Climate Change Research, McSweeny et al. (2011)
KAMPALA RAINFALL DATA…
Detailed rainfall data not available
Two sources: Outspan Primary School in Bwaise III area
Automatic weather station at Makarere University campus
RAINFALL LEADING TO FLASH FLOODS
Design discharge based on 1:10 year event = 100 mm/day.
We do NOT know what this rainstorm looks like.
We measured a 1:2 year rainstorm with 66.2 mm and
intensities > 100 mm/h, and scaled this event up
Important: the Kampala drainage master plan uses a design rainstorm
infiltration
based on a USDA-SCS method that generates aAverage
longer,
lessrates
intense
than measured.
openLISEM model system
Spatial, event based, high
resolution
Rainfall data
Gauge areas
Rainfall
Runoff, erosion, flooding
Vegetation
Buildings
Rain harvesting
INTERCEPTION
opensource, freeware
http://blogs.itc.nl/lisem
INFILTRATION
SPLASH
DETACHMENT
SURFACE
STORAGE
Soil cohesion
Texture D50
Roads, Structures
DETACHMENT /
DEPOSITION
SEDIMENT
TRANSPORT
OVERLAND
FLOW 1D
Channel cohesion
Texture D50
DETACHMENT /
DEPOSITION
SEDIMENT
TRANSPORT
CHANNEL
FLOW 1D
Sediment
Discharge
Channel
Discharge
Soil properties
Land use
Roads
Buildings
DEM
Roughness
Obstructions
FLOODING
2D
Flood
Discharge
DEM
Channel properties
Culverts
ORDER OF FLOW PROCESSES
kw
1
kw
2
sv
kw
kw
3
4
kw
sv
sv
kw = kinematic
wave
sv = Saint Venant
kw
kw
2D ‘FULLSWOF’ SHALLOW FLOOD SIMULATION
Freeware open-source University of Orleans
Finite volume solution of saint Venant equations,
with
“Modified Upwind Scheme for Conservation Laws” (MUSCL)
scheme and to avoid oscillations
“Harten, Lax, van Leer” scheme for “shock proof” differential
equation solutions
Fast and stable:
2-step iteration with varying timesteps to ensure
stability (~10-0.1 sec)
Olivier Delestre, Stéphane Cordier, Francois James, Frédéric Darboux
COUPLING 1D AND 2D
Two moments of coupling:
a. 1D runoff reaching the 2D flood zone
Mixing principle (turbulence): runoff continues in the floodzone but
the velocity decreases rapidly: Manning’s n resistance increases
with flood depth: n = n*exp(-ah)
b. Flood zone emptying through the channel :
Each timestep make one water level between channel and
surroundings
=>
THE REAL PROBLEM: DATA ON URBAN HYDROLOGY
Rooftop
some interception
no infiltration
max runoff
Vegetation, bare soil
interception
infiltration
less runoff
Drain
no interception
some infiltration
guided runoff
Murrum road
no interception
min. infiltration
less runoff
Dealing with sub-pixel information
Combine into one infiltration/runoff response
Channel information:
Dimensions, flow network
Road information:
Cover, flow resistance, impermeable
Building information:
Cover, roof storage, raindrums
Vegetation information:
Cover, canopy storage, flow resistance
Soil structure:
Crusting, Compaction (infiltration)
Soil physical information:
Ksat, porosity, suction, moisture content
FROM SATELLITE IMAGE TO FRACTIONS OF SURFACE
PROPERTY PER PIXELS
1m land use map
10 m
10m Vegetation fraction
10m Bare surface fraction
0.5 m resolution
Worldview image
Worldview 2 is out,
0.2m resolution 8 bands
800 US$ for 20 km2
10m House fraction
ACCURATE DRAINAGE SYSTEM NETWORK
Tertiary
secondary
primary drains
Tertiary drains badly maintained, blocked by sediment/garbage
Secondary drains are being cleaned
Primary drain is being improved
SCENARIO 1 - PRIMARY DRAIN IMPROVED, 1:10 YEAR EVENT
Include Simulation here
Simulation here
culverts
SOME KEY GOVERNANCE FACTS
People living in slums (former wetlands) are there partly illegally
(but they rent land from the King of Kampala so there is some form of
tenancy)
Complicated tenancy system between “traditional” Buganda land board
and Kampala municipality, 4 tenure systems exist next to each other
Unplanned, unregistered development
Slums are extremely dynamic, people live there from a day to a few
months, day time/nighttime and no recorded figures for that, no statistics
Some good schools in slums (twinning with UK nand US schools)! One of
the reasons for migration to the city, apart from economics
Many NGO’s, people know exactly the required answers to
questionnaires
Key question: are people upstream prepared to take action for people
downstream (who are there illegally in their view)
WHAT ABOUT RISK?
There is hardly any physical damage, there are no physical
vulnerability curves to be made
There is social vulnerability in the slums: poverty, health,
environment
FULL RISK ANALYSIS WAS NOT DONE
Risk as “expected losses” is almost impossible:
No direct physical damage
health problems related to standing water, waste management,
social disruption etc.
poor methodology for that
3 Stakeholder meetings throughout the project (“slum dwellers”,
authorities, engineers, planners etc) to identify “quick wins” and
long term strategies, and identify who is responsible and can take
action
TWO SOLUTIONS TO DECREASE VULNERABILITY
Improve drainage system
City engineering
larger drainage channels
Community self-help
cleaning
Increase resilience at house level
Elevate houses
Small dikes surrounding house
Bricked-up doorways
DRAINAGE SYSTEM ASPECTS
RESILIENCE STRATEGIES
CITY GROWTH, MODELED HOUSING DENSITIES 2020
Growth will take place in the vicinity of existing buildings
6.5%
4.2% growth per year
Infiltration with 1:10 year rainfall (100 mm)
2013 situation
Infil = 63.31 mm (1,775,000 m3)
2020 trend growth
Infil = 51.86 mm
(1,453,000 m3 = -22%)
SUSTAINABLE URBAN DRIANAGE SYSTEMS (SUDS)
Advantages:
Infiltration and slow down flow,
Avoid erosion
Filter function for groundwater
(leading to wells and sources)
Double function: grazing,
agriculture, recreation?
Buffers as temporary storage
ESTIMATED SCENARIO EFFECTS
now
Master plan
Grassed
waterway
Flood
zonation
Structures in the
Flood zone to be
removed
IMPLICATIONS FOR PHYSICAL PLANNING
Improve drainage with “more concrete”, wider channels, will only
evacuate water faster from the slopes to the valley
Grassed waterways will work
city repsosibility
Construction and maintenance
Displacement of people, eviction, possibly many lawsuits
Not all land is owned by the municipality, so major planning
requires agreement between all parties
Water harvesting at house level people’s responsibility
Behavioral change, maintenance of grass in plots
Subsidy for water tanks needed
Awareness programme
CONCLUSIONS
Integrated Flood Management is really needed here:
upstream-downstream
Because of the high infiltration capacity, avoid
surface sealing at all cost contrary to planning
policy that wants densification
Risk as a central concept
expected losses or
potential damages, but hard to establish because no
direct damages, we need research and better
methods (social risk)
Every solution needs a mix of engineering and
planning, and has major planning and governance
implications
THANK YOU