THE HYDROLOGY OF FLOODS

IN MALAWI CITIESTHE ROLE OF WETLANDS

Kenneth Wiyo, Bunda Campus, LUANAR

Seminar OutlineFlooding in Malawi January 2015

Introduction

Flood definition

Hydrology of flooding

Underlying drivers of floods

Case of cities of Blantyre & Lilongwe

Role of Wetlands/ Dambos in cities

Conclusions

Flooding DefinitionFlooding in Malawi January 2015

Def: Excess surface runoff exceeding the capacity of channels, streams and rivers resulting in the channel bursting its banks/ overflowing

Destruction of life, livestock, crops, property, infrastructure

Destructive force

Hydrology of Flooding (multi-facetted)

Excessive surface runoff

(SRO)

High rainfall within or

outside the catchment,

prolonged

Climate change effects

resulting in extreme

rainfall within or outside

the catchment (Jan 2015

floods)

Land use changes in the catchment resulting in low infiltration and high SRO

Underlying Drivers of Floods

Reduced capacity of channels/rivers to contain the excess SRO

Soil erosion, reduced infiltration, high SRO

Siltation and sedimentation

(A)Extreme or high rainfall events (natural or climate change)

(B) Destruction of the Environment

land pressure for agric

River bank cultivation

Cultivation of marginal lands

Deforestation-trees cut in river banks and forest reserves

Population pressure- more land for human settlements

And then came the floods (3)

Flooding, water

everywhere

Too much SRO nowhere

to go

Rivers/channels burst

their banks

Flooding everywhere, water everywhere

Life, livestock, crops property destroyed

Roads, infrastructure

damaged

Flooding in Malawi Cities (LL & BT)

Flooding in the cities of Blantyre and Lilongwe was very rare in the past

Now increasingly common (Ndirande, Area 36, Biwi).

What has changed? Land Use has changed

Land Use Changes Within Cities

Blantyre City Lilongwe City

Population growth, demand for human settlements

Housing in steep marginal areas (e.g Ndirande, Soche hills)

Forest reserves converted to settlements, agriculture

Houses close to river banks (Nasolo)

Storm drains sewers blocked by solid waste

Population growth, demand for human settlements

Gmelina forests converted to housing (areas 36, 22, 23 etc)

Prime agriculture land converted to housing- LL expanding eastwards

Settlements have now expanded into protected wetlands/ dambos (Kawale, Area 47, Kauma etc).

Hydrology of flooding in cities

Housing settlements, Infrastructure reduce infiltration and increase SRO (more roofs, impermeable surfaces)

Storm drains, culverts designed to take a certain design SRO flow. If SRO increases, capacity of storm drains, culverts exceeded

More SRO, more flooding

Role of Wetlands/Dambos in Flood Control

Functions of Wetlands Destruction of Dambos

Provide extra storage capacity for excess SRO

Thus act as a buffer zone for floods, slowly releasing the water , ameliorating peak flows

Can trap sediments, less enter the rivers

Clean the water, and trap pollutants

Encourage biodiversity in our cities, areas of wildlife refuge.

Protected Lilongwe City dambos encroached and destroyed-housing settlements.

Flood control/buffer function no longer there

More flooding in Lilongwe City

Summary of Flooding in Cities

Causes of Floods in General

Flood Drivers in Cities

Floods are multi-facetted.

Can either be caused by prolonged high rainfall events or extreme events due to climate change resulting in excess SRO.

Land use changes in a catchment can trigger floods by increasing erosion, decreasing infiltration and increasing SRO.

Increase in housing settlements into marginal forested areas, dambo and river banks.

Conversion of forested areas into human settlements.

Housing into protected wetlands/ dambos- no flood buffer.

Expect more flooding in Lilongwe and Blantyre Cities

FLOOD MITIGATION REQUIRE

TEAM WORK

THANK YOU