Buzi River Flood Risk Mapping

Joining technology and local knowledge to mitigate the effects of climate change

In recent years, due to the worsening effects of climate change, Mozambique has been buffeted by cyclones, tropical storms, and other tropical weather depressions.

The human, infrastructure and financial consequences of these events leave the country devastated, due to the lack of resources for prevention, mitigation and response policy and programmes’ implementation.

Only in the last three years, Mozambique has experienced a high number of damaging climatic events, such as:

March 2019

  • Tropical Cyclone Idai

April 2019

  • Tropical Cyclone Kenneth

December 2020

  • Tropical Cyclone Chalane

January 2021

  • Tropical Cyclone Eloise

January 2022

  • Tropical Cyclone Ana

March 2022

  • Tropical Cyclone Gombe

The district of Buzi is located downstream of the river with the same name. The riverbed runs along the district's village, where you can find many public and private infrastructures as well as a large part of the district's population and their respective areas of agricultural and cattle production, which are the main economic activities carried out in the district. According to the district health service, at the time of the occurrence of Idai (2019), the population was of 188.914 inhabitants.

During the cyclone, water coming from upstream carried by the Buzi River was one of the main risk factors that caused extensive floods and heavily impacted the population.

According to the secretariate of the Buzi district directorate, Idai caused:

  • 110 deaths
  • destruction of 11,480 houses of which 4,060 total and 7,420 partially
  • loss of 73,765 hectares of various crops
  • loss of 7,850 heads of cattle
  • loss of 25,169 poultry

Given the increasing frequency of cyclones and storms that are hitting the area, the National Disaster Management Agency (INGD) in partnership with the World Food Programme (WFP) and funded by the European Civil Protection and Humanitarian Operations (ECHO), conducted an extensive mapping exercise of the Buzi river basin  using Unmanned Aircraft Systems (also known as drones).

The use of drones in emergency preparedness and response is becoming a common approach and has been a success in terms of results achieved, due to the significant cost reduction in search and rescue operations. 

The main objective of the Buzi mapping exercise was to produce very high-resolution Digital Elevation Models (DEM), to identify low topographical areas which are more at risk of flooding during intense rainfall periods, and to inform subsequent discussion and identification of mitigation measures with local communities that help reducing floods impact in the district of Buzi.

Data collection with drones is an extremely important means of acquiring very high-resolution data, allowing the pilot the possibility of precisely delimiting the study area and the definition of the intended products.

INGD and WFP started data collection over the Buzi river in August 2021, rolling out the project funded by ECHO in 4 stages:

1)      Data collection

2)      Data processing

3)      Validation of preliminary results with local communities and participatory mapping

4)      Validation with local government and implementation of mitigation actions

 

Data Collection

On August 8, 2021, for about 40 days the teams were in the field collecting data. For this purpose, flight plans were made to cover the entire area of interest of the basin. 

 

Despite some technical and climatic constraints, the activity was carried out successfully. About 800 km² were mapped, using about 600k photos, 7TB of high resolution imagery.

Data Processing

The second and most time-consuming stage of this exercise, was the data processing. Data processing lasted for about 8 months, resulting in orthomosaics and Digital Terrain Models (DTM's).

Orthomosaics – with the orthomosaics it was possible to have an aerial view of the area of interest, allowing for a clear identification of the vulnerabilities of the district, mainly flood-related vulnerabilities.

It was possible to correlate the risk factors and, above all, to propose solutions to mitigate the identified vulnerabilities.

Problems such as clogged drainage ditches, inappropriate construction in inappropriate areas, poor solid waste management, among others were also identified through this exercise.

In addition to the identification of problems, the aerial view of the study area allowed the identification of points of interest in the district, i.e. potential places of refuge for the population in emergency situations and their evacuation routes.  In emergency situations, panic often takes over and people are not able to reason and make decisions such as searching for a safe place and a safe route to that place.

Therefore, with a bird's view of the study area, it was possible, through participatory mapping (community mapping) to design evacuation routes to the already known shelter centres and/or new safe places, identified with the help of the relief map.

Digital Terrain Models - with the DTMs it was possible to produce the flood risk maps according to the relief layout.  The DTMs allowed for the identification of the low areas with high risk of flooding, the intermediate areas with medium risk of flooding, and the higher areas with lower risk of flooding. 

It was possible to produce about 124 DTM maps with a scale of 1:2500.

Data Validation

The third part of this exercise consisted in validating the results with the community and some members of the local government.

After data processing was complete, the orthomosaics produced over the communities were printed to be used for the community mapping process.  INGD and WFP teams went to the field to present maps to local communities and start discussing with them emergency preparedness activities. The issues plaguing the district, the safe places or the places that have habitually been used as collective shelter during emergency situations were identified through the aerial images. 

Parallel to the participatory mapping exercise, the teams met with the different departments of the local government: education, health, administration, etc.

Returning to the field

The last stage of the risk mapping in the Buzi district consisted in returning to the field to present the results to the local government, as well as to place signs along the evacuation routes and on the previously identified infrastructures.

With the help of this risk mapping exercise, the government is more confident about mitigating the negative impact of extreme events. Dredging the Buzi river to facilitate the water flow is already being discussed together with implemention of additional measures to respond to each of the problems identified.

After consultations with the local government to present project results, the team worked with residents of areas at most risk to install signposts that highlight and help to locate safe areas and evacuation routes.

All this was possible thanks to the generous funding from the European Civil Protection and Humanitarian Aid Operations (ECHO).

© INGD / Antonio Beleza

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WFP Mozambique

In partnership with INGD

Funded by the European Union Humanitarian Aid