Torrential rain in the late hours of Sunday (14/08/2107) into Monday (15/082017) resulted in hundreds of deaths and loss of properties due to mudslides and flooding in Regent, Freetown, Sierra Leone. A national emergency has been declared and the flood disaster is thought to be the worst in Africa in the past two decades according to the Sierra Leone government and international rescue agencies.
Challenges for Early Warning Systems
Despite the concern around climate change, understanding hydro-meteorological hazards still faces many challenges. Studies have shown that combining future projections of various climate variables with hydro-meteorological hazard event characteristics need to be undertaken to improve our understanding of climate change hazards.
However, flooding and landslides are rare events, which require a long-time record of both the climate and the hazardous events. But the lack of some meteorological equipment (especially radar), data gap, flood forecasting software and few trained meteorological technicians at the Sierra Leone Meteorological Agency have been the major challenge in providing early warning systems in Sierra Leone. Even if the historical record of climate data is available, the completeness of hazard inventories may be questionable and inhomogeneous.
Rainfall and Slope Stability
Furthermore, while heavy rainfall is a precursor for many flood and landslide events, the occurrence also depends on the spatial variability, antecedent precipitation and temperature, and properties of the landscape.
One of the causes of the mudslide was the decreased stability of slope material, related particularly in the case of unconsolidated sediments to a sudden change in water content.
In general, the moisture content of slopes depends mainly on climate, and in particular on weather conditions. The latter explains why landslide is discontinuous in time and space, even where conditions are favourable. Mount Sugarloaf on the edge of Regent town gave way and caused catastrophic damage. Measurement by Sierra Leone Meteorological Agency Weather station in Freetown (Wilberforce) recorded a significant rain event in the region on 14 August, 2017. Nearly 81.3 mm fell in 12 hours, of which 70 mm fell in about 7 hours.
The monthly total at this station (10.83 inches / 275.08mm) through 15 August, 2017 was over 300% above average; creating the higher likelihood for dangerous mudslides in a significant rain event due to saturated soil. The forecast on the 13th August 2017 issued by the Sierra Leone Meteorological Agency indicated a significant rainfall was possible with high rainfall probabilities over the next 24 hours period.
Forecasts and Warnings
The lack of a radar equipment is the primary challenge to forecast exact rainfall amounts. There is an urgent need for radar and forecasting software to be installed at the Meteorological forecasting studio in order to beef up the early warning systems for the country.
A query of the Sierra Leone Meteorological Agency weather station thunder and lightning network pulses detected over the last four days over Sierra Leone (Lat 7 to 9.5 and long -14 to -10) shows 167,754 pulses. Unfortunately, the query did not indicate the event was caused by thunderstorms as there was no lightning detected in the exact region of the mudslide over the four days examined.
The rainfall was from warm, tropical, low-topped convection in a supersaturated atmosphere. Probably caused by the Intertropical Convergence Zone (ITCZ) disturbance interacting with a sea breeze front that moved inland off the warm, tropical Atlantic.
Human Causes
Floods occur due to human causes as well as natural causes. The leading causes of floods in Sierra Leone include intense precipitation, inadequate capacity within river banks to contain high flows, and silting of riverbeds. Another increasing and significant cause of flooding is deforestation.
The city of Freetown, especially the mountainous areas, has been observing increasing deforestation with residents cutting down the trees for firewood and charcoal. Deforestation plays several roles in the flooding equation because trees prevent sediment runoff, the unimpeded raindrop impact on bare ground leads to heavy erosion and quick runoff. Other factors include landslides leading to obstruction of flow and change in the river course, poor natural drainage, and heavy rainstorms.
Funding and Investment
With the increasing need for better measurements driven by the changes in our atmosphere, there is urgent need to fund the Sierra Leone Meteorological Agency with seed money to suit a broad range of issues. These vary from the state of the art weather stations, forecasting software, training of technicians to weather radar and regional collaboration.
Source: Floodlist