Accra, the capital city of Ghana, is located in subtropical West Africa. This rapidly growing capital city is surrounded by hills to the north and the Atlantic ocean to the south, bringing in moist air with the trade winds. During the rainy season (April – June and September – October), these conditions often result in heavy thunderstorms. Rainwater from these intense convective storms regularly floods Accra’s water systems, including hills just north of the city and numerous small streams running through the city itself. These floods lead to many disruptions across the Accra metropolitan area.
The urgency of effective flood early warning systems became painfully clear in 2015, when severe flooding of the Odaw River, one of the rivers in Accra, caused devastating loss of life and widespread damage.
A flood early warning system for Accra
Flood Early Warning System (FEWS) Accra is designed to help the city anticipate major flood events, improve preparedness, and ultimately protect communities against future flood risks. FEWS Accra brings together innovative satellite-based rainfall nowcasts with hydrological and hydraulic models to rapidly calculate where a flood can take place in Accra. Beyond the technical innovations in FEWS Accra, other crucial componens of the flood early warning chain are dissemination, warning and action protocols. To improve this part of the warning chain, the Accra Forecasting Center (AFC) has been set up. The AFC brings together all involved agencies and serves as a platform to work together with FEWS Accra, learn from each other through workshops and form standard operating procedures in which cross-agency communication is crucial.
Who are the users of FEWS Accra?
In Ghana, the National Disaster Management Organization (NADMO) is responsible for disaster response before, during and after events. NADMO’s response is organized through National, Regional, and District Emergency Operations Centers (EOCs). District EOCs disseminate flood warnings to vulnerable communities and organize local response efforts. NADMO’s district EOCs are key in getting warnings to the neighborhood level and act timely, but they can still be aided by information about timing, location and especially the possible impact of flooding in the hours to come. FEWS Accra will enhance NADMO’s capacity to anticipate impacts of heavy rainfall and flooding by providing timely water information and forecasts.
The key agencies that work with and collaborate through FEWS Accra are:
- Ghana Meteorological Agency (GMet), Ghana’s leading meteorological authority. It issues warnings for heavy rainfall and other severe weather events, including potential flooding, to citizens in Accra and beyond.
- National Disaster Management Organisation (NADMO), which disseminates warnings to the public and coordinates and mobilizes emergency response efforts.
- Hydrological Services Authority (HYDRO), which hosts flood forecasting systems for the White Volta and Oti Rivers, and issues related flood warnings.
- Water Resources Commission (WRC), the agency responsible for water resource management in Ghana. Within the project, WRC provides supporting services for FEWS Accra, such as IT staff, communication, project experts.
The main components of FEWS Accra: bringing together meteorology, hydrology, warning dissemination and stakeholder engagement
FEWS Accra brings together four critical components that strengthen Accra’s ability to anticipate and respond to flooding: a rainfall nowcasting system, hydrological and hydraulic flood forecasts, risk communication at the neighborhood level, and stakeholder engagement. Accra’s urban water network typically responds within 30 minutes to a few hours to rainfall. This means that when it rains, a flood can take place within that timeframe. This leaves very little time to respond. The goal of the system is to increase pre-rainfall warning time in order to make better decisions with sufficient time left for the actual response. This requires a system that can provide forecasts with a low latency, a high accuracy to pinpoint districts where a response is needed, and disaster management procedures that can take place in these short timeframes.
Improved flood forecasts start with better rainfall forecasts for short lead times, which should aid in increasing the anticipation times to at least a few extra hours. In the absence of a dense hydrometeorological network and weather radars in Ghana, remotely sensed information from the Meteosat Third Generation (MTG) satellites is used for rainfall estimation and forecasting. The information from the MTG satellites is available every ten minutes at a 2-kilometre resolution, with a maximum latency of approximately 30 minutes, providing high-resolution rainfall information with a good spatial coverage for Ghana. This makes it useful for forecasting, but requires a fast forecasting method, too. Nowcasting, a method which uses computer models to rapidly extrapolate remotely sensed rainfall estimates to the future, is well suited for this job. Given that heavy (tropical) thunderstorms are typically hard to capture with traditional weather models, such an observations-driven forecasting method is a strong alternative for the first hours into the future.
That is why a cutting-edge rainfall nowcasting system based on the open-source pySTEPS toolkit is at the core of FEWS Accra. With the satellite information, the nowcasting tool produces rainfall forecasts for the coming three hours ahead every ten minutes. This high frequency and good spatial coverage make it well-suited for Accra’s fast-responding urban basins. Hydrological and hydraulic models (HEC-HMS and HEC-RAS) then translate rainfall forecasts to on-the-ground impacts in terms of flood extent and depth. FEWS Accra uses this information to automatically generate flood alerts at the neighborhood scale. These flood alerts are verified by HYDRO and then passed on to NADMO for timely actions.
A crucial next step is how this information is subsequently picked up by NADMO on both national and district levels, to ensure the tool supports NADMO’s response. This requires a strong collaboration between all involved institutes (GMet, HYDRO, WRC and NADMO) through the AFC and engagement with local stakeholders—ensuring the tool fits real-world needs. A large part of the project has focused on how to communicate risks clearly between institutes and to the public, how to support timely action to strengthen community resilience, which is the last mile in the flood forecasting and response chain. An example of warning messages that directly can be sent through text messages and via television channels is shown below, and was designed together with NADMO.
Building bridges between institutions to ensure FEWS Accra has impact
The collaboration between the institutes was not straightforward from the start, as some had either not yet worked much together or had no procedures in place to communicate warnings and information with each other. It was very valuable to bring everyone together in workshops and to use these moments to develop standard operating procedures together, with the goal to understand the procedures the other organizations have to take and what they need from your institute to do so. Through these workshops, all institutes gain experience with the forecasting platform, the models and the operating procedures. This should build capacity and a longer-term sustainability for FEWS Accra.
Now that FEWS Accra is operational, the four institutions are integrating the use of this system in their day-to-day activities. The project will host a joint workshop during the next rainy season to provide more hands-on experience and focus on sustainable long-term collaboration. From next rainy season onward, we hope that FEWS Accra will make a difference in Accra’s flood resilience.
