# Recent Advances in Internet of Things Solutions for Early Warning Systems: A Review
## 4 Floods
### IoT Architectures for Flood EW Systems  
Referring to the layered architecture described in the Introduction, Table 2 lists the sensing units, communications technologies, and prediction methods employed, and other applications developed in each reviewed paper. The table also highlights if an Edge/Fog computing paradigm has been adopted or not in each of the reviewed use cases. Different meteorological and hydraulic quantities can be measured for flash floods predictions, such as: water level, water velocity, amount of rainfall, temperature, and atmospheric pressure. Alongside these quantities, soil composition, topology, and soil moisture \begin{table}
\begin{tabular}{p{12.9 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt}} \hline \hline
**Article** & **Perception Layer** & **Communication Layer** & **Application and Edge Layer** & **Edge/Fog Computing** \\ \hline
[46] & Water Level, Water flow & Zigbee & Data analysis and threshold-based warning at remote server; Android smartphone application & No. \\ \hline
[26] & Water flow sensor, water level sensor and rain value. & None proposed & Holt-Winter method for prediction at remote server; early processing and prediction at sensors/gateway level & Yes. \\ \hline
[47] & Monitoring, meteorological, and mobile stations & 3G/GSM, LoRA & IoT Cloud Platform to receive, manage, and store sensor data; semaphore alert system & No. \\ \hline
[48] & HMS stations & GSM, LoRA & Data processing and storage at remote server & No. \\ \hline
[49] & Ultrasonic Sensor & EnOcean & Water level monitoring at remote server & No. \\ \hline
[50] & Resistive Sensors & LoRa & Threshold-based alarm system at Cloud level; data storage and analysis on separate website & No. \\ \hline
[44] & Water pressure, rain gauge & 6 LoWPAN & Machine Learning forecasting at Cloud and sensor level; Visual and sound alarms system & Yes. \\ \hline
[51] & Wind sensor, humidity sensor, temperature sensor and water level sensor & LoRa & SVM model embedded in concentrator nodes for risk classification; final decision at Cloud server & Yes. \\ \hline
[52] & CMOS Cameras & Zigbee, GSM & Web platform; algorithms to get information such as rank of calamity and precision & No. \\ \hline
[53] & Water Level, Water Volume & None proposed & Threshold-based alert system; Cloud solution to manage actuators & Yes. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Technologies and solutions used for each layer of the reviewed papers for Flood EW. might influence floods, and weather forecast data can be used to enhance predictions. The sensors in the Perception Layer can be integrated with other sensors such as Global Positioning System (GPS) units. Smart camera systems can be used alone or alongside sensors that monitor hydraulic and meteorological quantities [56], but they require deeper data analyses and deep learning or image processing algorithms on the Application Layer (a comprehensive review on computer vision methods for flood monitoring by [PERSON] et al. is reported in [20]). The presence of pre-existing weather stations or hydro-geological monitoring stations can facilitate the deployment of the EW system. The correct positioning and installation of IoT devices is essential to achieve good results, and depending on the number of installed devices the solution might have drawbacks: a large number of IoT devices will increase cost and will generate redundant data; too few will decrease the effectiveness of predictions [26]. When it comes to the Communication layer, some of the reviewed solutions still rely on GSM, 3G or GPRS modules for long range communications, while new cellular solutions (such as 5G or Narrowband-IoT) could provide better coverage and less latency and an overall better performance. Ad hoc networks supported by protocols with long range such as LoRa are also a solution, and they are able to obtain high link budgets in difficult environments, but they require the installation of a gateway.