# Wireless Sensor Network-Based Rockfall and Landslide Monitoring Systems: A Review
## 3 Rockfall and Landslide Monitoring Based on WSNs
### WSN-Based Monitoring Systems
A so-called cluster head was designated for each area. The purpose of the cluster head was to aggregate data from all nodes of the cluster area and to transmit data to a gateway node. Finally, through the gateway, collected data were transferred to the data analysis center by using GPRS. Transport pathways are particularly at risk during a landslide. A WSN for practical use on expressways was developed in [102] in light of the rise in unexpected heavy rainfall events. The system adopted a monitoring station and multiple sensor nodes with several sensing elements. Data measured by the sensors were sent to the monitoring station that aggregated all the arriving data and periodically transferred it to a web server from which they could be downloaded. When different monitoring sites were established, the readings of each were automatically registered on the web server for monitoring the conditions of multiple slopes over the internet. The authors conducted several communication tests to test and improve the robustness of the system. A trial environment was established before the deployment of the full network. ZigBee WSNs are often used in the field of geohazard monitoring. In [103], the authors presented the hardware design of a system based on ZICM2410 [104] modules. The nodes were built to be mounted on a slope and were equipped with a tilt sensor, temperature and moisture sensors, an extensometer rod for displacement, and a capacitive rainfall sensor. A remote management unit was connected using GSM to a base station that gathered the nodes' data. A low power schedule of the working cycles was used to have better management of the battery power: the nodes sleep for several hours and have an active period of about 1 min where the power is increased. We do not have information of whether the system was deployed in a real scenario; however, the authors focused on the description of this low-cost solution for the sensor nodes. Similar hardware as the aforementioned system is found in [105], where the authors focused on the study of the hardware performances of a monitoring system that was installed in Guizhou, China. The system has been proven able to work consistently for a long period of time in a harsh environment. In [106], the authors presented a smart landslide monitoring device called \"SMARTMODE\", which could be inserted into a WSN scenario. The hardware consists of a micro-controller, a communication module, general-purpose input-output (GPIO) channels, and power circuitry. In particular, the node can be used in a ZigBee-based WSN, and is supplied using rechargeable batteries. The sensor components on the device were connected through plug-and-play connectors to allow the faulty parts to be replaced. SMARTMODE monitored the slope displacement using a MEMS-based three-axis in-place inclinometer (IPI). The slope-related parameters, which are soil moisture and soil pore-pressure (PP) were also monitored using capacitive sensors and pressure sensors. The software was responsible for data aggregation, context-adaptive operations, and energy optimization of the SMARTMODE. One of the software features was the filtering of the sensor data from noise and outliers to prevent false warnings. The authors faced one common difficulty that comes with soil monitoring, as was observed in previously mentioned research. Soil moisture sensors are sensitive to changes in soil properties; therefore, it can become difficult to decide whether a small change in the read values is due to rain or environmental fluctuations. This can happen until a significant change in sensing values has been observed. It is known in research that clustering helps to increase a network's lifetime and its routing performance. To this end, in [107], an autonomous structural monitoring system for dynamic events was presented. The system was composed of nodes capable of point-to-multi-point transmission to form a redundant routing pattern. This was needed due to harsh environmental conditions because the network was deployed on rockfall barriers.