# Optimal Arrangement Strategy of IoT Sensors in Urban Drainage Networks: A Review
## 3 Arrangement of IoT Sensors in the Drainage Network
### Optimization of Monitoring Point Locations  
#### 3.3.1 Design Evaluation Methods Based on Engineering Experience  
Decision makers need to comprehensively consider the overall study area and objectives when selecting the location of monitoring points based on engineering experience. The general principles considered include the principle of controllability, as well as the requirements for uniformity, feasibility, representativeness, safety, and installation and maintenance costs [5; 6; 10; 25]. Important nodes in the drainage network [12; 63], other than inspection wells, or the selection of important drainage facilities, such as sewage treatment plants [64], discharge outlets [24; 65], pump stations [6], and weirs [26], all require special attention. In addition, in some cases, where monitoring points are arranged in a UDN [34; 66], the classification of branch pipelines, main sewers, and main drains is conducted based  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Type of Study** & **Type of Node** \\ \hline \hline \multirow{8}{*}{N Nodes in certain locations have a higher necessity for monitoring} & \multirow{8}{*}{C Calibration of hydrodynamic models} & Priority is given to areas with a high population density such as hospitals, nursing homes, community centers, and inspection wells connected to buildings [12; 56]. \\  & & The location of potentially infected nodes is reflected through Bayesian prior probabilities [13] \\ \cline{1-1}  & & The pipeline where the flow rate changes is the “potential measurement location” [57] \\ \cline{1-1}  & & Most pollution events occur in nodes located downstream [32; 58] \\ \cline{1-1}  & & Different levels of priority are assigned to each potential monitoring site considering CSO frequency, flow rate, and mass of pollutant [9] \\ \cline{1-1}  & & A pre-screening procedure based on the concept of pollution matrix is introduced to screen some nodes [59] \\ \cline{1-1}  & & Nodes in key topological locations, nodes located in areas with high residential density [31] \\ \hline Nodes have the same necessity for monitoring [17; 33; 35] & Monitoring and source tracing of pollutants in drainage networks & Any node has an equal probability of being the source \\ \hline \hline \end{tabular}
\end{table}
Table 2: List of monitoring necessities associated with different types of nodes.  
on several factors. These include the connection structure of the network, the location of monitoring points, and monitored parameters. They are determined step by step, and the monitoring points are classified. [PERSON] et al. [67] considered the high-impact scenarios of sensor monitoring deployment, the high-risk locations of the monitored objects, and the high coverage of the sensor network as the objectives. They set different weights for the objectives based on expert opinions and optimized the objective function to calculate the sensor layout scheme.  
In certain UDNs with a long service life, detailed basic data and relevant information are often missing, making it difficult to establish high-precision models. Therefore, expert experience and engineering practice are relied on to select monitoring point locations, and extensive data and information on the operation of drainage networks are acquired. However, for UDNs with complex topology and a large number of pipelines and nodes, it is difficult to directly determine the monitoring point locations based on human experience. For example, the collected data at the discharge outlet and the downstream area can reflect the summary information of the system and facilitate useful observational utility [56; 67]. However, the merging of different branch pipelines blurs the details of the spatial information [57]. Thus, the layout of monitoring points should be more scientific and reasonable.