Proper Usage of Urban Waterlogging Early Warning Under Complex Operating Conditions

01 Introduction: Refined Operation and Maintenance

In recent years, short-duration heavy rainfall, outer rainbands of typhoons, and localized severe convective weather have occurred frequently. Urban low-lying sections, underpass interchanges, tunnel entrances, and entrances/exits of underground spaces are more prone to sudden water accumulation. For urban flood control management, waterlogging risk depends not only on rainfall but is also closely related to drainage capacity, surface runoff, pipe network backwater, river water levels, and on-site traffic organization. Once water accumulation exceeds the safe boundary for road traffic, it can quickly escalate into public safety issues such as trapped vehicles, endangered personnel, and traffic disruptions. Therefore, urban waterlogging warning is shifting from "post-rain patrols" to "in-rain monitoring, early warning, and rapid response."

In actual construction, radar water level meters, radar ranging sensors, electronic staff gauges, video surveillance, and rain gauges are widely used in water accumulation monitoring scenarios. Among these, radar devices have become a crucial perception means for urban water accumulation warning systems due to their non-contact measurement, minimal impact from water quality, ability to operate at night, and suitability for long-term outdoor deployment. However, equipment selection, installation location, debugging parameters, power and communication supply, and subsequent maintenance directly affect warning quality. Improper installation may lead to data jumps; unreasonable threshold settings may cause missed or false alarms; and inadequate maintenance may render the warning system ineffective during critical rainfall events.

02 System Composition and Selection Points

A complete urban water accumulation warning system typically includes the front-end perception layer, data transmission layer, platform application layer, and linkage disposal layer. The core of water accumulation warning is not merely "seeing the water," but translating water level changes into actionable warning responses.

Sensor Selection Strategy

• 80GHz FMCW Radar: Recommended for urban underpass interchanges, tunnel entrances, and low-lying intersections. Typical measuring range covers 0.3m to 15m with accuracy of ±3mm to ±10mm and a narrow beam angle (approx. 3 degrees), suitable for environments with numerous obstructions.
• 26GHz Radar: Suitable for more open areas but generally less effective than 80GHz in complex scenarios with bridges, culverts, railings, and green belts where anti-interference is key.
• Protection Rating: IP67 is standard; IP68 is advisable for long-term outdoor points. Operating temperature should cover -20°C to 70°C.

Power and Communication

Municipal power is stable for pump stations and existing pole resources. Solar panels (20W-60W) combined with lithium batteries (LiFePO4 or gel) are ideal for intersections without grid access, designed for 5-7 days of continuous rainy weather endurance. Communication methods include 4G, NB-IoT, LoRa, fiber optics, or wired Ethernet. For key points, local storage caching for over 7 days is recommended to handle communication interruptions.

Threshold Configuration

Thresholds are management parameters, not just equipment settings. Common four-level thresholds for ordinary roads are 50mm, 100mm, 200mm, and 300mm. For high-risk areas like underpasses, stricter grading (e.g., 150mm, 250mm, 350mm, 450mm) is adopted. It is critical to link these thresholds with traffic control, pump station operations, and emergency dispatch to form a closed loop.

03 Pre-installation Survey and Point Layout

The on-site survey determines the quality of monitoring points. Key considerations include historical water accumulation data, deepest water locations, drainage outlet positions, and traffic impact.

Point Layout Principles:

• Representativeness: Deploy sensors where water first reaches dangerous depths, avoiding high/edge locations chosen solely for convenience.
• Easy Maintenance: Ensure safe access for personnel, avoiding fast-traffic zones without safety islands.
• Convenient Linkage: Align points with on-site warning screens, traffic signals, and video surveillance for rapid response.

For typical underpasses, set up three types of points: bridge/culvert lowest point (radar sensor), entrance approach road (trend identification), and pump station forebay (linkage). If the length exceeds 80 meters, add points at both ends and the middle.

04 On-site Installation Steps

1. Foundation and Pole Erection: Use C25 concrete foundations (400mm×400mm×600mm). Anchor bolts (M16/M20) calculated based on wind load. Verticality deviation < 3/1000. Check load-bearing capacity if mounting on existing structures.
2. Radar Sensor Installation: Aim vertically/near-vertically. Bottom of antenna should be ≥0.5m from max water surface and 2.5m–4.0m from ground/deck. Avoid facing drainage outlets, vortex areas, or dense railings directly (offset ≥0.5m).
3. Beam Path Inspection: Ensure no continuous obstruction. With a 4m height and 3-degree angle, beam radius is ~0.1m–0.2m. Keep ≥0.5m clearance around the beam center. Verify strongest echo comes from the water surface, not obstacles.
4. Power and Cable Laying: Use galvanized/PVC/PE pipes. Outdoor joints must be IP67/IP68 waterproof. Label cables clearly. Solar panel tilt angle based on latitude; increase for self-cleaning in dusty areas.
5. Lightning Protection and Grounding: Install surge protectors. Ground resistance ≤10 ohms (≤4 ohms for key points). Use hot-dip galvanized steel or copper-clad rods. Ensure equipotential bonding on metal bridge structures.
6. Identification and Archiving: Attach nameplates with point name, number, contact, and date. Retain photos, coordinates, elevation, and wiring records for future troubleshooting.

05 Debugging and Acceptance Methods

Making water accumulation warning data trustworthy, usable, and linkable involves five critical steps:

• Parameter Configuration: Set reporting frequency (5-10 min normal, 1 min rain, 10-30 sec alarm). Input installation height and datum for accurate depth conversion.
• Static and Dynamic Calibration: Static: Collect 30+ sets of data on still water. Dynamic: Compare with staff gauge/video during controlled discharge. Target error: ±1cm to ±3cm.
• Threshold Setting: Define 4-level warnings (Prompt, Alert, Control, Emergency) based on road type and traffic. Add "duration" logic for underpasses (e.g., >250mm for 5 mins).
• Linkage Testing: Test full-link delay (sensor to action) within 30s–2 mins. Verify SMS/APP pushes, LED screens, traffic signals, and pump station starts.
• Acceptance: Conduct 72-hour trial operation. Metrics: Data integrity ≥99%, Alarm accuracy ≥95%, False alarm rate ≤5%.

06 Routine Maintenance and Pre-flood Inspection

Maintenance cycles must match the rainy season. Establish a mechanism of "monthly routine inspection, pre-flood special inspection, in-rain key patrol, and post-rain review."

Pre-flood inspections should use a checklist covering cleaning, tightening, voltage, signal, thresholds, and grounding. Rectify issues affecting data accuracy within 48 hours and flood-season alarm issues within 24 hours.

07 Common Troubleshooting and Handling

08 Case Reference: Practical Effects

A southern city renovated a waterlogging-prone area with 3 underpasses, 2 riverside roads, and 5 garage entrances. The project deployed 48 radar units, 12 rain gauges, and 26 video channels linked to the municipal platform.

• Graded Warnings: 150mm (District alert), 250mm (LED/Traffic police), 350mm (Pump full-load/Closure prep), 450mm (Barrier gates/Municipal command).
• Results: Duty personnel awareness time reduced from 28 mins to 6 mins. Response arrival time shortened by 40%. False alarm rate dropped from 11.6% to 3.2% due to video/rainfall linkage.
• Lessons Learned: Radars near drainage outlets caused echo disorder; relocating them 1.2m away stabilized data. Solar shading by trees caused battery drain; relocating panels solved charging issues.

09 Concluding Outlook: Digital Twin Integration

Future urban waterlogging warning will integrate rainfall forecasts, pipe network operations, and traffic response into a "Digital Twin." Multi-source data fusion (radar + rain gauge + pipe level + video AI) will enhance reliability. Edge computing will allow front-end filtering and alarming even during network outages.

Management processes must standardize point grading, acceptance, and maintenance. Only by transforming technical parameters into management systems can urban flood control move from passive response to proactive prevention. The value of radar lies not just in installation numbers, but in providing trustworthy data and traceable response closed loops.