3. Key analysis of urban drainage and waterlogging control system

The main cause of the Zhengzhou heavy rain incident was apparently blocked by more than twice the safety standard for waterlogging.

However, although the magnitude characteristics are different, excessive rainstorms can occur in any city. How can other cities learn the lesson?

To do this, we need to first understand the functional design of drainage and waterlogging systems. In essence, the drainage and waterlogging system is about the spatial balance and orderly arrangement of urban rainwater retention and discharge. The time and space estimation and risk control of waterlogging amount are important contents to solve the urban flood problem.

From the system point of view, different engineering means to achieve different design objectives. The engineering means include source emission reduction and drainage pipe system, waterlogging control system, and emergency prevention and response system of excessive rainfall.

The figure below shows how each system level is designed to respond to different storm characteristics and has different key functions.

? Drainage pipes (Figure A) - The design standard is usually 5 years or less. Main facilities include: source sponge facilities, surface diversion, pipe network, storage facilities, urban rivers and lakes, etc

? Waterlogging prevention and control system (FIG. B) -- mainly to cope with the designed rainstorm beyond the capacity of the pipeline network (such as once every 5 to 50 years). The facilities mainly include: pipeline network, regulation and storage facilities, urban rivers and lakes, discharge channels, and safe water accumulation areas on the surface.

? Exceedance system (Figure C) - For rainfall events (e.g., greater than once in 50 years), the main facilities include: storage facilities, urban rivers and lakes, exceedance discharge channels, surface inundation areas, pre-planning and emergency response non-engineering mitigation means.

Any drainage system (or subsystem) in which rainfall runoff enters the system and is a balance between drainage capacity and storage capacity of the system. Waterlogging occurs when the drainage capacity is limited and the system storage capacity exceeds the safe storage capacity of the design standard for flood control and drainage (see Figure 11).

Drainage and waterlogging control planning has clear design standards and index requirements for pipe channels, waterlogging prevention and control standards and sponge cities.

In the waterlogging control system, the space organization and safety regulation and storage of surface runoff exceeding the capacity of pipelines and canals in the design standard, as well as the risk assessment and countermeasures of waterlogging outside the standard, are the key points of project planning and management. These surface runoff organization and waterlogging outlet need urban space. In this work, the industry often provides very important flood risk map results for urban planning and drainage and flood prevention through flood risk identification and assessment.

The risk map can provide the basis for the spatial layout of flood control and drainage facilities, identify the inundation objects and evaluate the consequences, so as to arrange reasonable avoidance measures in the project and connect with the urban planning in space.

FIG. 11 Schematic diagram of hourly inflow and outflow flow-storage balance for a typical system

4. Cognition and inspiration

According to the above research, we can get the following inspirations:

The model simulation shows that the main cause of the disaster in the main urban area of Zhengzhou during the July 20 rainstorm is the superposition of the upstream rainstorm flood and the extreme hourly rainstorm in the city center, resulting in the formation of a large area of rapid slope confluence with different water depths.

The rainstorm amount far exceeded the urban watercourse flood control standard and the urban drainage and waterlogging control standard.

Does the city have a flood and waterlogging risk map according to the standard? Does the city have a risk map for excess rainstorm studies and plans? Where will excess flood or waterlogging be flooded in the planning works? How will the water be stored? How to discharge? I'm afraid many cities are still unprepared for these problems;

What about flooding?

The Zhengzhou incident exposed a very common risk management problem.

Watershed flood control projects manage river embankments and pumping stations, etc., but do not pay attention to urban details; Urban drainage and waterlogging prevention pay attention to urban details, but only assume the safety of river channels, and only limited to the standards of canal and waterlogging prevention and control;

Is it a flood? Was it a flood? Or flood?

The city is the source of water, water catchment, surface slope confluence, and expected to be protected objects, neither flood control projects can be protected, and not the usual waterlogging phenomenon, but also far beyond the capacity of drainage and waterlogging.

In fact, the concept is not important, what is important is the judgment, identification and evaluation of risk characteristics, such as rainstorm magnitude, inundation depth, velocity, inundation object, etc.

With global warming, climate uncertainty increases, the limitations of point rainstorm formula, insufficient research on exceedence events and weak flood risk assessment may be no small technical risks in today's urban planning and management.

Research on extreme meteorological conditions, reliable and refined in-standard and out-of-standard flood risk assessment and risk maps provide an important basis for identifying risk sources and important protection objects, as well as for urban spatial planning, major project avoidance and emergency response management. Urban drainage and flood control planning is the only technical way to make up for the limitations of today's technical system and provide these authoritative results;

This study builds a flood model based on a large amount of public information, and obtains professional analysis results that are more in line with the reality, indicating that under the condition of having good basic data, it should be the basic requirement to support the technical means of drainage and waterlogging control planning or waterlogging control through professional model technology.

Further, the integration of meteorological information and flood system simulation technology can provide assistance for rapid and efficient engineering decision-making, early warning and forecasting and emergency response.

With the improvement of the system's application inspection and error correction ability, this kind of early-warning, forecasting and emergency decision-making technology supported by real-time simulation will become more and more mature, which is an inevitable trend.