In stormwater runoff pollution control and urban flooding prevention projects, the selection of detention and storage ponds is a critical step that determines the technical and economic feasibility of the project.

When comparing the two technical approaches—traditional cast-in-place reinforced concrete structures and modular steel structures—from a professional perspective, there are striking differences in initial investment costs, construction efficiency, and site requirements. These factors need to be carefully evaluated based on project scale, site constraints, and schedule objectives.

This article focuses on the core elements— Initial Investment Cost Structure and Economies of Scale Through quantitative comparisons and scenario analysis, this study provides a basis for the scientific selection of two types of regulating and storage ponds. The analysis will cover cost dimensions such as materials, labor, machinery, construction duration, and earthwork support, and will be supported by data from real-world case studies.

I. Initial Investment Costs: Balancing Scale and Scenarios

Initial investment (including design, materials, and construction) is the primary consideration in project decision-making.

1. Prefabricated regulating reservoir:

Material cost: The unit price of steel structures is typically higher than that of concrete structures.

Construction cost: The construction period is shorter, the procedures are simpler, and the input of labor and machinery is reduced, resulting in a significant decrease in overall construction costs. The volume of earthwork, support requirements, and costs for temporary on-site facilities are also lower than those associated with conventional construction methods.

Transportation cost: The transportation costs of modules need to be factored into the considerations, especially for projects in remote areas.

Figure 1: Schematic Diagram of a Modular Detention Basin

2. Traditional detention basin:

Material cost: The primary materials (rebar and concrete) have relatively low and transparent unit prices.

Construction cost: The on-site construction process is complex, involving substantial costs for formwork, shoring, labor, and large-scale machinery usage. Labor costs account for a high proportion of the total expenses and are trending upward. Deep excavations, complex shoring systems, or specialized geological treatments can significantly increase project costs.

Economies of scale: In large and extra-large projects (with capacities exceeding 10,000 cubic meters), the material cost advantage is amplified, and the construction cost per unit volume (yuan/m³) is lower than that of prefabricated storage ponds.

Figure 2: Schematic Diagram of a Traditional Detention Basin

Cost Comparison: Taking a comprehensive water environment management project in central China as an example, the 4,500-cubic-meter and 9,000-cubic-meter regulating reservoirs constructed under this project are both reinforced concrete structures.

According to the construction drawing budget

The direct costs for the 4,500 m³ regulating reservoir project amount to 14.53 million yuan—of which 6.28 million yuan is allocated to the reservoir structure itself, 3.35 million yuan covers earthworks and excavation support, 280,000 yuan is for anti-floating anchor rods, and 4.62 million yuan is for equipment. If the project were to be redesigned as a prefabricated steel-structure regulating reservoir, the direct costs would drop to 10.82 million yuan, resulting in cost savings of 3.71 million yuan, or 25.5%.

The direct costs for the 9,000 m³ regulating reservoir project amount to 22.56 million yuan (with the pool structure accounting for 8.29 million yuan, earthworks and excavation support totaling 7.46 million yuan, anti-floating anchor rods at 780,000 yuan, and equipment costs at 6.03 million yuan). If the design were changed to a prefabricated steel-structure regulating reservoir, costs could be reduced by 4.63 million yuan, or 20.5%.

Table 1: Comparison of the Composition of Initial Investment Costs for Two Types of Regulation and Storage Ponds

Core difference: In large-scale projects, traditional detention ponds may have an advantage in terms of material costs. However, prefabricated detention ponds, thanks to their high construction efficiency and adaptability, tend to offer cost advantages at the overall project level, especially for medium- and small-scale projects and sites with space constraints. (For example, taking a 3,000 m³ detention pond as an instance, the construction cost of a prefabricated steel structure detention pond can be reduced by more than 30% compared to that of a reinforced concrete detention pond of the same size.)

Prefabricated detention ponds and traditionally cast-in-place detention ponds represent two fundamentally different technological approaches. The core difference stems from the essential distinctions in structural form and construction methods:

Ø Format: Modular (assembly-based, highly flexible) vs. traditional (monolithic, with limited form).

Ø Construction period: Modular (significantly shorter) vs. traditional (longer cycle).

Ø Initial investment: Modular (small- and medium-sized/limited projects have a comprehensive cost advantage) vs. traditional (large-scale projects have an advantage in material costs).

Ø Maintenance costs: Modular (steel structure with smooth inner walls, resistant to scaling, requiring infrequent cleaning, and featuring lower maintenance costs compared to concrete structures) vs. Traditional (concrete structures prone to cracking and leakage, with higher long-term maintenance costs).

II. Engineering Selection Recommendations

1. Scenarios that prioritize modular detention ponds:

Ø Strict schedule requirements (e.g., emergency projects, high-traffic areas).

Ø Small- and medium-sized projects (volume < 10,000 m³).

Ø Areas with limited space, complex underground utility lines, and where construction disturbance needs to be minimized (such as urban redevelopment projects).

Ø Projects with high requirements for long-term operational convenience and security.

Ø The geological conditions are relatively favorable.

2. Prioritize scenarios involving traditional detention and storage ponds:

Ø Ultra-large-scale regulating reservoir project (capacity > 10,000 m³).

Ø Requires special irregular structural designs or extremely high backfill/load requirements.

Ø The geological conditions are complex.

Ø Local labor costs are extremely low, and there is little pressure regarding project timelines.