Carbon Footprint Quantification and Reduction Potential of Ecological Revetment in Water Net Region of China: Case Study in Yancheng, Jiangsu Province

Abstract

With emphasis on constructing low-carbon cities, the renovation of the riverbank highlights energy conservation and carbon reduction. However, methods and standards for quantifying carbon emissions during ecological river channel construction are currently lacking. There is a scientific gap in research into carbon footprint assessment and reduction potential in ecological revetment technologies in water networks of China. This study attempts to clarify the carbon emission factors of different ecological revetment technologies and explore the carbon reduction potential during the construction stage of ecological rivers from the river revetment design, construction process and materials. The results show that in the carbon emission factors of six ecological revetment technologies, building materials have the largest adjusting potential for carbon reduction. The concrete material is responsible for 55.37–95.86% of carbon emissions in six ecological river technologies, with an average proportion of 69.96%. Accordingly, the concrete material emerges as the primary contributor to carbon emissions in ecological river engineering, followed by gasoline truck transportation and earthwork excavation. Moreover, the carbon emissions from ecological frame structures were the largest, followed by those of block structures, gabion structures, planted concrete and interlocking blocks and the wooden stake structure has the smallest carbon footprint. The choice of ecological revetment technologies is not only related to the realisation of regional water conservancy functions, but it also affects the carbon emissions of water conservancy projects. Engineers and decision-makers should pay great attention to the optimal design of the project, selection of low-carbon materials, energy saving and emission reduction in the construction process. This research not only provides guidance for design units in selecting appropriate river revetment technologies but also offers a theoretical foundation and data support for construction units to optimise their construction process management.

1. Introduction

The advancement of the global economy, the promotion of urban development and the significant improvement in residents’ living standards worldwide have led to an intensified awareness among people regarding environmental protection. The concept of living in harmony with nature has gained increasing popularity in recent years. In 2015, China’s Ministry of Housing and Urban-Rural Development introduced the concept of ‘urban repair and ecological restoration’ for the first time. The essence of this concept lies in honouring the interconnectedness between humanity and nature while adhering to the path of achieving harmonious and sustainable development of both the social economy and ecological environment [1]. Moreover, the ‘National New Urbanization Plan (2021–2035)’ of China emphasises that the active promotion of green and low-carbon lifestyles, as well as the implementation of sustainable urban construction planning and operational models, are crucial means to achieve the objectives of creating a beautiful China and attaining carbon neutrality. The plan further highlights that the development of green cities should be manifested through ecological restoration, environmental protection and the promotion of low-carbon production and living practises [2].

Urban development and watershed occupation usually modify the natural landscape, introducing river control works to ensure safe territory conditions, to guarantee the use of productive land and allow mobility [3]. River restoration is a complex process [4,5] in which the large modifications undergone by the riverine areas and its watershed, mainly caused by urbanisation itself [6], modify the water cycle and limit the space occupied by rivers. In urban settings, with all the difficulties taken into consideration, the main focus can include the restoration of lateral connectivity with the riverbanks, an increase in the river’s degrees of freedom, restoration of the natural flow, rebalancing of the geomorphological dynamics, reduction in water pollution, decontamination of soil and reactivation of areas along the river. Even if a river cannot be restored to its original situation, good opportunities will arise to improve the ecological functioning of the urban fluvial network [7].

With the continuous increase in global temperatures, climate change has emerged as a pressing issue of international concern [8,9]. The expectations of residents regarding rivers have expanded beyond rivers’ fundamental functions such as electricity generation, flood control and drainage, water supply, irrigation and transportation [10,11,12]. Instead, there is a growing demand for the harmonious coexistence between humans and water bodies, necessitating enhanced ecological and landscape functionalities from rivers. Rising global temperatures are closely linked to greenhouse gas emissions. The intensification of human activities resulting from urbanisation not only disrupts the carbon cycle equilibrium within river systems [13,14,15], but also amplifies the flux of carbon emissions from these systems [16,17,18,19], thereby exerting a detrimental impact on global efforts to reduce carbon emissions. Although the influencing factors of carbon emissions have been categorised as water environment, hydrological characteristics, meteorology and seasonality in relation to the impact of urbanisation on river systems [20], limitations in terms of enhancing these factors exist.

The construction of rivers, as a crucial component of urban development, not only plays a vital role in the ecosystem but also serves as an imperative means to implement low-carbon environmental protection. With emphasis on constructing low-carbon cities, the renovation of the riverbank highlights energy conservation and emission reduction. However, most studies have stayed at the periphery of carbon reduction and emission reduction in river channels and have not yet involved the carbon reduction potential of ecological river channels. For instance, the riparian zone, which serves as the transitional interface between terrestrial and aquatic environments, plays a pivotal role in facilitating the circulation and exchange of water and land materials and energy. It serves vital ecological functions, including slope protection, water and soil conservation, pollutant interception, water purification, water retention and habitat creation [21,22,23]. To enhance these functions, Li et al. (2018) developed a new generation of planting concrete made from recycled aggregates of demolished concrete to improve plant growth on the bank slope [24]. He et al. (2020) recommends that strategies should focus on design, maintenance and enhancement technologies to improve the sustained non-point source pollution control of riparian zones [25]. Zhu et al. (2021) hydrothermally synthesised a novel revetment material from sediment enriched with biochar made from waste biomass to improve the adsorption performance and enhance the strength of materials, which could be utilised for river ecological restoration [26]. It indicated that in the process of developing new materials and methods, low-carbon factors, or low-carbon measures, are involved, but this alone cannot achieve carbon reduction.

The urban river can be regarded as an ecosystem model. When the process factors of this system are unchangeable, we can initiate a study on carbon reduction strategies starting from the input aspect of the system. Thus, it is necessary to study the carbon emission during the construction period of river projects. However, most existing studies on low-carbon measures focus on general building works, such as prefabricated concrete buildings [27], old industrial buildings [28] and green building [29]. For water conservancy projects, compared with the high energy consumption of thermal power, hydropower is a clean energy, which can effectively reduce the consumption of fossil energy and is an important path toward achieving the goal of “double carbon” [30]. Carbon reduction potential in water-power engineering is popular [31], while carbon reduction studies on river channel projects, especially riverbank slope protection works, are few. Existing articles only study one or two riverbank forms [32,33]. The design of riparian areas plays a crucial role in ensuring their landscape function and determining greenhouse gas emissions during the construction period [33]. Moreover, the implementation of ecologically and low-carbon river revetment forms can effectively enhance the sustainability of the river ecosystem. New types of bank slope carbon emissions, especially the comparison between different forms of carbon emissions, need to be further studied, which can be helpful for selecting low-carbon forms. Currently, no standardised approach or methodology for quantifying the carbon emission associated with river revetment construction is available.

The Yangtze River Delta metropolitan area is one of the most developed areas in China, and this area is a typical water network area because of the developed water system. This study focuses on the commonly and newly used ecological revetment technologies in water network areas. Based on the life cycle assessment method, the life cycle process of river revetment engineering was divided into five units, including material production, material transportation, construction, maintenance and management, demolishment and disposal or recovery [32]. Through comparing the total carbon emissions in the construction stage and the maintenance stage, the carbon emissions in the construction stage of water conservancy projects account for 92.3% of the carbon emissions in the whole life cycle [31]. In drawing upon life cycle theory, the objectives of this study are to (1) clarify the carbon footprint factors associated with river revetment technologies, (2) establish a calculation and evaluation model for the carbon footprint during the revetment construction process to enable the effective management of carbon emissions and (3) explore the carbon reduction pathways of ecological revetment technologies from the river revetment design, construction process and materials. This study could offer theoretical support and a decision-making framework for establishing carbon emission reduction targets and assessing the potential of energy saving and emission reduction in ecological river course construction. In addition, it can provide practical guidance and serves as a reference for evaluating low-carbon ecological river course construction.

2. Materials and Methods

2.1. Research Area

The plain water network area in China is characterised by abundant rivers and lakes, resulting in slow flow rates that are often unpredictable due to both rainfall runoff and human intervention through water conservancy projects. Consequently, the regular dredging of natural channels is required every three to five years due to long-term soil erosion. Against this backdrop, engineers have been increasingly focusing on ecological river revetment to protect bank.

Bufeng Town is situated in the coastal reclamation area of the lower reaches of the Yangtze River within Yancheng, Jiangsu Province, China (Figure 1). The region boasts a well-developed river system with an intricate network of waterways. It falls under the category of a coastal facies accumulation plain area characterised by a flat topography. Tongyu River, Doulonggang River, Xichao River and numerous other river channels traverse through this town (Figure 1). Approximately 5% of the area is covered by water surfaces, making it a typical water network zone in China. The average elevation of natural ground predominantly ranges between 2.0 and 4.0 m while exhibiting a ground slope of less than 1/10,000. This area lies within the transitional zone from the north subtropical to warm temperate zones and experiences a humid monsoon climate conducive to agriculture, forestry, animal husbandry and fishing.

2.2. River Revetment Technologies

Based on the specific construction requirements of the project area and with the current state of the river and geological conditions taken into consideration, six types of river revetment technologies were selected: (1) wooden stake structure, (2) self-embedded block structure, (3) gabion (stone cage) structure, (4) ecological frame structure, (5) planting concrete and (6) industrial-type interlocking block. When the construction technology was determined, the subsequent calculation process was carried out according to the process in Figure 2.

(1)

Wooden stake structure

For the ecological revetment of the wooden stake structure, pine stakes measuring 3 m in length and with a tip diameter of no less than 12 cm are utilised (Figure 3). A semi-circular stake system beam is installed along with a layer of geotextile on the inside of the pile top, while hot-dip galvanised iron wire is employed for fixation. A soil platform with a width of approximately 0.4 m is positioned behind the stake. With the topography and existing greening conditions taken into consideration, the slope and above need to be connected to the existing ground using a gradient not steeper than a 1:2 ratio, ensuring proper arrangement for landscape greening.

(2)

Self-inlaid block structure

For the ecological revetment of the self-inlaid block structure, a 1.2 m wide reinforced concrete foundation is utilised (Figure 4). The bottom plate has an elevation of 0.60 m and a thickness of 30 cm. The wall body is arranged in a staggered position using five prefabricated blocks with a thickness of 20 cm each. An inner filtration layer consisting of geotextile and graded gravel, both measuring 30 cm in width, is incorporated within the blocks. The roof is constructed with reinforced concrete, boasting a thickness of 20 cm and reaching an elevation of 1.40 m at its highest point. A slope not exceeding a ratio of 1:2 connects the wall to the existing ground while also incorporating landscaping elements.

(3)

Stone cage structure

The stone cage structure utilises a prefabricated stone cage net pad and landfill stone structure, with the stone cage box forming a 2 × 0.5 × (0.5~1) m cuboidal structure (double stack) (Figure 5). The foundation consists of a 1.5 m wide reinforced concrete bottom plate, with an elevation of 0.30 m and thickness of 30 cm. The mesh size for the stone cage is 8 × 10 cm, with a mesh wire diameter of 2.5 mm, edge wire diameter of 3.2 mm and binding wire diameter of 2.2 mm. Hot plating using a steel wire made from a 10% aluminium zinc alloy is used for construction purposes. The wall is connected to the existing ground by means of slope not steeper than a 1:2 ratio while landscaping is arranged.

(4)

Ecological frame structure

The ecological frame structure is a prefabricated framework filled with stone and soil (Figure 6). The ecological box is a cuboidal body with dimensions of 2 × 1 × 0.5 m, arranged in a double-step configuration. The foundation consists of a reinforced concrete bottom plate with a width of 1.5 m, an elevation of 0.30 m and a thickness of 30 cm. Along the sides of the frame body, a geotextile filter layer is placed on top of sand for long-term stability and support. The upper part of the frame is filled with ploughing soil with a thickness of 20 cm to facilitate greening activities. The wall connects to the existing ground through a slope no steeper than a 1:2 ratio while incorporating landscaping elements.

(5)

Planted concrete structure

The planted concrete structure is ecologically reinforced with gravel bedding after levelling the river slope (Figure 7). Anti-filter concrete is used to prevent soil scour and leakage, lying at an elevation of 0.00–0.80 m. Planted concrete is laid from an elevation of 0.80–2.00 m using a cast-in-place form for the concrete frame, of which its width is controlled at approximately 3.0 m. The box is filled with tillage soil and seeded with grass seeds. The strength grade of the planted concrete is C15 or higher, with a porosity range of 25–30% and permeability coefficient of 0.1 cm/s. The filtered concrete has a strength grade of C20.

(6)

I-shaped interlocking block structure

The I-shaped interlocking block structure is ecologically reinforced, and gravel bedding is applied after levelling the river slope (Figure 8). Non-woven geotextile, gravel cushion and interlocking chain blocks are laid from bottom to top in an elevation range of 0.00–2.0 m. The concrete blocks used have a prefabricated thickness of 10 cm and a concrete strength grade of C25. Ploughing soil is used to fill the pores of the blocks, which are then seeded with grass at elevations ranging from 1.0 to 2.0 m.

2.3. Carbon Emission Calculation

2.3.1. Life Cycle Theory

The life cycle encompasses a series of consecutive stages within the product system, starting from the extraction and procurement of raw materials to the ultimate disposal of waste [34]. Life cycle assessment (LCA) involves compiling and evaluating the material and energy inputs and outputs of a product system, along with its potential environmental impact throughout its entire life cycle [35]. The fundamental objective of LCA is to quantify and evaluate the resources, energy consumption and environmental burden associated with the complete life cycle of the research subject while proposing corresponding measures for reducing its environmental impact [36].

The life cycle carbon emissions of riverbank protection projects refer to the direct or indirect release of CO₂-equivalent emissions into the external environment resulting from energy and resource consumption throughout various project stages. The entire life cycle process of riverbank protection projects can be divided into four key components: (1) engineering design; (2) the production and transportation of bank protection materials and construction of bank protections; (3) operation, repair and maintenance; and (4) demolition and recycling (Figure 9). To compare and evaluate ecological revetment technologies in terms of low-carbon footprint and environmental friendliness, this study primarily focuses on the ecological river construction phase. In this study, the entire construction period of ecological revetment could be further divided into three main stages: (1) the production stage of revetment materials, (2) the transportation stage of materials and (3) the engineering construction stage.

2.3.2. System Boundary Determination

The process boundary for the construction of ecological revetment needs to be defined so that carbon emission factors can be determined accurately (Figure 8). The calculation of river ecological revetment primarily involves two methods: the longitudinal length calculation method and the transverse area calculation method. When calculating the engineering quantity based on the longitudinal length of the river, comparing carbon footprint quantities under different section forms of the entire river project is convenient. However, a drawback is that this method fails to consider specific design aspects of the river section. On the contrary, while utilising area calculation in the transverse direction can quantify carbon footprints for different section designs themselves, this approach becomes difficult when determining carbon footprints for the entire river project. In this study, different river sections could not be compared effectively by considering the area in the transverse direction as a fundamental unit because of variations in cross sections among various bank slope protection technologies. Therefore, given that size requirements are met for each river section design, the longitudinal length along with its associated calculations serves as an appropriate unit for quantifying carbon footprints.

Control variables need to be established to compare the carbon emissions of different types of river ecological revetment technology. This study focuses on a typical section where an ecological river project is being constructed. The slope ratio of this section is 1:2, with a slope length of 15.6 m, a slope height of 6.98 m, a slope width of 13.95 m, a bottom width of 5 m and a design water depth of 2 m. The roughness coefficient is set at 0.025, and the channel bottom ratio is 0.002. The flow rate in the channel is 38.18 m³/s with a corresponding flow velocity of 2.12 m/s (Figure 10).

2.3.3. Calculation Process

The carbon emission calculation method in the life cycle construction stage of a river revetment project is formulated as follows, aiming to enhance the practicality of the carbon emission factor pool system within the context of river ecological revetment. This formulation is based on the principles of process boundary division and utilises the carbon emission coefficient method [37]:

$$\mathrm{E}=E_1+E_2+E_3$$

(1)

where E is the total carbon emissions in the construction period of the river ecological revetment project (kg); and E₁, E₂ and E₃ are the carbon emissions of the revetment material production stage, revetment material transportation stage and construction stages, respectively (kg).

(1)

The carbon emission of the revetment material production stage is calculated as follows:

$$E_1=\sum _{i=1}^n{M}_i\times F_{BM,i}$$

(2)

where M_i represents the consumption amount of type i revetment materials (t or m³ according to the attribute of materials), and F_BM,i represents the production carbon emission factor of type i materials. By default, all kinds of materials are commodities or prefabricated products.

(2)

The carbon emission in the transportation stage of materials is calculated as follows:

$$E_2=\sum _{i=1,p=1}^n{F}_{T,p}\times B_i\times L_i$$

(3)

where $F_{T,p}$ represents the carbon emission factor of material transportation when type p transportation mode (i.e., highway transportation by gasoline truck) is adopted; B_i represents the total weight of type i materials (t); and L_i represents the distance of type i materials from the origin to the construction site (km).

(3)

The carbon emission in the construction stage of the ecological revetment project is calculated as follows:

$$E_3=\sum _{j=1}^n{F}_{MC,j}{ \times X}_j{ \times N}_j$$

(4)

where F_MC,j represents the carbon emission factor of type j construction machinery and equipment; X_j represents the total workload of type j machinery and equipment (work shift); and N_j represents the number of type j machinery and equipment.

In this study, the design drawings for river revetment were thoroughly examined, and a comprehensive bill of quantities was obtained. Integrating the technical table enabled the bill of quantities to be transformed into an actual project consumption list that can be directly utilised in carbon emission calculations. With the use of the carbon emission coefficient method, the calculation formula for carbon emissions at each stage incorporated the actual project consumption list to determine the carbon emissions during each phase. The total carbon emissions during the prefabricated component stage were also computed.

2.4. Carbon Emission Factors

From the carbon footprint calculation formula, it can be concluded that the amount of carbon footprints is mainly determined by the engineering quantity and carbon emission factor. On the basis of the accuracy of screening carbon emission factors, our focus is primarily on six specific carbon footprint emission factors in relation to (1) earth and rock engineering, (2) building material, (3) transportation engineering, (4) construction machinery and equipment, (5) substrate engineering and (6) labour. Through analysing the material and process characteristics associated with various ecological revetment technologies, these carbon emission factors were successfully screened and determined (

Table 1. List of carbon emission factors.

Carbon Emission Source	Carbon Emission Factor	Carbon Emission Factor Value	Carbon Emission Factor Unit	Reference
A Earth and rock engineering	A1 Levelling land (m3)	0.023	kg·m−3	[38]
	A2 Earthwork excavation (m3)	1.086	kg·m−3	[38]
	A3 Earthwork backfill (m3)	0.128	kg·m−3	[38]
B Building material	B1 Precast concrete block (m3)	146	kg·m−3	[39]
	B2 C25 Concrete (m3)	250.54	kg·m−3	[39]
	B3 Geotextile (m2)	0.16	kg·m−2	[40]
	B4 Steel (t)	1789.06	kg·t−1	[41]
	B5 Wood (m3)	10.45	kg·m−3	[39]
	B6 Polypropylene B (kg)	0.6114	kg·kg−1	[39]
	B7 Polyester (kg)	0.51	kg·kg−1	[42]
	B8 Polyethylene (kg)	2.81	kg·kg−1	[43]
	B9 Mortar block stone (m3)	114.412	kg·m−3	[40]
	B10 Dry block stone (m3)	12.556	kg·m−3	[40]
	B11 M10 mortar (m3)	315.39	kg·m−3	[40]
	B12 Block stone (m3)	6.05	kg·m−3	[38]
	B13 Bitumen (t)	586.52	kg·t−1	[44]
	B14 Sand (t)	2.51	kg·t−1	[40]
C Transportation engineering	C1 Gasoline truck transport (100 t·km)	14.21	kg/100 t·km	[45,46]
D Construction machinery and equipment	D1 Mixer (one-shift)	10.3	kg/Stage crew	[37]
	D2 Grout machine (one-shift)	16.72	kg/Stage crew	[37]
E Substrate engineering	E1 Fine gravel cushion (m3)	8.76	kg·m−3	[37]
	E2 Grit cushion (m3)	2.51	kg·m−3	[37]
F Labour	F1 Workforce	0.3	kg·d−1	[47]

). All the carbon footprint emission factors fall under the category of traceable sources [31].

In these factors, the difference in magnitude is in the millions, such as between A1 levelling land with 0.023 to B4 steel with 1789.06. The top five factors with the highest value are B4 steel, B13 bitumen, B11 M10 mortar, B2 C25 concrete and B9 mortar block stone, in order. They all come from building material. The values of A1 levelling land, A3 earthwork backfill, B3 geotextile, B6 Polypropylene, B7 Polyester and F1 workforce are less than 1 (Table 1).

2.5. Data Analysis

The data were recorded and processed using Microsoft Excel 2019, and relevant charts were created. The data were compared and analysed using SPSS25.0 statistical software. AutoCAD2016 and Microsoft PowerPoint 2019 were utilised for drawing.

3. Results

3.1. Engineering Quantity Analysis

A comparison shows that the earthwork amount for land levelling remains consistent due to the assumption of an identical river channel section (

Table 2. Engineering quantities of different bank slope protection technologies.

Carbon Emission Source	Bill of Quantities	Engineering Quantities
		Wooden Pile Construction	Block Masonry Structure	Stone Cage Structure	Ecological Frame Structure	Porous Concrete for Plant-Growing	Interlocking Segment Structure
A Earth and rock engineering	A1 Levelling land (m3)	300.00	300.00	300.00	300.00	300.00	300.00
	A2 Earthwork excavation (m3)	56.52	703.42	1251.00	893.99	286.20	241.60
	A3 Earthwork backfill (m3)	12.50	344.00	539.00	403.25	47.00	4.70
B Building material	B1 Precast concrete block (m3)	\	50.62	\	105.00	\	78.10
	B2 C25 Concrete (m3)	\	165.60	114.00	140.00	163.20	76.00
	B3 Geotextile (m2)	100.00	200.00	300.00	644.00	\	828.00
	B4 Steel (t)	0.10	15.79	11.09	12.96	\	\
	B5 Wood (m3)	56.52	\	\	\	\	\
	B6 Polypropylene B (kg)	\	\	\	\	\	\
	B7 Polyester (kg)	\	\	\	\	\	\
	B8 Polyethylene (kg)	\	\	\	\	\	\
	B9 Mortar block stone (m3)	\	\	\	\	\	\
	B10 Dry block stone (m3)	\	\	\	\	\	\
	B11 M10 mortar (m3)	\	\	\	\	\	\
	B12 Block stone (m3)	\	\	150.00	144.00	\	\
	B13 Bitumen (m3)	\	\	\	\	\	\
	B14 Sand (t)	\	\	\	\	\	\
C Transportation engineering	C1 Gasoline truck transport (100 t·km)	4.53	53.48	50.98	96.98	50.57	49.43
D Construction machinery and equipment	D1 Mixer (one-shift)	\	2.00	2.00	2.00	2.00	1.00
	D2 Grout machine (one-shift)	\	2.00	2.00	2.00	2.00	1.00
E Substrate engineering	E1 Fine gravel cushion (m3)	\	59.20	\	101.74	76.00	82.80
	E2 Grit cushion (m3)	\	\	\	\	\	\
F Labour	F1 Workforce	10.00	10.00	10.00	10.00	5.00	10.00

). Stake protection requires the least earthwork, totalling only 56.52 m³, while gabion structure protection demands the largest amount of earthwork at 1251.00 m³. All protective measures involve backfilling with quantities ranging from 4.70 m³ for interlock block protection technology to 539.00 m³ for stone cage protection technology.

Among the building materials, only the block structure, ecological frame structure and interlocking block structure incorporate precast concrete blocks, with concrete consumptions of 50.62, 105.00 and 78.10 m³, respectively. Apart from the pile structure, all other revetment protection technologies utilise cast-in-place concrete materials. The block structure requires a total of 165.60 m³ of concrete material, while planted concrete amounts to 163.20 m³. The interlocking block structure has the lowest consumption at 76.00 m³. In addition to the implemented concrete protection technology, all other slope protection technologies utilise geogrids. Among these, the interlocking block method has the largest coverage area at 828.00 m², while the wooden stake structure has the smallest coverage area at 100 m². Steel in ecological slope protection materials primarily serves as a fixed connection for precast blocks and reinforces the foundation. The quantities of block structures, stone cage structures and ecological frame structures are equivalent, amounting to 15.79, 11.09 and 12.96 t, respectively. For stone cage structures and ecological frame structures, block stones are used to fill their internal structures with quantities of 150.00 and 144.00 m³, respectively.

Each protection technology utilises automobile transportation, with a primary focus on ensuring the quality of transportation. Therefore, for the purpose of this study, the transportation distance is assumed to remain constant at 10 km. Among these technologies, the wood structure has the smallest transportation volume, which amounts to 4.53 (100 t·km), while the ecological frame structure has the largest transportation volume at 96.98 (100 t·km). The other protection technologies have a similar transportation volume of approximately 50 (100 t·km). The mixer and grouting machine are synchronised with cast-in-place concrete, while stake structures are excluded from the use of other protective technologies. Apart from wooden stakes and stone cage structures, fine gravel cushion is employed in all other protection technologies. Notably, the ecological frame structure requires the highest amount of fine gravel cushion at 101.74 t, whereas the block structure necessitates only 59.20 t. This indicates that the engineering quantity varies greatly for different protection technologies, which implies a large variation in their carbon footprints.

3.2. Carbon Footprint Analysis

Under the context of carbon dioxide emissions and carbon neutrality, the quantity of carbon footprint to some extent reflects the ecological characteristics of different ecological river channel protection technologies. The magnitude of carbon footprints varies greatly due to variations in emission coefficients among different factors (

Table 3. Carbon footprint of different bank slope protection technologies.

Carbon Emission Source	Carbon Emission Factor	Carbon Footprint (kg)
		Wooden Pile Construction	Block Masonry Structure	Stone Cage Structure	Ecological Frame Structure	Porous Concrete for Plant-Growing	Interlocking Segment Structure
A Earth and rock engineering	A1 Levelling land	6.90	6.90	6.90	6.90	6.90	6.90
	A2 Earthwork excavation	61.38	763.91	1358.59	970.87	310.81	262.38
	A3 Earthwork backfill	1.60	44.03	68.99	51.62	6.02	0.60
B Building material	B1 Precast concrete block	\	7390.52	\	15,330.00	\	11,402.09
	B2 C25 concrete	\	41,489.42	28,561.56	35,075.60	40,888.13	19,041.04
	B3 Geotextile	16.00	32.00	48.00	103.04	\	132.48
	B4 Steel	178.91	28,252.84	19,845.15	23,186.22	\	\
	B5 Wood	590.63	\	\	\	\	\
	B6 Polypropylene B	\	\	\	\	\	\
	B7 Polyester	\	\	\	\	\	\
	B8 Polyethylene	\	\	\	\	\	\
	B9 Mortar block stone	\	\	\	\	\	\
	B10 Dry block stone	\	\	\	\	\	\
	B11 M10 mortar	\	\	\	\	\	\
	B12 Block stone	\	\	907.50	871.20	\	\
	B13 Bitumen	\	\	\	\	\	\
	B14 Sand	\	\	\	\	\	\
C Transportation engineering	C1 Gasoline truck transport	64.44	759.92	724.40	1378.03	718.57	702.37
D Construction machinery and equipment	D1 Mixer	\	20.60	20.60	20.60	20.60	10.30
	D2 Grout machine	\	33.44	33.44	33.44	33.44	16.72
E Substrate engineering	E1 Fine gravel cushion	\	518.59	\	891.24	665.76	725.33
	E2 Grit cushion	\	\	\	\	\	\
F Labour	F1 Workforce	7.30	7.30	7.30	7.30	3.65	7.30
	Total (kg)	927.16	79,319.48	51,582.43	77,926.06	42,653.88	32,307.51

). The amount of carbon footprint caused by land levelling is less than 10 kg CO₂; artificial activities also result in a carbon footprint less than 10 kg CO₂; the amount of carbon footprint generated by mixers and grouting machines fluctuates within the range of 10–40 kg CO₂; and earthwork backfilling contributes to a carbon footprint ranging from 10 to 100 kg CO₂. The carbon footprint attributed to geotextiles ranges from 10 to 100 kg CO₂. The carbon footprint associated with earthwork excavation varies between 100 and 1000 kg CO₂, while the carbon footprint resulting from fine gravel bedding fluctuates within the range of 500–900 kg CO₂. Lumps contribute around 1000 kg CO₂ to the carbon footprint, and vehicle transportation accounts for approximately 1000 kg CO₂. Concrete exhibits a significant increase in carbon footprint, with precast concrete blocks reaching about 10,000 kg CO₂ and cast-in-place concrete (C25) ranging from 20,000 to 40,000 kg CO₂. Similarly, steel contributes approximately 20,000 kg CO₂ to the overall carbon footprint. In terms of total carbon footprint, the carbon emissions from both block structure and ecological frame structure exceed 75,000 kg CO₂, with the former reaching nearly 80,000 kg CO₂. Following closely is the gabion structure (51,582 kg CO₂), followed by planted concrete (42,654 kg CO₂) and interlocking block (32,307 kg CO₂). The wooden stake structure has the lowest carbon footprint at only 927 kg CO₂ (Table 3).

An analysis of the carbon footprint distribution for each slope protection technology shows that, with the exception of wooden stake structures, other technologies exhibit a significant reliance on concrete materials. Notably, planted concrete structures (including cast-in-place and precast concrete) contribute 95.86% of the carbon footprint, while interlocking block concrete materials account for 94.23% (Figure 11). The carbon footprint of concrete materials in ecological frame structures accounts for 64.68%, while that of concrete materials in block structures accounts for 61.62%, and that of concrete materials in stone cage structures accounts for 55.37%. In terms of steel, the carbon footprint is as follows: 38.47% for steel used in stone cage structures, 35.62% for steel used in block structures and 29.75% for steel used in ecological frame structures. In addition, the carbon footprint of stake structure steel is 19.30%, while other constructions do not utilise any steel materials.

4. Discussion

4.1. Source of Carbon Emission Factor

In the process of ecological river construction, it is inevitable to encounter various carbon emission processes. Greenhouse gases are emitted during fossil fuel combustion, electricity consumption, chemical reactions and organic carbon combustion in the production and processing of building materials. During the transportation stage of materials, vehicles used for transportation consume gasoline, diesel and other fuels, which generate greenhouse gas emissions. Greenhouse gas emissions also arise from workers’ activities and living conditions as well as electricity and fuel usage while operating machinery and equipment. When we explore the carbon reduction potential of ecological channels, we must distinguish which factors are available and adjustable.

For the engineering quantity in the river construction project, part of it is determined by the location and natural conditions of the project. For instance, excavation and backfilling are determined by the section of the river channel. In this case, this type of engineering has very little potential for carbon reduction. Similar factors also include truck transport, mixer, grout machine and labour. These engineering quantities are difficult to adjust with the current engineering construction technology. Therefore, the carbon footprints of these factors are in the similar order of magnitude (Table 3).

In excluding these above factors, the remaining factors are mainly building materials. The selection of ecological revetment technologies would determine the building materials. In this studied case, there is a large difference between the engineering quantity and material type of six different types of ecological revetment technologies, as well as their carbon emissions. Therefore, in China, the choice of river section bank slope form in the water conservancy planning of cities and counties is not only related to the realisation of regional water conservancy functions, but it also affects the carbon emissions of water conservancy projects in the next few years. It should receive great attention from planners, designers and government decision-makers.

4.2. Design Process

From the current process of water conservancy projects, the design and construction process are the two most important links that determine the selection and function of materials.

In ensuring the functionality and safety of engineering, design optimisation should be greatly emphasised. Currently, the process of ecological river engineering design, ranging from bank slope selection to material parameters and safety factor determination, tends to be conservative. This is appropriate from the perspective of operational safety and functional realisation in engineering. However, with advancements in science and technology as well as economic and social progress, the original design concept needs to be refined and enhanced further. On the basis of estimations, achieving a 10% reduction in building material consumption can lead to an approximately 4% reduction in overall carbon emissions—a significant impact on emission reduction [31]. Therefore, designers should prioritise energy conservation and emission reduction awareness.

Moreover, except for reducing carbon sources, increasing carbon sinks has become another important path to realising the goal of “double carbon” in water conservancy projects [30]. Due to the large randomness in slope plant species and quantity, the carbon footprint of plants in ecological revetment technologies is ignored in this study. While for the carbon sink calculation, plants have a significant role, as they are the main carbon sinks in the ecological revetment technologies. During the long-term operation of ecological river channels, on the one hand, the development of plant roots enhances the toughness of bank slopes [48]; on the other hand, plants absorb carbon dioxide to achieve the purpose of carbon fixation [49]. Therefore, during the designing stage, engineers should focus on increasing the optimal allocation of plants and increasing the carbon sequestration capacity of ecological revetment technologies.

4.3. Construction Process

Construction process control should be enhanced. Despite the exclusion of carbon emissions from materials, energy consumption and other factors still have a high contribution to total emissions. Process analysis and adjustment should be implemented for riverbank slope engineering to reduce high-carbon emission processes, eliminate unnecessary steps and optimise energy consumption by adjusting its structure and increasing the use of green energy while minimising fossil fuel usage. These measures should be integrated into an intelligent control system that can sense and regulate carbon emissions during the project’s life cycle.

The ready-mixed concrete sector holds a significant market share within the concrete industry [50]. To systematically explore the pathway toward low-carbon concrete, Jin et al. (2023) uses ready-mixed concrete as an example. Enterprise A’s ready-mixed concrete product was selected as a representative object, and its carbon footprint composition was analysed across various stages, including raw material and energy acquisition, transportation, product production, transportation and pumping [51]. The calculated carbon footprint of 1 m³ of ready-mixed concrete products amounts to 262 kg CO₂eq. Notably, the carbon emissions from cement used in the production of these products account for approximately 76% (200 kg CO₂eq) of their overall carbon footprint. In addition, the transportation of raw materials and finished products contributes to carbon emissions totalling 24.2 kg CO₂eq due to energy consumption, representing around 9% of the total carbon footprint. The loss rate of building materials during the physical and chemical stages of prefabricated components is estimated to be between 1.5% and 2%, while that for in cast-in-place projects is at least 2% [52]. The utilisation of prefabrication methods can effectively reduce the loss of building materials by more than 0.5%. Therefore, enhancing the level of prefabrication in construction can possibly reduce the overall quantity of building materials required. An improved level of prefabrication leads to a decrease in construction machinery usage and an acceleration in project schedules, resulting in a reduced carbon footprint for prefabricated buildings. In addition, the selection of cast-in-place concrete or precast concrete is often determined by the construction level of the construction unit. Therefore, improving the construction technology level of construction units has also become an important means of carbon reduction.

4.4. Materials

The renewal and development of building materials is also an important pathway to reduce carbon emissions. The research and development of green production technology need to be enhanced, and the utilisation of eco-friendly building materials must be promoted. Currently, conventional materials that are commonly used in riverbank slope engineering, such as reinforced steel bars, cement and sand, have high carbon emissions per unit. The material system poses both a challenge and an opportunity for the project’s “double carbon” strategy. This study reveals that traditional materials (primarily concrete and steel) contribute at least 55% of the total emissions. The primary source of carbon emissions from these materials lies not within the construction site but rather during their production process. Therefore, construction technicians can enhance the circulation of building materials in future [30]. For instance, they can employ construction waste recycling technology in bank slope engineering to mitigate carbon emissions from banks [28]. Production technicians need to innovate and optimise the production process of traditional building materials while strengthening emission reduction efforts in raw material production [53]. In addition, scientific and technological personnel should expedite research and development on green building materials, actively exploring new methods and materials as substitutes for conventional building materials to expand avenues for emission reduction [29].

4.5. Implications

Water conservancy projects, being a crucial component of the national economy, contribute significantly to carbon emissions [54,55]. Traditional revetment measures primarily rely on slurry masonry and concrete materials, which not only incur high costs but also lack aesthetic appeal [56,57]. The ecological river revetment approach in general emphasises the preservation of biodiversity and erosion resistance, improves the quality of life of the inhabitants and reduces hydraulic risk within river ecosystems while ensuring the maintenance of fundamental functions of river courses. Ecological river revetment can fully harness the significant roles of riparian zones in maintaining ecology, thus preserving water resources and soil quality, and considers comprehensive safety measures, stability enhancement and landscape aesthetics. Given the technological characteristics and practical management context of ecological river revetment, ecological revetment technologies can effectively reduce resource consumption and minimise carbon emissions during the design and construction processes. In the future, establishing and enhancing a greenhouse gas control mechanism for river revetment construction and achieving effective coordination between pollution reduction and carbon mitigation will become focal points for researchers.

5. Conclusions

In this study, the carbon emission coefficient method was used to calculate the carbon footprints of six ecological revetment technologies; a calculation and evaluation model for carbon footprint during the revetment construction process was established; and then, the carbon reduction potential was analysed from the river revetment design, construction process and materials. The following was concluded:

(1)

Carbon emission factors for ecological revetment technologies could be divided into six sources: earth and rock engineering, building material, transportation engineering, construction machinery and equipment, substrate engineering and labour. In the carbon emission factors of six ecological revetment technologies, building materials have the largest adjusting potential for carbon reduction.

(2)

The concrete material emerges as the primary contributor to carbon emissions in ecological river engineering, followed by gasoline truck transportation and earthwork excavation. The concrete material is responsible for 55.37–95.86% of carbon emissions in various ecological river protection technologies, with an average proportion of 69.96%.

(3)

In terms of total carbon footprint, the carbon emissions from ecological frame structure were the largest, followed by those of block structures, gabion structures, planted concrete and interlocking blocks. The wooden stake structure has the smallest carbon footprint. Engineers should focus on increasing the optimal allocation of plants to increase the carbon sequestration capacity of ecological revetment technologies.

(4)

The choice of ecological revetment technologies is not only related to the realisation of regional water conservancy ecological functions, but it also affects the carbon emissions of water conservancy projects. Engineers and decision-makers should pay great attention to the optimal design of the project, selection of low-carbon materials, and energy saving and emission reduction during construction processes in the future.