Evaluating the concrete grade-control structures built by modified fish-nest bricks in the river restoration: A lab-based case study

https://doi.org/10.1016/j.jenvman.2022.115056Get rights and content

Highlights

  • Concrete grade-control structures (CGCSs) were examined in a lab-based study.

  • Two indicators are proposed to evaluate the CGCSs.

  • Elements among the CGCSs laid in staggered manner have a better performance.

Abstract

Concrete grade-control structures (CGCSs) have broad application prospects in the restoration of large rivers. But there is a lack of indicators to evaluate CGCSs at laboratory study. In this study, we proposed two evaluation indicators from the perspective of the impact of CGCSs on geomorphology change and fish habitat, namely the spatial-averaged occurrence probability of sweep events near the bed and flow diversity. To verify the reasonableness of these indicators, flume experiments were conducted with CGCSs built by modified fish-nest bricks in different Reynolds number and layout condition. Data of the flow field around structures in streamwise, transverse and vertical direction was obtained and analyzed. Results of mean flow field show that large recirculation zones are found in the cavity and behind the element. The mechanism of suspended sediment deposition around CGCSs in the flow can be further clarified by combining sweep and ejection according to quadrant analysis. In the vertical direction, the ratio of sweep to total events near bed after spatial-averaged processing is found to be higher for the staggered array. According to the Shannon's entropy, water flow diversity was calculated to quantify the fish habitat. The water flow diversity index around the CGCSs is higher for the staggered. It can be concluded that the elements of CGCSs in staggered manners have a better protection for riverbed and can provide a more stable fish habitat suitability. The results anticipated by the spatial-averaged occurrence probability of sweep events near bed and flow diversity in the experiment are consistent with the result of previous research on landform change and fish habitat. The research could provide a theoretical basis for the application of CGCSs for river restoration.

Introduction

Due to the human influences, river ecosystems continue to deteriorate, including the lowering of the groundwater level in adjacent areas, conversion of floodplains into terraces, simplification of river habitat conditions, declining in biodiversity, reducing ecological quality, and increased risk of flooding in river reaches located downstream of eroded reaches. (Korpak et al., 2021; Shahady and Cleary, 2021; Petit et al., 1996; Kondolf, 1995; Hajdukiewicz and Wyzga, 2019; Kuo et al., 2021). As a result, there is a growing interest in river restoration. River restoration is used to describe channel renaturalisation, diverse modifications of river channels and adjacent riparian zones and floodplains, as well as modifications to the water, sediment, and solute inputs to the river. It is one of the most prominent areas of applied water-resources science (Wohl et al., 2015). The common goal of these modifications is to improve hydrologic, geomorphic, and/or ecological processes within a degraded watershed and replace lost, damaged, or impaired parts of the natural system. Modern approaches to river restoration increasingly focus on reconstructing natural geomorphic processes, including erosion and deposition, to create aquatic and floodplain habitat (Beechie et al., 2010; Palmer et al., 2005; Thompson et al., 2016; Schwindt et al., 2019), promoting the intention of stabilizing banks and creating a stable channel form (Bernhardt et al., 2005).

Techniques for controlling the riverbed, stabilizing channel alignment, protecting stream banks and reestablishing the natural habitat are important in river restoration projects. Habitat-improvement structures have been widely used in restoration efforts. They can enhance the aquatic habitat represented by the pool and riffle sequence, which has been proved to be the important criteria for evaluating the success of restoration projects (Kondolf, 1995; Gordon et al., 1992). In these instream structures with habitat-improvement function, grade-control structures (GCSs) are generally designed. Grade control structure (GCSs), known as “hard structure”, are usually constructed to manage torrential streams, preventing vertical incision along the length of the streams and creating localized pool habitats (Seehorn, 1992). GCSs are usually made by earth, gravel, wood, concrete, gabion, and other materials. It can reduce the original bed slope by creating an equilibrium slope between each constituent structure, thus ensuring an equilibrium the balance of erosion and accumulation processes in a specific river section (Galia et al., 2016). The more a GCS sequence mimics a natural step-pool system, the higher the probability that the river will achieve stability and maintain good ecological condition (Marion et al., 2004). Many kinds of GCSs, like low-head hydraulic structures (block ramps, rock sills), three-dimensional grade control structures (rock weirs) and open hydraulic structures (vanes), are popular now in river restoration projects (Pagliara et al., 2012; Pagliara and Kurdistani, 2013, 2015; Salant et al., 2012; Lenzi et al., 2002; Radecki-Pawlik et al., 2018; Plesiński and Radecki-Pawlik, 2017).

Based on material of composed elements, these structures can be classified into wooden, natural boulder and concrete grade-control structures (WGCSs, BGCSs and CGCSs) (Conesa-Garcia and Lenzi, 2013). For the small to medium-sized rivers, BGCSs and WGCSs are commonly applied. Plenty of research has shown that the ecological state of select rivers and streams regulated with BGCSs is improving (Galia et al., 2016; Bylak et al., 2017; Plesinski et al., 2020; Korpak et al., 2021; Wang et al., 2022). According to Tullos and Walter (2015), WGCSs can providing shelter for resting and higher drift densities for more efficient feeding and higher rates of energy gain. But in large and high energy rivers like inland waterways, there are numerous uncertainties, obstacles and unquantified risks that limit WGCSs to being used with confidence more widely. Therefore, BGCSs and CGCSs, like bed sills, ramps, dikes, river bed and beach protection belts, are commonly applied on these large rivers, such as the Rhine and the Yangtze River (Yang et al., 2013; Grabowski et al., 2019; Winkels et al., 2021; Luo, 2021). The results of the application in engineering show that BGCSs can protect riverbeds effectively and get naturally consolidated with the sediments (Radecki-Pawlik, 2013; Chardon et al., 2021). Nevertheless, the permeability between boulder is low, which severely decreases ecology function and self-purification ability of the rivers. And it is high-cost to transform large rocks from the far materials resources to be brought to the engineering site in some situations. Considering such problems, concrete materials as an alternative to gravel should be taken into consideration (Rashki Ghaleh Nou et al., 2020). Elements were designed and constructed in several shapes and dimensions, such as tetrahedral permeable frames (TPFs), and A-Jack concrete blocks (Wang et al., 2017a; Khalifehei et al., 2021; Liu et al., 2020). Some typical characteristics are as follows: high permeability, constructing easily and more energy dissipation by dispersing concentrated jets into smaller ones.

It should be mentioned that not all GCSs have a positive effect in river restoration. An improperly designed system threatens not only the integrity of the GCSs but also the overall stability of the river bed, and generates certain ecological problems. For example, if distance between structures is too large, it may destabilize river bank and bed. Such phenomenon can also be observed for vegetation restoration (Nepf, 2012). Therefore, it is necessary to set guidelines for design a proper GCSs system. The guidelines for the WGCSs and BGCSs have been explored over the past decades. Design guidelines on engineered log jam have been summarized in several publications (Abbe and Brooks, 2011; USBR and ERDC, 2016). There are plenty of theory studies on various fields supporting the application of wooden and boulder elements, such as flow characteristic and scour mechanisms (Ismail et al., 2021; L'Hommedieu et al., 2020; Cooper and Tait, 2010). However, guidelines for the elements in CGCSs are rarely available from the literature. What is the characteristics of flow structure around them? And how to arrange them in order to have a better effect in restoration engineering? These questions should be taken into consideration. There should be some evaluation indicators for CGCSs to supply guidelines for using elements.

Combing the former research, there are two perspectives to investigate the indicators. One of perspectives is riverbed protection. Turbulence plays an important role in sediment transport, and it get reduced near the bed due to the “shelter” from structures. Both flume experiments and in-situ surveys so far confirmed that these elements had a good effect on river bed protection (Fig. 1). The TPFs can retard the flow and reduce the shear stress near bed, thus reducing the probability of sediment entrainment and even promoting sediment deposition (Wang et al., 2018). Bejestan et al. (2017) reported that the scour of trapezoidal crest can be reduced by up to 100% by using A-Jacks armors. The other perspective is ecology effect. CGCSs can enhance the habitat by creating heterogeneous flow, such as vertical and horizontal jets through porous structures. In-situ tests showed that the TPFs have a fish-aggregation effect (Wang et al., 2017b). Whereas, to the best of our knowledge, few studies investigated the evaluation indicators for CGCSs from the two perspectives. It is urgent to conduct the exploration.

In this study, we analyzed the mean flow field around the structures, and two indicators were proposed to evaluate the CGCSs according to the effect of elements on geomorphology change and aquatic organism habitat. Sediment entrainment was demonstrated by the spatial-averaged occurrence probability of sweep near bed and the ecology effect was quantified by flow diversity. The reasons are as follows:

  • 1.

    The concept of bursting phenomenon is a mean to describe the transfer of momentum. Quadrant analysis is a potent technique to recognize the structure of the bursting phenomenon. The sensitive regions for sediment entrainment can be consequently found by exploring predominate flow type (Kline et al., 1967; Izadinia et al., 2013).

  • 2.

    Like “functional diversity” of an ecosystem, flow diversity can be used to predicted how ecosystem functions are related to structural characteristics (Barakoti et al., 2019; Villéger et al., 2008; Harvolk et al., 2015).

A modification of fish-nest bricks (MFNBs) was taken as the object of study, which was proposed as a new type of elements for building CGCSs in the Yangtze River based on the successful application of the fish-nest bricks in freshwater fishery resources and habitat restoration (Wang et al., 2007; Xu, 2020). For a single MFNB, it is a low aspect-ratio prism with a vertical cavity. Fig. 2 shows schematic of the turbulent flow over the MFNBs. The advantages of the MFNBs are as follows: (1) The shape of the structure is uniform and it is easy to construct. (2) The center of gravity for the structure is low due to the low value of h/L, which makes it stable in the river. (3) The sharp edge of the square prism will increase the dissipation of flow energy. (4) An internal cavity provides habitat for fish and benthic organisms.

In order to evaluate the reasonableness of these indexes, CGCSs built by MNFBs were selected as an example to conduct flume experiments. We carried out hydraulic tests to study the impact of CGCSs on the riverbed protection and ecological effect in different incoming flow conditions (low and high velocity) and layout forms (in-lined and staggered manner).

Section snippets

Experimental set-up

Experiments of the flow field were conducted in a rectangular flow facility 20 m long, 0.6 m wide, and 0.5 m deep, with a bed slope of 1‰. To obtain a steady uniform state, the water depth can be modified using the tailgate. The discharge was measured by an electromagnetic flowmeter installed in front of the flume. For the model of a MFNB in Fig. 3, the dimensions of the structure are shown as follows: length of side L = 5 cm, height h = 0.5L = 2.5 cm, the length of the square cavity Le = 4 cm.

Vertical plane parallel to the mean approach flow

As Fig. 6 shows, in the vertical x-z plane (corresponding to y+ = 0), the distribution of u + for different flow conditions is similar in the same array type but is various under the different arrays (the structure is outlined in white). For in-lined arrays (C1, C3), a large recirculation zone exists in front of the structure. Disturbed flow has insufficient distance to readjust before encountering the next obstacle, which indicates that the regime of flow field in this area is like a wake

Sediment transport and ecology effect of structures in different manners

In the study, quadrant analysis was adopted to explore the turbulent events around the MFNBs in different manners. Both entrainment and deposition affect sediment transport near the bed. Results showed that the percentage of sweep to the total events is higher for the staggered, indicating that more downward vortex motion exists. It should be mentioned that there are some research exploring the influence of the structures layout on sediment erosion. The ‘corridors’ between structures produce

Conclusion

In this study, the MFNB, a new kind of elements for building the CGCSs, was selected for flume experiments to investigate the flow characteristic and explore indicators to evaluate CGCSs at laboratory study. Effect of the Reynolds number and layout on flow characteristic were also analyzed. Some conclusions can be drawn from the results:

  • 1.

    Large recirculation zones exist in the cavity and behind the MFNBs, and velocity decreases apparently in the area, where sediment settles and aquatic animals

Credit authorship statement

Jianhua Liu: Conceptualization, Methodology, Data curation, Writing – original draft. Zhonghua Yang: Conceptualization, Supervision, Funding acquisition, Writing: Review & Editing. Ming Li: Funding acquisition, Writing: Review & Editing. Kunkun Lu: Data curation. Da Li: Writing: Review & Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This research was supported by the National Natural Science Foundation of China (Grant No. 52020105006and 51879199) and Department of Science and Technology of Hubei Province (Grant No. 2019ACA154). The authors thank the assistance from the Changjiang Waterway Bureau, which provides two projects from Wuhan to Anqing section in the Yangtze River, namely “Research on ecological bottom protection structure and ecological restoration mechanism of waterway engineering” and “Study on the impact of

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