Research on the Sustainable Development of the Bistrita Ardeleana River in Order to Stop the Erosion of the Riverbanks and the Thalweg

Abstract

The impact of dams and reservoirs on the aquatic ecosystem of rivers is a very important topic for water resource management. These hydrotechnical facilities change the natural hydromorphological regime of the rivers. This paper analyzed the hydrodynamic characteristics of an undeveloped riverbed section downstream of the Colibița reservoir, from the Bistrita Ardeleana River hydrographic basin. After processing the data obtained on the field, two hydraulic models were made using the MIKE 11 program, which aimed to identify the hydraulic parameters such as the wet section, the depth, and the water velocity. The first modeling was used for the flow rate of Q = 54.5 m³/s: the water depth was between 1.952 m and 2.559 m; and the water velocity varied between 1.148 m/s and 1.849 m/s. The second modeling was used for a flow rate of Q = 178 m³/s and showed that the water depth had values between 3.701 m and 4.427 m; and the water velocity varied between 1.316 m/s and 2.223 m/s. Following the granulometric analysis, the average diameter of the particle in the thalweg was D50 = 25.18 mm. The conclusion reached as a result of hydraulic modeling and granulometric analyses indicated that hydromorphological processes take place along the length of the analyzed sector, which have negative effects on water quality as well as on the instability of the riverbed. To make the riverbed safe along the entire studied length, we managed to identify some alternative solutions that have the role of stabilizing the banks, respectively, to stop the deepening of the thalweg. The alternative hydrotechnical constructions will increase the roughness of the riverbed, essentially reducing the water speed and increasing the favorable conditions for the retention of alluvium.

1. Introduction

Water made life possible on Earth; it represents a critical resource for humans and animals whenever human settlements develop. Climate change influences the water cycle in nature. Extreme weather events, such as droughts and heavy rainfall that increase in proportion to the extent of climate change, exert a negative impact on water resources. Water scarcity limits economic development in many regions of the world [1]. 71% of the Earth’s surface is covered with water. The oceans (Atlantic, Arctic, Antarctic, Pacific, and Indian) represent 96.5% of Earth’s water, 2.5% represents the total amount of fresh water, while the totality of the water in the hydrographic basins of the rivers represents 1% [1,2].

Rivers play a key role in providing drinking water to the population. They act as filters and provide a variety of habitats for a wide range of plants and animals. Globally, only 37% of rivers longer than 1000 km remain free-flowing throughout their length, and 23% flow uninterruptedly to the ocean [3,4,5].

The quality of water in rivers depends on the integrity of water bodies and the quality of life in rivers. Human interventions that reduce the longitudinal connectivity of rivers and streams, which have increased in number, significantly affect the biodiversity of rivers and, implicitly, the quality of water [3].

When watercourses are degraded or ecologically damaged, many of the ecosystem services important to human society are lost. The high fragility of rivers crossing urban areas to anthropogenic disturbances leads to the restricted use of natural resources [6,7].

A degraded stream is defined as a river that does not function to its biological hydrological potential [8].

The alteration of the hydrological and morphological processes of the watercourses causes the deterioration of the habitats for the biota and disturbs the functionality of the ecosystem [4].

Human activities such as the construction of diversions, weirs, and dams represent a persistent threat to freshwater biodiversity [9] and exert a negative impact on rivers, implicitly on the functionality of aquatic ecosystems [10,11]. The impact of dams and reservoirs on the aquatic environment of rivers is an important topic in global water resources management. These hydrotechnical constructions change the natural hydromorphological regime of rivers, negatively affecting their functionality and are associated with several environmental impacts, such as habitat fragmentation, poor water quality, nutrient depletion, loss or modification of biodiversity, remodeling of trophic networks, ichthyofauna, reduction of riparian biodiversity the river [5,9,10,11,12,13,14,15,16,17,18]. Worldwide, the decline of river biodiversity has recently been linked to the reduced functionality of the riverbed substrate. The riverbed substrate (hyporheic zone) plays an important role in the life cycle of many aquatic species, therefore for aquatic biodiversity [19].

The impact of large dams is not only limited to direct effects on the hydromorphology of the aquatic or riverine ecosystem but also on downstream localities, which extend to ecological environments [16].

In most rivers around the world, varying degrees of sediment reduction have been observed due to transverse hydrotechnical constructions [15]. Transversal hydrotechnical constructions negatively affect natural aggregate transport, creating imbalances between hydromorphological processes and the storage process in the coastal area [3,20]. Structural degradation of rivers can lead to limited availability of alluvial material [19].

Hydromorphological changes to the river due to dams are substantially better understood in the downstream sections of the river than in the sections of the river upstream of accumulations, especially on mountain watercourses [5].

Rivers are under huge threat all over the world and huge amounts of money are being invested in restoring them [21]. The watercourse restoration, also called river recovery, represents a set of measures to bring back, as close as possible, the original state of the ecosystems along the entire length of a heavily anthropized water course [7]. In general, river restoration aims at hydromorphological and ecological changes that improve the natural state of aquatic and riparian ecosystems through a variety of restoration methods [18,21,22,23].

It is essential to distinguish between restoration projects designed to reconnect rivers and projects designed to reconfigure rivers. It is important to distinguish the differences between ecological restoration of the river that helps restore ecological integrity and restoration intended for other outcomes such as aesthetic or recreational enhancements that do not necessarily improve ecological functions [23].

The definition of watercourse restoration is not limited to structural engineering (stabilizing of riverbanks) but also opens ways to include other aspects of river restoration as part of river management [24].

In the last two decades, the revitalization of anthropogenic rivers has been established in Europe as a measure to achieve the good ecological status of water bodies, as required by the EU Water Framework Directive (European Commission, 2000), while protecting the objectives downstream of floods. The implementation of the Water Framework Directive in ensuring sustainable water management has been implemented taking into account river basins [1,25,26]. These legislative efforts and necessary actions conclusively aim at increasing the heterogeneity of ecosystems. Worldwide, the number of restoration projects and the use of public financial resources to finance these projects has increased significantly and is expected to further increase [25].

Most countries currently promote projects that include alternative solutions with natural components and that have the role of incorporating the recreational activities etc. of human society [27]. Ecologically focused watercourse restoration projects may involve the use of alternative technical solutions that use local materials such as trees, boulders, shrubs, and timber for landscape development, flood mitigation, reducing hydromorphological processes, and increasing the chemical and biological quality of water and the variety of the biotope [7,28].

Alternative hydrotechnical constructions that have clear or gray-green elements are promoted and designed to: cope with high flows recorded during extreme floods or flash floods; create longitudinal and transversal connectivity of ecosystems in the riverbeds that cross human settlements [29,30,31].

Working with natural hydro-technical systems, which are powered by a diversity of life within them, provides a range of benefits to society, ranging from carbon storage, clean water and air, to the reduction of climate change impacts and protection against floods and other environmental hazards [32].

The concern within the European Union as well as at the global level regarding the sustainability of watercourses and the environment has encouraged the improvement and acceleration of the practices used in the recovery of water courses, but even so, specialists in the field of water management encounter problems in terms of promoting environmentally friendly or green infrastructure methods [11].

In order to restore or remake aquatic ecosystems, it would also be important to control the amount of water released from the reservoirs and to calculate the environmental flow, corresponding to most fish species. It is known that flow forecasting is one of the most difficult tasks for the owners in the management of water supply or energy production. Research on the flow required to conserve aquatic habitat began in the 1970s and has been commonly used for river management, including dam operation, water regulation, and hydrographic basin management [33,34].

In the present paper, we analyzed the hydrodynamic characteristics of a river bed sector located in the municipality of Bistrița are analyzed. Two flow rates recorded on the Bistrita Ardeleana water course at the Bistrita hydrometric station were used for the hydraulic modeling. The first flow rate used had a value of Q = 54.5 m³/s and was recorded following heavy rainfall and water discharges from the Colibița reservoir. The second flow was Q = 178 m³/s, being the highest flow in the last 15 years.

After performing the two simulations, it was possible to identify the values of the local hydraulic parameters for the analyzed sector. In order to be able to determine the instability of the riverbed due to hydromorphological processes, it was necessary to determine the size of the particles in the thalweg, this being done with the help of granulometric analysis.

The results obtained from hydraulic modeling and granulometric analyses indicate that hydromorphological processes take place along the length of the studied section, processes that have negative effects on water quality as well as on the objectives located in the immediate vicinity due to the instability of the riverbed.

In order to strengthen the riverbed along the entire studied length as well as to improve the water quality, we managed to identify some alternative solutions. These are used to withstand erosion speeds and have the role of stabilizing the riverbanks, respectively to reduce the water speeds, and to stop the deepening of the thalweg. The alternative hydrotechnical constructions will increase the roughness of the bed, implicitly reducing the water speed, and increasing the favorable conditions for the retention of alluvium.

2. Materials and Methods

2.1. Study Area

The hydrographic basin of the Bistrita Ardeleana River is located in the north-east of Transylvania, more precisely on the administrative territory of Bistrita–Nasaud county, geographically located in the central-northern area of Romania, between coordinates 46°45′–47°37′ north latitude and 23°27′–25°36′ east longitude.

The Bistrita Ardeleana River springs from the central group of the Eastern Carpathians (the Calimani Mountains) crosses the Livezi–Bargau Depression and before pouring into the Sieu River, it crosses the hills of Bistrita, a subunit of the Transylvanian Subcarpathian (Figure 1). The hills and the depression are made up of clayey, marly, tuff formations, trapped in a crease system. The catchment area of the river and its tributaries is 650 km², and its elevation varies between 350 m and 1990 m [11,35,36,37,38].

During winter, this area experiences a climate regime influenced by air currents of polar origin from the west. The transitional seasons (spring, autumn) are shorter compared to those in southern Romania, and the summers are warm and quite humid. The average annual air temperature is between 0 and 2 °C on the mountain peaks and 9.1 °C in the municipality of Bistrița. Atmospheric precipitation is particularly high, in the mountain area the average annual amounts are between 850 and 1400 mm [39].

2.2. Methods Used

In the present paper, RTK measurements were made for the area under study with the help of the Leica GS14 GNSS GPS and the Leica CS10 controller, being of high accuracy, and thanks to the differential precision corrections transmitted by the permanent stations in the ROMPOS network. The topographical measurements were made upstream to downstream. After carrying out the measurements on the riverbed of the Bistrita River, along the length of the analyzed sector, the procedure of data processing followed, this being carried out in the AutoCAD 2023 software. The profiles formed by the points from the measurements were imported, after which the drawing of the characteristic lines was followed. Phantom 4 Pro drone flights were conducted to obtain an updated orthophotoplan and a more realistic image of the section of the watercourse studied in this paper (Figure 2). A sample of alluvial material was taken from the thalweg of the riverbed for granulometric analysis. Sampling of natural aggregates was carried out from the area considered representative of the entire analyzed sector. The surface considered representative is located between transversal profile 5 and transversal profile 6. The sample of alluvial material was taken to demonstrate that on the analyzed river bank section, thalweg erosions occur at certain flow rates. The processing of the alluvial material was carried out with the electromagnetic sieving machine model A 059-3.

The processing of the images captured with the professional drone was carried out with the Agisoft Metashape program. In order to achieve correct georeferencing, GCPs (Ground Control Points) were used [40]. For the georeferencing of the images, three GCPs (Ground Control Points) were used, being raised in the field with the Rover GNSS South Galaxy G1 equipment. Thanks to the images captured by the drone, a cloud of topographical points was obtained, after which the studied sector was represented in 3D format (Figure 3).

Based on the data obtained from the field, hydraulic modeling was carried out for a constant flow, which has a permanent and non-uniform movement. The purpose of this modeling was to identify the hydraulic parameters such as the wet section and the depth and the velocity of the water, which vary spatially, more precisely along the stream.

In order to be sure of the need to promote hydraulic constructions with environmentally friendly materials, two hydraulic models were made. The length of the sector analyzed in hydraulic modeling is 198 m.

MIKE 11 software was used for hydraulic modeling; this is an engineering software capable of simulating water speed, water level, flow, sediment transport, etc. [41]. Saint-Venant mathematical equations were used to perform hydraulic simulations in the one-dimensional system.

In the one-dimensional system, they are as follows:

The continuity Equation (1) is

$$\frac{\partial Q}{\partial x}+b_S\frac{\partial h}{\partial t}=q$$

(1)

The momentum Equation (2) is

$$\frac{\partial Q}{\partial t}+\frac{\partial \left(\alpha \frac{Q^2}{A}\right)}{\partial x}+gA\frac{\partial h}{\partial x}=0$$

(2)

where x is the longitudinal distance along the channel (m), h is the flow depth, t is the time (s), A is the cross-sectional area of flow (m²), B is the width of the channel (m), Q is the integrated discharge (m³/s), α the vertical velocity distribution coefficient, g is the acceleration of the gravity (m/s²), and q leads to the basic equations used in MIKE 11 into these equations [41]:

$$\frac{\partial Q}{\partial x}+\frac{\partial A}{\partial t}=q$$

(3)

Two hydraulic models were made for two flows recorded in the hydrographic basin of the Bistrița Ardeleana River:

Q = 54.5 m³/s represents the flow recorded following the heavy rainfall recorded in the hydrographic basin of the Bistrița Ardelena River in September 2022, this flow also being influenced by the controlled water discharges from the Colibița reservoir;

• Q = 178 m³/s represents a flow recorded at the Bistrita hydrometric station in 2019, this being the highest flow recorded in the last 15 years [42].

The simulations being in the one-dimensional system, the obtained results were represented graphically in the MIKE View program.

Following the interpretation of the results, we managed to identify alternative solutions or methods (environmentally friendly) that will contribute to the restoration of the water course.

ArcGIS 10.6 software was used to cartographically represent the critical sector, and the graphic representation of the identified green alternative methods was made using AutoCAD Map 3D 2023 software.

What would the alternative methods of restoration or restoring the watercourse of the studied section consist of?

The bioengineering methods used in hydrotechnical constructions with the role of defense or protection of the riverbanks and stopping the degradation of the thalweg are as follows:

• live fascines or branches;
• boulder revetment;
• vegetated rock gabions;
• crib wall;
• live stakes;
• vegetated rock rolls;
• coir rolls;
• shrubs;
• root wad;
• mats reinforced with grass.

All the methods listed above will reduce the negative effects of variations in flows and levels due to natural floods or resulting from the production of electricity on the riverbanks as well as on the thalweg in the studied sector (Figure 4).

Hydrotechnical constructions are needed to stop erosions and reduce the degradation of the thalweg on both banks of the river. The green hydro-technical constructions with the role of protecting the banks and stopping the degradation of the thalweg have the following lengths:

• right bank—217.5 m;
• left bank—149.5 m.

The hydrotechnical constructions proposed for the studied sector start on the right bank. The proposed solution for the right bank of the Bistrita Ardeleana River has the role of stopping the erosion of the bank and reducing the degradation of the thalweg.

The works will start upstream to downstream and will start with the vegetated rock gabions on a length of 46 m, being placed on a bed of live fascines or branches. Down-stream of the vegetated rock gabions for a length of 88 m, work will be done to divert the watercourse with the help of 18 root wad of the following species: wicker (Salix fragilis), willow (Salix cinerea), or white willow (Salix alba).

The distance between trees should be 3 times the diameter of the roots. Trunks can be between 2 m and 3 m long with a diameter between 30 cm and 60 cm. They need to be placed or positioned in such a way that they will interpolate the direction of the river flow [32,43]. After the root wads are installed in the trenches, they will be covered with boulders, gravel, and soil to anchor them [32]. In order to control erosion due to meteoric water, mats reinforced with grass will be installed (Figure 5).

A third component of the hydrotechnical construction on the right bank for a length of 55.5 m consists of the following: reprofiling the bank and the installation of two rollers. One is a vegetated rock roll and the second one will be a coir roll, both anchored with the help of stakes, so that at the end they are covered with earth mixed with gravel.

Along the entire length of the hydrotechnical construction proposed for the right bank, live stakes or shrubs can be planted, so that they can increase the effectiveness of the construction in the long term.

As for the methods or solutions proposed for the left bank, they will start like the works on the right bank, from upstream to downstream.

Given the height of the eroded bank (between 3.38 m and 4.97 m) which has an almost vertical slope, one solution is to defend the bank with crib walls with living material plus reprofiling the riverbank to have greater stability.

In fact, the hydrotechnical construction on this bank will include several environmentally friendly methods such as live fascines or branches, boulder revetment, crib wall, vegetated rock rolls, coir rolls, mats reinforced with grass, and planting live stakes or shrubs.

The crib walls with living material will be installed over a length of 94 m. These are box-shaped constructions made of soft wood, logs with a diameter between 20 cm and 40 cm, or square timber filled with local material [44,45,46].

When the works begin, a trench of 2 m width and 0.5–1 m depth will be made. The ditch must be made outwards because the crib walls are located in steps facing the water course. To provide stability to the crib walls, they will be secured at the corners by pillars driven into the ground. On the side facing the watercourse of the trench, bundles of live fascines will be placed from place to place. Later, a boulder revetment will be arranged along the entire length of the trench. Additionally, along the ditch, logs or square timber will be placed parallel at a distance of 1.5 m. On top of them along the entire length of the trench, logs or square timber with a length of 2 m each will be placed transversely, these being fixed at a distance of 1.5 m between them. This process will be repeated two or three times, the formed cell will be filled with soil mixed with stone with a diameter of 10 cm. The next step is to form a layer of fascines cut to a length of 2 m, placed with the cut towards the bank, these being covered with a layer of a mixture consisting of stone and soil. The process will be repeated with the laying of the logs longitudinally and transversely and that of forming the fascine bed. Both processes will continue until a height of 2 m is reached (Figure 5) [43,44].

After the completion of the crib walls downstream of them, the installation of vegetated rock rolls will continue for a length of 55.5 m, respectively coir rolls. At the end, mats reinforced with grass will be mounted and live stakes or shrubs will be planted along the entire length of the hydrotechnical construction proposed for the left bank.

The last necessary step after the completion of the hydrotechnical constructions would be the installation of a fence to restrict the access of animals and citizens to the water.

3. Results

The results will be presented first for the flow rate Q = 54.5 m³/s and then for the flow rate Q = 178 m³/s. In both variants, an initial water depth of 0.3 m was taken into account for the modeled river sector.

Table 1. The mileage of the transversal sections made in the studied sector.

Section	Kilometre (m)
P1	0.0
P2	13.41
P3	33.28
P4	56.53
P5	89.70
P6	119.62
P7	150.79
P8	176.19
P9	198

shows the mileage of the cross-sections used in the hydraulic simulations to obtain the values of the local parameters. The graphic representation of the transverse profile 6 is used to represent the maximum water level for the two analyzed situations.

Considering the above, the hydraulic simulations were carried out for two situations in which the flow rates had the values of Q = 54.5 m³/s recorded following heavy rainfall, this being also influenced by the controlled water discharges from the Colibița reservoir and Q = 178 m³/s representing a flow recorded at the Bistrita hydrometric station in 2019, this being the highest flow recorded in the last 15 years [42].

In the graphs presented below for both analyzed hydraulic models, the lines have the following meanings:

• the continuous black line positioned in the upper part of the graph represents the upper elevation of the right bank of the river from P 1 upstream to P 9 downstream;
• the dotted black line represents the upper level of the left bank of the river from P 1 upstream to P 9 downstream;
• the blue contour represents the water level in the longitudinal profile;
• the dotted line inside the blue contour represents the minimum water level for the hydraulic models, the minimum depth of 0.3 m from which the modeling started;
• the lower continuous black line that supports the water level represents the river valley;
• the continuous blue line indicates the maximum water level in the cross-section;
• the broken green line indicates the minimum level in the model, more precisely the minimum depth of 0.3 m taken into account along the entire length of the river sector;
• the continuous and thick red line represents the water speed in the longitudinal profile;
• the dotted red line indicates the initial speed of the water at the very beginning of the modeling until the water fills the bed up to the maximum level corresponding to the studied flow rate.

3.1. Hydraulic Modeling on the River Bistrita Ardeleana, Where the Flow Has a Value of Q = 54.5 m³/s

In Figure 6 we can see the water level in the longitudinal profile of the river sector taken into account for the flow rate Q = 54.5 m³/s.

In Figure 7 we can see the maximum water level in one of the cross sections for the flow rate Q = 54.5 m³/s.

In Figure 8, the water depth in each cross-section is represented graphically, respectively, for the maximum value of the depth for the flow rate Q = 54.5 m³/s.

In Figure 9, we can see the water speed, in the longitudinal profile along the sector studied in the modeling.

In the table below (

Table 2. Water level, depth, and velocity in each cross-section for flow rate Q = 54.5 m³/s.

Section/Q		Q = 54.5 m3/s
Section	Kilometer (m)	Water Level (m)	Water Depth (m)	Water Velocity (m/s)
P1	0.00	381.682	1.952	1.698
P2	13.41	381.635	1.985	1.819
P3	33.28	381.646	2.096	1.426
P4	56.53	381.611	2.381	1.455
P5	89.70	381.540	2.400	1.661
P6	119.62	381.563	2.573	1.148
P7	150.79	381.514	2.534	1.338
P8	176.19	381.505	2.545	1.235
P9	198	381.499	2.559	1.158

) the values recorded in all 9 cross-sections for the following parameters are transcribed: water depth, water level, and water velocity for the flow rate Q = 54.5 m³/s.

3.2. Hydraulic Modeling on the Ardelean Bistrita River, Where the Flow Has a Value of Q = 178 m³/s

In Figure 10 you can see the water level in the longitudinal profile of the analyzed river sector for the flow rate Q = 178 m³/s.

In Figure 11 we can see the maximum water level in one of the cross sections for the flow rate Q = 178 m³/s.

In Figure 12, the water depth in each cross-section is represented graphically, respectively, for the maximum value of the depth for the flow rate Q = 178 m³/s.

In Figure 13, the water speed can be seen in the longitudinal profile along the sector studied in the modeling.

In the table below (

Table 3. Water level, depth, and velocity in each cross-section for flow rate Q = 178 m³/s.

Section/Q		Q = 178 m3/s
Section	Kilometer (m)	Water Level (m)	Water Depth (m)	Water Velocity (m/s)
P1	0.00	383.431	3.701	2.163
P2	13.41	383.401	3.751	2.223
P3	33.28	383.445	3.895	1.819
P4	56.53	383.385	4.155	2.031
P5	89.70	383.318	4.178	2.182
P6	119.62	383.417	4.427	1.316
P7	150.79	383.366	4.386	1.559
P8	176.19	383.279	4.319	1.995
P9	198	383.339	4.399	1.509

), the values recorded in all nine cross-sections for the following parameters are transcribed: water depth, water level, and water velocity for the flow Q = 178 m³/s.

4. Discussion

Hydraulic modeling for the two flows Q = 54.5 m³/s and Q = 178 m³/s was carried out for a constant flow, having a permanent and non-uniform movement. Local parameters such as wetted section, depth, and velocity vary spatially, specifically along the stream. With the help of simulation, the average water speed and water depth were determined in all nine transverse profiles measured in the analyzed sector.

In each section, based on the study of the sector in the field, the rugosity was chosen both in the minor and in the major riverbed, consulting the specialized literature. The main factors that influence the flow and modify the roughness coefficient are the microrelief of the riverbed, the covering or protection of the riverbed with vegetation, and the irregularity in the plan and the section of the riverbed [47,48].

According to the simulation for the flow rate Q = 54.5 m³/s, the water depth in the studied sector is between 1.952 m (transverse profile 1) and 2.559 m (transverse profile 9). The water velocity in this sector at the flow rate Q = 54.5 m³/s varies lengthwise, depending on the shape of the riverbed or sections of the riverbed, with values between 1.148 m/s (transverse profile 6) and 1.849 m/s (transverse profile 2).

The modeling carried out for the flow rate Q = 178 m³/s shows us that the water depth for the analyzed water body had values between 3.701 m (transverse profile 1) and 4.427 m (transverse profile 6). The recorded water velocity for the flow rate Q = 178 m³/s had values between 1.316 m/s (transverse profile 6) and 2.223 m/s (transverse profile 2).

In order to be able to determine the existence of hydromorphological processes, it was necessary to take a sample of alluvial material from the section considered representative of the analyzed sector. Following the granulometric analysis, the average diameter of the particle in the thalweg is determined. According to the analysis, the sample of natural aggregates from the fluvial sector, the sieve through which 50% of the sample passed has a diameter of 25.18 mm (D50) (Figure 14).

According to

Table 4. Table with the average particle entrainment speed Va (m/s) [49].

The Name of the Constitutive Lands of the Thalweg	D50 (mm)	Average Water Depth (haverage, in m)
		2	3	4	5	6	8	10	12	14	16
Fine sand	0.15	0.6	0.7	0.7	0.8	0.9	1.0	1.1	1.2	1.3	1.3
Small sand	0.5	0.7	0.9	1.0	1.0	1.1	1.3	1.4	1.5	1.6	1.7
Pearly sand and small sand with gravel	1.0	0.9	1.0	1.2	1.3	1.4	1.5	1.7	1.8	1.9	2.0
Lage sand and pearly sand with gravel	2.5	1.1	1.3	1.4	1.6	1.7	1.9	2.0	2.2	2.3	2.5
Gravel with large sand	6.0	1.4	1.6	1.7	1.9	2.0	2.2	2.4	2.6	2.7	-
Fine river gravel	15.0	1.7	1.9	2.1	2.3	2.4	2.6	2.8	3.0	3.2	-
Medium river gravel	25.0	2.0	2.3	2.6	2.7	2.9	3.1	3.4	3.6	-	-
Large river gravel	60.0	2.5	2.8	3.0	3.2	3.3	3.6	3.9	4.1	-	-
Very small boulder	140.0	3.0	3.4	3.6	3.8	4.0	4.4	4.6	-	-	-
Medium boulder	250.0	3.6	4.0	5.2	4.5	4.7	5.0	5.3	-	-	-
Large boulder	450.0	4.2	4.6	4.9	5.1	5.3	5.7	-	-	-	-
Very large boulder	750.0	4.9	5.3	5.6	5.9	6.1	6.4	-	-	-	-
Clays and sandy loams with poor composition γ = 1.0 t/m3		1.0	1.0	1.0	1.2	1.3	1.5	1.6	1.8	1.9	2.0
Clays and sandy loams with poor composition γ = 1.4 t/m3		1.2	1.3	1.4	1.5	1.6	1.8	1.8	1.8	2.2	-
Clays and sandy loams with poor composition γ = 1.8 t/m3		1.5	1.7	1.8	1.9	2.0	2.2	2.4	2.4	2.5	-

, which shows the average particle entrainment speed according to PD 95-2002- Normative regarding the hydraulic design of bridges and footbridges, the sample taken from the bottom of the riverbed had a D50 = 25.18 m, representing medium river gravel for which, at a water depth of 2 m, the average particle entrainment speed was 2 m/s. Thus, according to the modeling for the flow rate Q = 54.5 m³/s, the water velocity in certain sections (transverse profiles 1, 2, and 3) can move particles with a diameter close to D50. The rest of the particles with a diameter smaller than D50 for depths greater than 2 m (transverse profiles 4–9) will all be set in motion and thus the hydromorphological process of bank erosion and valley deepening occurs. In the case of the simulation for the flow rate Q = 178 m³/s, the water velocity can set in motion the particles with a diameter close to D50 only in cross-section 2. In the rest of the cross-sections, the water velocity sets in motion all the particles smaller than D50.

It is very clear that at these depths and water velocities, a lot of thalweg particles with a diameter of less than 25 mm, which represent approximately half of the sample according to the study, are set in motion and significant erosion occurs on the thalweg and both river banks. Of course, once they are set in motion, even larger particles will be set in motion, by rolling or dragging, because they lose their stability through the mechanical connection they have with the other particles already set in motion.

It can also be observed that the entire studied sector is set in an area of pronounced curvature and not in alignment. Thus, in this section, in addition to the speed of the water, it acts very strongly through a process of force and accentuated erosion and the centrifugal force. Centrifugal force, as it is known, it is one of the main factors that influence the formation and evolution of minor riverbeds. It can be seen from the orthophotoplan that the right bank of the meander is a concave bank, being heavily eroded, the erosion evolving after each flood. On the left bank which is convex, we can see alluvium deposits, which are deposited after each flood when the depth and the speed of the water decreases in this section.

According to

Table 5. Values of the unit speed of moving isolated particles for non-cohesive soils [50].

Particle Diameter (mm)	Vli (m/s)	Particle Diameter (mm)	Vli (m/s)	Particle Diameter (mm)	Vli (m/s)
10	0.83	42	1.38	65	1.69
15	0.86	44	1.41	70	1.73
20	0.90	46	1.44	75	1.76
25	0.98	48	1.47	80	1.80
30	1.04	50	1.50	85	1.84
32	1.11	52	1.54	90	1.88
34	1.17	54	1.56	95	1.91
36	1.24	56	1.59	100	1.95
38	1.29	58	1.62	150	2.40
40	1.35	60	1.65	200	2.60

, a water velocity of 0.98 m/s is required to move the isolated particles for non-cohesive soils with a diameter of 25 mm. In the simulation performed for the flow rate of Q = 54.5 m³/s, the lowest value of the speed was 1.148 m/s and for the flow rate Q = 178m³/s the lowest recorded water speed was 1.316 m/s.

According to the table below (

Table 6. Unit mass entrainment speeds of alluvium for non-cohesive soils [50].

The Name of Alluvium	Particle Diameter (mm)	Vlm m/s
Dust and mud	0.005–0.05	0.15–0.21
Fine sand	0.05–0.25	0.21–0.32
Medium sand	0.25–1.0	0.32–0.57
Large sand	1.0–2.5	0.57–0.65
Small pebbles	2.5–5.0	0.65–0.80
Medium pebbles	5.0–10	0.80–1.00
Coarse pebbles	10–15	1.00–1.20
Small gravel	15–25	1.20–1.40
Medium gravel	25–40	1.40–1.80
Large gravel	40–75	1.80–2.40
Small boulders	75–100	2.40–2.80
Medium boulders	100–150	2.80–3.40
Big boulders	150–200	3.40–3.90
Small blocks	200–300	3.90–4.40
Medium blocks	400–300	4.40–4.80

), the mass entrainment speed of alluvium for non-cohesive soils with a diameter of 25 mm to 40 mm is between 1.4 m/s and 1.8 m/s.

After analyzing the results obtained from the hydraulic modeling for the two flow rates recorded on the Bistrita Ardeleana watercourse, it was observed that for the flow rate of 54.5 m³/s, the unit mass entrainment speeds of the alluvium were below 1.4 m/s and only at a few sections (cross profiles 6–9). For the flow rate of 178 m³/s, unit mass entrainment velocities of alluvium below 1.4 m/s were recorded only in transverse profile 6, the rest having high velocity values.

Previous studies such as the research paper by Florek, J. and Wyrębek, M. used 1D modeling carried out in the HEC-RAS program for mountain rivers in southern Poland. Following the interpretation of the results obtained by them, five sections were found where the riverbed is unstable; three of them are in the transverse profiles 16, 18, and 20 where the average diameter of the particles is 68 mm and 46 mm respectively. These dimensions are characteristic of the alluvial material consisting of large gravel. The flow rates considered were 4.69 m³/s, 42.38 m³/s, and 3.5 m³/s [51]. According to Table 6, the mass entrainment speed of alluvium with a diameter of 46–48 mm is between 1.8 m/s and 2.40 m/s.

As in the case of the sector analyzed in our work, in some cross-sectional profiles, the particles with a diameter smaller than the average diameter of the particles may be set in motion by rolling or crawling when the recorded flow rates are lower than those analyzed.

Even if different programs were used to create the hydraulic model, as well as the values of the monitored parameters differing, the purpose of the modeling was to identify or confirm the existence of erosions along the length of the analyzed sectors and to emphasize the importance of promoting green hydrotechnical constructions to return to the state of the river as natural as possible.

It is obvious that floods with increased flows over time can accelerate hydromorphological processes compared to low recorded flows [52]. The length of the unstable river bed is consistent with the controlled discharges from the reservoir, topography, and geological structure [53].

The application of the solutions identified for the defense of the banks to withstand erosion speeds and to stabilize the banks, respectively, and the application of the green solution to reduce the water speeds and stop the deepening of the thalweg are necessary for this analyzed sector. The hydrotechnical constructions identified for the safety of the two banks will increase the roughness of the riverbed, thus reducing the water speed, creating favorable conditions for the retention of alluvium. The vegetation in the minor river bed has a very important role in protecting the surface of the bed from instability through the root system [54].

The green methods proposed for the analyzed sector will be part of future research activity, in which their hydrodynamic behavior will be analyzed as well as the hydrodynamic characteristics of the studied body of water, having as examples similar research carried out by Lama, G. F. C. et al. and Pasquino V. et al. [55,56].

5. Conclusions

According to the modeling for the flow rate Q = 54.5 m³/s, the water velocity in certain sections can move particles with a diameter close to D50. The rest of the particles with a diameter smaller than D50 for depths greater than 2 m will all be set in motion. In the case of the simulation for the flow rate Q = 178 m³/s, the water velocity can set in motion the particles with a diameter close to D50 only in cross-section 2. In the rest of the cross-sections, the water velocity sets in motion all the particles smaller than D50.

In regard to the movement of isolated particles for non-cohesive soils with a diameter of 25 mm, it is necessary to have the velocity of the water at a value of 0.98 m/s. In the simulation performed for the flow rate of Q = 54.5 m³/s, the lowest speed is 1.148 m/s and for the flow rate Q = 178m³/s the lowest recorded water speed is 1.316 m/s.

In conclusion, in the analyzed sector, strong erosions occur both on the thalweg and on the banks at high flows due to abundant precipitation and controlled water discharges from the Colibița reservoir, at high flows recorded during extreme floods, the erosions are large-scale. Thus, it is recommended that hydrotechnical constructions with the role of protection against erosion should be carried out in this sector.

The hydrotechnical constructions aim to limit the morphological modeling or to consolidate the river bed artificially. These conventional constructions have a negative impact on the longitudinal and vertical connectivity of rivers.

Critical or unstable sectors on watercourses can become safe with the help of bioengineering, more precisely with green methods. This is possible given that these alternative constructions use natural elements that have positive effects on the ecosystem.

Morphological phenomena occur in the studied section that modify the river bed even at normal flows. At high flows also influenced by water discharges from the Colibita reservoir or at high flows recorded during extreme floods, the erosions or degradations are large-scale.

Thus, it is recommended to promote hydrotechnical constructions with the role of stopping the morphological modifications of the minor riverbed. This should be done taking into consideration that these erosions or degradations endanger the stability of the sector, since on the right bank in the immediate vicinity is the national or European road as well as a residential area, which over time could be affected due to the instability of the sector.

The bioengineering methods identified can be used to stop river banks or thalweg erosion of the Bistrita Ardeleana River. Green methods can significantly increase the diversity of the aquatic ecosystem. It helps to the accumulation of alluvium and the removal of excess moisture accumulated in the soil on the two banks of the river.

The profitability of these alternative constructions is very high due to the materials used, they are found in the immediate vicinity of the studied water course and the low labor force. Their resistance during natural floods as well as during the period of increased flows resulting from the production of energy (with daily frequency) gives them increased effectiveness.

These hydrotechnical constructions have a flexibility that allows them to withstand a slight instability of the foundation without damage unlike conventional methods (monolithic concrete).

Hydrotechnical constructions with natural elements could be used to replace hydrotechnical constructions destroyed or damaged by floods or the passage of time.