NB/T 10390-2020Code for design of desilting basin for hydropower projects (English PDF)

Foreword

This document was issued on 23 October 2020 by the National Energy Administration of the PRC and takes effect on 1 February 2021.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.140, Chinese classification P59.

It replaces DL/T 5107-1999, which is superseded.

In accordance with the requirements of the Notice of the National Energy Administration on Issuing the 2015 Plan for Formulation and Revision of Energy Sector Industry Standards (Guo Neng Ke Ji [2015] No. 283), the drafting group of this code revised this code through extensive investigation and research, careful summarization of practical experience, reference to relevant advanced domestic standards and on the basis of wide solicitation of opinions.

The main technical contents of this code are: general provisions, terms, setting conditions and design criteria for desilting basins, selection of desilting basin types, layout of desilting basins, hydraulic design and main dimension calculation, structural design, operation design and sediment monitoring design.

The main technical contents revised in this code are as follows:

Calculation requirements related to the structural design of desilting basins have been added.

The scope of application of the code has been modified: from desilting basins of Grade 3 and above in large and medium-sized hydropower and water resources projects to desilting basins of hydropower projects.

The setting criteria for desilting basins have been modified.

The setting criteria for desilting basins of water resources projects have been deleted.

The National Energy Administration is in charge of the administration of this code. China Renewable Energy Engineering Institute has proposed this code and is responsible for its routine management, and the Energy Industry Standardization Technical Committee on Hydropower Investigation and Design is responsible for the explanation of specific technical contents. Comments or suggestions arising during implementation should be sent to China Renewable Energy Engineering Institute (Address: No. 2 Liupukang North Xiaojie, Xicheng District, Beijing; Postal code: 100120).

Chief development organization of this code: PowerChina Chengdu Engineering Corporation Limited.

Chief drafting staff of this code: Zhu Wanqiang, Huang Yankun, Hao Yuanlin, Jiang Dequan, He Xianpei, Liu Ding, Tian Xun, Lü Jinbo, Lu Peican, Yuan Liyi, Chen Qiang, Ju Lin.

Chief reviewers of this code: Dang Lincai, Fang Guangda, Zhang Hongwu, Liu Xingnian, Li Guobin, Long Qihuang, Cao Yubo, Wang Yi, Guan Lihai, Hu Wangxing, Jiang Hongjun, Wu Caiping, Yang Jinying, Zhao Tie, Liu Rongli, Li Shisheng.

Publication information

Energy Industry Standard of the People's Republic of China NB/T 10390-2020, Code for Design of Desilting Basin for Hydropower Projects, replacing DL/T 5107-1999. Issued on 2020-10-23 and implemented from 2021-02-01 by the National Energy Administration.

Chief development department: China Renewable Energy Engineering Institute. Approval department: National Energy Administration. Implementation date: 1 February 2021. Published by China Water & Power Press, Beijing, 2021.

Announcement of the National Energy Administration, 2020 No. 5: the National Energy Administration approved 502 energy industry standards, including the Technical Code for Real-time Monitoring System of Ecological Flow of Hydropower Projects (Annex 1), and 35 English versions of energy industry standards, including Series Parameters for Horizontal Hydraulic Hoist (Cylinder) (Annex 2), and hereby issues them. Dated 23 October 2020.

Annex 1, Catalogue of Industry Standards, entry No. 6: NB/T 10390-2020, Code for Design of Desilting Basin for Hydropower Projects, replacing DL/T 5107-1999, approved on 2020-10-23, implemented on 2021-02-01.

1 Scope

NB/T 10390-2020 is the Chinese design code for desilting basins on hydropower projects, replacing DL/T 5107-1999. On rivers carrying heavy sediment loads, typical of the mountain rivers of western China, the fine sand drawn into a run-of-river intake will erode turbine runners, guide vanes and seals within a season if it is not removed first. A desilting basin slows the flow enough for the harmful particles to settle out before the water reaches the headrace and the machines. The code sets the general provisions and defined terms and symbols, then the basic requirements: the design sediment data, the particle size that must be removed, the removal efficiency required, and the design discharge. It covers the choice and arrangement of basin types - continuous flushing, periodic flushing and mechanically cleaned basins, open and underground, single and multiple chambers - together with the inlet transition that distributes the flow evenly and the outlet structure. Hydraulic design follows, with the calculation of settling length, width and depth, the settling velocity of the design particle, the turbulence and non-uniformity corrections, and the verification of removal efficiency. Flushing design covers the sediment storage volume, the flushing galleries and gates, and the flushing flow and duration. Structural design, operation rules and the monitoring of sediment close the code.

1.0.1 This code is formulated with a view to standardizing the design of desilting basins in water diversion structures of hydropower projects.

1.0.2 This code is applicable to the design of desilting basins treating suspended sediment in hydropower projects.

1.0.3 In addition to this code, the design of desilting basins for hydropower projects shall also comply with the provisions of the current relevant standards of the nation.

2 Terms

2.0.1 desilting basin: a structure used to settle suspended sediment with particle sizes larger than the design settling particle size from sediment-laden flow, thereby reducing the sediment concentration of the flow.

2.0.2 periodic flushing desilting basin: a desilting basin in which, after deposition has reached a certain extent, the flushing gate is opened for flushing, so that sediment settling and flushing operate alternately.

2.0.3 continuous flushing desilting basin: a desilting basin in which the settled sediment is continuously flushed into the downstream river channel while water supply is maintained continuously.

2.0.4 desilting channel: a wide and shallow earth-channel desilting basin of considerable length formed by making use of a natural depression.

2.0.5 working section: the main section of a desilting basin used for settling sediment.

2.0.6 working length: the length of the working section of a desilting basin used for sediment deposition.

2.0.7 working width: the width of the working section of a desilting basin used for sediment deposition.

2.0.8 working depth: the water depth between the normal water level of a desilting basin and the design sediment deposition elevation.

2.0.9 working flow: for a periodic flushing desilting basin of a hydropower station, the diversion discharge; for a periodic flushing desilting basin provided with sediment releasing orifices, the sum of the diversion discharge and the sediment releasing discharge through the bottom orifices at the end of the basin; for a continuous flushing desilting basin, the sum of the diversion discharge and the flushing discharge.

2.0.10 flushing flow: for a periodic flushing desilting basin, in which sediment settling and flushing operate alternately, the discharge admitted through the intake gate for flushing sediment when a basin chamber (tank) is being flushed.

2.0.11 basin chamber: each of the several troughs into which the working section is divided parallel to the flow direction by partition walls higher than the normal water level, according to the working flow, working width and working depth.

2.0.12 basin tank: in a periodic flushing desilting basin, each of the several troughs into which a basin chamber is divided parallel to the flow direction by partition walls higher than the flushing water level above the design deposition surface, according to the flushing flow.

2.0.13 overflow area: the zone at the tail of the working section of a periodic flushing desilting basin where an overflow weir is provided to draw off surface water.

2.0.14 mean annual sediment concentration through turbine: the ratio of the mean annual sediment load passing through the turbines to the mean annual volume of water diverted through the turbines.

2.0.15 coarse sediment concentration: the concentration of suspended sediment with particle sizes larger than the design minimum settling particle size of the desilting basin.

3 Setting Conditions and Design Criteria for Desilting Basin

3.1.1 The following basic data shall be collected for the design of a desilting basin:

1 Topographical and geological data.

2 Measured discharge, suspended sediment concentration, particle size distribution and its analysis method, and water temperature data of the river reach where the project is located.

3 Data on the mineral composition and hardness of the suspended sediment.

4 Where the outlet of the flushing and sediment releasing channel of the desilting basin is a natural river channel, the stage-discharge relationship of the natural river channel at the outlet.

5 Where the side wall of the desilting basin is close to a natural river channel, the water surface profiles of that river reach for discharges of various magnitudes.

6 River ice regime and air temperature data.

7 Sources and characteristics of river debris and pollutants.

3.1.2 The following relevant design data of other disciplines shall be collected for the design of a desilting basin:

1 Water levels of the headrace channel and water conveyance channel of the desilting basin.

2 Runoff design results for the river reach where the project is located.

3 Design diversion discharge.

4 Sediment carrying capacity of the flow in the headrace channel and water conveyance channel.

5 Abrasion resistance of turbine flow passage components, anti-abrasion measures and the interval between turbine overhauls, and the turbine working head.

6 Operation and dispatching modes of the reservoir and the power station.

3.2.1 The period-average inflow sediment concentration of the basin shall be calculated from the daily average inflow sediment concentration and the daily average diversion discharge during the diversion period of the intake headworks, using the following formula: S = (sum for i = 1 to T of Qi multiplied by Si) divided by (sum for i = 1 to T of Qi) (3.2.1).

Where S is the period-average inflow sediment concentration of the basin (kg/m3);

T is the number of days in the period;

Qi is the daily average diversion discharge on day i (m3/s);

Si is the daily average inflow sediment concentration of the basin on day i (kg/m3).

3.2.2 The representative inflow suspended sediment concentration and particle size distribution used as the basis for the design of a desilting basin shall be selected after analysis and demonstration according to the type of desilting basin, the sediment transport characteristics of the river and the effect of reservoir sedimentation.

3.3.1 The preliminary determination of whether to set a desilting basin (Figure 3.3.1) shall be made according to the intersection points of the mean annual sediment concentration through turbine Sp and of the mean annual coarse sediment concentration through turbine S'p with the rated head of the turbine Hr. Where both intersection points fall in Zone A of the figure, a desilting basin may not be provided; where both fall in Zone C, a desilting basin should be provided; where both fall in Zone B, a desilting basin may not be provided if reliable anti-abrasion measures have been adopted in the hydraulic design, structural design and materials of the turbine flow passage components.

Figure 3.3.1 (a), Hr-Sp: the chart plots Sp (g/m3) against Hr (m) and is divided into Zones A, B and C; the solid curves apply to power stations with Francis and axial-flow turbines, and the dash-dot curves apply to power stations with Pelton turbines.

Figure 3.3.1 (b), Hr-S'p: the chart plots S'p (g/m3) against Hr (m) and is divided into Zones A, B and C by boundary curves for four head ranges.

Dashed curves: applicable to hydropower projects with Hr less than 100 m, d not less than 0.35 mm.

Thick solid curves: applicable to hydropower projects with Hr from 100 m to less than 300 m, d not less than 0.25 mm.

Thin solid curves: applicable to hydropower projects with Hr from 300 m to less than 500 m, d not less than 0.15 mm.

Dash-dot curves: applicable to hydropower projects with Hr not less than 500 m, d not less than 0.10 mm.

3.3.2 The decision to set a desilting basin shall be made on the basis of the preliminary determination and according to the degree of sediment abrasion of the turbines and the results of the analysis and calculation of abrasion damage, after a comprehensive technical and economic comparison taking into account the role of the power station in the power system, the layout conditions of the headworks, the investment in the desilting basin, the sediment characteristics, the abrasion resistance level of the turbines, the benefits of the power station and the operation and maintenance requirements.

3.3.3 Where a desilting basin is provided, the settling rate within the basin of sediment with particle sizes equal to or larger than the design minimum settling particle size should be 80% to 85%. The design minimum settling particle size of the desilting basin may be determined according to the rated head of the turbine in accordance with Table 3.3.3.

Table 3.3.3 Design minimum settling particle size of desilting basin: rated head Hr less than 100 m, design minimum settling particle size 0.35 mm.

Rated head Hr from 100 m to less than 300 m: design minimum settling particle size 0.25 mm.

Rated head Hr from 300 m to less than 500 m: design minimum settling particle size 0.15 mm.

Rated head Hr not less than 500 m: design minimum settling particle size 0.1 mm.

4 Selection of Desilting Basin Types

4.0.1 Hydropower projects may adopt hydraulic flushing or non-flushing desilting basins according to topographical and head conditions. A hydraulic flushing desilting basin may be a periodic flushing desilting basin (Figure 4.0.1-1) or a continuous flushing desilting basin (Figure 4.0.1-2), and continuous flushing desilting basins may be divided into the single-chamber type and the multi-chamber type. A non-flushing desilting basin may be a silting-type desilting basin or a mechanical dredging desilting basin; desilting basins of other types may also be adopted according to project needs.

Figure 4.0.1-1 Periodic flushing desilting basin: (a) plan, showing the headrace channel, the upstream transition section, the working section and the overflow weir area; (b) section I-I.

Key to Figure 4.0.1-1: 1 basin chamber intake gate; 2 basin tank intake gate; 3 basin tank partition wall; 4 flow distribution pier; 5 water conveyance channel; 6 flushing gate; 7 sediment releasing channel; 8 sediment releasing orifice; 9 transverse collecting trough; 10 lateral collecting trough; 11 centerline of desilting basin; 12 basin chamber partition wall; 13 flushing gate opening.

Figure 4.0.1-2 Continuous flushing desilting basin, (a) plan of single-chamber continuous flushing desilting basin, showing the headrace channel, the upstream transition section, the working section, the downstream transition section and the water conveyance channel. Key: 1 intake gate; 2 diffusion section; 3 flushing branch gallery; 4 flow distribution pier; 5 flow straightening rack section; 6 main flushing gallery; 7 trapezoidal trough wall; 8 emergency flushing gate; 9 outlet gate; 10 centerline of desilting basin.

Figure 4.0.1-2 (b) Cross section of single-chamber type. Key: 1 desilting chamber; 2 partition wall; 3 flushing branch gallery; 4 trapezoidal trough wall; 5 centerline of desilting basin.

Figure 4.0.1-2 (c) Cross section of multi-chamber type. Key: 1 desilting chamber; 2 partition wall; 3 flushing branch gallery.

4.0.2 A hydraulic flushing desilting basin shall have sufficient flushing head and flushing discharge. Where the terrain is open, a periodic flushing desilting basin should be adopted; where the terrain is narrow, a continuous flushing desilting basin should be adopted.

4.0.3 A mechanical dredging desilting basin should be adopted for hydropower projects with insufficient flushing head. A desilting basin cleared by mechanical dredging shall have a sufficient sediment stockpiling site and shall not cause adverse effects on the environment of the area or on the project.

4.0.4 In low-lying plain areas a desilting channel (Figure 4.0.4) should be adopted. According to topographical and operating conditions, types such as gravity-flow settling in channels excavated in advance of deposition, settling with pumped water, or a combination of gravity flow and pumping implemented in stages may be adopted.

Figure 4.0.4 Desilting channel: (a) lake type; (b) strip type; (c) shuttle type I; (d) shuttle type II. Key: 1 intake gate; 2 channel; 3 settling strip channel; 4 control gate.

5 Layout of Desilting Basin

5.1.1 The layout of a desilting basin shall be coordinated with the layout of the project headworks and shall be determined comprehensively, making reasonable use of the topography, avoiding unfavorable sites, and taking into account factors such as geological conditions, project characteristics, operating conditions and economic indicators.

5.1.2 The inlet of a desilting basin should be located in a river reach with a stable channel and a smooth, gentle flow pattern; where necessary, the river channel may be suitably modified.

5.1.3 A desilting basin should be arranged adjacent to the intake of the headworks. Where restricted by topographical conditions or where the flushing head cannot meet the requirements, it may be moved downstream along the water diversion route to a suitable location, or it may be arranged underground.

5.1.4 The axis of a desilting basin should coincide with the axis of the headrace channel ahead of the basin inlet. Where there is an angle between them, measures shall be taken to ensure uniform transverse and vertical velocity distribution of the flow entering the working section of the desilting basin.

5.1.5 The main structures of a desilting basin shall be arranged so that the flow spreads uniformly into the working section of the desilting basin, and shall comply with the following requirements:

1 The headrace channel of the desilting basin shall meet the water conveyance and anti-silting requirements at the design diversion discharge; facilities for intercepting and releasing bed load should be provided at a suitable location in the headrace channel.

2 The plan layout of the diffusion section should be of the symmetrical diffusion type, with a diffusion angle on each side not greater than 12 degrees. Where the asymmetrical diffusion type is adopted, the sum of the diffusion angles on both sides should not be greater than 24 degrees. There should be no drop step at the junction between the bottom slab of this section and the bottom slab of the working section.

3 Flow distribution piers shall be arranged within the diffusion section; their position, size and direction, and the row spacing and bar spacing of the flow straightening racks, should be determined through hydraulic model tests.

4 The sill tops of the intake gates of basin chambers and basin tanks shall be level with or slightly higher than the upstream bottom slab, and should be higher than the design sediment deposition surface of the basin chambers and basin tanks. Where there are sources of debris, trash racks and cleaning facilities shall be provided at the intake gates. The gates shall meet the requirements of partial opening operation.

5 The side walls and partition walls of basin chambers shall connect smoothly with the gate piers of the corresponding basin chamber intake gates, and the slope of the upstream face of the side walls and partition walls should not be gentler than 1:0.1.

6 Lateral overflow weirs may be provided as needed at suitable locations of the headrace channel, the working section or the water conveyance channel of the desilting basin, with the weir crest elevation slightly higher than the operating water level of the desilting basin. The overflow capacity shall be determined according to factors such as operating requirements.

5.1.6 The sediment releasing channel should adopt free-surface sediment releasing, and its longitudinal slope shall not be gentler than the longitudinal slope of the working section of the desilting basin. The outlet of the sediment releasing channel shall be provided with anti-scour and anti-silting measures, and sediment releasing shall remain unobstructed at the river water level of the 2-year flood return period.

5.2.1 A periodic flushing desilting basin shall be provided with basin chamber intake gates; where basin tanks are provided within a basin chamber, basin tank intake gates shall be provided, and the number of openings of the intake gates shall be consistent with the number of basin tanks. The partition walls between basin tanks shall connect with the gate piers of the corresponding basin tank intake gates, and the wall top shall be 0.5 m higher than the flushing water surface elevation above the design sediment deposition surface.

5.2.2 Lateral and transverse overflow weirs and collecting troughs may be provided at the tail of the working section of the desilting basin. The water depth over the overflow weir crest should be less than 0.2 m, and both the overflow weirs and the collecting troughs shall meet free outflow conditions. The cross sections of the lateral and transverse collecting troughs shall be determined by hydraulic calculation.

5.2.3 Sediment releasing orifices should be provided at the bottom of the transverse wall at the end of the desilting basin; the discharge through the sediment releasing orifices should be 5% to 8% of the working flow. There should be not fewer than 2 sediment releasing orifices in each basin tank.

5.2.4 Flushing gates shall be provided at the end of the working section of the desilting basin, and the gates and hoists of the flushing gates shall meet the requirements of partial opening operation.

5.2.5 An outlet gate shall be provided on the downstream water conveyance channel of the desilting basin.

5.3.1 The flushing system of the desilting basin shall be provided with a number of branch galleries and a main gallery.

5.3.2 The bottom of the working section of the desilting basin may be formed into a number of inverted trapezoidal troughs across the width of the basin, and the angle between the trough wall and the horizontal should be greater than the underwater angle of repose of the sediment.

5.3.3 The branch galleries shall be arranged along the flow direction beneath the trapezoidal troughs of the desilting basin, and the bottom slabs of the trapezoidal troughs shall be provided with sediment inlet orifices connected to the flushing branch galleries. The branch galleries should each join a main gallery. Several flushing systems should be provided within the working section of the desilting basin, and the branch gallery length of a front flushing system should be shorter than that of a rear flushing system.

5.3.4 The main gallery and branch galleries shall meet the hydraulic requirements for sediment releasing, and anti-abrasion measures shall be taken. The outlet of the main flushing gallery shall be provided with anti-scour and anti-silting measures, and unobstructed sediment releasing shall be ensured.

5.3.5 A single-chamber continuous flushing desilting basin should be provided with an emergency flushing gate at the end of the working section of the desilting basin. The gate and hoist of the flushing gate shall meet the requirements of partial opening operation.

5.3.6 The downstream transition section of the desilting basin should be of the gradually contracting type.

5.4.1 A desilting channel may be arranged in the lake type, strip type or shuttle type according to topographical conditions.

5.4.2 Control gates should be provided at the inlet and outlet of a desilting channel, arranged in upper, middle and lower segments.

5.4.3 The slopes of the enclosing embankments and dividing embankments of a desilting channel shall be determined according to factors such as the characteristics of the embankment filling material and the scale of the channel.

5.4.4 The embankment crest elevation shall be determined as the water surface elevation plus the wave run-up height and the safety freeboard.

5.4.5 The embankment crest width shall meet traffic and protection requirements.

5.4.6 Effective seepage prevention, seepage cutoff and seepage drainage measures shall be taken for the layout of a desilting channel.

6 Hydraulic Design and Main Dimension Calculation

6.1.1 The main hydraulic section dimensions of a desilting basin shall be determined by drawing up different schemes according to topographical and geological conditions and making a technical and economic comparison.

6.1.2 A periodic flushing desilting basin shall mainly comprise the upstream transition section, the working section with its overflow weir area, and the water collecting and sediment releasing systems.

6.1.3 A continuous flushing desilting basin shall mainly comprise the upstream transition section, the working section, the downstream transition section and the sediment releasing gallery system.

6.1.4 The hydraulic calculation of a desilting basin should include the following:

1 Discharge capacity calculation.

2 Water surface profile calculation.

3 Sediment settling rate calculation.

4 Sediment deposition calculation.

5 Flushing hydraulic calculation.

6.1.5 The sediment settling calculation shall comply with the provisions of Appendix A of this code.

6.1.6 A desilting basin with complex hydraulic conditions should be verified by model tests.

6.2.1 The hydraulic design of the headrace channel and the upstream transition section shall make the flow in the channel smooth and stable, with water surface fluctuations gradually decreasing, and shall avoid backflow and vortices.

6.2.2 The design velocity of the headrace channel transition section shall be greater than the non-silting velocity of the suspended load and less than the non-scouring velocity of the channel.

6.3.1 The dimensions of the working section of a desilting basin shall be determined comprehensively according to factors such as discharge, sediment settling velocity, sediment deposition volume, deposition time, critical flushing velocity, flushing period and flushing mode.

6.3.2 The inlet of the working section of a periodic flushing desilting basin shall have sufficient working depth, taking into account the effect of sediment deposition during operation. The water depth at the inlet of the working section should not be excessive. The working depth and the inlet water depth may be calculated using the following formulas:

He = H - delta Hk (6.3.2-1)

H not greater than delta Z + q / vc - (i multiplied by Lw + i0 multiplied by L0) (6.3.2-2)

Where: He is the working depth at the inlet of the working section (m);

H is the water depth at the inlet of the working section (m);

delta Hk is the design sediment deposition thickness at the inlet of the working section (m), which may be taken as 25% H to 30% H when drawing up preliminary schemes;

delta Z is the difference between the water level at the inlet of the working section and the water level of the natural river channel at the outlet of the sediment releasing channel (m);

q is the unit-width flushing discharge of the sediment releasing channel [m3/(s.m)];

vc is the flushing velocity at the outlet of the sediment releasing channel (m/s);

i is the bottom slope of the working section of the desilting basin;

Lw is the length of the working section of the desilting basin (m);

i0 is the bottom slope of the sediment releasing channel;

L0 is the length of the sediment releasing channel (m).

6.3.3 The working depth at the inlet of the working section of a continuous flushing desilting basin shall ensure a sufficient flushing velocity and may be calculated using the following formula:

He not greater than delta Z1 - (1 + sum of xi) multiplied by vc squared / (2g) - vc squared multiplied by the integral from 0 to L of dl / (C squared multiplied by R) (6.3.3)

Where: delta Z1 is the height difference between the operating water level of the desilting basin and the top of the gallery outlet (m);

sum of xi is the sum of the local head loss coefficients;

L is the total length of the branch galleries and the main gallery (m);

g is the gravitational acceleration (m/s2);

C is the Chezy coefficient;

R is the hydraulic radius (m).

6.3.4 The water depth of a desilting channel should be 2.0 m to 3.5 m.

6.3.5 The single-chamber working section width of a hydraulic flushing desilting basin is determined by the discharge, water depth and flow velocity, and may be calculated using the following formula:

B = Q / (Hw multiplied by v) (6.3.5)

Where: B is the working width (m);

Q is the working flow (m3/s);

v is the mean flow velocity in the working section (m/s); when drawing up preliminary schemes, where the minimum settling particle size is 0.05 mm to 0.10 mm its value may be taken as 0.05 m/s to 0.15 m/s; where the minimum settling particle size is 0.25 mm its value may be taken as 0.25 m/s to 0.55 m/s; where the minimum settling particle size is 0.35 mm its value may be taken as 0.40 m/s to 0.80 m/s;

Hw is the mean working depth (m).

6.3.6 The ratio of width to depth of the single-chamber working section of a hydraulic flushing desilting basin should not be greater than 4.5.

6.3.7 The basin tank width of the working section of a periodic flushing desilting basin may be calculated using the following formula:

bs = Q's / qs (6.3.7)

Where: bs is the basin tank width of the working section of the periodic flushing desilting basin (m);

Q's is the basin tank flushing discharge (m3/s);

qs is the unit-width flushing discharge of the basin tank [m3/(s.m)].

6.3.8 The working width of a desilting channel may be preliminarily sized using Formula (6.3.5) of this code; where the allowable particle size of sediment leaving the basin is not greater than 0.05 mm, v may be taken as 0.20 m/s to 0.40 m/s.

6.3.9 The determination of the working length of a desilting basin shall comply with the following requirements:

1 Several main dimension schemes shall be drawn up, and the working length shall be determined by sediment settling calculation in accordance with Appendix A of this code.

2 The calculation of the settling velocity of suspended sediment shall comply with the provisions of Appendix B of this code.

3 The design length of the working section of the desilting basin should be taken as 1.2 times the calculated length.

6.3.10 For a periodic flushing desilting basin provided with lateral overflow weirs, the sediment concentration by particle size group leaving the basin may be calculated from the vertical distribution of sediment concentration in front of the weirs in the overflow weir area at the tail of the working section. The total length of the lateral overflow weirs of each basin chamber may be calculated using the following formula:

Le = (1 / beta) multiplied by [Qf / (m multiplied by the square root of 2g multiplied by hf to the power 3/2) - bf] (6.3.10)

Where: Le is the total length of the lateral overflow weirs of the basin chamber (m);

Qf is the diversion discharge of the basin chamber (m3/s);

bf is the width of the transverse front weir of the basin chamber (m);

hf is the mean water depth over the weir crest (m);

m is the discharge coefficient;

beta is the lateral influence coefficient of the overflow weir.

6.3.11 The sediment concentration by particle size group at the exit section of a desilting channel at the end of each period may be calculated in accordance with the relevant provisions of Section A.2 of this code. Where the particle size gradation and sediment concentration at the exit section approach the upper limit of the design criterion, the cumulative operating time may be taken as the service life of the desilting channel.

6.3.12 The working section of a periodic flushing desilting basin shall have a longitudinal bottom slope that ensures the velocity required for the flushing flow; the longitudinal bottom slope may be calculated using the following formula:

i not less than v1c squared / (C squared multiplied by R) (6.3.12)

Where: v1c is the flushing velocity in the working section (m/s).

6.4.1 For pressurized water diversion, the water depth of the transition section and the water conveyance channel shall meet the minimum submergence depth requirement of the pressurized intake, so as to avoid vertical-axis vortices and air entrainment.

6.4.2 The maximum water level of the transition section and the water conveyance channel shall be determined as the maximum surge water level when the hydropower station suddenly rejects its full load during normal operation at the design discharge.

6.4.3 Where overflow weirs are provided for the desilting basin, energy dissipation facilities shall be provided for the overflowing water.