NB/T 10392-2020Guide for design of energy dissipation and erosion control for water release structures of 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 P 59.

This guide has been prepared in accordance with the requirements of the Notice of the National Energy Administration on Issuing the First Batch of Formulation (Revision) Plans for Energy Sector Industry Standards in 2009 (Guo Neng Ke Ji [2009] No. 163). The drafting group carried out extensive investigation and research, carefully summarized practical experience, and prepared this guide on the basis of wide solicitation of opinions.

The main technical contents of this guide are: basic requirements; type and arrangement of energy dissipation and erosion control; design for hydraulic jump energy dissipation and erosion control; design for ski-jump energy dissipation and erosion control; design for surface flow and roller bucket type flow energy dissipation and erosion control; design for inside-tunnel energy dissipation and erosion control; downstream protection design; hydraulic model test and hydraulic numerical simulation; safety monitoring design; and operation and maintenance requirements.

This guide is under the administration of the National Energy Administration. It was proposed by, and is under the routine administration of, China Renewable Energy Engineering Institute. The Standardization Technical Committee of Hydropower Investigation and Design of the Energy Industry is responsible for the interpretation of its specific technical contents. Comments and suggestions arising in the course of implementation should be addressed to China Renewable Energy Engineering Institute (Address: No. 2 Liupukang Beixiaojie, Xicheng District, Beijing; Postcode: 100120).

Chief editing organization of this guide: PowerChina Kunming Engineering Corporation Limited.

Participating organizations of this guide: PowerChina Chengdu Engineering Corporation Limited; PowerChina Beijing Engineering Corporation Limited; PowerChina Northwest Engineering Corporation Limited; PowerChina Zhongnan Engineering Corporation Limited; PowerChina Guiyang Engineering Corporation Limited; PowerChina Huadong Engineering Corporation Limited.

Chief drafting staff of this guide: Zhang Zongliang, Yang Zaihong, Liu Shaochuan, You Xiang, Kong Caifen, He Qiyong, Zhang Shaochun, Luo Yongqin, Zhang Libing, Yang Jijian, Ou Hongguang, Li Yannong, He Wei, Chen Ruihua, Yang Hua, Zhao Shiming, Meng Fuqiang, Liu Lijuan, Lin Jianyong, Peng Chengjia, Yao Yuancheng, Xue Wenqiang, Lu Chuanyin, Zhou Heng.

Chief reviewing staff of this guide: Dang Lincai, Su Liqun, Wang Yiming, Wang Ke, Liu Zhanping, Zhao Hongmin, Ning Huawan, Zhao Yonggang, Huang Tairen, Qiu Huanfeng, Yang Huaide, Li Yuetao, Peng Yu, Dai Xiaobing, Liao Renqiang, Wu Wenping, Huang Guobing, Wu Weiwei, Piao Cen, Zhao Tie, Du Gang, Li Shisheng.

1 Scope

NB/T 10392-2020 is the Chinese design guide for energy dissipation and erosion control at the water release structures of hydropower projects. When a high dam discharges, the water arriving downstream carries enough energy to scour the riverbed and undermine the works, and the whole art is to spend that energy where it can do no harm. The guide covers the full family of dissipation arrangements: hydraulic jump stilling basins with their baffle blocks and end sills, ski-jump and flip bucket dissipation with the trajectory and plunge pool that follow, submerged jet and surface flow regimes, slotted and differential buckets, plunge pools with and without a lining, and the internal dissipation used inside outlet tunnels. For each it sets the hydraulic design method, the geometry, the flow conditions across the discharge range, and the criteria for judging that dissipation is adequate. Erosion control is treated alongside: the estimation of scour depth in rock and in alluvium, the protection of the riverbed and the banks, the stability of the downstream channel, and the interaction between adjacent outlets discharging together. Cavitation and aeration, the abrasion caused by sediment, and the vibration induced in the structure are addressed as design constraints. Hydraulic model testing, prototype observation and the monitoring expected in service close the document. It applies to the design of spillways, outlets and discharge tunnels on hydropower projects in China.

1.0.1 This guide is formulated in order to unify the requirements for the arrangement of energy dissipation and erosion control works, the configuration design, the hydraulic design, the structural design and the operation management of water release structures of hydropower projects, so as to achieve operational safety, technical advancement and economic rationality.

1.0.2 This guide is applicable to the design of energy dissipation and erosion control for water release structures of hydropower projects.

1.0.3 For small hydropower projects, the design of energy dissipation and erosion control of water release structures may be carried out in a simplified way in accordance with this guide.

1.0.4 For hydropower projects in plain areas, the design of energy dissipation and erosion control of water release structures may be carried out in accordance with the relevant provisions of the current industry standard Design Code for Sluice SL 265.

1.0.5 In addition to complying with this guide, the design of energy dissipation and erosion control for water release structures of hydropower projects shall also comply with the provisions of the relevant current national standards of China.

2 Terms

2.0.1 water release structure. A hydraulic structure provided for discharging flood water or waterlogging water in excess of the storage or carrying capacity of a reservoir, river course, canal or waterlogged area, and for releasing the water stored in a reservoir or canal in order to facilitate safety protection or inspection and maintenance.

2.0.2 hydraulic structure of energy dissipation and erosion control. A hydraulic structure arranged along the water release structure or at its outlet so that the high-velocity discharge flow is well connected with the downstream river channel, forming local hydraulic energy dissipation flow patterns such as a jet nappe, a hydraulic jump, a roller, sharp contraction and expansion, or spiral flow, thereby dissipating the energy of the discharge flow; it is also a hydraulic structure that prevents harmful scouring and undermining of the downstream riverbed and bank slopes.

2.0.3 stilling basin. An energy dissipation facility located downstream of a dam body, a sluice or a water release structure, composed of side walls, a baseplate, an end sill, an apron and the like, and set out according to the requirements of hydraulic jump energy dissipation.

2.0.4 plunge pool. A structure provided downstream of a dam body or a spillway to form a sufficient water area and water depth so as to satisfy the requirements of ski-jump and drop energy dissipation.

2.0.5 combined energy dissipation. An arrangement type of energy dissipation and erosion control in which several energy dissipation works are used simultaneously at one single water release structure, or two or more water release structures are operated jointly so that the discharge flows collide with, impact upon and shear one another, intensifying the turbulence of the water body and the dissipation of energy, thus further raising the energy dissipation ratio, reducing the scale of the energy dissipation works and mitigating downstream scouring.

2.0.6 outlet energy dissipation. An arrangement type in which hydraulic jump, ski-jump, surface flow or similar means are adopted at the outlet or the end of a water release structure to dissipate the energy of the discharge flow.

2.0.7 inside-tunnel energy dissipation. An arrangement type in which hole plugs, orifice plates, swirl devices or similar means are provided inside a flood discharge tunnel to dissipate the energy of the discharge flow.

2.0.8 hydraulic jump energy dissipation. An energy dissipation mode in which a hydraulic jump is used to dissipate the residual energy of the supercritical flow discharged along the bottom of the water release structure, transforming the supercritical flow into subcritical flow that connects with the downstream flow.

2.0.9 ski-jump energy dissipation. An energy dissipation mode in which a flip bucket is provided at the outflow section of a water release structure so that the discharged supercritical flow is thrown into the air, forming an aerated jet that falls into the downstream water cushion.

2.0.10 surface flow energy dissipation. An energy dissipation mode in which the drop sill or the roller bucket at the end of a water release structure is used to lift the main body of the descending supercritical flow to the water surface, and the residual energy is removed through the spreading of the main flow at the surface together with the bottom roller and the surface roller.

2.0.11 bucket-type flow energy dissipation. An energy dissipation mode in which a roller bucket is provided at the end of a water release structure to lift the discharge flow to the water surface, and the energy of the discharge flow is dissipated by the roller inside the bucket, the bottom roller and the surface roller.

2.0.12 end-flared pier. An overflow dam sluice pier whose rear part is widened into a fishtail shape.

2.0.13 auxiliary energy dissipator. An auxiliary structure used in combination with the main energy dissipation works, which partly changes the flow regime or the flow pattern of the discharge flow and intensifies the turbulence and shearing of the flow, thus further raising the energy dissipation ratio.

2.0.14 drop energy dissipation. An energy dissipation mode in which the flow falls from the outlet of a water release structure down into a water cushion.

2.0.15 shaft swirl energy dissipation. An inside-tunnel energy dissipation type in which a volute chamber makes the flow form a downward spiral motion closely attached to the wall surface inside a shaft, entraining air and falling into the deep water cushion at the bottom of the shaft.

2.0.16 horizontal vortex energy dissipation. An energy dissipation type in which a starting chamber at the bottom of a pressurized flow shaft makes the discharge flow generate a spiral motion closely attached to the wall surface inside a horizontal vortex tunnel, the energy of the discharge flow being dissipated through a plunge pool.

2.0.17 orifice plate energy dissipation. An energy dissipation type in which several stages of orifice plates are provided inside a pressurized flood discharge tunnel to locally contract the tunnel section, the energy of the discharge flow being dissipated by the sudden contraction and sudden expansion of the flow.

2.0.18 hole plug energy dissipation. An energy dissipation type in which one or several stages of hole plugs are provided inside a pressurized flood discharge tunnel, so that the sudden contraction and sudden expansion of the section when the flow passes through the hole plugs cause intense turbulence of the flow and dissipate the energy of the discharge flow.

3 Basic Requirements

3.0.1 The design of energy dissipation and erosion control for water release structures of hydropower projects shall include the arrangement, configuration design, hydraulic design, structural design and safety monitoring design of the energy dissipation and erosion control structures and of the protection works for the downstream river channel and bank slopes.

3.0.2 For the design of energy dissipation and erosion control for water release structures of hydropower projects, the following data shall be collected and analyzed:

3.0.2-1 Natural conditions such as meteorology, hydrology, sediment, topography and geology.

3.0.2-2 Requirements concerning reservoir operation, the social and ecological environment, flood control and navigation.

3.0.3 The structure grade and the flood standard of the energy dissipation and erosion control structures shall comply with the relevant provisions of the current industry standard Standard for Rank Classification and Flood Standard of Hydropower Projects DL 5180.

3.0.4 The hydraulic design of the energy dissipation and erosion control structures shall meet the following requirements:

3.0.4-1 Under design flood operation there shall be a stable energy dissipation flow regime and a good energy dissipation effect; under frequent flood operation there shall in addition be good hydrodynamic characteristics.

3.0.4-2 The outflow shall be coordinated with the requirements of the adjacent structures of the project complex and with those of downstream protection, navigation and the like, and shall not affect the normal operation of the other structures of the project complex or of important downstream facilities.

3.0.4-3 The conditions for inspection and repair during the operation period should be taken into account.

3.0.4-4 Where the hydraulic conditions are complex or new types of energy dissipation works are adopted, the preliminary hydraulic design may be carried out on the basis of the results of hydraulic numerical simulation and analysis.

3.0.5 The structural design of the energy dissipation and erosion control structures shall meet the following requirements:

3.0.5-1 The structural configuration, the material properties, the foundation anchorage and the foundation treatment shall be designed according to the hydraulic characteristics and the requirements for uplift resistance and sliding stability.

3.0.5-2 For concrete structures the spacing of the structural joints should be increased. Reliable closed water stops shall be provided on the faces of the structural joints, and keyways should be provided in the joints of the baseplate structure. Clear technical requirements shall be laid down for the treatment of construction joints.

3.0.5-3 Temperature control and crack prevention design should be carried out for concrete structures.

3.0.5-4 Seepage control and drainage measures shall be adopted according to the engineering geology, the hydrogeological conditions and the requirements of operation, inspection and repair. A closed pumping and drainage system should be provided for the stilling basin or the plunge pool behind the dam.

3.0.5-5 The standard value of the anchoring force of the foundation anchor bars shall be calculated according to the relevant provisions on the effective weight of the anchored foundation given in the current industry standard Design Code for Spillway DL/T 5166, and the sectional area of the anchor bar body and the length of the anchoring section shall be checked in accordance with the relevant provisions of the current national standard Technical Code for Engineering of Ground Anchoring and Shotcrete Support GB 50086.

3.0.5-6 Foundation treatment measures shall be proposed for the foundation rock mass.

3.0.6 The design for abrasion resistance and cavitation erosion prevention of the energy dissipation and erosion control structures shall take comprehensive account of the sediment characteristics and the hydraulic characteristics of the project, the structure of the works, the raw materials of the concrete, the requirements for temperature control and crack prevention, the construction technology and other factors, and shall comply with the following provisions:

3.0.6-1 The configuration of the structure shall be reasonably determined, and aeration facilities for reducing cavitation erosion shall be provided.

3.0.6-2 The surface irregularity of the flow passage face shall comply with the relevant provisions of the current industry standard Design Code for Spillway DL/T 5166.

3.0.6-3 The strength grade of the abrasion-resistant and cavitation-resistant concrete shall be determined according to the flow velocity; it shall not be lower than C30, but should not be higher than C50.

3.0.6-4 The concrete of the baseplate of the energy dissipation works should be placed continuously. Where abrasion-resistant concrete is provided as the surface layer of the baseplate, it shall be placed continuously with the concrete beneath it, without layering.

3.0.6-5 Where new types of energy dissipation works are adopted, a reduced-pressure model test should be carried out.

4 Type and Arrangement of Energy Dissipation and Erosion Control

4.1 General Requirements

4.1.1 The overall arrangement of energy dissipation and erosion control for water release structures shall be determined by a comprehensive technical and economic comparison, according to the requirements of the development tasks of the project, the reservoir operation and the environmental conditions, and taking comprehensive account of the topographic and geological conditions, the layout of the project complex, the connection of the outlet flow, the downstream scouring resistance capacity, the bank slope stability, the construction conditions, the operation and maintenance, the project investment and other factors.

4.1.2 For large and medium-sized hydropower projects with complex hydraulic conditions, and for water release structures using combined energy dissipation or new types of energy dissipation works, a hydraulic model test shall be carried out for the arrangement of energy dissipation and erosion control.

4.2 Selection of Energy Dissipation Types

4.2.1 According to the type of the water release structure and the position where the energy dissipation works are located, the energy dissipation type may be either outlet energy dissipation or inside-tunnel energy dissipation.

4.2.2 Outlet energy dissipation may adopt hydraulic jump, ski-jump, drop, surface flow, bucket-type flow and other energy dissipation modes; the specific selection should meet the following requirements:

4.2.2-1 Where there are navigation requirements downstream, where unfavourable geological bodies exist, or where atomization-sensitive factors are present, hydraulic jump energy dissipation should be adopted.

4.2.2-2 For high head and medium head cases in which the scour hole in the outlet energy dissipation zone does not affect the safety of the structures and of the bank slopes, ski-jump energy dissipation should be adopted.

4.2.2-3 Where the downstream tailwater is comparatively deep and the water level variation is comparatively small, surface flow or bucket-type flow energy dissipation may be adopted.

4.2.3 Where hydraulic jump energy dissipation is adopted as the energy dissipation mode, the following provisions shall be complied with:

4.2.3-1 When the height of a gravity dam is greater than 100 m, the provision of auxiliary energy dissipation works such as end-flared piers, stepped overflow faces and drop sills should be studied.

4.2.3-2 When the dam height exceeds 150 m or the flow velocity entering the basin is greater than 40 m/s, the adoption of hydraulic jump energy dissipation shall be subject to a special study and demonstration.

4.2.4 Where ski-jump energy dissipation is adopted as the energy dissipation mode, the following requirements shall be met:

4.2.4-1 The arrangement type and the configuration parameters of the flip bucket shall be determined according to the topography and geological conditions of the downstream river valley and to their suitability, so as to disperse the landing points of the jet nappe; the foundation of the structures and the toe of the bank slopes shall not be undermined.

4.2.4-2 Where the scour hole affects the safety of the structures and of the bank slopes, the adoption of engineering measures such as a plunge pool, a secondary dam, bank protection and bottom protection shall be studied.

4.2.5 Inside-tunnel energy dissipation may adopt swirl, orifice plate, hole plug and other energy dissipation types; the specific selection shall meet the following requirements:

4.2.5-1 For swirl energy dissipation, the downstream tunnel section should be connected by free-surface flow.

4.2.5-2 Orifice plate and hole plug inside-tunnel energy dissipation works shall be arranged in the pressurized flow section of the tunnel.

4.2.6 Where the discharge flow rate is comparatively large, the head comparatively high or the river valley narrow, so that a single energy dissipation type is difficult to arrange or the energy dissipation effect is not satisfactory, a combined energy dissipation arrangement type may be adopted, and the following requirements shall be met:

4.2.6-1 For one and the same water release structure, auxiliary energy dissipation works may be added to raise the energy dissipation ratio, or a type combining two or more energy dissipation modes may be adopted.

4.2.6-2 For different water release structures, the outflows may be made to collide with, impact upon and shear one another so as to increase the energy dissipation ratio.

4.2.6-3 A dispersed arrangement with zoned energy dissipation may be adopted.

4.2.6-4 Where hydraulic jump or bucket-type flow energy dissipation is adopted for an overflow dam, end-flared piers may be arranged on the sluice piers and steps may be arranged on the overflow face.

4.2.6-5 Where ski-jump or drop energy dissipation is adopted for the surface orifices of an arch dam spillway, they may be operated jointly with the ski-jump flows of the middle orifices and bottom orifices, forming a combined energy dissipation type with left-right opposed impact and upper-lower collision; however, the influence of flood discharge atomization on the other structures shall be fully taken into account. Where downstream atomization control factors exist, a laterally contracted, non-colliding type may be adopted.

4.3 Downstream Protection

4.3.1 The protection extent and the protection type for the downstream riverbed and bank slopes at the outlet of the energy dissipation works shall be determined by analysis according to the topographic and geological conditions, the energy dissipation mode, the hydraulic conditions of the river channel, or the protection requirements of the downstream objects affected.

4.3.2 Where ski-jump energy dissipation is adopted, the adverse effects of flood discharge atomization on the surrounding environment, on bank slope stability, on traffic conditions and on the operation of the other structures shall be analyzed and evaluated, and protective measures shall be taken.

5 Design for Hydraulic Jump Energy Dissipation and Erosion Control

5.1 General Requirements

5.1.1 The design for hydraulic jump energy dissipation and erosion control shall include the following contents:

5.1.1-1 The configuration design of the stilling basin and of the auxiliary energy dissipation works.

5.1.1-2 The hydraulic design of the stilling basin with respect to the hydraulic jump pattern, the water surface, the flow velocity, the hydrodynamic pressure and the like.

5.1.1-3 The structural design of the energy dissipation works, including the material properties and temperature control of the concrete, the structural joints and water stops, the uplift stability of the baseplate, the overall stability of the side walls, the foundation anchorage, the structural reinforcement, the foundation treatment, and seepage control and drainage.

5.1.2 The types of auxiliary energy dissipation works such as end-flared piers, stepped overflow faces, drop sills and small flip buckets shall be determined by comprehensive analysis according to the flood discharge head, the discharge per unit width, the downstream water depth and factors such as abrasion resistance and cavitation erosion prevention.

5.2 Configuration Design

5.2.1 The configuration design of the stilling basin should meet the following requirements:

5.2.1-1 The plan of the stilling basin should be symmetrical, straight and of equal width. Where the discharge per unit width entering the basin is comparatively large, the scouring resistance of the bedrock comparatively poor, or the downstream water depth comparatively shallow, a gradually expanding design may also be adopted.

5.2.1-2 The longitudinal profile of a rectangular stilling basin may be determined according to the results of the hydraulic calculation for hydraulic jump energy dissipation; that calculation should comply with the relevant provisions of Appendix A of this guide. Where the downstream water depth is insufficient, measures such as excavating deeper or providing an end sill may be adopted to increase the water depth inside the basin.

5.2.1-3 The cross section of the stilling basin should be a rectangular or trapezoidal section, determined in combination with the bank slope excavation and the configuration requirements of the side wall structure.

5.2.2 The configuration design of the end-flared piers shall meet the following requirements:

5.2.2-1 The configuration parameters of the end-flared piers may be preliminarily drawn up according to the relevant provisions of Appendix B of this guide and to engineering experience, and shall finally be determined by a hydraulic model test.

5.2.2-2 The configuration of the end-flared piers shall be coordinated with the arrangement of the sluice chamber section of the overflow dam; it shall not affect the discharge capacity nor obstruct the arrangement of the hinged supports of the radial gates.

5.2.2-3 An asymmetric type should be adopted for the configuration of the end-flared piers of the side orifices, and the contracted nappe shall not impinge on the downstream side walls.

5.2.3 Where a stepped overflow face is used in combination with end-flared piers, the following provisions shall be complied with:

5.2.3-1 An aeration cavity should be formed by the nappe issuing from the bottom of the end-flared piers; a small flip bucket may be provided in front of the first step of the stepped overflow face, and the height of the first few steps should be 1.5 m to 2.0 m.

5.2.3-2 The lower part of the stepped overflow face should be connected with the baseplate of the stilling basin by a circular arc or a small flip angle.