5.5 Water Conveyance Structure
5.5.1 The types of diversion structures shall be determined after techno-economic comparison of the development mode of the hydropower station，operational requirement，topographical and geological conditions and types of water retaining structures，with consideration of general layout of the project and construction conditions.
5.5.2 Design of intake shall meet the following requirements:
1 Under various operating water levels，the water flow is smooth with steady flow pattern and uniform inflow，and meets the requirements for diversion flow.
2 Air-entraining funnel vortices shall be avoided.
3 If water drawing or safe operation of units is impacted by sedimentation，sediment prevention and sediment scouring facilities shall be installed.
4 On rivers with plenty of floating debris，debris prevention and discharge facilities shall be installed；and in frigid regions，ice prevention and discharge facilities shall be installed.
5.5.3 The open-style intake on river bank should be located on a stable river reach.For a sediment-laden river，the intake should be located downstream of the pointed end of the concaved bank on a curved course；for a river with heavy floating debris or ices，the intake should be located on a straight river reach.The elevation of intake base slab should be 1.0m higher than the base slab of scour sluice and the inlet of scour gallery.
5.5.4 The elevation of the base slab of a submerged intake shall be higher than the balanced scouring and silting elevation of the reservoir at the front edge of the inlet.The submerged depth of its top edge under the minimum upstream operation water level shall meet the requirement that no air-entraining funnel vortex or negative pressure occurs at the intake，and shall not be less than 1.0m.
5.5.5 The sand trap in front of the open-style intake shall meet the requirement for water inflow，while its height shall be determined by integrating the submerged depth of intake，the sediment in river course and the layout of scour structure，which should not be less than 1.5m，or be around 50% of the water depth in the scour channel.The angle between the front edge of the sand trap and that of the scour sluice should be 105°-110°.
5.5.6 If water is diverted by using a grille-type bottom，the grille bars shall be set along the flow direction.The bar spacing should be 1-1.5cm，and the bar should have a trapezoidal cross-section with its width being 1.2-2.0cm.
5.5.7 For the intake，hydraulic calculations such as water head loss，drainage flow，area of air vent at the pressure intake，and water hammer pressure at pipes upstream of the shaft intake shall be conducted.
5.5.8 The intake structures shall meet the requirements for stability，strength，stiffness and durability，and the following calculations shall be conducted according to the types of structures:
1 Integral stability of intake against sliding and floating.
2 Stress of dam-type intake openings.
3 Structural strength and stiffness of tower base and tower body of tower intake and bank-tower intake，and structural strength of open-style intake chambers.
4 Structural strength of shafts for intake with inclined gates on bank or shaft intake.
5.5.9 In the selection of the diversion tunnel route，the following requirements shall be met:
1 The tunnel route should be straight，the turning radius should not be less than 5 times the tunnel diameter（or width），the turning angle should be less than 60°，and straight sections should be placed at both the start and end of any bend section and their length should be greater than 5 times the tunnel diameter（or width）.
2 The intake and outlet should be located at places with simple geological structures，stable slopes，shallow weathering or overburden.Excavation on steep and high banks shall be avoided.
3 The tunnel axis and rock strata should intersect at a relatively larger angle，so do the tectonic fracture plane and the strike of major weak zone.For an integral block-structured or thick-stratum rock mass，the intersection angle should not be smaller than 30°；for a thin layered rock mass，the angle should not be less than 45°.Heavily fractured zone，weak structured plane and areas with plenty of groundwater should be avoided，and engineering measures shall be taken if unavoidable.
4 The thickness of rock mass between two neighboring tunnels should not be less than 2 times the tunnel diameter（or width）.The thickness may be reduced if the rock mass is good，but shall not be less than tunnel diameter（or tunnel width）.
5 The setup shall be favorable for the layout of adits.
5.5.10 The minimum thickness of vertical and lateral rock masses of a diversion tunnel shall be determined after comprehensive analysis of geological conditions，shape and dimensions of tunnel crosssection，engineering or tunneling conditions，internal water pressure，lining type，and permeability characteristics of surrounding rocks，and shall meet the following requirements:
1 For the intake and outlet of tunnel and the trunk of free-flow tunnel，under the prerequisite that the safety during construction and operation could be guaranteed with reasonable construction methods and engineering measures，there is no particular requirement for the minimum thickness of vertical and lateral rock masses.
2 For a pressure tunnel，the thickness of vertical and lateral rock masses in the tunnel may be controlled at a water head not less than 40% of the internal water pressure if the surrounding rock is sound without any unfavorable structural plane and is applied with concrete or reinforced concrete lining；the thickness may be controlled at a water head not less than the internal water pressure if the rock is not lined or lined with anchoring and shotcreting concrete.
5.5.11 The longitudinal slope of diversion tunnel shall be determined based on comprehensive analysis of operational requirements，connection between the upstream side and downstream side，bottom elevations of structures along the axis，construction conditions，and overhaul conditions，etc.Neither horizontal nor adverse slopes should be set along the route.
5.5.12 The pressure head above the tunnel roof along the whole length of a pressure diversion tunnel shall not be less than 2.0m under the most unfavorable operating condition.
5.5.13 The cross-section of a diversion tunnel shall be designed in accordance with the following requirements:
1 Pressure tunnels should be circular.The cross-sectional dimension shall be determined based on comprehensive analysis and comparison of investments in tunnel project and electrical energy losses.The minimum inner diameter of the tunnel should not be less than 1.8m.For small-sectional tunnel with relatively good geological conditions，other types of cross-sections may be taken as well.
2 For a free-flow tunnel，a circular-arch upright-wall section or a horseshoe section should be adopted.The central angle of the circular arch sector at the circular-arch upright-wall section may be 90°-180°，and the ratio of height to width may be 1-1.5.The tunnel width should not be less than 1.5m，and the tunnel height should not be less than 1.8m.Under steady flow conditions，the space above the water surface level in the tunnel should have an area not less than 15% of the cross-sectional area of the tunnel，and its height should not be less than 0.4m；under unsteady flow conditions，the above-mentioned figures may be reduced.
5.5.14 Hydraulic calculation shall be conducted for diversion tunnels，including flow capacity，connection of upstream and downstream flows，water head loss，water hammer pressure，hydraulic grade line and flow profile，etc.
5.5.15 According to the strength，integrity and permeability of the surrounding rocks，the diversion tunnel may be applied with shotcrete-anchorage lining，concrete lining，reinforced concrete lining or steel plate lining.
5.5.16 The strength of concrete and reinforced concrete lining in diversion tunnels shall not be lower than grade C20.The thickness of monolayer reinforced concrete lining should not be less than 25cm，while the thickness of double-layer reinforced concrete lining should not be less than 30cm.The allowable maximum width of cracks shall not exceed 0.30mm，which should not exceed 0.25mm if water is corrosive.
5.5.17 For diversion tunnels applied with shotcrete-bolt lining，the allowable flow velocity in the tunnel should not be higher than 8m/s.The shotcrete thickness should be 8-12cm if no mesh reinforcement is applied，and be 10-25cm if there is mesh reinforcement.The shotcrete strength shall not be lower than Grade C25.
5.5.18 In diversion tunnels，backfill grouting shall be applied to the top of concrete and reinforced concrete lining.
The grouting scope，hole spacing，row distance，grouting pressure and grout concentration shall be determined after analyzing the types of lining structure，work conditions in the tunnel and construction method，etc.The grouting holes shall have a depth of more than 5cm into the surrounding rock.For sections with poor geological conditions，consolidation grouting should be applied to deal with it.The parameters of consolidation grouting may be determined after engineering analogy or on-site testing.
5.5.19 For earth-rock fill dams，within-dam embedded pipes should not be used for water diversion.If conditions permit，the pipes embedded in the strengthened defective earth-rock fill dams shall be plugged，while diversion tunnels are constructed in the mountain on the bank.If it is absolutely necessary to arrange embedded pipes for water diversion in an earth-rock fill dam，the following requirements shall be met:
1 Pipe bedding shall be located on a uniform and hard rock foundation，or a uniform and compact soil foundation.
2 Strength and stiffness of diversion pipes shall meet the requirements.
3 Axes of diversion pipes shall be perpendicular to dam axis.
4 Expansion joints and settlement joints shall be set for diversion pipes，and the length of each joint section should be 15-20m.Two layers of water stop shall be applied within the joint section for reinforced concrete pipes，and filter shall be applied to the exterior side of joints.
5 The quality of dam filling around the diversion pipes shall meet the requirements for stable seepage of dam body and dam foundation.A cutoff collar shall be placed where the diversion pipe passes through the anti-seepage structure and the section dimensions of the anti-seepage structure shall also be enlarged.The contact between the downstream side of the anti-seepage structure and the embedded pipes shall be applied with filter while the pipes are wrapped.
6 Gates shall be installed upstream of the dam.
5.5.20 The layout of surge chamber shall be designed based on the calculation of regulation assurance of units and the analysis of operating conditions，and finalized after techno-economic comparison of the function of the hydropower station in the electric power system，the topographical and geological conditions，and the layout of penstocks，etc.
The initial judgment of conditions for setting up a surge chamber may be made according to the time constant of water flow inertia in the penstock.If the inertia is larger than the tolerance，a surge chamber shall be installed with a tolerance of 2-4s.If the hydropower station is operated in isolation or the proportion of its unit capacity in the electric power system exceeds 50%，the lower value should be adopted as the tolerance；if the proportion of its unit capacity in the electric power system is less than 20%，the higher value should be taken as the tolerance.
5.5.21 The location of the surge chamber should be close to the powerhouse and shall be determined after techno-economic comparison with consideration of topographical and geological conditions，and layout of the penstock，etc.
5.5.22 The type of surge chamber shall be determined after techno-economic comparison according to operating characteristics of the hydropower station and with consideration of topographical and geological conditions，and characteristics of various kinds of surge chambers.
5.5.23 The cross-sectional area and height of the surge chamber shall respectively meet the requirements for fluctuation stability and surge wave.
5.5.24 When the maximum surge wave in the surge chamber is calculated，the lower value of roughness of the headrace should be adopted.If the water level of reservoir is at its normal storage level，the rejection of full load when all units sharing the same surge chamber are operated at full load shall be adopted as the design operating condition；if the water level of reservoir is the check flood level，the corresponding operating condition shall be rectified.
5.5.25 When the minimum surge wave in the surge chamber is calculated，the higher value of roughness of the headrace should be adopted.If the reservoir is at dead water level，the calculation is made as that with all n units in use of the same surge chamber，the load increases from（n-1）units to n units，or all n units suddenly come on full load from 2/3 load；and the second amplitude may also be rechecked under the situation that the full load of all units is suddenly rejected.
5.5.26 When the surge level in the surge chamber is calculated，the unfavorable operating condition of possible superposition of surges shall be rechecked.If the superimposed surge wave goes beyond the maximum surge wave level or is lower than the minimum surge wave level，the operation mode may be adjusted or the cross-section dimensions of the surge chamber be revised.
5.5.27 The freeboard above the maximum level of surge wave in surge chambers should not be less than 1.0m.The safe height between the minimum surge wave level in surge chambers and the top of penstock shall not be less than 2.0m.A safe water depth of no less than 1.0m shall be kept for the base slab of surge chambers.
5.5.28 With regard to the lining of surge chamber，rockbolt mesh reinforcement shotcrete or reinforced concrete lining shall be adopted based on the types of surrounding rocks.The surrounding rocks should be reinforced by consolidation grouting.Anti-freezing measures shall also be taken in frigid regions.(https://www.daowen.com)
5.5.29 For slopes above and outside the surge chamber，stability analysis and reinforcement treatment shall be carried out.Drainage facilities should be installed at adjacent areas，and safety protection facilities shall be installed on the top of surge chamber.Thermal insulation facilities shall also be installed in frigid regions.
5.5.30 The operational requirement of surge chamber shall be determined according to upstream and downstream water levels，operating characteristics，and structures of penstocks and surge chamber，etc.
5.5.31 The selection and layout of diversion channel route shall meet the following requirements:
1 Areas with complicated geological structural features，high permeability，land collapse and landslides，（wet）subsidence and mud-rock flow should be avoided；moreover，deep-cut and high-fill quantities shall be avoided，and land occupation and house demolition shall be controlled to the minimum.
2 The channel route should be straight.If a turning is necessary，the bending radius of lined channel should not be less than 2.5 times the channel water surface width，and the bending radius of un-lined channels should not be less than 5 times the water surface width.In frigid regions，the route of channel should be set along the sunny slope，and the bending radius should not be less than 5 times the water surface width.
3 The optimum positions and types of channel structures shall be selected.
5.5.32 In the selection of the type of diversion channel，consideration shall be given to the topographical and geological conditions，operation and general layout of the project，etc.Self-regulating channel，non-selfregulating channels，or the combination of both may be selected respectively after techno-economic comparison.
5.5.33 For the hydraulic design of diversion channel，the following calculations shall be conducted:
1 Under the normal operating condition of the hydropower station，the basic size of channel and the characteristic water level of the fore bay shall be defined according to the uniform flow in an open channel，and then the water depth，flow velocity and water surface elevation at each section are derived.
2 If the hydropower station load increases suddenly，the minimum water level at the end of channel is calculated by the unsteady flow method；if the load of the units is rejected completely，the water surface profile of the self-regulating channel is derived by the unsteady flow method.
3 Hydraulic calculation of water release structures.
5.5.34 The longitudinal slope and cross-section of diversion channel shall be defined after technoeconomic analysis of topographical，geological and hydraulic conditions.In hilly regions with steep surface gradient，major undulations and low groundwater tables as well as in frigid regions，a deep，narrow section should be adopted；for channels in plain areas with high groundwater tables，strong frost heave foundation soil and requirement for multi-purpose utilization，a wide，shallow section should be adopted；for hillside channels in mountainous regions，an enclosed rectangular box-type section should be adopted.
5.5.35 For the types of water release structures on non-self-regulating channels，discharge sluice，side weir or siphon sluiceway should be adopted.Regulating sluices shall be installed on the diversion channel where there are requirements for water level control，flow regulating and water distribution.Drainage facilities shall be installed on both sides of the diversion channel；and anti-freezing measures and ice discharge facilities shall be installed in frigid regions.For a diversion channel on a sediment-laden river，de-silting and sediment releasing facilities shall be installed.
5.5.36 The flow in a non-lined diversion channel shall be kept within the unscouring and unsilting velocity.For a lined or ice-transporting channel，the velocity should be between 1.0-2.0m/s.
5.5.37 Concrete lining，mortar rubble（gravel）works or geo-textiles may be adopted for seepage prevention of diversion channels.
5.5.38 The fore bay shall be arranged to meet the following requirements:
1 The fore bay should be located away from the areas with landslide，fracture developed along a slope and high slopes.Taking into account the route of penstock and the powerhouse location，the fore bay shall be located on sound and steady foundations with small permeability.The impact of hydrological and geological changes on the stability of slopes after construction of the fore bay shall be analyzed.
2 The volume and depth of the fore bay shall meet the requirements for minor fluctuations of water level and sedimentation when the power load changes.The regulating capacity shall also meet the requirement if the fore bay serves as a regulation pond.
3 The connection part of diversion channel and fore bay should be set up symmetrically，while the expansion angle should not be larger than 12°，and the longitudinal gradient at the bottom should be less than or equal to 1∶5.
4 The minimum submerged depth of the top edge of penstock inlet shall be consistent with the specification in Article 5.5.4 of this code.The elevation of base floor at the end of fore bay shall be at least 0.5m lower than the base slab of the intake chamber.
5 Sediment releasing and emptying facilities shall be installed for the fore bay.For the type of these facilities，the flush gallery（tunnel）should be adopted.In frigid regions，ice retaining，ice diversion and ice discharging facilities shall also be installed.
6 Hydropower station intakes in the fore bay may be controlled by gates or siphon-type water abstraction.
7 For water release structures of fore bay for a hydropower station with non-self-regulating channels，a side-weir sluice way should be adopted，and its discharge capacity shall meet the requirement for the maximum flow if the load of all units of the hydropower station is rejected and it shall be ensured that the energy dissipation is safe.
5.5.39 The regulation pond shall be arranged based on the demand of the hydropower station and upon techno-economic analysis of topographical and geological conditions，etc.The layout shall meet the following requirements:
1 The location of regulation pond shall be selected based on the required regulating volume and depth of level fluctuations，and with consideration of topographical and geological conditions，and natural depressions should be made use of.
2 The layout of regulation pond shall be chosen based on topographical and geological conditions，and the methods to be adopted include the use of or connecting with the diversion channel，the use of or connecting with the fore bay，or supplying water to penstock or fore bay directly through a connection pipe（channel），etc.Connection of flow between a regulation pond and various connection structures shall be defined after hydraulic calculation.
5.5.40 Calculations shall be conducted for the fore bay both for the maximum surge wave if load is rejected suddenly during normal operation of the hydropower station and for the minimum surge wave if load is increased suddenly.For self-regulating channels，the maximum water level of fore bay is the maximum surge wave level；for non-self-regulating channels，the maximum water level of fore bay is the maximum water level on spillway weir.The free board for the crest of fore bay may be taken as 0.1-0.3m plus the free board of the channel top.
5.5.41 Fore bay and regulation pond structures shall meet the requirements for stability，strength，deformation，crack resistance，anti-seepage and anti-freezing，etc.For the retaining wall，stability and strength calculations shall be made according to the requirements for retaining structures.
5.5.42 The types of penstocks shall be selected based on water head of the hydropower station and application conditions，and then determined，after techno-economic comparison，among reinforced concrete pipe，steel pipe，fiberglass reinforced plastic pipe，steel sleeve concrete pipe or steel sleeve pre-stressed concrete pipe，etc.
5.5.43 The route of penstock shall be selected through techno-economic comparison based on general layout of project and with consideration of topographical and geological condition，construction and operating conditions.The route should be short and straight.
5.5.44 The modes of supplying water by penstock shall be respectively selected among unit supply，composite supply or grouped supply after techno-economic comparison of water head of the hydropower station，development mode，diversion discharge and type of pipes，as well as topographical and geological conditions and layout of project，etc.The number of units to be connected with each penstock should not be more than three.
5.5.45 The inner diameter of penstock shall be defined after techno-economic comparison of water head of the hydropower station，pipe type，construction quantity，investment and loss of electric energy，etc.With regard to the economical flow velocity in the pipe，it may be 2.5-3.5m/s for reinforced concrete pipes and 3.0-6.0m/s for steel pipes.
5.5.46 The open penstocks（exposed penstocks）shall be arranged in accordance with the following requirements:
1 The pipeline shall be arranged away from landslides and collapsed areas；if a few pipe sections cannot be kept away from mountain flood or falling stones，they may be installed as exposed penstocks in a tunnel，underground penstocks or concrete-packed penstocks.
2 An anchorage block shall be built where the penstock turns or divides，or a tunnel is connected with a steel pipe，or a concrete pipe is connected with a steel pipe，and an expansion joint shall be built downstream of the anchorage block.If a straight penstock is longer than 150m，an anchorage block shall be installed in this length.The penstock between two anchorage blocks may be supported with buttress or pipe holders，and the intervals between buttresses should be 6-12m.The foundation of anchorage block and pipe holders shall be sound and stable.
3 The bottom of penstock shall be at least 0.6m above the ground surface，and the top of penstock shall be 2m lower than the minimum pressure line.
4 Longitudinal drainage ditches shall be built at both sides of exposed penstocks and connected with horizontal ditches；a maintenance sidewalk shall be built along the pipeline.
5.5.47 The wall thickness of penstock shall meet the requirements for strength and stability against external pressure and shall be defined after stress analysis.The maximum internal water pressure to be endured by the penstock shall be defined after water hammer analysis and calculation.
5.5.48 The bifurcated pipe of a penstock may be asymmetrically branch-shaped，symmetrically Y-shpaed or three-branch-shaped.The bifurcation angle may be defined based on the shape and material of the bifurcated pipe，which should be 30°-60° for reinforced concrete bifurcated pipes and 45°-90° for steel bifurcated pipes.
5.5.49 The expansion joint of a penstock may be of socket type，sleeve type or corrugated pipe type.Water stop material for the expansion joint of the former two types shall be characterized by high elasticity，durability and low friction coefficient.If the water head is lower than 300m，rubber asbestos packing may be used；if the water head is higher than 300m，PTFE（polytetrafluoroethylene）asbestos packing should be used.
5.5.50 The anchorage block shall be calculated for its stability against sliding and foundation stress，and settlement calculation shall also be conducted for the anchorage block on non-rock foundation.Corresponding engineering measures shall be taken for foundation of anchorage block and buttress with likelihood of differential settlement.
The structure type of penstock support may be selected based on pipe diameter，which may be saddleshaped，plain sliding type，rolling type or swing type.
5.5.51 Manholes shall be set for steel penstock，whose diameter shall not be smaller than 450mm and the intervals should not be larger than 150m.Drainage devices should be set up at the lowest point of the steel penstock；and energy dissipation facilities should be set up at the drainage outlet of high-water-head steel penstock.
5.5.52 The inner surface of penstocks must be sprayed with wear-resistant，anti-rust and anticorrosion coatings；and the external surface shall be treated against corrosion in accordance with the laying-out method of penstock.Moreover，an anti-freezing facility shall be installed in frigid regions.
5.5.53 For weld-molded steel pipes，welding-seam detection and water pressure test shall be performed.The test pressure value shall neither be less than 1.25 times the highest internal water pressure under normal working condition，nor be less than the highest internal water pressure under special working conditions.
5.5.54 The layout of underground embedded penstock shall meet the following requirements:
1 The underground pipelines shall be located in an area with favorable topographical and geological conditions.
2 For underground pipes，the method of supplying water by one pipe should be adopted；if supplying water by multiple pipes，the spacing between two neighboring pipes should not be less than 2 times the pipe diameter.
3 The type of tunnel shaft and the gradient of the penstock shall be defined after comprehensive analysis of the layout requirement，geological conditions and construction conditions.
4 Drainage facilities should be set up at areas where the groundwater table is high，and an observation shaft or piezometer shall be installed as well.
5.5.55 The strength grade of lining concrete for underground pipes shall not be lower than C20.Backfill grouting shall be applied to the crowns of adits and inclined shafts，with the grouting pressure not less than 0.2MPa.For underground pipes where the pressure is jointly withstood by the steel pipe and surrounding rocks，joint grouting shall be applied between the steel pipe and concrete，and between the concrete and rocks，with the grouting pressure at 0.2MPa.Consolidation grouting should be applied to the surrounding rock of the underground pipe，with the grouting pressure not less than 0.5MPa.
5.5.56 In an underground pipe，water-stop rings welded on the end of steel pipe shall be installed at the connection part with the concrete linings of diversion tunnel or surge chamber.The size of clear space between the steel pipe wall and the surrounding rock shall conform to construction requirement.
