application of non-linear alignment is permitted but must meet below requirements:

a)At the bent, inter-section angle of a tunnel is not smaller than 60º (regarding to a flow velocity lower than 10 m/s) and curve radius is not five time smaller than water depth in the tunnel;

b)If the flow velocity in a tunnel exceeds 10 m/s, model experiments are needed to determine values of inter-section angle and curve radius.

It is desirable that a tunnel route be straight connecting an entrance and exit of the tunnel to get the shortest length, however, in casecurves are inevitabledue to topographic conditions, geological conditions and other requirements related to layoutdesign, a tunnel route shall follow provisions in this article.

8.9.4The depth of rock layer on the top of the tunnel must be three time higher than the tunnel

width.

8.9.5Remove the tunnel alignment crossing the swelling rock layers.

8.9.6Energy of flow after the tunnel outlet must be absolutely dissipated.

8.9.7Dimension of tunnel cross-section should meet requirements of utilization aswell as

constructional, repaired and maintained conditions of a construction. However, its height is not lower than 2,0 m and the width is not smaller than 1,5 m.

As stipulated in Technical Regulation Article 8.9.4 to 8.9.7. 7.5.4. As stipulated in Technical Regulation 7.5.4.

7.6Intakes

7.6.1. Technical Regulation Article 7.6.1 describes technical requirements on intakes.

Technical requirements are described quoting provisions of Articlef QCVN8.4o 04 - 05 : 2012/BNNPTN.

8.4 Water intake works

8.4.1 The design computation of the water intake work should guarantee the following requirements:

a)Safety and stability working conditions in the design and examined situations;

b)Receiving enough flow and total volume of water regarding to requirements of water consumption sectors;

c)Having the control ability for volume of water supply and actively stop providing water in case of examination, amendment regarding to the operating regulation or emergency circumstances;

d)Trash rack, rubbish collecting measurements, inlet sill, sand trap and scour gallery should be placed to prevent and remove the encroachment of settlement, garbage and floating objects into the conduit;

e)Be convenience for the construction, management, maintenance, amendment and application of modern technology such as electrification and automation.

As stipulated in Technical Regulation 8.4.1.

8.4.2 Structural type and general layout of the selected intake work should be in coherence with the construction’s function, type of conduit (pressure, free or combination; regulation and not self-regulation); characteristic of water collector (with or without dam); natural conditions such as states of hydrology and muddy-sand current, bank morphology, the appearance of rubbish and floating objects as well as the operating and sedimentation regimes in upstream.The collection of water into a pressure line should guarantee that air will not imbibe into a pipe and the head loss is

95

Guideline for QCVN xxxx : 2013/BXD

the lowest. The intake gate should be designed with several sections sothat it can be separated into individual sections, which support the amendment or dredge activities.

This article describes technical requirements on an intakeasic.B technical requirements on a typical intake for hydropower plant are as follows.

(1) Type

There are two types of intake, non-pressure and pressure, according to the type of headrace or penstock to which it connects.

(2)Location

a)Non-pressure type intake

A non-pressure type intake shall be located off a mainriver course, and free from inflow of driftwood or inflow and accumulation of sediment.

A trash rack is installed at the entrance of an intake to prevent driftwood and dust from flowing into a settling basin and headrace.

Considering head loss andvibration due to flowing water, it is desirable that a flowvelocity passing through a trash rack be 0.3 to 1.0 m/s empirically.

b) Pressure type intake

A pressure type intake is located at any place because stored water reduces flow velocity. Usually it is located adjacent to a dam, off a slope or at the bottom of a reservoir.

Fig.7.6-1 Representative type of pressure type intake

8.4.3 Apart from regulations in the article 8.4.1, the water intake fromreservoira should guarantee below requirements:

a)During the exploiting period, stipulating in the table 10, the sill of water intake will not be covered by sediment and sludge. The re-generation of bank lines should not influence the conduit alignment.

b)Regarding to the sewer (culvert):

-The body of a culvert should place directly on the parent rock or the trench in the foundation (under condition of adequate loadbearingand deformation capacity of the ground base, which meet the design requirements). The culvert will not be arranged on the earth-fill base;

-The flow regime in a culvert can be pressure, free or semi-pressure flow. The semi-pressure state is not permitted to occur in case of submerged inlet and outlet of that culvert (pressure flow) and free flow in the middle of a culvert;

-The water inlet, placing under the earth dam or rock-fill dam of a reservoir, which has a storage capacity of 20 million m3 and above, will be arranged in the gallery below a dam to create the

advantages for the testing and amendment activities. It will also guarantee the safety working condition for culvert and dam;

96

Guideline for QCVN xxxx : 2013/BXD

-The culvert obtaining a high inside waterpressure- (for example culvert intake using to generate electricity or discharge flood) should have a round section;

-The water intake type tunnel should meet requirements in the article 8.10;

c) The horizontal water intake inside the body of concrete or reinforced concrete dam should meet requirements stipulating in the construction of concrete and reinforced concrete dam.

As stipulated in Technical Regulation Article 8.4.3.

8.4.4 Selection for the type of water intake from a river depends on types of typical water levels on a river and required levels in the major conduit in accordance with the onsite hydrological, topographic and geologic conditions. The water intake without dam is applied in case the water level

in a river always guarantees to be higher than required water level in the major conduit. If water level at the construction alignment is lower thanrequired level of the major conduit, the water intake should be in coherence with the appearance of dam. The intake in accordance with dam can be replaced by pump stations through the comparison calculation about investment effectiveness.

As stipulated in Technical Regulation Article 8.4.4.This article involves the stipulations prepared for intakes for irrigation facilities , so the stipulations related to waterways for the hydropower works shall be referred

8.4.5The computed water level in upstream of a construction is stipulated as below:

a)Regarding to the water intake without dam: the water level regarding to the maximum calculating design and examined water levels at the construction alignment will be determined corresponding to requirements in the article 5.2.1 of this regulation;

b)Regarding to the water intake with the appearance of dam: the water level in accordance with upstream of a dam incase of discharging the maximum calculating designed and examined flow.

8.4.6 In order to guarantee the operating and exploiting conditions and prevent failures for the construction itself, conduit line and technical equipments of rear works, adequate valves pursue each receiving gate will be installed. Type of valve gate, quantity and position will be determined regarding the specific task of each construction.

As stipulated in Technical Regulation Article 8.

8.4.7 The design will propose sand trap in accordance with adequate installations so as to guarantee the better quality of input water (i.e. necessary clearness)It will. also bases on the economic – engineering calculations.

Technical requirements on a sand trap shall refer to Technical Regulation Article 7.7.

8.4.8 The design for water intake and collection works of domestic water supply system and other production sectors, regulations about the design of external network and relevant water supply works will be complied.

In case an intake has a purpose of not only power generation but also other water use,design criteria for other water users shall be applied.

7.6.2. Technical Regulation Article 7.6.2 describes the ruleon avoidance of intake construction on foundation consisting of alternation layers consisting of rock and soil and solutions in case construction of an intake on such weak foundation is inevitableto secure stability and safety of hydropower civil works.

In general, constructing an intake on alternation layer is not recommended because differential settlement will damage its stability of the structure.

97

Guideline for QCVN xxxx : 2013/BXD

FSL

RWL

Air-entrained vortex

Fig.7.6-2 Submerge depth of pressure type intake

7.7Settling basins

Guideline for Technical Regulation Article 7.7 describes basic technical requirements on a settling basin. Technical requirements are described quoting provisions rticleof A8.5 fo QCVN 04 - 05 : 2012/BNNPTN.

8.5 Sand trap

8.5.1The design of sand trap and relevant installations should guarantee below requirements:

a)Silt of large grain, which exceeds the permitted value, will be remained in the sand trap in order to provide clear water in accordanceithw requirements about quality. Sizes of silts, which are permitted to provide into a conduit, are determined basing on taking advantages of useful alluviums. It will limit or not lead to the sedimentation or erosion situations in the channel flow; not dreasec the life-span of technical installations to below regulated level;

b)Guarantee to provide enough water of adequate clearness in order to meet requirements of water consumption sectors;

c)Actively remove deposited silt in a sedimentation compartment if necessary.

This article describes requirements on a function of settling basinsIncase. of a run-of-river type hydropower plant, water containing suspended sand and silt particles is taken from a weir and flowsinto a headrace, settles on the bottom of a headrace and reduces the sectional area of the tunnel, and remaining particles wear the surface of a headrace and penstock. To avoid deposition and wearing, a settling basin is located just downstream an intake to trap sand and silt particles suspended in water.

An example of design criteria of a settling basin in Japan is as follows:

The first step to determine the capacity of a settling basin is to define the minimum size of the soil and sand to be settled down.

The acceptable size is determined by considering following condition.

(1)To prevent the surface of the waterways and penstocks from the wearing

(2)To prevent turbines from damaging and the runner from wearing.

After finding the soil or sand particle size for the design, the length of a settling basin is obtained as follows:

H Q

L ≥ Vg·V = B·Vg

Where,

98

Guideline for QCVN xxxx : 2013/BXD

L : necessary length (m)

H: water depth (m)

B: width of the settling basin (m)

V : mean flow velocity (m/s) = Q/B*H

Vg : settling velocity (m/s)

Q: water flow rate (m3/s)

Relationship between Vg and d (particle size, i.e. a diameter of sediment such as soil and sand) In practice:

Vg is 0.1 m/s in accordance with diameter of sand, 0.5 or 1 mm; V shall be less than 0.3 m/s, and;

A settling basin shall be twice longer than the length obtained abovementioned formula.

V

H

Vg

Fig.7.7-1 Settling of soil or sand particle

Fig.7.7-2 Relationship between settling velocity and particle size

8.5.2The design for a sand trap on the channel of an irrigated system should be on a ground

of the siltcontents in a year with an average turbidity level as well as the examination about operating capacity of that pond in a year of the highest turbidity level in a correlation with the working state of a channel.

As stipulated in Article 8.5.2. This article is prepared for irrigation facilities, so thestipulations in Article

8.5.1shall be referred to for hydropower works.

8.5.3Sand trap should be placed on the scale of the headworkunit or at the beginning of a major conduit in a relationship with below requirements:

a) Onsite topographic and geologic conditions allow the arrangement of water conduit to the sand

trap with appropriate dimension and flow state in order to let harmful silt deposit in th sedimentation basin;

b) It has the ability of releasing deposited silt out of the sedimentation basin or accumulating in a basin so as to periodically dredge by a mechanical method.

Sand and silt deposited at the bottom of a settling basin shall bedischarged out of the basin periodically to maintain its function.

99

Guideline for QCVN xxxx : 2013/BXD

8.5.4 Choosing type of sedimentation compartment, which is continuously or periodically cleaned by the hydraulic or mechanical method, should be implemented on the ground of economic and engineering comparisons. The selection in the type of sand trap bases on below fundaments:

a)The sedimentation chamber cleaning by hydraulic method will be applied in the area of abundant water resources as well as adequate hydraulic gradient;

b)If the different in level is not high enough to clean the whole deposit layer in a basin, a compound of sedimentation and wash chamber should be used. In this compound measure, fine silt is removed by hydraulic method while coarse sediment is cleaned by mechanic measurements;

c)Sand trap type one periodic cleaning chamber will only be applied if water supply is allowed to be completely stopped, or raw water is allowed to be used during the cleaning period.

To avoid suspension of power generation during cleaning works of a settlingbasin, it is recommended that two sets of settling basinbe prepared and maximum discharge for power generation be supplied by one side of the settling basin.

Fig. 7.7-3 shows cleaning of a settling basin.

Washing away sediment remaining at the bottom of the settling basin

Fine particles discharged through bottom gates

Fig.7.7-3 Cleaning of settling basin

100