7.Undermining around the spillway outlet due to discharge which may damage safety of dam and appurtenant structures located near the spillway outlet is not observed;

Advance in undermining around the spillway outlet may make a spillway chute and/or a dam unstable. The hollow due to undermining must be filled with concrete or other suitable materials and measures must be taken to prevent the undermined part from collision of water discharged from the spillway.

8.Drainage system is workable properly, in case spillway chute or walls are equipped with drainage system to reduce back pressure;

Clogging of drainage system may increase back pressure and uplift, and lead to displacement of the spillway chute and side walls. In case drainage holes are clogged, these must be washed by pressurized water. In case it is difficult or impossible to recover drainage system, drainage holes must be newly drilled or anchors must be installed so that the spillway chute and side walls may not displace.

9.Inspection on spillway gates must be conducted following the provisions in Article 114, Article 115 and Article 116 of Section 4-5 “Hydromechanical Equipment”.

Spillway gates must be opened to discharge flood when necessary. Any inoperative state of spillway gate(s) in flood may cause overtopping of a dam.

Section 3 Periodic Inspection for Dam Safety

Article 100-a5 General provision

This article stipulates the provisions according to Article 8 of Circular No.34/2010/TT-BCT. Frequency and inspection items vary according to reservoir capacity. Inspection items are provided in Articles 100-a8 and 100-a9.

According to Article 8 of Circular No.34/2010/TT-BCT, the following items are involved in the periodic inspection for dam safety.

(1)Assessment on results of dam management works;

a.The implementation of hydropower reservoir operation procedure is approved by the competent authorities;

b.The implementation of operation procedure of spillway’s gate (if any), intake gate; records of operation procedure, gate operation test of structures (operation diaries);

c.The organization of monitoring, collecting, documenting of the meteorological, hydrological

elements in reservoirs area, the condition of seepage, leakage at dam body, dam foundation,

dam abutment, dam translocation, crack condition, slide condition occurred at body, foundation and surroundings of structures; sedimentation status of reservoir;

d.The provisions execution on maintenance, overhaul for each structure, part of structure and equipment related to dam safety;

e. Dam inspection: routine inspection, periodic inspection before and after the flood season; sudden inspection, detailed observations for dam must be carried out; and

f.The repair, improvement of dam.

(2)Check and analysis of dam monitoring data;

a.Collecting monitoring data of dam, headworks, waterways from construction, operation phase to the report preparation time for inspection;

b. Listing out the installed monitoring instruments, quantity, working status, repair time, evaluating measurement methods, reliability of methods, its measurement interval;

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c.Analyzing and evaluating the measured and monitored data at each monitoring location, monitoring data are shown as tables and charts. Accordingly, carrying out analysis and evaluation for working status of the structures, and forecasting the development tendency of dam translocation (vertical or horizontal translocation…), finding out mainreasons impacting

on the value if sudden changes of monitoring data occur;

d.Setting the actual saturation line and comparing with design saturation line for case of earth dams, rock dams; evaluating uplift at foundation of concrete dams;

e. Based on thedata monitored since the construction phase or the latest inspection time, evaluating the dam safety status and forecasting the safety decrease of dam (if any); and

f. Recommending or proposing on the monitoring, measurement works in the future regarding instruments, additional monitoring items (type, quantity), repair, reliability improvement for existing instruments, measurement interval.

(3) Check and assessment of quality and safety of a dam

a. Calculating, checking the dam stability based on structureurrentc status corresponding to normal water level, surcharge water level (design flood, examined flood level) and other cases in accordance with structure design regulations;

b.Calculating and checking dams or part of structures of which working status change suddenly, abnormally based on their monitoring results of seepage, dam translocation, or for cases of dams which are damaged seriously or were damaged earlier and developed under negative tendency; and

c.Proposing measures to ensure the stability and safety for construction works.

(4)Check of the situation of reservoir sedimentation

a.Analyzing and evaluating the sedimentation status of reservoir based on monitoring and measurements data collected in the past; distributing sedimentation based on the observation section of reservoir sedimentation, forecasting sedimentation and service life of the reservoir ;

b.Analyzing and evaluating the causes of the increase or decrease in the amount of sedimentation

in the reservoir; and

c.Proposing measurement, monitoring interval for reservoir sedimentation: number and location of the sedimentation measuring, monitoring.

(5)Recalculation of flood flow into a reservoir and review on flood control capacity of a reservoir according to the present dam design regulations and to the latest meteorological and hydrological data

a.Collecting and supplementing data of meteorology, hydrology and changes in topography, geomorphology, land cover of vegetation in the reservoir area from the design phase or from the latest inspecting times to report preparation time for the dam safety inspection;

b.Calculating and checking again the designed flood flow, examined flood (including flood model,

flood-peak flow, total flood flow) and comparing with the update of monitoring data of meteorology, hydrology in the operating phase; and

c.Calculating and checking the flood discharge capability of the spillway and comparing with designed flood flow and examined flood which have been inspected.

(6)Assessment of works in relation to prevention and protection of dam against floods and typhoons.

a.Collecting latest data of meteorology and hydrology, and records of typhoons;

b.Estimating the probable most unfavorable flood pattern and flood discharge plan against the flood;

c.Estimating flood in case of a dam breach;

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d.Investigating distribution of population and social infrastructure downstream of a reservoir;

e.Preparing a hazard map in case of the maximum flood discharge and a dam breach; and

f.Preparing an evacuation program in case of severe flood discharge from a dam which may inundate the downstream area and in case of a dam breach in cooperation with therlevant municipalities.

Article 100-a6 Frequency of inspection

This article is prepared according to Article 7 of Circular No.34/2010/TT-BCT. Frequency varies according to reservoir capacity.

The first inspection must be conducted during the one year period starting from the date one year after the date when the reservoir water level reaches the normal high water levelas shown in Figure 100-

a6-1 regardless of reservoir capacity. It is recommended that the first inspection is conducted in earliest stages as the initial conditions of a dam must be confirmed as early as possible.

Figure 100-a6-1 Timing of first periodic inspection for dam safety

Time for thesecond or subsequent inspection varies with reservoir capacity and smaller dams and reservoirs have shorter frequency of inspection because the conditions of those dams and reservoirs tend to change faster than larger dams and reservoirs.

Article 100-a7 (Article 100-a7) Inspection process for dams

Only visual inspection of dams cannot recognize progress of aging or changing in conditions of a dam and reservoir though visual inspection is important for safety inspection of a dam. It is important to analyze data of dam monitoring which isconducted periodically so that the Owner can recognize the

extent of aging or change in conditions of a dam and reservoir and confirm their status related to safety. Basically, all monitoring items which are conducted in the dam and reservoir must be analyzed. Among the monitoring data, important data are settlement anddeformation of a dam, leakage, uplift

and sedimentation of a reservoir. As for a fill dam, data of pore pressure and seepage line are also important, and change in groundwater level of the surrounding area of a dam is also important.

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Article 100-a8 Concrete dams

Major inspection items and a measurement method are recommended as follows.

1)Leakage and seepage

−Leakage and seepage from drains in concrete dams

Leakage and seepage from drains is measured manually, for example, directly at a drain hole with a graduated cylinder.

−Leakage from joints in concrete dams

Leakage from joints is measured manually, for example, directly at a joint with a graduated cylinder.

−Leakage and seepage from inspection galleries

Leakage and seepage water iscollected to a ditch and quantity of water is measured by a triangular weir (V-notch weir) which isequipped in suitable locations sothat quantity of leakage water from each area of a dam can be measured. Quantity of leakage and seepage water is measured manuallyor automatically, for example, with an automatic water level recorder of a float type, water pressure meter type, or other types.

2)Uplift

Uplift is measured by a water pressure auge. An inspector of the Owner closes a valve of a drain hole in a gallery, leaves it closed fora sufficient time until the scale on a pressure gauge installed in a valve gets settled and reads the scale in case a valve is installed in a drain hole to measure uplift.

3)Displacement of dam body

−Displacement of downstream slope

Displacement of downstream slope is measured by targets installed on the surface of the slope.

−Displacement of dam body

Displacement of a dam body is measured by collimation and/or plumb lines installedin a dam body.

−Opening of joints in concrete dams

Opening of transverse and longitudinal joints in a concrete dam is measured by a joint gauge installed on a joint of blocks or a mark put across the blocks. Relative displacement of adjoining blocks is measured manually or automatically.

Generally, stress and strain varyaccording to a local situation, whichindicate a local phenomenon, while displacement consists of cumulativestrain of each part of a structure, whichindicates the overall situation.

Inspection items are divided into the following two categories;

a)Measured values showing a situation in the whole structure of a dam such as total amount of leakage (item 1) above) and displacement (item 3) above)

b)Measured values showing a situation in a local point of a dam such as stress and uplift (item 2) above)

Even if inspection items of b) show abnormal values, it is judged that these values may not show disorder in the whole structures of a dam as long as these abnormal values notar remarkable or

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inspection items of a) do not show abnormal values. However, in case inspection items of b) show extremely abnormal values and the part where extremely abnormal values are measured has serious defects, detailed inspection and investigation must be conducted and immediate measures must be taken as it is estimated that the wholestructure is in a critical condition. In that case, measurement values of b) are indispensable to investigate causes of the defects and to study repair works of them.

Some of the above values are caused by plural factors. For example, displacement of a dam body of concrete gravity dams or arch dams in upstream and downstream direction is mainly caused by hydrostatic pressure of the reservoir water and temperature. In such cases,influence of each factor

must be analyzed separately in order to detect the major cause of irregular phenomenon.

The state of hydro civilstructure must be evaluated not only by monitoring data itself but also by its tendency in variation. Frequency of official inspection which may be 7 years or 10 years is far beyond the required frequency for monitoring to analyze safety and stability of the structure and the results of routine inspection stipulated in Technical Regulation Vol.4 must be used to supplement information. It

is important notonly to continue routine monitoring of hydro civil structure but also to analyze a tendency of change in monitoring data, and the Owner must take propermeasures against faults and disorders found in the monitoring records, if any.

Article 100-a9 Fill dams

Major inspection items and a measurement method are recommended as follows. 1) Leakage and seepage

− Leakage and seepage from inspection galleries

Leakage and seepage water iscollected to a ditch and quantity ofwater is measured by a triangular weir (V-notch weir) which isequipped in suitable locations sothat quantity of leakage and seepage water from each area of a dam can be measured. Quantity of leakage and seepage water is measured manuallyor automatically, for example, with an automatic water level recorder of a float type, water pressure meter type, or other types.

2)Pore pressure

Pore pressure is measured by a pore pressure gauge or piezometer embedded in an impervious zone of a fill dam.

3)Displacement of dam body

−Displacement of downstream slope

Displacement of downstream slope is measured by targets installed on the surface of the slope.

−Settlement of crest in fill dams

Settlement of the crest in a fill dam is measured by targets installed on the crest.

Generally, stress and strain varyaccording to a local situation, whichindicate a local phenomenon, while displacement consists of cumulativestrain of each part of a structure, whichindicates the overall situation.

Inspection items are divided into two categories;

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