CIRCULAR

Technical regulations on sea level observation methods

Pursuant to the Law on Meteorology and Hydrology dated November 23 2015;

Pursuant to Decree No. 38/2016/NĐ-CP dated May 15, 2016 of the Government detailing certain provisions of the Law on Meteorology and Hydrology;i paragraph one of ArticleNo. of the Law on Meteorology and Hydrology;

Decree No. 36/2017/NĐ-CP dated April 4 Article 2. The receipt, handling of reflections and petitions from individuals and organizations concerning administrative regulations shall be carried out in accordance with Decree No. 20/2008/NĐ-CP dated February 14, 2008 of the Government on the receipt, handling of reflections and petitions from individuals and organizations concerning administrative regulations (amended and supplemented by Decree No. 48/2013/NĐ-CP dated May 14, 2013 on amending and supplementing certain articles of decrees related to administrative procedure control and Decree No. 92/2017/NĐ-CP dated August 7, 2017 on amending and supplementing certain articles of decrees related to administrative procedure control).7 of the Government stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment; tr48/2020/NĐ-CP

Decree No. dated November 38/2016/NĐ-CP dated May 15, 2016 of the Government detailing certain provisions of the Law on Meteorology and Hydrology; 4 Pursuant to Decree No. 32/2019/NĐ-CP dated April 10, 2019 of the Government on assigning tasks, procurement or tendering for the supply of products and services using state budget from regular operating expenses;20 of the Government Article 1. Amending and supplementing some articles of Circular No. 10/2018/TT-BCT dated May 24, 2018 issued by the Minister of Industry and Trade detailing certain provisions of Decree No. 40/2018/NĐ-CP dated March 12, 2018 of the Government on the management of multi-level marketing business activities (hereinafter referred to as Circular No. 10/2018/TT-BCT) as follows: 38/2016/ND-CP dated May 15, 2016 of the Government stipulatingi paragraph one of ArticleNo. of the Law on Meteorology and Hydrology;

At the proposal of the Director of the General Bureau of Meteorology and Hydrology, the Head of the Science and Technology Department, and the Head of the Legal Department;, amended and supplemented by Decree No. 109/2025/NĐ-CP and Decree No. 193/2025/NĐ-CPenergy The Minister of Natural Resources and Environment issues this Circular stipulating technical methods for sea level observations. This Circular stipulates technical methods for sea level observations. and This Circular applies to agencies, organizations, and individuals related to sea level observation activities.

AND tr1. High tide: the highest water level of the tidal cycle expressed by height value and time of occurrence. 2. Low tide: the lowest water level of the tidal cycle expressed by height value and time of occurrence..

PART I

GENERAL PROVISIONS

Article 1. Scope of Regulation

3. Rising tide: the period from low tide to the next high tide.

Article 2. Applicability

4. Falling tide: the period from high tide to the next low tide.

Article 3. Explanation of Terms

In this Circular, the following terms shall be understood as follows:

5. Observation depth: the vertical distance from the still water surface to the observation point.

6. Wave period: the time between two consecutive wave crests at a point.

7. Whole hour: hours at times from 00:00; 01:00; 02:00; up to 23:00.

GENERAL PROVISIONS ON OBSERVATION METHODS

Principles for conducting observations at stations

observer

1. Directly conduct meteorological and marine observations according to the provisions from Article 7 to Article 30 of this Circular.

Chapter II

2. Record data truthfully, objectively, and accurately reflect the nature of phenomena. When there are suspicious phenomena, re-observe and note in the observation logbook.

Article 4. 3. Use measuring instruments with valid certification and within calibration validity period. for 4. Check measuring instruments before conducting observations.

5. Conduct observations on time and in the correct sequence.

6. Maintain and store measuring instruments according to regulations.

7. Update new information into station technical files; submit observation materials to higher-level management units as prescribed.

8. Timely report to local authorities and higher-level management units when abnormal dangerous meteorological and marine phenomena are detected.

9. Promptly report violations of technical construction project boundaries, materials, and other facilities.

Article 5. Recording and Preliminary Adjustment of Observation Data

1. Observation results are recorded in the observation logbook with black pencil and entered into software as prescribed.

2. The cover and explanatory notes of the observation logbook must be written in black or dark blue ink; do not soil, wrinkle, or tear.

3. After the daily observation time of 19:00, the observer must record and enter the results from the observation logbook into the report.

4. For stations equipped with automatic water level recording devices, graphs must be plotted, adjusted, and data entered into the report.

5. Correct readings on measuring instruments, check calculations, make preliminary adjustments to results, and select characteristic values.

. Providing Information and Data

1. Daily at observation times of 01:00, 07:00, 13:00, and 19:00, the observer must send coded telegrams and telegrams to the Meteorological and Hydrological Information and Data Center.

2. Submission deadline for paper documents: stations submit to the Regional Meteorological and Hydrological Station before the 5th day of the following month; Regional Meteorological and Hydrological Stations submit documents to the National Meteorological and Hydrological Observation Center before the last day of the month.

3. Submission deadline for digital documents: stations send before the 5th day of the following month to the Regional Meteorological and Hydrological Station and the National Meteorological and Hydrological Observation Center.

Article 64. For automatic stations: data must be continuously transmitted to the specified addresses. Follow the prescribed model, configuration, file format, and installation time.

5. Owners of specialized meteorological and hydrological works not subject to Article 3, Clause 3 of the Law on Meteorology and Hydrology are encouraged to comply with this Circular's provisions.

TECHNICAL REGULATIONS ON OBSERVATION METHODS

DISTANT VIEW OBSERVATION

distant view

1. Daytime distant view observation

PART III

a) Daytime distant view observation towards the mainland is conducted at designated positions; sequentially observe the distant view across all nine reference points, starting from the nearest to the farthest; determine which reference points are visible and which are not; the distant view grade is determined based on the distance between two consecutive reference points and classified according to grades 0 to 9 as specified in Table 2 of Appendix II issued with this Circular;

Section 1

b) In cases where sufficient reference points for distant view observation towards the sea are available, selected reference points include land spits, islands, buoys, lighthouses, and ship smokestacks or other objects when their distances are known; if there are insufficient or missing reference points towards the sea, the distant view grade is determined based on the clarity of the horizon as specified in Table 3 of Appendix II issued with this Circular;

Article 7. Observation methods c) If the distant view does not reach the horizon, estimate the distant view over the sea surface by eye or binoculars;

d) If the distant views differ in various directions, the worst distant view and its limit are recorded in the observation logbook;

đ) When the distant view is less than 4 km from grade 5 downwards, additional symbols indicating the limiting factors (fog, mist, rain) must be recorded.

2. Nighttime distant view observation

a) Nighttime distant view observation towards the mainland and sea when reference points are available: Observers familiarize themselves with darkness for 10 to 15 minutes before observation at the designated position and then conduct the observation; the method follows the provisions of Point a, Clause 1, Article 7 of this Circular;

d) For sea visibility in different directions, the worst sea visibility and its limit shall be recorded in the observation logbook;

e) When sea visibility is less than 4 km at wind force level 5 or below, the symbol for the limiting factor of sea visibility (fog, mist, rain) must also be recorded.

2. Nighttime Visibility Observation

a) Observations towards the mainland and towards the sea with reference points: The observer must acclimate to darkness at the designated location for 10 to 15 minutes before conducting observations; the method of observation shall be carried out according to the provisions of point a, Clause 1, Article 7 of this Circular;

b) In case of missing observation points, determine the visibility before sunset one or two hours (depending on weather conditions); if there is no phenomenon reducing visibility at the 1-hour observation point, take the visibility at the 19-hour observation point; the observation method shall be carried out in accordance with point a, Clause 1, Article 7 of this Circular.

3. Visibility observation using automatic equipment: automatically measure and transmit data 24 times/day or according to usage requirements.

Article 8Position for observation

1. Open towards the sea.

2. Convenient and safe for observation.

3. Ensure long-term observation.

Article 9. Observation Frequency

1. Observe four times/day at 1 hour, 7 hours, 13 hours, and 19 hours.

2. In dangerous weather conditions at sea, continuous observation once/hour shall be conducted.

3. Besides the observation frequency prescribed in Clauses 1 and 2 of this Article, the competent authority may decide to supplement the observation frequency based on needs.

4. For automatic observation: automatically measure and transmit data 24 times/day or according to usage requirements.

Section 2

WIND OBSERVATION

Article 10. Observation Method wind

1. The observer arrives at the facility, stands in the designated position, simultaneously observes the direction and speed of the wind for 2 minutes.

a) Wind observation using the Beaufort scale: use a 1-meter-long, 0.15-meter-wide fabric wind ribbon to observe the direction and rely on the behavior of surrounding plants and objects to determine the wind speed; wind speed observation according to the Beaufort scale is conducted for 10 minutes, determine the wind direction using the wind ribbon, and determine the wind speed according to the Beaufort scale specified in Table 4 of Appendix III issued together with this Circular;

b) Wind observation using a handheld anemometer: install the observation device on a wooden, concrete, or stainless steel pole; mount the handheld anemometer at the top of the vertically fixed pole, at least 2 meters above ground level; lock the anemometer's brake to prevent the needle from moving, record the initial reading, then start the timer and release the brake to allow the anemometer needle to move; after 100 seconds, lock the brake again and read the needle values; calibrate the speed through calibration documentation to obtain the actual measured wind speed value; calculate the uncalibrated average wind speed value using the formula:

Where: V is the average wind speed, measured in m/s.

c) Wind observation using the EL self-recording anemometer: simultaneously observe the direction and speed for 2 minutes; read the direction and speed on the display screen of the measuring device.

d) Wind observation using the Munro self-recording anemometer: replace and install the chart daily at 9:10 am, mark the chart with a pencil at 10:00 am, 13:00 pm, 19:00 pm, 1:00 am, and 7:00 am; during Synop or Typh observation periods, read the wind direction and speed on the chart before reading the barometer; the wind speed and direction are the average values over a 10-minute period before the full hour:

Procedure for calculating the chart over 24 hours: adjust the time on the self-recording chart; determine the wind direction and speed for each hour; determine the maximum average wind direction and speed at each 10-minute interval; determine the instantaneous maximum wind direction and speed.

e) Wind observation using the Young self-recording anemometer: simultaneously observe the direction and speed for 2 minutes; read the direction and speed on the display screen of the measuring device.

2. Wind observation using automatic equipment: automatically measure and transmit data 24 times/day or according to usage requirements.

Article 11Position, person Installation for observation

1. The observation position must be open, facing all prevailing wind directions; ensure technical clearance; have sufficient area to install the observation facility; ensure long-term maintenance of observation activities.

2. The wind observation facility is constructed and installed in the form of a round pillar or triangular shape.

a) Round pillar: made of stainless steel, resistant to chemical corrosion and suitable for marine environments; height from 10 m to 12 m connected to a cable anchoring system; pipe thickness greater than or equal to 0.003 m, pipe diameter greater than or equal to 0.049 m; the base of the pillar is securely installed.

b) Triangular pillar: made of stainless steel, resistant to chemical corrosion and suitable for marine environments; height from 10 m to 12 m; the triangular pillar consists of three main pipes at the three corners, each side length 0.2 m, and crossbars with a diameter greater than or equal to 0.015 m; pipe thickness greater than or equal to 0.003 m, pipe diameter greater than or equal to 0.036 m; the base of the pillar is firmly attached; auxiliary equipment includes lightning protection devices, cables, tensioners, and e-cus.

Article 12. Frequency of observation    

1. Observe four times/day at 1 hour, 7 hours, 13 hours, and 19 hours.

2. In case of dangerous weather conditions, observations shall be carried out according to the requirements of the competent authority.

3. For automatic observations: automatic measurement and data transmission 24 times/day or as needed.

Section 3

WAVE OBSERVATION

Article 13. Methods of wave observation

1. Estimating wave observation by eye: The observer goes to the structure, stands at the correct position, and conducts the observation; within 5 minutes, the observer looks at the sea surface to determine the type of waves, wave form, sea surface state, wave height, period, and wave direction.

a) Wave type observation: Wind waves at the time of observation, wind still directly affects the waves. The slope of the wave on the leeward side is steeper than on the windward side; the crest of the wave falls down forming white foam; wind waves are observed when the wind is small and calm; wind waves are determined from another location transmitted to the observation point; wind waves have a gentle, even, and long shape like plowed fields; the wave types are specified in Table 5 of Appendix IV issued together with this Circular;

b) Wave form observation: Regular waves have long crests parallel to each other like plowed fields; the distance between two consecutive crests is less than the length of the wave; irregular waves have crests breaking up into segments, with crests and troughs alternating; the distance between two consecutive crests is greater than the length of the wave;

c) Sea surface state observation: Indicated by a scale from 0 to 9 due to the effect of wind causing changes in the sea surface; the sea surface state scale is defined in Table 6 of Appendix IV issued together with this Circular;

d) Wave height observation: By eye or using binoculars for 5 minutes to determine the height of the largest visible waves and record in the logbook from 10 to 15 large waves, select the five largest waves and record in the observation logbook; estimated wave height by eye is classified from 0 to 9, recording method specified in Table 7 of Appendix IV issued together with this Circular;

đ) Wave period observation: At a fixed point on the sea surface, the observer uses a stopwatch to track 11 wave crests passing through the fixed point, stop the stopwatch; the wave period is equal to the total time determined divided by 10; at each observation station, determine three times the time it takes for 11 wave crests to pass through a fixed point consecutively.

Where: t is the wave period, measured in seconds (s); t₁ is the time taken for the first 11 wave crests; t₂ is the time taken for the second 11 wave crests; t₃ is the time taken for the third 11 wave crests.

e) Wave direction observation by eye: Determined by the direction from which they come and divided into eight main compass directions, as specified in Table 8 of Appendix IV issued together with this Circular.

2. Observation using a stereoscopic instrument

a) Conduct wave type, wave form, and sea surface state observation applying points a, b, and c of Clause 1, Article 13 of this Circular;

b) Height observation: When wave height is 0.25 meters or more; adjust the sight so that the wave gauge aligns with the height scale, determine the number of divisions from the base of the wave to the top of the gauge float, the fractional part of the division is estimated; within 5 minutes, determine the height of the largest waves based on the height scale, select the five largest waves and record in the observation logbook. Knowing the number of divisions and the value of each division, we can determine the wave height using the following formula:

h = H x i x k (m)

Where: h is the wave height, measured in meters (m), accurate to 1 cm; H is the number of divisions on the height scale, machine H10 has 48 divisions, H40 has 60 divisions; i is the value of each division, machine H10 one division is 0.5 m, H40 one division is 1 m; k is the coefficient of the machine depending on the water level at the time of observation;

c) Wave period observation: Aim the sight so that the waves travel towards the observer, the wave crests coincide with the horizontal lines of the stereoscopic grid, determine the first wave crest passing through a certain horizontal line of the scale, simultaneously start the stopwatch; observe ten consecutive wave crests passing through and stop the stopwatch, observe continuously three times; the wave period is calculated as the average value of three observations; unit is seconds (s);

d) Wave length observation: Aim the sight so that the waves travel straight towards the observer, based on the wave length scale on the machine to determine the distance between two consecutive wave crests occupying how many divisions; the actual length of the wave is calculated as follows:

l = d x n x k (m)

Where: l is the wave length (meters); d is the length of the division (meters), specified in Table 9 of Appendix IV issued together with this Circular; n is the number of divisions; k is the coefficient of the machine depending on the water level at the time of observation;

đ) Wave propagation speed observation: Aim the lens so that the waves travel towards the observer, use a stopwatch to determine the time it takes for a wave crest to pass through one or more divisions of the length scale; the wave propagation speed is calculated as follows:

Where: C is the wave propagation speed (m/s); l is the distance the wave crest travels 100m (meters); t is the time it takes for the wave to travel the distance (seconds); k is the coefficient of the machine depending on the water level at the time of observation;

e) Wave propagation direction observation: Aim the sight of the machine along the direction parallel to the wave propagation direction; the wave propagation direction is determined according to the directional disc with degree markings and rounded to eight main directions; in case the waves propagate towards the observer, record the direct reading on the directional disc, in the opposite case, the reading is added 180^°.

3. Automatic wave observation: automatic measurement and data transmission 24 times/day or as needed.

Article 14Position, construction for observation

1. Located in the coastal area, facing the sea must be unobstructed in the direction of the prevailing winds and ensure natural marine conditions.

2. Must be high enough to observe waves from multiple directions.

3. The depth of the sea in the observation area must be the deepest in the coastal zone, avoiding areas where the shore is too steep and deeply indented.

4. The shoreline at the observation site should not be too winding causing local changes in wave direction and wave shape; if the shore is too low and cannot place the observation site at the required height, then a wave observation tower must be erected.

5. Not subject to being limited or having wave patterns from the sea altered by sandbars, shoals, submerged rocks, or other obstacles; not far from the station, ensuring observation under all weather conditions.

6. Auxiliary structures for wave measurement using radar

a) The house for the wave measuring machine shall be constructed solidly with a minimum size of 1.5 m x 1.5 m x 2.0 m (length, width, and height respectively), having three doors facing the sea. Inside the house, there shall be a sturdy wooden, brick, or concrete pillar for mounting the machine, with the machine base securely fastened to the pillar using bolts.

b) The wave-measuring buoy shall be shaped like a pear, with a diameter of 1 m and length of 1.8 m, coated with anti-rust paint both inside and out; the upper part shall be painted red and marked with the name of the managing unit, while the lower part shall be painted black; the bolts securing the buoy must be firmly attached, with rubber seals to prevent water seepage into the buoy; the submerged part should weigh two to three times more than the floating part to ensure balance when the buoy is released into the water.

c) The steel chain shall have a diameter of 0.018 m to 0.020 m, with oval-shaped links that are securely formed, with internal dimensions of 0.06 m x 0.03 m; the chain ends shall have two pins with a diameter of 0.028 m to 0.030 m; the bolts shall have a diameter of 0.028 m to 0.030 m and shall be drilled with holes for fixing nails; the length of the chain shall be 1.5 to 2.0 times the depth at which the buoy is placed.

d) The anchor shall be made of reinforced concrete, shaped like a truncated cone, with a height of 0.5 m, a bottom surface of 0.9 m x 0.9 m, and a top surface of 0.7 m x 0.7 m, with an iron handle with a diameter of 0.035 m to 0.040 m; depending on the terrain, waves, and currents in the observation area, the mass may be increased.

7. Radar observation structures shall be built solidly, meeting the technical requirements of the measuring equipment.

Article 15. Frequency of Observation

1. Observations shall be conducted three times daily at 7 a.m., 1 p.m., and 7 p.m. The 7 p.m. observation can be adjusted seasonally but not later than 2 hours before sunset.

2. In case of dangerous weather conditions, observations shall be carried out according to the requirements of the competent authority.

3. For automatic observations: automatic measurement and data transmission 24 times/day or as needed.

Section 4

WATER TEMPERATURE OBSERVATION

Article 16. Methods for Observing Sea Water Temperature

1. Sea water temperature measured with a thermometer

a) In cases of small waves: The observer shall proceed to the designated location and conduct the observation; using a rope to submerge the thermometer 0.5 m underwater vertically, quickly pulling up the thermometer, pouring out the water from the bulb, and re-submerging it at the initial depth; the thermometer must remain underwater for a minimum of 3 minutes; quickly pull up the thermometer; read the value on the thermometer, recording the decimal part first and then the whole number, accurate to 0.1°C; record in the observation log (during daytime, face away from the sun; at night, shine a flashlight from behind the thermometer).^°C; record in the observation logbook (during daytime, facing away from the sun; during nighttime, using a flashlight from behind the thermometer);

b) In cases of large waves: The observer shall proceed to the designated location and conduct the observation; using a tool to collect water samples at depths less than 0.5 m, shielding the sample from sunlight and rain during observation to avoid altering the actual data value; immerse the thermometer in the water and gently shake it two to three times; lift the thermometer, pour out the water from the bulb, and re-submerge it for a minimum of 3 minutes; read the value on the thermometer, recording the decimal part first and then the whole number, accurate to 0.1°C; record in the observation log (must shield the sample from sunlight and rain to avoid altering the actual data value).^°C; record in the observation logbook.

2. Sea water temperature measured with a handheld device

a) In cases of small waves: Submerge the sensor directly 0.5 m underwater for a minimum of 3 minutes; press the button to activate the temperature measurement mode, record the stable reading on the device in the observation log.

b) In cases of large waves: Use a tool to collect water samples at depths less than 0.5 m; submerge the sensor in the sample collection tool and gently shake it, for a minimum of 3 minutes; press the button to activate the temperature measurement mode, record the stable reading on the device in the observation log (must shield the sample from sunlight and rain to avoid altering the actual data value).

3. Automatic measurement of sea water temperature: Automatically measures and transmits data 24 times a day or according to usage needs.

Article 17. Position, monitoring facilities

1. The monitoring area must be accessible to the outside.

2. Locations with depths of 0.5 meters or more.

3. Fixed locations that ensure long-term monitoring.

4. Not affected by polluted water or hot water from production facilities.

5. Must not be near concrete or rock structures.

6. Free from trees, grass, debris, or floating objects that could affect temperature measurement equipment.

7. Must be convenient and safe for personnel and equipment during monitoring.

8. For automatic monitoring stations: the mounting frame for measuring instruments must meet technical specifications; the position and monitoring facilities must be securely attached to piers, platforms, wells, buoys, and turtles.

Article 18. Frequency of Monitoring

1. Monitor four times a day at 1 hour, 7 hours, 13 hours, and 19 hours. Conduct monitoring ten minutes before each whole hour.

2. For automatic monitoring: automatically measure and transmit data 24 times a day or as needed.

Section 5

WATER SALINITY MONITORING

Article 19. Methods of Water Salinity Monitoring

1) When sea waves are small: submerge the measuring head directly into the water at a depth of 0.5 meters, minimum time of 3 minutes; press the button to activate the salinity measurement mode, record the value when it stabilizes on the device in the monitoring logbook.

2) When sea waves are large: use a tool to collect water samples at a depth less than 0.5 meters; place the measuring head into the sample container and gently shake it, minimum time of 3 minutes; press the button to activate the salinity measurement mode, record the value when it stabilizes on the device in the monitoring logbook (must shield the sample from sunlight and rain to prevent changes in the actual value of the data).

3. Automatic monitoring of seawater salinity: automatically measure and transmit data 24 times a day or as needed.

Article 20. Position, Monitoring Facilities

1. The monitoring area must be accessible to the outside.

2. Locations with depths of 0.5 meters or more.

3. Fixed locations that ensure long-term monitoring.

4. Not affected by polluted water or hot water from production facilities.

5. Must not be near concrete or rock structures.

6. Free from trees, grass, debris, or floating objects that could affect temperature measurement equipment.

7. Must be convenient and safe for personnel and equipment during monitoring.

8. For automatic monitoring stations: the mounting frame for measuring instruments must meet technical specifications; the position and monitoring facilities must be securely attached to piers, platforms, wells, buoys, and turtles.

Article 21. Frequency of Monitoring

1. Monitor four times a day at 1 hour, 7 hours, 13 hours, and 19 hours. Conduct monitoring ten minutes before each whole hour.

2. For automatic monitoring: automatically measure and transmit data 24 times a day or as needed.

Chapter 6

SEA LIGHT MONITORING

Article 22. Methods of Sea Light Monitoring

1. Beam light monitoring: at the monitoring location, observe the mechanical effect of waves on rock outcrops using the eye, or use a stick or ruler to stir the seawater and observe the needle-like beams of light emitted.

2. Milk light monitoring: at the monitoring location, observe the presence of milky white light with high brightness for a short period of time.

3. Large bioluminescent cluster light monitoring: at the monitoring location, observe extended, wide, and moving light phenomena on the surface of the sea (light emission from large organisms such as octopuses, jellyfish, fish, and other animals).

4. When observing sea light types specified in Clause 1, 2, and 3 of Article 22, refer to Table 10 in Appendix V to determine the intensity of sea light and record it in the monitoring logbook.

5. Recording of Sea Light Monitoring Data

a) If no sea light is observed during monitoring, record the symbol "0".

b) Beam light type: record the letter "T" in the monitoring logbook and include the corresponding sea light intensity level. In case of beam light with intensity level 1, record T, intensity level 2 record T, other levels record similarly.₁c) Milky light type: record the letter "S" in the monitoring logbook and include the corresponding sea light intensity level. In case of milky light with intensity level 1, record S, intensity level 2 record S,₂d) Large bioluminescent cluster light type: record the letters "SVL" in the monitoring logbook and include the corresponding intensity level; In case of large bioluminescent cluster light with intensity level 1, record SVL, intensity level 2 record SVL,

e) On nights with moonlight: if sea light cannot be monitored, record the symbol "OST"; if sea light can be monitored, add the symbol (Tr) to the right of the sea light type.₁intensity level 2 records S₂d) Large bioluminescent cluster light type: record the letters "SVL" in the monitoring logbook and include the corresponding intensity level; In case of large bioluminescent cluster light with intensity level 1, record SVL, intensity level 2 record SVL,

d) Type of large bioluminescent light: Record the symbol SVL in the observation logbook along with the corresponding intensity level; In case of type SVL with intensity level 1, record SVL ₁intensity level 2 records SVL ₂d) Large bioluminescent cluster light type: record the letters "SVL" in the monitoring logbook and include the corresponding intensity level; In case of large bioluminescent cluster light with intensity level 1, record SVL, intensity level 2 record SVL,

e) On nights with moonlight: If marine luminescence cannot be observed, record the symbol "OST"; if marine luminescence can be observed, add the symbol (Tr) to the right of the marine luminescence symbol;

Article 23. Monitoring Position

1. Located in an open sea area, ensuring stable and long-term monitoring.

2. The monitoring location should not be affected by light from the shore and ships.

3. Do not conduct monitoring at places where salinity is frequently low and freshwater is mixed.

"d) Within no more than one working day from the date of receiving the dossier submitted for administrative procedures by the specialized agency assigned by the Provincial People's Committee, the Chairman of the Provincial People's Committee shall issue a notification of the result of the inspection of plant-based food exports or a certificate at the request of the importing country.". Frequency of Monitoring

Conduct monitoring twice a day at 1 hour and 19 hours. Perform monitoring five minutes before the whole hour.

Section 7

CURRENT MONITORING

Article 25. Current Monitoring Method

1. Determine the depth of the measurement point, lower the instrument to the specified measurement layer, measure the direction and speed of the current; depths from 5 meters to less than 10 meters are measured using a single-layer method (0.2 or 0.5); depths from 10 meters to less than 30 meters are measured using a two or three-layer method (0.2 and 0.8 or 0.2; 0.5; 0.8); depths of 30 meters or more require at least three layers; or according to requirements, more measurement layers can be arranged; the surface layer must be submerged under water by at least 0.5 meters; the bottom layer must be at least 1 meter above the seabed; current speed units are cm/s, m/s; current direction is measured in degrees from 0^° to 359^°.

2. Automatic current monitoring: automatically measures and transmits data 24 times a day or as needed.

Article 26. Monitoring Location and Structures

1. The sea area must be open, with flat seabed terrain, without submerged reefs or shoals that distort current direction.

2. The monitoring area must be the deepest within the monitoring zone, with a minimum depth of 5 meters during the lowest tide level.

3. Structures for manual and automatic measuring instruments must meet technical specifications; monitoring locations and structures are fixed to bridges, piers, buoys, platforms, and vessels.

Article 27. Frequency of Monitoring

1. Monitor four times a day at 1 hour, 7 hours, 13 hours, and 19 hours; monitor eight times a day at 1 hour, 4 hours, 7 hours, 10 hours, 13 hours, 16 hours, 19 hours, and 22 hours; monitor twelve times a day at odd hours from 1 hour to 23 hours.

2. For automatic monitoring: automatically measure and transmit data 24 times a day or as needed.

Section 8

SEA LEVEL MONITORING

Article 28. Sea Level Monitoring Method

1. Monitor sea level using water gauges, stakes

a) When there is no wave: The observer must look straight at the water gauge, the water level ruler placed vertically on top of the stake, read the mark closest to the water surface; if the water surface lies between two marks on the water gauge or ruler, round off to the nearest centimeter; if the water surface is below 0.5 of a division, take the lower value, from 0.5 of a division upwards, take the upper value; must read six times and record in the logbook.

b) When there are waves: Look at the water gauge, the water level ruler at the peak and trough of each wave; must monitor three pairs of values (peak, trough), the average of six readings is the reading.

c) Record all items "Stake Number", "Height of Stake Top", "Reading" in the logbook.

d) Convert the reading to "O" station of each measurement by adding "Height of Stake Top" and "Reading".

2. Monitor sea level on self-recording float-type equipment

a) Read the water level on the stake and water gauge at 1 hour, 7 hours, 13 hours, and 19 hours; mark the graph by gently moving the pen arm or turning the float wheel to draw a vertical line 3 mm wide on both sides of the water level recording line, separate the pen arm to interrupt the self-recording line for 2 to 3 minutes; when marking the graph at the designated times, if there is a time difference, adjust the clock on the self-recording device to match the Vietnam Radio Station time; determine the difference in water level between the stake, water gauge, and the self-recording graph.

b) Cut the self-recording graph of water level every month three times, first at 8 am on the 11th, second at 8 am on the 21st, and third at midnight on the 1st of the following month; the time to replace and cut the graph must not exceed 5 minutes, if it exceeds the specified time, read the water level on the stake and water gauge again and mark additional points.

c) Check and correct the water level recording line on the graph: if the recording line is smooth and steady but interrupted for less than or equal to 3 hours, restore the missing section based on the trend of the recording line; if interrupted for more than 3 hours but less than 6 hours, restore the missing section based on the current trend, the previous day's trend, and the next day's trend; if the recording line has a stepped or sawtooth pattern, smooth it by drawing a straight line through the midpoint of each step or tooth.

d) Calculate the graph: use a black pencil to draw a line about 3 mm long perpendicular to the self-recording water level line, at the intersection of the hourly division line and the self-recording line, read the accurate water level value to 1 cm and record the result to the right of the self-recording line.

đ) Determine high and low water levels on the graph: determine the value of high water and low water and the corresponding time satisfying two conditions: water level difference greater than or equal to 10 cm and time greater than or equal to 3 hours; if high water or low water stops for several hours, take the value at the start of the stop; record the high water and low water values and the time of occurrence in fractional form.

e) Determine the duration of flood tide and ebb tide: the duration of flood tide equals the time of high water minus the time of the preceding low water; the duration of ebb tide equals the time of low water minus the time of the preceding high water; if the duration of flood tide or ebb tide spans two consecutive days but the number of hours is different, choose the day with the larger number of hours; if the duration of flood tide or ebb tide spans both consecutive days but the number of hours is the same, choose the day with the smaller total duration of flood tide and ebb tide; if the total duration of flood tide and ebb tide of the two days is the same, choose the following day.

3. Correct the water level on the graph

a) If the difference in water level between the stake, water gauge, and the self-recording graph is 2 cm or more, immediately correct the water level after calculating the graph.

b) Determine the difference in water level between two consecutive observation periods, calculate the difference in water level between two periods using the formula:

Where: a is the difference in water level between the graph and the stake, water gauge at the previous observation period; b is the difference in water level between the graph and the stake, water gauge at the subsequent observation period; c is the difference in water level between the two periods.

c) Determine the hourly water level difference between two observation stations according to the formula:

In which: c is the difference in water levels between two stations; d is the hourly water level difference between two observation stations.

d) The corrected hourly water level is obtained by adding the corresponding corrections to the calculated hourly water level on the algebraic addition chart, as specified in Table 11 of Appendix VI issued together with this Circular.

4. Elevation adjustment for tide gauges, benchmarks, automatic recording devices, and automatic equipment.

a) National elevation (absolute elevation) at the main benchmark station;

b) Determining the zero elevation of tide gauges, benchmarks, automatic recording devices, and automatic equipment; referencing to the zero elevation of the station, national elevation immediately upon construction or repair;

c) Checking the elevation of tide gauges, benchmarks, automatic recording devices against the check benchmark once a year; checking the elevation of the main benchmark and check benchmark against the benchmarks, tide gauges, automatic recording devices, and automatic equipment, for newly established stations every six months, for stations established for five years or more once a year; for the main benchmark, it must be checked every five years; the results of the checks shall be reported to the direct management authority as prescribed.

5. Water level monitoring using automatic equipment: automatically measuring and transmitting data 24 times a day or as required.

Article 29. Location and Monitoring Structures

1. The location of monitoring should be within the area accessible to the sea, convenient for monitoring, maintenance, preservation, and long-term stability. Minimize the impact of waves, capable of measuring the lowest and highest water levels occurring at the monitoring site.

2. The well structure includes auxiliary structures interconnected such as: wells, houses for automatic water level recorders, water inlet pipes to the well.

a) Wells made of reinforced concrete, stainless steel pipe, high-grade plastic pipe, diameter ranging from 0.6 m to 0.8 m; well walls made of stainless steel pipe thickness of 0.005 m; high-grade plastic pipe thickness of 0.01 m; reinforced concrete thickness from 0.1 m to 0.15 m; bottom of the well lower than 1 m below the lowest possible water level at the site, top of the well higher than the designed water level from 1 m to 1.5 m and higher than the floor of the house by 0.8 m;

b) House for automatic water level recorders classified as Class III construction, built solidly, closely integrated and synchronized with the well, walls constructed of bricks, roof made of reinforced concrete, area 3.5 m x 3.5 m;

c) Water inlet pipe to the well: horizontal pipe type made of concrete, non-corrosive metal, or high-grade plastic with high durability, circular pipe, inner diameter from 0.065 m to 0.09 m, length of the pipe equal to the actual distance from the well to the lowest water level plus 1 m; siphon type made of non-corrosive metal or high-grade plastic, circular pipe, inner diameter from 0.065 m to 0.09 m, parallel tube type, inverted U-shaped with both ends, one end placed inside the well, the other end placed along the coastline slope; the position of both ends of the siphon pipe must be lower than 0.5 m to 1.0 m below the lowest water level ever occurred at the site, the regulation system and siphon pipe must be absolutely sealed; water directly enters the well through the bottom or wall of the well, the bottom of the well concave or flat.

3. The line structure of benchmarks made of wood, reinforced concrete, or non-corrosive metal, ensuring stability and elevation adjustment.

a) Wooden benchmarks group 2: size 0.1 m x 0.1 m, top of the benchmark higher than the step surface from 0.02 m to 0.05 m, benchmarks constructed along the slope of the bank, height difference between adjacent benchmarks from 0.3 m to 0.5 m, benchmarks driven at least 0.8 m deep, sandy soil driven from 1 m to 1.5 m deep;

b) Benchmarks made of concrete or non-corrosive metal: size 0.15 m x 0.15 m, height selected appropriately based on the geological conditions of the monitoring line.

4. Tide gauge structures: tide gauges made of concrete, wood, or galvanized iron, ensuring resistance to waves, wind, corrosion, minimal expansion and contraction due to temperature changes, shape must be straight, markings on the tide gauge up to 1 cm, markings must be accurate and clear.

a) Wooden tide gauge: width from 0.1 m to 0.2 m, thickness from 0.03 m to 0.06 m, length from 1 m to 2.5 m;

b) Galvanized iron tide gauge: width 0.1 m, divided into multiple sections, each section 0.5 m long;

c) Concrete tide gauge: width from 0.15 m to 0.2 m, thickness 0.1 m, length from 1 m to 2.5 m;

d) Tide gauge support: made of iron size 0.18 m x 0.18 m x 1.5 m; made of concrete base size 0.6 m x 0.6 m, bottom size 0.8 m x 0.8 m, height 0.6 m; made of wood size 0.2 m x 0.2 m x 1.5 m.

5. Equipment structures for contact and non-contact water level measurement

a) Made of stainless steel, round pipe, square pipe, or V-shaped;

b) Frame for installing equipment designed in a right-angled triangle or lever arm form, suitable for the technical characteristics of the equipment, attached to existing construction structures (concrete pillars, steel pillars, wharves, or platforms), one end attached to the equipment, the other end attached to the frame base in a rotating shaft form, ensuring stability;

c) The length of the lever arm for installing equipment is one-tenth of the height measured from the measurement point down to the sea surface at the lowest tide.

6. For automatic stations: use auxiliary equipment made of stainless steel, meeting the technical requirements of the measuring equipment, attached to existing construction structures (concrete pillars, steel pillars, wharves, or platforms).

Article 30. Frequency of monitoring

1. Monitoring shall be conducted four times a day at the hours of 1 AM, 7 AM, 1 PM, and 7 PM.

2. In case of dangerous weather conditions, observations shall be carried out according to the requirements of the competent authority.

3. For automatic observations: automatic measurement and data transmission 24 times/day or as needed.

Chapter IV

IMPLEMENTATION

Article 31. Effective Date

This Circular takes effect from October 26, 2020.

Article 32. Implementation Organization

1. The Director of the General Department of Meteorology and Hydrology shall be responsible for inspecting and guiding the implementation of this Circular.

2. Ministries, ministerial-level agencies, People's Committees of provinces, Departments of Natural Resources and Environment of provinces and centrally governed cities, and related organizations and individuals are responsible for implementing this Circular./.