Flash flood disaster in Bayangol district, Ulaanbaatar

Janchivdorj.L, Institute of Geoecology,MAS

Advanced Training Workshop on Reservoir Sedimentation Management10-16 October 2007. IRTCES, Beijing China

Total area: 1.565 million sq km

Mongolia resides on a high mountainous plateau ( aver.1580m)some 4,653 meters above the sea level

CHINACHINA

RUSSIARUSSIA

• Mongolia receives very limited precipitation with an annual mean value ranging 50mm southern part to 400 mm in the northern area.

• Also flow regime is not very stable and changes a lot depending on location, season and year. Seasonal variability is more predominant in rivers in Asian Internal Basin, while those in Pacific Ocean Basins show somewhat more stable regime.

• The water quality has been classified with the water quality index. Most rivers are still very clear, although some river cities, larger villages and gold and Cu. mines are with polluted water

• The exact these trends is (are) not yet clear, but human activities and recovery from the past human activities combination with global climatic change are suspected as possible reasons.

Mongolia is divided into three ocean basins in the Central and Eastern Asia, namely:

Water ResourcesWater Resources

AOBADB

Pacific Ocean Basin (POB) 12% of the territory)

Central Asian Internal Drainage Basin (ADB) (68% of the territory)

Northern Arctic Ocean Basin (AOB) 20.% of the territory)

POB

The Gobi Desert occupies more than 400,200 sq. km

Gobi Desert region

High Mountains region

Forest region

Steppe region • Forest region

• High Mountainsregion

• Steppe region

• Gobi Desertregion

• Flood caused calamities from generation to generation worldwide. That is why it is the one of the relatively well-explored natural phenomenon.

• There are a two type of floods.- One is a river flood, the other is a flash flood.

We will develop the methodology, make a correlation on the example of the Bayangoldistrict, Ulaanbaatar. In this district the flash flood is dominant. That is why we should like to explain the flash flood in detail.

FLOOD DISASTER…

• Since the systematic observation (1935) period, serious floods have been observed at Mongolia rivers and they caused severe property damages and loss of life.

• Flash flood is becoming one of the main disaster in Mongolia.

-About 18 flash flood events were observed from 1996-2003 and resulted in 56 lives lost and lot of property damages.

FLOOD DISASTER…

Due to Mongolia’s geographical location and its geomorphology flash flood probability is high in our country.

• One issue we should pay attention is a selection of the calculation method for the flood disaster protection, flood riskand disaster assessment.

• This is the complex consideration of the flood occurrence, its spread and scatter. Because of it this is a time consuming work and significant resources need to mobilize here. We need to follow the principle of the water accumulation basin in orderto conduct a flood risk assessment.

• For example in case of Ulaanbaatar we need to model the process of the raining, flood occurrence and its scattering on the basis of the probability theory in the 6900 km2 area which is the water accumulation area of the Tuul river. We also need to assess the load bearing capacity of the structures. We can get from the map the water accumulation area and the other related information and conduct a calculation on the basis of these information.

FLOOD DISASTER…

• The water and flood do not know administrative boundaries. It flows following the slopes of land by a natural law, and cause a distress to who lives on the down stream.

• Water flows, following the slopes of land by gravity. The flowing water possesses a huge energy by the effects of nature and human being. Because of it special professional staff design and construct this unique structures of flood protection on the basis on engineering calculation.

• Mongolia is situated on the central and northeastern Asian high land. It has a specific climate and nature. Its vegetation cover is sparse. By the result of many million years erosion. It has formed the gravel surface. The rainwater runs off on this surface without absorption forming a huge destructive force of flood. Because of it there are permanent erosion and damage on the earth and soil forming different kind and size of ditches. In steep ditches sedimentary gravel flows, slipping down. Its almost impossible to stop it. We need to take into account its flow intensity and volume when we plan the flood protection measures. This is inconvenient in case of cities and settlements. In the countryside it is a simple natural process to form a deposit of sediment. The people do not notice that gravel and dust make a movement to the downward by the wind, water and human beings, animals and form a deposit on the low lands.

• During a design of the flood protection structures we take into account it and plan specific measures for it. We do not need to seek a measure to withstand this powerful stream. If we make an error in the designing process the flood will cause a natural destruction to us. Because of it we build a pond to deposit the sediment. We will also fix and fasten the ditches which are under erosion.

FLOOD DISASTER…

• Ulaanbaatar city has built in the wide valley of River Tuulat the beginning and in later it is expanding to the top of the hills of the four mountains. This specific feature of the city has an effect on the rain distribution and intensity, and on the heavy rain’s following phenomena such as lightning and wind storm.

• The water accumulation areas of the rivers; creeks and dry beds between the four mountains which surround the Ulaanbaatar is flake alluvial. Capital city’s territory belongs to the heavy flash flood zone. The average precipitation amount of year is 258,8 mm in Ulaanbaatar.

• The floodwater carries down the mud, gravel even 15-20 cm diameter stones, hugely increasing its destructive force. This is called flash flood wave. The urbanization changes the earth and soil. The soil’s absorption ability is decreasing by the urbanization. By this way it increases the rate and volume of the floodwater.

• We must forget never that Ulaanbaatar is under a flood flash risk due the capital city’s geographical location and its geomorphology and the errors in the city planning. The historic record shows this. The base to live in the secure environment is to protect from the river and flash floods.

FLOOD DISASTER…

flood flash in a street at Bayangol district ,UB in 2003

Some houses are built on the floodplains , which are usually the pass way of a extra

ordinary flood flash, increased flow

resistance.

Flood control Dam and diversion channel

• We have calculated the maximum amount of the precipitation in one day, maximum flow rate of the rain flood, comparing it to the discharge capacity of the Western mountain’s flood channel (with its branch channels) by the Bayangol district’s flood hazard assessment.

• We have made a hazard map on the basis of this assessment. The maximum amount of the precipitation in one day is calculated on the basis of the historic data of the rain which fell on Ulaanbaatar and many year’s serial data of Ulaanbaatar’s 3 meteorological monitoring station

Disaster assessment of small reservoir for flood control in Bayangol district Structural works can not play fully it’s functions

The gher districts have settled down around on the up stream of this channel and around the dam. Here the vegetation cover is destroyed. House holds around it throw their wastes into the channel. Deposited sediment has not been cleaned up for years.

Flood control Dam and sediment reservoir

Map of flood disaster trace and inundation

The average precipitation amount of year in warm season is 233,8 mm. Significant amount of it fall in the form of heavy rain.The rain falls for 50-60 days in a year. (But in last the 10 year it is 69-82 days in a year).In the study period of time the rain’s 57-65 5 % was sustained rain, 29-41 % of it was heavy rain and 1-8 % of it was light rain which fell on Ulaanbaatar. During the heavy and sustained rain, the flood occurs in the ditches, on the slopes of the land surface and along the gradients of the roads and inundate the Gers, rise difficulties in the road traffic. The significant part of the rain falls between July and August. It is 70-75% of total precipitation (180-200 mm). The most intense rain fell in August 3rd 1982. During this event 44 mm rain fell within 17 minutes. Met. station it was 54 mm) fell on Ulaanbaatar in July 18th, 2003, causing a flood. Heavy rains fell on the Chingeltei mountains within an hour in of June 20th, 2000. Bayangol district of Ulaanbaatar is located on the feet and foot hills of Sambalkhundev mountain which is 1525.7 m above sea level. Here some flash protection measures has taken. But it is not perfect. Due to city planning distortion, someone’s willful activity, the flood risk is increasing rapidly in recent years.100 houses of the families which settled down on the floodway, over flow during the flood and spread on the streets and play grounds of children.

Probability curve of the maximum precipitation for a day, Ulaanbaatar met. station

H = 125,25 p -0,407

0

20

40

60

80

100

120

1,0 10,0 100,0

precipitation

0

50

100

150

200

250

300

350

400

450

1930 1940 1950 1960 1970 1980 1990 2000 2010years

T,m

m

Theoretical curve of the max. rain intensity, Ulaanbaatar Met. station

I= 8,7381Ln(t) + 45,565

0

10

20

30

40

50

60

70

80

0 4 8 12 16 20 24

T,hour

H,m

m

Conclusion an discussion• We should use the methodology approach that divides the flood into

two groups: natural (countryside) and city flood.-Rain and flash flood occurrence, its escalation, volume, duration

and its monitoring technology is different, because of it we should deal with the issue of river flooding and flash flooding separately.

• In case of flash flooding the way of prevention is limited. Here mostly the meteorological forecasting information for very short period of time is needed.

• According to our submitted methodology we have conducted a risk assessment for the Bayangol district’s flood protection structures. – Most of the flood protection structures of Bayangol district have

been built in 1966-1980. Because the insufficient maintenance and service, man’s ill natured activity for many years, it can not fulfill their purposes increasing the flood vulnerability. This channel did not transmit the flood water some times and the flood water overflowed the dam and flooded the ghers which were settled in front of this dam.

– First part of this channel’s discharge capacity is 5.7 m3/sec. But the coming water’s discharge is 9.5-16 m3/sec. We need to think that if we do not increase the discharge capacity of this channel the hazard will remain as the same before. 16 m3/sec dischargespassed in that channel during the flood which occurred in July 2003. It is obvious that more flood water will pass this channel.

Conclusion an discussion

• In case of Bayamgol district which is located in an extremely hazardous zone of flash flood and flood risk, we should take the next measures.

– We need to plan and accomplish the flood protection measures in the new settlement areas where the gher districts are expanding, besides that we need to fix and fasten the dry beds of ditches which are under erosion by the flash flood and water.

– To renew the old flood protection structures on the basis of thecalculation for the water discharge capacity, to improve the water discharge capacity of old flood protection structures. (Western mountain channel).

– In case of west dry bed of National Television Station which can not bear its load, we need to heighten the side walls by 0.6-1 m. There is need to develop a technical solution to take away the excess water.

– We will recommended that remove the Dam and clean up sediments

THANK YOU FOR YOUR ATTENTION !