EROSION CONTROL IN VIETNAM USING JAPANESE RIVER ENGINEERING

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1 Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015 September 7 th -8 th, Hoi An, Vietnam EROSION CONTROL IN VIETNAM USING JAPANESE RIVER ENGINEERING Hirotada MATSUKI, Dr.Eng. JICA Expert, MARD, Vietnam Address, So 2 Ngoc Ha, Ba Dinh, Ha Noi, Vietnam matsuki84@gmail.com Abstract This paper is a report of riverbank protection projects under a concept to use natural power of rivers. Riverbank erosion is one of the most severe problems in the rural area of Vietnam. JICA has been supporting river management using Japanese river engineering. In 2010, JICA conducted 2 riverbank protection works at Kim Ngoc Village and Thanh Xuyen Village in the central Vietnam. Main structure at one site was bamboo foot protection to prevent scour holes in form of the riverbank. Another was masonry groyne to deflect flood flow away from the riverbank. These techniques have a purpose to avoid thalweg away from riverbank to change flow conditions from erosion to sedimentation. In 5 years after construction, flood flow had changed to put sediment on riverbanks to increase their stability. Structural specifications in this paper are designed to meet each site conditions, but these are referable for other projects. We can scale up this technique to erosion control in Vietnam. From these trials, finally, this paper shows some considerations as lessons learn from rivers, cost validity, reparability, people s participation and engineer s capacity. Keywords: Erosion, Deposition, Equilibrium, Thalweg, Foot protection, Guide wall and Groyne 1. INTRODUCTION Yearly typhoons bring vast rainfall to cause a large amount of runoff. Rivers in Asia monsoon region have a high and rapid water level rises which can trigger severe riverbank erosions. Locan authorities in Vietnam have been tackling such natural hazards, but suffering financial burden. Many donors also try to contribute for erosion control, however, total budget is insufficient to cover all of less-populated communities. They are often ableto invest for erosion control saying lack of fund, moreover, some of their protective facilities are inviting budgetary needs for maintenance. In , JICA had conducted the Project for Building Disaster Resilient Societies in the Central Region in Vietnam to enhance community-based disaster risk management. At 2 villages which had typical erosion problems, JICA worked with engineers of local provinces to install Japanese river engineering named small-scale and low-cost method. Because of insufficient numerical datinformation of site conditions, they had to design facilities depending much on experimental judgment. In 2010, they constructed bamboo foot protection for Kim Ngoc Village and masonry single groyne for Thanh Xuyen Village. The authoer surveyed 2 sites in 2014 and 2015 to evaluate conditions and effectiveness of the facilities. This paper shows results of the riverbank protections and lessons learned from the projects. 1

2 2. BACKGROUND HISTORICAL, THEORICAL AND LEGAL Japanese traditional river engineering has 3 major backgrounds as follows: 1) trials and lessens in 400 years, 2) learnings from of natural theory and 3) technical standards on legal basis. In the end of the 16 th century, feudal lords applied military engineering into rivers to enrich their territory. After national unification in the 17 th century, they became local authorities to enhance rice cropping and water communication for economic activity of their provinces. Their river management has been carried on basically up to now. Natural floods become severer in the 18 th century, so that the central government tried to standardize river engineering and reduce administrative costs. In each province, engineers could select facilities from manuals, however, economic damage increased and recovery cost became a burden to local authorities in the 19 th century. They had to manage rivers within limited financial capacity and brushed up their river engineering. The advanced engineering had unique features as: - to manage a river by interaction with the river - to minimize damage in the river basin - to modify techniques in accordance with local conditions - to maintain facilities in daily/yearly livelihood of people Japanese river engineering has been modernized throughout the 20th century. Engineers had manualized advanced technology which often use concrete and metal. Investment for river management succeeded in reducing flood damage frequency and ensuring economic development in flood prone areas. In parallel, river maintenance cost increased and river environment degraded. On this occasion, Japan River Council summarized a report Progression of river culture in ). The report stressed to combine traditional and modern engineering to meet the latest demands of changing society. The traditional engineering utilizes natural theory of rivers. Rivers bring water and sediment. Each sediment keeps its equilibrium among underwater gravity, tractive force and frictional force, to formulate river morphology as a result. At outer banks of river bends, longitudinal flow and transverse secondary flow combine each other to make a spiraling flow. This flow washes away sediment on outer riverbed and puts it on inner riverbed downwards 2). Outer scouring leaves a thalweg. It should be notified that a thalweg is moving during a flood. Flood flow is much powerful to shift a thalweg deeper and outer than usual. When a thalweg on riverbed develops well and approaches riverbank, materials on the slope slide down into the scour hole. This is a major cause of riverbank erosion. If we put a small change on the flow conditions, rivers would response to reach different equilibrium to formulate another thalweg. Starting from present equilibrium, by shifting thalweg, it is possible to encourage rivers to make safer riverbed and riverbank. This concept is summered in a phrase River Makes River. To apply this river engineering, it is important how to use groynes 3). River management facilities in Japan are stipulated in River Law. Under the law, a Decree describes Structural Standards for River Facilities which stipulates 3 major structure namely dyke, revetment and groyne 4). - Continuous dykes or unconnected dykes prevent outflow of flood. - Dykes, revetment, groyens and others should keep safe from design flood. - Revetments cover dykes directly from flood flow, if necessary. - Groynes deflect or decelerates flood flow to protect dykes, if necessary. These articles is so general and not specific because field conditions have a wide variety. To supplement the standards, MLIT has circulated technical guidelines and responsible institutes have issued technical commentaries. These documents recommend that - Revetments should consist of slope protection, foundation and foot protection. - Groynes must be designed to accomplish defensive effects and environmental impacts. Using these regulations, river engineers have to design dykes, revetments and groynes, which is effective and harmonious to characteristic conditions of each river. 2

3. BAMBOO FOOT PROTECTION JICA assisted riverbank protection works in 2010 at 2 sites in the central Vietnam: Kim Ngoc Village on Ta Trach River in Thua Thien Hue Province and Thanh Xuyen Village on

3 3. BAMBOO FOOT PROTECTION JICA assisted riverbank protection works in 2010 at 2 sites in the central Vietnam: Kim Ngoc Village on Ta Trach River in Thua Thien Hue Province and Thanh Xuyen Village on Thu Bon River in Quang Nam Province. Fig.1 Map of Kim Ngoc Thanh Xuyen At Kim Ngoc, Ta Trach River rose year after year 2m on a riverside community road. The river flow eroded 300m of the riverbank during the 2007 flood and the erosion went forward 15m in the next 3 years. People reported that further erosion in 2009 typhoon Ketsane threatened 120 houses and the community road. This site was selected to apply Japanese river engineering 5). In a field survey, JICA experts confirmed a 250m eroding bank and a typical spiral flow. The flow looked to be deeper and stronger during flood and to accelerate the erosion (photo-1). In order to weaken the spiral flow, the experts suggested to put an effective foot protection which could prevent local scour on the riverbed, although it was often neglected in Vietnam. For the main structure, they selected bamboo gabion because it had high quality for underwater works. All materials for bamboo gabion were locally available in low price. Vietnamese engineers designed other parts of foundation and slope protection by their conventional methods. Each structure was designed as followings: - Riprap foot protection for deep part - Bamboo gabion foot protection - Bamboo crib foundation at water edge - Riprap on eroded lower riverbank - Grassing on non-damage upper riverbank The bamboo gabion was made by local workers. They stitched split bamboo to make a sheet on the slope. Then they fastened it like a sausage on the water and put stone inside to sink it down on the riverbed (See Photo.2). Including this work, averaged cost for the riverbank protection was 400 USD/m. The bamboo structure didn t decay under the water (See Photo.3). In 4 years after the construction, the river level rose up every year to submerge the riverbank. As a result, the riverbank got 0.3m thickness of sediment until The sediment transported by the river indicated that flood flow had smaller velocity than before because the foot protection kept water depth shallower (See Photo.4). To maintain this function, people should monitor and repair the foot protection according to its conditions after every flood. The foot protection and the slope protection were covered by natural vegetation and human cultivation that would make water flow slower and slower. However high trees 3

The stronger bamboo gabion was used, the more stable and long-life foot protection could be constructed. Photo.1 Erosion at Kim Ngoc (2009) Photo.

4 might make the slope fragile because their roots would vibrate the soil under strong wind. People should maintain the trees in low height. In addition, local people looked to have good skill of bamboo fabrication. The stronger bamboo gabion was used, the more stable and long-life foot protection could be constructed. Photo.1 Erosion at Kim Ngoc (2009) Photo.2 Bamboo gabion at Kim Ngoc (2010) Photo.3 Bamboo gabion at Kim Ngoc (2012) Photo.4 Sediment at Kim Ngoc years after (2014) Slope protection Foundation Foot protection Fig.2 Basic structure of revetment-slope protection, foundation and foot protection 6) 4

4. MASONRY SINGLE GROYNE AND BAMBOO GUIDE WALL In 2007 flood, Thu Bon River eroded riverbank at Thanh Xuyen village for 500m, especially severely for 250m.

JICA decided to put structural measures at this site expecting to be a pilot project among many erosion sites in Thu Bon river basin 5).

To avoid damage at this section, the experts suggested to control thalweg farther than the present condition.

5 4. MASONRY SINGLE GROYNE AND BAMBOO GUIDE WALL In 2007 flood, Thu Bon River eroded riverbank at Thanh Xuyen village for 500m, especially severely for 250m. People reported its expansion after 2009 typhoon Ketsana. Behind the riverbank, there are a community road and 250 houses (See Photo.5). JICA decided to put structural measures at this site expecting to be a pilot project among many erosion sites in Thu Bon river basin 5). JICA experts surveyed river morphology and found that the main flow came closer to 250m section and backed away at natural rock at downstream. To avoid damage at this section, the experts suggested to control thalweg farther than the present condition. For thalweg control, groynes were effective and Vietnamese used masonry groynes in many rivers. The experts selected a single groyne to deflect strong flow at upper-end of eroding bank and an underwater wall to guide the inflected flow downwards smoothly. The guide wall was made of bamboo baskets and connected to the riverbank by several bamboo barricades. The wall and the barricades would catch sediment and form a wide foot protection under the water. Each structure was designed as followings: - Single masonry groyne at the most crucial eroding point - Bamboo guide wall for deflected flow downwards - Bamboo barricades to connect the wall and the bank - Bamboo crib foundation at toe of eroding river edge - Grassing or vegetation on the eroded slope A single masonry groyne was built at the most crucial eroding point to deflect river flow. The structure had enough length of 20m because it can keep functions after getting collapse at its head part (See Photo.6). And the underwater guide wall was expected to shutout diffracted flow and send deflected flow downwards. Behind the wall, several barricade reduced flow velocity to stop erosion and attract deposition, so that slope protection on the bank was neglected (See Photo.7). These structures cost 400 USD/m as a whole. Photo.5 Erosion at Thanh Xuyen (2009) Photo.6 Masonry groyne at Thanh Xuyen (2010) Photo.7 Design thalweg at Thanh Xuyen (2010) Photo.8 Sediment at Thanh Xuyen years after (2014) 5

6 In 4 years, the river flow deposited soil on the downstream field of the groyne. The thickness was over 1m and the width was 15m at a maximum, leaving 5m of the groyne on the water. New vegetation grew up on the sediment. This fact showed that the groyne and the underwater structure had succeeded to make the water flow go slower (See Photo.8). One concern was a vortex at upstream side of the groyne. If the groyne tilted downstream a little, the flow would not make the vortex which may damage the bank. As well, more morphological information, such as bathymetric survey, would help more adequate configuration of structures. According to washing away of sediment or deformation of the groyne, additional groynes or guide wall should be designed. Groyne Fig.3 Structure of single masonry groyne 6) Barricade Guide wall Fig.4 Structure of bamboo barricade and bamboo basket guide wall 6) 6

7 5. CONCLUSIONS The author can evaluate the trials at Kim Ngoc Village and Thanh Xyuen Village are successful at the moment 5 year after construction. At the 2 eroded riverbank, thalweg was avoided away from the eroded riverbank. As a result, flow conditions had changed from erosion to sedimentation, so repeating flood is expected to make the riverbanks more and more stable. These are outputs of the concept to use natural power of rivers. From these experiences, we can share some recommendations to develop river engineering. (1) Lessons learnt from rivers Under the concept, engineers should give intentional impact to rivers and wait for expected reaction by rivers. Large impact, however, may bring back unexpected reaction which damages riverbanks. Moreover needless facilities often leave waste of budget and negative impact on ecological conditions. It is recommended for river engineers to learn from rivers in advance and take try-andcheck procedure. (2) Cost validity Construction cost of the 2 riverbank protection were 400 USD/m equally. These were relatively cheap as initial cost because engineers tried to achieve the maximum effect within limited budget. Moreover the facilities required minimum maintenance, almost maintenance-free. In other cases, however, cost-cut investment often invites costly maintenance or fruitless reconstruction. Engineers should design facilities building in maintenance work. (3) Reparability Rock and bamboo were selected as main construction materials. Local labors were hired in the construction work. These approaches contribute not only to save budget but also to prepare for repairing work. After getting damage on some facilities in the future, experienced local labor are able to repair them using available materials. Utilization of local material and local labors is a key factor to secure reparability of facilities. (4) People s participation Local people are the first beneficiaries of defensive infrastructure. And they are able to exam any abnormality in their daily life and repair small damage on the spot. Their sense of ownership is essentially important for facilities to keep functions long time. River engineers should enhance people s awareness through construction and maintenance of riverbank protection. (5) River engineer s ability For river management, experienced river engineers are indispensable. While workers hold highquality construction capacity, most of river engineers have less chances to train themselves to originate and arrange river facility design. It is recommended for local authorities to give opportunities for river engineers to brush up their skills and foster their successors in the next generation. That is indispensable to sustain local communities and economic activity in flood-prone regions. This paper reports 2 trials of erosion control by thalweg avoidance from riverbank. Lessons learned are usable promptly for other riverbanks which have similar conditions. We can scale up this technique to erosion control under at other rivers different conditions in Vietnam. As well, the core technique is to control thalweg on riverbed. This is also applicable for deposition control, such as flashing away sediment from intakes by flood flow. In other cases, this technique is turned over to different purposes as navigation management, sand excavation, vegetation control and ecological diversity. This concept is called River Makes River and has a wide range of application. River engineers should understand mechanism of rivers and develop engineering to manage rivers safely, beneficially and naturally. After studying basic methods in manuals, river engineers should originate a unique technique, adapt it at each site and share experience with other engineers and people living with rivers. 7

8 6. REFERENCES 1) River Advisory Commission: Inheritance and development of traditional river engineering for our life and culture, 1999 (in Japanese) 2) Oregon State University: Streambank erosion protection and channel scour manipulation using rockfill dukes and gabions, ) Shubun Fukudome: Study of the technical approach and the applicability on river works aiming at harmony between the flood control and the river environment, 2011 (in Japanese) 4) Commentary on Structural Standards for River Facilities, 2008 (in Japanese) 5) Takayuki Nobe: JICA project annual report for riverbank erosion management, ) Ministry of Agriculture and Rural Development in Vietnam: Guideline on Riverbank Erosion Management, pp.4-5,

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