Firstly, the project has built numerical hydraulic and salt transport models for Tra Ly River and Thai Binh rivers. The models can be used to serve the forecasting development of saltwater intrusion in different concrete hydrological and hydraulic conditions, especially the varied sluice gate operating mode condition. The models' results showed that:
- The hydraulics of salinization in Tra Ly and Hoa rivers particularly depends on the hydrological, hydraulic and tidal conditions and occurs in a high complexity. With the frequency of river flow of 85% in the dry season the salinity intrusion in Tra Ly river over a length up to 19.4km and in Hoa river up to 14km from their mouths;
- Salinity intrusion in the dry season months in the beginning of year is stronger than in the last two months of dry season. Although saltwater intrusion is strong during dry seasons, during the first months of the dry season there is a lot of the time when the rivers have salt content lower than 1ppt, during which it is likely could be abstracted for domestic water supply and agriculture;
- The predictive results of the models have very good role in building low salinity content water at different locations in Tra Ly and Hoa rivers over the time. This has great practical significance for the supply of domestic and agricultural water in two districts Thai Thuy and Tien Hai, where there is extreme difficult conditions of fresh water resources, and for contributing to the building of water pumping plan at Tra Ly and Hoa rivers over the time .
Secondly, the Project has built model of groundwater movement and salinity transport in groundwater for water extraction from the upper Holocene aquifer along Tra Ly and Hoa rivers to serve the planning and development of fresh groundwater resource, the combined use of surface water is to prevent saltwater intrusion in Tra Ly and Hoa rivers and to sustain the development of fresh groundwater resource in the area. The modeling method is the finite element method and dynamic programming to determine the optimal pumping. The results of the specific groundwater movement and salinity transport have showed that:
- Salt intrusion prevention dams in Tra Ly and Hoa rivers provide primary conditions for increasing groundwater and contributing groundwater de-salinization of the upper Holocene aquifer, and the groundwater wells along the rivers may provide from 130m3/day or more for a given rural domestic water supply plant;
- The groundwater abstraction from the 3rd year onwards would reach a steady-state, and the flow from the river provides from 70 % to over 80 % of the water abstraction volume for the wells located 400m from the river bank;
- Initially the groundwater is salty or brackish, after the water is then de-salinized by the rainwater infiltration and water flow from the river; the desalinization happens slowly during the first years from the start of the abstraction, the desalinization process mainly light occurs in near river area, and gradually moves further to the pumping wells;
- The entire area between the river and the pumping wells would be completely exploited from the 20th year since the abstraction start and the fresh groundwater abstraction would last continuously forever in future .
Thirdly, the advances and disadvantages of the two salinization prevention dams on the two rivers:
- The proposed locations of the two dams are the most suitable and ensure the maximum benefit upstream dams' areas, as well as the area downstream of the dams has a narrow width of the downstream area, so the fresh water conduit to the area requires only a small scale construction;
- The two dams' locations are relatively close to the sea so should not significantly affect the hydraulic regime change than the case there are no dams, which ensures no adverse impacts to the flood planning for Tra Ly and Hoa rivers' catchment areas;
- The presence of two dams would support the interior drainage culverts at upstream of the two rivers (at low tides) if there are some accidents to the culverts and/or decrease of drainage capacity due to deterioration;
- Two dams would create a dozen of kilometers in length of fresh water in the rivers, which has a great effect to improve water supply to areas where fresh surface water and groundwater is relatively limited;
- The presence of two salt-intrusion prevention dams would facilitate the development of long-term fresh groundwater supply for the future, particularly in the context of sea level rise;
- The combination of inland transportation bridges with the dams will create favorable conditions for the transportation, contributing socio-economic development and enhance the spiritual life of the people through the ease visiting, trading and the business partnership etc.
- The presence of the constructions will alter the natural conditions surrounding areas, in addition to the overwhelming benefits that constructions would bring, it could not avoid some limitations and disadvantages, specifically the risk of flooding (especially if the operation incidents occur) , alteration of the balance of alluvial transport, difficulty to waterways, affecting the residence and migration of aquatic lives etc. However, from the overall review, the economic and social benefits are overwhelming.
Fourthly, the operation regulation regime of the salt-intrusion prevention dams in Tra Ly and Hoa rivers had been studied and recommended is:
- For the Tra Ly dam's sluice gate: real-time operation through hydrological-hydraulic and salinity transport model with relationship between water level and flow rates; with the installation of electronic monitoring system that automatically measure water pressure, water column pressure to the construction etc.; the charts of water levels upstream and tidal fluctuations should be built before and supplied to the operation unit; with proper communication system with the central committee of flood and storm prevention and Thai Binh province to promptly intervene in the emergency situation during the rainy flooding seasons.
- For the Hoa river sluice gate, two operation modes proposed are: 1 ) real-time operation (built in the same manner as Tra Ly river dam) ; 2 ) Rigid operation (but by installing sluice equipment of automatic operation). The rigid operation of Hoa river sluice gate shall be determined through hydrological-hydraulic and salt-intrusion modeling results, based on which the system for automatic drainage from the upstream of the dam to the seaside is designed to ensure water flow in a given range of values; with proper communication system with the central committee of flood and storm prevention and Thai Binh province to promptly intervene in the emergency situation during the rainy flooding seasons.
Fifthly, about the dynamic engineering geological phenomena that affect the Tra Ly and Hoa rivers' dams, the Project had given the results:
- The study areas have presence of some risk of disadvantages in engineering geology such as the underground erosion and ground surface boiling and weakness by groundwater pressure, which can cause indirect effects to pile foundation of the constructions, and is represented by concrete instability parameters specifically to the areas;
- The foundation conditions of the depth up to 40m overall have major weak soil types (clay, silts) mainly of soft state to plastic state with low loading capacity. In the area of proposed Tra Ly dam up to a depth of 40m it still has not meet higher bearing capacity soil. In the area of proposed Hoa river dam, a little higher bearing capacity soils had been reached from a depth of 35m;
- The soil layers are relatively strong varied in space and depth, having large variability in physical and mechanical properties, so the analysis of the bearing capacity had been conducted by the division of the soil layers into thin slices. The calculation results of the load bearing capacity of reinforced concrete single pile of section 0.3m bay 0.3m and 40-m-long by different methods have given different results, ranging from the lowest value of approximately 25tons to highest value of 48tons.
Within the framework of the projects, two scientific articles have been prepared and submitted to the journal The Earth Sciences, one of which is currently under publication:
- Nguyễn Văn Hoàng, Nguyễn Thành Công, Lê Quang Đạo, Ứng Quốc Khang. Study of Tra Ly river salt intrusion modeling for assessment of salt intrusion prediction. Under publication;
- Nguyễn Văn Hoàng, Ứng Quốc Khang, Lê Quang Đạo, Phạm Lan Hoa, Lê Thanh Tùng. Study of desalinization of upper Holocene aquifer thanks to salt-intrusion prevention dam in Tra Ly river in Thái Bình province. Under review.