6 The need for big scale measures 133
6.4 The need for big scale measures
6.4.2 River Channel Dimensioning Recommendation
The new artificial Chao Phraya River:
Section one of the new artificial river channel from lower Nakhon Sawan to Ayutthaya city is 153.22 km long. This area has mostly mixed-use activities in the rural area. This research strongly recommends the first section as the main concept for flood prevention, development of land activities and adaptation to climate change.
The section two from Ayutthaya to the Gulf of Thailand is surrounded with high density population and human settlement area. The length of the new artificial river channel is 122.99 km. The difficult circumstances around that area lead to the consideration of two different approaches for the new artificial channel between Ayutthaya and Bangkok. The two options include:
1. One channel with a concrete riverbed
2. Two separate channels with soil riverbeds which split below Ayutthaya. Those two options are described in detail below after the calculation of the sections of the new artificial river line.
Pasak River converges with the new artificial river
Pasak River will use as tributary of the new artificial river
Present Pasak River section will use as retention and recreation area
Figure 6.9 The new artificial Chao Phraya River and cross section calculation
Figure 6.10 Inclination of the new artificial river channel
Used Formulas and given values (See Appendix 5 for the derivation of the formulas):
River length (𝑙): Section 1: 153.22 km
Coefficient of roughness (𝑘): Soil with coarse stone: 25
Concrete: 70
Mean velocity of water (𝝂): 𝑘 ∗ (𝑅)23∗ (𝐼)12
Discharge (𝑸): 𝜈 ∗ 𝐴
Option 1:
The depth of the river line in this version is 10 m. The riverbed is made of soil and coarse stones in the first section and concrete in the second section.
First section (Nakhon Sawan to Ayutthaya; Upstream area)
River length: 153.22 km
Height difference of the river line: 26 m
Mean inclination of river: 0.17 m/km = 17 cm/km Main Channel total width: 100 m
Water depth of main channel: 10 m
Coefficient of roughness (soil): 25 Mean velocity of water: 1.34 m/s
Discharge: 1,202.62 m3/s
Second section (Ayutthaya to the shoreline; Downstream area)
River length: 122.99 km
Height difference of the river line: 7 m
Mean inclination of river: 0.057 m/km = 5.7 cm/km Main Channel total width: 100 m
Water depth of main channel: 10 m
Slope of embankment: 1:1
Cross section area: 900 m2
Volume of soil/water: 110,691,000 m3 = 111 million m3
Wetted perimeter: 108.28 m
Hydraulic radius: 8.31 m
Coefficient of roughness (concrete with timber lining):
70 Mean velocity of water: 2.17 m/s
Discharge: 1,950.17 m3s
Option 2:
The main difference of version two to version one is that the river line splits into two lines after the level of Ayutthaya. With two lines, it is possible to construct both lines with soil instead of concrete.
First section (Nakhon Sawan to Ayutthaya; Upstream area)
River length: 153.22 km
Height difference of the river line: 26 m
Mean inclination of river: 0.17 m/km = 17 cm/km Main Channel total width: 100 m
Water depth of main channel: 10 m
Slope of embankment: 1:1
Cross section area: 900 m2
Volume of soil/water: 137,898,000 m3 = 137 million m3
Wetted perimeter: 108.28 m
Hydraulic radius: 8.31 m
Coefficient of roughness (soil): 25 Mean velocity of water: 1.34 m/s
Discharge: 1,202.62 m3/s
Second section (Ayutthaya to the shoreline; Downstream area)
River length: 122.99 km
Height difference of the river line: 7 m
Mean inclination of river: 0.057 m/km = 5.7 cm/km Main Channel total width: 100 m
Water depth of main channel: 10 m
Slope of embankment: 1:1
Cross section area: 900 m2
Volume of soil/water: 110,691,000 m3 = 111 million m3
Wetted perimeter: 108.28 m
Hydraulic radius: 8.31 m
Coefficient of roughness (soil): 25 Mean velocity of water: 0.77 m/s Discharge per river: 696.46 m3/s Discharge of both river: 1,392.92 m3/s
From the collected data of the previous flooding events, it can be assumed, that this amount of discharge is suitable for a sufficient flood protection of the central area of Thailand.
This research suggest two options for the artificial river lines:
Figure 6.11 Options of the new artificial Chao Phraya River
Option No.1
One continuous river channel, where for the two sections use different materials for the riverbed. For the first section a riverbed use of soil and coarse stone is sufficient for the rural areas, whereas for the second line a concrete riverbed has to be used which increases the coefficient of roughness and therefore the discharge of the channel in urban areas. The second section has to have a higher discharge because of the consideration of heavy rainfall around the area of the river line. Also, the new artificial river converges with the Pasak River, hence it is necessary for the future to prepare an area for high river discharge downstream.
Option No.2
One river channel that leads from Nakhon Sawan to Ayutthaya and splits into two river lines after Ayutthaya. Two river channels have the benefit, that for the riverbed no concrete has to be used. The riverbeds can be constructed with a soil riverbed which decreases the maximum discharge, but with the addition of another channel, the same amount of discharge can be achieved. This option tries to have as less impacts on the natural habitats as possible, hence the soil riverbed and the lower discharge.
The research suggests that section one should be considered as the main support for flood protection, whereas section two in the high density human settlement area is an additional solution.
The slope of the riverbed can affect the soil erosion of the river channel. Hence, in concrete riverbeds steeper slopes can be used. In the following calculation a lower slope of the riverbed in the first section of the new artificial river is considered where soil is used for the river channel.
Example of the First section (Nakhon Sawan to Ayutthaya; Upstream area)
Coefficient of roughness (soil): 25 Mean velocity of water: 0.91 m/s
Discharge: 310.10 m3/s
A slope of 1:1 has the benefit of a high discharge, but it also has impacts on the soil of the riverbed. However, the soil erosion can be solved with vetiver grass that strengthens the ground (see Chapter 5.8 Number 4, Vetiver grass / Landslide or Land erosion prevention).