Environmental Control During Reclamation
Environmental Control During Reclamation
This chapter focuses mainly on monitoring and measurement. In order to minimize the environmental impact, hydrodynamic modeling and silt-transport modeling need to be done prior to reclamation. Erosion or sedimentation that occurs at a certain location depends on the reclamation layout and changes in the direction of the current resulting from the reclamation. Models can be done for each specific site. Usually measurements are taken at the feasibility stage for the purpose of calibrating the model. In creating land at foreshore locations, changes in the environment will occur. In order to minimize the environmental impact of land reclamation, several measures need to be taken. Environmental changes are likely to occur on the seabed, the water mass, the coastal line and marine life. To control the environmental impact, the following measures are necessary.
In creating new land at foreshore locations, the pattern of the waves and currents is likely to change. Changes may also occur to the seabed because of erosion or sediment movement. In order to monitor these changes, seabed sampling is carried out at several locations. These types of seabed samplings are taken using vibro-coring methods and the collected samples are sent to the laboratory for grain size distribution analysis. Figure 7.1 shows vibro-core equipment, and Figure 7.2 shows vibro-core sampling in progress. Figure 7.3 compares the grain size distribution of the seabed soil before and after reclamation at one particular reclamation project.
It can be seen that at most locations, deposits of finer types of sand with silt was found after reclamation especially at sampling points in front of natural slopes without any shore protection. However, the location adjacent to reclamation with shore protection does not show any changes in the grain size distribution of seabed soil. It is clear therefore that shore
protection structures protect erosion and also protect soil from being washed away.
Owing to erosion and transportation of sediments not only materials on the seabed can be changed but the seabed topography can also be altered. Therefore, seabed monitoring is necessary. A hydrographic survey to monitor the seabed morphology is usually carried out at certain time intervals during the reclamation process. Figure 7.4 (a) to (e) shows the change in seabed morphology some time after reclamation. It was found that most areas had heaved up and only few areas had settled. Most of the heaved-up areas were found at the toe of the slope.
Due to the action explained earlier, the geometry of the beach around the reclamation area can be changed. The changes can be in the form of either erosion or deposition. Land and hydrographic surveying are necessary to monitor the changes of the beach profile up and downstream of the reclamation site since erosion and deposition occur at such locations. Figure 7.5 shows erosion of a beach close to a reclamation site, whereas Figure 7.6 shows deposition at a beach downstream from the reclamation area.
The geometry and size of the channel around the reclamation area will be changed during the reclamation process because the current regime in the channel around the area will change. In turn, this may lead to change of the three features explained in the earlier sections. Therefore, current measurements are usually taken at several locations suggested by the hydraulic modeling. The currents are measured at several levels along the water column, and generally an automatic real-time monitoring system with data transmission is used.
The typical instrument used in the Changi East reclamation project is called a sentinel. The instrument consists of a mooring frame with two spherical buoys to hold it to the top of the mooring while allowing the sentinel to profile the water above. The buoys are made up of a load frame and a synthetic foam sphere. Several velocity measuring cells are installed along the profile. The number of cells for various depths for different sentinels are shown in Table 7.1. Specifications of three types of sentinels are shown in Table 7.2.
|Table 7.1 Some typical deployments of sentinels.|
|Water depth (m)||10||50||120|
|No. of cells||8||12||16|
|Cell size (m)||1||4||8|
|Ens. Interval (min)||15||30||60|
Figure 7.7 shows a typical design of a sentinel, its installation and its features. Figure 7.8 shows an increase in current movement during and after reclamation. It was also found that there were fluctuations in current velocity during the reclamation period and an increase in velocity after reclamation.
|Table 7.2 Specifications of three types of sentinels.|
± 2.5% of water velocity
± 0.5% of water velocity
|Velocity resolution||—||1 mm/s|
|Velocity range||—||± 5m/s (default)|
± 20m/s (maximum)
|Echo Dynamic Range||—||80dB with ± 1.5dB|
|Temperature sensor||—||-5∞ to 45∞ with ± 0.4∞ C|
precision and 0.01∞ resolution
|Tilt||—||± 15∞ range with ± 0.5∞ accuracy,|
± 0.5∞ precision and 0.1∞ resolution
|Campus||—||± 2∞ accuracy ± 0.5∞ precision and|
0.01∞ resolution maximum tilt ± 15∞
The pattern of waves rarely change because of reclamation but may change slightly because of shore protection structures. Therefore, wave measurements are also carried out from time to time. Figure 7.9 shows equipment used in wave measurement. The measurements are usually based on water pressure.
The typical parameters that are derived from the measurement are:
Hs(significant wave height), Hmax(maximum wave height)
Tp(peak crossing period), Tz(zero-crossing wave height)
Depending upon the quality of the material imported, the quality of water around the reclamation area may change. Another reason for water quality changes could be the change in the surface drainage pattern. Therefore, water quality surrounding the reclaimed area needs to be monitored from time to time. Water samples are collected from time to time and sent to the laboratory for mechanical, chemical, and biological analysis. There are some instruments which can measure and determine the quality of water in-situ. They measure the amount of dissolved oxygen in the water to determine the quality. Figure 7.10 shows the measurement of dissolved oxygen in the water during reclamation. It was found that the dissolved oxygen was reduced during reclamation. The clarity of water is also measured from time to time. Water clarity is measured in terms of visibility to the depth as shown in Figure 7.11. It was also found that water clarity was reduced during the reclamation process. Sometimes, instead of clarity, turbidity is measured.
Turbidity could be increased when there is an increase in suspended solid. Therefore, water samples are collected and sent to the laboratory to determine the amount of suspended solids. An increase in suspended solids will affect marine life. Figure 7.12 shows an increase in suspended solid content during reclamation.
In order to prevent the migration of fine material to the surrounding area silt barricades are usually installed around the working area of the reclamation. An example of such an installation can be found in Kansai International Airport, phase II, where the whole site was surrounded by a silt barricade throughout the profile of the water column (Figure 7.13). In order to protect the profile of the water column one silt barricade could be dropped from the sea level and another could be floated up from the sea bottom (Figure 7.14). Silt barricades are usually made with geofabric. Some reclamation projects use a temporary sheet pile instead of a silt barricade. Figure 7.15 shows a reclamation site surrounded by a temporary sheet pile to prevent the drifting of silt. For some projects, a silt barricade is installed downstream of the rehandling pit to prevent the drifting of silt while rehandling (Figure 7.16).