TO ANTICIPATE IMPACTS OF RECLAMATION OF JAKARTA BAY

- A groundwater conservation's perspective - ^*)

by : Soetrisno S., Satriyo H., and Haryadi T. ^**)

3. Impacts

The main reclamation activities will be fill in the Jakarta bay with allogenous materials to certain depth and certain extension of the bay.

Depending on the type/size of the materials and its dimensions, the reclaimed land will give impacts to the groundwater setting, among others the important are :

1. Changing of interface between fresh and salt water of shallow groundwater.

Since the shore line will move to seaward, hence, the interface will more away from land. The hydraulic pressure of salt water will awaken by reclaimed land. However, it depends on the grain size of materials which are used for filling in. If the function of the reclaimed land is dominated by fine materials, then it conducts as barrier from salt water flow to inland and will give an advantage in decreasing of shallow groundwater salinity, instead otherwise.

By using simulation modelling MODFLOW, and assuming that reclaimed land surface will be 3 m above sea level in average, it can be predicted that fresh-salt water boundary moved about 1000 m toward the sea from its existing position (Fig 1 and Fig 2).

2. Changing of recharge - discharge pattern.

Actual groundwater flow in North Jakarta, due to lowering of piezometric head, is lasting from shallow to deep aquifer system. Downward leakage may occur almost everywhere in entire basin of Jakarta.

As reclaimed land will conduct as barrier flow, then, it will influent the actual flow. However it is still depending on materials filling, whether the reclaimed land will act as barrier or conduit. Further research is still needed to get a detail knowledge concerning those matter.

3. Changing of water balance.

There will be an additional space of infiltration from precipatation and surface water in the northern part of Jakarta, after reclamation are finished.

Water balance, then, will change as the new infiltration and surface run-off occurs.

If the everage precipitation (P) of Jakarta is 1,730 mm year^-1 and potencial evaporation (Ep) is 1,606 mm year^-1, then assuming that material filling has horizontal permeability (Kh) 2.64 m day^-1 and vertical permeability (Kv) 1/5000 kh as existing basin filling in north Jakarta (Schmidt and Tirtomiharjo, 1986), whereas 60% of reclaimed land is still open space, hence, infiltration rate (Rgw) may be calculated using MODFLOW will be:

Rgw = 18.25 mm year^-1

or equivalent with 1.05% of P.

For the whole area of reclaimed land (2,700 ha), then total recharge will be :

å Rgw = 18.25 x 10^-3 x 27 x 10⁶

= 492,750 m³/year^-1

This figure is insignificant compare to the total availability of groundwater in the basin, however, since it occurred in coastal area, it will give influence to the hydrostatic balance between fresh and salt water.

Surface run-off (Ro) calculated as 105.75 mm year^-1 or equivalent with 6.11% of P, then total surface run-off for the whole area of reclaimed land will be available :

å Ro = 105.75 x 10^-3 x 27 x 10⁶

= 2,225,250 m³ year^-1

This total amount of surface run-off is relatively high figure, therefore to build ponds or lakes in that area will give a benefit for increasing recharge and controlling flood in north Jakarta.