Thursday, 24 July 2008 9:28 AM

Lake Tengano Residents Say Lake Level Increasing

Halo editor mi laek sharem samfala idea lo dis fala subject tu.

The best way to understand such problem is take a look at the overall geomorphology. Landscape systems are dynamic and interrelated. When left on their own they are in equilibrium but once changes are made to the landscape other aspects of the system change to return to equilibrium.

Appropriate questions include: has the lake shorelines been artificially retreated? Are there any changes to the catchment areas? Have dams (natural or man made) built across tributaries that recharge the lake? Has there been any dredging in the lake bed (which I doubt)? Was there prolong rainfall that resulted in a huge recharge. Was there a period of neap high tide (extreme high tide)?

If the claims by Mr Tango are accurate, that the lake rose 2-3 feet (0.6-0.9m), there is room for some serious concern. Lakes are generally at the same level as the sea (sea being the base level). So in theory the lake should rise if there is a rise in sea level as the locals infer. Alternatively lake level may rise due to high recharge or due to neap high tide. But as Mr Fairfax correctly pointed out, sea level change is not a localised issue, alarm bells would be ringing in other parts of the world should a 2-3 feet rise in water occur.

Suggestion by Mr Fairfax that the island may have sunk is possible but difficult to ascertain. I may be wrong here but I gathered Rennel & Bellona are uplifted coral islands with volcanic or metamorphic basement rocks (?). Undeniably the Solomons is tectonically active area but for such a change to take place there has to be a tectonic event such as fault raptures say along a subduction zone margin. Was there such an event prior to the rise in lake level that could have caused the island to subside? Mr Fairfax correctly referred to the drop in some reefs during the Western Province Tsunami but forgot to mention the rise in other areas. Without getting bogged down in technical jargons, fault raptures can result in uplift on one side of the fault trace and subsidence on the other wall (that is for a classical strike-slip fault).

So how do we reconcile this rather odd event? Well perhaps try to answer questions posed earlier. May be residents should try and accurately measure the rise in lake level again. If there is a known datum on the island (Geology Department or Met Service could help), the actual drop in level of either land or lake can be confirmed.

Disclaimer: The views and opinions expressed in this letter/article are those of Warwick Sitana and do not necessarily reflect the official policy or position of Solomon Times Online.

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