＊Lobster Isle
Tamsui locals say that there is a peculiar double fork flow around Lobster Isle in Zhouzhiwan bay . During low tide one can distinctively see two wave trains moving respectively from the two different sides of Zhouzhiwan bay and meeting together at the end of Lobster Isle.
And soon after the two join together and gain momentum, they become one strong flow flushing out back into the ocean along both sides of Lobster Isle . The peculiar flow looks most spectacular when it is splited in two near the two claws of Lobster Isle.
＊The two wave trains moving respectively from the two different sides of Zhouzhiwan bay
Some say that the double fork flow has faded away since Taipei Port was built. while others believe that it is still there, only it has become less powerful than it used to be.
What causes the mysterious double fork flow to occur and how is it that it dissapears ? What does it signify when it is being there or not being there? why does its existence has anything to do with Lobster Isle and Taipei Port ?
By exploring the above-mentioned issues we expect to drow public attention to the current critical situation of Lobster Isle and the deteriorated ecological environment around the isle, and by putting our heads together we hope we might find out answers to these questions through joint efforts.
Double fork flow is a common expression widely used among local fishermen for calling the specific ocean current. As a metter of fact, it is termed rip current in oceaneering.
A rip current is a narrow, powerful current of water which moves directly away from the shore, cutting through the lines of breaking waves like a river running out to sea, and is strongest near the surface of the water.
Rip currents form because breaking waves push surface water towards the land, and this causes a slight rise in the water level along the shore, and which will tend to flow rapidly back to the ocean.
When waves travel from deep to shallow water, they break near the shoreline and generate currents., which run parallel to the shoreline. This type of currents is called “Coastal currents.” A rip current is generated by coastal breaking waves and it forms when a coastal current travels in an offshore direction.
A rip currents run almost perpendicular to the beach, and it can speed up dramatically in a short time but does not last long. It mostly occurs during periods of intense wave action；otherwise its strength can be too weak to gather enough momentum to move itself seaward across the breaker zone and spread out to the ocean.
The location of a rip current is subject to change due to the effects of wave height, wave period, submarine terrain and the shape of the coastline and it plays a vital role in promoting the transport and the erosion of coastal sediment.
Western Taiwan coastal current ebbs and floods just in opposite directions. When the north-bound Western Taiwan coastal current meets the right turning Tamsui river stream being discharged into the sea at low tide, the two soon become one and keep going ahead,and finally surge into Zhouzhiwan bay.
＊ Western Taiwan coastal current ebbs and floods just in opposite directions.
Western Taiwan coastal current at high tide
Western Taiwan coastal current at low tide
After all the water from the two streams converge and heap up in one place at the shore of Zhouzhiwan bay, a rip current with the effect of jetties forms and quickly flow out along both sides of Lobster Isle into the ocean. And this rip current is called double fork flow by local fishermen.
Now if we liken the curved Zhouzhiwan bay to a crossbow , then Lobster Isle is its barrel with a bowstring rail ,and then the coastal current is the archer’s drow -hand, and finlly the rip current is the shooting arrow with a double fork arrowhead.
So we can further deduce that the strong sea currents ,including the coastal current , Tamsui river stream and the rip current in Zhouzhiwan bay, must have a significant impact on the little isle and causes the amazing large quantity of boulders on it to pile up high like an artificial embankment .
According to the locals, the speed of double fork flow used to be very strong and fast, but now it has become weaker and weaker. Besides, the embankment of Lobster Isle was much higher, and the water on the south side ,which faces Taipei Port, was much deeper than now. But things has totally been changed since Taipei Port was built.
Is Taipei Port really the culprit to blame for all this? Is this a fair and just criticism? Now let’s first take a look at the satellite map of Taipei Port and the satellite image of Tamsui River estuary, so that we may have an objective judgement and derive a conclusion from facts.
If we compare Western Taiwan coastal current and Tamsui River stream to a man's fists swingng at Zhouzhiwan bay, then Western Taiwan coastal current is his left fist and Tamsui River stream is his right fist. And from the satellite map we can clearly see the left fist indeed being hecked by Taipei Port so that it misses its target and has lost part of its strength .
And next let's take another look at the satellite image of Tamsui River estuary, you will find that Tamsui River stream just looks like a powerful right punch being thrown at Zhouzhiwan bay. But try to imagine that without a strong coastal current to push the wave and add to the billows, the driftsands in Tamsui River certainly couldn't be carried too far away because the heavy river sands would make Tamsui River stream’s flow speed slow down.
*The right turning Tamsui river stream
*The satellite map of Taipei Port
As the coastal sea current has become weakened .a lot of driftsands fromTamsui River certainly would be blocked by Lobster Isle lying across in Zhouzhiwan bay ; consequently more and more migrating sands would keep piling up on the shore near Lobster Isle, while on the other side of the bay the sea current with much less sand has kept eating away much of the shore . Therefore , a striking contrast is formed between the two.
Without a strong support from the weakening current , the embankment of Lobster Isle seems a little bit collapsed, and part of it has even been buried by the sand. Furthermore, Double fork flow is no longer as strong as it used to be.
Frankly speaking, the building of Taipei Port no doubt has a lot to do with the change of the coast current. Besides , sea level rise which is caused by Greenhouse effect is also one of the many other factors that contribute to the the coast current change and Lobster Isle’s becoming lower.
But in the final analysis, for decades the lsle has been deserted without protection , so a large number of boulders on the isle were carried away and used as construction materials.This is also one of the main causes why its embankment has become so low now.
*Lobster Isle is sinking into the sea
Lobster Isle, the storm rider, the natural treasure , have evolved over Hundreds of thousands of years, but now has lost its old charm. A good deal of his blustering spirit seem to have evaporated with the removal of his fine panoply. Alas! How sad it is that he is no longer able to drive monstrous waves.
But whether the double fork flow is only weakening for the time being, or totally disappears forever is in fact a serious warning. It reminds us that we should pay more attention to Lobster Isle's critical situation and make much more efforts to preserve its fragile marine environment; otherwise it would simply sink into the sea, and be reduced to a hidden reef. And if we don’t want all this to happen before our own eyes, we must work together and discover a way to do something about it.
