March 29, 2019 is Brexit Day — the day when the UK officially leaves the European Union. With just 140 days left until then, there is still a distinct possibility that the UK will leave the bloc in the hardest possible way, without a deal and without any kind of transitional arrangements.

At the moment, the sticking point in negotiations with the EU relates to the border in Ireland. However, should the doomsday scenario unfold next March, another "border" will immediately come into sharp focus — the English Channel, which separates England from France.

In the event of a so-called "no deal" Brexit, the scale of potential chaos across a multitude of areas is difficult to quantify.

In terms of trade, the UK would be forced to trade with the EU under WTO rules, meaning that customs, regulatory and other checks would need to take place at EU-UK borders while tariffs would be charged on goods exported between the jurisdictions.

Read more: Brexit: What's the 'no deal' fallout for the UK and EU?

That would mean dramatically changed circumstances at the main pressure points of EU-UK trade. There is no greater physical pressure point than the English Channel. Whether it is ship crossings from the Port of Dover to the Port of Calais, or the extensive land bridge crossings via the Channel Tunnel, the area is the ground zero of trade relations between the EU and the UK.

From here to an eternity

For more than 25 years, trade between the UK and the rest of the EU has been close to seamless. Up to 10,000 trucks roll on and off ferries either side of the crossing each day. Another 6,000 trucks cross the Channel Tunnel daily, as do trains, vans and cars. Delays are rare, because shared membership of the EU customs union and single market means there is no need for checks.

Estimates vary as to how long delays that future checks could place on each vehicle will be, but even wildly optimistic estimates of minimal delays of just a few minutes long could create massive tailbacks on either side of the Channel. Ian Robertson, long-serving BMW board member and the group's special representative in the UK, told DW this time last year of his worries about the damage delays could do to the car industry, to take just one sector.

"I have seen a study that put a notional three minute delay on every truck on either side of the English Channel, and that would create 17 kilometers (10.5 miles) of tailbacks," he said.

A deep trading relationship

According to a source no less august than the Guinness Book of Records, the Dover Strait in the Channel is the world's busiest shipping lane. Between the Dover-Calais ferry route which crosses the Channel, and the Channel Tunnel — the 50 kilometer rail tunnel which links England with France — the vast bulk of goods passing between the UK and the EU traverses this route.

Read more: Brexit causing concern for business on both sides of the Irish border

Understandably, given the UK's 45-year membership of the EU, the EU is by a distance the UK's most important trade partner. UK trade with the EU totaled €703 billion ($803 billion in 2017).

In the Tunnel

Multiple sectors across the UK and the EU are dependent on the current frictionless arrangement. Since internal EU customs barriers were abolished by the Maastricht treaty in 1992, the number of trucks leaving the UK for Europe has quadrupled.

Several manufacturing sectors have their production models based on free movement across the Channel, especially the car industry and its so-called "just-in-time" model, whereby parts arrive at production facilities across the continent down to the hour. For the food sector, the need for speed is equally clear.

Yet to take the goods that traverse the Channel Tunnel for example, there is a wide spread of sectors represented in the top 10, from postal and courier freight, to computer parts, meat and food, textiles and metals. An extensive study by the multinational EY, released earlier this year, revealed the reliance that UK-EU trade currently has on the Channel Tunnel alone.

According to a report in Bloomberg this week, the UK government is currently looking into re-opening old sea crossings between the south of England and ports in Belgium and the Netherlands, in order to limit the pressure the Dover-Calais crossing and the Channel Tunnel will come under in the event of a no-deal Brexit.

It is not just the UK that is worried. Business owners across the continental EU are also concerned about what could happen to their supply routes come March 2019.

Xavier Bertrand, president of the Hauts-de-France region, the area of France closest to England, told the Financial Times recently that he believed border checks in France would need to be softened in the event of a no-deal Brexit, in order to save the region from economic disaster. "The trucks, companies and factories that will be blocked will be those of the north of France, the whole of France and Germany," he said.

Joachim Lang, director of the Federation of German Industry (BDI), said last month that a no-deal Brexit would be "a disaster that would cause great difficulties for tens of thousands of firms and hundreds of thousands of workers on both sides of the English Channel."

Including sea freight sent from European ports into Dover, particularly from Calais, the volumes are intense. In terms of the Channel Tunnel crossing alone, several EU countries send billions of euros worth of exports across to the UK every year.

German exports across the Channel Tunnel in 2016 were worth €15 billion, the most of any one EU country. Yet the load is relatively evenly spread across several countries (see infographic).

Cars, trucks, buses and trains

According to Getlink, the company which operates the Channel Tunnel, 1.64 million trucks traveled between the UK and the EU using the crossing last year. More than 2.5 million cars made the crossing, as did 51,229 buses and 2,012 freight trains.

Added to the even greater volumes of truck crossings made on ferries — the Port of Dover handled a record 2.6 million trucks in 2017, according to its own figures — it accounts for a massive annual volume.

The other border

While the outcome of Brexit remains very much uncertain, deal or no deal, there are some pointers as to how the Channel could look if the UK leaves the bloc without a deal.

Every day, around 500 trucks from non-EU countries arrive to clear customs at Dover, either via ferries or the Channel Tunnel. Delays for each truck are considerable, and often take longer than an hour for each vehicle, depending on its origins.

Another glimpse of the capacity for chaos came in 2015 when the migrant crisis combined with French ferry operator strikes to cause long tailbacks at both Dover and Calais. That chaos would likely be dwarfed by that which could unfold in the event of a no-deal Brexit.

And while this "border" is very different from the Irish one currently commanding most of the attention, the one at the English Channel has massive implications for Ireland as well.

A massive 80 percent of Ireland's road freight to the rest of the EU goes through Britain and on to continental Europe from there, via the Channel. After Brexit, Irish hauliers still using this route could potentially have to clear customs four times. Keeping that in mind, this really is Brexit's other border.