The French aren't spending all this money for fun:
http://www.senat.fr/leg/pjl18-009.html
[Helped by Google translate !]
"The withdrawal of the United Kingdom from the European Union
will lead to the restoration of veterinary, sanitary, phytosanitary and safety controls and customs formalities.
These checks will have to be put in place on the day of withdrawal in case of exit without agreement,
or at the end of the transition period in case of entry into force of the withdrawal agreement.
At present, with regard to the Channel / North Sea seaboard, border posts of veterinary and phytosanitary border inspection services (SIVEP) are present in Le Havre, Dunkirk, Saint Malo and Brest.
However, they are not sized to control all the batches coming from the United Kingdom
or do not have facilities approved for all types of flows (absence of control facilities for live animals in Le Havre and Dunkirk in particular).
In addition, the entry points to, among others, Roscoff, Cherbourg, Caen-Ouistreham, Dieppe and Calais as well as the Channel Tunnel do not have border posts to date.
To fulfill their obligations, these services need the provision by the operators of premises adapted to the nature and volume of the flows introduced from the United Kingdom.
In addition, a location in a restricted access zone could be retained in certain entry points, which should be subject to appropriate security.
In addition, carrying out these checks will take a significant amount of time, which will slow down the flow of traffic and may therefore quickly lead to congestion in the port, rail and road infrastructures.
< with corresponding queues at the British end >
In order to maintain the fluidity of the port and ensure the safety of the various convoys, adequate parking areas will have to be created.
Improvements may therefore include the construction of roads, car parks, buildings, control areas
or the installation of any equipment necessary for carrying out the controls concerned."