In Catholicism, in case of emergency Is it valid to baptise a large assembly of people with a fire hose? In an emergency is it possible to baptise a large group of catechumens (around 100 or more) with a firehose or some other "group baptism" method, such as walking outside during rain? The catechumens would presumably consent to being baptised.

I imagine that if a group was baptised with a firehose, the Trinitarian formula would only be pronounced once, but I have a hunch that who ever is doing the baptising has to pronounce the Trinitarian formula each time for every individual in the congregation, but I'm not sure: it seems inconvenient, inefficient, infeasible and impractical in an emergency situation because it would take too long to baptise them all individually. It seems to be that a "group baptism" would be the most effective thing to do in an emergency, assuming that it is allowed.
According to the Catechism, any person can perform baptisms in an emergency, but it is doubtful whether the use of a firehose would be considered valid or, more importantly, necessary:

1256: The ordinary ministers of Baptism are the bishop and priest and, in the Latin Church, also the deacon.57 In case of necessity, anyone, even a non-baptized person, with the required intention, can baptize58 , by using the Trinitarian baptismal formula. The intention required is to will to do what the Church does when she baptizes. The Church finds the reason for this possibility in the universal saving will of God and the necessity of Baptism for salvation.59
12:58: The Church has always held the firm conviction that those who suffer death for the sake of the faith without having received Baptism are baptized by their death for and with Christ. This Baptism of blood, like the desire for Baptism, brings about the fruits of Baptism without being a sacrament.
1259 For catechumens who die before their Baptism, their explicit desire to receive it, together with repentance for their sins, and charity, assures them the salvation that they were not able to receive through the sacrament.
1281: Those who die for the faith, those who are catechumens, and all those who, without knowing of the Church but acting under the inspiration of grace, seek God sincerely and strive to fulfill his will, can be saved even if they have not been baptized (cf. LG 16).