Last time I did what could be called draining was 2003/2004 in NW England only....not really done any since then, nor really any urban exploring. Would like to get back into it, but no drains where I am, and no pals to go exploring with.
I had a gander at that map link. Got me a bit confused because of the .co.uk domain.
I think that is an irrigation canal, lots of them in this part of spain, the farmers use the water which comes from a big reservoir to irrigate their crops in the fields, you will notice small penstocks along the way. But it seems a little bit polluted in one part, so I could be wrong, it could be a stormwater channel/culvert with some industrial/sewage overflow. How far that underground part goes is anybodys guess, but the signs say "Bathing Prohibited" that water can have strong undercurrents, be very deep, and surprisingly cold. Not sure if it could be safe to attempt really, but looks promisiing. Cheers again, pb