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One of the hose clamps for the cooling spiral malfunctioned, so my cooled wort got contaminated by roughly 10 liters of tap water (in a 55 L batch). This is a lager beer which will be stored for five months before drinking, so ignoring the contamination is out of the question.
The wort is being re-boiled and re-hopped, but is there any consequences of re-boiling (apart from losing hop aroma)?
Reboiling will increase bitterness of all the hops that went in 'late' in the kettle. Obviously, as you said you'll lose your aroma charge will decrease in proportion to the length of the reboil.
You may likely increase the maillard profile of the malt character depending on how long you boil. The complexity of the original grist will dictate the extent of that impact on flavor.
And depending on how much of the 10L you plan to boil off, your mineral content will certainly change as well. Again, depends of the mineral profile of your base water as to the total effect on the beer.
Depending on how long you plan to reboil you may expect to re-dissolve some protein into the beer. (Some of the early Lambic and Belgian brewers would conduct really long boils that drove some portion of the protein in the hot break to break down further allowing it to no longer be break but be soluble peptides in the beer, changing the mouthfeel). This again is base beer dependent along with length of boil.
When you say it got contaminated you mean that some tap water went in contact (mixed) with your wort, right? I wouldn't say that is contamination. IMHO, contamination is that some bacteria has started to grow and eat the nutrients of your wort thus producing some compounds and off-flavours. In this case I wouldn't boil since these compounds are already in your wort and cannot be removed by boiling. Otherwise, I would boil and maybe add DME (dry malt extract) to compensate the gravity points you lose with tap water.
Unless your tap water is bad, I wouldn't call dilution "contamination". If this happened to me, I would do a small boil with DME or LME (just enough to get everybody dissolved), cool it down and dump it in. If you don't want to do that, you can ferment it, bottle/keg it and send it to me. I'll be more than happy to take care of it for you.
Seriously, relax. It's just beer. Don't make it into some sort of rocket surgery.
Sounds like you know of and are dealing with the alpha acid issues.
The tap water may impact flavor if it was chlorinated, regardless of boil time.
Reboiling or longer boiling helps with DMS removal.
I would boil off the 10 liters to keep your gravities on point, or accept the dilution if you're only doing a short reboil for sanitation.
You say this is a lager, so I'm going to assume a high percentage of pilsner. In that case, I would not recommend re-boiling anything. Some extract brewers tap up from the faucet on every brew (I used to). If you re-heat the wort, you are going to create DMS, which will be a much bigger problem than any potential contamination from unboiled tap water, never mind the difficulty in retaining the desired flavor profile. You could potentially start the boil over, adding more water to account for a full boil (to allow the new DMS to be boiled off), but the hop utilization and maillard reactions will be different from intended. I think a safer alternative would be to rack to two vessels (to prevent the fermentation from overflowing) and just ferment as normal. You'll produce a thinner, less ABV beer than intended, but otherwise, should be fine.
Better to dump it than to take the chance of bad fermentation and contaminated beer..
