I've been cleaning culverts for over 20 years & have used many things to clean them from wisk brooms to my home-made leaf blower vac pac until now.
A couple years back (year before last, or the beginning of last year I think) I was in Home Depot and seen a neat gadget...... AN 18v. Battery operated vac by RYOBI..... Since I use RYOBI equipment and have almost every peice of 18v. equip. they make I had to have one. I didn't hold much promice for it, but figured I would give it a try....
While out west last few times I hit about 20 - 35 culverts each time and did a lot of crevasing when ever we would stop. To my supprise, the thing had as much power (or close to it) as my gas leaf blower vac, was relitavely quiet AND NO FUMES when I was more than 80' back inside a culvert that was a few hundred feet long..... This is important when it's a small culvert or a long one where carbon monoxide may build up from a gas model AND KILL YOU.
The draw back is that the batteries only last about 20 min. of continuous running, or if ran for 5 minutes at a time (giveing the batteries a cool down period of 15min. or so between runs) last about 1 1/2 hours. For me this wasn't a big problem since I have SO MANY peices of the equipment and have 2 batteries for each peice I took about a dozen batteries and 2 duel chargers with me. Never used up all the batteries, found LOTS of gold.....
The big advantages are that the unit only costs $49.00 (I've seen them for as low as $29.00 on sale) and it is designed for both wet and dry use.... It actually has a drain plug on the top / back of the unit so you can use it in a running culvert and just open the plug, drain off the water without loosing the material or taking it apart and continue through the culvert....
The other people who responded have given you some very useful info and know what they are talking about, they have pretty much covered things, I'm giving you an idea of another peice of (cheep) equipment that works extreamly well for culverts.
Good luck and hope you find LOTS of gold,
Coyote.