So long as you keep a nice beefy stack of your best units, along with a good deal of Eastern Archers astride the bridge that leads into the province of Thracia you should be able to hold back the tide of any hordes that are foolish enough to attempt a crossing. Be sure to hire a few of the horse archers in the area as well as they can come in handy.

Make sure to form a nice semi circle a decent distance from the bridge rather than making a huge clump around the very end of the bridge. The steppe hordes are VERY fond of their missile weapons, ESPECIALLY their horsearchers, but the Vandals and Goths BOTH have some long ranged archers that you want to stay well away from so that you can keep your casualty rate at an acceptable level. Seeing as the majority of any cavalry they bring will be able to ford, you'll want to make sure your semi-circle also faces a good deal of the water surrounding the bridge itself. Set your legions to defensive mode and fire at will and keep gaps to an absolute minimum. Place your Eastern Archers on the flanks so that they can fire on any who attempt to ford the river or cross the bridge but far enough away so that any archers on the opposite banks are unable to fire upon them. Keep your general right behind your legions and a unit of the heaviest available cavalry on each flank. Once they start contacting your legionary line you should see that the sheer amount of missile fire going around will even make some enemy units route before they make it to your side, but the majority will continue charging. Should they last long enough to start pushing, wrap your semi-circle around them so that they are surrounded, and charge your cavalry in around from the flanks into their rear.

The only time I have found it worth straying from that tactic is when facing the huns, you dont have to stay so far away as they lack any long range foot archers and so you are able to actually position both you lines closer and tighter and your archers close enough to rake the enemy horsearchers with missile fire without fear of return fire. In fact you'll even see a great deal of the enemy will lay dead before they reach the water, and many more will be drifting downstream before the fighting even begins.