No, that is not normal, it's an unusually high kill rate in my experience. When I've been in parties that hit those levels, there have usually been one or two kills per campaign that require resurrection (though more close saves with resurgences and whatnot).
It may be due to bad player tactics, weak characters, or the GM runs things tougher than the average GM - either "higher CR" or just "better", some GMs (like myself) manage to squeeze a lot more kill per hit die out of opponents. Sometimes a GM runs a harder game, which is fine.
But of course the thing about high level D&D is that you have multiple levels of defense against death, so as long as it's not impacting the fun, there's nothing really wrong with it. You get to play croquet with the angels on a regular basis, heck, that could be part of the plotline (Order of the Stick, anyone?)
If you do want to do something about it, and don't want to embrace character optimization, I would review the results of combats with the rest of the group and see if the deaths were preventable - do party fighters repeatedly charge recklessly into combat, ruining the ability of mages to area effect; do you get spread out all over the battlefield where the cleric can't effectively help you when you're in trouble; do characters not participate well (the tank fighter who hides in the back, the cleric who's too busy fighting to heal)?
Tactics In Depth
I had to coach one player group that was getting demoralized about how tough combats were going in one D&D campaign. In general they didn't have any coordination - they'd open a door and see bad guys. One guy would run in (before the mages could cast damage or battlefield control spells), others would back off, people didn't have mobility or line of sight. They'd decide to run, except most of the group would retreat, but one guy would want just one more round of full attacking, and then be cut off, and then half of the rest of the party would keep fleeing but the other half would come back and help them, and get cut off themselves... I taught them to do simple things like NOT go in the door, but back off and form an inverted triangle around the door and let them come to the party (for dumb melee opponents of course) so that they are the ones getting surrounded and losing actions and being separated. One PC that was taking on a leadership role took it upon themselves to "call the shots" in combat so that a retreat or attack was performed by the whole group. There's a lot of D&D-specific ticky combat tactics stuff, but I always see the greatest difference being made by system-independent "having your crap together" kinds of basic techniques. Have patience, don't get split up, coordinate maneuver to take the fewest attacks but deliver the most, concentrate fire.
I remember in one five year long 2e campaign I ran, the team leader went so far as to run IA drills and basic response plans. When they would come into contact, there were set formations they would move into automatically. The shouted command "Blue" told PCs to close their eyes to avoid the mage's Color Spray. The only PC death in that campaign came from PC on PC action.
I'm sorely lacking in 4th ed knowledge, but here are some basics for you to consider:
1 - EXPOSURE: That bridge is brutally open, making it a natural murder zone if the defending force is prepared with missiles or skirmishers. Reducing line of sight to the bridge is of critical import. Consider using fire from your artillery (clay pots filled with flammables and/or greek fire, will get stuff burning but good) to create a smoke/hazard from a distance (building 3 is a convenient target). That will hopefully create a natural wall of fire/smoke to screen your crossing. Various spell effects will help you get troops across while taking a minimum of casualties.
Good use of your ambushers from the east to create a skirmish/distraction will help pull attention away from your crossing as well.
Anyway you slice it, though, you're going to have to cross under fire and into resistance. Consider pavise-style portable-cover on wheels to screen from the worse of it, or other variants on a battering ram/siege tower to cross the river if they go with missiles.
2 - PUSH OF PIKE: Once the crossing is secured from missile fire, they'll seek to deny you the physical ability to cross. Pike phalanx would be my go-to option with swordsmen mixed up among the front rank to handle anyone who got under the spears. It should go without saying that you're going to want your heaviest vanguard on point for the crossing. Think 'wrecking ball' and you're in good shape - but a slow moving one. The good news for pike formations is that they're bunched up - prime AoE ability fodder.
3 - AREA DENIAL COUNTERMEASURES: Expect them to consider blowing the bridge as a real option, especially if they're out-classed. You'll want counters to obvious spells and abilities that could damage the bridge - ESPECIALLY when your vanguard is crossing it.
4 - AIR SUPPORT: I don't know if Tar Devils can fly but if they can, or if you have any other airborne units, use that. Drop buckets of burning pitch, greek fire, whatever you can to disrupt enemy formations, break down their line of communication/sight, and otherwise foul up their day. Ideally use something you can selectively extinguish, to prevent your own forces from falling into the same trap.
5 - ONCE ACROSS: Feed your own pike wall equivalents and establish an expanding schiltron. You are at your most vulnerable when half of your forces are across, that's when a savvy enemy will hit you, and try to bust the bridge if they haven't already. If they can split your force in half they can apply their full pressure to half your strength and then mop you up. An educated commander will expect this.
6 - ALTERNATE CROSSINGS: Consult spells and magical abilities you may have available which can effect the flow of the river or its depth (make a ford). Consider the forest in the southwest as a source of wood. Log jams could be constructed or similar makeshift crossings secured. A particularly determined/unscrupulous commander could simply go for artillery-fired ropes of sufficient length and order troops to ford the river, using the rope to keep themselves above water. It's slow going and low through-put, and losses will be heavy but See Also: Evil. :P
7 - ARTILLERY SUPPRESSION: Consider parking artillery on the near bank and using it to suppress the far bank. While completely anachronistic, it should be quite effective until they can silence your battery. The idea is to smash stuff and make fire and generally convince defending archery that its better if they just pack it up and go home. :)
YMMV. Have fun!