Index: src/tunnelbridge_cmd.cpp
===================================================================
--- src/tunnelbridge_cmd.cpp	(revision 27233)
+++ src/tunnelbridge_cmd.cpp	(working copy)
@@ -435,7 +435,8 @@
 
 			switch (GetTileType(tile)) {
 				case MP_WATER:
-					if (!IsWater(tile) && !IsCoast(tile)) goto not_valid_below;
+					if (!IsWater(tile) && !IsCoast(tile) && !IsLock(tile)) goto not_valid_below;
+					if (IsLock(tile) && GetLockPart(tile) == LOCK_PART_LOWER && GetTileMaxZ(tile) + 1 > z_start) return_cmd_error(STR_ERROR_BRIDGE_TOO_LOW_FOR_TERRAIN);
 					break;
 
 				case MP_RAILWAY:
Index: src/water_cmd.cpp
===================================================================
--- src/water_cmd.cpp	(revision 27233)
+++ src/water_cmd.cpp	(working copy)
@@ -288,8 +288,9 @@
 		return_cmd_error(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
 	}
 
-	if (IsBridgeAbove(tile) || IsBridgeAbove(tile - delta) || IsBridgeAbove(tile + delta)) {
-		return_cmd_error(STR_ERROR_MUST_DEMOLISH_BRIDGE_FIRST);
+	if (IsBridgeAbove(tile - delta)) {
+		int z_lower_bridge = GetBridgeHeight(GetNorthernBridgeEnd(tile - delta));
+		if (z_lower_bridge <= GetTileMaxZ(tile - delta) + 1) return_cmd_error(STR_ERROR_BRIDGE_TOO_LOW_FOR_TERRAIN);
 	}
 
 	if (flags & DC_EXEC) {
@@ -859,6 +860,7 @@
 
 		case WATER_TILE_LOCK:
 			DrawWaterLock(ti);
+			DrawBridgeMiddle(ti);
 			break;
 
 		case WATER_TILE_DEPOT: