Within the first two to four months of the bombings, the acute effects killed 90,000–166,000 people in Hiroshima and 60,000–80,000 in Nagasaki, with roughly half of the deaths in each city occurring on the first day. Of the people who died on the day of the explosion, 60% died from flash or flame burns, 30% from falling debris and 10% from other causes. During the following months, large numbers died from the effect of burns, radiation sickness, and other injuries, compounded by illness. In both cities, most of the dead were civilians.
In Hiroshima, the radius of total destruction was about one mile (1.6 km), with resulting fires across 4.4 square miles (11 square km). The residents of Hiroshima were given no notice of the atomic bomb.
Let’s focus on the area near ground zero, the hypocenter of the explosion, which is this part of the map above (the green lines are the rivers):
Below are a few 3D maps/maquettes of this area – which obviously suffered the most destruction – taken from an exhibition in the Hiroshima museum. They show the area before and after the explosion. In each one, you can see the famous dome structure which has become iconic for the event (I marked it on the images).
From another viewpoint: