My acquaintance with I. K. Brunel terminated, I think before I was eight years old. I remember him as a good-tempered, kind-hearted lad, much esteemed and beloved by his schoolfellows. His complexion was rather dark; his eyes beamed with intelligence and kindliness; and his manner readily engaged affection and confidence. His companions, though some were older than himself, seemed to feel him to be more than their equal in the discussion of any difficulty which had to be overcome by skill rather than brute force.
Fanny goes on to relate a Smallville-style foreshadowing of things to come, witnessed when she accompanied her father and the boys for a morning walk along the river. (At that time, c.1817,** note, there was no Chelsea Embankment.)
In passing by a grating in the road, over a sewer, near the foot of Battersea Bridge, a dog was heard to howl from below, where he could be seen but could not be reached. The other boys would have passed on, with only a word of pity for the poor animal, whose fate the next rising tide seemed likely to seal; but, my father coming up to the spot, Brunel asked him to sanction an endeavour to save the dog. Leave, you may be sure, was readily granted to him; and Brunel, going down to the shore, opened the trap of the drain and called the poor dog, which came to him and was taken by him to his father's house, close at hand in Lindsay Row, there to be kept and cared for.
I remember my father was very much pleased, and warmly commended Brunel as one whose heart was ever as ready to prompt, as his head and hand were to devise and effectuate, plans of kindness.
She goes on to compare this act with Brunel's behaviour on the occasion of the Thames Tunnel flood in 1828:
On this occasion... many of the workmen were, in a moment, buried alive in the invading waters. Young Brunel, who was then, I think, about twenty years of age, dived five times from the dark shaft into the deep and muddy water, and each time brought up a living man, who but for his determined self-devotion and courage, must there have found a grave.
He was about to plunge in again, when those who stood by withheld him by force; he exclaiming that they ought not to prevent him from saving more lives, or, at least, trying to do so, as he could dive and swim, while many of the poor men could do neither.
But though upheld by strong determination and excitement for the work he had already done, he was now exhausted; so they took him away to bed, and when my sister Anne and I called at the Tunnel to inquire after him, his old nurse, who was in attendance upon him there, told us he was ill with fever, brought on by his exertion and the intense sorrow he felt at not having been able to save all the poor men who were working under his superintendence when the rushing water broke in upon them.
Accounts that I've seen of the accident elsewhere on the web don't have these heroic details, by the way. Here, for example, we read that:
Six men were killed and Brunel, who had been working on one of the frames with two of the men who drowned, was saved only by sheer luck. His leg was badly crushed and he was knocked half unconscious, but the wave of flood water swept him towards an exit shaft and into the arms of the useful Beamish, who was rushing to the scene.
The useful Beamish, here discovered rushing open-armed to the shaft, was IKB's assistant on the project - and yes, I have noticed the slashy possibilities. What's less plausible is that, unable to walk on his crushed knee, Brunel was nevertheless able to dive back into the waters and save five men. Possibly Fanny was recalling the earlier flood of 1827, but on that occasion no one seems to have died. There's another problem, too: while the web source says that Brunel was sent to Brighton to recover, Fanny has him staying - with his old nurse! - at the Tunnel itself; while Wikipedia dispatches him to convalesce in Clifton, where he just happened to hear about a competition to build a bridge...
I think the most romantic option is to have him saving his men and then, after a short stay with dear old nurse, going to Bristol, so that's what I choose to believe until I see compelling evidence to the contrary.
* Not Fanny Butler the medical missionary: this is her aunt.
** The date is an estimate based on the fact that IKB spent some time at boarding school in Hove before being sent to France at the age of 16. The incident may have happened rather earlier.