Was actually having a chat with Ken Titmuss (@oldmapman on twitter) about exactly this the other day: when exactly was the Somerset House bit "embanked"? As I understand it, Bazalgette was doing his thing in the 1860s, yet there is a quite well-known painting by Thomas Shepherd from 1871 showing SH unembanked. Either Shepherd was making an idealised rendition or Bazalgette's project hadn't reached that part of the Thames. I guess.
Well, here's the Daily News from July 21, 1864:"Passers over the bridges of Westminster and Waterloo have for some time been in the habit of stopping to notice the rapid progress which was being made in the work. The whole of the line of the embankment has long since been distinctly marked out by wooden scaffolding of massive size and strength, within which the river wall was and is being construced. There were great and rapidly accumulating heaps of 'ballast' under the Adelphi and Somerset House ..."So I can't imagine it was unembanked in 1870. Also, a quick google suggests you might be confusing 1871 with 1817? http://commons.wikimedia.org/wiki/File:Somerset_House_THS_1817_edited.jpgalthough I wouldn't always take wikipedia as gospel.best wishes,Lee