The Hornby Vellard was a project to build a causeway uniting all seven islands of Bombay into a single island with a deep natural harbour. The project was started by the governor William Hornby in 1782 and all islands were linked by 1838. The word vellard appears to be a local corruption of the Portuguese word vallado meaning fence or embankment.
The purpose of this causeway was to block the Worli creek and prevent the low-lying areas of Bombay from being flooded at high tide. The cost was estimated at about ₹. 100,000. It was completed in 1784 and was one of the first major civil engineering projects which transformed the original seven islands of Bombay into one island.
According to some accounts, Hornby ordered the work to be started after the East India Company turned down his proposal; and continued as Governor till the end of his term in 1785, ignoring the suspension notice sent to him. 
All of above is just folklore and popular notion carried over the last two centuries, it gained currency around early 19th century, but real facts are very different. There is no record or evidence to prove the above story.
In fact the work on this embankment was started in 1720, full 50 years before Willian Hornby became the Governor of Bombay, and it was completed by 1727. At that time it was simply being referred to as the Vellard. Detailed records of the period are available as the consultations (minutes) of the meetings of the Bombay Council during this period, to attest to these facts.
In fact, the land parcels were already being leased out, east of the Great Breach, in the 1730s and 1740s.
Therefore, calling this Hornby Vellard is a misnomer.
- Hobson-Jobson: A Glossary of Colloquial Anglo-Indian Words and Phrases By Henry Yule, Arthur Coke Burnell, William Crooke; pub. 1903 John Murray, London
- p. 23 ff. A History of the Mahrattas By James Grant Duff, Published 1921 H. Milford, Oxford,
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- Materials Towards Statistical Account of the Town and Island of Bombay Vol. III, pages 264-266, 316-321, 446-447 and 501-503.
- Bombay by Samuel T Sheppard, 1932