West Yorkshire-based urban regeneration specialist Maslen Environmental joined JBA Consulting, with almost 200 staff, which also has headquarters in Yorkshire.
Maslen recently tackled three big regenerations, including a former steelworks in Derbyshire and a wetland creation in Humberside that won a British Urban Regeneration Association award.
The 440ha project for the Environment Agency forms part of the area's flood management. Managing director Steve Maslen said JBA would add useful engineering and softer environmental skills.
"The merger is a case of one and one makes three," he said. "Lots of our work is related to climate change. The risk of flooding is going to be a big area and JBA's skills (in this area) are critical."
Maslen, whose 10-strong staff is based in Saltaire, West Yorkshire, has a £400,000 turnover. JBA, based 16km away in Skipton, North Yorkshire, is headed up by Jeremy Benn and has a £9.3m turnover.
"We are not massively ambitious but want to see steady and sustainable growth," said Benn. "Maslen is small, and joining forces with us will make it easier for it to pre-qualify for a lot of public-sector tenders."