Why are outside "consultants" always assumed to know more than the guys on staff who have been doing this work for years??? Most technical consulting firms have a handful of experienced people at the top and lots of newbies at the bottom.
The US/NYS have done the arerial flood plain mapping that towns need to do this kind of work, so all they have to do is download it into into their planning databases. The town has the engineers, the planners, and others on staff who know how to do the math, use the computer software, and recognize drainage/floodways. Moreover, these people are very familiar with the specific area.
Engineering is Mr Mohan's field of expertise, so I think he can judge the expertise that the staff members have in this matter (and it is an engineering matter, btw).
Keep in mind that it could take six months just getting an RFP written, replies solicitted, and a bid accepted (ie, consulting firm selected) while the town staff could have the study done in that time. The town is saving money on the bidding and selecting costs and on the contract itself. Plus, builders and contractors could be up and running on their projects again at least a half-year sooner, bringing in more money to the town coffers through permit fees, taxes, etc