I found a photograph of the sign at the entrance to the town that shows two flags flying and a similar one of Nagua city hall also with two flags flying. One is the Dominican Republic national flag. The other I assume is likely the Nagua municipal flag. Unfortunately, neither photograph gives a good look at the details of the flag. At the very least, it appears to be a vertical tricolor of green-blue-yellow, with a thick white fimbriation between each color and a thin white border around the whole flag. I can't tell from the photos if there is anything else to the design - such as an inclusion of the coat of arms or an inscription - but there may be. I'm hoping that someone who has visited the Dominican Republic and travel to Nagua may remember seeing something that would help me refine this flag. The photographs that I found online with the tricolor flag can be seen here and here. Randy Young, 23 December 2015
Nagua has the coat of arms on the flag, too. It can be seen here and here. Tomislav ipek, 28 December 2015
Nagua is the capital of María Trinidad Sánchez province on the northern shore of the Dominican Republic. Established as a municipality in 1938, most of the town actually lies below sea level, making it susceptible to flooding. As of 2012, Nagua had a recorded population of just over 79,000.
Source: Wikipedia article Randy Young, 23 December 2015
While searching online, I found a graphic of a flag that shows the Nagua coat of arms centered on a white field. Unfortunately, though, I was unable to find any photographic evidence to support that design. Randy Young, 23 December 2015