The phrase "a flat section of" is correct and usable in written English.
It can be used to describe a specific part of a surface or area that is level and even, often in contexts related to geography, architecture, or design.
Example: "The architect designed a flat section of the roof to allow for easy installation of solar panels."
Alternatives: "a level area of" or "an even part of".
Snapping on our skis, we slid over to a flat section of perfect corduroy.
We practiced first on a flat section of the Deerfield River, and then moved on after lunch to a stretch with a few rocks.
In the early 1960s Cormack showed how details of a flat section of soft tissues could be calculated from measurements of the attenuation of X rays passing through it from many different angles.
We're on a flat section of the route that approaches the final climb of the stage and the race itself: a brutal ascent of 3,000 vertical feet, known as Empire, with grades of 15%to20%0% over seven miles — a hors catégorie (HC) or beyond category climb, which means it's off the scale.
One suggestion was to put the plaque on a flat section of the depiction of Laguna's iconic Lifeguard Tower, which would never be allowed on the real tower.
The train then follows a flat section of track before going down a small drop leading into the station and back up the lift where the train stops.
And he was whizzing through a relatively flat section of a course he loved.
Cut a large, flat section of log for a splitting block, to place the segments you are splitting on.
I spoke with a Hawaiian who started telling me about his marital troubles in New York, before spending a flat section with a native of Belfast who bemoaned the lack of a good fish and chip shop in Oxford.
The next year, a team from the Woods Hole Oceanographic Institution in Massachusetts, using autonomous underwater search vessels, discovered a large debris field from the flight on a relatively flat section of ocean bottom at depths between 12,500 and 13,100 feet.
At a flat section at the base of the mountain we did bike-handling drills, riding along the perimeter of a large box etched in the sand, and weaving in and out of rocks spaced roughly four feet apart.
