Superelevation is the banking or tilting of a road or pavement surface towards the inner edge of a curve. It is done to counteract the centrifugal force acting on vehicles as they travel around a curve, providing a safer and more comfortable driving experience. When a road is superelevated, the outer edge of the pavement is higher than the inner edge, creating a slight slope towards the center of the curve.

The attainment of superelevation can be achieved by rotation of the pavement about the inner edge. This means that the pavement is built with a gradual slope or cant towards the inner edge of the curve. This rotation of the pavement has several advantages, one of which is that it helps to avoid drainage problems in flat terrain.

Here's why superelevation by rotation of pavement avoids drainage problems in flat terrain:

1. Improved surface drainage: When the pavement is rotated about the inner edge, it creates a slope towards the center of the curve. This slope allows water to drain towards the inner edge, preventing the accumulation of water on the road surface. In flat terrain, where natural drainage may be limited, this rotation of pavement helps to improve surface drainage and reduce the risk of water ponding on the road.

2. Reduced hydroplaning risk: Hydroplaning occurs when a layer of water forms between the tires of a vehicle and the road surface, leading to loss of control. By ensuring effective surface drainage, the rotation of pavement helps to minimize the risk of hydroplaning, particularly in flat terrain where water may pool on the road.

3. Prevention of erosion: In flat terrain, water runoff can cause erosion of the road surface. By directing water towards the inner edge of the curve, the rotation of pavement helps to prevent erosion and maintain the integrity of the road.

4. Enhanced pavement durability: Proper drainage is essential for the longevity of road pavements. By avoiding drainage problems in flat terrain, the rotation of pavement helps to prevent water-related damage such as cracking, potholes, and rutting, thereby improving the durability of the road.

In conclusion, the attainment of superelevation by rotation of pavement about the inner edge is preferable in flat terrain as it helps to avoid drainage problems and provides a safer and more durable road surface.