TERRAINS Terrain, or land relief, is the vertical and horizontal dimension of land surface. Terrain is used as a general term in physical geography, referring to the lay of the land. This is usually expressed in terms of the elevation, slope, and orientation of terrain features. Terrain affects surface water flow and distribution. Over a large area, it can affect weather and climate patterns.

TYPES OF TERRAINS The most basic classification of terrain divides it into flat and non-flat terrain Flat terrain is terrain with small irregularities such as forest, shelter belts, etc. Non-flat terrain which are also known as complex terrain is defined as an area where terrain effects are significant on the flow over the land area being considered. Non-flat terrain has large-scale elevations or depressions such as hills, ridges, valleys, and canyons.

CLASSIFICATION OF TERRAINS NON FLAT TERRAIN Large Scale Mountains, Valleys Small Scale Depressions Elevations FLAT TERRAIN Small Irregularities CLASSIFICATION OF TERRAINS

FLAT TERRAIN NON FLAT TERRAIN

wind turbine geometry: To quantify as flat terrain, the following conditions must hold. Note that some of these rules include Elevation differences between the wind turbine site and the surrounding terrain are not greater than about 60 m anywhere in an 11.5 km diameter circle around the turbine site. No hill has an aspect ratio (height to width) greater than 1/50 within 4 km upstream and downstream of the site. The elevation difference between the lower end of the rotor disk and the lowest elevation on the terrain is greater than three times the maximum elevation difference (h) within 4 km, upstream

FLOW OVER FLAT TERRAIN WITH OBSTACLES Flow over flat terrain, includes especially with man-made and natural obstacles. Man-made obstacles are defined as buildings, silos, etc. Natural obstacles include rows of trees, shelter belts, etc. For man-made obstacles, a common approach is to consider the obstacle to be a rectangular block and to consider the flow to be two dimensional. This type of flow, produces a momentum wake, and a free shear separates from the leading edge and reattaches downwind, forming a boundary between an inner re-circulating flow region (eddy) and the outer flow region.

FLOW IN FLAT TERRAIN WITH A CHANGE IN SURFACE ROUGHNESS In most natural terrain, the surface of the earth is not uniform and changes significantly from location to location. This affects the local wind profile. The figure shows that the downwind profile changes significantly in going from a smooth to a rough surface.

CHARACTERISTICS OF NON-FLAT TERRAIN: SMALL-SCALE FEATURES Researchers have divided non-flat terrain into isolated and mountainous terrain. where the first refers to terrain of small-scale features and the latter refers to large-scale features. For small-scale flows this classification is further divided into elevations and depressions.

ELEVATIONS Flow over elevated terrain features resembles flow around obstacles Ridges are elongated hills that are less than or equal to 600 m (2000 ft) above the surrounding terrain and have little or no flat area on the summit. The ratio of length to height should be at least 10. When the prevailing wind is not perpendicular, the ridge will not be as attractive a site. Also, when the ridge is concavity in the windward direction enhances speed-up, and convexity reduces speed-up by deflecting the wind flow around the ridge.

EFFECT OF RIDGE ORIENTATION AND SHAPE ON SITE SUITABILITY The slope of a ridge is also an important parameter. Steeper slopes give rise to stronger wind flow, but on the lee of ridges steeper slopes give rise to high turbulence

DEPRESSIONS Depressions are characterized by a terrain feature lower than the surroundings. The speed-up of the wind is greatly increased if depressions can effectively channel the wind. Shallow valleys and canyons (< 50 m) are considered small-scale depressions, and other features such as basins, gaps, etc. are considered as large-scale depressions There are many factors that influence the flow in depressions. These include orientation of the wind in relation to the depression, atmospheric stability, the width, length, slope, and roughness of the depression, and the regularity of the section of valley or canyon.

CHARACTERISTICS OF NON-FLAT TERRAIN: LARGE-SCALE FEATURES Large-scale features are ones where the vertical dimension is significant in relation to the planetary boundary layer. The flow over these features is the most complex, and flow predictions for this category of terrain classification are the least quantified. Valley and canyons A canyon is a deep valley often curved from the Earth by a river.  A valley is a landform, which can range from a few square miles (square kilometers) to hundreds or even thousands of square miles in area. It is typically a low-lying area of land, surrounded by higher areas such as mountains or hills.

2. Slope winds Slope winds are local diurnal winds present on all sloping surfaces. They flow upslope during the day as the result of surface heating, and downslope at night because of surface cooling. Slope winds are produced by the local pressure gradient caused by the difference in temperature between air near the slope and air at the same elevation away from the slope.

3. Prevailing winds in alignment 4. Prevailing winds in non-alignment