Terrain Analysis

Shape of the Land

Wind does not blow the same everywhere. Hills make it faster. Valleys slow it down. Watch how terrain shape changes the wind your building must resist.

Calculate Topographic Effects

Kzt Factor

1.00

No speed-up

Hill Height 50 ft

Hill Steepness 0.25

Terrain Types

Different terrain features create different wind effects. Here are the three types that matter for building design.

Hill

Kzt up to 1.85

A bump in the terrain with slopes on all sides. Wind accelerates over the top as it is squeezed between the ground and upper air.

Ridge

Kzt up to 2.01

An elongated hill with a long axis. Creates the highest speed-up because wind must go over, not around. Common in mountain areas.

Escarpment

Kzt up to 1.57

A cliff or steep slope separating two flat levels. Wind accelerates at the top edge. Think of coastal bluffs or plateau edges.

Where Speed-Up Happens

Wind does not speed up everywhere on a hill. Location matters. See where the highest and lowest wind speeds occur.

Crest (Maximum Speed-Up)

Highest Kzt. Wind is fastest at the top where it crests the hill. This is the worst location for wind loads.

Upper Windward Slope

Moderate speed-up. Wind begins accelerating as it rises over the hill. Still elevated Kzt values.

Base & Leeward Side

Minimal effect. Kzt approaches 1.0. Wind is either approaching flat or in the wind shadow.

Topographic Questions

Common questions about how terrain affects wind load calculations.

Kzt is a multiplier that accounts for wind speed-up over hills, ridges, and escarpments. When wind flows over terrain features, it accelerates as it crests the high point. Kzt ranges from 1.0 (flat terrain) to over 2.0 for steep hills, meaning wind pressures can more than double due to topography alone. The calculation depends on hill height, steepness, and your building position relative to the crest.

The Florida Keys are mostly flat with minimal elevation changes, so topographic effects are usually not significant. Most Keys projects use Kzt = 1.0. However, if your site is on elevated fill, a bridge approach, or near any man-made or natural ridge, you should evaluate topographic effects. Even a 15-foot elevation change can trigger Kzt requirements under certain conditions.

Wind speed is highest at the crest of a hill or ridge and in the upper half of the windward slope. The speed-up effect decreases as you move down the leeward side or away from the crest horizontally. Buildings at the top of a hill experience significantly higher wind loads than those at the base. If you can choose where to build, the base of the windward side or far down the leeward side will have lower wind loads.

Topographic speed-up can increase wind pressures by 50% to over 100% in extreme cases. Since pressure is proportional to velocity squared, a Kzt of 1.5 means about 2.25 times the pressure. This is why hilltop buildings often suffer more damage in storms than valley buildings. Even modest hills can significantly increase design requirements.