Clause 6.3.3 defines the topography factor k3, which accounts for wind speed acceleration over hills, ridges, cliffs, and escarpments. For flat terrain (slope < 3°), k3 = 1.0. Where the upwind slope exceeds 3°, wind accelerates near the crest of the feature, and k3 > 1.0 applies. The factor depends on the shape of the topographic feature (hill, ridge, or escarpment), the upwind slope, and the position of the structure relative to the crest. Detailed calculation procedures are given in Appendix C.
Key Requirements
•k3 = 1.0 for flat terrain where the upwind ground slope is less than 3°
•For upwind slopes between 3° and 17°, k3 shall be calculated using the factor s from Appendix C
•For upwind slopes greater than 17°, k3 shall be taken as the value for 17° slope
•k3 applies within a zone extending from 1.5L upwind to 2.5L downwind of the crest (L = effective horizontal length of the hill)
•The maximum value of k3 occurs at the crest and reduces with distance from the crest
Reference Tables
Topography Factor k3 — Summary of Key Values
Condition
k3 Value
Flat terrain (upwind slope < 3°)
1.00
Crest of a low hill (slope 3°–17°)
1.0 + 0.36 × s (varies by slope)
Crest of a steep escarpment (slope ≥ 17°)
Up to 1.36
Downwind of crest (within 2.5L)
Reduces from crest value to 1.0
s is a factor depending on the slope and shape of the topographic feature. Detailed values are in Appendix C, Figs. 14 and 15 of IS 875 Part 3:2015.
Formulas
k3 = 1 + C × s
Topography factor at or near the crest of a hill, ridge, or escarpment
k3 = Topography factorC = Coefficient: 1.2 for a 2D ridge or escarpment, 0.8 for a 3D axisymmetric hills = Factor from Appendix C depending on upwind slope (θ), position relative to crest, and height above ground
Practical Notes
✓For the vast majority of Indian building sites (flat ground, plains), k3 = 1.0. This factor becomes significant only for buildings on or near hilltops, ridges, or cliff edges.
✓At the crest of a steep ridge, k3 can reach up to 1.36 — this means a 36% increase in wind speed and an 85% increase in wind pressure compared to flat terrain.
✓Hill stations (Shimla, Ooty, Munnar) and telecom towers on ridges are the most common cases where k3 > 1.0.
✓The wind acceleration effect diminishes rapidly with distance from the crest — beyond 2.5L downwind, k3 = 1.0 again.
Common Mistakes
⚠Assuming k3 = 1.0 for a building on a hilltop without checking the upwind slope angle.
⚠Not applying k3 for communication towers sited on ridges for better signal coverage — these are exactly the locations where k3 is significant.
⚠Using k3 > 1.0 for buildings in valleys — the topography factor addresses speed-up over crests, not valley effects.
⚠Applying the escarpment factor when the feature is actually a symmetric hill — C differs (1.2 vs 0.8).