What is a design wave?

A hypothetical wave defined by its characteristics (height, period, direction) used for designing a structure to withstand specific sea conditions associated with a certain return period (e.g., 1 in 100 years).

What is the SMB method?

The Sverdrup-Munk-Bretschneider (SMB) method is an empirical technique detailed in Clause 4.3 for predicting wave height and period based on wind speed, fetch length, and wind duration.

How do I choose between a fetch-limited or duration-limited condition?

It is a fetch-limited condition if the wind blows for a long enough time over a given fetch. It is duration-limited if the wind stops or changes before the waves have had time to fully develop over that fetch. The one that results in smaller wave parameters governs the design.

Does this code apply to shallow water?

The primary forecasting methods are for deep water. Clause 6 provides guidance on wave transformation, which is the process of modifying deep-water wave parameters to account for shallow water effects like shoaling, refraction, and diffraction.

IS 10398

: 1982

Guidelines for Determination of Design Waves in Coastal Waters

CurrentSpecializedGuidelinesWater Resources · Coastal and Marine Engineering

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This standard provides guidelines for determining the characteristics of design waves, such as significant wave height and peak period, for the design of coastal and marine structures. It covers empirical wave forecasting methods based on wind data (like SMB) and the statistical analysis of measured wave records to establish design conditions.

Provides methods and guidelines for determining design wave parameters for coastal and marine engineering projects.

Quick Reference Values

Significant wave height (Hs) definitionAverage of the highest one-third (33.3%) of waves in a record

Relationship between Hmax and Hs (approx)Hmax ≈ 1.8 * Hs

Relationship between H(1/10) and HsH(1/10) ≈ 1.27 * Hs

Typical design return period for major structures50 to 100 years

Deep water condition definitionWater depth (d) > L/2, where L is wavelength

Key Formulas

gH / Ua^2 = f(gF / Ua^2) — Functional form of SMB method for fetch-limited wave height.

gT / (2π Ua) = f(gF / Ua^2) — Functional form of SMB method for fetch-limited wave period.

C0 = gT / (2π) = 1.56 T^2 — Deep water wave celerity (speed) in m/s.

L0 = gT^2 / (2π) = 1.56 T^2 — Deep water wavelength in meters.

Practical Notes

The wave forecasting methods (SMB, PNJ) presented are empirical and now largely superseded by numerical models (e.g., SWAN, MIKE) for detailed design, but remain useful for preliminary estimates.

The code focuses on wind-generated waves and does not cover wave generation from other events like tsunamis or landslides.

Accurate determination of wind speed (U), fetch length (F), and wind duration (t) is critical for the reliability of the forecasting methods.