Longshore Drift Rate Calculator
Estimate the potential sand moved along a beach by obliquely breaking waves, or combine two wave classes into a net and gross annual drift budget.
🏖️ What is Longshore Drift?
Longshore drift, also called littoral drift, is the movement of sand and gravel along a coast, parallel to the shoreline. It happens because waves usually reach the beach at a slight angle. The breaking waves push water and sediment up the beach at that angle, and the backwash and the longshore current in the surf zone carry them along the shore. Over a year this zigzag movement can add up to hundreds of thousands or even millions of cubic metres of sand passing a single point. The longshore drift rate is the volume or mass of sediment that crosses a fixed line perpendicular to the beach per unit time.
Coastal engineers and geomorphologists estimate the rate to design harbour entrances and jetties, size beach nourishment projects, plan sand bypassing, predict how fast a spit will grow, and judge whether a stretch of coast is likely to gain or lose sand. This calculator uses the CERC formula from the Shore Protection Manual, which assumes the transport is proportional to the longshore component of wave energy flux at the breaker line. It works from the breaker height and the angle between the breaking crests and the shoreline, and it turns the answer into cubic metres per second, thousand cubic metres per year and tonnes per year.
Three distinctions matter. First, the result is a potential rate for an unlimited supply of loose sand, not a measured quantity. Second, it is not the same as coastal erosion or beach profile change: sand can move along the coast without any net loss, and a beach erodes only when more leaves than arrives. Third, the formula uses the bulk volume of sand including pore space, so the mass of grains is smaller by the factor (1 minus porosity) times the grain density. The CERC formula is usually accurate to within a factor of about two unless K is calibrated against local data.
The second mode adds an annual view. You enter two wave classes, each with a breaker height, a signed angle and the share of the year it occurs, and the calculator adds the signed contributions. It reports the net drift, the gross drift and the drift ratio, which tells you whether the coast is one-directional or has opposing transports that largely cancel. To find the breaker height from deep water waves, start from the significant wave height calculator, and for wave speeds and wavelengths see the ocean wave speed calculator.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Moderate Swell
Hb = 1.5 m, θb = 10°, seawater, quartz sand, Hs with K = 0.39
Example 2 — Storm Waves
Hb = 3.5 m, θb = 25°, seawater, quartz sand
Example 3 — Small Lake
Hb = 0.4 m, θb = 20° in fresh water (ρ = 1000 kg/m³)
Example 4 — Two-Class Annual Budget
Class 1: 1.2 m at +12° for 55% of the year. Class 2: 1.8 m at −15° for 25% of the year.
❓ Frequently Asked Questions
🔗 Related Calculators
What is the CERC formula for longshore transport?
The CERC formula says the volume of sand moved along a beach is proportional to the longshore component of wave energy flux at breaking. In the form used here, Q = K rho sqrt(g) / (16 sqrt(kappa) (rho_s - rho) (1 - p)) times Hb^2.5 times sin(2 theta). It gives the potential rate in cubic metres per second.
What value of K should I use in the CERC formula?
Use K = 0.39 when Hb is the significant breaker wave height, the value recommended by the Shore Protection Manual and derived from the Komar and Inman 1970 tracer study. Use K = 0.78 when Hb is the root-mean-square breaker height. The two give the same answer for the same sea state.
At what angle is longshore drift greatest?
At 45 degrees between the breaking wave crests and the shoreline. The rate follows sin(2 theta), which reaches 1 at 45 degrees and falls to zero at 0 degrees (crests parallel to shore) and at 90 degrees (waves running along the beach).
How do you convert m3/s of longshore drift to m3/year?
Multiply by the number of seconds in a year, 3.156 x 10^7. A potential drift of 0.0856 m3/s becomes about 2.70 million m3 per year if the same waves acted all year, though real wave climates vary, so the budget mode is better for annual totals.
What is the difference between net and gross longshore drift?
Gross drift is the sum of the sand moved in both directions, counted as positive amounts. Net drift is the signed sum, so transport one way cancels transport the other way. A coast with large gross but small net drift moves a lot of sand back and forth yet loses little overall.
What is the drift ratio?
The drift ratio is net divided by gross. It runs from minus 1 to plus 1. A value near plus or minus 1 means waves push sand almost entirely one way, while a value near 0 means two opposing directions nearly cancel. Many exposed coasts have ratios between 0.3 and 0.9 in magnitude.
What is the breaker index kappa?
The breaker index is the ratio of breaker height to the water depth where the wave breaks, Hb / hb. A common value is 0.78. It turns the breaker height into a breaking depth and wave speed, so a smaller kappa implies deeper breaking and slightly more transport.
Does this calculator give the true volume of sand or the bulk volume?
It gives the bulk volume, which includes the pore space between grains, through the (1 - p) term with p = 0.4. Multiply the bulk volume by rho_s (1 - p) to get the mass in tonnes, which the calculator also reports so you can compare with dredging records.
How accurate is the CERC formula?
Without calibration it is good to about a factor of two. With site measurements, such as trapped sand against a jetty, K can be calibrated and the formula then predicts within roughly 50 percent. Laboratory and field studies show that the standard K often overpredicts.
Does longshore drift cause coastal erosion?
Drift moves sand but does not itself erode a beach. Erosion occurs when a stretch of coast loses more sand downdrift than it receives updrift, which this calculator does not evaluate. The net drift shows the imbalance only if sand supply and barriers are known.