Longshore Drift Rate Calculator
Our oceanography & coastal science calculator computes longshore drift rate accurately. Enter measurements for results with formulas and error analysis.
Formula
Q = K x Pl / ((rhoS - rhoW) x g x (1-p)) | Pl = (rho x g x Hb2 / 16) x Cb x sin(2*alpha)
Where Q is volumetric transport rate in m3/s, K is the CERC empirical coefficient (0.39), Pl is longshore wave power in W/m, rhoS is sediment density, rhoW is water density, g is gravity, p is porosity, Hb is breaker height, Cb is breaker celerity, and alpha is the wave approach angle at breaking.
Worked Examples
Example 1: Moderate Energy Beach Transport
Problem: Calculate the annual longshore transport rate for a beach with 1.2 m breaker height, 10 s wave period, and 12-degree breaker angle. Beach slope is 0.04.
Solution: Breaker celerity: Cb = sqrt(9.81 x 1.2 / 0.78) = 3.88 m/s\nLongshore wave power: Pl = (1025 x 9.81 x 1.44 / 16) x 3.88 x sin(24)\n= 904.4 x 3.88 x 0.4067 = 1,426 W/m\nImmersed weight rate: Il = 0.39 x 1,426 = 556.2 N/s\nVolumetric rate: Q = 556.2 / ((2650-1025) x 9.81 x 0.6) = 0.0584 m3/s\nAnnual rate: 0.0584 x 31,557,600 = 1,842,969 m3/yr
Result: Annual transport: ~1,843,000 m3/yr | Very High | Longshore velocity: 0.77 m/s
Example 2: Low Energy Sheltered Beach
Problem: A sheltered beach has 0.5 m breaker height, 6 s period, and 8-degree breaker angle. Estimate annual transport and classify beach state.
Solution: Breaker celerity: Cb = sqrt(9.81 x 0.5 / 0.78) = 2.51 m/s\nLongshore wave power: Pl = (1025 x 9.81 x 0.25 / 16) x 2.51 x sin(16)\n= 157.0 x 2.51 x 0.2756 = 108.6 W/m\nIl = 0.39 x 108.6 = 42.3 N/s\nQ = 42.3 / ((2650-1025) x 9.81 x 0.6) = 0.00445 m3/s\nAnnual: 140,362 m3/yr (Moderate)
Result: Annual transport: ~140,000 m3/yr | Moderate | Low-energy coast
Frequently Asked Questions
What is longshore drift and how does it transport sediment along coastlines?
Longshore drift (also called littoral drift or longshore sediment transport) is the movement of sediment along a coastline driven by waves approaching the shore at an oblique angle. When waves break at an angle to the shoreline, they push sediment particles up the beach face in the direction of wave propagation during swash, but gravity pulls the water and sediment directly back down the slope during backwash. This zigzag pattern of sediment movement produces a net displacement along the shore in the direction of wave approach. Longshore drift rates vary from near zero on sheltered coasts to over one million cubic meters per year on exposed, high-energy coastlines. This process is fundamental to coastal geomorphology because it shapes beaches, builds spits and barriers, fills harbors, and controls shoreline evolution over engineering and geological timescales.
What is the longshore current and how fast does it flow?
The longshore current is a shore-parallel flow generated within the surf zone by the longshore component of wave momentum flux (radiation stress). When waves break at an angle, they transfer momentum in the along-shore direction, driving a current that typically flows between 0.1 and 1.5 meters per second, though velocities up to 2 m/s have been measured during storms. The Longuet-Higgins formula estimates longshore current velocity as V = 20.7 times beach slope times the square root of (g times Hb) times sin(alpha) times cos(alpha), where alpha is the breaker angle. The current is strongest near the breaker line and decreases toward shore and seaward. Longshore currents are important not only for sediment transport but also as a swimming hazard, as they can carry swimmers rapidly along the beach away from their entry point.
How do groins and jetties affect longshore sediment transport?
Groins and jetties are shore-perpendicular structures that interrupt longshore sediment transport by creating physical barriers across the transport pathway. On the updrift side of a groin, sediment accumulates because the structure traps material moving along the shore, building out the beach. On the downdrift side, the beach erodes because the sediment supply from updrift has been intercepted, creating a characteristic asymmetric shoreline pattern. This downdrift erosion (called terminal scour or the groin effect) can extend hundreds of meters beyond the structure. Groin fields (multiple groins in series) attempt to minimize this effect by distributing trapping over a longer shoreline segment. Jetties at harbor entrances often trap large volumes of sediment, requiring periodic dredging of navigation channels. Modern coastal engineering increasingly favors beach nourishment and living shorelines over hard structures.
How is longshore drift measured in the field?
Field measurement of longshore sediment transport employs both direct and indirect methods with varying accuracy and coverage. Sediment tracer studies use fluorescent-coated or naturally distinctive sediment grains released at a known point, with subsequent sampling to track their dispersion pattern and transport rate. Sand trap instruments placed on the seabed collect sediment flowing past a fixed point over time. Impoundment measurements at groins, jetties, or natural barriers quantify transport by measuring volume changes in trapped sediment. Repeat bathymetric and beach profile surveys detect volume changes that indicate net transport patterns. Optical and acoustic backscatter sensors measure suspended sediment concentrations in the water column. More recently, satellite imagery and drone surveys combined with machine learning algorithms enable large-scale shoreline change analysis. Each method has limitations in spatial coverage, temporal resolution, and the ability to capture both bedload and suspended load components.
What is the relationship between longshore drift and coastal landforms?
Longshore drift is the primary process responsible for creating and maintaining many distinctive coastal landforms. Spits form where longshore transport continues past a change in coastline orientation, depositing sediment into open water that progressively extends the shoreline. Barrier islands develop from spit growth and sediment accumulation parallel to the mainland coast, eventually enclosing lagoons. Tombolos connect offshore islands to the mainland where wave diffraction creates convergent transport patterns. Cuspate forelands (cape-like features) develop where opposing drift directions meet. Beach ridges record former shoreline positions and transport patterns preserved in the geological record. River mouth bars and deltas are shaped by the interaction of fluvial sediment delivery and longshore redistribution. Understanding longshore transport patterns is essential for predicting how these landforms will evolve under changing wave climates and sea level rise.
How does beach nourishment interact with longshore drift processes?
Beach nourishment (the artificial placement of sand on an eroding beach) interacts directly with longshore drift because the added sediment becomes part of the littoral transport system immediately upon placement. Nourished beaches typically lose sand at higher rates initially because the artificially widened beach profile is out of equilibrium with the ambient wave climate and longshore transport regime. The placed sand gradually disperses downdrift through longshore transport, benefiting adjacent beaches but reducing the design life of the nourishment project. Project design must account for expected longshore transport rates to estimate renourishment intervals, which typically range from 3 to 10 years depending on wave energy and project volume. Terminal structures like groins at the downdrift end of nourishment projects help retain sand but can create downdrift erosion. The grain size of nourishment material strongly affects retention, with coarser material persisting longer than the native sediment.