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Swell Decay Tables

These tables provide theoretical decay rates for swells as they move away from the fetch area that generated them. There are many factors that can affect the real decay rates including interaction with opposing winds, currents, other swells, land, shallow waters, whether the fetch area is aimed right at you, etc. These are best used as a rough guide only. Experience has shown that depending on conditions, one can normally expect actual swell heights at your location to be some percentage less than what is indicated below.

Instructions:
Using a wave model or a sea height estimation table, determine the sea heights for a fetch area you are interested in. Calculate the distance (in nautical miles) from your beach to the center of the fetch area. Then find the table(s) below that best represents the sea heights in the fetch area. Locate the row that best fit's the distance to your beach and look up the expected swell height. For sea heights and distances not defined in the table, interpolate as required. The result represents the average height of the highest 1/3 of all swells generated for the fetch area at the specified time and location.

Example: If a storm is 1700 nmiles away from your beach with seas of 30 ft (aimed straight at you), multiply the sea height (30 ft) times the decay factor (.2325) to determine resulting swell size. Result: 30 ft X .2325 = 6.98 ft

Other Considerations
It's interesting to note that nearly 50% of the swell height is lost as it travels the first 200-300 nmiles away from the fetch area. Another 50% is lost as it moves between 1000-1600 nmiles away.

Also note that the larger the seas are, the more gradual the decay rates are. A 5 ft sea has a decay factor of .088 over 5000 nmiles while a 45 ft sea has a decay factor of .142, a 1.6 time difference. The inference is that larger seas have more energy, and the resulting seas decay less over time as a result of that energy. This is obvious when one considers the summer swells that hit the coast of Southern California. These swells travel over 5000 nmiles and virtually all of them have a period of at least 15-16 secs, indicating they were generated by larger storms rather than small fetch areas. Swells generated by weaker systems would not survive the journey.

5 ft Seas

Distance

Decay Factor

Swell
Height

50

.73

3.65

100

.60

3.00

200

.48

2.40

300

.42

2.10

400

.375

1.88

500

.35

1.75

600

.32

1.60

700

.295

1.48

800

.27

1.35

900

.26

1.30

1,000

.25

1.25

1,100

.2375

1.19

1200

.225

1.13

1300

.2125

1.08

1400

.2

1.00

1500

.192

.96

1600

.185

.93

1700

.175

.88

1800

.165

.83

1900

.16

.80

2,000

.155

.78

2,500

.14

.70

3,000

.125

.63

3,500

.113

.57

4,000

.1

.50

5,000

.088

.44

6,000

.075

.38

7,000

.069

.35

8,000

.063

.32

9,000

.057

.29

10,000

.05

.25

10 ft Seas

Distance

Decay Factor

Swell
Height

50

.75

7.58

100

.62

6.28

200

.5

5.80

300

.453

4.53

400

.4067

4.07

500

.36

3.60

600

.342

3.42

700

.324

3.24

800

.295

2.95

900

.2825

2.83

1,000

.27

2.70

1,100

.2575

2.58

1200

.245

2.45

1300

.2325

2.33

1400

.22

2.20

1500

.215

2.15

1600

.21

2.10

1700

.201

2.01

1800

.192

1.92

1900

.186

1.86

2,000

.18

1.80

2,500

.164

1.64

3,000

.148

1.48

3,500

.134

1.34

4,000

.12

1.20

5,000

.1025

1.03

6,000

.085

.858

7,000

.0775

.775

8,000

.07

.70

9,000

.0625

.63

10,000

.055

.55

15 ft Seas

Distance

Decay Factor

Swell Height

50

.76

11.10

100

.63

9.45

200

.5125

7.69

300

.465

6.98

400

.4175

6.26

500

.37

5.55

600

.353

5.29

700

.336

5.04

800

.319

4.79

900

.303

4.55

1,000

.285

4.28

1,100

.272

4.08

1200

.259

3.89

1300

.252

3.78

1400

.245

3.68

1500

.235

3.53

1600

.225

3.38

1700

.220

3.30

1800

.210

3.23

1900

.200

3.15

2,000

.190

2.85

2,500

.178

2.67

3,000

.155

2.33

3,500

.1425

2.14

4,000

.13

1.95

5,000

.104

1.56

6,000

.095

1.43

7,000

.09

1.35

8,000

.085

1.28

9,000

.0738

1.11

10,000

.0625

0.94

----

20 ft Seas

Distance

Decay Factor

Swell Height

50

.77

15.4

100

.64

12.8

200

.525

10.5

300

.465

9.30

400

.4283

8.57

500

.38

7.60

600

.361

7.22

700

.342

6.84

800

.323

6.46

900

.304

6.08

1,000

.29

5.80

1,100

.28

5.60

1200

.27

5.40

1300

.26

5.20

1400

.25

5.00

1500

.242

4.84

1600

.233

4.66

1700

.225

4.50

1800

.217

4.34

1900

.208

4.16

2,000

.20

4.00

2,500

.188

3.76

3,000

.175

3.50

3,500

.1575

3.15

4,000

.14

2.80

5,000

.12

2.57

6,000

.1

2.00

7,000

.095

1.9

8,000

.09

1.8

9,000

.08

1.6

10,000

.07

1.40

25 ft Seas

Distance

Decay Factor

Swell Height

50

.775

19.31

100

.645

16.13

200

.535

13.38

300

.475

11.88

400

.4346

10.87

500

.398

9.95

600

.368

9.20

700

.3485

8.71

800

.329

8.23

900

.3095

7.74

1,000

.295

7.38

1,100

.285

7.13

1200

.275

6.88

1300

.2625

6.56

1400

.25

6.25

1500

.245

6.13

1600

.24

6.00

1700

.232

5.80

1800

.225

5.63

1900

.218

5.45

2,000

.21

5.25

2,500

.1975

4.94

3,000

.185

4.63

3,500

.165

4.13

4,000

.145

3.63

5,000

.128

3.20

6,000

.11

2.75

7,000

.102

2.55

8,000

.094

2.35

9,000

.084

2.10

10,000

.075

1.88

30 ft Seas

Distance

Decay Factor

Swell Height

50

.775

23.25

100

.645

19.35

200

.54

16.20

300

.485

14.55

400

.445

13.35

500

.4

12.00

600

.375

11.25

700

.354

10.62

800

.333

9.99

900

.316

9.48

1,000

.3

9.00

1,100

.29

8.70

1200

.28

8.40

1300

.27

8.10

1400

.26

7.80

1500

.25

7.50

1600

.24

7.20

1700

.2325

6.98

1800

.225

6.75

1900

.218

6.54

2,000

.21

6.30

2,500

.20

6.00

3,000

.19

5.70

3,500

.17

5.10

4,000

.15

4.50

5,000

.13

3.90

6,000

.11

3.30

7,000

.1033

3.10

8,000

.0966

2.89

9,000

.0858

2.57

10,000

.075

2.25

----

35 ft Seas

Distance

Decay Factor

Swell Height

50

.78

27.3

100

.655

22.93

200

.545

19.08

300

.49

17.15

400

.445

15.58

500

.405

14.18

600

.375

13.13

700

.356

12.46

800

.337

11.80

900

.321

11.24

1,000

.305

10.68

1,100

.2925

10.24

1200

.28

9.80

1300

.272

9.52

1400

.263

9.21

1500

.255

8.93

1600

.248

8.68

1700

.2393

8.38

1800

.231

8.09

1900

.222

7.77

2,000

.21

7.35

2,500

.20

7.00

3,000

.19

6.65

3,500

.17

5.95

4,000

.15

5.25

5,000

.135

4.73

6,000

.12

4.20

7,000

.1094

3.83

8,000

.0988

3.46

9,000

.0881

3.08

10,000

.0775

2.71

40 ft Seas

Distance

Decay Factor

Swell Height

50

.785

31.4

100

.658

26.32

200

.545

21.80

300

.4967

19.87

400

.449

17.96

500

.405

16.20

600

.379

15.16

700

.36

14.40

800

.34

13.60

900

.326

13.04

1,000

.310

12.40

1,100

.296

11.84

1200

.282

11.28

1300

.268

10.70

1400

.265

10.60

1500

.26

10.40

1600

.256

10.24

1700

.246

9.84

1800

.237

9.48

1900

.227

9.08

2,000

.217

8.68

2,500

.207

8.28

3,000

.1975

7.90

3,500

.1713

6.86

4,000

.155

6.20

5,000

.14

5.68

6,000

.125

5.24

7,000

.12

4.80

8,000

.108

4.32

9,000

.096

3.84

10,000

.08

3.20

45 ft Seas

Distance

Decay Factor

Swell Height

50

.79

35.55

100

.658

26.61

200

.548

24.66

300

.497

22.37

400

.45

20.25

500

.41

18.45

600

.38

17.10

700

.362

16.29

800

.345

15.53

900

.33

14.85

1,000

.315

14.18

1,100

.301

13.55

1200

.287

12.92

1300

.273

12.29

1400

.27

12.15

1500

.265

11.93

1600

.26

11.70

1700

.25

11.25

1800

.24

10.80

1900

.23

10.35

2,000

.22

9.90

2,500

.21

9.45

3,000

.2

9.00

3,500

.1713

7.71

4,000

.155

6.98

5,000

.142

6.39

6,000

.131

5.90

7,000

.12

5.40

8,000

.108

4.86

9,000

.096

4.32

10,000

.08

3.60

 

12963

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