Wave growth at short fetches

Jamie Morris, Caroline Gautier, Gerbrant van Vledder

Chair(s): dr. Makropoulos

Tuesday 30 june 2015

16:45 - 17:00h at Central America (level 0)

Themes: (T) Special session, (ST) Coasts at threat in Europe

Parallel session: 7H. Special session: Coasts at threat in Europe

For short-fetch areas (typically 500-5000m), such as lakes and wide rivers, the performance of SWAN is poorer than for longer fetches. Deltares (2013) showed an under prediction of wave height (order of 15%) for short fetches at Lake IJssel (Netherlands). This inconsistency in growth behavior suggests a shortcoming in the parameterizations of the physical processes responsible for wave evolution. To increase the reliability of SWAN model results, we investigated the parameters controlling wave growth at short fetches. Wind and wave data was collected at Lake IJssel where near ideal fetch-limited conditions can be observed during offshore winds. Observations along a cross-shore transect at three locations (FL49, FL48, and FL47) with fetches of 800 m, 1600 m and 10 000 m were analyzed. Observations of wave height were compared to a number of empirical growth curves (e.g. Kahma and Calkoen (1992)). Results indicate that for similar dimensionless fetches (gF/U10^2), the wave observations show a great deal of variability in dimensionless wave height (gHm0/U10^2). This can be partially attributed to measurement error, and due to simplification of the wind speed to one moment in time. However, it can also imply that parameters other than the typically used wind speed and fetch must be used to scale wave growth when the fetch is limited. Furthermore, the observations (at FL48 and FL49) generally lie above the theoretical growth curves. As SWAN is calibrated using the theoretical growth curve of Kahma and Calkoen (1992), this explains the under-prediction in the SWAN results. The role of a number of parameters in influencing wave growth was investigated (inverse wave age, wave steepness, temporal wind speed gradients, gustiness, and atmospheric stability). It has been determined that wind gustiness and wave steepness can explain some of the scatter in the wave observations, where steeper waves, and gustier winds result in higher wave heights than the contrary. Deltares, 2013b: SWAN uncertainties for short fetches, dd Dec, 2013. Report 1207807-001-HYE-0008. Kahma, K.K., and C.J. Calkoen, 1992: Reconciling discrepancies in the observed growth of wind-generated waves. J. Phys. Oceanogr., 22, 138901405.