Comparison of sea wave measurements using HF Radar and numerical model (IllSW A) hindcast

Comparison of sea wave measurements using HF Radar and numerical model (IllSW A) hindcast

Aghababazadeh, F.

Battjes, J.A. (mentor)
Holthuijsen, H. (mentor)
Booij, N. (mentor)
Verhagen, H.J. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

Coastal Engineering

1994-06-01

The study of waves and current, especially in the nearshore region is of particular importance for nearshore or coastal works. During the last decades, the formulations to describe the wave propagation as the waves move towards the shore has advanced very much. Many operational models for the hindeast and forecast of waves have been developed. On the other hand the development of measurement techniques has led to wider, more frequent and more accurate measurements of the sea state, both in terms of current and wave field. Hence there is a growing need for the verification and calibration of models and measuring techniques. A recent development in measurement of waves is the use of High Frequency (HF) Radar. Such a method is already established as a powerful tool for measuring the pattem of surface current, but its use in wave measurements especially in the dual arrangement is recent. In this method two land based radars are mounted. Measurement of the backscatter of H F wave at each point on the sea provides the raw dataset which is analyzed to give directional spectra of surface elevation at each point. More details on H F Radar wave measurement technique are given in appendix B of the present report. Two numerical models are used, both developed at the Delft University of Technology. They are HISWA (HIndcast Shallow water WAves) and DUCHESS which perform wave and current calculations respectively. The performance of the wave model has been examined against point measurements of waverider buoys. Two such studies are mentioned here. Vogel, et al. (1988) reported an inter-comparison between two wave models; HISWA and CREDIZ, using Haringvliet area wave data where the influence of local wind is significant. They conclude that Hiswa performs very weIl in predicting the significant wave height (Hs) but the changes in mean frequency need more consideration. Their figure lO-b (not reproduced here), which is a scatter diagram for measured and Hiswa computed wave heights is of particular interest for this study since it resembles the Radar and Hiswa scatter diagram 1 (chapter 4). Soras et al. (1987) reported a verification study of HISWA for the shallow waters of the Norwegian seas. The latter case study area is for sheltered shallow waters where no current is present but it is very sensitive to the deep water incident wave heights. Comparison of significant wave height (Hs) and mean wave period (T) with waverider results at one point showed an agreement within 10% in H, but too long periods for the hindcast. DUCHESS is a 2-dimensional horizontal model for estuary and sea surges (current and water levels). lts output can be used as input in HISWA. In this way the wave model HISWA will account for the effect of both bottom topography and current field on waves. A short explanation of the models is given in Appendix A. Few investigations have been carried out for the purpose of verifying the HF Radar wave measurements and in particular the inversion algorithms to obtain the wave parameters. Wyatt (1986) reported on promising experiments when only one radar was employed for the measurements and suggested the use of two radars, which was reported later by Shearman (1987) and Wyatt (1989, 1990). They claimed accurate measurements up to a range of 100 km and wave heights within 10% of those measured with a directional wave buoy. Powell, and Khandekar (1993) reported on the wave measuring capabilities of one ground based radar and evaluated the significant wave height and one-dimensional spectra at five points with hindeast values generated by an operational ocean wave model in the Canadian North Atlantic shelf. Their wave model used two spherical grids, a coarse grid spacing of 1° in latitude (110 km) and a fine grid spacing of one third of the coarse grid (37 km). They claimed 'reasonable' agreement between the radar-measured wave data and model generated hindcasts.

coastal works
wave measurements

http://resolver.tudelft.nl/uuid:f2639195-779d-4ed0-9536-844d718399a6

Student theses

master thesis

(c) 1994 Aghababazadeh, F.