RESEARCH ARTICLE Spectral Wave Modeling in Very Shallow Water at Southern Coast of Caspian Sea Seyed Masoud Mahmoudof 1 & Peyman Badiei 2 & Seyed Mostafa Siadatmousavi 3 & Vahid Chegini 1 Received: 9 January 2017 /Accepted: 4 July 2017 # Harbin Engineering University and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract This study evaluates the capability of the Simulating WAves Nearshore (SWAN) wave model (version 41.01) in predicting significant wave height and spectral peak energy content for swell waves in very shallow water of surf zone during depth-induced wave breaking and dissipation. The model results were compared with field measurements at five nearshore stations. The results demonstrated that some breaker index formulations were successful for significant wave height prediction in surf zones. However, an incorrect shape of the energy spectrum and overestimated near spectral peak energy content at shallow water stations were obtained using all of the embedded depth-induced wave breaking formulations in SWAN. The dependent breaker index on relative depth (K p d) formulation, which was successful in predicting near spectral peak energy content, resulted in an average error of 30%. Finally, this formulation was modified to enhance the model performance in reproducing the spectral peak energy content. Keywords Caspian Sea . Shallow water . SWAN . Spectral peak energy . Coastal processes 1 Introduction The main objective of this study is to evaluate the validity and performance of the formulations for depth-induced wave breaking and dissipation processes in very shallow waters. For this purpose, the results of the Simulating WAves Nearshore (SWAN) model (Booij et al. 1999) are compared with the data obtained from a series of field measurements conducted at the southern coast of the Caspian Sea. The SWAN model is briefly described in Section 2 and various formulations of wave breaking and dissipation terms implemented in the model are introduced and discussed. Field measurement program and results are presented in Section 3. The overall evaluation methods are discussed in Section 4 and the simulation results in comparison with the field data are evaluated in Section 5. The study is summarized and conclud- ed in the final section. 2 Model Description In this study, the phase-averaged third-generation wave model SWAN version 41.01 was employed to simulate spectral evo- lution of waves in shallow water in the surf zone. The model numerically solves the conservation of wave action density equation as follows: ∂N ∂t þ ∂ c x N ð Þ ∂x þ ∂ c y N   ∂y þ ∂ c σ N ð Þ ∂σ þ ∂ c θ N ð Þ ∂θ ¼ S σ; θ; x; y; t ð Þ σ ð1Þ where N = E/σ is action density, and E is energy density de- pendent on relative frequency σ and mean direction θ. In ad- dition, c x , c y , c θ and c σ represent the wave group velocities in x, y , θ and σ directions, respectively. The S on the right side denotes the total sum of sink and source terms and can be written as follows: * Seyed Masoud Mahmoudof m_mahmoudof@inio.ac.ir 1 Iranian National Institute for Oceanography and Atmospheric Sciences (INIOAS), Tehran 1411813389, Islamic Republic of Iran 2 School of Civil Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran 3 School of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran 1684613114, Iran Journal of Marine Science and Application https://doi.org/10.1007/s11804-018-0011-y