Nature-Based Solutions for Coastal Management PDF

Nature-based solutions for coastal management FLOW ENERGY ATTENUATION – PHYSICAL PROCESSES Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Drag force Wave transformation Waves and currents Shear stresses Nature Ba...

Nature-based solutions for coastal management FLOW ENERGY ATTENUATION – PHYSICAL PROCESSES Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Drag force Wave transformation Waves and currents Shear stresses Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Multidisciplinary problem Climate sciences Economics Engineering Sociology Nature-based solutions Hydrodynamics Ecology Morphodynamics Biology Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Rigid Waves Flexible Emerged Submerge d Non linear processes Hence, make it harder to understand Currents Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Drag force Wave transformation Waves and currents Shear stresses Nature Based Solutions for Coastal Management Maria Maza, 2024  Drag Force Analogy: walking in the beach against water flow Drag force – uniform flow & cylinder bond between fluid molecules reducing frontal area [water, honey] will, lower drag force [aerodinamic car] Flow-cylinder interaction depends on the Re: o For Re1 viscous forces are negligible except close to the boundary: irrotational flow + boundary layer 1 𝐹! = 𝜌𝐶! 𝐴" 𝑈 𝑈 2 Nature Based Solutions for Coastal Management Maria Maza, 2024  Drag Force Re < 3 x 105: the BL remains laminar, although the wake may be completely turbulent. The laminar BL separates at 82º. Pressure in the wake is nearly constant and lower than the upstream pressure. This asymmetry in pressure leads to a nearly constant drag force with C D ≈ 1.2 3 x 105 < Re < 3 x 106: laminar BL becomes turbulent and it separates at 125º. C D ≈ 0.33 flow separation behind the area FIGURE 9.16 Depiction of some of the flow regimes for a circular cylinder in a steady Maria Nature Based Solutions for Coastal Management uniform cross flow. Here Maza, 2024 Re ¼ UNd/n is the Reynolds number based on free-stream speed UN and cylinder diameter d. At the lowest Re, the streamlines approach perfect fore-aft symmetry. As Re increases, asymmetry increases and steady wake vortices  Drag Force Pressures within the wake are higher when the BL is turbulent. This results in a sudden drop in the drag coefficient from 1.2 to 0.33 at the point of 394 transition. 9. BOUNDARY LAYERS AND RELATED TOPICS not really applicable to real case of vegetation Non-linear relationship between Re and drag coefficient why? 1.2 3x105 Nature Based Solutions for Coastal Management Maria Maza, 2024 FIGURE 9.21 Measured drag coefficient, CD, of a smooth circular cylinder vs. Re ¼ UNd/n. The sharp dip in CD near Recr is due to the transition of the boundary layer to turbulence, and the consequent downstream movement of  Drag Force Drag force – more complex (and realistic) cases Flow-vegetation interaction: o Complex geometries o Submerged / Emergent o Flexible / Rigid 1 𝐹! = 𝜌𝐶! 𝐴" 𝑁𝑈 𝑈 2 Nature Based Solutions for Coastal Management Maria Maza, 2024  Drag Force Drag force – more complex (and realistic) cases Flow-vegetation interaction: o Complex geometries o Submerged / Emergent o Flexible / Rigid 1 𝐹! = 𝜌𝐶! 𝐴" 𝑁𝑈 𝑈 2 Skimming flow FD FD Nature Based Solutions for Coastal Management Maria Maza, 2024  Drag Force Drag force – more complex (and realistic) cases Flow-vegetation interaction: o Complex geometries o Submerged / Emergent o Flexible / Rigid 1 𝐹! = 𝜌𝐶! 𝐴" 𝑁𝑈 𝑈 2 FD FD Nature Based Solutions for Coastal Management Maria Maza, 2024  Drag Force Introduce the importance of mechanical diffusion in vegetated flows --> model predicts that turbulence intensity increases Energy attenuation – Uniform flow with the introduction of sparse vegetation owing to the addition of wake production, but then decreases with increasing population density as mean flow speed is reduced Force balance between free surface gradient and drag force: surface slope 𝜕ℎ 𝑔ℎ = 𝐹! 𝜕𝑥 Nepf, H. (1999). Drag, turbulence, and diffusion in flow through emergent vegetation. Water Resources Research 35.2 extends the cylinder-based model for vegetative resistance by including the dependence of the drag coefficient, CD on the stem population density. supported by laboratory and field observations, Introduce stem Reynolds' numbers Nature Based Solutions for Coastal Management Maria Maza, 2024  Drag Force Energy attenuation – Waves Dissipation is considered to be only due to drag force: equation is based on regular wave derivative of the group velocity of 𝐻' wave height Dalrymple, R.A., Kirby, J.T., Hwang, P.A., 1984. Wave the wave, to 𝐻= diffraction due to areas of energy dis- sipation. J. distance along 1 + 𝛽𝑥 Waterw. Port Coast. Ocean Eng. 110, 67–79. 𝜕𝐸𝑐# dissipation x: distance vegetation beta: wave damping coefficient, function of drag force = −𝜀! 𝐻*+,,' wave decay formulation 𝜕𝑥 𝐻*+, = Mendez, F.J., Losada, I.J., 2004. An empirical model to estimate the propagation of random breaking and depth-integrated and time-averaged 1 + 𝛽′𝑥 non-breaking waves over vegetation fields. Coast. integrate from -h energy dissipation per unit horizontal area to the height of Eng. 52, 103–118. extension for random waves vegetation (hv) $%&%! 𝐻' 𝜀! = 2 𝐹! 𝑈𝑑𝑧 𝐻= Losada, I. J., Maza, M., & Lara, J. L. (2016). A new $% 1 + 𝛽() 𝑥 formulation for vegetation-induced damping under 𝐻*+,,' combined waves and currents. Coastal Engineering, 𝐻*+, = 107, 1–13. drag force 1 + 𝛽′() 𝑥 Nature Based Solutions for Coastal Management Maria Maza, 2024  validation shows good agreement How? By choosing correct Cd Energy attenuation – Waves --> calibrating tools Nature Based Solutions for Coastal Management Maria Maza, 2024  Energy attenuation – Waves waves interact with meadow of seagrass seaweed attenuates wave energy Nature Based Solutions for Coastal Management Maria Maza, 2024  Drag Force Drag force ➞ CD? Energy attenuation ➞ CD? β? Numerical modelling Physical modelling Field campaigns Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Drag force Wave transformation Waves and currents Shear stresses Nature Based Solutions for Coastal Management Maria Maza, 2024  Refraction Transformation of wave characteristics ( height, direction) due to changes in wave celerity generated by changes in water depth. Wave height can also be estimated assuming energy flux Wave direction can be determined using the conservation within wave propagation equation. wave rays. Nature Based Solutions for Coastal Management Maria Maza, 2024  Refraction In intermediate and shallow waters, wave length is related with water depth wave length decreases as water depth decreases Consequently, wave These changes in wave celerity bend the wave celerity decreases as water depth crest decreases Nature Based Solutions for Coastal Management Maria Maza, 2024  Refraction Wave rays Wave crests tend to be parallel to the shoreline Nature Based Solutions for Coastal Management Maria Maza, 2024  Refraction Wave ray Wave crest Refraction of a SWELL due to the bathymetry Nature Based Solutions for Coastal Management Maria Maza, 2024  Refraction When we have a vegetation field in a particular area, it can lead to refraction phenomena because it decreases the effective depth in the area. The phenomenon of refraction is not significant for small ecosystems or those that do not have sufficient density to influence the flow. what this image tells? lower energy after passing vegetation (?) vegetated patches Nature Based Solutions for Coastal Management Maria Maza, 2024  Diffraction Energy transfer across wave rays due to gradients in wave height It is clearly observed behind islands and breakwaters but is present all over the wave propagation. If there is a gradient of wave height, diffraction tries to compensate the gradient. Nature Based Solutions for Coastal Management Maria Maza, 2024  Diffraction Diffraction generates a typical circular wave crest pattern behind breakwaters. Nature Based Solutions for Coastal Management Maria Maza, 2024  Diffraction When we have an emerged and sufficiently dense vegetation field, the energy behind the field is lower than in the surrounding areas. Therefore, behind the field there is a lateral energy transfer. Nature Based Solutions for Coastal Management Maria Maza, 2024  Reflection H m0 R KR = H m0i Structure Reflection coefficient Vertical Breakwater 0.7 – 1 Mound Breakwater (1:2 to 1:3) 0.3 - 0.6 Natural Beach 0.05 - 0.2 Dique en talud de bloques artificiales de hormigón 0.3 - 0.5 Dique vertical disipador de energía 0.3 - 0.8 Playa 0.05 - 0.2 Nature Based Solutions for Coastal Management Maria Maza, 2024  Reflection When a wave interacts with an ecosystem part of the energy is reflected, especially for dense and emerged vegetation fields. Nature Based Solutions for Coastal Management Maria Maza, 2024  Breaking Wave steepness exceed a physical limit an particles velocities are larger than wave celerity Wave energy is dissipated by turbulence and wave height decreases suddenly Nature Based Solutions for Coastal Management Maria Maza, 2024  Breaking There are 4 different types of breakers depending on the Iribarren´s number (also know as surf similarity parameter) Spilling tan a I ro = H Lo Breaker Types Iribarren number, Ir0 Plunging Spilling < 0.4 Plunging 0.4 – 2 Collapsing Collapsing 2 – 3.1 Surging > 3.1 Oscilación > 3.1 Surging Nature Based Solutions for Coastal Management Maria Maza, 2024  Breaking Depending on the depth and the characteristics of the ecosystem, breakage may occur. This is a crucial process in coral reefs. tidal terrace with reefs? Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Drag force Wave transformation Waves and currents Shear stresses Nature Based Solutions for Coastal Management Maria Maza, 2024  Waves and currents Nature Based Solutions for Coastal Management Maria Maza, 2024  Waves and currents Drag force changes FD_W FD_WC FD_WC C+W on the same dir. drag force for only the current A smaller wave damping is found for waves and current acting in the same direction and an increase in the wave damping rate is obtained for waves and current flowing in the opposite direction Nature Based Solutions for Coastal Management Maria Maza, 2024  Waves and currents Nature Based Solutions for Coastal Management Maria Maza, 2024  Waves and currents Conservation of energy: Nature Based Solutions for Coastal Management Maria Maza, 2024  Introduction Drag force Wave transformation Waves and currents Shear stresses Nature Based Solutions for Coastal Management Maria Maza, 2024  Shear stresses Shear stresses are very important in vegetation fields and coral reefs: they will determine sediment transport and coral/plants loss by uprooting. Nature Based Solutions for Coastal Management Maria Maza, 2024  Shear stresses There are some analytical formulas for bottom shear stresses: b ub = σ ( H cosh k ( h − h) )! L $ cos # k − σ t & = Ab = ( ) H cosh k ( h − h) = H 1 2 sinh ( kh) " 4 % 2 sinh ( kh) 2 sinh ( kh) H 1 = σ sin (σ t ) = U m sin (σ t ) = Abσ sin (σ t ) 2 sinh ( kh) Linear wave theory assumes no viscous fluid èwater particle movement on the bottom èno true èboundary layer Nature Based Solutions for Coastal Management Maria Maza, 2024  Shear stresses For flat bottom and laminar boundary layer: second viscous Newton law: ∂u τ = ρν ∂z The maximum bottom shear stress results: ρν U m τ b,max = ν /σ This shear stress can be expressed as a function of a friction coefficient (Jonsson, 1966): 1 1 τ b = ρ fcU 2 τ b,max = ρ fwU m2 2 2 Nature Based Solutions for Coastal Management Maria Maza, 2024  Shear stresses Shear stress around ecosystem elements is high and can result in loss of ecosystem elements through a combination of sediment loss and force. Nature Based Solutions for Coastal Management Maria Maza, 2024  Shear stresses Additionally, shear stress on top of submerged canopies influences sediment, nutrients and oxigen transport. The importance of having gaps in between meadow Nature Based Solutions for Coastal Management Maria Maza, 2024  Nature-based solutions for coastal management FLOW ENERGY ATTENUATION – PHYSICAL PROCESSES Nature Based Solutions for Coastal Management Maria Maza, 2024