## 1. Introduction

[2] Salt marsh vegetation influences coastal geomorphology [*Hacker and Dethier*, 2006] and promotes sediment deposition [*Kastler and Wiberg*, 1996; *Bartholdy et al.*, 2004] by dissipating waves [*Knutson et al.*, 1982; *Mendez and Losada*, 2004] and currents [*Nepf*, 1999]. Productive saltmarsh ecosystems [*Zedler et al.*, 2001] can retain and remove nutrients [*Gribsholt et al.*, 2007] and pollutants [*Nepf et al.*, 1997; *Gaylord et al.*, 2007].

[3] When vegetation is rigid, wave dissipation can be simulated by calculating drag on individual stems, vertically-integrating the resulting dissipation, and summing over all stems [*Dalrymple et al.*, 1984]. For sufficiently sparse canopies (for sufficiently low *λ*_{p} = proportion of bed area occupied by stems), the required depth-variability of velocity can be estimated from frictionless linear wave theory (higher-density canopies, often found very near the bed, can cause leading-order local departures from frictionless theory [*Lowe et al.*, 2005, 2007]). When vegetation is flexible (e.g., giant kelp [*Elwany et al.*, 1995]), wave dissipation is reduced owing to a tendency of stems to move with the surrounding water. This reduced dissipation has been quantified by fitting a rigid vegetation model [*Dalrymple et al.*, 1984] with a reduced effective drag coefficient. Experiments with flexible vegetation [*Bradley and Houser*, 2009; *Kobayashi et al.*, 1993; *Mendez et al.*, 1999; *Mendez and Losada*, 2004; *Augustin et al.*, 2009] show that the fitted effective drag coefficient decreases with increasing Kuelegan-Carpenter number *KC* = *ut*_{0}/*d* (where *u* is water velocity, *t*_{0} is wave period and *d* is stem diameter) and Reynolds number *Re* = *ud*/*ν* (where *ν* is kinematic viscosity). However, stem flexibility depends on parameters not included in *Re* or *KC*, such as the Young's modulus *E*. A theoretical model [*Mullarney and Henderson*, 2010] for bending of linearly-elastic stems by linear waves predicts that stem motion and wave dissipation are controlled by a dimensionless stiffness *S* = *Ed*^{3}*t*_{0}/4*ρC*_{D}*L*^{4}*u* (where *ρ* is water density, *C*_{D} is drag coefficient and *L* is stem length). This theory successfully predicted the observed motion of two intermediate-stiffness stems in a natural saltmarsh. The theory yields previously untested analytic predictions of the increase in wave dissipation with increasing stiffness, from zero dissipation in the fully flexible limit (*S* → 0) to rapid dissipation in the rigid limit (*S* → ∞). An alternative theoretical model for dissipation (which represents mobile stems as rigid beams with an elastic hinge at the bed) has been tested in the laboratory [*Mendez et al.*, 1999], but no theoretical model for dissipation by flexible vegetation has previously been tested against field observations of dissipation.

[4] We combine measurements of vegetation geometry and wave attenuation in a natural saltmarsh (Section 2) with a wave energy balance equation (Section 3) to quantify dissipation (Section 4). If standard drag coefficient values are used, then a theoretical model [*Mullarney and Henderson*, 2010] accounting for stem motion predicts wave dissipation more accurately than a rigid-vegetation model [*Dalrymple et al.*, 1984].