This is a cross-section of the same tidal marsh transect the 3D version renders as a height-field grid — here plotted directly as an elevation profile from the tidal creek (left) to the marsh interior (right), so the physics that matters (how far each point sits above or below the waterline) is the y-axis itself instead of a bar height you have to read off a 3D scene. Each column follows the Morris et al. (2002) marsh-equilibrium model: accretion is a parabolic function of inundation depth D, the depth of water over the marsh surface at mean high water:
D = MHW − E (inundation depth, clamped ≥ 0)
a(D) = k1·D − k2·D² (vertical accretion, m/yr)
dE/dt = a(D) − c (c = slow background compaction)
Negative feedback: a column sitting below its optimum depth traps more sediment and rises faster, pulling it back toward equilibrium; a column already near the optimum grows slowly. k1 scales with the sediment-supply slider and with proximity to the creek (left edge); k2 is fixed, so a(D) has a maximum sustainable rate amax = k1²/4k2 at D = k1/2k2, and a(D) reaches zero at Dmax = k1/k2.
- If sea-level rise stays below amax, the marsh surface tracks mean high water indefinitely — the profile line stays near the waterline.
- Once SLR exceeds amax, inundation depth grows every year faster than accretion can close the gap — the profile sinks below the waterline and that column is reclassified as drowned mudflat.
- Columns near the creek (left) carry higher local sediment supply, so real marshes typically drown from the interior (right) outward — reproduced here as a receding profile, not a uniform one.
This threshold behaviour — a sediment-supply-dependent "tipping point" rate of sea-level rise beyond which marshes convert to open water — is the central prediction used in coastal-wetland vulnerability assessments worldwide.