Paid tier only. Calling this without an authenticated CivilQuants account returns TIER_INSUFFICIENT — sign up at https://civilquants.com/pricing or use the free-tier alternative compute_attenuation_tank. Vegetated, geotextile-reinforced or rip-rap-lined linear drainage swale per CIRIA C753. Trapezoidal prismatic channel with three lining strategies covering the UK design palette from low-velocity amenity grass channels (1V:3H, 1-3% gradient) to high-velocity rip-rap-lined stretches. Optional check-dams (stone or concrete) for steeper sections. Renders cleanly across all four standards using existing earthworks / geosynthetics / concrete handlers — no PC items, all contractor-full supply route. Example params: bed_width=0.5 m (0.2–3), left_side_slope_h_per_v=3 (1.5–6), right_side_slope_h_per_v=3 (1.5–6). Example call: {"params": {"bed_width": 0.5, "left_side_slope_h_per_v": 3, "right_side_slope_h_per_v": 3}, "standard": "MMHW"}. Omitted parameters use sensible engineering defaults. Pass deliverables=["xlsx","dxf","pdf"] (any subset) to also receive one-shot download URLs in the same call: Excel BoQ (both tiers, watermarked free) plus the dimensioned DXF (CAD) and PDF drawing sheets (paid tier).
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