Spillway Design Toolkit

Side Weir / Lateral Spillway module (De Marchi constant-specific-energy method). Light theme, clean labels, and non-overlapping SVG visuals.

Module: Side Weir (De Marchi)

Inputs

Spillway module

Dropdown ready for future modules

Channel geometry

Rectangular default, trapezoidal available
De Marchi method assumes constant specific energy along the side-weir reach (friction/slope neglected in the ideal theory). Treat results as preliminary unless validated/calibrated.

Upstream conditions & design mode

SI units

Discharge coefficient (C)

Custom • Literature presets • Calibration
Literature presets are only a starting point. Side-weir coefficients vary with geometry, approach flow, Froude number, crest details, sub/supercritical regime, aeration, etc. Prefer calibration when possible.
Tip: start with Rectangular + Subcritical + Typical C and test.

Results

Key outputs

Computed with De Marchi constant-energy method
Coefficient used
—
Specific energy (constant)
—
Weir length
—
Diverted discharge
—
Remaining discharge
—
Downstream depth (end of weir)
—
Froude number (upstream)
—
Froude number (downstream)
—
Checks: —

Visuals, Method & References

Visual schematic (non-overlapping labels)

Plan view + cross-section
Auto-updated from inputs/results Units: m, m³/s
PLAN VIEW CROSS-SECTION Main channel Flow → Side weir (crest line) Overflow out Side outlet L = — m B = — m Bed y₁ y₂ p p = — m Upstream water level (y₁) Downstream water level (y₂)
Label policy: All labels are placed outside or on dedicated guide lines to avoid overlap. Solid blue = y₁, dashed blue = y₂ (computed).
Method notes (what the tool is doing) Expand

This module implements the classic De Marchi side-weir concept: side outflow causes spatially varied flow with decreasing discharge along the main channel, and the analysis assumes constant specific energy along the weir reach (idealized: friction and slope neglected).

Core relationships used (SI units):

  • Lateral outflow per unit length (Poleni-type): dQ/dx = -(2/3)·C·√(2g)·(y − p)^(3/2)
  • Specific energy: E = y + V²/(2g), with V = Q/A(y)
  • Rectangular closed-form (De Marchi): length uses Φ(y) with L = (3B/(2C))·(Φ(y₂) − Φ(y₁))
  • Trapezoidal: uses a numeric integration consistent with the same constant-energy + side-outflow assumptions.

Important: coefficients for side weirs are highly case-dependent. Use calibration when possible and verify results against project criteria / physical modeling / CFD / 1D–2D hydraulic modeling as needed.

References (shown at the end, as requested) Expand/Collapse
  1. A. Y. Mohammed (2022). Review of Discharge Coefficients of Side Weirs. PDF
  2. M. Di Bacco & A. R. Scorzini (2019). Are We Correctly Using Discharge Coefficients for Side Weirs? Insights from a Numerical Investigation, Water (MDPI). Author PDF mirror · Publisher page
  3. De Marchi (1934). Foundational analytical side-weir theory introducing the constant-specific-energy assumption (original publication commonly cited in side-weir literature).
  4. USACE HEC‑RAS documentation: Lateral structures (lateral weirs, gated spillways, etc.), useful for hydraulic modeling context. HEC‑RAS User Manual section
Note: This page is a calculator/UI prototype. For final design, verify with your agency/owner standards, stability/structural checks, freeboard, debris & sediment considerations, approach flow conditions, and an appropriate model study if required.