Size swales accurately with configurable shapes and hydraulic checks. Get discharge, velocity, and geometry instantly, then export results. Use outputs for quick field documentation.
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| Case | Units | Shape | b | d | zL | zR | S | n | Expected Notes |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Metric | Trapezoid | 0.60 m | 0.45 m | 3.0 | 3.0 | 0.010 | 0.035 | Moderate capacity, stable velocity. |
| 2 | Metric | Triangle | 0.00 m | 0.35 m | 2.5 | 2.5 | 0.008 | 0.040 | Narrow section, watch depth increase. |
| 3 | Imperial | Rectangle | 2.00 ft | 1.00 ft | 0.0 | 0.0 | 0.005 | 0.030 | Higher hydraulic radius, efficient conveyance. |
Use the table to sanity-check outputs before design decisions.
Top width at water depth
T = b + (zL + zR)·d
Cross-sectional area
A = d · [ b + ( (zL + zR)·d ) / 2 ]
Wetted perimeter
P = b + d·√(1+zL²) + d·√(1+zR²)
Hydraulic radius
R = A / P
Manning discharge capacity
Q = (K/n) · A · R^(2/3) · S^(1/2), where K = 1.0 (metric) or 1.49 (imperial)
Design wisely, verify assumptions, document results for approval always.
Swales are shallow, engineered channels used to convey stormwater while reducing peak runoff, filtering sediment, and improving site resilience. A good cross-section must satisfy hydraulic capacity, fit within the grading envelope, and remain safe to inspect and maintain. This calculator supports those decisions by computing geometric properties (top width, flow area, wetted perimeter, hydraulic radius) and a uniform-flow capacity estimate using Manning’s equation.
Begin with the section type that matches constructability. Trapezoidal swales are common because the flat bottom is easy to shape and supports low flows without concentrating erosion at a single point. Triangular sections can be practical in narrow corridors, but depth rises rapidly as flow increases. Rectangular channels can be efficient hydraulically, yet they often require lining and careful safety treatment because side slopes are vertical.
Geometry drives performance. For a given depth, wider bottoms increase area, while flatter side slopes increase top width and right-of-way needs. Wetted perimeter increases with side slopes, which can reduce hydraulic radius and offset some gains from area. The calculator reports these values so you can see how changing b, d, zL, and zR affects the balance between capacity and footprint.
The hydraulic check uses Manning flow capacity, which is appropriate for steady, uniform flow in a prismatic channel. Capacity increases as roughness decreases, slope increases, and hydraulic radius increases. Roughness is highly variable for vegetated swales; select n values that reflect expected vegetation height, density, and seasonal conditions. Use a conservative approach when long-term maintenance or mowing frequency is uncertain.
Example using the built-in table (Case 1, metric): b = 0.60 m, d = 0.45 m, zL = zR = 3, S = 0.010, n = 0.035. The computed top width is about 3.30 m and the flow area is about 0.878 m². The estimated discharge capacity is approximately 1.01 m³/s, producing an average velocity near 1.15 m/s. Adding 0.15 m freeboard gives a check depth of 0.60 m, a top width near 4.20 m, and an area about 1.44 m² for overtopping review.
Use results as a screening tool, then confirm details that can change water depth in practice: inlet controls, check dams, transitions, and downstream tailwater. If velocity is too high for the lining, iterate by flattening slopes, widening the bottom, reducing slope with grade control, or selecting a more resistant lining. Export the CSV or PDF to document assumptions, share with reviewers, and maintain a clear calculation record.
1) What does the side slope z mean?
z is the horizontal-to-vertical ratio (H:V) of each side. For example, z = 3 means the bank moves 3 units horizontally for every 1 unit vertically.
2) When should I use the custom asymmetric option?
Use it when one bank must be flatter or steeper due to space, utilities, or tie-in grades. The calculator accepts different zL and zR and reports geometry accordingly.
3) Is the discharge value a design flow or a maximum capacity?
It is a capacity estimate for the entered depth, slope, and roughness. Compare it to your required design flow and ensure freeboard and erosion limits are satisfied.
4) How do I pick Manning roughness n?
Select n based on lining and vegetation conditions. Short grass, dense grass, riprap, or concrete all differ. Use project standards or references, and apply a conservative value if conditions are uncertain.
5) Why does velocity matter?
High velocity increases erosion risk and may require flatter slopes, wider sections, check structures, or lining. Low velocity can encourage sedimentation; consider maintenance access and sediment forebays where appropriate.
6) Does this account for backwater or inlet controls?
No. It assumes uniform flow controlled by bed slope and roughness. If downstream controls or structures influence depth, perform a profile check using appropriate hydraulic methods.
7) What should I include in my report?
Include inputs, assumptions, computed geometry, discharge, velocity, and freeboard check. Also note lining/vegetation, construction tolerances, and any downstream constraints or transition details.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.