Tools › User guides › Scour Depth Calculator
The Scour Depth Calculator works out the design discharge (RDSO RBF-16 for catchments under 25 km², or area × velocity), the foundation design discharge Qf, the normal and maximum scour depth by the IRS Bridge Sub-structure & Foundation Code, the scour level and a foundation check, with a PDF calculation sheet. The page links the full RBF-16 report and the IRS code at the right clauses.
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| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| HFL (m) | Highest flood level for Q (afflux included). | ||
| Water level at Qf (m) — optional | Blank = HFL. | Scour depth D is measured from the Qf level (Cl. 2.4 / 4.6.3). | |
| Bed level (m) | Lowest bed level at the site. | ||
| Catchment area A (km²) | From the topo sheet. | Under 25 km²: RBF-16. Also sets the Qf increase (Cl. 4.4). | |
| Design discharge Q from | Auto, RBF-16, Area × velocity, or enter Q directly. | Auto | |
| C/s area at HFL, velocity V | For Q = A × V. | ||
| Design discharge Q (m³/s) | When entered directly. |
| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| Hydro-meteorological subzone | From RBF-16 Fig. 2 (linked on the page). | e.g. 1(g) Lower Gangetic Plains | Sets the Fig. 4 zone and the 1-h rainfall method. |
| Length of longest stream L (km), Height H (m) | From the topo sheet. | tc by Bhatnagar: (L³/H)^0.345. | |
| Time of concentration tc — optional | Blank = formula. | ||
| Catchment description | Soil / cover class for the runoff coefficient C. | Red soil / alluvium … | |
| 50-yr 24-h rainfall R50(24) (mm) | From the RBF-16 rainfall map of your subzone. | ||
| 50-yr 1-h rainfall from | Map ratio, map value, or 0.37 × R50(24). | Ratio | |
| Zone for Fig. 4, ratios, C override | Optional overrides. | Blank | Typed values replace the curve readings. |
| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| Structure type | RCC box, Pipe, or Bridge with piers. | ||
| Number of cells / vents, clear span, clear height | For a box. | ||
| Internal diameter of each pipe | For pipes; mean width = πd/4. | ||
| Intermediate wall thickness | Effective width = cells × span − (cells − 1) × wall (Cl. 4.5.6). | 0.30 m | |
| Total waterway, number of piers, pier width | For a bridge: effective width = total − 2 × Σb. |
| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| Bed type | Alluvial / sandy, Clayey, Gravel / boulder. | ||
| Bed material / silt factor | Pick the sand size (f = 1.76√m) or enter m or a custom f. | Medium sand m = 0.3 mm → f = 0.96 | |
| Normal scour formula | Auto (compares waterway with Lacey regime width), or force constricted (Cl. 4.6.4) or Lacey (Cl. 4.6.3). | Auto | |
| Lacey coefficient C | Regime width W = 1.811 C √Q. | 2.67 | |
| Observed maximum scour, remark | From field observation, if any. | Cl. 4.6.7: use actual observations wherever possible. |
| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| River condition | Straight reach 1.25D … severe swirls 2.75D (Cl. 4.6.6). | 1.5D (moderate bend) | |
| Measure scour depth from | HFL / Qf level (code) or bed level (more conservative). | HFL | |
| Foundation type | Box / pipe floor, open foundation (≥ 1.75 m below scour, Cl. 6.1 iv), well / pile (1.33 × max scour, Cl. 6.9.1). | ||
| Foundation check by | Bottom RL, depth below bed, or no check. | RL | |
| Foundation / box bottom level, depth below bed | Your proposed foundation. |
Press Calculate: Q, Qf, Lacey width, waterway, formula used, normal and maximum scour depth, scour level and the foundation verdict appear, with Download PDF calculation.
For catchments smaller than 25 km², as IRS Cl. 4.3.4 requires; above that, use area × velocity or a hydrological study.
If the waterway is less than the Lacey regime width, the constricted formula D = 1.338 (qf²/f)^⅓ is used; otherwise Lacey D = 0.473 (Qf/f)^⅓. Auto chooses for you.