Dam Freeboard Calculator

Required freeboard is computed as:

F_req = R_u + A_settle + A_safety + A_other

Required crest elevation is:

E_crest,req = E_DFL + F_req

Provided freeboard (when crest is entered) is:

F_prov = E_crest − E_DFL

Wave estimation (optional) is a screening approach:

• Estimate significant wave height H_s from wind speed and fetch.
• Approximate run-up R_u using slope and roughness, with a cap.

1. Enter the Design Flood Level elevation for your dam.
2. Enter crest elevation if you want a compliance check.
3. Add settlement, safety, and other allowances as required.
4. Choose wave mode: enter run-up or estimate from wind.
5. Press Calculate to view freeboard and required crest.
6. Download results as CSV or PDF for reporting.

Freeboard as a safety reserve

Dam freeboard is the vertical buffer between the design flood water surface and the crest. It protects the embankment from overtopping, erosion, and rapid loss of stability. For earthfill and rockfill dams, adequate freeboard also helps limit saturation of the downstream face during wind setup, reducing the likelihood of sloughing and seepage related deformation during long storms and emergency gate operations. Designers typically separate hydrologic loading from additional effects, so the freeboard allowance can be reviewed and updated without changing the flood study.

Key inputs that drive freeboard

The calculator combines wave run-up with settlement, safety, and other allowances. Wave run-up represents short-term water surface excursions caused by wind, fetch, and slope. Settlement covers long-term consolidation and construction tolerances that reduce crest elevation. Safety allowance addresses uncertainty in inflow estimates, routing, and operations. Other allowance can represent seiche, ice shove, debris, or localized setup near spillways.

Interpreting wave run-up estimates

When you select estimation mode, the tool produces a screening significant wave height and converts it to a run-up value using slope and a roughness factor. Use this feature for early options and sensitivity checks. For final design, replace the run-up input with values from your governing manual, physical modeling, or site-specific numerical analysis.

Using margins to verify crest elevation

If you enter a proposed crest elevation, the output includes provided freeboard and margin. A positive margin means the crest meets the required buffer for the entered assumptions. A negative margin indicates the crest may be vulnerable during the design event, and you should increase crest level, reduce uncertainty, or revise wave protection details.

Documentation and design review notes

Record the datum used for elevations, the flood event definition, and the basis for each allowance. Keep wave assumptions consistent with reservoir operating level and wind direction. Export the results to attach in design memos, and update the calculation whenever slope protection, operating rules, or freeboard criteria change.

FAQs

1. What does the calculator report as required crest elevation?

It adds the required freeboard to the Design Flood Level elevation. The required freeboard equals wave run-up plus settlement, safety, and other allowances you enter.

2. Should I use estimated wave run-up for final design?

Use the estimate for early screening and sensitivity checks. For final design, enter run-up from your governing standards, detailed analysis, or physical modeling, and document the basis.

3. Why is settlement included as an allowance?

Embankments can settle due to consolidation, construction tolerances, and long-term deformation. Adding a settlement allowance helps keep the effective crest above the design flood level for the project life.

4. What does a negative margin mean?

A negative margin means the entered crest elevation provides less freeboard than required for the selected assumptions. Consider raising the crest, improving wave protection, or revising allowances using project criteria.

5. Can I use different elevation datums?

Yes, but all elevations must use the same datum and reference. Mixing survey datums will produce misleading margins and required crest elevations. Verify inputs against the latest as-built and design drawings.

6. How do I include site-specific effects like seiche or ice?

Enter them under Other allowance as a single combined value, or split them into components externally and sum them. Keep notes on how each component was derived for design review.