Flood loads
Nominal flood loads Fa on a wall or a pile by ASCE 7-22 Chapter 5: hydrostatic, hydrodynamic, breaking waves, debris impact, local scour, flotation and breakaway walls. Copy the loads into the design of the element: nothing links them. The type reads Calculated, though flotation and the breakaway limit give advisories.
Inputs of the flood loads #
- Flood zone: V-Zone, Coastal A-Zone, noncoastal A-Zone. It sets whether breaking waves apply, the depth coefficient and the factors on Fa.
- Risk Category: sets Cp (Table 5.4-1) and CI (Table C5.4-1).
- Water: saltwater 64.0 pcf, freshwater 62.4 pcf.
- G: the eroded ground, after storm erosion and before local scour (5.3.2).
- DFE: BFE plus any freeboard your community sets. The hydrostatic and hydrodynamic loads go to here.
- Stillwater depth: 0.65 (BFE − G) by Eq. 5.4-3, or a stillwater elevation from the Flood Insurance Study. FEMA P-55 uses the study's elevation.
- Element: a wall, per foot, or a square or round pile of width D. Eq. 5.4-4 takes 1.4 D for a square. Piles in the row only multiplies the breaking wave on a row.
- Surface exposed to free water: a foot more of depth (5.4.2). On by default on a wall; a pile takes no extra foot here.
- Flow velocity: upper bound √(g ds), lower bound ds / 1 s, or entered. C5.4.3 gives the range as interim guidance for coastal areas, and tells you to use a better source if you have one.
- Shape factor a: not less than 1.25 (Eq. 5.4-1). Cd: 0 uses 2.0 square, 1.2 round, 1.25 wall; FEMA P-55 Table 8-2 runs from 1.25 to 2.0.
- Waves: dry behind the wall or water on both sides; surface angle from horizontal; wave approach angle.
- Debris: weight W (1,000 lb prefilled, a reasonable average in C5.4.5), by Eq. C5.4-3 with the natural period, duration 0.03 s and CO 0.8, or by FEMA P-55 Eq. 8.9 with Cstr. Upstream screening sets CB.
- Flotation: the displaced volume and dead weight. 0 leaves it out.
- Breakaway wall: wind and earthquake on it, and its intended collapse pressure.
The design method sets the factors on Fa only.
Two depths are used on purpose. Hydrostatic pressure, and the surcharge and drag on a wall, go to the DFE (ASCE 7-22 5.4.2, 5.4.3). Drag on a pile uses D × ds, the stillwater depth, as FEMA P-55 Eq. 8.8 and its examples do, while ASCE 7-22 5.4.3 leaves moving water to fluid mechanics. Breaking waves use ds (5.4.4).
Checks performed #
| Load | Basis |
|---|---|
| ds, Hb | Eq. 5.4-3, 5.4-2 |
| Velocity | Eq. C5.4-1, C5.4-2, or entered |
| Hydrostatic, on a wall | 5.4.2 |
| Hydrodynamic, wall up to 10 ft/s | Eq. 5.4-1, uniform over the projected area |
| Hydrodynamic, wall over 10 ft/s and any pile | ½ Cd ρ V² A, FEMA P-55 Eq. 8.8 |
| Breaking wave, pile | Eq. 5.4-4, at the stillwater level |
| Breaking wave, wall | Eq. 5.4-5, -6 or -7, then Eq. 5.4-8, -9. Holds the hydrostatic part: do not add 5.4.2 to it |
| Debris impact | 5.4.5, Eq. C5.4-3 or FEMA P-55 Eq. 8.9 |
| Local scour, pile | FEMA P-55 Eq. 8.10, 8.11 |
| Flotation (advisory) | 5.3.1, combinations 5b or 7b: 0.9 D or 0.6 D against the factored force |
| Breakaway wall | 5.3.3: largest of wind, earthquake and 10 psf; collapse at no more than 20 psf (advisory) |
Limitations #
- Loads only: the element, its embedment and anchorage are designed elsewhere.
- Wave slam, nonbreaking wave analysis, wave run-up and uplift under a floor (5.4.4 names them) are out. Tsunami (Chapter 6) is out. The breaking wave on a wall is depth limited, so it is the code's method and no wave model.
- The depth is one value: no erosion analysis, no dune loss. Enter G after that study.
- Local scour at a wall comes from a coastal or geotechnical engineer: FEMA P-55's method for walls is not built.
- Scour is reported and not carried into the depths: FEMA P-55's example does the same.
Verification examples #
Verification status: 24 values compared in 11 examples (6 published, 5 hand calculations from the cited equations): 24 within tolerance, 0 recorded deviations. Code basis: the provisions it uses were read in the target edition.
| Example | Source | Values |
|---|---|---|
| 8 in. square pile, oceanfront | FEMA P-55 Vol. II Example 8.4, pp. 8-40 to 8-43 | ds 4.6, V 4.6 and 12.2, Hb 3.6 ft; drag 909, wave 868 lb, row of 7 6,076 lb, debris 2,440 lb; scour 1.76, 5.28 ft |
| 10 in. round pile | Example 8.3, pp. 8-30, 8-31 | Drag 509, 2,037 lb; wave 1,816 lb |
FEMA P-55 is to ASCE 7-10; the equations used are the same in 7-22. Its row total reads "(625 lb)(7)", a leftover: 868 × 7 is the 6,076 it prints. Its scour diagonal uses a 7.5 in. dressed pile though the 8 in. goes into the load equations, so the scour values run at 7.5 in.
No published example was found for a wall. Eq. 5.4-5 to -9, 5.4.2, Eq. 5.4-1 and Eq. C5.4-3 are hand calculations from the code equations, which only guards against a typing error. The tables are read from the code. ASCE 7-22 is the only source for Table C5.4-2's depth coefficient at 2 ft and 3 ft, used here as a straight line, as Figure C5.4-1 shows.