Also in: Disaster Response and Recovery
Flood Hydrostatic and Hydrodynamic Load on a Wall
Floodwater against a wall pushes with a triangle of still-water pressure and, where it flows, with drag.
Example
You enter
- Still-water flood depth (ft) 4
- Wall or obstruction width (ft) 10
- Water fresh
- Velocity basis lower
- Entered velocity (ft/s) 0
- Obstruction type wall
You get
- Hydrostatic (lb/ft) 499.2
- Hydrostatic (lb) 4992
- Hydrostatic height (ft) 1.333
- Velocity lower (fps) 4
- Velocity upper (fps) 11.35
- Drag coefficient 1.25
- Hydrodynamic lower (lb) 776
- Hydrodynamic upper (lb) 6247
- Hydrodynamic height (ft) 2
Checked against published worked examples from: FEMA.
Details, formula, and sources
ScopeFloodwater against a wall pushes with a triangle of still-water pressure and, where it flows, with drag -- and at the fast end of FEMA's velocity range the drag can exceed the still-water load. The hydrostatic load is half the unit weight times the depth squared per foot of wall (62.4 pcf fresh, 64.0 salt), acting a third of the way up, which is why flood-resistant design requires openings in enclosure walls: letting water in equalizes the pressure. FEMA P-55 brackets the velocity between the depth per second and the shallow-water wave speed, sqrt(g ds), and the calculator computes both, selects the drag coefficient from FEMA's width-to-depth table or the pile values, and reports the hydrodynamic load at mid-depth. Fresh water 4 ft deep against a 10 ft wall is 499.2 lb/ft, 4,992 lb at 1.33 ft; drag is 776 lb at the 4 ft/s lower bound and 6,247 lb at the 11.35 ft/s upper bound, where it exceeds the still-water load. The velocity choice depends on the flood zone, the slope, and the distance from the source, and has to be justified rather than defaulted. A 20 ft wall against 6 ft of saltwater carries 23,040 lb of hydrostatic load alone -- why a dry-floodproofed wall is engineered, not improvised. Loads are unfactored. It does not include breaking waves, debris impact, buoyancy and uplift, saturated-soil pressure on a basement wall, scour, or the ASCE 7 load factors and combinations, and it does not design the wall or barrier. FEMA P-55, ASCE 7, the engineer of record, and the floodplain administrator govern.
Formula or tablehydrostatic f_sta = 1/2 gamma_w ds^2 per ft of width (62.4 pcf fresh, 64.0 salt), at ds/3; velocity bounds V = ds / 1 s and V = sqrt(g ds); hydrodynamic F_dyn = 1/2 Cd rho V^2 (width x ds) (rho 1.94 fresh, 1.99 salt slugs/cu ft), at ds/2; Cd from Table 8-2 by width/depth, or 2.0 square and 1.2 round piles.
Source and editionFEMA P-55, Coastal Construction Manual, Volume II, Chapter 8 -- Eq. 8.2, 8.3, 8.8 and Table 8-2; ASCE 7 Chapter 5 cited by number. The engineer of record and the floodplain administrator govern.
Free accessFEMA P-55 is a free federal publication; ASCE 7 is cited by chapter only and no ASCE text is reproduced.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namesdepth_ft, width_ft, water, velocity_basis, entered_velocity_fps, obstruction, hydrostatic_lb_per_ft, hydrostatic_lb, hydrostatic_height_ft, velocity_lower_fps, velocity_upper_fps, drag_coefficient, hydrodynamic_lower_lb, hydrodynamic_upper_lb, hydrodynamic_height_ft
- Water properties 62.4 pcf and 1.94 slugs/cu ft fresh; 64.0 pcf and 1.99 slugs/cu ft salt; g = 32.2 ft/s^2FEMA P-55 Chapter 8
- Velocity bounds lower V = ds / 1 s, upper V = sqrt(g ds)FEMA P-55 Eq. 8.2
- Drag coefficient 1.25 for width/depth up to 12 rising to 2.0 above 120; piles 2.0 square, 1.2 roundFEMA P-55 Table 8-2