FEMA Flood Debris Impact Load

ScopeA flood carries debris -- pieces of other buildings, poles, tanks -- and FEMA P-55's simplified method gives a single impact force from the object's weight, the water's speed, and three coefficients: Fi = W x V x C_D x C_B x C_Str. It is an impulse turned into a force, and the structure coefficient is the counterintuitive part: a stiff reinforced concrete wall takes four times the force of a flexible timber pile from the same object, because it stops it faster. The depth coefficient is 1.0 in a floodway, Zone V, or Zone A deeper than 5 ft, and otherwise follows FEMA's table rows, which lie on 0.25 (ds - 1); the blockage coefficient drops from 1.0 to 0 as upstream trees and buildings screen the flow. The velocity defaults to FEMA's half the shallow-water wave speed and accepts an entered value, flagged if it falls outside the Eq. 8.2 bounds. A reinforced concrete foundation wall in Zone A with 4 ft of still water, no screening, and FEMA's recommended 1,000 lb debris takes 5.67 ft/s x 0.75 x 1.0 x 0.8 = 3,405 lb at the still-water level; a timber-pile house takes 851 lb from the same object. A pile house in Zone V with 8 ft of water takes 1,605 lb. The force is concentrated at one point, unlike the spread hydrostatic load. It does not address logs, vessels, or containers, multiple impacts, the strength of the element struck, tsunami loads, or load factors and combinations, and local guidance on debris weight supersedes the default. FEMA P-55, ASCE 7, and the engineer of record govern.

Formula or tableFi = W x V x C_D x C_B x C_Str at the still-water level; V default = 1/2 sqrt(g ds); C_D = 1.0 in a floodway or Zone V, else 0.25 (ds - 1) clamped to 0-1 (Table 8-3 rows); C_B = 1.0, 0.6, 0.2, 0.0 by upstream screening; C_Str = 0.2, 0.4, 0.8 s/ft by structure.

Source and editionFEMA P-55, Coastal Construction Manual, Volume II, Chapter 8 -- Eq. 8.9, Tables 8-3 and 8-4, and the building structure coefficients; ASCE 7 Chapter C5 cited by number. The engineer of record governs.

Free accessFEMA P-55 is a free federal publication; its coefficients are transcribed; ASCE 7 is cited by chapter only.

What governsEstimate. AHJ and licensed structural engineer govern.

API field namesdebris_weight_lb, zone, depth_ft, velocity_basis, entered_velocity_fps, screening, structure, velocity_used_fps, depth_coefficient, blockage_coefficient, structure_coefficient_s_per_ft, impact_force_lb, impact_elevation_ft

• Debris weight 1,000 lb recommended where local debris is unknownFEMA P-55 Chapter 8
• Depth coefficient Table 8-3 rows (0 at 1 ft to 1.0 at 5 ft) on the line 0.25 (ds - 1)FEMA P-55 Table 8-3
• Structure coefficient 0.2 timber pile or masonry column, 0.4 concrete pile or moment frame, 0.8 reinforced concrete wallFEMA P-55 Chapter 8