Outflow Structures

Outlets and other discharge structures can only be modeled individually when using the Outflow Structures routing method. The table below lists available outflow structure types and subtypes, which include outlets, spillways, gates, pumps, and dam tops, breaks, and seepage. Evaporation is also a specific type of release that requires the outflow structures routing method.

Table 2.Outflow Structures available in HEC-HMS

Structure Type	Description
Outlet	Lower elevation pipes, etc.
Culvert	Allows for partially full and submerged, pressure flow
Orifice	Assumes a large, submerged outlet
Spillway - ungated	Structures on dam top, typically uncontrolled
Broad-Crested	Uncontrolled flow using weir assumptions
Ogee	Uncontrolled outlet with an approach channel, designed for a specific head. Weir flow assumptions, adjusts based on upstream head
Specified	Allows for an elevation-discharge relationship
Spillway - gated	Allows for controlled spillways
Sluice	Gate moves on a vertical plane
Radial	Gate rotates on a horizontal axis
Dam Top	Allows for uncontrolled overtopping
Level	Constant elevation dam top
Non-Level	Represents several different segments of a cross section. Can be used for separate sections of dam, separated by other outlets.
Pump	All pumps use the head-discharge relationship to calculate flow
Dam Break	Dam failures. Allows for defined trigger time and development
Overtop	Dam is overtopped and experiences erosion
Piping	Piping occurs through dam and eventually cuts away the dam top
Dam Seepage	Tabular
Additional Release	User specified tabular release
Evaporation	Modeled as its own outflow structure based on user-defined relationship