Breach (Water Overlay)

A Breach area can be used to simulate a breach in a levee or dam.

Overview

A Breach area can be defined via the BREACH_HEIGHT attribute. It can have an input area or external area with a water body outside of the hydrological model. Water can flow through the breach form the input into the Breach area just behind the levee and vise versa.

The breach is simulated as a generic weir with breach characteristics that models the energy loss, the weir input/output is dynamically mapped onto the 2D grid cells. Using the Verheij–van der Knaap method the breach can also grow over time from phase one deepening to phase two widening. Below a simple schematic of the different breach elements.

Side View

Top View

Features

The breach can grow over time, based on its initial width, critical speed and inner and outer water level, for more details see Verheij–van der Knaap growth formula.

With larger breaches the water level inside the breach area can become unstable. Therefor the inner water level (used for breach growth) is measured at a distance defined in BREACH_MEASUREMENT_DISTANCE_M behind the breach area when an BREACH_ANGLE is provided.

Instead of using an input area you can also define a fictitious external area, this can represent a larger water body outside the project area, e.g. the entire sea. The external area also has it's own Surface level, Water level and Area.

If a BREACH_WIDTH attribute is defined, the breach's polygon is intersected with a circle emanating from the center-point of the polygon. It is only in that intersection, called "active", that water will flow in from the input area. The radius of the circle defining the intersection will expand as the breach grows over time.

Water flowing through the breach will flow in the direction defined by the BREACH_ANGLE attribute, regardless of whether the water flowed onto the breach from elsewhere in the water model, or from the simulated external water body. When no BREACH_ANGLE is defined, water can flow in any direction. Adding this attribute with also convert the breach advection speed (m/s) into the 2D cell of the breach area. It is recommended to always use this attribute for optimal flow.

If no BREACH_SPEED attribute is defined, the active breach will never grow.

If no width is defined, the width is assumed to be very large, using the full Breach area polygon.

The BREACH_HEIGHT attribute can also have multiple values over time, this allows you this simulate the breach vertical growth.

Notes

The elevation model covered by the entire breach area is lowered to at least the minimum height defined by its BREACH_HEIGHT attribute. This is to prevent the terrain height from interfering with the flow.

When using breach growth the inner water level is computed from the average value in the active breach area. For optimal flow simulation it is therefor recommended to place the breach area just behind the levee on a flat space.

Water on a breach area is automatically spread across the entire active breach area equally.

The inner/outer water levels and breach width are visualized per timeframe on the breach object in the 3D map, these can be very useful to see how the breach is progressing.

The Base Types Result child overlay can be used to see which part of entire the breach area is considered "active". Breach growth can be seen when checking multiple timeframes.

You can also optionally change the default breach weir coefficient to model energy loss by adding a WEIR_COEFFICIENT attribute to the Breach area.

When the water level of in the input area drops over time the flow can also reverse, flowing back into the e.g. the river/sea.

Attributes

Key	Unit	Range	Description	Default value
BREACH_HEIGHT	m + datum		The new terrain height at the location of the breach.	n/a
BREACH_WIDTH	m		The initial width of the breach.	10000
BREACH_ANGLE	geo angle (0-360°)	-1.7976931348623157E308 to 1.7976931348623157E308	The angle at which water flows out through the breach.	n/a
BREACH_SPEED	m/s		The critical speed factor in the breach width grow-formula (e.g. 0.2 for sand and 0.5 for clay).	n/a
BREACH_INPUT_AREA	ID of an Area		The Area item representing the influx area for the breach.	none
EXTERNAL_SURFACE_LEVEL	m + datum		The height of the bottom of the external water body behind the breach. The lowest level the external water level can lower to.	0
EXTERNAL_WATER_LEVEL	m + datum		The water level of the external water body behind the breach. The initial level the water level is set to.	0
EXTERNAL_AREA	m²		The surface area of the external water body behind the breach. The larger the external water body, the more water can flow from it (or into it), and the smaller the effect of the inflow or outflow of water on the external water level. When provided with a second value, this key will also define a bottom area for the external water body, transforming its cross-section from a rectangle to a trapezoid.	n/a