Levees, also known as embankments, are earthen structures built along rivers and coastlines to protect surrounding areas from flooding. While levees provide a vital defense against floods, they are not impervious to failure. Levee breaches can occur due to overtopping, erosion, and internal instability. When a levee breaches, water flows uncontrollably into surrounding areas, causing flooding and potential damage to surrounding areas.
Importance of Levee Breach Modeling
Levee breach modeling is essential for flood risk assessment and mitigation planning. It involves simulating levee failure to predict the extent of flood inundation in downstream areas. Accurate modeling helps emergency planners and engineers evaluate potential impacts and develop effective response strategies.
Defining Levee Structure
In GeoHECRAS, levee breaching can be analyzed by modeling the levee as a lateral structure. When defining a levee, the area behind it should not be included in the main river cross section data. Instead, cross sections stop at the top of the levee. The lateral structure (levee) can be connected to a 2D flow area, a storage area, or another river reach. Refer to this article in our knowledge base to learn how to draw lateral structures.
The strategy for modeling the area behind the levee depends on how water behaves during overtopping or a piping breach.
- 2D Flow Area: Suitable when water follows multiple paths with varying water surface elevations.
- Storage Area: Suitable when water accumulates behind the levee like a reservoir.
- Separate River Reach: Suitable when water flows in a defined downstream direction before rejoining the main river.
Entering Levee Breach Data
Levee breach data can be entered in the Levee Breach panel of the Lateral Structure Data dialog box. Levee breach data defines the nature of a breach, including breach location, breach width, breach side slopes, breach height, breach progression (linear or non-linear), breach formation time, and failure mode (overtopping or piping). The shape and progression of a levee breach depend on user-defined inputs such as final bottom width, structure height, and breach formation time. Refer to this article in our knowledge base to learn how to enter levee breach data.
Levee Breach Types
In GeoHECRAS, the following types of levee breaches can be defined:
- Overtopping Failure
- Piping Failure
Overtopping Failure
Overtopping failure occurs when water exceeds the levee crest and erodes the soil, causing the levee to collapse. This can happen during a flood event when water levels rise above the levee.
Causes of Overtopping Failure
- High water levels
When the water rises above the levee, it overtops and erodes the soil.
- Wind waves
Wind waves increase the height of the water level at the levee, making it more likely that the levee will be overtopped.
- Debris accumulation
Floating debris, such as trees and logs, can accumulate against the levee and increase the height of the water level.
Preventing Overtopping Failure
- Constructing levees at sufficient elevations
- Protecting levee surfaces with vegetation or riprap
- Monitoring for signs of erosion or overtopping
Example: Overtopping Failure Mode
Consider a levee modeled as a lateral structure connected to a storage area to represent the area behind the levee. The user defines the levee by entering a series of station and elevation points that represent the top of the levee profile. This station and elevation data is then used as a weir profile for calculating the amount of water flowing over the levee. For example, the image below shows the levee modeled as a lateral structure. As the levee overtops and/or breaches, the storage area fills up until it reaches the same elevation as the water in the river.
The levee information is entered as station and elevation data in the Overflow Weir panel of the Lateral Structure dialog box. The station elevation data represents the top of the levee. The levee information is entered from the upstream to the downstream end of the levee. For example, the information entered for the overflow weir is shown below.
The following information must be entered in the Overflow Weir panel:
- Station and elevation data representing the top of the levee
- Headwater distance from the upstream end of the levee to the nearest upstream cross section
- Width of the levee structure
- Weir flow reference elevation
- Weir crest shape
- Weir coefficient (Cd)
Once the physical data are entered, the Levee Breach panel of the Lateral Structure dialog box is used to define breach parameters. For overtopping levee failure mode, consider a breach with the following parameters:
- Breach centerline station: 211.29 ft
- Final breach bottom width: 320.00 ft
- Final breach bottom elevation: 732.66 ft
- Formation time: 0.50 hours
- Breach trigger elevation: 739.29 ft
In the case of linear breach progression, the breach will start as a tiny trapezoid (or rectangle if side slopes are zero) at the top of the weir based on the center station. If the breach progression is non-linear, the horizontal growth will be adjusted as needed. Progression in the vertical direction will match the horizontal growth.
Piping Failure
Piping failure results from seepage erosion, forming subsurface channels that transport soil particles through porous media. Piping failure begins on the land-facing side as water pressure forces a slit to develop, eventually eroding backward under the levee and causing collapse.
Causes of Piping Failure
- High hydraulic gradients
High hydraulic gradients are the force of water pushing through the soil. When the hydraulic gradient is too high, it can erode the soil grains and create a hole in the levee.
- Fine-grained soils
Fine-grained soils, such as clays, are more susceptible to piping than coarse-grained soils. This is because fine-grained soils have more surface area, which makes them more vulnerable to erosion.
- Wetted perimeter
The wetted perimeter is the length of the levee that is in contact with water. The longer the wetted perimeter, the more vulnerable the levee is to piping.
- Levee seepage
Seepage is the movement of water through the soil. If there is too much seepage through a levee, it can increase the risk of piping.
Preventing Piping Failure
- Designing levees with a low hydraulic gradient
- Using coarse-grained soils to reduce piping susceptibility
- Minimizing the wetted perimeter
- Managing seepage through drainage pipes or geosynthetics
Example: Piping Failure Mode
For piping levee failure mode, users input breach progression data similar to overtopping levee failure mode with additional parameters for internal erosion. Consider a breach with the following parameters:
- Breach centerline station: 211.29 ft
- Final breach bottom width: 320.00 ft
- Final breach bottom elevation: 732.66 ft
- Formation time: 0.50 hours
- Piping coefficient: 0.5
- Initial piping elevation: 736.29 ft
- Breach trigger elevation: 739.29 ft
In the case of linear breach progression, the breach will start as a tiny square (or rectangle) based on the center station and initial piping elevation. If the breach progression is non-linear, the piping breach would be a rectangle that grows vertically.
Estimating Levee Breach Parameters
Estimating the location, dimensions, and development time of a levee breach is crucial to assess the potential risks associated with levee failure. While breach parameters can vary for each levee, engineers typically use standard estimation methods to:
- Predict peak flow from a breach
- Assess potential warning times for downstream communities
- Refine breach estimates using geotechnical analyses and site-specific data
In any levee safety study, it is important to consider a range of parameter estimates for the breach size and development time for each failure event. After that, sensitivity analyses of the breach parameters should be conducted to determine their effect on the outflow hydrograph, downstream water levels, and evacuation planning.
Output for Levee Breach Analysis
GeoHECRAS provides various plots and tables to evaluate breach analysis results, such as profile plots, lateral structure hydrographs, and storage area hydrographs. These plots can be animated on a time step by time step basis to visualize the flood wave propagation.
An example of a profile plot of a levee breach is shown below. Refer to this article in our knowledge base to learn how to view water surface profile plots.
The user can view hydrographs at any location where hydrograph output is required. These hydrographs represent the flow leaving the levee and subsequent flow at downstream locations as the flood wave moves through the river system. The example image below shows a series of hydrographs generated for the modeled levee breaches.
The user can also plot the stage and flow hydrographs for the storage area. This allows the user to easily see the amount of flow coming into and out of the storage area, and the change in water surface elevation.
Refer to this article in our knowledge base to learn how to view the stage and flow hydrographs.
In addition to the graphical plot, users can access tabular results for lateral structures, selecting a specific timeline for detailed analysis by selecting a specific profile. The profiles are labeled by the date and time they occurred in the model simulation. Refer to this link to learn how to view detailed hydraulic information on the lateral structure.