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  2. Activity : N-OF06 Overflow Test (RTM)
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Activity Name:

N-OF06 Overflow Test (RTM)

Activity Numbre:

70

Activity type

Strength test

Involved Workpackage

Flood defences
Emergency response
Knowledge infrastructure

Objective

The N-OF06 overflow test is part of a series of tests that involve Reinforced Turf Mats (RTMs).

Activity Brief descriptor

The RTMs are a form of protection that has been applied to a section of the levee after the top vegetation layer has been removed. After this removal, the underlying clay cover layer was exposed and subsequently covered with different solutions.

Linked to theme(s):

Breaching
Animal- & vegetation-induced anomalies
Erosion processes
Temporary levee repairs
Levee survey & monitoring technique
Prolonged collaboration

Activity Time

Start Date:

Nov. 22, 2021, 8 a.m.

End Date:

Nov. 22, 2021, 5 p.m.

Active time length:

Days:

None

Hours:

2.0

Other:

Total overflow time: 1 hour + 5 minutes.

Location

Levee stretch:

VI

Coordinates/ Latitude (WGS):

51.34852

Coordinates/ Longitude (WGS):

4.22899

Coordinates/ Altitude (mTAW):

12.0

Descriptor:

Landward side of levee.

Other references:

Lambert '72 coordinates: X=140262.0 m; Y=226530.5 m.

Data

Data: 1

Data type: Photo imagery
Description: 2 portals with in total 4 cameras are used to take surface photographs of the levee slope at different times during and in between overflow activity. The surface images in between overflow blocks show damage and vegetation evolution.
Date availability: None
Find at: https://stowanl-my.sharepoint.com/:f:/g/personal/llhpp_stowa_nl/Eh9dKC1RLURJgVRH7RpZNP4BuF5jA3kBXIGHdCQj9WpX1Q?e=Oqeu0H
Media avaliable:
Find media:

Relationships with other activities

Detaled information about the activity

Pre-conditions

The RTMs are a form of protection that has been applied to a section of the levee after the top vegetation layer has been removed. After this removal, the underlying clay cover layer was exposed and subsequently covered with different solutions.

Activity Descriptor

Total run time 1:05 with 10 m³/min

Equipments

Equipment: 1

Overflow generator.
The setup is described in Vercruysse et al. (2022): "Design and application of an overflow generator." (Ref WL2022R20_047_1)

Monitoring & Sensors

- Overhead cameras (4) - Acoustic water height sensors - Electromagnetic current velocity sensors - Discharge sensor (EM or acoustic)
Description of the hydraulic conditions of the overflow. The setup is described in Vercruysse et al. (2022): "Design and application of an overflow generator." (Ref WL2022R20_047_1)

this activity does not have sensors

Limits

During Activity

From the onset of the overflow test, it was observed that the water was flowing through and under the RTM solution. Discharge water appearing at the toe was heavily sediment laden, which was an indication of strong erosion below the RTM. During the experiment, it was observed that water was flowing out along two lines from the top to the bottom of the crest all the way down. These lines were about 40 cm wide and were separated by 2.5 m. It was considered and confirmed afterwards that these lines were actually corresponding to the tracks of the crane that was used during the removal of the grass cover and installation of the RTM solution. In this process, gullies were formed that were filled with (less compacted) soil afterwards. It is thought that this soil was quickly eroded and formed very deep (0.5 m) erosion channels. Simultaneously, the vegetation that is growing through the RTM solution was being torn away so that the geogrid itself is exposed on many places.

Post-activity conditions

Strongly eroded cover layer/

Immediate Results

The emergency repairs do not prevent from erosion occurring during a new phase or event of overflow.

Results /Conclusion

Given the artifact created by the crane track, it is considered unreasonable to make any conclusions regarding the RTM solution for slope stabilization and therefore, the test should be considered as a failure. Factual data report: https://stowanl-my.sharepoint.com/:w:/g/personal/llhpp_stowa_nl/EX5yYnuJ-EBBjoKAxAwg8LkBef8ra9V_-GHjzbkz0Was8g?e=pyaUSq Annexes: https://stowanl-my.sharepoint.com/:w:/g/personal/llhpp_stowa_nl/EWIzoxIhxopCryc6744YgHYBBWr_0fhoGpJ_1pSy0_tM2w?e=pY8Hc6

Infrastructure Knowledge

An integration report will be added later (February 2023) as knowledge document.
The integration report discusses insights gained from both the steady overflow and wave overtopping tests.
under development

Other key information