Activity_create

Activity Name:

N-OF07 Overflow Test (RTM)

Activity Numbre:

Activity type

Strength test

Involved Workpackage

Flood defences

Emergency response

Knowledge infrastructure

Objective

Perform a test on a levee stretch from which the top vegetation is removed, and a repair is performed using a geogrid "reinforced turf mat' solution (in this case: Tensar grid)

Activity Brief descriptor

The N-OF07 RTM overflow test has been executed on the landward side of a Dutch levee in the Hedwige Prosper Polder. The entire section has been stripped from its original vegetation and soil layer (approx. 10 to 15 cm of soil was removed). On the entire section, the RTM solution has been installed. The setup for the execution of the test consists of a 2 m wide section that is boarded by wooden plates but the boarding is limited to the top of the slope. Further down, the water is left free to search its way down the slope, over or though the geogrid. No sensors have been place, other than 1 camera frame. The reason for this limited setup is that the goal is to observe the performance of the RTM rather than assessing damage evolution or hydraulic behaviour.

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. 23, 2021, 12:40 p.m.

End Date:

Nov. 23, 2021, 1:10 p.m.

Active time length:

Days:

Hours:

1.0

Other:

1 block of overflow with 30 minutes duration at a discharge of 11 m³/min.

Location

Levee stretch:

Coordinates/ Latitude (WGS):

51.34832

Coordinates/ Longitude (WGS):

4.22918

Coordinates/ Altitude (mTAW):

12.0

Descriptor:

Landward slope of levee.

Other references:

Lambert '72 coordinates: X=140275.0 m; Y=226513.9 m.

Data

Data: 1

Data type: Photo imagery

Description: 1 portal with in total 2 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: June 15, 2022

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Media avaliable:

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Relationships with other activities

Activities that are linked to 71/ N-OF07 Overflow Test (RTM) :

No activities
DB_id: 71/ N-OF07 Overflow Test (RTM)
DB_id: 71/ N-OF07 Overflow Test (RTM) linked to the following activities:
DB_id: 88/ P2C Overflow tests

Detaled information about the activity

Pre-conditions

Levee slope with top vegetation removed and Tensar Geogrid RTM solution installed.

Activity Descriptor

The execution of the overflow test was performed on 23/11/2021 between 12:40 and 13:10, and 1 block with a duration of 30 minutes. A discharge of 11 m³/min was applied. The test was stopped due to severe erosion of sediments.

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

1 overhead camera portal to take pictures. No hydraulic sensors.

Visual observation of stability/damage evolution.

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 geogrid 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

Post-activity conditions

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 (0.25 - 0.5 m) erosion channels. It was also observed that some of the anchors became partly loosened due to the erosion and the current action that created a drag force on the RTM solution.

Immediate Results

Geogrid solutions may not be adequate in protecting a clay cover. Crane tracks severed the test and installation technique may have to be revised.

Results /Conclusion

Factual data report: https://stowanl-my.sharepoint.com/:w:/g/personal/llhpp_stowa_nl/EX5yYnuJ-EBBjoKAxAwg8LkBef8ra9V_-GHjzbkz0Was8g?e=pyaUSq Factual data report annexes: https://stowanl-my.sharepoint.com/:w:/g/personal/llhpp_stowa_nl/EWIzoxIhxopCryc6744YgHYBBWr_0fhoGpJ_1pSy0_tM2w?e=pY8Hc6 An integration report: Will be added later.

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