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A new method for assessing non-flood defence objects on primary flood defences

 
Iv-Infra

Reliable assessment

Primary flood defences in the Netherlands must be assessed every few years for conformity to safety standards. A new assessment system, the Statutory Assessment Instrument (WBI2017) has been in place since 2017. Non-flood defence objects on a flood defence, such as trees, buildings, cables and pipelines, all form a part of this assessment as these objects may have an influence on the safety of the flood defence. For example, when a fallen tree has created an indentation in the ground, this may cause instability and damage to the surface area. However, the instruments of the assessment system have not yet been fully developed for non-flood defence objects and are also unsuitable for assessing large quantities of objects. So, how should thousands of these objects be assessed? Is there a way to approach this smartly and make it manageable?

6000 trees and 3000 buildings

The District Water Control Board of Schieland and Krimpenerwaard has around 6000 trees and 3000 buildings on its 70 kilometres of primary flood defences. According to the new legal safety standards, these objects must all be assessed before 2023. Iv-Infra and Tauw are currently developing a method to efficiently evaluate large quantities of objects. Because these quantities are so large, it is important that these can be efficiently assessed and that the method is traceable and reproducible. Reliability is also an important aspect of this new method, but how can reliability be substantiated? Iv-Infra and Tauw work with three main pillars in this method: secure information, apply knowledge & expertise and automation. 
 

Secure information

Existing data from the water board is verified, validated and, where necessary, supplemented. The digital register is loaded with objects and is, of course, subject to change. To update this register, the unique Iv-Infra scan car was deployed to accurately map the non-flood defence objects. The additional data collected by the scan car allows objects to be approved quickly, for example; it can be demonstrated that the dimensions of an object are so minute that any occurrence of failure will not influence water safety. The testing of all non-flood defence objects is thus improved and made more efficient.
 

Apply knowledge and expertise

Iv-Infra and Tauw have extensive experience in assessing primary and regional flood defences. The specific knowledge is bundled and applied in the development of this new method.
 

Automation

To make the assessment process optimally efficient, as much as possible is automated. The large quantities of collected GIS data and the scan car data are supplemented with current elevation data measured by helicopter. The relevant parameters for each object are then determined by specially developed GIS scripts which are used to perform the simple test automatically. Automating this process decreases the probability of errors and reduces labour-intensive operations.
 

A two-step assessment

The assessment of the non-flood defence objects consists of two steps. The first step is a simple assessment, which is approximately 90% automated. The remaining 10% is carried out by consultants of Tauw and Iv-Infra and consists of creating an elevation profile by adding height lines in a GIS model, checking data, validation and sampling in the field. To prevent a labour-intensive process, an attempt is made to approve as many objects as possible with the simple test, but it is inevitable that additional calculations will also be necessary. For these additional calculations, a detailed test is carried out, after which it is concluded whether the non-flood defence object is safe or unsafe. The District Water Control Board of Schieland and Krimpenerwaard then takes the necessary measures based on this safety assessment.

 

An individual judgement for each non-flood defence object

To arrive at this new method, all data is thoroughly processed, additional data is collected, and innovative solutions are devised. The process is still under development, but the path to the final method is going well. The reliability and traceability of the data and the reproducibility of the method are vital to the development and will ensure that the data can be utilised in future tests or reinforcements. Although customisation will still be required, the development of this method is an important step towards an accurate overview of the individual safety of all non-flood defence objects, where previously only a generic judgement was possible.

This article is also published in Land + Water.

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