| Description | Scottish Flood Defence Asset Database Final Report |
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| ISBN | (Web Only) |
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| Official Print Publication Date | |
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| Website Publication Date | August 20, 2007 |
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Contents |
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CHAPTER 3: DATA COLLECTION
Project overview
3.32 The project was divided into stages and a methodology established (Figure 3 1) to allow different levels of effort to be applied depending upon data availability.
Figure 3-1: Methodology Flowchart
Scheme Categorisation
3.33 The desktop study and questionnaire established that no schemes had all of the information available to populate the database. Where possible some information could be taken from 'as-built' drawings and design reports but a large number of schemes required collection of new data and modelling to determine how well the schemes would perform under current design standards.
3.34 The schemes were ranked on the number of properties protected, the (declared) standard of protection and the availability of a flood extent map. The schemes were then prioritised by age and information availability.
- Category A - very recent schemes with large amounts of available information to be used to complete the database without further study.
- Category B - schemes with some information and a reasonable likelihood of protecting properties. More detailed studies were undertaken on the majority of these schemes.
- Category C - little or no information (usually name and location only). The location was confirmed and any information available entered into the database.
3.35 The full prioritised list of schemes can be found in Appendix A, most of the schemes categorised within C were in areas with few properties 5. Analysis of most of the schemes within category B was made including a detailed condition assessment, topographic survey, hydraulic assessment, economic assessment, flood outlines and model report.
'On Site' Condition Assessment
3.36 Information from the desktop study and from the returned questionnaires was transferred into an Interim Asset Database. For those schemes that required a condition assessment, information was transferred from the Database onto a pocket PC so that the data gathered could be checked and cross referenced whilst carrying out the site inspection for each scheme.
3.37 The detail and accuracy of asset location and data gathering was enhanced by the use of global positioning satellite software based on handheld computers. The known location of the Flood Prevention Schemes and related defences, Ordnance Survey 1:10,000 scale maps and Landline datasets were also downloaded to the PC. A GPS connected to a GIS application ( GISmo) developed for the study provided grid references onsite allowing the exact location of photographs, faults and defence components to be recorded.
3.38 The conditions of the assets at each Flood Prevention Scheme were assessed in accordance with the Environment Agency's National Sea and River Defence Survey's manual 6. This manual provides a grading system for each type of defence on a scale of 1 to 5, as shown in Table 3-1. The descriptions for each defence type are typically based on the scale of defects, the requirement for maintenance or remedial works, the urgency of work required and a quantification of the defects in terms of length, area and height. The manual provides descriptions and includes photographic examples, to help give each asset a condition score.
Table 3-1: Sea and River Defence Condition Assessment
Grade | Description |
1 | Very Good | In good condition Fully serviceable, no remedial work required Maintenance to continue as present No significant defect |
2 | Good | Minor defects non urgent Minor routine maintenance work required In reasonable condition, some increase in maintenance, probably not more than 5% affected with slight defect |
3 | Fair | Some cause for concern, requires careful monitoring Significant maintenance works required Average condition, some minor repairs needed & moderate 5%- 20% affected |
4 | Poor | Structurally unsound now or in the near future Major remedial works required or replacement (1-5 years) Extensive repairs required in short term Extensive defect 20-50% affected |
5 | Very Poor | Completely failed or derelict requires completes reconstruction. Major urgent repairs or replacement needed without delay to avoid failure probably beyond repair. Extensive defect >50% |
(source: Environment Agency, Ibid)
3.39 Following the condition assessment survey a report was produced and uploaded to the project website where it could be viewed and downloaded. The reports were also passed to the relevant local authority to check and ensure any faults (examples in Figure 3-2) could be checked and rectified immediately.
Figure 3-2: Example Defence Faults
Damage to Embankment
Gap and displaced wall
Survey
3.40 On completion of the condition assessment, the schemes were reassessed and the requirements for further work determined. Where survey work was deemed necessary, survey specifications were drawn up. The surveys used the Ordnance Survey National Grid ( OSGB 36) with controls established from the Ordnance Survey GPS Network and heights related to Ordnance Survey Newlyn datum.
River Cross Section Survey
3.41 Cross sections are required of watercourses in order to allow the estimation of hydraulic capacity. The cross sections were taken along the length of the modelled watercourse at right angles to the direction of flow in the watercourse or floodplain. These sections included the flood defence where applicable as well as elevations of structures such as bridges, culverts, trash screens and weir crests. The survey data has been recorded in engineering drawing format using CAD (location plans and cross-sections) and included in the database.
Location Accuracy
3.42 The detailed surveys can show assets in different location to those on an OS survey map. This is due to the relative accuracy differences between the survey equipment used and the base OS mapping. The Landline maps, the most detailed available have a resolution of ±2.8 m in the horizontal plane at 1:2,500 scale and ±0.5 m at the 1:1,250 scale. This compares to centimetre resolution from the kinematic GPS equipment used for the survey. An example is shown in Figure 3 3 where a floodbank appears to cross the river. The Ordnance Survey have an on going programme for improving positional accuracy which should reduce these inconsistencies.
Figure3-3: Alignment of surveyed assets and OS background
Threshold Survey
3.43 In order to accurately determine potential damages, property levels were surveyed. Doorstep levels were typically used unless there was a lower level of entry for floodwater. The area covered by threshold levelling was determined from existing flood outlines, estimation or from a preliminary JFLOW model.
Survey Drawings
3.44 The majority of the survey drawings on the database have one drawing for the cross sections and one drawing showing the threshold levels, assets and cross section locations. A number of drawings, where applicable, have typical sections drawn as an insert showing construction details.
3.45 The drawings are presented in PDF format on the database and the original CAD drawings supplied on CD- ROM to both the appropriate local authority and Scottish Executive.
Information from Detailed Study
3.46 Following the condition assessment and survey, where required, a detailed study was completed for each Flood Prevention Scheme and more information is included in the following chapters. A detailed study encompassed the following stages:
- Flood flow estimation
- Hydraulic modelling to estimate flood depth
- Flood extent mapping
- Determination of standard of protection
- Economic assessment
Contents |