Technical Note on Assessment of Required Compensatory Flood Storage

Transcription

1 Crewe Green Link Road Technical Note on Assessment of Required Compensatory Flood Storage Note Date: November 2011 Note Status: Issue D03 Note ID: _B _Tec_PrelimStorageNote_D03_App.doc

2 Document Control Sheet BPP 04 F8 Version 7 April 2011 Client: Cheshire East Council Project No: Project: Crewe Green Link Road Document Title: Technical Note on Assessment of Required Compensatory Flood Storage Ref. No: _B _Tec_PrelimStorageNote_D03_App.doc Originated by Checked by Reviewed by Approved by ORIGINAL NAME NAME NAME NAME DATE EDR 23/11/2011 Document Status Issue REVISION NAME NAME NAME NAME DATE EDR 28/11/2011 Document Status Issue 02 updated following comments REVISION NAME NAME NAME NAME DATE EDR 02/02/2012 Document Status Issue 03 updated following completion of modelling REVISION NAME NAME NAME NAME DATE Document Status Jacobs Engineering U.K. Limited This document has been prepared by a division, subsidiary or affiliate of Jacobs Engineering U.K. Limited ( Jacobs ) in its professional capacity as consultants in accordance with the terms and conditions of Jacobs contract with the commissioning party (the Client ). Regard should be had to those terms and conditions when considering and/or placing any reliance on this document. No part of this document may be copied or reproduced by any means without prior written permission from Jacobs. If you have received this document in error, please destroy all copies in your possession or control and notify Jacobs. Any advice, opinions, or recommendations within this document (a) should be read and relied upon only in the context of the document as a whole; (b) do not, in any way, purport to include any manner of legal advice or opinion; (c) are based upon the information made available to Jacobs at the date of this document and on current UK standards, codes, technology and construction practices as at the date of this document. It should be noted and it is expressly stated that no independent verification of any of the documents or information supplied to Jacobs has been made. No liability is accepted by Jacobs for any use of this document, other than for the purposes for which it was originally prepared and provided. Following final delivery of this document to the Client, Jacobs will have no further obligations or duty to advise the Client on any matters, including development affecting the information or advice provided in this document. This document has been prepared for the exclusive use of the Client and unless otherwise agreed in writing by Jacobs, no other party may use, make use of or rely on the contents of this document. Should the Client wish to release this document to a third party, Jacobs may, at its discretion, agree to such release provided that (a) Jacobs written agreement is obtained prior to such release; and (b) by release of the document to the third party, that third party does not acquire any rights, contractual or otherwise, whatsoever against Jacobs and Jacobs, accordingly, assume no duties, liabilities or obligations to that third party; and (c) Jacobs accepts no responsibility for any loss or damage incurred by the Client or for any conflict of Jacobs interests arising out of the Client's release of this document to the third party _B _Tec_PrelimStorageNote_D06_App.doc

3 Contents 1 Introduction Background to the study Purpose of this Note 1 2 Methodology Data Sources and Analysis Tools Methodology 3 3 Results Comparison Against Previous Estimate Future Work 6 List of Tables Table 1 - Results of the Preliminary Analysis of Compensatory Storage for a 1 in 100+cc Flood Event 6 List of Diagrams Diagram 1 - Compensatory Flood Storage 1 List of Appendices Appendix A Figures _B _Tec_PrelimStorageNote_D06_App.doc

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5 1 Introduction 1.1 Background to the study Cheshire East Council is proposing to construct a new road to link the existing A500 and A5020. The proposed road will be constructed partly on a raised embankment alongside Basford Brook, an Environment Agency Main River. The study area and proposed layout are shown in Figure 01 in Appendix A. A Flood Risk Assessment (FRA) has been prepared for the proposed link road and has identified that the development will be at risk from fluvial flooding from Basford Brook. The modelled flood extents prepared for the FRA are shown in Figure 02 in Appendix A. The construction of the embankment will reduce the available storage within the floodplain of Basford Brook. This has the potential to increase water levels downstream and consequently increase flood risk. Compensatory flood storage will be required to offset the loss of floodplain resulting from the construction of the embankment. Compensatory flood storage must become effective at the same point during a flood event as the lost storage would have done. It should provide the same volume, and be at the same level relative to flood level, as the lost storage. This requirement is often called level for level or direct compensation. A visual example of how this typically works is shown in Diagram 1-A. Diagram 1 - Compensatory Flood Storage Ideally, level for level compensatory storage should be provided immediately next to the location where the floodplain has been removed Purpose of this Note This note outlines work carried out to provide a preliminary, desk-based evaluation of the storage that is likely to be required to compensate for the loss of floodplain resulting from the construction of the link road. 1 For further information on level for level storage please refer to CIRIA C624 Development and Flood Risk - guidance for the construction industry _B _Tec_PrelimStorageNote_D06_App.doc 1

6 This note updates the findings of a previously prepared note 2. The previous note used existing modelling, which did not include the minor tributary of Basford Brook that affects the northern end of the site, nor the tributary to the south of the site, joining on the right bank to the north of Crotia Mill Farm. This note has been completed following an update to the hydraulic model which now includes these minor tributaries. This Technical Note outlines the data and methodology used for the analysis and presents the results for a flood event 1 in 100 plus climate change chance of occurrence in any year. 2 Please refer to document _BXXXXXXX_Tec_PrelimStorageNote_D02_App _B _Tec_PrelimStorageNote_D06_App.doc 2

7 2 Methodology This section outlines the data sources and methodology used for preparing the preliminary flood storage assessment. 2.1 Data Sources and Analysis Tools The following data was used to prepare the compensatory storage assessment: Proposed link road design Modelled Flood Levels from updated hydraulic model Environment Agency LiDAR topographic data Ordnance Survey 1:1250 scale MasterMap Data The following programs were used for the analysis: ESRI ArcView GIS with the Spatial and 3d Analyst extension packages Microsoft Excel 2.2 Methodology Division of the Study Area The proposed link road runs from south to north along the course of Basford Brook. There are number of existing topographic features that divide the existing floodplain into 3 areas. Flood levels within these 3 areas are different, and it is therefore necessary to divide the calculation of storage volumes up into 3 separate zones, Areas 01, 02 and 03 (see Figure 03 in Appendix A) Modelled Flood Risk Data Flood level grids were obtained from the updated hydraulic model as.asc grids and converted into ArcGIS format using the ArcGIS Convert Ascii to Grid tool. The modelled level grids were clipped to the outline of the proposed scheme, to extract the area of lost floodplain, and then divided between the 3 areas _B _Tec_PrelimStorageNote_D06_App.doc 3

8 Key Assumptions The following assumptions regarding water levels and the construction of the proposed scheme have been made. It has been assumed that, within each area, storage will be required up to the maximum water level, regardless of any change in water level across each area 3. It has been assumed that the embankment is a square section. The above assumptions will result in a conservative estimate of the volume of compensatory storage required. No further, more detailed, calculations are considered to be necessary, as using a conservative estimate of the storage volume will account for uncertainty in the hydraulic model and the topographic data used. It is further assumed that flood relief culverts will be built into the road embankment to maintain connectivity between the river and the floodplain. The area to the east of the road embankment is assumed to be permitted to flood and therefore compensation for this flood area is not required Topographic Data 2m horizontal resolution LiDAR data was obtained for the study area. The clipped flood outline datasets were used to clip the LiDAR dataset to the outline of the areas of floodplain that are to be lost. The clipped LiDAR dataset formed the basis for the assessment of volume as it represents the before development topography Preliminary Assessment of Flood Storage The volume of storage required in a 1 in 100 (1%) annual probability flood event, including an allowance for climate change, was calculated using the 3d Analyst Surface Volume tool. This tool applies a target level across a specified raster grid dataset and calculates the volume between the target level and the grid surface. For level for level compensatory storage, the volume of storage provided must match the composition of the storage lost. It is therefore necessary to identify the volume of storage at different levels within the flood event. To do this, the Surface Volume calculation was carried out at 100mm intervals between the minimum elevation level and the maximum water level. This provides a cumulative volume of storage as the water level increases. This data was imported into Microsoft Excel in order to calculate the volume of storage required between each slice. The results of the analysis are presented in the Section 3. 3 Analysis of the new modelled water levels indicates that this predominantly affects area 03 where a change in level of ~0.2m occurs. In Areas 01 and 02 water level changes across each site are <0.1m _B _Tec_PrelimStorageNote_D06_App.doc 4

9 3 Results The results of the Preliminary Analysis of Compensatory Flood Storage are shown below. Figure 03 in Appendix A shows the areas of floodplain that will be lost if the proposed link road is constructed. An estimated total of 6,950m 3 of compensatory storage will be required across all 3 areas. To satisfy the requirement to provide level for level compensatory storage it will be necessary to provide the same volume of storage being lost at each stage slice, as indicated in the table below. Area 01 Slice Storage Volumes (m 3 ) Cumulative By Slice Area 02 Slice (maod) Storage Volumes (m 3 ) Cumulative By Slice (cont. on next page) _B _Tec_PrelimStorageNote_D06_App.doc 5

10 Area 03 Slice (maod) Storage Volumes (m 3 ) Cumulative By Slice Table 1 - Results of the Preliminary Analysis of Compensatory Storage for a 1 in 100+cc Flood Event 3.1 Comparison Against Previous Estimate The original outline estimate of storage volumes, calculated prior to the updated hydraulic modelling was carried out, calculated that a total of between 4,000m 3 and 6,000m 3 of compensatory storage would be required. The total estimate following the updated hydraulic modelling is 6950m 3. The estimate has increased because the original modelling data did not include two tributaries, as described in the note in Section 1.1. The flooding from these tributaries had to be estimated for the outline assessment, without the benefit of hydraulic model results. Once the model was updated and the tributaries included, the subsequent predicted flooding was greater than originally anticipated and exceeded the 50% uncertainty included as part of the outline assessment. 3.2 Future Work The modelling work carried out to date has indicated that the road itself is at risk from flooding during the 100 year event, including an allowance for Climate Change. Also, the compensation volumes described in this report assume that the areas of flooding that occur on the opposite side of the embankment would still occur following road construction. A number of flood relief culverts will therefore need to be incorporated into the design. The model will need to be used in an optioneering process, to locate and size these culverts, help size watercourse crossings and drainage requirements, and provide flood levels to allow the road to be designed such that it is not at risk from flooding _B _Tec_PrelimStorageNote_D06_App.doc 6

11 Appendix A Figures _B _Tec_PrelimStorageNote_D06_App.doc

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13 Metres Legend Watercourse Proposed Scheme Footprint A4 1:5,000 Dwg ID: B /Store/Fig01 Date Rev Wrk Chk App 18/11/11 00 EDR 02/02/12 02 EDR Figure Name: Project: Figure 01 - Study Area Crewe Green Link Road Preliminary Storage Volume Assessment

14 Metres Legend Watercourse Proposed Scheme Footprint Dwg ID: B /Store/Fig02 Date Rev Wrk Chk App Figure Name: Figure 02 - Modelled Flood Risk 18/11/11 00 EDR Project: 02/02/12 02 EDR 1 in CC Flood Extent Crewe Green Link Road A4 1:5,000 Preliminary Storage Volume Assessment

15 Area 03 Area 02 Area Metres Legend Watercourse Compensatory Storage Volume Assessment Areas Proposed Scheme Footprint 1 in 100+cc Floodplain Lost A4 1:5,000 Dwg ID: B /Store/Fig03 Date Rev Wrk Chk App 18/11/11 00 EDR 02/02/12 02 EDR Figure Name: Figure 03 - Areas of Floodplain Lost as a Result of Road Construction Project: Crewe Green Link Road Preliminary Storage Volume Assessment