2016 Update of South Holland Strategic Flood Risk Assessment - Modelling and Mapping Note
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REPORT 2016 Update of South Holland Strategic Flood Risk Assessment - Modelling and Mapping Note 2016 Modelling and mapping Update Note Client: South Holland District Council Reference: WATPB3271R001D01 Revision: 01/Final Date: 28 February 2017 Project related HASKONINGDHV UK LTD. Rightwell House Rightwell East Bretton Peterborough PE3 8DW Water VAT registration number: 792428892 +44 1733 334455 T +44 1733 262243 F email E royalhaskoningdhv.com W Document title: 2016 Update of South Holland Strategic Flood Risk Assessment - Modelling and Mapping Note Document short title: Reference: WATPB3271R001D01 Revision: 01/Final Date: 28 February 2017 Project name: South Holland SFRA Extension Project number: PB3271 Author(s): Lee Garratt and Fola Ogunyoye Classification Project related Disclaimer No part of these specifications/printed matter may be reproduced and/or published by print, photocopy, microfilm or by any other means, without the prior written permission of HaskoningDHV UK Ltd.; nor may they be used, without such permission, for any purposes other than that for which they were produced. HaskoningDHV UK Ltd. accepts no responsibility or liability for these specifications/printed matter to any party other than the persons by whom it was commissioned and as concluded under that Appointment. The integrated QHSE management system of HaskoningDHV UK Ltd. has been certified in accordance with ISO 9001, ISO 14001 and OHSAS 18001. 28 February 2017 WATPB3271R001D01 i Project related Table of Contents 1 Introduction 1 1.1 General 1 1.2 Methodology 1 1.3 Model Inputs 2 2 Mike11 Modelling - Surfleet 4 2.1 Methodology 4 2.2 Surfleet Breach Parameters 5 2.3 Mike11 Model Runs 5 2.4 Surfleet Overtopping Modelling 5 3 Crowland to Cowbit Breaching 6 3.1 Methodology 6 4 TUFLOW Modelling 11 4.1 General 11 4.2 2D Domains 11 4.3 Topography 12 4.4 Raised Features and Buildings 13 4.5 Roughness Coefficients 13 4.6 Model Runs 14 5 Mapping 15 5.1 General 15 5.2 Detailed Residual Mapping for Selected Settlements 15 5.3 District-wide Residual Flood Risk Mapping 16 5.4 Comparison of Mapped Outputs for 2016 and 2010 Updates 17 Table of Tables Table 1 - Mike11 Model Breach Parameters ............................................................................ 5 Table 2 - Surfleet Mike11 Model Runs ..................................................................................... 5 Table 3 - Crowland to Cowbit Maximum Water Levels ........................................................... 9 Table 4- Crowland to Cowbit Washes Breach Parameters ..................................................... 9 Table 5 - Model Roughness Coefficients ............................................................................... 13 28 February 2017 WATPB3271R001D01 ii Project related Table 6 - Model Runs .............................................................................................................. 14 Table 7 – Comparison of 2010 and 2016 Mapped Outputs ................................................... 17 Table of Figures Figure 1 - Mike11 Breach Locations at Surfleet ...................................................................... 4 Figure 2 - Reservoir Flood Maps Hydrograph Methodology .................................................. 7 Figure 3 - Extent of Crowland to Cowbit Washes ................................................................... 8 Figure 4 - Crowland Breach Locations .................................................................................. 10 Figure 5 - Crowland 2D Model Extent .................................................................................... 11 Figure 6 - Surfleet Model Locations ....................................................................................... 12 Figure 7 - Delineation of Mastermap Data (Left) and Associated Model Roughness Coefficient Delineation (Right) ............................................................................................... 13 28 February 2017 WATPB3271R001D01 iii Project related 1 Introduction 1.1 General This report sets out the process and parameters used in generating additional 2D flood modelling outputs that have been used in the update (2016) of the South Holland Strategic Flood Risk Assessment. This report is supplementary to the June 2009 Modelling Report1 of the previous SFRA study. The information presented here is supplementary to that in the main SFRA report. The outputs from the modelling study have been used to map maximum flood depth, velocity, hazard and extent for a range of flood probabilities. To achieve these outputs, two new detailed TUFLOW models have been developed to represent likely flood extents as a result of breaching of the defences from the Crowland and Cowbit Washes and from the River Glen at Surfleet. This has been presented in addition to the detailed modelling for Spalding and Sutton Bridge developed as part of the 2010 SFRA. Only fluvial flooding has been considered in this modelling update. Fluvial boundaries have been generated utilising the Environment Agency’s 2007 version of the MIKE11 River Welland and Glen Catchment Strategic Model2. Further details of these inputs are discussed below. In addition to the new modelling outlined above, a review of the detailed modelling for Spalding was carried out to address anomalies identified in the results of the previous SFRA outputs. This led to an update of a couple of breaches where they were not applied consistently across different flood probabilities. A location plan showing all breach locations is provided as a separate Map, Figure MA1. The whole of the tidal flooding information has been replaced by the mapping from the latest Environment Agency Tidal Hazard mapping. Where the mapping for an area contains both fluvial and tidal components, the more severe of the two has been presented. 1.2 Methodology TUFLOW modelling has been utilised to simulate flood propagation in the South Holland District from the new fluvial breaching of defences. The sources of fluvial flooding considered are the Rivers Glen and Welland, including the Crowland to Cowbit Washes. No new or updated information from Internal Drainage Boards were used within this update. The previous study and update undertook TUFLOW modelling in two forms. A District-wide assessment was undertaken for present-day (2006) and year 2115 scenarios (the latter allowing for the prospective impact of climate change) considering a fluvial flood event of 1% Annual Probability of Exceedance (AP) or a tidal event of 0.5% AP as appropriate to location. 0.1% modelling was not carried out at the District- wide scale. The District-wide modelling used a 50m terrain grid. Additionally, more detailed assessments were made for both the Spalding/Pinchbeck area and the Sutton Bridge area. These were expected to be the principal areas for future development in the District. The detailed modelling used a10m terrain grid. As well as considering the same scenarios taken for the District-wide modelling, it also considered fluvial and tidal events of 0.1% AP for the present-day and year 2115. This study has utilised a similar approach to the former detailed modelling (10m grid resolution) for the new modelling for Crowland and Surfleet. 1 Update of Strategic Flood Risk Assessment, Modelling Report, June 2009, Royal Haskoning 2 Welland and Glen Catchment Strategic Model – 2007. Environment Agency 28 February 2017 WATPB3271R001D01 1 Project related 1.3 Model Inputs To derive the various outputs for the study, a series of breaches have been modelled in the embankment of the Crowland Washes (to assess risk at Crowland) and from the River Glen (to assess risk at Surfleet). Only fluvial breaching has been considered. A modified reservoir breach methodology has been adopted in order to determine risk at Crowland. The flows through the breaches for the River Glen and Crowland Washes have been used as inputs in the TUFLOW models. For Surfleet, the flows have been derived using the River Welland and Glen Mike11 model. Details of the Mike11 modelling are described in Section 2. For Crowland Washes, breach modelling at Crowland has been carried out as described in Section 3. The flood propagation is described in Section 4 and the creation of the mapping outputs is described in Section 5. The 2007 River Welland Glen Catchment strategic Model and results have been utilised in this study as an ongoing study updating the hydrology and geometry of the various watercourses had not been completed. The intention of this study was that the updated version of the model and its results would have been utilised in this study however the Environment Agency were happy that the older model and results would be suitable for use in this study, as any difference in channel levels was likely to be small and unlikely to have a significant impact on hazard mapping results. The outputs presenting climate change considerations within this study are represented as the 2115 scenarios. The modelling and mapping represent the outputs from the previous modelling which is in line with the previous PPS 25 guidance and therefore is not in line with the latest guidance published by the Environment Agency in 2016. The new modelling also used the previous guidance for consistency. The table below shows the comparison between the PPS 25 increases used to account for 100 years of climate change (Year 2115) in this update compared to the present day (2006) and the current Environment Agency 2016 guide. Climate change increases for New climate change increases Parameter