REPORT. Coastal Inundation Hazard Zone Mapping. Addendum to Coastal Protection Areas Re-assessment Stage Two

Transcription

1 REPORT Coastal Inundation Hazard Zone Mapping Addendum to Coastal Protection Areas Re-assessment Stage Two Prepared for Prepared by Date Job Number

2 Distribution: (FILE) 1 copy 1 copy

3 Table of contents 1 Purpose 1 2 Coastal Inundation Hazard Zone (CIHZ) levels 1 3 CIHZ mapping method applied (Tonkin & Taylor April 2015) 1 4 Mapping detail Waihi Beach Pukehina 2 5 Results verification by dynamic model for Waihi Beach 3 6 Reference documents 4 7 Applicability 4 Appendix A : Figures

4 1 1 Purpose The purpose of this addendum report is to explain the methods and judgements used by Tonkin & Taylor (T+T) to map the CIHZ levels for Waihi Beach and Pukehina over a 100 year timeframe (2115). These maps are shown in Appendix B of the Coastal Protection Areas Re-assessment Stage Two report ( the report ). Judgement was applied to manually adjust some parts of the initially developed maps. The reasons and explanations for this are summarised in this addendum report. 2 Coastal Inundation Hazard Zone (CIHZ) levels Peak tide levels of 3.7mRL and 2.8mRL were used for mapping the CIHZ for Waihi Beach and Pukehina respectively (as highlighted). The calculations for these levels are shown in Table 4-10 of the report which is replicated below. The areas of land likely to be inundated when the sea level reaches the levels in the table above comprise the CIHZ. 3 CIHZ mapping method applied (Tonkin & Taylor April 2015) The following process was followed to map the CIHZ: Using the 2012 LiDAR-derived digital elevation model (DEM), map all areas where ground level is below 3.7mRL for Waihi Beach or 2.8mRL for Pukehina. For Waihi Beach the area where ground level is below 3.7mRL is shown in Figure 1 (Appendix A). For Pukehina the area where ground level is below 2.8mRL is shown in Figure 26 (Appendix A). Apply the connected bathtub approach. This assumes that land, even if it is below a 3.7mRL or 2.8mRL ground level, can only be inundated if it is actually directly connected to the open coast i.e. the water has a way of getting there. The connected bathtub method removes any land which is not directly connected to the open coast and therefore not susceptible to inundation. An example of this is shown in Figure 2. Allow for head loss / damping of tidal time series and for physical obstructions / constrictions not shown in the DEM to manually adjust the potential inundation extents using engineering judgement. This allowance is to more closely mimic the effects that actual obstructions have on the propagation of inundation landwards. Distance from the open coast and depth of inundation were taken into account in making these allowances. This item was not fully described in the report.

5 2 Apply smoothing to the gridded inundation extents. Further detail on exactly where and for what reason areas in the DEM lower than 3.7mRL and 2.8mRL were removed from the CIHZ is provided below. 4 Mapping detail 4.1 Waihi Beach Figures have been attached (Appendix A) to this addendum to illustrate where areas in the DEM lower than 3.7mRL have been removed. In Figure 1 the entire Waihi Beach area is shown, with the April 2015 CIHZ extent mapped together with an indication of where current (2014 LiDAR) ground level is at, or below, the design peak tide level of 3.7mRL determined by Tonkin & Taylor. Using the 3.7mRL level it can be seen that there is a greater extent of land area below 3.7mRL than shown in the CIHZ. Some of these differences are due to there being no direct connection to the open coast from low lying areas. Other differences are due to damping and obstructions as outlined above. In Figures 3 to 13 inclusive, detailed maps showing DEM levels and the mapped CIHZ extents at 1:2000 scale covering the Waihi Beach area are shown. These maps have been annotated with reasons for areas shown where DEM level is below 3.7mRL have not been included in the CIHZ. A summary of the mapping is provided in the table below. Figure number Reasons for exclusion of areas where DEM elevation is lower than 3.7mRL 1 Peak level damping due to shallow and constricted flow over dunes 2 Peak level damping due to shallow and constricted flow over dunes No connection with open coast 3 Peak level damping due to long flow distance at shallow depth from open coast No connection with open coast 4 Limited connectivity via constrained waterway, together with peak level damping from open coast Peak level damping due to long flow distance from open coast via constricted waterway 5 Peak level damping due to long flow distance at shallow depth from open coast Difference between 2012 and 2014 LiDAR (land has been filled since 2012) 6 Physical obstruction to flow at shallow depth running through two rows of commercial development alongside Wilson Road Peak level damping due to shallow and constricted flow over dunes Note that in the Bowentown area there is no CIHZ identified because there is no area where there is a DEM level below 3.7mRL and a direct connection with the open coast. Examination of these maps and the explanations provided in the table above led to a further analysis being carried out that is described below (Section 5). 4.2 Pukehina For the Pukehina area all of the CIHZ as mapped was hydraulically connected with the estuary coast, and therefore no judgement was required to manually manipulate the CIHZ as indicated by land level alone. This is shown in Figure 26 (Appendix A), where the mapped CIHZ is shown overlying the areas where existing ground level is lower than 2.8mRL, and a clear direct correlation can be seen. Coastal Inundation Hazard Zone Mapping Addendum to Coastal Protection Areas Re assessment Stage Two

6 3 5 Results verification by dynamic model for Waihi Beach For verification purposes, T+T undertook a further mapping exercise for the Waihi Beach area whereby an existing hydrodynamic model, used for assessment of stormwater flooding, was used with a dynamic tidal boundary applied and the resulting inundation extent compared with the extent presented in the report. This analysis represents a different methodology to that used for the initial mapping, and was undertaken as a check of the previously developed maps. The alternative methodology could only be applied to the Waihi Beach area because a suitable model covering Pukehina was not available. The principal area in Waihi Beach where the mapped CIHZ and the area where the DEM was below 3.7mRL showed significant differences is that area shown in Figure 8. In this area, overflow from Two-Mile Creek towards the south is shown to occur in the dynamic model, but has been cut off in the mapped CIHZ. A sample plot from this analysis is presented below, and this shows inundation depths and extents at the time of the peak tide level having been reached. In Figure 14 the peak inundation depths derived from the dynamic model are shown overlying the mapped CIHZ and the DEM levels. Evident from this are the following: All inundation in this area comes from the overflow from Two-Mile Creek. The overflow passes through two rows of shops. The DEM used in the model has all above-ground features stripped out, with ground levels beneath existing buildings (dwellings and shops etc.) being interpolated from adjacent ground levels. Building platforms were therefore omitted from the DEM, and actual ground levels in these areas are likely to be higher than as represented in the DEM. Based on the above it is our professional view that it was reasonable to remove this area from the CIHZ, as was done in the report, as it is likely that ground levels as represented in the DEM are lower than actual and that these represent a real barrier to the overflow at this location. Peak water levels and inundation extents from the dynamic model simulation of an extreme tidal event for the entire area are given in Figures 15 to 25 inclusive. These can be compared directly with Figures 3 to 13. In general it can be seen that the inundation extents using the dynamic model closely match the inundation extent presented in the report, with the differences in extent being able to be explained as above.

7 4 In conclusion the inundation extent included in the report closely matches the expected inundation extent derived from a dynamic tidal boundary model, verifying the inundation extent included in the report. 6 Reference documents Tonkin & Taylor (April 2015), Coastal Protection Areas Re-assessment, Stage Two, Prepared for WBoPDC. 7 Applicability This report has been prepared for the exclusive use of our client Western Bay of Plenty District Council, with respect to the particular brief given to us and it may not be relied upon in other contexts or for any other purpose, or by any person other than our client, without our prior written agreement. Report prepared by: Authorised for by: Mark Pennington Senior Water Resources Engineer pp Richard Reinen-Hamill Project Director MSP \\tgadc\data\rep\live\tt\projects\851833\ \issueddocuments\addendum report\ msp.addendumreport.rep.docx