Layer: Greater Utuhina Catchment Model (BOPRC) (ID: 280)

Parent Layer: Flood Modelling 1%AEP

Name: Greater Utuhina Catchment Model (BOPRC)

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Description: <DIV STYLE="text-align:Left;"><DIV><DIV>Reference Document File: A4132542RDC GIS Project SDP62933Note: This dataset is a polygon version of the raster supplied by BOPRC. It is intended for temporary use only for LIM Maps until raster source is ready.Data Set NameGreater Utuhina Catchment Model (GUCM)Data Set AbstractThese map layers represent data from the Greater Utuhina Catchment flood study (a Bay of Plenty Regional Council contracted analysis) and shows a set of spatial data layers that indicate areas of land at risk of fluvial flooding from the Utuhina Stream, Mangakakahe Stream and Otamatea Stream.The GUCM deliverables depict and quantify the flood susceptibility from fluvial (riverine) flooding for the study area, e.g. the river channels are no longer able to hold the volume of water and breach the banks. The data set contains:Current built environment<UL STYLE="margin:0 0 0 0;padding:0 0 0 0;"><LI>Design flood levels predicted during 1% AEP RCP8.5 2130 scenario data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations (Moturiki Datum).</LI><LI>Design max flood depths predicted during 1% AEP RCP8.5 2130 scenario data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations.</LI><LI>Flood extent for design flood 1% AEP RCP8.5 2130 scenario data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations.</LI></UL>Potential future built environmentTwo future development scenarios for the 1% AEP RCP8.5 2130 as supporting information for RLCs infill plan change.Scenario a - Current allowance in the district planmaximum impervious surface standards in residential zones –Residential 1 Zone: 80%–Residential 2 Zone: from 100%Scenario b - Proposed reduced allowance in the district planmaximum impervious surface standards in residential zones –Residential 1 Zone: 70%–Residential 2 Zone: from 80%<UL STYLE="margin:0 0 0 0;padding:0 0 0 0;"><LI>Design flood levels predicted during 1% AEP RCP8.5 2130 scenario data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations (Moturiki Datum)-infill scenario a.</LI><LI>Design flood levels predicted during 1% AEP RCP8.5 2130 scenario data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations (Moturiki Datum)- infill scenario b. </LI><LI>Design max flood depths predicted during 1% AEP RCP8.5 2130 scenario data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations - infill scenario a.</LI><LI>Design max flood depths predicted during 1% AEP RCP8.5 2130 scenario data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations - infill scenario b.</LI></UL>Meta dataAll data layers need to be saved with reference to the following metadata. A limited number of the supplied data will be available in Bay hazards viewer for external parties.Meta data Text to be finalisedOverview of ModelRegional Council has engaged Blue Duck Consultants and DHI Water and Environment Ltd to prepare a hydrological and hydraulic model, which combined we refer to as the Greater Utuhina Catchment Model (GUCM).The hydrological model (a Non-Linear Reservoir model) is intended for flood forecasting purposes and support design investigations within the catchment. The hydrological model calculates runoff from rainfall at 122 separate sub-catchments within the wider Utuhina Stream catchment. Stream flows from runoff are tracked through a network of routing branches to Lake Rotorua. DHI extended the existing 2014 hydraulic model of the lower Utuhina reach to include the Utuhina Stream and its tributaries (Mangakakahi Stream and Otamatea Stream) upstream of the urban environment. The hydraulic model does not explicitly include the Rotorua Lakes Council's stormwater pipe network. Inflow hydrographs, each representing the flow contribution from a sub-catchment, were derived from the hydrological model and applied as a point or distributed inflows to the hydraulic model. These two models were built and calibrated in unison for current land use and current climate change scenarios, following which climate change prediction where made to the year 2130.For further information refer to DHI “Utuhina - Phases 2 and 3 Numerical Modelling”, August 2021.Note an addendum to the report is currently being prepared for the design flood data including an allowance for estimate imprecision and phenomena not explicitly included in the calculations.Purpose of ModelThe GUCM has primarily been built to undertake a capacity review of the lower Utuhina Stream flood protection scheme and to assess the relative impacts of any potential flood mitigation measures on a catchment-wide scale. Its secondary purpose is to provide information on areas susceptible to fluvial (riverine) flooding for emergency planning, provide flood level advice in the vicinity of the streams, and provide boundary conditions for the stormwater network assessments.Limitation of dataThe fluvial (riverine) flood maps derived in DHI “Utuhina - Phases 2 and 3 Numerical Modelling”, August 2021do not include data include any allowance for estimate imprecision and phenomena not explicitly included in the calculations.There are multiple sources of uncertainty in deriving these maps including:<UL STYLE="margin:0 0 0 0;padding:0 0 0 0;"><LI>The accuracy of gauging data and rating curves and associated estimated flood discharge for the chosen design events.</LI><LI>The definition of floodplain topography and channel cross-sections.</LI><LI>The choice of effective hydraulic roughness coefficients.</LI><LI>The choice of the hydraulic model and its representation of the physics.</LI><LI>The treatment of floodplain infrastructure.</LI><LI>The consideration of the performance of flood defences.</LI><LI>The potential for non-stationarity processes arising from both catchment change (land-use and/or land management changes) and climate variability and/or change.</LI></UL>No detailed uncertainty analysis has been carried out. However, an allowance for estimate imprecision and phenomena not explicitly included in the calculations (factor of safety) has been applied to the design flood levelsto account for uncertainties. Areas of greater uncertainties received a greater allowance for the design levels, such as the lower portion of the catchment.The flood maps shownarefluvial (riverine)flooding maps only, e.g. flooding from the streams but do not explicitly show flooding from direct localised rainfall or from pipe surcharges. Urban flooding from direct localised rainfall and from pipe surcharges are beyond this study and would need to be established by a suitable rain-on-grid model.As such, the design flood levels cannot solemnly relied upon for finished floor level determination within the urbanised subcatchment area as these areas may have other contributing (localised) factors that are not considered.Please note that both the estimates for estimate imprecision and phenomena not explicitly included in the calculations scenario as well as the future scenarios are not covered within the modelling report and an addendum report covering these scenarios is not yet available.</DIV></DIV></DIV>

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