Parent Layer: Greater Utuhina Catchment Model (GUCM)

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Description: Regional 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 were made to the year 2130.For further information refer to DHI “Utuhina - Phases 2 and 3 Numerical Modelling”, August 2021 and DHI “Utuhina – Additional Modelling and Mapping(Addendum Report)”, August 2022.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 data layers are generated from catchment level modelling based on the datasets, requirements and technology available at the time of model build.There are multiple sources of uncertainty in deriving these maps including:The accuracy of gauging data and rating curves and associated estimated flood discharge for the chosen design events.Hydrologic assumptions, including antecedent rainfallThe definition of floodplain topography and channel cross-sections.The choice of effective hydraulic roughness coefficients.The choice of the hydraulic and hydrologic models and their representation of the physics.The treatment of floodplain infrastructure.The consideration of the performance of flood defences.The potential for non-stationarity processes arising from both catchment change (land-use and/or land management changes) and climate variability and/or change.No detailed uncertainty analysis has been carried out. However, a freeboard margin (factor of safety) has been applied to the design flood levels to account for uncertainties. Areas of greater uncertainties received a greater freeboard margin 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.

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Copyright Text: Bay of Plenty Regional Council

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