Urban Flood Mapping and Stormwater Management Planning for Coastal Cities Below Sea Level: Fid-Bau Portal

Urban Flood Mapping and Stormwater Management Planning for Coastal Cities Below Sea Level

Nabi, Abdul / Cody, Neal / Paine, Ali / Gaafar, Mohamed

Napier is a city of 66,000 located on the eastern coast of the North Island of New Zealand, in the Hawke's Bay region. The city sits astride an estuary and the Pacific coastline and is separated from the adjacent waters in many places by a long stop bank (i.e. a levee). This stop bank is necessary as some parts of the City of Napier are below mean sea level. Napier's stormwater infrastructure comprises storm pipes, overland conveyance routes, detention ponds, open drainage channels, ocean outfalls, flood gates and many force mains and stormwater pumping stations. Napier City Council engaged Stantec’s Canadian Team to undertake a stormwater modelling and master planning initiative to represent its hydrological and hydraulic systems, assess flooding damages and recommend flood mitigation options. The authors used DHI MIKE FLOOD to conduct a 2D-integrated stormwater modelling exercise and create flood mapping for various return periods. The combination of numerous stormwater pumping stations, force mains, groundwater intrusion and tidal impacts created challenging conditions for modelling and planning. Added to this is the fact that the area is seismically active and experienced a devastating earthquake in 1931, meaning that much of the infrastructure is at increased risk due to ground movement or tsunamis. Modelling and design of flood mitigation measures accounted for anticipated precipitation escalation due to climate change and sea-level rise impacts on a 100-year timescale. Mitigation measures to address future development intensification and new suburban communities were also investigated, and climate change adaptation strategies such as retreating from areas where land is expected to be permanently inundated below sea level were also explored. Ultimately, green infrastructure and volume retention measures were recommended to complement traditional grey infrastructure in the flooding mitigation strategy. The total stormwater system improvements (capital expenditure) recommended over the initial mitigation horizon are projected to be approximately NZ$401.8 M (Napier City stormwater master plan by Stantec, 2020, [1]). Two months following submission of the model and master plan, Napier was struck by an intense downpour which saw 420% of the average monthly precipitation fall in just 24 h, exceeding a 250-year return period on some durations. Observed flooding locations confirmed the model-predicted locations closely.