Urban Design Storm Hyetograph Analysis Based on a New Method Considering Spatiotemporal Variations

Abstract

Design storms guarantee uniformity regarding quality and operation standards of engineering projects and have been employed widely in urban drainage system design. Commonly used urban design storms, such as the Chicago (K-C) storm and Pilgrim and Cordery (P-C) storm, are calculated using prescribed or historical hyetographs. A prescribed hyetograph is unsuitable for a particular urban region in most cases, and a historical hyetograph takes no account of the spatiotemporal variations between the rainfall pattern at the rain station and that within the calculated region. Additionally, neither method can make adaptive adjustments for climate change. To obtain a more practical design storm with consideration of spatiotemporal variations and climatic changes, this study introduced proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) to urban design storm study. To demonstrate the feasibility and advantages of the proposed methodology, four cities (London, New York, Sydney, Wuhan) in different continents with different climatic characteristics were selected as case studies. The principal results are as follows. (1) Breaking the assumption of a uniform precipitation distribution, the proposed DMD-POD method is effective in capturing design storms under current climatic conditions and is sufficiently flexible to adapt to climate change. (2) The low-order representation of the rainfall field indicates substantial change in the storm patterns in urban areas. The peak discharge in New York and Wuhan is almost 10% higher than before urbanization, while that in Sydney and London is more than 10% lower on average. The peak time is largely unchanged in New York and Sydney, while it is 4 and 5 h later in London and Wuhan, respectively. (3) Compared with the K-C storm and P-C storm, the flood peak of POD storm increases and appears slightly earlier. The flood peak time in Wuhan simulated with the POD storm is 1 h (2 h) earlier than that simulated with the K-C storm (P-C storm). The peak flow obtained by the POD storm is 9.55% (25.05%) greater than that obtained by the K-C storm (P-C storm), which means that the POD design storm demands a higher level of safety for an engineering project under the same return period.