Low impact development (LID) facilities and the urban drainage network jointly determine the performance of an urban drainage system, but the synergistic interaction between them has been characterized only for a single case catchment with a static sewer network and one LID type. The open problem is to generalize this characterization: to determine how the synergistic effect between LID placement and the network depends on sewer topology, conduit capacities, design return periods, and the choice between combined and separate systems, how it behaves under broader rainfall features (duration and spatio-temporal patterns) and other LID measures and cities, and to consolidate the result into transferable placement rules within a systematic optimization framework for urban drainage planning.

Background

Low impact development (LID) denotes decentralized stormwater measures — green roofs, permeable pavement, bioretention cells, rain gardens — that retain, infiltrate, and delay runoff near its source, in contrast with 'grey' conveyance infrastructure (the piped sewer network). The urban drainage system (UDS) composed of LID facilities plus the drainage network must simultaneously serve several functions: reducing urban flooding (its social performance), limiting combined sewer overflow (CSO — the spillage of untreated stormwater–sewage mixture that occurs when a combined sewer's capacity is exceeded, its environmental performance), and limiting shock loading to the wastewater treatment plant (WWTP), i.e. the excess inflow above dry-weather operating conditions that degrades treatment (its technological performance). In China this agenda is part of the Sponge City Program, and internationally of green–grey (or blue–green–grey) infrastructure planning.

Most LID studies before 2019 evaluated LID practices by their own scale, type, coverage, or local placement while treating the drainage network as a fixed background, so it was unclear whether LID performance depended only on the facilities themselves or also on their spatial relationship to network hydraulics and topology. Zeng, Guo and Dong (2019, Journal of Environmental Management 246:849–859) addressed this with Storm Water Management Model (SWMM) simulations — SWMM being the standard one-dimensional hydrological–hydraulic model for urban runoff and sewer routing — of a 10.4 km² catchment in Kunming, China with a hybrid combined/separate sewer system. They compared four green-roof siting strategies of equal total LID area (facilities placed only in upstream, midstream, or downstream sub-catchments, or distributed uniformly), scoring each by total flooding volume, total CSO volume, and WWTP shock loading, and sweeping rainfall intensity from 1 to 80 mm/h. They confirmed that a synergistic effect exists — the spatial relationship between LID placement and network characteristics can aggravate or mitigate conflicts among the three functions — that no siting strategy dominates across all three criteria, and that rankings shift with rainfall intensity and with the spatial distribution uncertainty of LID allocation.

The study's own limitations define the open problem. The drainage network was held static, so how the synergy changes when the sewer topology, conduit capacities, design return periods (the average recurrence interval of the design storm), or the combined-versus-separate system choice vary is unknown; only green roofs in one Kunming catchment were tested, so transferability to other cities, land-use patterns, and LID types is unestablished; rainfall duration was pinned at one hour with a uniform temporal pattern, leaving duration and spatio-temporal storm structure unresolved; and the authors note that the combination of LID characteristics and network characteristics is too large to treat exhaustively, leaving the generalization of placement rules into a full optimization framework open.

Problem Statement

The problem, as left open by Zeng, Guo and Dong (2019), is to generalize the demonstrated synergistic interaction between LID facility placement and the drainage network beyond a single static case. Specifically: determine how the synergistic effect between LID siting and the drainage network — measured across the social, environmental, and technological performance of the urban drainage system (total urban flooding volume, total combined sewer overflow volume, and shock loading to the wastewater treatment plant) — changes when (i) the sewer network itself varies in topology, conduit capacities, and design return period, and between combined, separate, and hybrid systems; (ii) rainfall features beyond intensity vary, namely duration and spatio-temporal patterns; and (iii) the LID type, land-use pattern, and city vary. The overarching objective is to consolidate these results into transferable placement rules and a systematic optimization framework for LID siting in diverse urban drainage systems, replacing the single-catchment, single-LID-type, static-network evidence base with a general characterization a planner can apply to a given network and storm climate. All three axes (i)–(iii) lie within scope of this generalization, and the transferable placement rules and optimization framework are the intended synthesis of the axis results rather than a separate stand-alone deliverable; a generalization established across a representative range of networks, rainfall features, LID types, and cities fulfils the objective, and exhaustive coverage of every possible combination is not required.

References

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