Methodology to simulate unsaturated zone hydrology in Storm Water Management Model (SWMM) for green infrastructure design and evaluation.

Min-Cheng Tu,Bridget Wadzuk,Robert Traver,Vassilis G Aschonitis

doi:10.1371/journal.pone.0235528

Min-Cheng Tu, Bridget Wadzuk + Show 2 more

Open Access

https://doi.org/10.1371/journal.pone.0235528

Copy DOI

Abstract

Hydrologic models such as the USEPA Stormwater Management Model (SWMM) are commonly used to assess the design and performance of green infrastructure (GI). To accurately represent GI performance models used in design need to be able to address both the hydrology/hydraulics of the catchment and the GI unsaturated (vadose) zone hydrology. While hydrologic models, such as SWMM, address the need for catchment hydrology/hydraulics, they often simplify the unsaturated zone hydrology. This paper presents a methodology utilizing existing components of SWMM to represent unsaturated zone hydrology in an accessible format that does not require adjustments to the SWMM source code. The methodology simulated the unsaturated soil water movement by considering flow caused by differences of soil matric head and flow caused by gravity between soil layers with finite depth/length. The flow flux related to the soil matric head is a function of soil water diffusivity (D) and the soil moisture gradient, where D can be represented by a pump curve in SWMM. The flow flux related to gravity was controlled by unsaturated hydraulic conductivity (K) only and was also simulated by a pump. The methodology was compared to another variably saturated model, HYDRUS, with theoretical soils (with single layers of sand, loam, silt, and clay, as well as dual-layer scenarios). Field data was used to compare the methodology to HYDRUS and the SWMM LID (Low Impact Development) module. In all comparisons the presented methodology and HYDRUS delivered similar results for the vadose zone response to a storm event, while the LID module of SWMM exhibited slower water movement. The results showed that under natural conditions, the approximation of the presented methodology yielded satisfactory results to simulate flow through the unsaturated vadose zone.

Highlights

Green Infrastructure (GI) is an effective tool to mitigate urban nonpoint source pollution and restore the natural water cycle, which incorporates media and plants that utilize all aspects of the hydrologic cycle and plant transpiration to maximize water removal
The proposed methodology was compared to HYDRUS 1-D for eight GI storm response scenarios shown in Fig 7 and Table 1
The soil moisture results of HYDRUS-1D were averaged from five observation points to represent the mean soil moisture of a soil block, results were comparable to the proposed Stormwater Management Model (SWMM) methodology

Summary

Introduction

Green Infrastructure (GI) is an effective tool to mitigate urban nonpoint source pollution and restore the natural water cycle, which incorporates media and plants that utilize all aspects of the hydrologic cycle and plant transpiration to maximize water removal. Recognizing the benefits of GI, many cities have started to adopt various types of GI at the city-wide scale [8,9], there remains a need for models that represent the vadose zone hydraulics to aid in design and understanding of system behavior. To accurately simulate soil moisture in the unsaturated (vadose) zone by the Richards equation [16,17,18] and Van Genuchten relation [19] during GI design and/or evaluation is the first challenge. A second challenge is that due to the nature of nonpoint source control, strategically placing GI on a large, watershed scale is crucial. The final challenge, which aligns with the future trend of stormwater management, is that the model should be compatible with the emerging trend of smart realtime control stormwater systems [20]

Objectives

Results

Conclusion