As global urbanization reaches unprecedented levels, there is a need to plan, design, and update infrastructure systems in cities, including water infrastructure. Urban infrastructure is likely to face a significant increase in weather-related risks due to factors such as climate change [1
]. Against this backdrop, urban designers have been increasingly looking to green infrastructure (GI) as a distributed way to retain and detain stormwater throughout an urban watershed. By slowing down the flow of runoff within the watershed, natural hydrologic processes such as infiltration, recharge, and evapotranspiration can be restored into the urban landscape, reducing the rate and volume of runoff (and associated pollutants) discharged to collection systems and the receiving water-bodies to which they drain.
An important question for urban drainage engineers is the rate at which runoff can be routed through engineered inlets into GI systems. Although tributary area characteristics such as pavement depression storage have been found to be important [2
], inlets are also key determinants of GI performance. Inefficient or clogged inlets can cause localized street flooding [3
] while underutilizing the engineered storage capacity of individual GI facilities [4
]. To the extent that GI is a key part of the urban drainage planning, they are also critical drivers of the hydrologic performance of the entire urban watershed.
Though municipal GI programs are well underway, research to improve GI facility design is minimal. There has been extensive laboratory research focusing on highway stormwater inlets [5
], but because their geometry differs from typical GI inlets, the transferability of these findings is limited. Additionally, most highway inlet research was conducted in laboratories under controlled conditions that are quite different from typical GI field conditions.
Flow through GI inlets can be modeled, though work in this area has also been minimal. Normative hydrologic and hydraulic (H&H) modeling tools used by urban drainage engineers—such as the stormwater management model—(SWMM) generally assume 100% efficient inlets [9
]. However, field observations indicate that the momentum of the approaching flow tends to carry it past the inlet opening [6
]. This discrepancy in observed and modeled inflow rates and volumes can result in inaccurate hydrologic mass balances, with implications on understanding of particulate matter transport [10
] and other unit operations in the urban watershed.
In the US, where vertical curbs are common, GI facilities are often engineered into the sidewalk with flow into them passing through a “curb-cut,” often modeled as flow over a side weir. However, when water flows out of a channel over a side weirs it induces changes in flow pattern, velocity distribution, and mass transport in the main channel, e.g., gutter, [11
] that make analytical solutions of the governing equations [12
] difficult to solve. Some researchers have compared analytical solutions with laboratory experiments. Focusing on rectangular or trapezoidal weirs, Madjid Delkash and Babak Ebrazi Bakhshayesh [14
], for example, modified DeMarchi’s equations derived from energy and momentum conservation to describe the flow regime. These equations, which utilize dimensionless parameters and the upstream Froude number to compute the coefficient of discharge, would need to be significantly modified to represent flow through actual GI inlet configurations, which have transverse bed slopes and are also subject to routine clogging.
With advances in computational capacity, researchers have recently begun using three-dimensional (3D) computational fluid dynamic (CFD) models to simulate inlet hydraulics as a cost-effective alternative to laboratory studies and analytical equations [15
]. These researchers [9
] have studied inlets, catch basins, and manholes with different geometries, using laboratory data for validation. CFD simulations of highway inlets suggest that upstream Froude number, flow rate, and profile (depth at various upstream locations), velocity and depth at the start of the inlet, and spread are the key physical determinants of inlet efficiency [13
]. These studies prove that CFD models can be used to simulate a range of physical conditions and better understand the impact of physical conditions on specific flow processes, for example, the hydraulics of a GI inlet.
However, while CFD models can be useful in examining specific simulated flow processes in detail, a practical application of these studies requires real-world validation of the insights these models provide [10
]. To date, CFD models are rarely compared to real-world experimental data. None of the above-mentioned studies utilized field observations in model (physical or CFD) construction, calibration, and validation. One advantage of field experiment data over laboratory experiments is that these observations include the full range of variability encountered in the real world. In the present context, field experiments can help study how actual morphological conditions of GI tributary areas and inlets, such as roughness, clogging, and as-built construction characteristics are related to inlet inflow and bypass.
Taking advantage of the increasingly recognized value of CFD models as a cost-effective means of better understanding key flow processes, this paper develops, calibrates, and validates a CFD model to describe field conditions at an actual GI facility. The model domain was constructed using surveyed field data, and the model was calibrated and validated using five measurements made in the field: The (i) inflow rate, (ii) intercepted flow rate, (iii) upstream velocity, (iv) upstream flow depth, and (v) flow conditions immediately upstream and downstream the inlet. A sensitivity analysis of the validated model was used to investigate how inlet performance would vary under field conditions that differ from those found at this particular site.
The paper is organized as follows. The procedure to gather observed data for these five parameters is described in Section 2.2
. The model development is included in Section 2.3
. Section 3
contains a comparison of the simulation results and observed data. Finally, the inlet hydraulics, effects of clogging, and increases in flow rate are discussed in Section 6
The study confirms that hydraulics play an important role in GI performance. This study underscores the importance of inlet clogging and other morphological differences in determining inlet efficiency at a range of flow rates. The research determined that inlet efficiency is likely to be lower at higher flow rates, as would be expected at higher tributary areas and increasing precipitation intensity. The research also reveals that inlet efficiency is reduced due to inlet clogging, emphasizing the importance of adequate maintenance of inlets in urban environments.
The field study underscores the importance of quality control during and after GI construction, and of careful monitoring of street resurfacing. At this site, field conditions differed significantly than those prescribed on the design, suggesting that performance predictions made based on the design drawings would deviate significantly from actual.
The CFD models were able to reasonably replicate inlet hydraulics and inlet efficiency. For sites with complex tributary area characteristics, CFD models thus offer a viable alternative to laboratory studies.
In conjunction with the CFD models, the hydrant tests were an effective approach to inlet monitoring that did not require installation of expensive on-site instrumentation. The hydrant test provided a controlled environment in the field that helped to calibrate and validate the model.
The research suggests that under real world conditions, stage discharge curves provided by flume manufacturers, developed in controlled laboratory environments, could deviate significantly from observed. In situ verification is recommended.
To increase the ability of GI systems to adapt to climate change, inlet length and apron should be designed to intercept 100% of the presented inflow during flow rates specified by local policy.
A comparison of the model validation attempts at the two inlets suggests that milder longitudinal street slopes (e.g., <1.5%) can cause silting upstream of the inlet. GI on mild street slopes should be designed with an inlet that minimizes silt deposition. If curb-cut inlets are used in such locations, they should have both additional apron depth in front of the inlet and into the GI to maintain a free outfall condition.
Inlet performance is reduced significantly with higher clogging under intense flow rates. Sediment deposition in the inlet channel and in front of the inlet (apron section) and clogging of the inlet with ordinary trash can both reduce inlet performance, especially under high intense rainfall conditions. The inlet and upstream tributary catchment areas should be cleaned frequently during the fall to remove leaves to improve the inlet performance.
Studies on other inlet design parameters such as apron design, inlet length under various street morphological conditions (transverse and longitudinal slope), and range of flow rate should be conducted to formulate design specifications for GI inlets to increase their hydraulic performance.
Differences in inlet performance suggest that efforts to upscale GI performance linearly could introduce significant errors into watershed estimates of stormwater capture.
GI design must consider local morphological conditions such as street slopes, inlet lengths, and inlet geometry. While upscaling the model results, inlet efficiency should be included. Sewers and canals are designed to have non-silting and non-scouring velocities. Such minimum and maximum velocity at the planned GI locations should be determined. The minimum velocity upstream of the GI inlet should be in the range of 0.6 to 0.9 m/s. GI on flatter street slopes or streets with undulations should be fitted with non-clogging inlets. Such GI with a lower upstream velocity should be fitted with rectangular inlets with free outfall conditions [5