Directly Connected Impervious Area Research Articles

An urban hydrological model for flood simulation in piedmont cities: Case study of Jinan City, China

Urbanization has great potential to adversely affect the local hydrological cycle, as well as result in severe ecological and environmental problems. Piedmont cities have a special topography and complicated overland flow processes, thus creating challenges for the simulation of urban flooding/waterlogging processes. The TVGM-USWM model, a new distributed urban hydrological model that considers the flow routing in urban drainage systems and complicated overland flow processes, was developed in this study. We applied this model to the Huangtaiqiao drainage basin of Jinan City, China and set up two simulation scenarios. The spatial heterogeneity of the proportion of directly connected impervious area (DCIA) and disconnected impervious area (DIA) in sub-catchments was considered in scenario B, but not in scenario A. We conducted sensitivity analysis on model parameters by using the Sobol method. Results showed that: (1) model parameters have a certain degree of uncertainty, and the parameter characterizing the proportion of the DCIA and DIA showed a relatively high sensitivity under rainstorms of different levels and different accuracy assessment standards; (2) The simulation of scenario A was poor (with a mean Nash coefficient of 0.59), which ignored the different flow paths followed by runoff from DCIA and DIA; and (3) Considering the spatial differences in the overland flow process, the prediction accuracy of scenario B significantly increased (with a mean Nash coefficient of 0.82). This indicated the importance of the disconnected impervious area and the lateral water exchange across the impervious-pervious interface on the flood simulation in a piedmont city. Compared to most urban hydrological models, the TVGM-USWM model can represent complex rainfall-runoff mechanisms in urban areas with varied terrain. The findings of this study can provide scientific decision-making for the planning and construction of a sponge city in Jinan City.

Assessing the Use of Dual-Drainage Modeling to Determine the Effects of Green Stormwater Infrastructure on Roadway Flooding and Traffic Performance

Green stormwater infrastructure (GSI) is increasingly used to reduce stormwater input to the subsurface stormwater network. This work investigated how GSI interacts with surface runoff and stormwater structures to affect the spatial extent and distribution of roadway flooding and subsequent effects on the performance of the traffic system using a dual-drainage model. The model simulated roadway flooding using PCSWMM (Personal Computer Stormwater Management Model) in Harvard Gulch, Denver, Colorado, and was then used in a microscopic traffic simulation using the Simulation of Urban Mobility Model (SUMO). We examined the effect of converting between 1% and 5% of directly connected impervious area (DCIA) to bioretention GSI on roadway flooding. The results showed that even for 1% of DCIA converted to GSI, the extent and mean depth of roadway flooding was reduced. Increasing GSI conversion further reduced roadway flooding depth and extent, although with diminishing returns per additional percentage of DCIA converted to GSI. Reduced roadway flooding led to increased average vehicle speeds and decreased percentage of roads impacted by flooding and total travel time. We found diminishing returns in the roadway flooding reduction per additional percentage of DCIA converted to GSI. Future work will be conducted to reduce the main limitations of insufficient data for model validation. Detailed dual-drainage modeling has the potential to better predict what GSI strategies will mitigate roadway flooding.

How does increasing impervious surfaces affect urban flooding in response to climate variability?

Total impervious area (TIA) is one of the most common measures for predicting runoff yield in hydrologic studies and regulating urbanization in land use policy. Directly connected impervious area (DCIA), a subset of TIA, represents the hydraulic connection between development and underground sewer systems. Which indicator to use in runoff prediction has been subject to debate. The effectiveness of TIA and DCIA in the face of climate variability also remains unclear. The present study empirically assessed the impacts of TIA and DCIA on urban runoff in three metropolitan statistical areas (MSAs) in the U.S. state of Texas. The total monthly runoff depths of 92 watersheds and peak flows of 43 watersheds were computed using streamflow monitored by the U.S. Geological Survey from 2010 to 2017. A series of ordinal logit regression models were developed to determine which imperviousness indicator better predicted probable runoff yield. Additionally, an average marginal effect analysis was performed to investigate how TIA and DCIA responded to changing precipitation depths. The results demonstrate that DCIA outperformed TIA in runoff depth prediction, whereas TIA predicted peak flow better than DCIA. However, for far-above-average runoff, the contribution of DCIA to runoff depth was far greater than that of TIA, but no difference was found for peak flow. The effectiveness of TIA and DCIA also varied by total and 24-hour peak depths of monthly precipitation. After reaching their maximum capacity, both TIA and DCIA became less effective in predicting runoff and did not correlate with rainfall depth in extremely wet months. Meanwhile, the control of TIA and DCIA for runoff volume reduction was most effective for monthly rainfall of a 5% to 10% probability of exceedance in all MSAs, whereas that of peak flow reduction was most effective if the 24-hour peak storm in a month had a 2% to 5% probability of exceedance. The findings of this study demonstrate the hydrologic significance of regulating DCIA over TIA for high-risk runoff under certain rainfall depths and return periods. The study expands the current knowledge of urban hydrology for effective stormwater management and mitigation of future flooding risk.

Sensitivity of Streamflow Metrics to Infiltration‐Based Stormwater Management Networks

AbstractAs stormwater control measures (SCMs) capture surface runoff from impervious areas, a shift in the water balance and flow regime components may emerge in urban watersheds, but the amount of SCM treatment needed to detectably shift these components may vary. We used the Soil and Water Assessment Tool (SWAT) hydrologic model to assess the sensitivity of 16 hydrologic metrics as an increasingly dense rain garden SCM network was applied across the West Creek watershed, near Cleveland, Ohio (USA). As the area treated by SCMs increased, annual baseflow increases matched decreases in surface runoff, while water yield and evapotranspiration changes remained small. The stream's peak response to rainfall decreased with SCM implementation across storm sizes, ranging from the threshold rainfall depth (4.8 mm) to values higher than the design storm of a single rain garden (19 mm). SCM networks draining >20% of directly connected impervious area (DCIA) significantly decreased the magnitude of discharges with a return period of less than 1 year, the percentage of time above mean flow, and flashiness. Recession slopes and annual 1‐ and 7‐day low flows exhibited a slight response that fell within uncertainty limits of the model. Water balance and rainfall response metrics exhibited the greatest sensitivity to different intensities of stormwater management, while infrequent high and low flows were resistant to detectable change even at high levels of SCM treatment when model uncertainty was included.

Determination of runoff response to variation in overland flow area by flow routes using UAV imagery

Surface runoff can be routed to both pervious areas (PAs) and drainage systems during an overland flow. Excessive runoff from an impervious area (IA) flowing into a drainage system causes an overload, which can be relieved by diverting runoff to PAs. However, the hydrological link between IAs and PAs, especially the runoff response to variation in overland flow areas (OFAs), has not well been considered in runoff simulations due to the complexity of routing. To understand how the OFA within an IA contributes to runoff generation, a novel classification approach was applied to categorize the IA in a study area in Nanjing University, Xianlin campus, China into directly connected impervious area (DCIA) and indirectly connected impervious area (ICIA) by flow routes using high-resolution ground-based images (0.5 m) from an unmanned aerial vehicle. The OFAs then include DCIA and the total impervious area (TIA), which is the sum of DCIA and ICIA. The runoff simulations were supported by Storm Water Management Model (SWMM) calibrated with observed rainfall and runoff data. The resulting proportions of DCIA and ICIA of the study area were 34.13% and 10.99%, respectively. The spatial distributions of DCIA and ICIA are characterized by the subcatchment landscape heterogeneity resulting from vegetation, imperviousness, and slope. The observed runoff coefficients and peak flows were positively correlated with the percentages of DCIA and TIA. The runoff coefficient was significantly correlated with the DCIA in a light rainfall event of 18.4 mm (R2 =0.82) and with the TIA in a heavy rainfall event of 119 mm (R2 =0.92). Runoff generation is affected by both the characteristics of the rainfall event and the accompaning variations in OFAs. Results indicate that increasing of the flow connectivity from IAs to PAs and increasing the water retention capacity of PAs may be effective strategies for optimizing landscape patterns for stormwater management.

Low-impact development for impervious surface connectivity mitigation: assessment of directly connected impervious areas (DCIAs)

Urbanization increases directly connected impervious area (DCIA), the impervious area that is hydraulically connected to downstream drainage by closed pipelines. Although the benefits of low-impact development (LID) have been examined in other studies, its effect on alleviating DCIA levels has seldom been assessed. This study measured the DCIA of urban watersheds in Houston, TX, USA. Five land-use types were categorized and the contribution of LID facilities to reducing DCIA in each type was estimated by using Sutherland's equations. The results showed (1) DCIA in commercial areas was greater than that in residential areas, especially for big-box retailers; (2) the percentage of DCIA reduction by LID varied by land-use type; and (3) optimal combinations of LID application could maximize the effectiveness of DCIA reduction. The results contribute to prioritizing land-use type for implementing LID practices and providing local governments with a useful measure to estimate runoff volume.

Exploring the Linkage between Urban Flood Risk and Spatial Patterns in Small Urbanized Catchments of Beijing, China.

In the context of global urbanization, urban flood risk in many cities has become a serious environmental issue, threatening the health of residents and the environment. A number of hydrological studies have linked urban flooding issues closely to the spectrum of spatial patterns of urbanization, but relatively little attention has been given to small-scale catchments within the realm of urban systems. This study aims to explore the hydrological effects of small-scaled urbanized catchments assigned with various landscape patterns. Twelve typical residential catchments in Beijing were selected as the study areas. Total Impervious Area (TIA), Directly Connected Impervious Area (DCIA), and a drainage index were used as the catchment spatial metrics. Three scenarios were designed as different spatial arrangement of catchment imperviousness. Runoff variables including total and peak runoff depth (Qt and Qp) were simulated by using Strom Water Management Model (SWMM). The relationship between catchment spatial patterns and runoff variables were determined, and the results demonstrated that, spatial patterns have inherent influences on flood risks in small urbanized catchments. Specifically: (1) imperviousness acts as an effective indicator in affecting both Qt and Qp; (2) reducing the number of rainwater inlets appropriately will benefit the catchment peak flow mitigation; (3) different spatial concentrations of impervious surfaces have inherent influences on Qp. These findings provide insights into the role of urban spatial patterns in driving rainfall-runoff processes in small urbanized catchments, which is essential for urban planning and flood management.

Improved methods to estimate the effective impervious area in urban catchments using rainfall-runoff data

Summary Impervious surfaces are useful indicators of the urbanization impacts on water resources. Effective impervious area (EIA), which is the portion of total impervious area (TIA) that is hydraulically connected to the drainage system, is a better catchment parameter in the determination of actual urban runoff. Development of reliable methods for quantifying EIA rather than TIA is currently one of the knowledge gaps in the rainfall-runoff modeling context. The objective of this study is to improve the rainfall-runoff data analysis method for estimating EIA fraction in urban catchments by eliminating the subjective part of the existing method and by reducing the uncertainty of EIA estimates. First, the theoretical framework is generalized using a general linear least square model and using a general criterion for categorizing runoff events. Issues with the existing method that reduce the precision of the EIA fraction estimates are then identified and discussed. Two improved methods, based on ordinary least square (OLS) and weighted least square (WLS) estimates, are proposed to address these issues. The proposed weighted least squares method is then applied to eleven urban catchments in Europe, Canada, and Australia. The results are compared to map measured directly connected impervious area (DCIA) and are shown to be consistent with DCIA values. In addition, both of the improved methods are applied to nine urban catchments in Minnesota, USA. Both methods were successful in removing the subjective component inherent in the analysis of rainfall-runoff data of the current method. The WLS method is more robust than the OLS method and generates results that are different and more precise than the OLS method in the presence of heteroscedastic residuals in our rainfall-runoff data.

How does imperviousness impact the urban rainfall-runoff process under various storm cases?

Dramatic changes in imperviousness exert significant influence on the rainfall-runoff process in urban catchments. In urban rainwater management, imperviousness is generally adopted as an effective indicator for assessing potential runoff risk. However, the effects of imperviousness on rainfall-runoff at the scale of small urbanized drainage areas have not been fully determined, particularly when various storm characteristics are considered. In this paper, a model-based analysis is conducted in a typical urban residential catchment in Beijing, China, in which 69 subareas are delineated within the catchment as the basic drainage units. Two metrics, total impervious area (TIA) and directly connected impervious area (DCIA), are employed to quantify the spatial characteristics of imperviousness of the subareas. Three runoff variables within the delineated subareas including total runoff depth (Qt), peak runoff depth (Qp), and lag time (Tlag) are simulated by using the Storm Water Management Model (SWMM) to represent the specific rainfall-runoff characteristics. Moreover, model input storms are designated to several typical flood-induced rainfall events with varying amounts, locations of rainfall peak, and durations for holistic assessment of imperviousness. Regression analyses are conducted to explore the contributions and relative significances of impervious metrics in predicting runoff variables under various storm cases. The results indicate that the performances of imperviousness with fine spatial scale (<1ha) and heavy rainfall conditions (>34mm) may vary markedly according to storm conditions. Specifically, TIA rather than DCIA acts as a dominate factor affecting total runoff, and its significance maintains relatively stable with various storm conditions. In addition, the combined use of both TIA and DCIA are more effective for predicting peak runoff than that using a single impervious metric; however, rainfall amount, peak location, and duration alter the contribution gaps between TIA and DCIA and the overall performance of the regression model. Moreover, DCIA is more likely to affect runoff lag time without the contribution of TIA; however, an increase in rainfall peak ratio or duration will significantly limit its performance. These results can provide insight into the hydrologic performance of imperviousness, which is essential for landscape design and runoff regulation in small urban catchments.

Retrofitting with innovative stormwater control measures: Hydrologic mitigation of impervious cover in the municipal right-of-way

Summary Impervious Cover (IC) has been shown to increase runoff volumes, peak discharges and pollutant loads to streams, which leads to degraded water quality and biological integrity. Stormwater Control Measures (SCMs) have been developed to mitigate the hydrologic and water quality impacts of urban areas and IC. This paired watershed study evaluated the impacts of SCM retrofits on hydrology for a small urban drainage area. In February 2012, a bioretention cell (BRC) street retrofit, four permeable pavement parking stalls and a tree filter device were installed to control and treat residential street runoff in Wilmington, North Carolina, USA. In the SCM-Retrofit catchment, 52% of the directly connected impervious area (DCIA) and 69% of the total drainage area was retrofitted for potential hydrologic mitigation. Underlying soils in the study area were urban sands. Peak discharge significantly decreased by 28%, while lag times in the catchment remained unchanged. Runoff depth significantly decreased by 52%. When compared to the control catchment, runoff depths in the SCM-Retrofit catchment were significantly less for events with low hourly rainfall intensities ( 7.4 mm/h). During post-retrofit monitoring, runoff thresholds in the SCM-Retrofit and control catchments were 5.2 mm and 3.5 mm, respectively. The SCM-Retrofit runoff coefficient decreased from 0.38 to 0.18 and was substantially less than other runoff coefficients reported in the literature for conventional residential development. This study illustrated how a limited number of SCM retrofits installed within the public right-of-way can mitigate some of the hydrologic impacts of existing residential development.

Retrofitting Residential Streets with Stormwater Control Measures over Sandy Soils for Water Quality Improvement at the Catchment Scale

Impervious cover (IC) has been shown to increase runoff volumes, peak discharges, and pollutant loads to streams, which can lead to degraded water quality and biological integrity. Stormwater control measures (SCMs) have been developed to mitigate the hydrologic and water quality impacts of urban areas and IC. This paired watershed study evaluated the impacts of multiple SCM retrofits on water quality at a catchment scale in a 0.53 ha urban residential drainage area. In February 2012, an in-street bioretention cell (BRC) retrofit, four permeable pavement parking stalls, and a tree filter device were installed to treat residential street runoff in Wilmington, North Carolina. In the retrofitted catchment, 94% of the directly connected impervious area (DCIA) and 91% of the total drainage area were retrofitted for water quality treatment. Underlying soils in the study area were sand. After the SCM retrofits were constructed, concentrations of total Kjeldahl nitrogen (TKN), total phosphorous (TP), total suspended solids (TSS), copper (Cu), lead (Pb), and zinc (Zn) significantly decreased by 62%, 38%, 82%, 62%, 89%, and 76%, respectively. Concentrations of dissolved pollutants [nitrate-nitrite-nitrogen (NO2,3-N), total ammoniacal nitrogen (TAN), and orthophosphate (O-PO43−)] did not change. Mass exports of TKN, TAN, O-PO43−, TP, TSS, Cu, Pb, and Zn significantly decreased by 79%, 60%, 54%, 72%, 91%, 54%, 88%, and 77%, respectively. Improvements in water quality were due to decreases in particulate and particulate-bound pollutant concentrations and loads. This study has shown that a limited number of SCMs installed within a street right-of-way can mitigate a substantial portion of the water quality impacts caused by existing residential development.

Comparison of Runoff Quality and Quantity from a Commercial Low-Impact and Conventional Development in Raleigh, North Carolina

Urbanization and its associated increased impervious footprint lead to stream impairment through erosion, flooding, and augmented pollutant loads. Low-impact development (LID) focuses on disconnecting impervious areas, increasing infiltration and evapotranspiration, and treating storm water on site through the use of storm water control measures (SCMs). In this study, a conventional development (centralized storm-water management) and an adjoining infiltration-based LID commercial site in Raleigh, North Carolina, were compared with respect to hydrology and water quality. The conventional development [2.76 ha, 61% directly connected impervious area (DCIA)] and the LID (2.53 ha, 84% DCIA) had underlying hydrologic soil group B soils. The LID was treated by a mix of green (aboveground) and grey (underground) infrastructure including an underground detention chamber and infiltration gallery, underground and aboveground cisterns, and aboveground swales and bioretention; the conventional development was treated with a dry detention basin and swales. Inflow and outflow runoff volumes and peak flows were normalized by DCIA. For the 47 hydrologic storms monitored, runoff coefficients of 0.02 at the LID site and 0.49 at the conventional site were recorded. The conventional development had an 11-fold higher median peak flow rate than the LID site. For the three storms more intense than the 10-year, 5-min average recurrence interval (ARI) event, the conventional site Qp was an average of 7.7 times higher than that of the LID. Flow proportional, composite water-quality samples were analyzed for total nitrogen (TN), total phosphorus (TP), total Kjeldahl nitrogen (TKN), total ammoniacal nitrogen (TAN), nitrite-nitrate nitrogen (NOX), organic nitrogen (ON), orthophosphate (Ortho-P), and total suspended solids (TSS). Generally, no significant difference in pollutant event mean concentrations (EMCs) was observed between sites. For the 20 water-quality storms sampled, the LID site produced pollutant loadings that were less than 5% of those at the conventional site for all species studied. Results demonstrated highly effective and space-saving solutions when green and grey infrastructure are merged, which is often the case when constructing on high land-cost properties. The exceptional results from this LID were due to (1) an overdesigned system capable of capturing the 77-mm storm, rather than a typical 25-mm storm, and (2) the high infiltration capacity of the type B soils coupled with a high driving head (∼3 m).

Assessing Impervious Surface Connectivity and Applications for Watershed Management1

Abstract: Although total impervious area (TIA) is often used as an indicator of urban disturbance, recent studies suggest that the subset of impervious surfaces that route stormwater runoff directly to streams via stormwater pipes, called directly connected impervious area (DCIA), may be a better predictor of stream ecosystem alteration. We evaluated the differences between TIA and DCIA in the Shepherd Creek catchment, a small (1.85‐km2), suburban basin in Cincinnati, Ohio. Imperviousness determinations were calculated based on publicly available geographic information system (GIS) data and parcel‐scale field assessments, and these direct assessments were compared to DCIA calculated from published, empirical relationships. Impervious and semi‐impervious area comprised 13.1% of the catchment area, with 56.3% of the impervious area connected. When summarized by subcatchments (0.26‐1.85 km2), TIA measured in the field (11‐23%) was considerably higher than that calculated from the National Land Cover Data Imperviousness Layer (7‐18%). In contrast, TIA calculated based on aerial photos was similar to TIA calculated from field assessments, thus indicating that photo interpretation may be adequate for catchment‐scale (&gt;25 ha) TIA determinations. While these GIS data sources can be used to calculate TIA, on‐site assessments were necessary to accurately determine DCIA within residential parcels. There was a wide variation in percent connectivity across parcels, and, subsequently, DCIA was not accurately predicted from empirical relationships with TIA. We discuss applications of DCIA data that highlight the importance of parcel‐scale field assessments for managing suburban watersheds.

Estimation of Urban Imperviousness and its Impacts on Storm Water Systems

Imperviousness is an important indicator of the impact of urbanization on storm water systems. A hydrologic analysis was performed to evaluate long-term impacts from an apartment area in Miami. The result shows that the directly connected impervious area (DCIA), which covers 44% of the catchment, contributes 72% of the total runoff volume during 52 years. Few studies have actually measured the DCIA with a high level of accuracy for residential areas that constitute the largest proportion of urban land. A detailed analysis of urban imperviousness was performed using geographic information systems and field investigations on a 5.81 ha residential area in Boulder, Colo. For this study area, the total impervious area is 35.9% and the DCIA is 13.0%. Transportation-related imperviousness comprises 97.2% of the DCIA. Hydrologic modeling of this area shows about a 265% difference in estimates of peak discharge with imperviousness measured at five different levels of accuracy. These results suggest the need to focus on DCIA as the key indicator of urbanization’s effect on storm water quantity and quality.