Journal of Hydrologic Engineering

The impervious cover model (ICM) has attracted considerable attention in recent years, with nearly 250 research studies testing its basic hypothesis that the behavior of urban stream indicators can be predicted on the basis of the percent impervious cover in their contributing subwatershed. The writers conducted a meta-analysis of 65 new research studies that bear on the ICM to determine the degree to which they met the assumptions of the ICM and supported or did not support its primary predictions. Results show that the majority of research published since 2003 has confirmed or reinforced the basic premise of the ICM, but has also revealed important caveats and limitations to its application. A reformulated conceptual impervious cover model is presented in this paper that is strengthened to reflect the most recent science and simplify it for watershed managers and policy makers. A future challenge is to test the hypothesis that widespread application of multiple management practices at the catchment level can improve the urban stream degradation gradient that has been repeatedly observed by researchers across the country.

Changes in impervious cover result in changes in both urban water quality and quantity. The changes in storm-water quantity have been studied for many years; however, the impacts of impervious cover on storm-water quality are not as well understood. This paper examines the impacts of impervious cover on both storm-water quality loads and concentrations using data collected from 38 storm-water monitoring stations in Austin, Tex., between 1985 and 2005. The writer determined whether there was a significant relationship between impervious cover and the mean concentrations of several pollutants, including nutrients, metals, solids, and bacteria. In cases where no significant relationship existed, the data were examined to determine whether a difference in storm-water runoff quality from developed and undeveloped areas existed. A final comparison was performed using impervious cover and land use as independent variables and mean watershed concentrations of several common pollutants as dependant variables. This paper reports a complete methodology to estimate mean storm-water concentrations and annual loads in the Austin, Tex., area. This methodology was also tested for predicting event loads. While the event predictions were poor, the results were similar to other event loads prediction methods.

Field measurements indicate a correlation between increased impervious cover and decreased stream base flow in and near the White Clay Creek Wild and Scenic River watershed near Newark, Del. A stream base flow monitoring network was established in 19 watersheds near the University of Delaware campus. The watersheds have land uses varying from heavily forested to highly urbanized with impervious cover ranging from 3 to 44%. Using geographic information system land use mapping, watershed impervious cover was estimated based on the ratio of pavement and roof area for each land cover condition. Stream base flows were calculated using the continuity equation $(Q=\nu A)$ from velocity and channel cross-section area measurements recorded on $5\text{days}$ during 2006 and 2007. Results from all five events indicate increased watershed impervious cover correlates with decreased stream base flows. For the five events, the coefficients of determination $\left(R^2\right)$ based on linear regression of impervious cover and base flow data are 0.33, 0.35, 0.32, 0.46, and 0.58; evidence of fair to reasonably good correlation. Increased watershed imperviousness can result in dwindling drinking water and aquatic resource flows especially during drought periods. Water resource protection area ordinances, recharge augmentation, and pavement reduction techniques are available to reduce the impacts of impervious cover on watershed hydrology.

The purpose of this study was to collect data to estimate curve number (CN) values for concrete and asphalt surfaces and to determine how these values are affected by rainfall intensity and duration, and pavement slope. Two concrete and two asphalt pavements of size $15.24\mathrm{m}\times 3.05\mathrm{m}$ at respective slopes of 2.2 and 4.6% and 2.4 and 5% were constructed to conduct field experiments. A water distribution system was specially designed and constructed to simulate rainfall at variable intensities. A total of 64 experiments were conducted. Rainfall intensities ranged from 0.635 to $36.29\mathrm{cm}∕\mathrm{h}$ $(0.25\text{to}14.28\mathrm{in.}∕\mathrm{h})$ , and rainfall durations ranged from 0.25 to $4\mathrm{h}$ . The CN for concrete was 100 for all conditions. The CN values for asphalt ranged from 97 to 100 with an average of 99. Statistical tests indicated that the average CN values for concrete and asphalt were not equal. As expected, there was a direct relationship between curve number and rainfall intensity and an inverse relationship between curve number; and rainfall duration. The slope did not affect the CN value for concrete but there was an inverse relationship between curve number and slope for asphalt. It is possible that this inverse relationship was due to construction or other defects in the 5% asphalt pavement. The presence of minor cracks in the concrete pavement lowered the CN value to 99 in some cases. The presence of an enhanced “porous” area in the 5% asphalt pavement, less than 1% of the total slab area, reduced the CN values by as much as 10% for relatively low-intensity events. It is likely that the presence of cracks due to construction defects or weathering, construction joints, or other construction defects can significantly reduced the CN value.

Hydrologic model estimates of runoff quantity and quality are appropriately sensitive to the extent and distribution of impervious area (IA) inputs. This research enhances land use maps of IA extracted from road networks and demonstrates its impact on hydrologic model parameters for 704 watersheds in New York. National Land Cover Database (NLCD) 1992 and 2001 data at $30\mathrm{m}\text{pixel}$ resolution provided baseline maps, and enhancement involved reclassifying pixels with overlapping high accuracy vector roads. NLCD 1992 is limited by its lack of explicit IA estimates, instead relying on wide ranges of IA for developed classes, and by the failure of its transportation class to capture most roads. NLCD 2001 has no transportation class, and its explicit IA estimates were found to underestimate IA. Road enhancement caused significant increases in hydrologic parameters such as the curve number, runoff coefficient, and event mean concentration based pollutant loads, with a $p<1\mathrm{E}\text{-}3$ for a paired, two-tailed $t$ -test. Magnitude of hydrological parameter changes increased with increasing watershed road density. Roads in the enhanced NLCD were treated as a directly connected impervious area, which caused large differences in pollutant loads generated by explicit runoff simulation. While NLCD allowed runoff to filter through wetlands and forests, enhanced NLCD directly discharged polluted road runoff and doubled phosphorus export coefficient estimated loads. This research suggests that watershed models use road enhanced NLCD to represent IA.

The objectives of this study were to characterize asphalt parking lot runoff quality and determine factors influencing nutrient concentrations and loads. Event mean concentrations (EMCs) and loads were measured from eight asphalt parking lots in North Carolina using automated flow meters and rain gauges. The number of water quality samples collected varied from 11 to 26 per site. EMCs and loads were statistically analyzed for six nutrient forms: total nitrogen, total Kjeldahl nitrogen, ammonia-nitrogen, nitrate-nitrogen, total phosphorus, and ortho-phosphate. The mean EMCs (in mg/L) were 1.57, 1.19, 0.32, 0.36, 0.19 and 0.07, respectively. Nitrogen species’ concentrations were slightly lower than those from highway runoff found in the literature; whereas, phosphorus EMCs were similar to those in highway runoff. Current load prediction models, generally based on highway or roadway nutrient concentrations, are therefore expected to over-estimate nitrogen loads from asphalt parking lots. Spring and summer presented the highest EMCs and loads, respectively. Significant seasonal differences in concentration $(\mathrm{p}<0.05)$ were found mainly between spring and the other three seasons, while loads in summer differed from those of fall and winter. In an attempt to determine the factors affecting EMCs and loads, Pearson correlation tests and multiple linear regression analyses were performed. Strong correlations were found among the variables of each group of factors referred to as climate, physical characteristics and surrounding land use. Rainfall depth, catchment area, the percentage of asphalt and natural surrounding land use were good predictors of nutrient concentrations and loads.

In this paper, we estimated impervious surface from United States census housing density data sets for the conterminous United States to examine the distribution and extent of impaired watersheds, and to estimate the risk to watersheds from development in the near future. We used regression tree methods to relate estimates of current housing density to the 2001 National Land Cover Database (NLCD) percent urban imperviousness. As of 2000, we estimate $\mathrm{83,749}{\mathrm{km}}^2$ of impervious surface (IS) cover across the United States (about 9.6% lower than the NLCD). We estimate that IS cover will expand to $\mathrm{114,070}{\mathrm{km}}^2$ by 2030. About 7% of eight-digit Hydrologic Unit Code (HUC) watersheds (3.6% of the conterminous United States) were stressed or degraded ( $>5\%$ IS) in 2000, and we estimated that this will increase to nearly double to 8.5% of watersheds by 2030 (6.3% of area). We explored the subtle differences of fine-grain pattern for different urban land use types by comparing our national estimates of IS to those developed for the Chesapeake Bay watershed. We also found important nonlinear affects of watershed scale and aggregation, whereby estimates of IS could differ by roughly ten-fold.

Given the relatively high cost of mapping impervious surfaces at regional scales, substantial effort is being expended in the development of moderate-resolution, satellite-based methods for estimating impervious surface area (ISA). To rigorously assess the accuracy of these data products high quality, independently derived validation data are needed. High-resolution data were collected across a gradient of development within the Mid-Atlantic region to assess the accuracy of National Land Cover Data (NLCD) Landsat-based ISA estimates. Absolute error (satellite predicted area—“reference area”) and relative error [satellite (predicted area—“reference area”)/”reference area”] were calculated for each of 240 sample regions that are each more than 15 Landsat pixels on a side. The ability to compile and examine ancillary data in a geographic information system environment provided for evaluation of both validation and NLCD data and afforded efficient exploration of observed errors. In a minority of cases, errors could be explained by temporal discontinuities between the date of satellite image capture and validation source data in rapidly changing places. In others, errors were created by vegetation cover over impervious surfaces and by other factors that bias the satellite processing algorithms. On average in the Mid-Atlantic region, the NLCD product underestimates ISA by approximately 5%. While the error range varies between 2 and 8%, this underestimation occurs regardless of development intensity. Through such analyses the errors, strengths, and weaknesses of particular satellite products can be explored to suggest appropriate uses for regional, satellite-based data in rapidly developing areas of environmental significance.

Impervious surfaces have been identified as a reliable indicator of the impacts of development on water resources. Research has found that imperviousness has a direct effect on local surface water as well as indirect effects on downstream receiving waters. Some of the affected characteristics of a given watershed due to the increase of impervious surfaces include hydrological impacts (the amount of runoff, peak discharge rates, and baseflow are altered), physical impacts (stream morphology and temperature are changed), water quality impacts (nutrient and pollutant loads increase), and biological impacts (stream biodiversity decreases). While attention has been focused on quantifying these relationships, little work has been done to assess the efficacy of various methods for estimating and mapping impervious surfaces. This paper assesses the results from six techniques for estimating the percent of impervious surface compared to photogrammetrically derived calibration and validation data from high spatial resolution digital planimetric datasets for 53 towns in Connecticut and New York. Impervious surface estimation layers and techniques examined include: (1) National Land Cover Dataset (NLCD) 2001 impervious surface layer, derived through regression tree classification of Landsat Enhanced Thermatic Mapper (ETM) data; (2) Connecticut’s Changing Landscape (CCL) 2002 impervious surface layer, derived through Landsat ETM subpixel classification and quantification of percent imperviousness; (3) and (4) land use-specific coefficients for two different land cover datasets, NLCD 2001 and CCL 2002, as modeled with the Impervious Surface Analysis Tool (ISAT); (5) and (6) a population density and a land cover-based regression model—Estimation Tool for Impervious Surfaces (ETIS)—using US Census Bureau population and CCL 2002 and NLCD 2001 land cover. In comparing results with the reference data, it has been found that estimates of census tract-wide imperviousness based on the direct Landsat-derived spectral methods tested yielded the lowest accuracy with an RMSE of 7.2% (NLCD) and 10.2% (CCL), respectively. The ISAT and ETIS, which leveraged readily available land cover information, and in the case of ETIS, population data, demonstrated the highest accuracy of the techniques examined with an RMSE of 4.4% (NLCD ISAT), 6.3% (CCL ISAT), 4.2% (NLCD ETIS), and 4.7% (CCL ETIS). An assessment of impervious surface estimates will provide decision makers and planners with useful information to guide them in selecting the optimal method of mapping imperviousness given their programmatic needs, available data, and technical resources.

In this paper, the variability of simulated flood discharges is evaluated using two methods for estimating impervious surfaces and common design methods in the Mission Creek watershed $\left(31{\mathrm{km}}^2\right)$ located in Santa Barbara, Calif. The two impervious estimation methods are characterized as high resolution from manually digitizing aerial photographs and medium resolution from automated interpretation of satellite data. In all cases, the high resolution fraction of impervious area is greater than the medium resolution. The difference between methods is greatest in urban regions having either small or large fractions of imperviousness (low- or high-intensity development). The largest difference observed at the modeling unit scale $(\sim 3.4{\mathrm{km}}^2)$ was 19 percentage points (26 versus 45%). To quantify discharge variability, four events (2- and $100\text{-}\mathrm{year}$ for 6- and $24\text{-}\mathrm{h}$ durations) and three models were used. The models used combinations of Green-Ampt and curve number (CN) losses and kinematic wave and unit hydrograph routing. The Green-Ampt model tended to overpredict peak discharge relative to the CN model, and kinematic wave routing and unit hydrograph agreed well except for the largest events, where the efficiency of the routing decreased travel time sufficiently to increase the peak discharge. The CN model was more sensitive to the impervious surface estimation method relative to the Green-Ampt model due to the mechanisms for incorporating imperviousness into the models. At the scale of the watershed used in this study, the effects of imperviousness estimation method on simulated peak discharge ranges from 16% for the $2\text{-}\mathrm{year}$ event to 9% for the $100\text{-}\mathrm{year}$ event. While the effects are present, they are not sufficient to shift the recurrence interval of the design discharges. At the model unit scale, the difference in peak discharge between impervious estimation methods exceeded 41 and 21% for the 2- and $100\text{-}\mathrm{year}$ events, respectively. Overall, the two impervious surface estimation methods result in different peak discharges but neither method consistently results in discharges less than flood frequency-based discharges. In terms of design risk, the results suggest that the design methods are sufficient to absorb the imperviousness variability from the two methods used here. For applications where a model is calibrated to a specific event, variability resulting from differences in estimation methods is likely to be further muted by adjusting model parameters in the calibration process.

A major impact on stream ecosystems in urban catchments is the increased frequency of runoff of poor quality storm water. This problem has not been commonly addressed in storm-water management, which usually focuses on pollutant load reduction or abatement of peak flows, or in some cases on minimization of impervious area. If stream ecosystems are to be restored or protected, a greater focus is required on the minimization of hydraulic connection between impervious surfaces and streams for small, frequent rain events. We propose new objectives for storm-water management. We suggest a primary objective of zero directly connected imperviousness for catchments where the ecological objective is to protect stream ecosystems, because the direct connection between impervious surfaces and streams is a severe stressor to stream ecosystems. We then propose an index to quantify the degree of connection of impervious surfaces, scaled to runoff frequency (retention capacity, RC), which can be used as a design objective for treatment measures. RC can be combined with total imperviousness to calculate attenuated imperviousness, analogous to effective imperviousness, but with an objective quantification of the degree of disconnection of impervious surfaces. RC emphasizes the importance of mimicking natural runoff behavior, with the greatest emphasis on small, frequent storms, to reduce the frequency of flow disturbance experienced by streams closer to the preurban frequency. It provides a practical design objective for storm-water treatment systems, which is better targeted toward the protection of stream ecosystems than the pollutant load or runoff volume reduction targets which are commonly used.

As an increasingly adopted storm water best management practice to remedy hydrologic impairment from urban imperviousness, bioretention facilities need rigorous field performance research and monitoring to confirm performance and improve design and maintenance recommendations. This study investigated hydrologic performance at six bioretention cells in Maryland [College Park (CP), a $181{\mathrm{m}}^2$ cell, $50–80\mathrm{cm}$ media depth, monitored for 22 events, and Silver Spring (SS), a $102{\mathrm{m}}^2$ cell, $90\mathrm{cm}$ media depth, monitored for 60 events] and North Carolina [Greensboro (G1 and G2), each approximately $317{\mathrm{m}}^2$ , $120\mathrm{cm}$ media depth, both monitored for 46 events, and Louisburg ( $\mathrm{L}1=\text{surface}$ area of $162{\mathrm{m}}^2$ , $\mathrm{L}2=\text{surface}$ area of $99{\mathrm{m}}^2$ ); each had $50–60\mathrm{cm}$ fill depths, monitored for 31 and 33 events, respectively] over $10–15\text{month}$ periods. Outflow from each cell was recorded and inflow was either recorded or calculated from rainfall data. In Louisburg, L2 was lined with an impermeable membrane to eliminate exfiltration while L1 was unlined to allow both exfiltration and evapotranspiration. Results indicate that bioretention facilities can achieve substantial hydrologic benefits through delaying and reducing peak flows and decreasing runoff volume. A large cell media volume: drainage area ratio, and adjustments to the drainage configuration appear to improve the performance. Media layer depth may be the primary design parameter controlling hydrologic performance. Performance diminishes as rainfall depths increase and rainfall durations become longer. Annual water budget analysis suggests that approximately 20–50% of runoff entering the bioretention cells was lost to exfiltration and evapotranspiration.

Urban development within a watershed can take on a wide and diverse range of spatial patterns. The terms “sprawl” and “clustered” development, for example, are frequent in the literature, spanning the spectrum of possible spatial patterns of urban development. The relationship between flood conditions and the spatial distribution of the urban development has been poorly studied, often because of limitations in streamflow data availability or the common use of lumped watershed models in urban hydrologic modeling. We study this relationship with an optimization-based approach that accounts directly for the spatial distribution of imperviousness to investigate how the urban spatial pattern will affect flood peaks and how it can be used to reduce or minimize undesirable impacts to water resources. We employ several water resources-based objective functions to perform optimizations that result in distinct spatial patterns of urbanization showing characteristics of both sprawl and clustered development, depending on the objective function used. We conclude that the approach followed here and the resulting optimized landscapes provide a helpful understanding of the important role played by the spatial form of the urban pattern when trying to minimize impacts to water resources. One objective function, crafted to approximate ecologically based imperviousness threshold policies, resulted in an optimized urbanization pattern suggesting that unintended consequences of low density sprawl may follow from such policies. This specific finding underscores the general value of our optimization-based approach for planning and managing new urban development around water resources.

Urban impacts to water quality and quantity have been a major focus of resource and ecosystem protection efforts since the early 1960s, focusing in the last decade on the impact of impervious thresholds. These are now commonly used as benchmarks of water quality planning and protection in local, watershed, and regional planning efforts. However, the relationship between urbanization and hydrologic impacts is much more complex than this cause-and-effect model would indicate, containing some weaknesses for effective growth management planning. This paper reviews the current literature to synthesize the development-related variables of hydrologic impairment, placing them in a context that is useful in growth management and development mitigation. Through this critical review of the literature, the paper focuses on an outstanding question in land planning: which best management practices, individually or in concert, are the most effective in dealing with the water quality impacts of urban growth and development? Research indicates two largely overlooked areas of potential improvement in water protection efforts: the location of impervious surfaces in the watershed, and the maintenance of adequate areas of forest stands and native vegetation.