NRCS Overland Flow Travel Time Calculation

Overland flow travel time given by the formula developed by the Natural Resources Conservation Service (NRCS) on the basis of kinematic wave theory can underpredict for short, steep, flow planes having low flow resistance and high degrees of imperviousness. Underestimation is caused by inaccurate representation of average rainfall intensity for durations shorter than approximately 15?min. Although the formula was developed based on only NRCS 24-h Type?2 and Type?3 rainfall distributions, it is used without restriction in regions that experience NRCS Type?1 and Type?1A storms. An alternate overland flow travel time formula based on rainfall intensity-duration relations developed previously by the writer to represent accurately high-intensity portions of each NRCS storm type is presented.     

Development of an Empirical Lag Time Equation

A regression analysis was performed on measured lag times from gauged watersheds to develop a lag time equation. The watersheds are part of the Agricultural Research Service’s database. They are located in several states and are comprised of varying terrain. The goal of the analysis was to develop a lag time equation that is useful in hydrologic modeling. The study included measurements from approximately 10,000 direct runoff events from 52 watersheds to determine which watershed parameters are best for predicting lag time. The lag time was found to correlate strongly with the longest hydraulic length of the watershed. Therefore an equation was developed that used only this parameter. The inclusion of any other watershed characteristics in the equation did not improve its ability to predict the lag time. Finally, the National Resource Conservation Service procedures for calculating watershed lag time were used to determine the lag times of the watersheds. These estimated lag times were then compared with the measured lag time of the watershed. It was found that the use of these methods generally underpredicted the true lag time of a watershed.     

Utility of LANDSAT-Derived Land Use Data for Estimating Storm-Water Pollutant Loads in an Urbanizing Area

In many watersheds located in southern California, efforts are being focused on urban runoff because of its adverse impact on receiving water quality. The Sweetwater River watershed is a good example, where the drainage area is rapidly urbanizing and deteriorating reservoir water quality. Contaminated storm water is captured and diverted but as urbanization increases, additional runoff will be generated which will overload the existing infrastructure. To better manage the diversion systems and minimize future construction, storm-water volumes and pollutant loadings need to be estimated. Due to the lack of real-time storm-water runoff monitoring data, pollutant loadings must be estimated from land use information. We used satellite imagery to estimate selected storm-water pollutant loads and compared the results to predictions using land use information from public records. Satellite imagery was useful in estimating storm-water pollutant loads and identifying high loading areas. Satellite imagery with appropriate classification is a promising tool for watershed management and for prioritizing best management practices.     

Rational Coefficients for Steeply Sloped Watersheds

When computing peak discharges for the design of drainage systems using the rational method, it is important to have an accurate value for the rational coefficient (C). For steeply sloped watersheds the origin of values of the rational coefficient are unknown and lack even modeling verification. A model that shows the relationship between the rational coefficient and watershed slope was developed for steeply sloped watersheds. Using Horton’s infiltration equation, Manning’s equation, the velocity method for computing times of concentration, and generalized intensity-duration-frequency curves, a model was developed to test the effect of variation of several watershed characteristics on the relationship between slope and the rational coefficient. Analyses with the model showed that both Manning’s coefficient and land use had the greatest effect on the relationship between C and slope. A mathematical function was then developed from data generated from the Horton–Manning model. This model allows C to be estimated for a given slope and a value of Manning’s coefficient for the land cover. A rational coefficient at a 6% slope is also required input. The model was tested using several watersheds with moderate to steep slopes. This relationship should be used to better estimate values of C on steep slopes, and thereby, lead to more accurately hydrologic designs.     

NRCS (SCS) Synthetic Curvilinear Dimensionless Unit Hydrograph

A continuous time function with two parameters for a curvilinear dimensionless unit hydrograph (CDUH) is derived from the unit hydrograph in the form of a gamma probability density function. This equation is then applied to the Natural Resources Conservation Service [formerly, Soil Conservation Service (SCS)] curvilinear dimensionless unit hydrograph [NRCS (SCS) CDUH]. The values of the parameters are slightly different for the rising and recessing limbs of the unit hydrograph. The NRCS unit hydrograph that expresses the time distribution of runoff of unit depth is shown to be a two-parameter gamma probability density function. Its mass curve is, therefore, an incomplete gamma function. This technical note explains mathematically that what was originally called the mass curve of the ratio of runoff volume to the total volume of the NRCS CDUH is shown to be, in fact, the mass curve of the unit hydrograph. The equations of continuous time functions derived in this study can be used in place of the NRCS CDUH tabulation and the NRCS linear triangular unit hydrograph. An example is shown to illustrate the application.     

Modeling Storm Water Mass Emissions to the Southern California Bight

Storm water runoff is perceived as a major source of pollutants that results in adverse environmental effects, but large-scale assessments are rarely conducted. The problem is particularly pronounced in southern California where 17 million people have rapidly developed coastal watersheds. The goal of this study was to make regionwide estimates of mass emissions, assess the relative contribution from urbanized watersheds, and compare pollutant flux from different land uses. A geographic information system-based storm water runoff model was used to estimate pollutant mass emissions based on land use, rainfall, runoff volume, and local water-quality information. Local monitoring data were used to derive runoff coefficients; over 1,700 storm water sampling events were used to calibrate and validate annual loadings. An average rainfall year produced 1,073×109?L of runoff, 118,000 metric tons (MT) of suspended solids, 1,940 MT of nitrate-N, 108 MT of zinc, and 15 kg of diazinon. The majority of mass emissions were from urbanized watersheds except for suspended solids, total DDT, and chlorpyrifos. Agricultural areas had the greatest fluxes for pesticides, including total DDT and chlorpyrifos while open areas typically had the smallest.     

Sensitivity of Watershed Runoff under Humid Conditions to Potential Climate Variations

A semidistributed watershed model is applied over the Mahantango Creek catchment in Pennsylvania to estimate future changes in direct runoff under 22 different climate scenarios. It is shown how different subcatchments of the watershed may respond to possible changes in the precipitation and temperature regimes. Subcatchments with the most unfavorable future runoff responses can be identified where possible changes in land use management practices may be suggested.     

Estimating Nutrient Outflow from Argicultural Watersheds to the River Kali in India

In India, fertilizers and chemicals are applied to different crops, which in turn, cause nonpoint source pollution of surface water and groundwater of the region. In the present work, extensive water quality surveys were done to estimate the nutrient outflow from three small agricultural watershed of the Kali Basin, Uttar Pradesh, India. A total of 576 field data sets have been collected during March 1999–February 2000 from four sampling stations. During the monsoon period the nutrient outflow from these agricultural watersheds were found to be orders of magnitude higher than during the nonmonsoon period. The percentage of nutrients outflow from each watershed was estimated on a monthly basis by obtaining periodical cropping patterns and the amounts of fertilizer applied for each watershed. A maximum of 85% of total nitrate and 70% of total orthophosphate applied in the field was found to be lost during the month of July from the third agricultural watershed having maximum slope and minimum watershed area. Using the data sets generated during field surveys, commonly used modeling approaches based on mass balance differential loading and decay fraction were tested for their applicability to estimate nonpoint source (NPS) pollution in the River Kali. The NPS concentration and load values computed from these approaches were compared with the NPS values measured in the field and the performances of different equations have been evaluated using error estimations such as standard error, normal mean error, mean multiplicative error, and correlation statistics. Further, a refined model based on reaction kinetics and mass balance differential loading has been proposed for the River Kali that minimizes error estimates and improves correlation between observed and computed nonpoint source loads.     

Experimental and Field Studies of Type I Settling for Particulate Matter Transported by Urban Runoff

Settling velocity is an important constitutive parameter of particulate matter (PM) transported by runoff. Settling velocity is either explicitly or implicitly utilized when designing or modeling unit operations, and in situ or watershed controls for urban rainfall-runoff. Utilizing two common settling devices, a settling column and an Imhoff cone, settling velocities of discrete noncolloidal particles in source area urban rainfall-runoff were measured. A comparison of settling models applicable to discrete (Type I) PM settling was developed. Models were compared to measured results across the noncohesive silt- and sand-size PM gradation from 2 to 2,000?μm, utilizing measured particle-size distributions (PSDs) and specific gravity. Results indicate that Newton’s Law can reproduce measured settling velocity when measured inputs of PM diameter, specific gravity, and temperature are utilized. Alternative models to Newton’s Law (in the Stokesian regime) did not improve agreement with measured settling velocities determined using PSDs from laser diffraction. Settling velocity distributions using Newton’s Law were applied for two limiting classes of storm events loading a screened hydrodynamic separator (HS) at an urban watershed. Results indicate that for a low flow and high flow event, Newton’s Law and a simple ideal overflow model of the HS could reproduce PM separation and the PSD of eluted PM (2 to ～ 250?μm) within 17% of measured results on a gravimetric basis.     

Upland Erosion Modeling in a Semihumid Environment via the Water Erosion Prediction Project Model

The major water quality impairment in the midwest United States is sediment eroded from agricultural lands. Yet, few understand the spatial and temporal variability of erosion, or soil erosion dynamics, in relation to precipitation, topography, land management, and severe events. The objectives of this paper are to (1) develop a methodology for estimating long-term spatial soil erosion and water runoff losses and (2) explore issues in applying an established physical-based process model, Water Erosion Prediction Project (WEPP), to a large area by establishing a prototype system for the state of Iowa. This study for the first time provides a comparison of the model predictions against long-term measurements of the sediment delivery ratio (SDR) in the South Amana Catchment of the Clear Creek Watershed (CCW), a heavily instrumented watershed that is roughly 10 times the maximum WEPP fold size. To further examine the performance of WEPP in a semihumid environment, such as the CCW, where runoff and raindrop impact to erosion may be significant, the SDR was plotted as a function of the runoff coefficient, defined as the runoff/rainfall ratio. In addition, the WEPP predictions are compared against the statistical relation of SDR vs. runoff coefficient developed by Piest et al. in 1975) for watersheds in Iowa. It is shown that WEPP follows the trend shown by Piest et al. quite closely and performs well for continuous simulations extended up to 300?years.     

Attenuation of Roadway-Derived Heavy Metals by Wood Chips

Wood chips were evaluated for their ability to attenuate heavy metals in roadway runoff. Column experiments with controlled synthetic runoff composition and flow rate were used to assess effects of flow rate (intercepted sheetflow from a 3-m wide roadway section), runoff salt concentration, wood exposure to alternating wetting and drying cycles, wood aging, competition among dissolved heavy metals, and removal of particle-associated heavy metals. Overall, wood chips damped the “pulse” of copper in the synthetic runoff such that the effluent was characterized by lower concentrations (3–25% of input) over longer periods of time, but with little retention of the total copper mass. The most effective treatment was wood chips aged up to 9 months. Increased aging and chip water content reduced effluent concentrations, relative to no treatment. Flow rate had no effect on effluent concentrations. The presence of salt (>2?mS/cm) or dissolved lead (500?μg/L) in the runoff caused greater copper effluent concentrations than the no treatment case. Removal of suspended particles (and associated contaminants) was greater than 85% with an estimated capacity of 0.16?g/gwood. Field evaluation with concentrated flow to a gutter containing a wood chip treatment showed little effect on total or dissolved copper and zinc runoff concentrations and indicated that wood chips may be a source of contaminants in subsequent storm events. Applications of wood chips to treat roadway runoff would not provide a significant decrease in total maximum daily load contributions (e.g., kg/d); however, there may be some scenarios for which wood chip treatments to decrease peak storm water concentrations of dissolved heavy metals in sheetflow runoff is desirable.     

Knowledge-Based Model for Supplementary Irrigation Assessment in Agricultural Watersheds

In this paper a knowledge-based model for supplementary irrigation assessment in rainfed agricultural watersheds is presented. The supplementary irrigation assessment problem is divided into different components and is modeled separately. Geographic Information System (GIS) is used to aggregate spatially varying attributes required for the modeling. A graphical user interface is developed in a GIS platform by using the ERDAS macro language tools. The model was applied to two case study areas in India: a subwatershed of Gandheshwari area (West Bengal), and Harsul watershed (Maharashtra). In the Gandheshwari subwatershed, the water availability was found to be inadequate to meet the irrigation requirement and hence the model identified the areas that can be irrigated with different outsource water supply. On the other hand, surface runoff generated in the Harsul watershed was found to be sufficient to meet the supplementary irrigation requirement, thereby showing the feasibility for supplementary irrigation in the area. Using the model, the effect of any rainfall condition can be simulated and hence appropriate measures can be taken in advance to reduce the risk of crop failure.     

Hydrodynamic Separation of Particulate Matter Transported by Source Area Runoff

Hydrodynamic separation is a preliminary unit operation frequently utilized in wastewater and more recently in storm water for separation of coarse particulate matter (PM) and gross solids. In order to examine the behavior and separation mechanisms of a screened hydrodynamic separator (HS) not influenced by scour, this study examined the event-based performance of an empty-bed (clean sump and volute) HS for PM fractions transported in eight runoff events from a 1,088?m2 paved source area urban watershed. Influent particle-size distributions (PSDs) {d50?m from 270?to?2,202?μm} and HS particle separation efficiency (PSE) (from 38 to 70% of mass) exhibited variations influenced by hydrology and previous loadings. When examined as PM size fractions, results demonstrate separation of the sediment fraction (>75?μm) ranging from 76 to 94% while for settleable and suspended (1–25?μm) fractions, the PSE was variable and significantly lower; from 3 to 57% and 2 to 43%, respectively. Results demonstrate a correlation between higher influent PM concentration and coarser PSDs, illustrating why higher PM concentrations promote higher PSEs; and why HS performance must be specified at a PM concentration, PSD, and flow rate. Results demonstrate that HS behavior is influenced by influent PSDs coupled with flow rate. Hydrodynamic separation is effective for high-rate gross solids control. However, current HS designs require incorporation of hydrologic control, methods of frequent sludge zone management before scour, and stored runoff management to control interevent redox conditions.     

钢铁制造流程铁钢界面时间参数“涨落”特征

 时间是钢铁制造流程物质流的基本参数之一,研究其“涨落”特征对钢铁制造流程的动态有序运行、动态精准设计和结构重构优化具有重要意义。以钢铁制造流程铁钢界面为例,采用以中位数和稳健变异系数为主要指标的稳健统计方法,对物质流运行过程各环节时间的集中性和离散性特征及其影响因素进行深入分析。研究结果表明,铁钢界面物质流运行过程中,各作业环节的时间集中性指标和离散性指标的差异均较大,主要与作业内容和主体独立性有关;而各等待环节的时间集中性指标差异较大而离散性指标差异较小,主要受系统结构及生产组织的综合影响。工序自身生产特点、流程静态物理结构和物质流动态运行程序是影响钢铁制造流程多维物质流时间参数“涨落”特征的主要因素。     

Evaluation and Optimization of Bioretention Media for Treatment of Urban Storm Water Runoff

Bioretention is a relatively new urban storm water best management practice. The objective of this study is to provide insight on media characteristics that control bioretention water management behavior. Eighteen bioretention columns and six existing bioretention facilities were evaluated employing synthetic runoff. In columns, the runoff infiltration rate through different media mixtures ranged from 0.28 to 8.15?cm/min at a fixed 15 cm head. For pollutant removals, the results showed excellent removal for oil/grease (>96%). Total lead removal (from 66 to >98%) decreased when the total suspended solids level in the effluent increased (removed from 29 to >96%). The removal efficiency of total phosphorus ranged widely (4–99%), apparently due to preferential flow patterns, and both nitrate and ammonium were moderate to poorly removed, with removals ranging from 1 to 43% and from 2 to 49%, respectively. Two more on-site experiments were conducted during a rainfall event to compare with laboratory investigation. For bioretention design, two media design profiles are proposed; >96%?TSS, >96%?O/G, >98%?lead, >70%?TP, >9%?nitrate, and >20%?ammonium removals are expected with these designs     

Three-Dimensional Modeling for Estimation of Hydraulic Retention Time in a Reservoir

A three-dimensional computational fluid dynamics model is used to estimate the hydraulic residence time for a portion of the Wachusett Reservoir in central Massachusetts. The basin under consideration has several major inflows and exhibits complex flow patterns. The basin is modeled using the FLUENT software package with particles used to track travel time in a steady-state flow field. A tetrahedral mesh with over 1.6 million cells is used with accurate depiction of basin bathymetry and inlet and outlet geometries. Modeling is performed to simulate behavior during a period when conditions are isothermal. It is determined that mean hydraulic residence time is 3–4?days; approximately half of what would be expected assuming strictly plug flow. The presence of a primary flow path, large scale eddies and stagnation zones contribute to the faster travel times. Reductions in inflow rates produce increased residence times and significant changes in flow patterns.     

First Flush Concepts for Suspended and Dissolved Solids in Small Impervious Watersheds

Eight rainfall-runoff events were examined from each of two small paved urban transportation land use watersheds (A = 544?m2 and 300?m2) in an attempt to distill multiple definitions of the first flush phenomenon into a consistent framework and examine common volumetric capture requirements. Results indicated that two separate criteria must be employed to describe the delivery of suspended sediment concentration (SSC) and total dissolved solids (TDS) as aggregate indices of entrained particulate and dissolved matter. The concentration-based first flush criterion is defined by high initial SSC or TDS concentration in the early portion of a rainfall-runoff event with a subsequent rapid concentration decline. In contrast, the mass-based first flush (MBFF) has several published forms, shown to be equivalent herein. The MBFF is defined generally as a disproportionately high mass delivery in relation to corresponding flow volume. For mass-limited events, mass delivery was skewed towards the initial portion of the event while the mass delivery in flow limited events tended to follow the hydrograph. This study also investigated published estimates of the water quality volume (WQV); assuming that an in-situ Control Strategy or Best Management Practice (BMP) captures and treats only this WQV, while flows in excess of this volume bypass the BMP. For the two watersheds, results indicate that a relatively large runoff volume must be captured to effect meaningful reductions in mass and concentrations (as event mean concentrations) despite a disproportionately high mass delivery early in the event. Results suggest the potential for misinterpretation of overall BMP effectiveness may be significant based on use of a number of these common published estimates based on a WQV.     

Evaluating Urban Pollutant Buildup/Wash-Off Models Using a Madison, Wisconsin Catchment

Buildup/wash-off (BUWO) models are widely used to estimate pollutant export from urban and suburban watersheds. Here, we propose that the mass of washed-off particulate during a storm event is insensitive to the time between storm events (the traditional predictor of particulate accumulation in BUWO models). Our analysis employed USGS data of total suspended solids and discharge data for nonsnow events in a 9.4-km2 suburban catchment in Madison, Wis. Kinetic energy of rainfall was calculated using National Weather Service NEXRAD radar reflectivity. A regression analysis found that storm event runoff volume and rainfall kinetic energy explained 81% of the variability in event particulate load; volume alone explained 69% of the variability in event loads. Time between storm events was not significant. Additionally, we simulated storm event particulate loads using a BUWO model and a model assuming a constant mass available for wash-off. Both models produced very similar predictions over a range of parameterizations, suggesting that buildup models could perhaps be simplified under many circumstances.     

Variably Saturated Flow in Storm-Water Partial Exfiltration Trench

Storm water from impervious urban areas can adversely impact water quality and quantity. The partial exfiltration trench (PET) is a control device designed to moderate both the quality and the quantity of urban runoff. This paper uses a 2D numerical model to evaluate variably saturated flow profiles and residence time distributions for a PET subject to storm water loading. Parameters estimated from laboratory experiments and hydrographs measured at a prototype PET are used to calibrate the numerical model. Simulation experiments show that flow through the PET is influenced strongly by the rate and duration of the hydraulic loading and by the type and properties of the surrounding soil. Unless the surrounding soil is nearly saturated or highly impermeable, propagation of the wetting front through an unlined PET occurs as 2D variably saturated flow. Variably saturated 2D flow through the PET is characterized by skewed residence time distributions, long mean travel times (relative to plug flow), high exfiltration losses to the surrounding soil, and low tracer mass recovery at the underdrain. These features of the PET performance are beneficial for storm-water treatment because the first flush of runoff often contains significantly higher pollutant concentrations than those present in later phases of the storm.