COMPARATIVE EVALUATION OF THREE URBAN RUNOFF MODELS1

ABSTRACT: Three urban runoff models, namely, the Road Research Laboratory Model (RRLM), the Storm Water Management Model (SWMM) and the University of Cincinnati Urban Runoff Model (UCURM), were examined by comparing the model simulated hydrographs with the hydrographs measured on several instrumented urban watersheds. This comparison was done for the hydrograph peak points as well as for the entire hydrographs using such statistical measures as the correlation coefficient, the special correlation coefficient and the integral square error. The results of the study indicated that, when applying the three selected non-calibrated models on small urban catchments, the SWM model performed marginally better than the RRL model and both these models were more accurate than the UCUR model. On larger watersheds, the comparisons between the SWM model and the other two models would be likely even more favourable for the SWM model, because it has the most advanced flow routing scheme among the studied models.     

ACCUMULATION OF SELECTED TRACE METALS IN SOILS OF URBAN RUNOFF DETENTION BASINS1

ABSTRACT Field investigations were conducted at three sites in the Washington, D.C., area to detect accumulation patterns of the trace metals, cadmium, copper, lead, and zinc in the soils of urban stormwater detention basins. The research results seemed to indicate that the use of detention basins to control urban stormwater runoff had few harmful effects to fine textured soils with respect to the study trace metals. Although the trace metals, especially lead and zinc, were found to accumulate in the surface soils of the basins, little significant downward movement of metals in the soil profiles had occurred. Accumulations of metals in the surface soils appeared to be a function of microtopography and the resultant residence time of standing water. The fractions of trace metals that were present in a leachable form in surface soils and stormwater solids were small, with median values ranging from 7.7 percent of the total concentration for Cd to 0.01 percent for Pb.     

ESTIMATING RUNOFF VOLUMES FROM URBAN AREAS1

ABSTRACT: Estimations of runoff volumes from urban areas can be made by the equation Q = a A σ(P_e– b), where Q is the runoff volume, a is the part of the total area A Contributing to runoff, P_e is the rainfall amount for a single event, and b is the initial rainfall losses. For the evaluation of a and b, rainfall/runoff measurements were made in five areas of sizes between 0.035 km² and 1.450 km². By linear regression analysis of rainfall volumes versus runoff volumes, the initial rainfall losses were found to vary from 0.38 mm to 0.70 mm for the different areas. The parts of the areas contributing to runoff were found to be proportional to the impermeable parts of the mas. The equation is applicable in urban areas with well defined paved surfaces and roofs and with a negligible amount of runoff from permeable areas.     

OBSERVATIONS ON KINEMATIC RESPONSE IN URBAN RUNOFF MODELS1

ABSTRACT: This paper first discusses the results of sensitivity analyses conducted on various parameters of the San Francisco Stormwater Model ta version of WREM) and the Penn State Runoff Model in terms of their impact on outflow hydrographs. The parameters considered within a idealized catchment include: basin shape, imperivous fraction, overland roughness and slope: deterntion depth; infiltration capacity; and hyetograph timing. Second, the results for the hypothetical catchment are extended to the lazzard laboratory surfaces (asphalt, grass, roofing material) as a mean of illustrating the need for changes in model structure, as opposed to continued parameter adjustment Finally the effect of altering the scale of hydraulic representation in the surface runoff and sewer transport calculations are demonstrated for two gaged watersheds in Hamburg, West Germany.     

SUSPENDED SEDIMENT AND METALS REMOVAL FROM URBAN RUNOFF BY A SMALL LAKE1

ABSTRACT: A small lake in the Chicago Metropolitan Area was from 91 to 95 percent efficient in removing suspended sediment and from 76 to 94 percent efficient in removing copper, iron, lead, and zinc from urban runoff. Sediments accumulated in the lake in the form of an organic-rich mud at an average rate of 20 millimeters per year; this reduced lake storage and covered potential habitat for aquatic organisms. Copper, lead, and zinc concentrations were closely associated with suspended-sediment concentrations and with silt- and clay-sized fractions of lake sediment. Although concentrations of mercury and cadmium were near detection limits in runoff, measurable concentrations of these metals accumulated in the lake sediments.     

CONTRIBUTIONS OF RAINFALL TO CONSTITUENT LOADS IN STORM RUNOFF FROM URBAN CATCHMENTS1

ABSTRACT: Rainfall is a significant source of some constituents, particularly nitrogen species, in storm runoff from urban catchments. Median contributions of rainfall to storm runoff loads of 12 constituents from 31 urban catchments, representing eight geographic locations within the United States, ranged from 2 percent for suspended solids to 74 percent for total nitrite plus nitrate nitrogen. The median contribution of total nitrogen in rainfall to runoff loads was 41 percent. Median contributions of total-recoverable lead in rainfall to runoff loads varied by as much as an order of magnitude between catchments in the same geographic location. This indicates that average estimates of rainfall contributions to constituent loading in storm runoff may not be suitable in studies requiring accurate constituent mass-balance computations.     

MONTHLY RUNOFF DISTRIBUTION RELATED TO BASIN GEOGRAPHY1

ABSTRACT: Mean monthly runoff from ungaged drainage basins that have significant snowpacks each year can be estimated quite well by assuming that the time duration between snowfall and snowmelt is the predominant factor in temporal runoff distribution. That time span is related to basin temperatures which are, in turn, functions of basin elevation and latitude. Regional hydrologic analyses of gaged basin data create regression equations for estimating runoff distribution by month. These equations then can be applied to ungaged basins. Basin latitude and mean elevation are two independent variables that can be used in estimating monthly runoff distributions.     

STATE VARIABLE MODEL FOR URBAN RAINFALL-RUNOFF PROCESS1

ABSTRACT: A nonlinear hydrologic system model has been developed for analyzing the urban rainfall-runoff process. The model is formulated as a state variable model consisting of several parameters. A search technique is employed to find the set of parameters for which the model's response best fits observed data. The model could be used in either a simulation or forecasting mode. The model is applied to observed data for the Waller Creek Watershed in Austin, Texas, to develop the model parameters for various levels of urbanization of the watershed. The trend of each parameter with respect to levels of urbanization is examined.     

CONTAMINANT LEVELS IN PRECIPITATION AND URBAN SURFACE RUNOFF1

ABSTRACT: Precipitation and resultant runoff were sampled for a series of storm events over the period of one year. The test site was the parking lot of a large suburban shopping mall in the Syracuse, New York, area. Both precipitation and runoff were tested for lead, zinc, copper, cadmium, and petroleum hydrocarbons: substantial amounts were detected in each. No correlation was found between precipitation contaminant concentration and the length of the antecedent dry period. A weak, but apparently inverse relationship was noticed between concentration and amount of precipitation. Poor correlations were obtained between runoff contaminant concentration and the antecedent dry period. The variability attributable to different precipitation volumes was removed by converting to a unit-area basis. The variability attributable to precipitation contaminant load was removed by subtraction. The resultant value, dryfall accumulation, then correlated well with the length of the antecedent dry period. Metal ions were found in both precipitation and runoff and were hypothesized to come from atmospheric fallout as a result of distant emissions and from very localized sources, primarily vehicle traffic on the parking facility. Petroleum residues were believed to be the sole result of automobile losses, since none could be detected in precipitation samples.     

DISTRIBUTION OF BENTHIC MACROINVERTEBRATES IN A STREAM EXPOSED TO URBAN RUNOFF1

ABSTRACT: A study of benthic macroinvertebrate community composition was conducted at eight sites along Shabakunk Creek, a small stream in Mercer County, New Jersey, which receives urban runoff. The relationship between changes in substrate composition and the nature of the benthic macroinvertebrate community has been examined. Organisms were collected seasonally from natural substrates in riffles. Attempts to employ artificial substrates for invertebrate collection proved unsuccessful, as the population on the samplers was not representative of that in the stream bed. Number of total benthic macroinvertebrate taxa collected declined from 13 in relatively undeveloped upstream areas to four below heavily developed areas, while population density decreased simultaneously in the same areas. Periphyton samples collected from natural substrates were analyzed for selected heavy metals. Significantly higher heavy metal concentrations are reported from substrates sampled below heavily developed areas, and changes in these values are discussed with regard to changes in benthic macroinvertebrate distribution.     

ESTIMATING PEAK RUNOFF FROM FIELD-SIZE WATERSHEDS1

ABSTRACT: A simple nonlinear runoff model was developed and tested for use on field-size agricultural watersheds. A Wooding idealization of the watershed topography was used. Kinematic wave equations were used with an assumed, instead of computed, overland flow, watersurface profile in order to simplify the numerical computations. The approach was used to synthesize runoff hydrographs for an agricultural watershed in Iowa. The accuracy of the synthesized hydro-graphs was judged by comparing the estimated and observed peak discharges and by comparing estimated and observed stages at the measuring weir. The mean errors were 0.01 in/hr and 0.05 ft, respectively. A qualitative comparison was also made with a detailed kinematic wave study. The largest variability occurred during the seedbed period for both models, which was attributed to changes in surface roughness. The roughness was more constant and the results more consistent for the canopy and ground residue periods.     

RELIABILITY OF THE DESIGN STORM CONCEPT IN EVALUATING RUNOFF PEAK FLOW1

A comparative study was undertaken to evaluate peak runoff flow rates using (1) a continuous series of actual rainfall events and (2) design storms. The ILLUDAS computer model was used to simulate runoff over a catchment within the city of Montreal, Canada. A ten-year period, five-minute increment rainfall data base was used to derive peak flow frequency curves. Two types of design storms were analyzed: one derived from intensity duration frequency curves (Chicago type), the other from averaging actual rainfall patterns (Huff type). Antecedent soil moisture conditions were considered in the analyses. It was found that the probability distribution of runoff peak flow was sensitive to the choice of design storm pattern and to the antecedent soil moisture condition. A symmetrical, Chicago-type design storm with antecedent dry soil moisture produced a flow frequency curve similar to the one obtained from a series of historical rainfall events.     

ALTERNATIVE STRATEGIES FOR STORMWATER DETENTION1

ABSTRACT: In a simulation experiment, stormwater flows are partially diverted, at various levels, to a detention basin in order to compare the recombined (i.e., undiverted flows and basin discharges) hydrograph to the response of the traditional, in-line design. The use of off-line detention basins is shown to be an effective technique for reducing peak flows from developed watersheds to pre-development levels with lower storage requirements. In addition, the discharge hydrographs produced by off-line detention are significantly different from those produced by the traditional design and may be more suited to certain stormwater management situations.     

AN EVALUATION OF FLOOD FREQUENCY ESTIMATES BASED ON RAINFALL/RUNOFF MODELING1

ABSTRACT: An evaluation of flood frequency estimates simulated from a rainfall/runoff model is based on (1) computation of the equivalent years of record for regional estimating equations based on 50 small stream sites in Oklahoma and (2) computation of the bias for synthetic flood estimates as compared to observed estimates at 97 small stream sites with at least 20 years of record in eight eastern states. Because of the high intercorrelation of synthetic flood estimates between watersheds, little or no regional (spatial) information may be added to the network as a result of the modeling activity. The equivalent years of record for the regional estimating equations based totally on synthetic flood discharges is shown to be considerably less than the length of rainfall record used to simulate the runoff. Furthermore, the flood estimates from the rainfall/runoff model consistently underestimate the flood discharges based on observed record, particularly for the larger floods. Depending on the way bias is computed, the synthetic estimate of the 100-year flood discharge varies from 11 to 29 percent less than the value based on observed record. In addition, the correlation between observed and synthetic flood frequency estimates at the same site is also investigated. The degree of correlation between these estimates appears to vary with recurrence interval. Unless the correlation between these two estimates is known, it is not possible to compute a weighted estimate with minimum variance.     

EFFECTS OF URBAN LAKES ON SURFACE RUNOFF AND WATER QUALITY1

ABSTRACT: The effects of an artificial lake system upon the runoff hydrology of a small watershed have been determined by comparing the quantity and quality of runoff with that of an adjacent and similar watershed containing no lakes. Lake storage reduced peak discharge and slowed flood recession rate downstream. Water stored within the lakes is generally of different quality than downstream surface runoff. Salt stored in the lakes from winter deicing is released during periods of surface runoff throughout the rest of the year. During summer or fall runoff events, lake outflow dominates the salt load of the outlet stream, generating double-peaked load hydrographs in which the second, or lake-induced, crest is many times larger than the peak which corresponds to maximum flow. On the other hand, the lakes cause a reduction of salt loads and concentration in winter runoff. The concentration and loads of ions which are not related to road salt are generally less affected by the lakes, although they are increased substantially in the fall.     

URBAN RUNOFF: POLLUTION SOURCES,CONTROL, AND TREATMENT1

This paper reviews progress on urban storm water management and pollution control, with emphasis on non- and low-structurally intensive techniques along with the total system approach encompassing control-treatment. Many of the U.S. Environmental Protection Agency's demonstration-evaluation projects are presented to exemplify: Land Management Techniques, i.e., land use planning, best use of natural drainage, dual use of retention and drainage facilities required for flood control designed concurrently or retrofitted for pollution control, porous pavement, surface sanitation, and chemical use control; Collection Systems Control, i.e., catchbasin cleaning, flow regulators (including swirl and helical devices), and the new concepts of elimination or reduction of unauthorized cross-connections, in-channel/conduit storage and/or other forms of storage for bleed-back to existing treatment plants; Storage including in-receiving water storage; Treatment, i.e., physical/chemical, disinfection, and a treatment-control planning and design guidebook; Sludge and Solids Residue from Treatment; and Integrated Systems, i.e., storage/treatment, dual-use wet-weather flow/dry-weather flow facilities, and reuse of stormwater for nonpotable purposes. Recommendations for the future in the areas of: control based on receiving water impacts, toxics characterization and their control, sewer system cross-connections, integrated stormwater management, and institutional/sociological/economic conflicts are also presented.     

NATIONWIDE ASSESSMENT OF URBAN RUNOFF IMPACT ON RECEIVING WATER QUALITY1

ABSTRACT: Urban stormwater runoff has been recognized as a potential major contributor of pollution to receiving waters. However the projected high costs of control have prompted an examination of the extent to which these impacts have been documented. A nationwide search was conducted for case studies demonstrating a cause-effect linkage between urban runoff and impairment of beneficial uses in receiving waters. The results indicate that numerous definitions of “impacts” are being used and that few substantive data exist to support many of these allegations. Results of a preliminary impact assessment are presented for the 248 urbanized areas of the United States. Then, the results of more recent efforts to assess these impacts in several case studies are described. This assessment demonstrates the critical need for additional short-term and long-term sampling programs.     

SIMULATION OF RUNOFF FROM URBAN WATERSHEDS1

ABSTRACT. .A mathematical model for urban watersheds is being developed in stages at the Utah Water Research Laboratory, Utah State University at Logan. In verifying the watershed as a unit, watershed coefficients are determined on the computer, and related to the urbanization characteristics. The second stage of verification consists of dividing the watershed into subzones, and determining the urban parameters within each subzone. Each subzone is then individually modeled, and outflow hydrographs are routed through succeeding downstream subzones to the gaging point. The model thus makes it possible to: (a) develop runoff models for subzone hydrographs within the urban watershed, and (b) account for spatial variations of storm and watershed characteristics. An attempt was also made to analytically model the outflow hydrograph based on storm and watershed characteristics.     

ACCUMULATION OF SELECTED TRACE METALS IN SOILS OF URBAN RUNOFF SWALE DRAINS1

ABSTRACT: Field investigations were conducted at three sites in the Washington, D.C., area to detect the accumulation patterns of the trace metals, cadmium, copper, lead and zinc in the soils of roadside grassed swale drains that had been receiving urban stormwater runoff. Two sites were residential areas and one site was an intensively used highway. The research results seem to indicate that the use of swale drains to control urban stormwater runoff had few harmful effects to fine textured soils with respect to the study metals. With the exception of zinc, typical roadside patterns of decreasing metal concentrations with increasing distance from roads were observed for the upper 5 cm of study soils. Zinc accumulated in residential grassed swales due to leachate from galvanized curverts. Sampling to a depth of 60 cm revealed no evidence of subsurface trace metal enrichment in the study swales. Although the percentage of soil zinc in leachable form was as high as 20 percent of total zinc concentrations, the other study metals had small leachable components. Leachable lead was always less than 1 percent of the total lead.     

A PARAMETRIC MODEL CALIBRATED WITH A PHYSICALLY BASED MODEL FOR RUNOFF PREDICTION FROM UNGAGED STREAMS1

ABSTRACT: Recent developments in the numerical solution of the governing partial differential equations for overland and channel flow should make possible physically based models which predict runoff from ungaged streams. However, these models, which represent the watershed by sets of intersecting planes, are complex and require much computer time. Parametric models exist that have the advantage of being relatively simple, and once calibrated are inexpensive to use and require limited data input. In this study, a procedure was developed for calibrating a parametric model against a physically based model, utilizing base areas of one acre and one square mile, with the expectation that base areas can be combined to model real watersheds. Simulation experiments with the physically based model showed that, for the one-acre base area, the dominant parameter (cell storage ratio, K) related to the slope and friction of the planes, whereas for one square-mile areas, the dominant parameters (K plus a lag factor, L) relate to channel properties. These parameters decreased exponentially as rainfall intensity increased.