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.     

THE EFFECT OF DATA INDEPENDENCE IN MODEL CALIBRATION AND MODEL TESTING1

ABSTRACT: With the increased use of models in hydrologic design, there is an immediate need for a comprehensive comparison of hydrologic models, especially those intended for use at ungaged locations (i.e., where measured data are either not available or inadequate for model calibration). But some past comparisons of hydrologic models have used the same data base for both calibration and testing of the different models or implied that the results of model calibration are indicative of the accuracy at ungaged locations. This practice was examined using both the regression equation approach to peak discharge estimation and a unit hydrograph model that was intended for use in urban areas. The results suggested that the lack of data independence in the calibration and testing of regression equations may lead to both biased results and misleading statements about prediction accuracy. Additionally, although split-sample testing is recognized as desirable, the split-samples should be selected using a systematic-random sampling scheme, rather than random sampling, because random sampling with small samples may lead to a testing sample that is not representative of the population. A systematic-random sampling technique should lead to more valid conclusions about model reliability. For models like a unit hydrograph model, which are more complex and for which calibration is a more involved process, data independence is not as critical because the data fitting error variation is not as dominant as the error variation due to the calibration process and the inability of the model structure to conform with data variability.     

UNIT HYDROGRAPHS – A COMPARATIVE STUDY1

ABSTRACT. Unit hydrographs derived by using two methods, linear programming and least squares, are compared. Test data comprise rainfall and runoff information from four storms over the North Branch Potomac River near Cumberland, Maryland. The mathematical bases of these methods for unit-hydrograph derivation are explained. The linear programming method minimizes the sum of absolute deviations, and the least squares method minimizes the sum of the squares of deviations. Computer subroutines are readily available for application of these methods. The unit hydrographs derived with the two methods are practically the same for storms 2 and 3, but differ somewhat for storms 1 and 4. However, the reconstituted direct surface runoff hydrographs are similar to those observed with the exception of the hydrograph for storm 4 which had a relatively more non-uniform rainfall excess of a considerably larger duration.     

Fitting a Gamma Distribution Over a Synthetic Unit Hydrograph1

ABSTRACT: Several methods for synthetic unit hydrographs are available in the literature. Most of these methods involve the hand fitting of a curve over a set of a few hydrograph points, which can sometimes be a subjective task. Besides, the user often finds it difficult or simply neglects to adjust the generated unit graph to a runoff volume of one unit (inch, cm, or mm). It is the purpose of this paper to present to the design hydrologist a simple method to fit a smooth gamma distribution over a single point specified by the unit hydrograph peak and the time to peak with a guaranteed unit depth of runoff.     

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.     

Automated Algorithms for Heuristic Base‐Flow Separation1

Abstract: The subjective nature of graphical base‐flow separation combined with the many applications of base‐flow time series derived from continuous streamflow data, motivates the development and application of automated algorithms for heuristic base‐flow separation. Base‐flow time series derived from gauged streamflow support diverse applications in engineering hydrology, catchment analysis, hydrogeologic investigations, regional low‐flow analysis, and recharge estimation. Whether based on graphical procedures for recession analysis or analytical expressions derived from fundamental equations of ground‐water flow, the variety of base‐flow separation algorithms belies the array of base‐flow definitions and interpretations that variously refer to dominant process, source, flow path, and characteristic response time. Algorithms that are invariant in their consistent – though heuristic – characterization of base‐flow response are particularly useful for interbasin comparisons of low‐flow characteristics and hydrologic regionalization. More adaptable algorithms provide application‐specific flexibility in allocating flow components like interflow to either quickflow or slowflow. Four widely used algorithms that produce consistent base‐flow time series using only gauged streamflow records are compared and contrasted with a complementary heuristic algorithm that incorporates hydrologic judgment explicitly, through manual parameterization. The utility of these inherently subjective algorithms is illustrated through a simple example of flow phase separation in a two‐component end‐member mixing model of dissolved chlorides in the Cuyahoga River.     

THE UNIT HYDROGRAPH: A SATISFACTORY MODEL OF WATERSHED RESPONSE?1

ABSTRACT: Some 96 flood events larger than the mean annual flood from 16 watersheds in the Commonwealth of Pennsylvania were used to derive unit hydrographs by the least-squares method. Analyses of the unit hydrographs were conducted to ascertain their response to watershed parameters, climatic and storm variables and locations within different hydrologic regions. Significant differences both within and among watersheds led to the formulation and testing of hypotheses stating that differences among watersheds are caused by physiographic differences while differences within watersheds result from climatic and storm differences. The analysis showed, that while many watersheds parameters strongly influence the shape of the unit hydrograph, only the storm variables duration and volume of precipitation excess produce significant differences. Seasonal differences were apparent but not proven statistically significant.     

UNIT HYDROGRAPHS FOR DEVELOPING DESIGN FLOOD HYDROGRAPHS1

ABSTRACT: In Illinois, a procedure has been developed to derive unit hydrographs for generating 100-year and probable maximum flood hydrographs, on the basis of 11 parameters that define the hydrograph shape very well. Regional regressions of these parameters with basin factors show very high correlation. Thus satisfactory values of parameters can be determined for ungaged areas or those with a few years' record. The nonlinearity in unit hydrographs derived from usual floods is largely attributed to mixing within-channel and overbank-flow flood events. To minimize the effects of nonlinearity and to derive unit hydrographa suitable for calculating spillway design floods, use of the proposed method of developing such hydrographs is recommended.     

ESTIMATION OF VOLUME-DURATION-FREQUENCY RELATIONS OF UNGAGED SMALL URBAN STREAMS IN OHIO1

ABSTRACT: This paper describes methods for estimating volume-duration-frequency relations of urban streams in Ohio with drainage areas less than 6.5 square miles. The methods were developed to assist engineers in the design of hydraulic structures on urban streams for which temporary storage of water is an important element of the design criteria. Multiple-regression equations were developed for estimating maximum flood volumes of d-hour duration and T-year recurrence interval (dV_T). Maximum annual flood-volume data for all combinations of six durations (1, 2, 4, 8, 16, and 32 hours) and six recurrence intervals (2, 5, 10, 25, 50, and 100 years) were analyzed. The significant explanatory variables in the resulting 36 volume-duration-frequency equations are drainage area, average annual precipitation, and basin-development factor. Standard errors of prediction for the 36 dV_T equations range from ±28 percent to ±44 percent.     

POND SIZING FOR RATIONAL FORMULA HYDROGRAPHS1

ABSTRACT: The Modified Rational formula hydrograph and the Yarnell generalized rainfall chart are generally accepted procedures for sizing storm water detention ponds for small drainage areas. A procedure has been developed to choose the rainfall duration which, for a chosen return period, will result in the largest required storage volume of a detention pond. A graphical solution has been provided and its use has been described by application to an example.