Paper by Albert H. Halff, Jose I. Novoa, and Louis M. Salced

Halff, Albert H.

Novoa, Jose I.

Salcedo, Louis M.

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EFFECT OF URBANIZATION AND OTHER FACTORS ON SYNTHETIC UNIT HYDROGRAPHS by Albert H. H a m  Jose I. Novoa, and Luis M. Salcedo The public and private sectors of the United States are demanding more and better flood studies from the engineering profession. Flood studies are requested for streams with and without flood discharge records. The studies on gauged streams use frequency analysis of the gauge records and hydrographic analysis. A synthetic flood analysis based on the unit hydrograph method is the engineering method currently used for flood studies on streams without gauge records. The Snyder method, a typical synthetic hydrograph technique, is used in this study to evaluate the urban- ization factors that are needed for calculating the 100-year flood level on Dallas area streams. The synthetic unit hydrograph procedure as presented by F. F. Snyder is based on his study of streams in the Appalachian Mountains region (Snyder, 1938). Snyder's technique develops the shape of the unit hydro- graph, which is the building block for all flood hydrographs. His method is widely used by engineers in private practice and by those working for gov- ernment agencies and has therefore been carefully evaluated. Many regional studies have been published that relate t, to various physical properties of a watershed. T ,  is defined as the time to peak of the synthetic unit hydrograph from the centroid of the unit rainfall. For ex- ample, Eagleson (1962), Vansickle (1962), Carter (1961), Espey (1966), and Nelson (1970) have published papers on this subject. On September 1-3, 1970, the Hydrologic Engineering Center of the Corps of Engineers held a seminar on Urban Hydrology. Several of the papers addressed the problem of the change in runoff with increasing urbanization. The dominant theme of the presentations was that channelization, not just urbanization, deter- mines the ultimate effect on t,. Mr. Halff is President, Albert H.  Halff Associates, Inc., Dallas, Texas. Mr.  Novoa is Vice President, Albert H. Halff Associates. Mr. Salcedo is Manager, Hydraulics Section, Albert H. Halff Associates. 18 RICE UNIVERSITY STUDIES An additional paper by T. L. Nelson specifically related t, to a shape factor for varying percentages of urbanization in Dallas-Fort Worth water- sheds. LL,,/So5 is defined as the shape factor, where L is the stream length, LC, is the stream length to the centroid of the basin, and S is the weighted slope of the stream, Nelson's paper aIso states that further study is required "to improve the relationships developed, and to isolate the effect that channel improvement produces." With Nelson's statement in mind, our study investigated the relationship between t, and LL,,/So5, urbanization, and channelization for Dallas area streams. The percentage of urbanization was determined from U.S. Geological Survey 7.5 minute quadrangle maps. The study uses storms covering a 49- year time span, thereby covering a significant change of urbanization within four of the individual stations. The amount of urbanization was evaluated for each storm in each basin. The amount of urbanization was measured by planimeter for both the original and the photo-revised U.S.G.S. quadrangle maps. The next variable introduced into the analysis is the percentage of the main channel length, L, that is channelized. The amount and date of com- pletion of channelization is based on information obtained from the City of Dallas Public Works files, the City of Richardson, and U.S.G.S. 7.5 minute quadrangle maps. Data were collected on 84 storms in the following eleven basins: 1) White Rock Creek at Keller Springs Road 2) White Rock Creek at Greenville Avenue 3) TurtIe Creek at Hall Street 4) Bachman Branch at Midway Road 5) Big Fossil Creek at Maltom City 6) Village Creek at Handley 7) Joe's Creek at Spur 348 8) Duck Creek at Belt Line Road 9) Joe's Creek at Royal Lane 10) Elam Creek at Seco Road 11) Rush Creek at Arapaho Road. The 84 storms were reproduced using the Hydrologic Engineering Center HEC-I Flood Hydrograph Package (1973). The HEC-1 computer program contains a subroutine, OPTIM, that derives unit hydrograph parameters, including t,, using the actual rainfall and several other parameters. The results of the optimization are shown in table 1 (pp. 19-21). Snyder's t, is tabulated for each storm along with the percentage error between the ob- served peak discharge and the HEC-1 calculated peak discharge, based upon the HEC-1 reproductions. TABLE 1 Creek Percentage Percentage Percentage Observed Reproduced of LL,, of of Date Q Q Error f So Urbanization Channelization White Rock at  KeIler Springs White Rock at Greenville 20 RICE UNIVERSITY STUDIES 0 0 0 0 0 0 0 0 0 0 0 0 0 0  0 " " ~ " " ~ " ~ " " " ~ "  Y " " 2 2 " 2 ° m m m m  m o o o r -  - W W W b  Duck Creek at Beit Line Road Big Fossil Creek at Haltom City t ,  from F.W.D.-C.O.E. Village Creek at Handley f,from F.W.D.-C.O.E. estimated Rush Creek at Arapaho Road Elam Creek at Seco Road Joe's Creek at Royal Lane TABLE 2 MULTIPLE REGRESSION RESULTS Regression Analysis Dependent Variable TP Residual Standard Deviation 0.3189 Standard Error of the Mean 0.0348 ?? 0 Multiple R 0.9089 m Multiple RSQR 0.8261 C z Variable Entered URB 2 Variable m B-Coef Std Error of B Partial-R Beta-Coef Std Error of Beta ;d CHAN -0.0030 0.0013 - 0.2401 -0.1605 0.0725 z -I URB - 0.0009 0.0017 - 0.0620 - 0.0528 0.0951 4 LL /S 0.3383 0.0339 0.7445 0.7589 0.0760 cn Constant -0.1051 2 u CI ANALYSIS OF VARIANCE TABLE E Source D.F. Sum of Square Mean Square F Mean 1 0.24418E 02 0.24418E 02 Regression 3 0.38684E 02 0.12894E 02 0.12672E 03 Error 80 0.81405E 01 0.10175E 00 EFFECT OF URBANIZATION ON HYDROGRAPHS 23 The data on 84 storms in 1 1  different drainage basins included the per- centage of urbanization and the percentage of channelization associated with each storm. The data were analyzed using a multiple regression method on the variabIes Int, (In is the natural logarithm) versus InLL,,/SO 5 ,  percent- age of urbanization, and percentage of channelization. The regression anal- ysis summarized on table 2 provides the following equation: where URB = percentage of urbanization, CHAN = percentage of chan- nelization, and e = 2.7182. A plot of the resulting equation is shown in figure 1. The curves show only the extremes, i.e., Curve No. 1: 0 %  URB, 0% CHAN; Curve No. 2: 100% URB, 0% CHAN; Curve No. 3: 0% URB, 100% CHAN; and Curve No. 4: 100% URB, 100% CHAN. The analysis shows that a total of 82.6% of the variance in tp is ex- plained by the three variables in conjunction, using the procedure defined in Nie (1975). But of the 82.6'70, 80.3% is explained by LL,,/SO alone. The addition of channelization and urbanization picked u p  only 2.3% and 0.1 To, respectively, of the remaining 17.4% unexplained variance in tp. Urbanization and channelization do not significantly affect t, for this set of data. This may be explained either because the data do not reflect urbaniza- tion and channelization correctly or because urbanization and channeliza- tion do  not affect tp for these streams. A further possible explanation may be that the interrelationship between channelization and the shape factor LL,,/SO is causing an incorrect relationship t o  be used. Future studies should also account for the fact that the data for the 84 storms show per- centage of urbanization and percentage of channelization to have a fairly high correlation coefficient of 0.766. The same data were then analyzed by using a regression method on the variables lnt, versus InLL,,/SO 5.  The 84 storms provided the following re- lationship between tp and LL,,/SO 5 :  with a correlation coefficient of 0.90. The statistical analysis shows that LL,,/SO is by far the dominant in- dependent variable in the relation. The shape factor, based on this work, can account for enough of the variance in t, to justify a single variable anal- ysis. RICE UNIVERSITY STUDIES EFFECT OF URBANIZATION ON HYDROGRAPHS 25 Examination of the data showed that White Rock at Greenville, Bach- man at Midway, Duck Creek at Belt Line Road, and Joe's Creek at Spur 328 had significant changes in urbanization. Joe's Creek was the only one of the eleven streams that had a range of variance in channelization. A linear regression was run on t, vs. percentage of urbanization for the four streams, with results as shown in table 3. TABLE 3 Number Equation Stream Of Int, = A + (m)(URB) Correlation Floods Slope Intercept Coefficient (MI (A 1 White Rock at Greenville 13 -0.03 + 1.6 -0.31 Bachman at Midway 12 - 0.003 + 0.345 -0.08 Duck Creek at Belt Line Road 1 I + 0.01 0.85 + 0.57 Joe's Creek at Spur 348 8 0.001 0.1 + 0.04 The slopes of the linear equations are flat, up to the right and down to the left, The correlation coefficients are low. Either the sample is too small, or urbanization does not significantly relate to t, for these creeks and these storms. A multiple regression was run of lnt, vs. urbanization and channeliza- tion on Joe's Creek. The resulting equation is: In t, = 0.06 URB - 0.15 CHAN - 2.5. The correlation coefficient of t, vs. urbanization alone is 0.04. The correla- tion coefficient of t ,  vs. channelization is - 0.14. The multiple R for both URB and CHAN is 0.46. The correlation coefficients are low. Either the sample is too small or channelization does not significantly relate to t,. Since the shape factor alone left 20% of the variance in t, unexpIained, a new variable, storm movement, was studied to try to improve the relation- ship. Storm movement data are available from the United States National Weather Service. The information is in the form of observation log sheets prepared by the radar operators and also of microfilm photographs of the radar screens. The radar operator Iog sheets for the Dallas area from 1961 to 1976 are located at the Stephenville, Texas, Radar Station. The radar op- erators note the velocity and the azimuth of the storms. Our investigations related the storm movement azimuth and velocity term, SMOV, to the t, 26 RICE UNIVERSITY STUDIES values of 68 of the 84 storm reproductions for which storm movement data were available. The equation developed was: t, = 0.853 (SMOV) Oo3' eMX)3URB (LL,,/S) 346 . 0036CHAN The final results of the regression analysis relating t, to storm move- ment indicate that a storm moving upstream would decrease the t,. The addition of storm movement to the relationship accounted for little, if any, of the variance in t,. Therefore, the value of the relationship was discounted for the available data. The validity of the relationship of t ,  vs. urbanization, channelization, and the shape factor shown in figure 1 was further analyzed by testing its ability to  reproduce recorded storms. The test was performed using the same techniques used in developing synthetic storm discharges. The curves were tested by trying to reproduce the 84 storms of table 1. The Corps of Engineers of the Fort Worth District developed a compu- ter program, "Dallas," to facilitate the development of discharges for the Flood Insurance Agency study of the Dallas area. The program incorpo- rates the Corps loss rates for Dallas County, the T. L. Nelson urbanization curves, the rainfall depths from Technical Paper 40, and the Snyder's stor- age coefficient (640 C,) values for Dallas County. The program was modi- fied to accept the time to peak vs. shape, urbanization and channelization curves, actual rainfall, and various loss rates. The modifications did not alter the original methods of determining rainfall from T.P. 40, Snyder's coefficients, unit hydrograph ordinates, or flood hydrograph discharges. The 84 recorded storms of table 1 were incorporated in the program. The rainfall excess, as optimized by HEC-1 in the t ,  optimization, was the input rainfall. The flood peaks were predicted to within 29.6% of the recorded values by using the time to peak vs. shape and urbanization and channelization curves. Several reasons may be offered to explain the 30% mean error. First, the testing technique uses the entire drainage area above the gauge without any flood routing procedure, A flood routing procedure would account for the effect of tributary flow on the timing of the flood peak. Second, the test procedure assumes a uniform rainfall over the basin. Any variation in rain- fall over the drainage area may affect the flood peak. Third, a constant C, value was used in the test procedure. The C, value may be a function of many variables, such as rainfall. A constant C, may not be applicable to the basin over the time span of the various storms. Fourth, as Leo R.  Beard (1977) pointed out in a critique of the study, the t, value may be influenced EFFECT OF URBANIZATION ON HYDROGRAPHS 27 by the magnitude of the flood. Fifth, Beard also pointed out that the rela- tive Manning's "n" factors, the roughness coefficient of creeks, may also affect the timing of the peak of the unit hydrograph. The T. L. Nelson curves were also tested against the same 84 storms with all other variables remaining the same. The Nelson curves use only the urbanization factor, not the channelization factor. They predicted the flood peaks to within 36.4% of the recorded values. Even given its slightly better prediction record, the channelization factor needs more study before it can be accepted as a worthwhile addition to the equation relating time to peak and shape. Conclusions The data from 84 storms on 11 streams in the DalIas area showed that a strong relationship existed between the lag time (t,) and the shape factor (LL,,/SO 9. The slope of 0.4 in the equation lnt, = 0.04 lnLL,,/So5 - 0.37 checks Nelson's work for the Dallas area. Analysis of the same data for the relationship between lag time and ur- banization, channelization, and direction of storms was inconclusive. A narrow interpretation of the best equation using shape factor, urbanization, and channelization shows that t, is reduced by increasing urbanization and channelization. Further, the equation shows that for the data analyzed, channelization has a greater effect on t ,  than does urbanization. A more in- tensive analysis of the data, however, shows that the addition of channeliza- tion and urbanization to the relation explains only 2% of the variance in t,. Therefore, the overall conclusion is that percentage of urbanization and percentage of channelization do not significantly affect t, for this set of data. More information is needed to establish the final relationship among the variables. REFERENCES CITED BEARD, LEO R., June 24, 1977. Review of Dallas Urban Hydrology Cri- teria, letter to the District Engineer, U. S. Army Corps of Engineers, Fort Worth District. CARTER, R. W., 1961. "Magnitude and Frequency of Floods in Suburban Areas," US Geological Survey Prof. Paper 424-B, pp. B9-BI 1. EAGLESON, P. S., 1962. "Unit Hydrograph Characteristics for Sewered Areas," Proc. ASCE J. Hydraulics Div. 88, No. HYZ: 1-25. B P E Y ,  WILLIAM HOWARD, 1966. "A Study of Some Effects of Urbaniza- tion on Storm Runoff from a Small Watershed," Texas Water Devel- opment Board, Report 23. 28 RICE UNIVERSITY STUDIES HYDROLOGIC ENGINEERING CENTER U. S. Army Corps of Engineers, 1973. Computer Program 723-X6-L2010 HEC-1 Flood Hydrograph Package Users' Manual, U. S. Army Corps of Engineers. NELSON, THOMAS L., 1970. "Synthetic Unit Hydrograph Relationship Trinity River Tributaries Fort Worth Dallas Urban Area," Proceed- ings of a Seminar on Urban Hydrology. U. S. Army Corps of Engi- neers. NIE, NORMAN J. et al., 1975. Statistical Package for the Social Sciences, St. Louis: McGraw-Hi11 Book Company. SNYDER, FRANKLIN F., 1938. "Synthetic Unit-Graph," Transaction, American Geophysical Union Reports and Papers. VANSICKLE, D., 1962. "The Effects of Urban Development on Storm Run- off ," The Texas Engineer 32 : 12. 

Effects of Urbanization and Other Factors on Synthetic Unit Hydrographs

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Effects of Urbanization and Other Factors on Synthetic Unit Hydrographs

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