AN ANALYSIS of the SCS METHOD in the SIMULATION of STORMWATER DISCONNECTION in an URBAN WATERSHED by EILEEN M. ALTHOUSE a Thesis
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AN ANALYSIS OF THE SCS METHOD IN THE SIMULATION OF STORMWATER DISCONNECTION IN AN URBAN WATERSHED By EILEEN M. ALTHOUSE A thesis submitted to the Graduate School-New Brunswick Rutgers, The State University of New Jersey in partial fulfillment of the requirements for the degree of Master of Science Graduate Program in Bioresource Engineering written under the direction of Dr. Christopher Obropta and approved by ___________________________________ Dr. Christopher Obropta __________________________________ Dr. Christopher Uchrin __________________________________ Dr. Eric Vowinkel New Brunswick, New Jersey May 2007 ABSTRACT OF THE THESIS An Analysis of the SCS Method in the Simulation of Stormwater Disconnection in an Urban Watershed by Eileen M. Althouse Thesis Director: Dr. Christopher C. Obropta Small storms can have adverse affects on downstream water quality in urbanizing watersheds because impervious surfaces convey greater volumes of runoff and lead to larger storm flows than under natural conditions (Booth 1990; Beard and Chang 1979). Therefore management of the water quality storm (1.25” of rain over 2 hours) has been targeted in water quality initiatives. This study examined whether reducing the effective impervious area that contributes runoff during the water quality storm by disconnecting it from the stormwater conveyance system could be a viable stormwater management solution in existing residential areas. Disconnection was examined in the Pompeston Creek Watershed, Burlington County, New Jersey on the lot, subdivision, and watershed scale. A calibrated HEC-HMS model of the watershed was used for the watershed scale analysis. The SCS equations to were applied to simulate disconnection by routing runoff from the disconnected impervious surface over an adjacent impervious surface. The 2, 10, and 100-year storms were examined in addition to the water quality storm. Three primary conclusions were made: 1) the composite curve number method, and therefore the composite curve numbers ii given in TR-55, under-predicts small storm runoff when compared to the volume weighted method because the composite curve number does not account for the runoff conveyed by directly connected impervious surfaces in urbanizing areas; 2) by disconnecting the runoff from the impervious areas by routing it over the pervious area, the runoff volume can be reduced for the water quality storm; 3) the effectiveness of disconnection in mitigating the runoff volumes relies on the infiltration capacity of the pervious area. Both the extent of the under-prediction of the composite curve number and the relative volume reduction achieved by disconnection decrease as storm depth increases. The adjustment of the basin curve number during model calibration for small observed storm events suggested that the original composite curve number method was inadequate in predicting runoff in the watershed scale model for small storms. Basin-wide reductions in runoff volumes with the application of basin-wide disconnection were consistent with the reductions predicted on the smaller scales. These results have a direct application to regional stormwater management planning as disconnection can be a suitable retrofit for the management of the small storm in existing residential areas. iii ACKNOWLEDGEMENTS Thank you to my father, Richard, and sisters, Cate and Georgine, for their unconditional support throughout my college experience. I would like to especially thank my advisor, Christopher Obropta, for his continued support over the years. I am grateful to all the members of my committee, Christopher Obropta, Christopher Uchrin, and Eric Vowinkel for their assistance and patience. An extra special thank you goes to Debra Lord for her vital assistance in the field and invaluable knowledge of the Pompeston Creek Watershed. I would also like to thank Sandra Goodrow, Katie Buckley, Robert Miskewitz, and Lisa Evrard for their technical guidance. Thank you to Amy Boyajian, Mike Mak, and Steve Yergeau for their assistance in the field. Thank you to all of my other colleagues from the Water Resources Program, including Greg Rusciano, Cheryl Burdick, Josef Kardos, Peter Kallin, Jim Moore, Mehran Niazi, and Sean Walsh for their support. I also appreciate the support and assistance of Rita Lehman, Jeanie Nicewicz, Dawn Skoube, and Veronica Tompkins. A special thank you goes to Martha Rajaei for her procedural guidance and support. I also appreciate the support of friends from the Environmental Science Graduate Student Association: Fang Liu, Priya Narasingarao, Imtiaz Rangwala and Samriti Sharma. This project was made possible through the New Jersey Department of Environmental Protection Division of Watershed Management 319(h) Funds. iv TABLE OF CONTENTS ABSTRACT..............................................................................................................................ii ACKNOLEDGEMENTS..........................................................................................................iv LIST OF TABLES....................................................................................................................vi LIST OF FIGURES.................................................................................................................vii INTRODUCTION .....................................................................................................................1 Problem definition ................................................................................................................ 1 Disconnection as a BMP....................................................................................................... 3 Purpose.................................................................................................................................. 6 LITERATURE REVIEW ..........................................................................................................8 Research on Watershed Impacts of Impervious Surface Cover............................................ 8 Research on Directly Connected Impervious Surfaces and Disconnection as a Management Strategy ............................................................................................................................... 16 TR-55 and the SCS Curve Number .................................................................................... 28 HEC-HMS....................................................................................................................... 31 Research on application of the SCS equation..................................................................... 32 MATERIALS AND METHODS.............................................................................................40 Lot Scale Disconnection ..................................................................................................... 45 Subdivision Scale Disconnection........................................................................................ 49 Watershed Scale Disconnection.......................................................................................... 61 HEC-HMS Model Calibration ........................................................................................ 61 RESULTS ................................................................................................................................71 Lot Scale Disconnection ..................................................................................................... 71 Subdivision Scale Disconnection........................................................................................ 79 HEC-HMS Calibration...................................................................................................... 101 Watershed Scale Disconnection........................................................................................ 113 DISCUSSION........................................................................................................................120 Comparison of the Two Methods at the Lot Scale and Subdivision Scale....................... 120 Sensitivity of runoff volume to curve number in the SCS equations ........................... 123 Influence of directly connected impervious surfaces.................................................... 128 Curve number in watershed calibration ........................................................................ 136 Small Storm Management through Disconnection........................................................... 140 Lot Scale Disconnection ............................................................................................... 140 Subdivision Scale Disconnection.................................................................................. 143 Application of Field Data to Subdivision Scale Disconnection.................................... 146 Subdivision Curve Number Analysis............................................................................ 152 A New Urban Curve Number Table ............................................................................. 153 Runoff Reduction through Disconnection on the Watershed Scale.............................. 154 CONCLUSIONS AND RECOMMENDATIONS ................................................................155 REFERENCES ......................................................................................................................162 APPENDIX............................................................................................................................166