2. The Integration of Water Quality and Drainage Design Objectives

Introduction ............................................................................................................................................................1 Rainfall and Runoff Characteristics for Urban Areas .............................................................................................2 Small Storm Hydrology..........................................................................................................................................9 Stormwater Receiving Water Problems..............................................................................................................9 Typical Problems with Assumptions Commonly Used in Urban Hydrology Analyses ....................................10 Most of the Annual Rain is Associated With Many Small Individual Events...............................................10 The Rainfall-Runoff Inter-Relationships for Different Urban Areas are Surprisingly Similar .....................21 Varying Contributing areas are Important in Urban Hydrology ...................................................................26 Observed Runoff Volumes Do Not Compare Well With Commonly Used Urban Runoff Prediction Methods ........................................................................................................................................................27 Actual Volumetric Runoff Coefficients (Rv) Vary With Storm Size............................................................36 Small Storm Hydrology Model.............................................................................................................................38 Runoff Process for Paved Surfaces...................................................................................................................38 Infiltration of Rain Water Through Pavement Can be a Substantial Portion of the Total Rain for Most Events ...........................................................................................................................................................38 Variable Runoff Losses as a Function of Time Indicate Very Different Infiltration Values for Different Rain Intensities .............................................................................................................................................38 Infiltration in Disturbed Urban Soils ................................................................................................................38 Disturbed Urban Soils Do Not Behave as Indicated by Typically Used Models..........................................38 Basic Characteristics of the Small Storm Hydrology Model ............................................................................40 Comparison of the Small Storm Hydrology Model with the Horton Infiltration Equation...............................45 Comparison of the Small Storm Hydrology Model with the NRCS Curve Number Procedure........................47 Volumetric Runoff Coefficients can be Calculated for Different Surfaces and Rains using the Small Storm Hydrology Model .............................................................................................................................................48 Excellent Verification of Small Storm Hydrology Model for Many Conditions ..............................................50 Example Application using the Small Storm Hydrology Model.......................................................................50 Predicting Runoff Yields from Different Source Areas................................................................................51 Conclusions ..........................................................................................................................................................54 References ............................................................................................................................................................55