City of West Lake Hills Drainage and Erosion Control Design Manual May 2020 TABLE OF CONTENTS Chapter 1 Introduction ....................................................................................................... 1 1.1 Purpose and Scope ....................................................................................................... 1 1.2 Applicability .................................................................................................................... 1 1.3 Waivers ............................................................................................................................ 1 1.4 Amending the Manual .................................................................................................. 1 1.5 References and Definition of Terms ............................................................................ 1 Chapter 2 Drainage Criteria .............................................................................................. 3 2.1 Permit Submittal Components ..................................................................................... 3 2.1.1 Preliminary Drainage Plan ......................................................................................... 3 2.1.2 Type I Development Submittal ................................................................................. 4 2.1.3 Type II Development Submittal ................................................................................ 4 2.1.4 Type III Development Submittal ............................................................................... 4 2.2 Finished Floor Elevations ................................................................................................ 5 2.3 Drainage Facility Design ............................................................................................... 5 2.3.1 Stream Bank Erosion Hazard Setbacks .................................................................... 6 2.3.2 Temporary Erosion Control ........................................................................................ 6 2.3.3 Drainage Easements and Right-of-Way ................................................................. 6 2.3.4 Freeboard.................................................................................................................... 7 2.4 Stormwater Mitigation ................................................................................................... 7 2.5 Development in the Floodplain ................................................................................... 7 2.6 Permanent Water Quality Controls ............................................................................. 8 2.7 Maintenance of Drainage Facilities ............................................................................ 8 2.7.1 Maintenance Access ................................................................................................ 8 Chapter 3 Determination of Storm Runoff ......................................................................... 9 3.1 General Requirements .................................................................................................. 9 3.2 Design Rainfall ................................................................................................................ 9 3.2.1 Rainfall Intensity Duration Frequency ...................................................................... 9 3.2.1 Rainfall Depth Duration Frequency ......................................................................... 9 3.3 The Rational Method ................................................................................................... 10 3.3.1 Runoff Coefficient .................................................................................................... 10 3.3.2 Time of Concentration ............................................................................................ 11 3.3.3 Rainfall Intensity ........................................................................................................ 13 3.4 NRCS Unit Hydrograph ................................................................................................ 13 3.4.1 Curve Number .......................................................................................................... 13 3.4.1 Lag Time .................................................................................................................... 14 Chapter 4 Design of Drainage Infrastructure .................................................................. 15 4.1 General Requirements ................................................................................................ 15 4.2 Street Flow ..................................................................................................................... 15 4.2.1 Flow at Intersections ................................................................................................ 15 4.2.2 Permissible Spread of Water ................................................................................... 15 4.2.3 Street Flow Calculations .......................................................................................... 15 4.3 Inlet Design ................................................................................................................... 16 4.3.1 Inlet Types and Descriptions ................................................................................... 16 4.3.2 Inlet Capacity Calculations .................................................................................... 17 City of West Lake Hills | Page i Drainage and Erosion Control Design Manual May 2020 4.4 Storm Drain Systems ..................................................................................................... 17 4.4.1 General Requirements ............................................................................................ 18 4.4.2 Design Criteria .......................................................................................................... 18 4.4.3 Calculation of the Hydraulic Grade Line .............................................................. 19 4.5 Open Channels ............................................................................................................ 19 4.5.1 Design Criteria .......................................................................................................... 19 4.5.2 Roughness Coefficients ........................................................................................... 20 4.5.3 Channel Analysis ...................................................................................................... 21 4.5.4 Supercritical Flow ..................................................................................................... 21 4.5.5 Shear Stress ................................................................................................................ 21 4.5.6 Energy Dissipators ..................................................................................................... 23 4.6 Bridges and Culverts .................................................................................................... 24 4.6.1 General Requirements ............................................................................................ 24 4.6.2 Bridge Design Criteria .............................................................................................. 24 4.6.3 Culvert Design Criteria ............................................................................................. 24 4.6.4 Culvert End Treatments ........................................................................................... 25 4.6.5 Culvert Hydraulics .................................................................................................... 25 4.6.6 Culvert Outlet Protection ........................................................................................ 27 4.6.7 Energy Dissipation .................................................................................................... 27 Chapter 5 Detention Facilities ......................................................................................... 28 5.1 General Requirements ................................................................................................ 28 5.2 Design Criteria .............................................................................................................. 29 5.3 Outlet Structure Design ............................................................................................... 29 5.3.1 Orifices ....................................................................................................................... 29 5.3.2 Weirs ........................................................................................................................... 30 5.3.3 Discharge Pipes ........................................................................................................ 32 5.3.4 Emergency Overflow Weirs ..................................................................................... 32 Chapter 6 Water Quality Controls ................................................................................... 34 6.1 Applicability .................................................................................................................. 34 6.2 Design Criteria .............................................................................................................. 34 6.3 Maintenance ...............................................................................................................