Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 200 TABLE OF CONTENTS F200 PROJECTION OF FLOWS AND HYDRAULICS OF SEWERS SECTION NO. SUBJECT DATE F 210 TERMINOLGY, DEFINITIONS AND ABBREVIATIONS June-92 F 220 PROJECTION OF FLOWS " F 221 TRIBUTARY AREA " F 222 DESIGN PERIOD " F 223 POPULATION ESTIMATE " F 224 LAND USE " F 225 PER CAPITA FLOWS " F 225.1 RESIDENTIAL FLOWS " F 225.2 COMMERCIAL FLOWS " F 226 INDUSTRIAL FLOWS " F 227 MAJOR POINT SOURCE DISCHARGES " F 228 INFILTRATION / INFLOW " F 230 DETERMINATION OF DESIGN FLOWS " F 231 MINIMUM VELOCITY " F 232 AVERAGE DRY WEATHER FLOW " F 233 PEAK DRY WEATHER FLOW " F 234 PEAK WET WEATHER FLOW " F 235 USE OF THE ADWF – PDWF CHART " F240 TYPES OF FLOW " F 241 TYPES OF OPEN CHANNEL FLOWS " F 242 SUPERCRITICAL AND SUBCRITICAL FLOW " F 250 DESIGN CRITERIA " Bureau of Engineering SEWER DESIGN Manual – Part F 6/92 F 200 SECTION NO. SUBJECT DATE F 251 CALCULATION OF PIPE SIZE June-92 F 252 MANNINGS ROUGHNESS COEFFICIENT " F 253 MINIMUM SLOPE " F 254 INVERT DROPS ACROSS MAINTENANCE HOLES (ALL " PIPES THE SAME SIZE) F 255 INVERT DROPS ACROSS MAINTENANCE HOLES " (OUTLET PIPE IS LARGER THAN THE INLET PIPE) F 256 SEWER DESIGN COMPUTATION SHEET " F 260 HYDRAULIC ANALYSIS OF JUNCTIONS " F 270 INVERTED SIPHONS " F 271 LOCATION DESIGN " F 272 SINGLE VS. MULTIPLE BARREL DESIGN " F 273 HYDRAULICS " F 274 HORIZONTAL ALIGNMENT " F 275 VERTICAL ALIGNMENT " F 276 HYDRAULIC DESIGN OF INVERTED SIPHONS " F 277 HYDRAULIC DESIGN OF AIRLINES " F 278 STRUCTURAL DESIGN " F 279 CORROSION RESISTANCE AND PREVENTION " F 280 MATERIAL SELECTION " F 281 APPURTENANT STRUCTURES FOR INVERTED SIPHONS " F 200 APPENDIX " EXAMPLE 1: EXAMPLE OF DESIGN FLOW CALCULATIONS " EXAMPEL 2: CALCULATIONS OF PIPE SIZE BY USE OF FLOW CHARTS EXAMPLE 3: PROJECTION OF FLOWS AND DESIGN OF " INTERCEPTOR SEWER PIPE Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 200 SECTION NO. SUBJECT DATE EXAMPLE 4: DESIGN OF NON-CIRCULAR SEWER PIPES June-92 Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 200 F 200 LIST OF TABLES TABLE NO. TITLE DATE F222 Minimum Design Periods for Wastewater Facility Components June-92 F229 Public & Commercial Facilities Average Daily Flow Projections " F250 Design Criteria for Sewer Pipe " F255 Additional Invert Drops Across MH when the Outlet is Larger " than the Inlet Sewer Pipe LIST OF FIGURES FIGURE NO. TITLE DATE F235 ADWF-PDWF Chart June-92 F237 Example Service Area " F242A Specific Energy Curve " F242B Hydraulic Jump at Change in Slope " F242C Critical Depth of Flow and Specific Head in Circular Pipes " F251A Sewer Drainage Basins (Tributary Area) of Interceptor/Sewer " AB F251B Superimposition of the CT Map on the Sewer Drainage Basin " Map F251A-F251O Flow Charts " F255 Invert Drops Across Maintenance Holes " F256 Gravity Sewer Design Computations Sheet " F260a-260o Hydraulic Analysis Junctions - Open Channel Flow " F272 Typical Two Barrel Siphon " Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 200 F 200 INTRODUCTION Section F 200 provides city standards, policy and procedures for the hydraulic design of sewers. It provides guidelines for the projections of residential, commercial and industrial wastewater flows. It also provides criteria for the inclusion of groundwater infiltration and extraneous inflow for sewer design. F 210 TERMINOLOGY, DEFINITIONS AND ABBREVIATIONS Terminology used in this Section is defined as follows: AVERAGE DRY WEATHER FLOW (ADWF): ADWF consists of average daily sewage flows and groundwater infiltration (GWI). CENSUS TRACT (CT): A defined area boundary developed for census purposes. The City is divided into approximately 700 CT areas. Population and employment projections are provided by CT. Each sewage drainage basin comprises CTs which are partially or wholly located within each basin boundary. DESIGN PERIOD: The length of time a sewage facility is anticipated to provide adequate service. Actual service life of the sewage facility may differ from the design period. DIURNAL FLOW: Fluctuation of flows over a 24 hour period. EQUALIZATION STORAGE: The storage of peaking flows to prevent overflows from the collection and conveyance systems. The stored wastewater is discharged back to the system during low flow periods. The storage can be online or offline. GROUNDWATER INFILTRATION (GWI): Groundwater that infiltrates pipeline and maintenance hole defects located below the ground surface. Groundwater infiltration is separate and distinguished from stormwater inflow. INFLOW: Drainage that enters the collection system through direct illegal or permitted connections, such as, catch basins, downspouts, area drains and MH covers. Inflow is separate and distinguished from infiltration. (See Stormwater Inflow) Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 210 INFILTRATION/INFLOW (I/I): The wastewater component caused by rainfall-dependent infiltration/inflow (RDI/I) and groundwater infiltration (GWI). PEAK DRY WEATHER FLOW (PDWF): PDWF consists of peak sewage flows plus GWI. PEAK WET WEATHER FLOW (PWWF): PWWF consists of PDWF plus RDI/I. PEAK FACTOR: Peak Factor is PDWF/ADWF. RAINFALL DEPENDENT INFILTRATION (RDI): RDI consists of rainfall that enters the collection system through GWI. RAINFALL DEPENDENT INFILTRATION/INFLOW (RDI/I): RDI/I consists of rainfall that enters the collection system through both RDI (infiltration) and SWI (inflow) sources. SERVICE AREA: The sewer service area served by a collection system or a wastewater treatment plant. The City of Los Angeles comprises the Hyperion Service Area (HSA) and the Terminal Island Service Area (TISA). SERVICE LIFE: The operational life of a sewage facility which should exceed the design period of the facility, provided it is designed, constructed and maintained properly. STORMWATER INFLOW (SWI): SWI consists of rainfall runoff that enters the system through direct connections such as catch basins, downspouts and area drains. SYSTEM ANALYSIS MODEL (SAM): SAM is a computer model used for modeling the City's interceptor system for various flow conditions. SEWERAGE DRAINAGE BASIN: A drainage area which boundaries are determined by gravity flow. The HSA comprises 20 basins and 206 sub-basins. The TISA is one basin with 8 sub-basins. TRIBUTARY AREA: The tributary area of a sewage system consists of all areas which contribute flow to the sewer by gravity and/or force main discharges. These include sanitary sewer as well as I/I flows. Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 210 WASTEWATER FLOW RATE UNITS: Commonly used flow rates are: cfs cubic feet per second gpcd gallons per capita per day gpd gallons per day gpapd gallons per acre per day gped gallons per employee per day gpm gallons per minute mgd million gallons per day Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 220 F 220 PROJECTION OF FLOWS Each service area has its own unique characteristics. For this reason, there is no one correct approach to the projection of flows within a service area. Procedures presented in this section for the development of flows are intended as guidelines. The Engineer is encouraged to use his own initiative and judgement for the projection of flows in conjunction with these guidelines. When possible, the Engineer should measure flows to verify parameters used to project future flows including residential, industrial, commercial and I/I flows. Key locations for monitoring flows include major interceptors, sewer outfalls, major point-source discharges and wastewater pumping plants. The Engineer may prepare a request form to WSED to measure flows. The procedure and information required to obtain flow measurements is outlined in Section F 512.14. The following parameters shall be evaluated to project wastewater flows and are discussed in further detail in this section. Also, example problems illustrating the projection of flows and hydraulic design of pipes are shown in the appendix of this section. a. Tributary Area b. Design Period c. Population Estimate d. Land Use e. Per Capita Flows f. Residential Flows g. Commercial Flows h. Industrial Flows i. Major Point source Discharges j. Infiltration/Inflow F 221 TRIBUTARY AREA The tributary area of a sewer includes all areas which will contribute flow to the system. It includes flows from the ultimately developed service area and basin to basin flow routings. Potential service areas, such as, areas served by septic tanks and local treatment facilities should also be assessed for possible inclusion in the tributary area. The area may be limited by Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 221 natural topography, natural or human-made barriers, political boundaries or economic factors. As- Built drawings and Wye and Sewer maps should be reviewed to help define the tributary area boundary. Although they may not be up-to-date, sewer maps (1" = 400' with contours) and drainage maps may be helpful in determining the tributary area. F 222 DESIGN PERIOD The design period is that length of time the capacity of the sewerage facility is anticipated to be adequate to service its tributary area. It must be determined before design of the facility is commenced. A standard for minimum design periods for various components of a sewer system are summarized in Table F 222. For force mains, pumping plants, and other items not documented herein, see Section F 700. TABLE F 222 MINIMUM DESIGN PERIODS FOR WASTEWATER FACILITY/COMPONENTS Wastewater Facility/Component Minimum Design Period (Yrs) Trunk, interceptor, outfall, 60 - 100 and relief sewers - sewers 18- inch in diameter and greater Lateral sewers - sewers less 100 than 18-inch in diameter F 223 POPULATION ESTIMATE The population estimate for the tributary area is the basis for computing the design flow. It is customary to multiply the estimated population by the estimated per capita wastewater contribution. Because the population estimate is the basis for the computation of design flows, it is important that it be as accurate as possible. Generally, population projections prepared for land use planning have shorter projection periods than are required for the design period for sewerage facilities. However, the Advance Bureau of Engineering SEWER DESIGN Manual - Part F 6/92 F 223 Planning Report (APR) projects resident and employment populations for the years 2010, 2050 and 2090. APR population projections are distributed based on Southern California Association of Governments (SCAG) population distributions and are shown by Census Tract (CT). This information is periodically updated. The Engineer may obtain this information from WPMD. F 224 LAND USE Land uses help define population densities and types of contributors to wastewater flows within the tributary area. Zoning maps and field review of land use can be used to verify the reasonableness of long range projections. However, because land use planning is typically done in increments for shorter time periods than the design period for a sewer, their use should be limited primarily to confirmation of shorter term flow projections. F 225 PER CAPITA FLOWS Per capita wastewater flows are less than per capita water consumption because of water lost to lawn irrigation, swimming pools, washing cars, etc. The unit flow rates will vary from area to area. The per capita flow rate can be estimated from flow measurement and census data. The average per capita wastewater flow in the Los Angeles area is estimated at 90 gpcd. The average employee wastewater flow is estimated at 30 gped. F 225.1 RESIDENTIAL FLOWS Residential flows may be estimated by multiplying the population times the per capita flow of 90 gpcd unless data is available to support a higher rate. F 225.2 COMMERCIAL FLOWS Commercial flows may generally be accounted for by multiplying the employment population times the estimated employee wastewater flow of 30 gped and utilizing flow rates in the Public and Commerical Facilities Average Daily Flow Table F 227 in the appendix to this section. Commercial operations which may contribute significantly greater flows, such as, car washes, laundry facilities, etc., shall be investigated by the Engineer. See F 227 for major point sources