Introduction to Storm Sewer Design

Transcription

1 A SunCam online continuing education course Introduction to Storm Sewer Design by David F. Carter

2 Introduction Storm sewer systems are vital in collection and conveyance of stormwater from the upstream contributing watershed area to a receiving downstream water body such as a lake or stormwater facility. Proper design of this system is essential in avoiding upstream flooding, erosion and sediment transport. A working knowledge of hydrology and hydraulics is required to produce an efficient design that prevents creation of adverse impacts. This course intends to provide the user with the basic tools needed to achieve the desired end result - a cost efficient and environmentally effective design. Rational Method System hydrology is usually analyzed using the Rational Method which is expressed in the equation: Where: Q = C x I x A Q = Peak runoff rate for a design return period (frequency) in cubic feet per second (cfs) C = Runoff coefficient for a design return period expressed as the dimensionless ratio of rainfall excess to total rainfall I = Rainfall intensity in inches per hour during a period of time equal to the time of concentration for the design return period. This value can be taken from an Intensity-Duration-Frequency (IDF) curve for the location being analyzed or computed from an equation derived from same. Tc = Time of concentration for the longest flow path in the contributing watershed area, in minutes, used to determine I A = Contributing watershed drainage area, in acres, tributary to the design point The usual procedure for calculating the time of concentration for a watershed involves at least three types of flow: overland or sheet, shallow concentrated and open channel. One acceptable way to account for each of these types of flow is through an approach that considers the average velocity for each particular flow segment. This method requires calculating a travel time using the equation: Copyright 2010 David F. Carter 2 of 20

3 Where: t= i L/(60)v i i t i = Travel time for velocity segment i, in minutes L i = Length of the flow path for segment i, in feet v i = Average velocity for segment i, in ft/sec The time of concentration is then calculated as: t c = t 1 + t 2 + t t i Where: t c = Time of concentration, in minutes t 1 = Overland flow travel time, in minutes t 2 = Shallow concentrated travel time, in minutes t 3 = Open channel travel time, in minutes t i = Travel time for the i th segment, in minutes Overland flow travel times can be evaluated using Figure 1-Overland Flow Velocities for Various Land Use Types to obtain an estimate of the average velocity following these steps: 1. Using available topographic and aerial mapping, determine the average slope of the overland flow path, in percent, and identify the land use cover. 2. Enter the x-axis with the average slope from Step 1 and run a vertical line to the curve which represents the land use cover of the overland flow path. 3. Run a horizontal line from the point of intersection determined in Step 2 to the y-axis and read the overland flow velocity in feet per minute. 4. Convert the velocity obtained in Step 3 to feet per second by dividing by Use the above equation to calculate the overland flow time (t 1 ). Copyright 2010 David F. Carter 3 of 20

4 Figure 1 Overland Flow Velocities for Various Land Use Types Copyright 2010 David F. Carter 4 of 20

5 Shallow concentrated flow travel times can be evaluated using Figure 2-Shallow Concentrated Flow Velocities to obtain an estimate of the average velocity following these steps: 1. Using available topographic mapping, determine the average slope of the shallow concentrated flow path, in percent, and determine whether it is rill or gutter flow. 2. Enter the y-axis with the average slope from Step 1 and run a horizontal line to the curve which represents either rill or shallow gutter flow. 3. Run a vertical line from the point of intersection determined in Step 2 to the x- axis and read the shallow concentrated flow velocity in feet per second 4. Use the above equation to calculate the shallow concentrated flow time (t 2 ). Other types of shallow concentrated flow, as well as open channel flow, can be evaluated using Manning s Equation. Manning s Equation System hydraulics is usually analyzed using Manning s Equation: Where: Q = (1.49/n) x A x 2/3 R x S Q = Discharge rate for design conditions in cfs n = Manning s roughness coefficient (dimensionless) A = Cross-sectional area of conduit in square feet R = Hydraulic radius A/P in feet. For a circular channel flowing either full or halffull, the hydraulic radius is D/4 where D is the diameter. P = Wetted perimeter in feet, used to determine the hydraulic radius R S = Slope of the energy grade line in ft/ft Copyright 2010 David F. Carter 5 of 20

6 Figure 2 Shallow Concentrated Flow Velocities Reference: USDA< SCS, TR-55 Draft (1984) Copyright 2010 David F. Carter 6 of 20

7 Storm Sewer Tabulations Table 1-Storm Sewer Tabulation, is an example of a storm sewer tabulation form recommended for tabulating the results of hydrologic and hydraulic calculations for storm drain systems. Design of this system usually follows these steps: 1. Select inlet locations and develop preliminary plan. 2. Conduct hydrologic calculations to determine design flow rates. 3. Conduct hydraulic calculations to determine culvert sizes needed to convey flow rates determined in Step Design outfall including required retention/detention facility with the goal of dissipating energy to prevent erosion and sediment transport. The parameters to be recorded on this form are described below: Runoff Coefficients (C) Available literature abounds with tables giving runoff coefficient values based on land use, soil type and watershed slope. Table 2-Runoff Coefficients for a Design Storm Return Period of 10 Years or Less gives such values for a design storm return period of 10 years or less. It is customary in many areas to design storm sewers based on the 5-year return period. For return periods longer than 10 years, a design storm frequency factor is applied. The designer has the choice of using a single coefficient value or different values for high and low coefficient areas contributing runoff to a specific inlet. For example, a highly impervious area would have a higher coefficient than a low impervious area such as a park. A combined business/park area might be 60 percent high and 40 percent low. A summary of runoff coefficient determinations for the example project appears in Table 3-Runoff Coefficients. Likewise, a summary of hydrologic parameters appears in Table 4-Hydrologic Parameters. Copyright 2010 David F. Carter 7 of 20

8 Table 1 FRANKLIN WOODS STORM SEWER TABULATION BY: CARTER STRUCT. STRUCT. LENGTH DRAINAGE AREA (AC.) MANHOLE ELEV. OF H.G. Notes TYPE NO. (FT) C= 0.4 TIME OF OR CROWN ELEVATION Florida Zone 4 IDF C= 0.2 SUB Tc FLOW TOTAL Q INLET FLOW LINE EL. 5 Year Storm C= TOTAL (MIN.) IN INTENSITY (CA) TOTAL TOP UPPER LOWER FALL DIAM. SLOPE Velocity Capacity 1 foot minimum cover INCRE- SUB (CA.) SECTION (IN/HR) RUNOFF ELEVATION (FT.) (IN.) (%) (fps) (CFS) MENT TOTAL (MIN) (CFS) (FT.) Remarks TYPE Minor Loss = " C.I. S Loss Coeff. = " C.I. S RCP COVER = 1.57 TYPE Minor Loss = " C.I. S Loss Coeff. = 0.60 "J" MH S-2A HDPE COVER = 1.43 TYPE Minor Loss = " C.I. S Loss Coeff. = 0.50 "J" MH S-2A RCP COVER = 1.74 TYPE Minor Loss = 0.03 "J" M.H. S-2A Loss Coeff. = " C.I. S HDPE COVER = 1.70 TYPE Minor Loss = " C.I. S Loss Coeff. = " C.I. S RCP COVER = 1.75 TYPE Minor Loss = " C.I. S Loss Coeff. = 0.80 "J" MH S-5A RCP COVER = 1.80 TYPE Minor Loss = 0.02 "J" M.H. S-5A Loss Coeff. = " C.I. S HDPE COVER = 2.67 TYPE Minor Loss = " C.I. S Loss Coeff. = 0.80 "J" MH S-6A HDPE COVER = 2.29 TYPE Minor Loss = 0.02 "J" M.H. S-6A Loss Coeff. = " M.E.S. S-6B HDPE COVER = Copyright 2010 David F. Carter 8 of 20

9 Copyright 2010 David F. Carter 9 of 20

10 Runoff Structure Basin Slope Land Use Soil Type Coefficient S-1 B-1A Flat (0-2%) *SFR: 1/4-acre lots Sandy 0.4 B-1B Flat (0-2%) pasture Sandy 0.2 S-2 B-2 Flat (0-2%) SFR: 1/4-acre lots Sandy 0.4 S-3 B-3 Flat (0-2%) SFR: 1/4-acre lots Sandy 0.4 S-4 B-4 Flat (0-2%) SFR: 1/4-acre lots Sandy 0.4 S-5 B-5 Flat (0-2%) SFR: 1/4-acre lots Sandy 0.4 S-6 B-6 Flat (0-2%) SFR: 1/4-acre lots Sandy 0.4 * SFR = Single Family Residential Table 3 Runoff Coefficients Drainage *Time of Rainfall Inlet Area Concentration Intensity Runoff Flow Rate Structure Type (acres) (min) (in/hr) Coefficient (cfs) S-1 curb inlet S-2 curb inlet S-2A manhole 0 NA NA NA NA S-3 curb inlet S-4 curb inlet S-5 curb inlet S-5A manhole 0 NA NA NA NA S-6 curb inlet S-6A manhole 0 NA NA NA NA S-6B mitered end section 0 NA NA NA NA TOTAL 5.48 Table 4 Hydrologic Parameters * This is the time of concentration for the longest hydraulic flow time to that particular point including overland flow, gutter flow (if applicable) and sewer flow. Copyright 2010 David F. Carter 10 of 20

11 Notes: Data related to the system such as rainfall zone, storm event frequency, minimum cover for culverts, etc. should appear here. Structure Type This data is usually given in abbreviated format such as C.I for curb inlet, D.B.I for ditch bottom inlet and J M.H. for type J manhole. Structure Number This is a unique identifier giving the sequential numbers of the drainage structures in the system. A common convention is to number them in ascending order from upstream to downstream. That is, the outfall might be S-10 while the upstream most structure might be S-1. Refer to Figure 3-Storm Sewer Schematic for an example storm sewer schematic. Length The distance in feet from the centerline of the structure in question to the centerline of the next structure. Since pipe is manufactured in standard 8-foot lengths, it is best practice to try to design the system using this incremental length if possible. Increment This is the incremental area contributing runoff to the inlet in question. Manholes have no incremental area as no runoff is being collected at that point. Subtotal This is the subtotal of all the incremental areas contributing runoff through the structure in question. Subtotal (CA) This is the subtotal of the contributing incremental areas multiplied by its respective runoff coefficient. Copyright 2010 David F. Carter 11 of 20

12 Figure 3 Storm Sewer Schematic Copyright 2010 David F. Carter 12 of 20

13 Tc (min) This is the time of concentration required for the flow contribution to travel from the most upstream point of the contributing drainage area to that particular point of the system. This time includes the initial overland flow and gutter flow plus the flow time within the sewer system. Time of Flow in Section (min) This is the flow time for the section of pipe in question. Intensity Values for this parameter depend on the design frequency and the time of concentration. If taken from an intensity-duration-frequency (IDF) curve, it is the intensity in inches per hour for the design frequency storm of duration equal to the time of concentration. An example of an IDF curve appears in Figure 4-Typical Rainfall Intensity-Duration-Frequency Curve. The following equation can be used to approximate values from this particular curve based on time of concentration for the 5-year return period storm event: 0.93 I= 145/(Tc+20) Total (CA) If more than a single runoff coefficient value is used for the structure being analyzed, this is the sum of the subtotal CA s determined for each value. Q Total Runoff (cfs) This is the product of the intensity and the total CA. Manhole or Inlet Top Elevation (ft) If the structure is a curb inlet, the gutter elevation is given. For a manhole, the top elevation is shown. If the structure is a ditch bottom inlet, the grate elevation and the slot elevation (if present) are shown and so noted. Some jurisdictions may require Copyright 2010 David F. Carter 13 of 20

14 Figure 4 Typical Rainfall Intensity-Duration-Frequency-Curve Copyright 2010 David F. Carter 14 of 20

15 compliance with a minimum grate or inlet elevation, such as the 25-year high water elevation in the receiving water body. Elevation of Hydraulic Gradient (H.G.)-Upper This is the elevation, under design conditions, to which water will rise in the various inlets and manholes. The elevation of the H.G. should be compared to the manhole or inlet top elevation to ensure that flow remains inside the designed structure and that any required freeboard is available during conditions of peak flow. Elevation of Hydraulic Gradient (H.G.)-Lower This is the elevation of the hydraulic gradient at the downstream end of the pipe section. Crown Elevation-Upper This is the inside top of the pipe elevation at the upstream end of the section under consideration. Normal convention is to give this number to even tenths of a foot for 18-inch pipes and larger, and to even five one-hundredths of a foot for 15-inch pipes. Crown Elevation-Lower This is the inside top of the pipe elevation at the downstream end of the section under consideration. The same convention mentioned above applies. Flow Line-Upper The invert elevation of the upstream end of the pipe is shown here. Flow Line-Lower The invert elevation of the downstream end of the pipe is shown here. Copyright 2010 David F. Carter 15 of 20

16 Fall (feet) This is the hydraulic gradient fall given to the nearest one-hundredth of a foot. It does not include minor losses as does the difference between upper and lower hydraulic gradient. Fall (feet) The total physical fall of the pipe section is given to the closest one-tenth of a foot. Diameter (inches) The diameter of the culvert is given in inches. If the culvert is elliptical, the rounded equivalent is used. If it is a box culvert, the width and height are given in feet. An abbreviated description of the pipe is shown below the diameter. For instance, RCP means reinforced concrete pipe; HDPE means high density polyethylene pipe. Note: When transitioning from a smaller upstream pipe to a larger downstream pipe, the most hydraulically efficient practice is to match crown elevations. Note: Local design criteria usually dictates minimum pipe size for certain applications, such as under roadways. Although HDPE is a good substitute for RCP in certain applications, local design criteria may limit the maximum size for certain applications, such as under roadways. HDPE is much lighter than RCP and easier and cheaper to install, but does not stand up to loading stress as well as RCP. Some of the newer material, such as Hardy pipe, may also be good for certain applications. Local design criteria should always be consulted before specifying these materials. Slope (%) The slope of the hydraulic gradient is shown over the physical slope of the pipe. The minimum physical slope insuring minimum flow velocity depends on the Manning s coefficient which is dependent on the pipe material and diameter. The hydraulic gradient is calculated as: S = [Q/((1.49/n) x A x 2/3 R )] 2 Copyright 2010 David F. Carter 16 of 20

17 Some typical values of Manning s n for various materials appear in Table 5- Manning s n Values for Various Pipe Materials. Velocity (fps) The velocity resulting from the slope of the hydraulic gradient is given over the velocity resulting from the physical slope of the pipe. Velocities less than 2.5 fps should be avoided as this will result in deposition and buildup of sediment inside the pipe, which will impede flow. Likewise, high velocities in excess of 7.5 fps will enhance erosion and should also be avoided. Velocity is determined by the relationship V = Q/A. Capacity (cfs) The capacity of the culvert on the slope of the hydraulic gradient is given over the capacity of the culvert on the physical slope. Minor Loss (feet) Table 5 Manning's 'n' Values for Various Pipe Material Surface Manning's 'n' Concrete High Density Polyethylene (HDPE) Polyvinyl Chloride (PVC) Uncoated cast iron Ductile iron Corrugated Metal Pipe (CMP) Wrought-iron, black Wrought-iron, galvanized Vitrified clay This is calculated by multiplying the velocity head (v 2 /2g) by the loss coefficient (K) to determine the loss due to the configuration of the system. Loss Coefficient This is the coefficient used to determine the minor loss described above. Refer to Figure 5-Head Loss Coefficients for Estimating Energy Losses Through Manholes/Junctions for these coefficients which are based on system configuration. Cover (feet) This is the difference between the Manhole or Inlet Top Elevation (ft) and the Crown Elevation-Upper. It does not consider the thickness of the pipe. Minimum cover is Copyright 2010 David F. Carter 17 of 20

18 Figure 5 Head Loss Coefficients for Estimating Energy Losses through Manholes/Junctions Copyright 2010 David F. Carter 18 of 20

19 usually dictated by the local permitting authority, but in no case should be less then one foot. Spreadsheet Storm sewer tabulations are easily done with a computer spreadsheet program such as Excel. A lookup table can be used interactively to import pipe data for the various calculations. An example of such a table can be seen in Table 6-Lookup Table for Spreadsheet Calculations. The flow rates shown here have not yet had the slope factor applied to them. This operation occurs after they are imported by the storm sewer tabulation spreadsheet Concerning the storm sewer tabulation spreadsheet (Table 1), the only parameter that needs to be adjusted for geographic location is the rainfall intensity-duration-frequency. This is because the intensity parameter used in the spreadsheet is the intensity in inches per hour for the design frequency storm of duration equal to the time of concentration for the watershed in question. This data can be either taken from the appropriate IDF curve or calculated by equation which can be entered into the spreadsheet program. Copyright 2010 David F. Carter 19 of 20

20 Table 6 Lookup Table for Spreadsheet Calculations D A R *Flow Rates(cfs) for Various Manning's n Values DIAM AREA HYDRAULIC RADIUS PVC RCP RCP(>36") RCP(>54") HDPE (INCHES) (SQ FT) (FEET) , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,588.0 EQUIV , , , , , X X X X X X X , , X , , , , , X , , , , , X , , , , , X , , , , , X , , , , ,755.0 * The flow rates shown here have not yet had the slope factor applied to them. This operation occurs after they are imported by the storm sewer tabulation spreadsheet. Copyright 2010 David F. Carter 20 of 20