Stormwater Conveyance Design 1 John Reimer City of Madison Engineering Department

Hydrologic Cycle http://www.mde.state.md.us/environment/wma/stormwatermanual/

Stormwater Runoff Water falls as rain, snow, or ice. Most seeps into ground. If ground is saturated, frozen, or has paved surfaces, water flows & is called stormwater runoff. Stormwater Flows over surfaces such as roads, driveways and parking lots.

Where does Stormwater Go? Stormwater flows to storm drains along streets. It may carry soil, pet waste, oil, pesticides, & other pollutants with it. This polluted runoff goes to streams & lakes untreated.

Stormwater Quantity Impacts Impervious surfaces cause higher runoff volume Increased flooding potential High velocity stormwater runoff causes stream erosion Stream erosion causes habitat and property loss Changes to streambed morphology Spring Street, Madison, WI 2008

Stormwater Quantity Impacts Stable Channel Downcutting Widening Sedimentation Stable, Entrenched Pre- to Post-Development

Growth and Development Urbanization happens… Madison, WI 1867 2013

Stormwater and Urbanization Arrow lengths indicate increase/decrease Urbanization Runoff Hydrograph Post-development Increases Peak Increase Volume Decreases Time to Peak Discharge Pre-development Time

Flow Hydrograph Area under hydrograph represents volume of surface runoff Qpeak

Separate Storm Sewer Street Runoff Inlet Pipe Flow Combined Sewer

Stormwater Management Stormwater Runoff Urbanization Stormwater Controls Design and Review Focus

Methods of Calculating Runoff Statistical analysis of streamflow records Regional methods Transfer methods Rational Method Tabular Method (TR-55) Computer Models

Statistical Analysis of Streamflow Records

Regional Methods Q100=48*A0.660*S0.349*I0.172 Correlation of a dependent variable, usually a discharge of specified recurrence interval, with causative or physically related factors such as watershed area and stream slope for a specified area Assumption: Underlying assumption in a regional method is that the watershed to which it is being applied is one of or similar to those waterhsed that were used to develop the method Procedure: Watershed parameters such as area, stream slope, and percent imperiousness are determined and quantified using appropriate data sources. These parameters are then substituted into the regional method equations and flows of specified recurrence interval are computed Example: Q100: 100-year recurrence interval discharge A: Drainage Area S : Channel Slope I: Impervious area Q100=48*A0.660*S0.349*I0.172

Transfer Method A discharge of specified recurrence interval for tributary area of known size and runoff characteristics is used to estimate a discharge of the same recurrence interval for a larger or smaller watershed having similar runoff characteristics.

Rational Method Qp = CiA C = runoff coefficient also called the “coefficient of imperviousness” i = rainfall intensity (in/hr) A = drainage area (acres) (tc = time of concentration < rainfall duration)

C-Typical Values (by surface) Forested 0.059-0.2 Asphalt 0.7-0.95 Brick 0.7-0.85 Concrete 0.8-0.95 Shingle roof 0.75-0.95 Lawns, well drained (sandy soil) up to 2% slope 0.05-0.1 2% to 7% slopes 0.10-0.15 over 7% slope 0.15-0.2 Lawns, poor drainage (clay soil) up to 2% slope 0.13-0.17 2% to 7% slopes 0.18-0.22 over 7% slope 0.25-0.35 Driveways, walkways 0.75-0.85 Ref: Civil Engr. Ref. Manual, 6th ed., Michael Lindeburg, PE, Professional Publications, Inc. ISBN: 0-921045-45-0

C-Typical Values (by use) Farmland or Pasture 0.05-0.3 Unimproved 0.1-0.3 Parks or Cemeteries 0.1-0.25 Railroad Yard 0.2-0.4 Playgrounds (not asph or conc) 0.2-0.35 Business Districts neighborhood 0.5-0.7 city (downtown) 0.7-0.95 Residential single family 0.3-0.5 multi-plexes, detached 0.4-0.6 multi-plexes attached 0.6-0.75 suburban 0.25-0.40 apartments, condominiums 0.50-0.70 Industrial light 0.5-0.8 heavy 0.6-0.9 Ref: Civil Engr. Ref. Manual, 6th ed., Michael Lindeburg, PE, Professional Publications, Inc. ISBN: 0-921045-45-0

Rational Coefficient C Must be weighted if you have different area types within the drainage area Drainage area = 8 acres: 2 acres; C=0.35 (residential suburban) 6 acres; C=0.2 (undeveloped-unimproved) Weighted C=[(2)(.35)+(6)(.2)]/8 = 0.24

TR-55 Approach (See previous lecture for detail calculations) Curve Number Approach Soil Group A, B, C, D from coarse grain to fine grain Land Cover Hydrologic Condition Poor-Higher runoff Good-Lower runoff Antecedent Moisture Condition Cropping practice Agriculture Download WinTR-55 http://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/water/?&cid=stelprdb1042901

Computer Models Storm Water Management Model (SWMM) Developed by EPA Rainfall-runoff simulation model used for single event or long-term simulations The routing portion of SWMM transports runoff through a conveyance system of pipes, channels, storage, pumps, and regulators

Stormwater Flow in Street The hydraulic capacity of a pavement cross section is the quantity of stormwater flow that does not exceed an allowable interference criteria for width of spread or depth at the curb Design approach is to selct a location for analysis and to determine the width T and depth d of flows Manning equation, with modification, is used. Because T may be 40 times the depth d, the hydraulic radius R can be set equal to the depth d. When integrated across the width of flow, the resulting modified equation is: Q = K(Z/n)So1/2d8/3 K=0.375 metric or 0.56 English n=Manning’s coefficient Q=Stormwater flow Z=Reciprocal of the cross slope T/d So=Longitudinal grade D=Stormwater flow depth

Catchbasins and Inlets Admit runoff to sewer system Should be designed not just placed Different types for different applications Inlet Catchbasin

Detail of Inlet

Inlet Design Orifice Weir Inlet computations involve three elements: Quantity of stormwater aproaching inlet Quantity of stormwater intercepted by the inlet Quantity of stormwater that bypasses inlet and is carried over Orifice Weir

Detail of Catch Basin

Storm Sewer Design (Inlet and Manholes) Inlet Spacing Is needed and depends on street slope, crown, permissible spread, and bypass flows Manhole Spacing Needed at junctions of pipes, grade changes, and alignment changes Commonly every 300’ to 400’

Storm Sewer Design (Energy Considerations) Energy grade line is assumed to be parallel to the sewer slope Generally slopes of the sewer are kept to a minimum sufficient to maintain proper flow situations. Storm slopes are frequently parallel to the ground surface Transition through junctions/manholes are made as streamline as possible in order to minimize turbulence

Storm Sewer Design (Minimum & Maximum Velocities) Velocities must be kept above the minimum scour velocity in order to prevent deposition of solids in the sewer For storm sewers, the minimum velocities should be 2 ft/sec or greater Maximum velocities at full flow should generally be less than 15 ft/sec in order to prevent excessive erosion of the pipe material

Storm Sewer Design (Pipe Materials) Various shapes are available: Corrugated Metal Pipe (CMP) Concrete (RCP) PVC High Density Polyethylene (HDPE)

Pipe Friction Losses Major head loss in a storm sewer collection system is that caused by friction along the periphery of the pipe Most apply Manning’s formula: Darcy-Weisbach equation (only used when a pipe is flowing under pressure): Sf=V2n2/2.22 R4/3 hf=SfL hf=f(L/D)(V2/2g)

Minor Losses Occur whenever turbulence, other than friction, is introduced into the system. Can occur at bends, entrances, junctions, manholes, outlets, and transitions Can be determined by using energy equations or can be estimated by using simpler methods as discussed in previous lectures with gravity sanitary sewer design

Minor Head Loss (Energy Equation) Q1V1 Q2V2

Minor Head Loss (Derived from Bernoulli and Darcy-Wisbach Equations) General Form No Change in Pipe Sizes Manhole Losses Flow Bend Losses Flow f Angle Less than 40o

Types of Flow Control Based on a variety of laboratory tests and field experience, two basic types of flow control have been defined for culverts: Inlet control Inlet control occurs when the culvert barrel is capable of conveying more flow than the inlet will accept. Upstream water surface elevation and inlet geometry are the primary factors influencing culvert capacity Outlet Control Outlet control occurs when the culvert barrel is not capable of conveying as much flow as the inlet opening will accept. All the geometric and hydraulic characteristics of the culvert play a role in determining culvert capacity.

Inlet Control The flow passes through critical depth just downstream of the culvert entrance and the flow in the barrel is supercritical. The submergence of the outlet end of the culvert does not assure outlet control. In this case, the flow just downstream of the inlet is supercritical and a hydraulic jump forms in the culvert barrel. The inlet end is submerged and the outlet end flows freely. Again, the flow is supercritical and the barrel flows partly full over its length. A hydraulic jump will form in the barrel. The median inlet provides ventilation of the culvert barrel. If the barrel were not ventilated, sub-atmospheric pressures could develop which might create an unstable condition during which the barrel would alternate between full flow and partly full flow.

Outlet Control Classic full flow condition, with both inlet and outlet submerged. The barrel is in pressure flow throughout its length. Outlet submerged with the inlet unsubmerged. For this case, the headwater is shallow so that the inlet crown is exposed as the flow contracts into the culvert Entrance submerged to such a degree that the culvert flows full throughout its entire length while the exit is unsubmerged. This is a rare condition, it requires an extremely high headwater to maintain full barrel flow with no tailwater. Outlet velocities are unusually high under this condition The culvert entrance is submerged by the headwater and the outlet end flows freely with a low tailwater. For this condition, the barrel flows partly full over at least part of its length (subcritical flow) and the flow passes through critical depth just upstream of the outlet With neither the inlet nor the outlet end of the culvert submerged. The barrel flows partly full over its entire length, and the flow profile is subcritical

Culvert Design Modeling Computer Model (HY 8) Developed by the Federal Highway Administration (FHWA) Interactive culvert analysis program to compute the culvert hydraulics and water surface profiles for various pipe shapes Users can analyze inlet and outlet control for full and partially full culverts, analyze tailwater, analyze flow over the roadway embankment, and balance flows through multiple culverts

Storm Sewer Design (Design Storm Frequency) Pipe Sizing Should base the pipe sizing so they just flow full at the design storm frequency Normally the design for the residential areas storm piping is based on a 10 year frequency event Normally the design for detention pond overtopping is based on a 100 year frequency event Always analyze the final design to determine the effect of larger storms

Procedure for Storm Sewer Design Hydrology Delineate Watersheds Determine quantity of stormwater runoff Select rainfall intensity and recurrence interval Apply Method Rational Method, TR-55, etc.

Procedure for Storm Sewer Design Storm Sewer Layout Sewer location should be determined by evaluation of construction, maintenance, replacement costs, and utility conflicts Sewer depth should be defined by hydraulics, structural loading, and utility conflict considerations

Procedure for Storm Sewer Design Computations Use hydrologic calculations to determine hydraulic conditions are satisfied (i.e. does the pipe convey the proper amount of water from the contributing area) Verify inlet flows are achieved and at low points Calculate pipe flows (Manning’s Equation)

Case Study: Storm Sewer Design Procedure Final Grading Plan with Flow Directions

Case Study: Storm Sewer Design Procedure Delineation of Basins Identify discharge points Determine drainage patterns based upon grading plan for each phase Identify off-site drainage and drainage from undisturbed areas Delineate drainage subbasins for each phase Determine area of each subbasin including bypass drainage

Case Study: Storm Sewer Design Procedure Delineation of Basins Schematic Layout

Case Study: Storm Sewer Design Procedure Design Criteria: Quantity Determination: Rational Method Storm Frequency: 10 Year Recurrence Interval Friction Flow Formula: Manning (n=0.013 for RCP) Minimum Velocity: 3 feet/second Minimum Pipe Diameter: 12 inch per code Materials: Reinforced Concrete Pipe

Case Study: Storm Sewer Design Procedure Computations From Manning’s Q=1.49/n A R2/3 S1/2 Q=CiA Reach Rational Method Peak Discharge Pipe Diameter 1-0 2.4 cfs 12 in 1-1 3.0 cfs 1-2 4.1 cfs 15 in 1-3 11.6 cfs 21 in 1-4 25.0 cfs 24 in 1-5 1-6 38.2 cfs 1-7 43.0 cfs 30 in

Questions