4 What is a Storm Sewer System? A storm sewer system is composed of surface components (e.g., gutters and inlets) and subsurface components (e.g., pipes, manholes, inlet boxes)
5 Storm Sewer DesignStorm sewer design and analysis consists of 2 basic parts:Surface flow calculationsSubsurface flow calculationsSurface flow calcs evaluate the capacity of gutters (i.e., gutter spread and depth) and inlets (for inlets on grade, part of the flow, called “bypass flow,” will not be picked up and will continue down to the next inlet).Subsurface flow calcs evaluate the capacity of the subsurface pipes to prevent flooding. Storm sewer pipes should always point downhill, and the systems are branched (i.e., typically, there should not be loops).
7 Gutters Flow typically travels to storm sewer inlets though gutters The engineer is interested in the width and depth of the gutter flow, and has to make sure that neither is excessive.Gutter flow is analyzed using a form of Manning’s equation
8 InletsThree common inlet types are grate inlets, curb inlets, and combination inlets.Inlets may be located on a continuous grade or in a sag locationInlets on grade do not intercept 100% of the flow that comes to them—some of the flow bypasses and goes to the next inlet downgradeHEC-22 calculation methods are used to determine gutter spread and depth and inlet capacity
17 A little more theory… Open Channel Flow GutterOpen channel flow is flow that has a free water surface open to the atmosphere.It occurs in natural rivers and streams, manmade ditches and channels, gutters, and gravity-flow pipes.
18 Conservation of Energy In open channel flow, the pressure head term (p/γ) is replaced by the vertical flow depth yThe energy equation between sections 1 and 2 for the channel shown is written as:
19 Open Channel Flow Definitions Normal FlowIf a channel shape remains constant for a long enough distance, the flow will reach a constant “normal depth.”Varied FlowFlow depth typically varies along the length of a channel due to factors like changing channel shape or flow depths other than normal depth on the upstream or downstream end.Varied flow can change gradually along a channel (“gradually varied flow” or GVF) or rapidly (in the case of a hydraulic jump).Supercritical vs. Subcritical FlowThese are the 2 basic flow types possible for flow in an open channel.Supercritical flow is shallow, high-velocity flowSubcritical flow is deeper, slower-velocity flowFlow can transition from subcritical to supercritical flow, or vice versaBetween supercritical flow and subcritical flow is the “critical depth.” This value can be determined for any channel and used to classify the flow type. Sometimes, it is also used as a starting point (boundary condition) in GVF calculations
20 Open Channel Flow Types Normal FlowTransition from Subcritical to Supercritical flowTransition from Supercritical to Subcritical Flow (Hydraulic Jump)
24 Rational Method Q = c × i × A To compute a peak flow rate from a watershed, a method such as the Rational Method can be used:Q = c × i × AQ is the peak discharge from the drainage areac is the “runoff coefficient” (the fraction of rainfall that is converted to runoff)i is the intensity of the rainfall for a design storm event having a duration equal to the drainage area “time of concentration”. This can be obtained using intensity-duration-frequency curves for the locale (see below).A is the area
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