CVE 341 Water ResourcesCHAPTER 16

Design of Hydraulic Controls & StructuresLecture Notes 6:

Design of Hydraulic Controls & StructuresVery Complex taskIncorporates many elements of engineering practiceConcerns when designing Hydraulic Structures

SafetyEfficiency ReliabilityCost effectiveness Environmental concern

CULVERT versus BRIDGEThe span length is the main criteria to demarcate between a culvert & a bridgethe span is less than 6.5m (or 20 ft), it is called a culvert and it is a bridge when the span is more than the specified lengthA culvert is a conduit for the passage of surface drainage water under a highway, railroad, canal, or other embankment;

A bridge is a structure carrying a pathway, roadway, or railway over a waterway. A culvert usually is either circular or rectangular in cross-section, so it has an all-enveloping structure (i.e. a roof, 2 walls and a floor Bridge sits on foundations on each bank (or on wing-walls), and as it does not have a floor, it is called a bridge.

Bridges

Culverts

Key Design ConsiderationsWe will focus on CULVERT.

CULVERTBRIDGEHydraulic performance Safety and security Conveyance of sediment, debris and trash Environmental acceptability Ease of construction Ease of O&MDesign flows Design flow depths & velocity Acceptable backwater Acceptable afflux Effects of blockage/ debris Acceptable clearance in flood for floating debris Acceptable scour depths If river is navigable, consideration of boat/ship strike.

Hydraulic characteristics of culvertsA culvert consists typically - an inlet, an outlet and between these a culvert barrel of length L and height D - a trash screenThe barrel is normally on a slope, So and the depths of water above the upstream and downstream invert levels are known as the headwater depth (HW) and tailwater depth (TW) respectivelyCIRIA Report 168, Culvert Design Guide

Flow ControlFlow control is a fundamental principle in hydraulic design and a control point is a feature that controls the water level/discharge relationship. It is the point in a channel or culvert which has the lowest capacity. In other words points upstream and downstream of the control point have a greater capacity than at the control point. In the context of culverts, both inlets and outlets can control the flow.

CIRIA Report 168, Culvert Design Guide

Inlet & Outlet ControlInlet Control

Flow is controlled by headwater depth and inlet geometryUsually occurs when slope of culvert is steep and outlet is not submergedSupercritical, high v, low dOutlet Control

When flow is governed by combination of headwater depth, entrance geometry, tailwater elevation, and slope, roughness, and length of culvert

Subcritical flow

Frequently occur on flat slopes

Concept is to find the required HW depth to sustain Q flow

Tail water depth often not known (need a model), so may not be able to estimate for outlet control conditions

TYPES OF FLOW THROUGH CULVERTSThe six principal types of flow which can occur in culvertsdepending on

the flow rate, the upstream and downstream water levels and the geometry of the culvert.

In each case, the flow is controlled either at the inlet/the outlet or downstream of the culvert.

Type I flow Critical depth at inletIn this case the culvert flows with a free surface and the flow is controlled at the inlet. The flow accelerates as it contracts into the culvert and the water surface passes through critical depth slightly downstream of the inlet face of the culvert. The water surface remains below critical depth for the full length of the barrel and the tailwater elevation is below critical depth at the outlet.CIRIA Report 168, Culvert Design Guide

Type II flow Critical depth at outletThe culvert flows with a free surface and the flow is controlled at the outlet. The culvert barrel slope is less than critical slope and the tailwater depth is less than critical depth at the outlet. Under these conditions the outlet behaves like a weir, and critical flow occursCIRIA Report 168, Culvert Design Guide

Type III flow Subcritical flow throughoutIn this case the culvert flows with a free surface and the water level is controlled downstream of the outlet (see Figure 2.5). This is the type of flow which occurs most frequently in culverts and the type for which the culvert should be designed. The tailwater depth is above critical depth at the outlet but the tailwater elevation is below the soffit of the culvert. CIRIA Report 168, Culvert Design Guide

Type IV flow Submerged outletIn this case both the inlet and outlet are fully submerged and the culvert barrel flows full. The water level is controlled at a point downstream of the outlet.CIRIA Report 168, Culvert Design Guide

Type V flow Rapid flow at inletA high headwater depth could cause Type V flow to occur where the flow separates at the inlet of the culvert. In this case the culvert inlet behaves as an orifice, and critical flow occurs just inside the culvert barrel. The water surface remains below critical depth for the full length of the barrel, and the tailwater depth is below critical depth at the outlet. A hydraulic jump will occur downstream of the outlet.CIRIA Report 168, Culvert Design Guide

Type VI flow Full flow with a free outletIn this case the culvert flows full throughout its length but the tailwater elevation is below the culvert soffit at the outlet. The control point occurs at or downstream of the outlet, and the culvert therefore operates under outlet control.A variant on Type VI flow is the case where the culvert flows full for part of its length but the water surface then separates from the culvert soffit. CIRIA Report 168, Culvert Design Guide

HYDRAULIC ANALYSIS OF CULVERTSThe engineer usually undertakes the hydraulic analysis of culverts for one of two reasons:

to determine a suitable size for a new culvert that satisfies the design discharge, culvert length, permissible headwater depth and outlet velocity

to calculate the peak capacity of an existing culvert for which all the dimensions are known.

Culvert DesignMost culverts operate under downstream control. This means that the hydraulic computations proceed from the downstream in the upstream direction. The design discharge and allowable headwater elevation are initially established. Other constraints such as culvert shape, material, aesthetics, etc., are specified.Assume a culvert size and check performance assuming both inlet and outlet control. Whichever gives the highest HW elevation controls the hydraulic performance.Compare culvert performance with design constraints, and select the smallest (least expensive) size that meets the criteria.

An Easy Way to Size CulvertsThere are several free software:

(for example from US Federal Highways administration Has been used for a long time and is very well respected New Graphical Interface Allows for multiple culverts to be specified at a variety of locations on a mapPlease see also Handout for Culvert (Hand) Design Procedurewww.riverconveyance.net/aes/partners.html

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