120 Sewer Flow Measurement
sediment and debris deposition, turbulence, confined space/hazardous conditions is-
sues, access, variable pipe slope along a reach resulting from differential settlement
of individual pipes, and different pipe sizes. Nevertheless, continually increasing
environmental concerns and the need to more optimally manage stormwater and
wastewater flows have increased the need to accurately monitor flows in storm, sani-
tary and combined sewers. These concerns are not new, e.g., North Rhine-Westphalia,
Germany, issued a decree that the most important detention facilities of the com-
bined sewer network were to be equipped with continuous monitoring devices more
than 20 years ago (Weyand, 1996). Fortunately, as the need for measurement devices
capable of high accuracy has increased new and improved measurement techniques
also have been developed over the last 20 years. This chapter attempts to summarize
the accuracy, advantages, and disadvantages of the available techniques.
There are basically two basic types of flow measurement techniques: (1) those that
rely on a relation between stage and discharge, e.g., Manning’s equation and flumes;
and (2) those that estimate average velocity by acoustic or electromagnetic means
and multiply this by the cross-sectional area obtained through a depth measurement
device and known conduit geometry. Most of these devices have two parts: (1) a
primary device that directly interacts with or controls the flowing water; and (2) a
secondary device for measuring water depth (Church et al., 1999).
This chapter focuses on the general characteristics of the various measurement
techniques of types 1 and 2 and does not provide a direct comparison of the com-
mercially available equipment for flow measurement in sewers that apply the various
measurement techniques. Due to the limited space available and the limited num-
ber of independent evaluations of measurement equipment, a proper comparison of
the equipment cannot be done here. Further, it is not the purpose of this chapter
to advocate or criticize any particular device, but rather to give the readers basic
information on the measurement techniques to aid in the selection of the appropriate
technique. When purchasing equipment readers should carefully review the liter-
ature provided by the manufacturers, discuss experience with the equipment with
professional colleagues, and apply the time-honoured principle of caveat emptor (let
the buyer beware).
2.2.1.1 Purposes of Flow Monitoring
There many reasons for flow monitoring, among the most common are:
(1) Real-time control (RTC) of the sewer system. Existing large sewers can be
controlled by gates, e.g., to increase storage capacity and prevent overburdening
of treatment plants (Curling et al., 2003). RTC also can optimize treatment
plant operation to ensure consent standards are met and to minimize the total
pollutant load reaching the environment (Watt and Jeffries, 1996) or improve
plant efficiency in order to provide capacity for future sewer extensions (Anon.,
1996).
Introduction 121
(2) Sewerage system operational considerations. Information about storage and dis-
charge conditions can, for example, be the basis for optimizing cleaning and
maintenance work (Weyand, 1996).
(3) In regional sewerage networks, flow monitoring can equitably allocate costs
among communities.
(4) Compliance with regulatory requirements.
(5) Provide data for calibration and verification of numerical models (Baughen and
Eadon, 1983).
(6) Identify inflow and infiltration (I/I) problems.
(7) Performance evaluations of pumps and hydraulic structures (e.g., overflow
structures).
Items (1)–(4) typically require long-term, effectively permanent, monitoring,
whereas items (5)–(7) generally require only temporary monitoring. On the basis
of 10 years of sewer monitoring experience in Germany, Weyand (1996) made two
very important, related observations: (1) it is important to start the planning of
monitoring systems with the formulation of necessary demands; and (2) experience
shows that the requirements of monitoring systems rise with their use. Thus, it is
quite possible that sites that originally were established for a temporary study may
become long-term sites, and careful selection of equipment and sites is necessary.
2.2.1.2 Equipment Selection Considerations
Huth (1998) prepared a list of considerations for selection of flow measurement
equipment. Six of his eight issues are:
(1) Know the relative strengths and weaknesses of the available equipment.
(2) Buy the level of accuracy required for the application. For example, high accu-
racy is needed for RTC, cost allocation, and model calibration and verification;
whereas lesser accuracy may be required for I/I studies, basic sewerage system
operation, and performance evaluations of hydraulic structures. However, it must
be remembered that requirements of monitoring systems rise with their use.
(3) Know your flow rate. Sanitary sewers may have fairly constant flows, whereas
storm and combined sewers have wider flow ranges and require equipment that
is accurate over a wide range of flow conditions. Curling et al. (2003) stressed
the importance of having high accuracy over the full range of flows noting that
varying accuracy will increase variation in modelling results and lead to poor
understanding of problem sites, improper estimation of capacity, and improper
allocation of capital improvement funds.
122 Sewer Flow Measurement
(4) Learn what is in the water. Debris may clog some equipment (flumes) and reduce
the performance of other equipment (acoustic transducers) requiring frequent
maintenance, also high particulate loads may affect the ability of sound waves
to penetrate the flow.
(5) Location, location, location (discussed in detail in Section 2.2.1.3).
(6) Make sure there is power.
Similarly, Church et al. (1999) noted that selection of the most appropriate method
for collection of accurate flow data that are representative of a particular site re-
quires knowledge of the flow regime(s), range of flow rate and depth, rapidity of
flow changes, channel geometry, and the capabilities and accuracies of the methods
available for measuring flow.
2.2.1.3 Monitoring Locations
The importance of proper site selection cannot be overstated (Church et al., 1999).
Most of the flow measurement techniques described in this chapter work best at sites
where fully developed, uniform, open channel flow not subject to backwater effects
is present composing optimal hydraulic conditions. Fully developed, uniform open
channel flow usually requires many diameters of straight, uniform, undisturbed
pipe upstream and downstream of the measurement location. For example, Johnson
(1995) notes that the British Standard 1042 recommends that upstream from the
measurement point a straight length of pipe equal to 30 to 50 diameters is sufficient
depending on the type of turbulence causing device, whereas downstream from
the measurement point 5 diameters of straight pipe should be present. Shorter
sections of straight pipe could affect flow measurement accuracy. The accuracy
of some methods also may decrease due to backwater effects and transitions from
open-channel to pressurized pipe-full flow.
Practical considerations may make it necessary to place a monitor at a location with
nonoptimal hydraulic conditions. For example, important locations, such as over-
flows, bifurcations, and known flooding points, may require individual monitoring
irrespective of hydraulic conditions (Baughen and Eadon, 1983). Borders between
communities may require monitoring irrespective of hydraulic conditions for ‘po-
litical reasons’ in cost allocation. Accurate model calibration and verification may
require monitoring of each subcatchment (Baughen and Eadon, 1983). Finally, local
constraints such as accessibility, power supply, and nonhydraulic goals of monitoring
may also necessitate using nonhydraulically optimal sites. For example, monitoring
locations might be selected for ease of pollutant sampling regardless of hydraulic
conditions, as was the case of combined sewer monitoring in the Chicago, USA,
area reported by Waite et al. (2002).
Some of the techniques discussed in this chapter are better at measuring flow
under nonhydraulically optimal conditions than others. Thus, once the monitoring
Introduction 123
locations are selected the following questions (after Church et al., 1999) must be
considered:
(1) Is the flow measuring technique applicable to the flow and channel characteristics
at the site?
(2) Is the flow measuring technique capable of measuring the full range of flows?
(3) Will the flow measurements be of sufficient accuracy to meet the objectives of
the study?
2.2.1.4 Characteristics of Ideal Sewer Flow Measurement Equipment
In order to deal with the complex hydraulic environment of sewer systems, Wenzel
(1975) recommended that the ideal device for flow measurement should have the
following characteristics:
(1) capability to operate under both open channel and full flow conditions;
(2) a known accuracy throughout the range of measurement;
(3) a minimum disturbance to the flow or reduction in pipe capacity;
(4) a minimum of field maintenance;
(5) compatibility with real-time remote data transmission;
(6) reasonable construction and installation costs.
Drake (1994) further suggested that the equipment must provide reliable and
accurate level and/or flow measurements within dynamic conditions, withstand a
corrosive environment, overcome turbulence, and resist entanglement with floating
matter.
2.2.1.5 Quality Assurance and Quality Control
For any flow monitoring, but particularly for sewer flow, detailed quality assurance
and quality control (QA/QC) programmes are necessary. Church et al. (1999) de-
scribe in detail the key components of a QA/QC programme for flow monitoring,
and their main QA/QC components are summarized as follows:
(1) Frequent and routine site visits by trained/experienced personnel to maintain
equipment and keep the site clean.
(2) Redundant methods for measuring flow.
124 Sewer Flow Measurement
(3) Technical training of project personnel. Weyand (1996) also stressed that it is
necessary to have specially trained and qualified staff for operating and calibrat-
ing the sewer flow meters.
(4) Frequent review by project personnel of data collected. Weyand (1996) also
noted that data quality must be continually checked to detect equipment
malfunctions.
(5) Quality audits, in the form of periodic internal reviews.
(6) Quality audits, in the form of periodic external reviews.
Church et al. (1999) noted that frequent calibration of equipment is necessary
because of the difficult monitoring environment, and that the difficulties of measuring
in this environment result in a high probability of incomplete record, even when
stations are well maintained and properly calibrated.
2.2.2 MANNING’S EQUATION
The simplest form of stage-discharge relation is obtained by assuming that Manning’s
equation is valid for the selected monitoring location. Using Manning’s equation
discharge, Q, is calculated as
Q=1
nA(h)R(h)2/3S1/2(2.2.1)
where nis Manning’s roughness coefficient, A(h) is the cross-sectional area of flow,
R(h) is the hydraulic radius of the flow, his the depth (or pressure head for full-pipe
flow) of flow, and Sis the energy slope of the flow. In this technique, his measured
using a pressure transducer or bubbler system, Aand Rare calculated as a function
of hfrom the known conduit geometry, Sis approximated as the pipe slope, and nis
estimated from standard tables on the basis of pipe material and condition. Soroko
(1973) noted that Manning’s equation may be appropriate for discharge calculation
in channels with a straight course of at least 61 m, preferably longer, the course being
free of rapids, abrupt falls, and sudden contractions or expansions.
The primary advantage of this technique is that only a stage measurement device
is needed to estimate flow. The primary disadvantages of this technique are that
proper estimates of Sand nare difficult to obtain. For steady, uniform flow in a
channel as specified by Soroko (1973) the bed slope equals the energy slope, how-
ever, for unsteady, nonuniform flow common in storm and combined sewers the bed
slope and energy slope diverge. Further, even in cases where the bed slope approxi-
mates the energy slope well, determination of the bed slope is difficult. Most often
the bed slope is estimated from design plans, but this can be substantially different
from the actual pipe slope. For example, Melching and Yen (1986) compared ‘as
built’ measurements of pipe slopes between manholes with the slope indicated on