Application of SWMM to the Flooding Problem in Truro Nova Scotia.pdf

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Canadian Water Resources Journal
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Application of SWMM to the Flooding Problem in
Truro, Nova Scotia
Ayad El-Sharif & David Hansen
To cite this article: Ayad El-Sharif & David Hansen (2001) Application of SWMM to the Flooding
Problem in Truro, Nova Scotia , Canadian Water Resources Journal, 26:4, 439-459, DOI: 10.4296/
cwrj2604439
To link to this article: https://doi.org/10.4296/cwrj2604439
Published online: 23 Jan 2013.
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Application of SWMM to the Flooding Problem
in Truro, Nova Scotia
Submitted September 2000; accepted May 2001
Written comments on this paper will be accepted until June 2002
Ayad El-Sharifland David Hansen2
ABSTRACT
The town of turo, Nova Scotia, has a long history of flooding problems, partly due
to its proximity to the Bay of Fundy and the extreme tidal variations that occur in
the Bay. The nature and extent of the unsteadiness of the flow in the downstream
reaches of the Salmon River, which flows through Truro, are lesser known. These
aspects of the hydraulics are complicated by the fact that many of the complaints
about water damage appear to be related to the storm sewer system, much of which
is o1d, and none of which could be considered to actually manage stormwater. Yet,
management of stormwater is indicated because the Salmon River is dyked along
much of its lower reaches, with aboiteau. present in some of these dykes. In this
study, the nature of the unsteadiness in the dynamic waves moving through the river
system and the relative importance of tidal effects were examined. The ability to
model the river and the storm sewer system in a unified manner was considered very
important, partly because of the aboiteau. The well-known U.S. trPA hydrologic/
hydraulic model 'SWMM' v.4.31. was selected for this purpose. It was of interest
to see how difficult it would be to adapt SWMM to the hydraulic problems of
Truro, even though SWMM is primarily used for designing/evaluating networks
of closed circular conduits. A representative part of the town's storm sewer system
was incorporated into the model to provide an indication of the severity of flooding.
Results were both interesting and favourable. It was found that the dynamic wave
caused by a 2-year flood event and a 100-year tide reached upstream as far the
Walker St. Bridge. This indicates that flood events result in a situation whereby a
one-way aboiteau, designed to permit the exit of Truro's stormwater, cannot do so
during the passage of large translatory waves. In addition, preliminary outcomes
of modelling Truro's stormwater system suggest that some of the flooding is often
caused not by an excess of water overtopping the dykes along the Salmon River,
but by the inadequacy of the storm sewers and the lack of any real management of
stormwater. Any future stormwater management plan should take into consideration
l Former Research Assistant and
2
Associate Professor, Dalhousie University, Sexton Campus, Department of Civil
E,ngineering, Halifax, NS
Vol.26, N0.4,2001
439

the need for the translatory wave to first pass through the lower reaches of the
Salmon River, and should incorporate a flood warning system. It is recommended
that a flood-warning system be implemented for Truro and that SWMM be
seriously considered as the platform for operating such a system. SWMM would
likely be the best choice of operational model for such a warning system because it
can handle the natural and artificial systems under one platform.
REsUME
La ville de turo, en Nouvelle-Ecosse, connait depuis fort longtemps des probldmes
d'inondation, en partie i. cause de sa proximitd i 1a baie de Fundy et des variations de la
mar6e extr€mes qui se produisent dans la baie. La nature et l'ampleur de I'instabilit6 des
ddbits dans les passages en aval de la Rividre Saumon, qui traversent Truro, sont moins
connues. Ces aspects de l'hydraulique sont compliqu6s par le fait que bon nombre des
plaintes formul6es i. propos des d6gdts caus6s par l'eau semblent se rapporter au rdseau
d'6gouts pluvial, en grande partie trCs vieux, et dont on ne pourrait pas consid6rer qu'i1
gdre vraiment les eaux pluviales. Pourtant, la gestion des eaux pluviales est indiqu6e car la
Rividre Saumon comporte des digues le long de la majeure partie de ses biefs inferieurs,
avec un aboiteau
*
dans certaines de ces digues. Dans la pr6sente 6tude, on s'est pench6
sur la nature de cette instabilit6 dans 1es vagues dynamiques qui se d6placent d travers ce
r6seau fluvia1 et sur I'importance relative des effets des mar6es. La capacit6 de mod6liser
la rividre et le r6seau d'6gouts pluvial d'une manidre unifide a 6t6 jug6e trds importante,
en partie i cause de l'aboiteau. Le trds fameux moddle de simulation hydrologique et
hydraulique US trPA - OESWMMI v. 4.31a 6td retenu pour les besoins de l'6tude. 11
6tait intdressant dbbserver combien il serait difficile d'adapter le moddle de gestion des
eaux pluviales (SWMM) aux probldmes hydrauliques de turo, mdme si le " SWMM
" sert principalement i concevoir ou )" 6valuer les rdseaux de conduits circulaires ferm6s.
Une partie repr6sentative du r6seau d'6gouts pluvial de 1a ville a 6t6 int6gr6e au moddle
pour fournir une indication de 1'ampleur de I'inondation. Les r6sultats ont 6t6 i la fois
intdressants et favorables. On a constat6 que la vague dynamique causde par une crue i
r6currence de 20 ans et une mar6e i. rdcurrence de 100 ans atteignait aussi loin en amont
que le Walker St. Bridge. Cela indique que les inondations entrainent une situation oi
un aboiteau unidirectionnel, congu pour laisser s'dchapper les eaux pluviales de Truro,
n'arrive pas i le faire au moment du passage de grandes vagues de translation. De plus, les
rdsultats pr6liminaires de la mod6lisation du r6seau dtgouts pluvial de Tiuro indiquent
qu'une certaine partie de f inondation est souvent causde non pas par l'excds d'eau qui
d6borde des digues le long de la Rividre Saumon, mais par f inefEcacit6 des dgouts
*A gated culvert which only permits flow in the downstream direction. The
downstream ends are, in this case, located mainly in the estuarial reaches of the
river. Modest high tides are thereby prevented from flooding the town, but neither
can urban runoff leave the town when the tide is uo. Aboiteau' is an Acadian word
probably arising from the French'abattre'(to put down) and'eau'(water). There
are other less-common spellings, such as abateau.
440
Vol.26, N0.4,2001
Revue canadienne des ressources hydriques

pluviaux et le manque de v6ritable gestion des eaux pluviales. Tout plan futur de gestion
des eaux pluviales doit tenir compte du fait que la vague de translation doit d'abord
traverser les biefs in{Erieurs de la Rividre Saumon, et doit 6galement comprendre un
systdme d'avertissement de crue. I1 est recommand6 que le systtme d'avertissement soit
adopt6 i Tiuro et que le moddle SWMM soit envisagd sdrieusement i la plate-forme
pour le fonctionnement d'un tel systdme. Le moddle SWMM serait probablement le
meilleur choix de moddle d'exploitation pour un tel systdme d'avertissement car il peut
g6rer les systdmes naturels et artificiels sous une m€me plate-forme.
INTRODUCTION
Known as the 'Hub of Nova Scotia', the town of Tiuro, located 85 km northwest
Halifax, is a small town of about 12,000 people popular with tourists. The Salmon
River runs through the centre of town (Figure 1). The North River is a major tributary.
The Salmon and North Rivers drain a combined area of about 730 km2, and flow into
Cobequid Bay at the eastern end of the Bay of Fundy. The catchment area is devoid
of iakes and is primarily forested, with small amounts of agriculture. The North River
drains into the Salmon River, and the Salmon River debouches into a tidal estuary
near Burntcoat Head. The principal tributaries of the Salmon-North River system are
McClures, Lepper, Christie, Farnham, Soley and McCurdy Brooks.
This study attempts i) to better understand the spatial hydrodynamics of the system;
ii) to evaluate the potential interaction between the storm sewer sub-system and its much
larger receiving watercourse, one that acts both independently and with a high degree of
variability and iii) to determine if SWMM could be adapted to this task, knowing that it
was primarily designed for prismatic dendritic sewer systems. The research described in
this paper was based largely on data collected for the 1988 Maclaren Plansearch report
prepared for the Town of Truro. The Maclaren Plansearch study set out to predict a
maximum water surface profile for the river system flowing through the Tiuro-Bible
Hill area and to determine the necessary extent of the implied flood-risk mapping.
The main outcome was the determination of the maximum water surface orofile of the
Salmon-North River system for the 20- and 100-year return period events and the
determination of the associated flood-risk zones.
OBJECTIVES
1) Determination ofthe flood-wave type.
There are three categories of flood waves: kinematic, diffusive and
fully dynamic. This paper outlines available criteria for flood-wave
differentiation and describes a different way ofidentifying the type(s) of
flood wave at work in a flooding problem. This is of interest because it
potentially affects the amount of field work, the hydraulic complexity of
the problem and the numerical effort.
Vol.26, N0.4,2001
441

a) Looking upstream and east. Shown here is the upstream limit ofthe study area, from approximately node 2600
to node 2400 in Figure 2. The river overflows toward the urban area shown at the bottom of photo (and beyond).
b) Floodplainjustdownstreamofthedownstreamlimitofthestudyarea(node2800inFigure2).Thereach
shown is close to (obequid Bay (on the Bay of Fundy) and is affected by the very large tidal variation in that bay.
Figure 1. Two Views of the Salmon River and 5ome of the Areas that Ftequently Experience Flood
Damage (Photos (ourtesy of Mr. Jon Noble, CBCL Ltd., Halifax).
442
Vol.26, N0.4,2001

2) Reproduction of the 1988 Maclaren Plansearch results'
This research set out to reproduce, using SWMM, the maximum water
surface profile of the Saimon-North River system for the. 20- and
100-yeai design hydrographs and various outlet boundary conditions'
3) Evaluation of different scenarios for the river system'
The extent of the effect of the tides from the Bay of Fundy on the
maximum water surface profile of the river was investigated. The relative
importance of tide .,ersl,s river discharge in the determination of the
maximum water surface profile was also examined'
4) Study of flooding and surcharging problems for a representative
neighbourhood.
ThJ urban planner for the Town of Truro commented that "most of the
water-related complaints apPear to be related to runoff having 'nowhere
to go' rather than to tht 1,-rgr.r, of water from the Salmon River"
(Nison, 1998). This would indicate that a unified approach-must be
taken in studying turo's flooding problems' The U'S' trPA SWMM
modei appeat"a I t" well suited to examining such a problem' A small
neighbouihood that drains into Doggett's Brook and thence into the
Salmon River was selected as a case study' The storm sewer network
of this neighbourhood was added to the SWMM model of the Salmon-
North River system using the Runoff block of swMM. This included
examination of surcharging in the storm sewer network (as distinct from
flooding of the river sYstem).
PREVIOUS STUDIES
The flooding problem in turo and Bible Hill has been studied quite extensively
over the past
-90
years. As early as 7917, a study by Bell-Dawson Jnc' for the
Department of Naval Services investigated the tidal regime of cobequid Bay and
Burntcoat Head. Cox (1938) recom-.r-rd"d a set of remedial measures that included
modifications to the dyke system and channel recti{rcation measures' Several other
studies were done in the 1970s and were concerned mainly with the determination
of the tidal regime at Burntcoat Head in the tidal estuary of cobeqrrid Bay and
its relationship to the tidal regime at saint John, New Brunswick. In 1978, the
government of Cu.rudu und tf,. province of Nova Scotia entered into a general
igreement regarding flood damage reduction, resulting in the prepa_ration of flood-
.ik -up, urrifrrrthJr studies regarding possible remedial measures. In 1980, a study
by M.M. Di1lon Ltd. and W.N. Horner &Assoc. Ltd. emphasized the astronomical
and meteorological effects on tides. one of the main outcomes of this repoftwas a
set of regressio*n equations for predicting the tidal magnitude at Truro. The report
"Hydrotichnical Siudy of the Truro and Area Flood Plain" (Maclaren Plansearch,
tld8) was a relatively complete study that set out to determine the maximum water
Vol.26, N0.4,2001
443

profile for the river system flowing through turo-Bible Hill, and the necessary
extent of the implied flood-risk mapping. Environment canada's one-D model
was used to generate these water surface profiles (Environment Canada, 1995;
Gunaratnam and Perkins, 1970).
A recent study entitled "turo Flood plain" (EDM, 1997) expanded somewhat
on the work done in the 1988 study, and also used the one-D -od.l. It attempted
to address the planning implications ofthe 1988 hydrotechnical study, and included
additional hydrotechnical work. The study area was revisited and the decades
of changes to the floodplain were included in the analysis. No comprehensive
stormwater management plan has ever been developed for the Town of turo.
METHOD
with respect to the choice of swMM, the authors are not aware of any other
single model that can simultaneously handle both the hydrology and hydraulics of
a network of storm sewers (that frequently surcharge) as well as the Mrology and
hydraulics of a natural receiving channel (with its own highly variable hydroiogy).
SWMM was also considered a good choice because there is a considerable amount
of experience, history and expertise with it in North America. The rainfall-runoff
algorithm used in swMM is called non-linear reservoir routing, in which the
so-called width of the sub-catchment and Mann tng's n are the key .orrtrolr. Table 1
gives the values of the various parameters. For sewer systems having relatively
steep slopes, the tansport block in SWMM then uses kinematic wave routins to
transmit inflow hydrographs downstream (Doggett's Brook and the salmon Rivlr).
The Extran block in swMM, used for modelling fu1ly dynamic translatory waves,
of the kind occurring in the salmon River, adopts a finite difference (FD) solution
of the st. venant equations. The FD numerical method used by swMM, known
as an enhanced explicit scheme, is accurate as long as the spatial and temporal
discretization is suitably fine (At and Ax not too large). In this case At was set equal
to 15 seconds and Ax to between 100 m (in and around upstream bridges) to g70b m
(downstream), with an average Ax of about 3000 m. The spacing w"as imposed by
the distance between cross-sections as provided by a previous sttidy that was based
on the Environment canada one-D model (which had no storm sewer comDonent).
The accuracy of the approximate finite difference method used by SWMMto solve
the St. venant equations was not examined in our work because this could not be
meaningfully assessed without hydraulic field data requiring the documentation of
actual dynamic waves of the Salmon River. This data-seidoes not yet exist but
should be collected if a flood-warning system is ro be considered by ihe Town of
turo. A11 three SWMM algorithms mentioned above are described in Bedient and
Huber (1992) and Huber and Dickinson (1988).
It was considered an important first step to investigate the degree to which
translatory waves moving through the salmon River aie indeed Jynamic. The
modelling of fu11y dynamic waves is computationally intensive and relatively difficult
in terms of calibration because true calibration requires simultaneous knowledge of
444
Vol.26, N0.4,2001

Area
No.
Area
(ha)
Length (L)
(m)
'widrh^
(A/L)
(m)
1.42.6
21.6.1
1.37.2
1,21..3
187.9
174.1
61..7
83.0
128.4
r-)z.J
24.6
83.7
1.1.5.6
55.8
42.5
75.0
69.3
Slope
(m/m)
Depression
storage for
pervious areas
(mm)'
1
2
3
4
5
o
7
8
9
10
11
12
l-)
1A
r-)
1/
IO
17
t.z)
1.40
l.l-)
1.71.
3.58
+.)r
0.47
1.06
2.30
1.84
0.30
1..02
2.82
2.21.
r. -)o
1.83
0.95
87.6
64.8
87.6
141.0
190.5
259.7
76.2
1.27.6
179.1.
139.7
121..9
1.21.9
243.8
396.2
320.0
243.8
1.37.2
0.0354
0.0074
0.0092
0.0109
0.0087
0.0367
0.1.444
0.0092
0.0087
0.0052
0.0797
0.0098
0.0082
0.0054
0.0131
0.0029
0.0262
4.0
8.5
7.7
7.0
7.8
3.9
2.0
7.7
7.8
10.1
5.3
7.4
8.1
10.0
6.4
1.3.4
4.6
Table 1. Characteristics of Doggett's Brook neighbourhood simulated using the Runoff block in
SWMM3.
3A Manning's n value of 0.011 was used for all impervious areas, and of 0.2 for
all pervious areas. The PC-SWMM'sensitivityWtzard'in the Runoff blockwas
used to determine that changes in n did not strongly affect the results, within
reasonable limits of z.
awhere L is the length of the path of a particle of water starting at the most distant
point in the sub-watershed.
s
calculated using Do = 0.0303 S 0 ae,
see Huber and Dickinson (1988)'
(anadian Water Resources Journal
Vol.26, N0.4,2001
445

the longitudinal and temporal variation in both flow and stage, and this is difficult
to obtain for even a single event. It is also known that the other extreme, kinematic
waves' are relatively easy to investigate and model. If conditions are kinematic, a
single flow of interest can be legitimately ascribed to an entire reach at a given
moment in time, and the implied field work is 1ess. Ponce (1989) has suggested two
criteria to determine whether a flood wave is kinematic, diffusive or fully dynamic.
The following dimensionless criteria to determine whether a flood wave is kinematic
or not were suggested:
tSV.
-ou>N 11 )
-;
- '/
where:
t, = time of rise of the input hydrograph (T);
s" = bottom slope of bed (dimensionless);
vo = average flow velocity (L/T);
do = flow depth (L); and
N = equal to 85,if 950/o accuracy is required over one period oftranslation.
The following criteria were suggested to determine whether a flood wave is diffusive
of not:
where:
g = gravitational acceleration (L/Tr); and
M = 15 (recommended).
If equation (2) is satisfied, then the flood wave associated with t_ is essentially
diffusive. When neither equation (1) nor (2) is true, then the wave is'dynamic, and
the use of lesser approximations is il1-advised. For events and boundary conditions of
interest these criteria were experimented with in order to determine the longitudinal
variation in the type of flood wave. The results of these numerical experiments (not
presented here) were found to be confusing and inconsistent, due partly to difficulty
of deciding what average depth and velocity to use. These quantities were found to
vary greatly over time for the Salmon River, as might be expected for a translatory
wave passing through a watercourse of this size. Thus, a more direct approach, based
on the temporal variation in friction slope s, relative to bed slope, was taken. This
quantity was examined for representative cross-sections at the beginning and near
the middle of the longitudinal profile, and near the outlet of the Salmon River.
For the reproduction of the various extreme water surface profiles generated
by the Maclaren Plansearch 1988 report, the Extran block in swMM was used.
'Extreme water surface pro6le'refers to a profile connecting the maximum water
surface elevations that occurred at different time steps: i.e. these water surface
446
Vol.26, N0.4,200'1
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elevations were not reaTized simultaneously. The generation of such profiles required
data collected for the 1988 studv: a complete set of cross-sections for the Salmon
River, the North River, and McClures Brook, as well as any additional cross-
sectional data that were deemed necessary by EDM (7997) for their study. The
1988 Maclaren Plansearch study also included design flood hydrographs for the
three major rivers involr,ed and a set of boundary conditions for the Salmon River at
McNutt Brook (node 2800, Figure 2).
The upstream extent of the effect of various high tide scenarios on the Salmon
River was studied by comparing these high tide outcomes against outcomes that
used constant-elevation outlet boundary conditions, prepared using separate runs of
SWMM. The relative importance of tide vs. flow on water surface profile elevations
was examined by considering two extreme combinations: 1) a large river flow event
(a T = 100-year flow event) and a sma11 tidal event (a typical spring tide evenr); 2) a
modest flow event (aT = 2}-year flow event) and a large tidal event (a T = 100-year
tidal event). The outcomes of combining nominally extreme events was considered
to be of interest as a hypothetical endpoint of the behaviour of the system, so no
assumptions or detailed anolyses were made regarding the statistical independence
or non-independence of extreme rainfall events in relation to extreme tidal events.
A smal1 neighbourhood that drains its stormwater into Doggett's Brook was
taken as the basis for the behaviour of the storm sewer system and its interaction
with the river in a preliminary manner (Figure 3). Table 1 gives the characteristics
of the Doggett's Brook neighbourhood as used by the Runoff block in SWMM. A
24-hour AES hyetograph (Hogg, 1982) with a 2-year return period was used as
the precipitation input. This simulation incorporated the complete river system with
the pipe network of the Doggett's Brook sub-watershed added, and involved the use
of a feature in SWMM called Dynamic Grade Line (DGL) to enable viewing of
flooding/surcharging during the event. The extent of flooding was also determined
by examining plots of flooded nodes at different 'snapshots' in time during the
event. No flow data were available for this area and records of sewer sizes and invert
elevations were incomplete. The planimetric shape of the area as well as the drainage
area and ground slopes were determined from 1:1200 maps available from the office
of Land Management for Nova Scotia. These maps were used to decide on the
drainage divides of the subcatchments, and to calculate the areas using a planimeter.
The length of flow path(s) and the percent perviousness,/imperviousness of each
subcatchment were also determined. Typical values for Manning's z were used in the
Runoff block of the model.
ANALYSIS
Figure 4 describes the variation in the friction slope with time in the Salmon
River as compared to the bed slope during the flooding event. It was expected
that upstream reaches, in general the more likely to be kinematic, would have
the smallest s, - s" differences, compared to a downstream reach over time. For
more upstream reaches of the Salmon River (Figure 4, a and b), Figure 4 indicates
448
Vol.26, N0.4,2001
Revue canadienne des ressources hvdrioues

il t --.:
L/ '+'8rF(
3
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=,
U
=
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Vol.26, N0.4,2001
449

0.0050
r
0.00,1-5
0.0040
0.0035
0.0030
0.0025
0.0020 1
0.00 t5 '
0.0010 r
S" = 0.000714
0.0005.
0.0000
r0 20 30 ,t0
Time (hrs)
a) for sub-reach at upstream limit of study.
50
0.0050]
:
0.0040 l
0 0030
0.(D20
0.00 r0
So = 0.0008
L0 20 30 40
Tirnc (hrs)
b) for sub-reach midway along the study area.
0.0006
0 0005
0.000,1
0.0003
0.0002
0.0001
0.0000
'i
10 20 30 .+0 50
Time (hrs)
c) for sub-reach at downstream limit of study.
Figure 4. Variation in Friction Slope S,Over Time, as Compared to the Bed Slope S., for Three Sub-
Reaches ofthe Salmon River.
450
Vol.26, N0.4,2001

that, although the friction slope (sr) was different than the bed slope (so), it
was fairly constant throughout the temporal progression of representative flood
waves. Maximum or near-maximum water surface profiles would be reached neady
simultaneously in these reaches. In the most downstream reach, the behaviour of the
friction slope (sr) during the progression of the flood wave was erratic (Figure 4, c),
due to the strong interaction of the tidal effects in the estuary with the incoming
flood hydrograph. This implies that numerical simulations must be based on the
complete statement of the unsteadpflow equations (the St. Venant equations) in and
near the lower reaches. The collection of stage data in the field for use in calibrating
SWMM into a flood-warning tool should also focus on the lower half of this river
system. The more kinematic behaviour observed upstream implies that field-data
collection resources could be spread out more thinly without significantly affecting
the overall understanding ofthe behaviour ofsevere floods.
As can be seen from Figures 5 and 6, the reproduction of the previous One-D
results (EDM 1997) using Extran for both the T = 201ear and the T = 100-year
return period is quite good for both the Salmon River and the North River. Some
locations along the longitudinal profile of the Salmon River and the North River
appear to need particular attention. Figure 7 indicates the locations where the
profiles generated by Extran and One-D were quite different.
For the extent of the influence of tides on this river system, the SWMM
outcomes for two boundarpcondition scenarios are shown in Figure 8. These two
scenarios were prepared using the T = 2O1ear return period conditions (hydrologic
input and tide), showing that the associated maximum water surface profiles are
similar upstream of a point near the Walker St. Bridge. This point (Figure 7) is
located about 11,000 m from the start of the profile of the Salmon River.
The study of the relative importance of tide vs. flow results indicated that the
second combination (modest flow and high tide) had a higher maximum water-surface
profile. This suggests that tide plays a more prominent role than river flow in the lower
reaches of the river up to the upstream limit of noticeable tidal effects (Figure 9). The
effects of extreme tides dramatically decrease upstream of this point.
The flooding problem within the storm sewer system in the Doggett's Brook area
was also examined. The river contributed to the backing up of the storm sewer system
in the manner dictated by the conditions computed by Extran at the outfail. This
flooding problem was examined visually by using the DGL feature in SWMM to view
the progression of the flood wave through the stormwater system, and to determine
those locations of the junctions (manholes) where flooding occurs. This pilot storm
sewer-system modeiling effort showed that it is not large enough to handle the
volumes of water associated with significant events because street flooding occurred at
almost all of the junctions in this small neighbourhood. Figure 10 shows a snaPshot
of the progression of the surcharging of, and flooding in, the Doggett's Rrook storm
sewer system. A surcharged storm sewer is one in which the obverts are under water,
so that the pipe is under some pressure. A flooded storm sewer is defined as one which
has completely fi1led the manholes at either end and is causing water to go out onto the
street. None of this svstem was surcharsed after t = 24 hours.
Vol.26, N0.4,2001
451

l
45.0
40.0
35.0
30.0
C zs.o
s 20.0
a
15.0
10.0
5.0
0.0
1
I
T
+
T
+
0.0
3s.0
30.0
25.0
E 20.0
il
q""
10.0
5.0
0.0
----*- wsp using ,Rtran'.
" "'-wspusing'1-D'.
-
locus olseclion inverls.
Salmon River
Rd. bridge
Walker St.
bridge
5000.0 10000.0 15000.0
Distance from node 2300 (m)
a) Salmon River.
Stewart
bridge
OnslowRd.
-_,--+
onoge
1000.0 2000.0 3000.0 4000.0 5000.0 6000.0 7000.0 8000.0 9000.0
b) North River.
Figute 5. Maximum Water Surface Profiles (WSP) Generated by Extran, as Compared to 0ne-D,
for T = 20-Year Return Period Conditions.
452
Vol.26, N0.4,2001

45.0
40.0
35.0
30.0
g 25.O
il
s 20.0
q
15.0
10.0
5.0
0.0
T
T
0.0
Salmon Rivel
Rd. bridge
Walker St
bridge
10000.0 15000.0
a) Salmon River.
35.00
30.00
25.00
E 20.00
a
A
I
I
T:,:ll
10.00
5.00
Onslow Rd
bridge
0.0
f
-
1ooo.o 2ooo.0 3000.0 4000.0 5000.0 6000.0 7000.0 8000.0 9000.0
b) North River.
----X-- *rp using 'Ertran'.
------wspusing'1-D'.
-
locus of section inverts.
Figure 6. Maximum Water Surface Profiles (WSP) Generated by Extran, as Compared to 0ne-D,
for T = 1 00-Year Return Period (onditions.
Vol.26, N0.4,2001
453

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Vol.26, N0.4,2001

a) At t = 36 hrs.
a nood.dnod.
O |'doodcd node
5105 node nrlmb.r
frfragr dud.
b) From t = 48 hrs onward.
Figure 10. Snapshots of the Progression of tlooding in the Doggett's Brook Storm Sewer Netwotk
(l-atter Half of Event).
Vol.26, N0.4,2001
457

SUMMARY AND (ONCLUSIONS
1. Friction slope plots and water surface profiles indicate that the Salmon
River probably makes the transition from nearly kinematic behaviour to
fully-dynamic behaviour over a fairly short longitudinal distance. The
portion of the Salmon-North River system considered was simulated
using a model based on the complete statement of the gradually-varied
unsteady flow equations (the St. Venant equations). Results indicate that
efforts to document events that could be used to calibrate the model
should focus on reaches downstream of the Walker St. Bridge. Conditions
become dramatically more unsteady downstream of this location.
2. SWMM was used successfullv to reoroduce the maximum water surface
profiles previously prepared .,ri,-rg ih. One-D model of Environment
Canada to an adequate degree ofaccuracy.
Tidal effects from the Salmon River estuary play a more important role
than flow in the determination of the maximum water surface orofile
elevations, especially downstream of a point 11,000 m from the beginning
ofthe profile.
The Doggett's Brook storm sewer system is completely incapable of
handling significant flood events (T = 201ear or T = 100-year hydrologic
and tidal events). This situation is probably indicative of the capacity of
most of Truro's storm sewer system.
The Nova Scotia Minister responsible for the Emergency Measures Organization
recently gave the Town of Truro a mark of 'F' for its level of flood preparedness
(Chronicle-Herald,1999).It is believed that, if properly calibrated against observed
events, SWMM would be a very suitable model on which to base a flood warning
system, one that could improve significantly turo's present level of flood readiness.
ACKNOWLEDGEMENTS
The financial assistance of the National Research Council of Canada is
acknowledged. The cooperation of Mr. Peter Nelson, MCIR Director of Planning
and Deveiopment, and Mr. Andrew MacKinnon, Assistant Town Engineer for
the municipality of Truro is gratefully acknowledged. The SWMM user's group
at SWMM-USERS@LISTStrRV.UOGUtrLPH.CA oroved to be a valuable
resource.
458
Vol.26, N0.4,2001
3.
4.

REFERENCES
Bedient, P.B. and W.C. Huber. 1992. Hydrology and FloodplainAnalysis,2nd edition.
Addison Wesley, Reading, MA.
Hogg, W.D. 7982. "Drstribution of Rainfall with Time: Design Considerations'"
Chapman Conference on Rainfall Rates, American Geophysical Union' Urbana
Illinois, Aprrl 2729. (Reference for what is sometimes referred to as the 24-hour
AES hyetograph'.)
chronicle-HenId. 1999. "turo Gets F on EMO Report card." saturday,
January 30 (S. March reporting).
Cox, O.S. 1938. Tiuro Flood Control Progrant Public Works Canada.
trDM. 1997. Tiuro Flaocl Plain. Prepared by EDM and Maclaren Plansearch
and Wallace Macdonald and Lively Ltd. for the Joint Committee on Floodplain
Management for Truro District.
Environment Canada. 1995. Inland Waters Directorate and B.C. Environment
Water Management Division 1995. The ONE-D Hydrodynamic Pragram, User's
Manual, March.
Gunaratnam, DJ. and F.E. Perkins. 1970. "Numerical Solution of Unsteady Flows
in Open Channels." Hydrodynamics Lab. Report No. 127, Department of Civil
Engineering, MIT, July.
Huber, W.C. and R.E. Dickinson. 1988. Storm Water Management Model, Wrsion 4:
(Jser's Manual. U.S. Environmental Protection Agency, Athens' GA.
Maclaren Plansearch. 1988. Hydrotechnical Study of Tiura and Area Flood Plain.
Preoared for the Province of Nova Scotia and Environment Canada under the
Federal Flood Damage Reduction Program (FDRP).
Nelson, P. 1998. Personal Communication.
Ponce, V.M. 1989. Engineering Hydrology: Principles and Practices. Prentice Hall,
Englewood Cliffs, NJ.
Vol.26, N0.4,2001
459

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