6. Presentation - Spring Creek Drainage StudyAgenda -September 2, 1997
AGENDA ITEM 6 - PRESENTATION -SPRING CREEK DRAINAGE STUDY -AUTHORITY
TO REQUEST ENGINEERING SERVICES, DESIGN PHASE I
BACKGROUND/CONSIDERATION: The Storm Drainage Analysis of the
Spring Creek drain area is now complete and has been submitted to
staff by Peccia and Associates. I have enclosed a copy of the
study for your review. Peccia Representatives will be here to
present their results to us. I believe we should discuss the
results. I am requesting authorization to proceed with the Phase
I design as proposed on Table 2 (Meridian Road option).
RECOMMENDATION: I recommend approval for requesting Engineering
Services for Phase I (Meridian Road option) as authorized within
the study.
i .�clum
-
Prepared For
The City of Kalispell, Montana
Prepared By
Robert Peccia & Associates, Helena, Montana
Meridian Road
�*Z'Rorm Drainage Analysis
Kalispell, Montana
Prepared for:
The City of Kalispell, Montana
July, 1997
Prepared by:
Robert Peccia and Associates
825 Custer Avenue
Helena, Montana 59601
Copyright Oo 1997
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
Table of Contents
Title Page
Table of Contents
Chapter 1: Introduction ........................................ i
Chapter 2: Computer Modeling of Storm Drain Systems .......... 3
Chapter 3: Existing Drainage System ........................... 5
Hydrology of the Study Area ................................ 5
Existing Storm Drain Flows ................................. 6
Chapter 4: Design Storm Improvements ........................ 10
Northridge Heights ....................................... 10
Lower Spring Creek ...................................... 15
Chapter 5: Detention Requirements ............................ 17
Northridge Heights ....................................... 17
Upper Spring Creek ...................................... 18
Chapter 6: Future Development ................................ 21
Impacts of Development .................................. 21
Policies................................................ 21
Chapter 7: Project Summary .................................. 23
List of Figures
Figure 1: SWWM Flowchart ....................................... 4
Figure 2: Subbasins & Hydrology ................................... 7
Figure 3: Existing Storm Drain Flows ................................ 9
Figure 4: Design Storm Improvements: Glenwood Drive Option .......... 12
Figure 5: Design Storm Improvements: Meridian Road Option ............ 13
Figure 6: Glenwood & Three Mile Road Upstream Detention ........... 19
Figure 7: Spring Creek & Idaho - Upstream Detention .................. 19
List of Tables
Table 1: Cost Estimates for Design Storm Improvements:
Northridge Heights ...................................... 14
Table 2: Storm Drain Improvements: Costs & Priorities ................. 24
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
The northwest area of Kalispell has a history of snow melt and storm
water runoff problems. This is primarily because the region's
generally flat topography inhibits runoff, a natural stream (Spring
Creek) meanders through the region, and there are several residential
areas that contain no underground storm drains. Many of the larger
floods occur during late winter or early spring when the ground is
frozen.
In recent years this area of Kalispell has been the focus of much
economic activity. Major highways intersect here, a large shopping
mall was constructed in 1985, several new motels have sprung up,
and many restaurants have recently been built. One of the more active
areas of development is along the lower reaches of Spring Creek. The
stream's periodic flooding and generally swampy conditions
complicate storm water runoff conditions in the area.
The Montana Department of Transportation is also planning a major
reconstruction of North Meridian Road, between Idaho Street (U.S.
2) and U.S. 93, in this section of town. The project includes the
installation of storm drain pipes along the new roadway. Kalispell is
responsible for addressing the impacts of storm water that runs onto
the road from outside the construction area. This will require the City
to install new storm drain lines and pay for size upgrades to some of
the proposed piping along Meridian Road.
Due to the urgency caused by the Meridian Road project, City
officials decided to make a detailed analysis of all drainage -related
issues affecting northwest Kalispell. The scope of the analysis is as
follows:
• Determine, using a computer modeling program, the storm
water runoff characteristics of the drainage basins that impact
the northwest area of Kalispell;
• Identify problem areas, especially those that affect the Spring
Creek drainage and the Meridian Road project;
Robert Peccia & Associates 1
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
• Recommend drainage improvement alternatives that mitigate
the problems;
• Prioritize and estimate the costs of the improvements;
• Consider the drainage -related impacts of future development
in the area;
• Recommend policies to be applied to future subdivisions and
site development; and
• Examine various financing methods to fund future storm
drain improvements;
Robert Peccia & Associates 2
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Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
0•
Most storm drain analysis today is done using computers. There are
many packages available to handle a variety of drainage -related
situations. Most of the software is limited in scope, however, and is
designed for specific applications. For example, some programs help
with culvert design, while others compute runoffs for very small
drainage areas such as parking lots.
There are a few programs that perform a comprehensive analysis of
a large drainage basin. These programs usually analyze a basin's
hydrology then use those results to model the hydraulics of a storm
drain network that lies in the basin. One such program is XP-SWMM,
which was used to prepare this report. Developed for the U.S.
Environmental Protection Agency, the program originally ran on a
large main frame computer and was called "Storm Water
Management Model" or SWMM.
XP-SWMM is conveniently organized into modules (see figure 1).
The RUNOFF component models the overall hydrology of the basin
using such parameters as area, topography, vegetation, and rainfall.
An interface file stores the results from the RUNOFF analysis which
then become input to the EXTRAN module. The EXTRAN module
contains the storm drain pipe network data which it uses to compute
the storm water flows, water levels, and velocities throughout the
system.
A potentially useful feature of the XP-SWMM program is its ability
to model storm water quality. Future government regulations will
likely require that water quality be considered when storm drain
systems are designed and built.
Robert Peccia & Associates 3
Figure 1
SWMM Flowchart
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Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
The northwest area of Kalispell is affected by two general drainage
systems, the Spring Creek drainage and the region above and
including the Northridge Heights subdivision. These two systems
meet where Spring Creek intersects Meridian Road just south of
Idaho Street. A smaller area east of Meridian Road drains toward the
same general area. (Since this region impacts the overall flows it was
included in the model. It is, however, outside the scope of this study.)
All of the water from these systems eventually makes its way into the
storm water detention structure south of Apple Way next to the
abandoned Burlington Northern Railroad spur.
Hydrology of the Study Area
To model the hydrology of the study area, the region's primary
drainages were divided into smaller subbasins. Each subbasin was
then measured and inventoried to determine important characteristics
that affect storm water runoff. These include subbasin area,
infiltration, topography, and regional rainfall data. The subbasins act
as small interconnected watersheds that transport rainfall from the
basin's upper reaches to its lower elevations (see Figure 2).
The Soil Conservation Service (SCS) method for classifying
watersheds was used because it takes into account the ability of the
ground to absorb water during a storm. Frozen ground conditions,
undeveloped land, and paved (or partially paved) areas can be
simulated. The SCS method also uses rainfall data that applies
specifically to the region, which we believe, produces more realistic
modeling results.
Three scenarios were modeled: a ten-year "design storm", a ten-year
storm with "frozen" ground, and a 100-year storm. The frozen ground
situation is simulated by decreasing the infiltration of the subbasins's
surface to practically zero (paved regions are treated much the same
way). To model the 100-year storm rainfall amounts are increased
accordingly.
Robert Peccia & Associates 5
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
The model computes the runoff generated by each subbasin and the
flows at various points in the drainage as a function of time. Figure
2 shows the maximum flows from one subbasin to another for each
scenario. This is the amount of runoff that must somehow be
accommodated by storm drain pipes, channels, streets, and detention
basins.
Existing Storm Drain Flows
Most of the Spring Creek drainage is forest, open fields, and
undeveloped grasslands. Spring Creek runoff enters the system at a
49-inch culvert that crosses Idaho Street two blocks west of Meridian
Road (see Figure 3). A natural, marshy, channel carries the water
from Idaho street to just west of Meridian Road where it is
intercepted into an underground 60-inch pipe.
Until the late 1970's Spring Creek flowed across Meridian Road
under a small bridge. The stream turned to the south where it was
captured in a ditch along the south side of the railroad tracks and
conveyed westward to the vicinity of the present outfall structure.
In 1979, storm sewer improvements provided a 60-inch pipe along
the west side of Meridian Road, flowing south and west to the outfall.
Spring Creek continues to flow in its natural open channel up to the
west edge of Meridian Road, where it was completely intercepted by
the new underground system.
Several years later the owner of the property west of Meridian Road
at Spring Creek installed approximately 250 linear feet of 60" RCP,
moving the capture point to his west property line and filling in the
creek bed to expand his parking lot. Recently, winter thaw events
have caused localized flooding immediately upstream from this point
of capture.
The Northridge Heights drainage passes though mostly developed
residential areas. At present storm water runs primarily above ground,
either in manmade channels, roadside ditches, or streets. A crucial
feature in this system is a 24-inch culvert that crosses Three Mile
Drive near Northern Lane (see Figure 3). Another 24-inch culvert lies
just downstream at Rosewood Drive. A nice grass -lined channel
Robert Peccia & Associates 6
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Area 7 = 350
Area 8 =102 .".
Area 9 = 150 ' '
Area 10 = 207 :". , j , g „r
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Area 11 = 39 v' '° ' ` ;' \ :� £ x,s a Q
Area 12 = 88 '
Area 13 = 7
Area 14 = 37 € ♦ n
E«er
Area 15 = 71
Area 16 141
Area 17 = 25
Area 18 = 82 _: A ` . X
{ y.
Area 19 = 28 ...�
4.6 cfs
6.2 cfs
,, ' 18 cfs
18 cfs 5.8 cfs °"' 31 cfs
7.7 cfs 62 cfs
25 cfs
11 cfs .:
17 cfs 4
55 cfs
33 cfs �4.7 cfs
47 cfs 42 cfs 8.3 cfs
15
140 cfs 74 cfs 18 cfs
147 cfs
15 cfs t
.,
20 cfs34 cfs 10 cfs
'
57 cfs 49 cfs 18 cfs
152 cfs; 41 cfs
........... 74 cfs 55 cfs 26 cfs
108 cfs
95 cfs 30 cfs
331 cfs 191 cfs 107 cfs 16
18
13
86 cfs 10 cfs
120 cfs 32 cfs 10 cfs
y 44 cfs 31 cfs
340 cfs 57 cfs
139 cfs
96 cfs
194 cfs 9
(7
SCHEMATIC 1
SUB- ASI S 45efs
103 cfs 71 cfs
156 cfs 155 cfs
450 cfs
Outfall Structure
134 cfs
201 cfs
542 cfs
RI 1,104-1
Spring Creek Drainage
Northridge Heights
East of Meridian
134 cfs -10 Year Flows
201 cfs - 10 Year Flows (Frozen Ground)
s42 cfs - 100 Year Flows
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
carries storm water from the upper reaches of the drainage in
Northridge Heights to Three Mile Drive, through the two culverts,
and onto Glenwood Drive north of Liberty Street. The water then runs
down a ditch along Glenwood until it hits Two Mile Drive. From here
the water heads directly east in roadside ditches (primarily on the
south side of Two Mile Road) until it reaches Meridian Road.
Modeled flows through the existing storm drain system for the ten-
year storm, ten-year storm with frozen ground, and 100-year storm
are shown in Figure 3. Places where pipes are filled to capacity and
backup (surcharge) are also marked on the figure.
It is readily apparent that the two culverts at Three Mile Drive and
Rosewood Drive are major restrictions in the system. Of the 43 cfs
that arrive at Three Mile Drive during a ten-year storm, only about 28
cfs pass through the culvert. The rest backs up on the north side of the
road.
The culverts that carry Spring Creek water, on the other hand, appear
to be of adequate size to handle the flows for the ten-year storm and
the frozen ground situation. This suggests that the flooding that
occurs on lower Spring Creek may be caused by other things.
Obstructions in the culverts or downstream pipes or the topography
of the lower Spring Creek area are possible causes.
The computer model indicates that the outfall retention structure near
the railroad spur next to Montana Forest Products functions as
designed, even for high -flow events. Water levels in the structure
don't become critical under any of the storm events modeled.
Robert Peccia & Associates 8
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
A storm of a magnitude that can be expected to occur, on average,
every ten years is called a ten-year storm. An extremely large storm,
one whose magnitude is seen about once every 100 years, is called a
100-year storm. Storm drain systems are typically designed to handle
flows up to the five or ten-year storm. This is known as the design
storm. We recommend that Kalispell use the ten-year storm as its
design storm.
All improvements to Kalispell's storm drain network, including
pipes, culverts, and channels should be based upon ten-year flows.
More severe runoff conditions such as a rapid winter thaw or a 100-
year storm occur less frequently. It is not, therefore, economically
justifiable to build a storm drain network large enough to handle these
events. A common practice is to build detention basins that fill when
flows exceeding the design levels occur. We recommend that
Kalispell plan detention facilities to accommodate the 100-year
storm.
Northridge Heights
At present, the Northridge Heights drainage system cannot handle the
ten-year storm. As shown in Figure 3, the culverts at Three Mile
Drive and Rosewood Drive are the primary restrictions. In addition,
most of the flow in this drainage is above ground. Although the
channels, ditches, and streets can carry most of the runoff, a fair
amount of localized flooding and nuisance ponding occurs. For
technical, safety, and esthetic reasons it is preferable to run storm
water through below -ground pipes whenever possible.
The Meridian Road reconstruction project will also have an impact
on Kalispell's storm drain requirements in this area. This is because
the Montana Department of Transportation will not allow any above-
ground flows along the new highway, including those that presently
occur. Currently about 30 cfs flows down the ditch along Meridian
Road (between Two Mile Drive and Idaho Street) during a ten-year
storm. Another 25 cfs or so would reach this point if the upstream
restriction at Three Mile Drive is removed. In either case, Kalispell
Robert Peccia & Associates 16
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
is required to pay the costs of upgrading the sizes of the culverts that
will be installed as part of the highway reconstruction project to
accommodate these flows.
There are two general ways to upgrade the current storm drain system
in the Northridge Heights area to handle a ten-year storm. One is to
install an additional culvert at Three Mile Drive and lay new storm
drain piping from there directly south, between homes and along
Glenwood Drive to the ditch south of Liberty Street. The other is to
intercept much of the runoff at Three Mile Drive and pipe it east to
Meridian Road. Figure 4 and Figure 5 show the improvements
associated with each option. A comparison of the costs is presented
in Table 1. Both options eliminate the restriction caused by the 24-
inch culvert at Three Mile Drive and reduce, to a manageable 17cfs,
the overland flow through the yards and streets in the Rosewood
Drive area.
Robert Peccia & Associates 11
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Meridian Road Option
North
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
Table 1
Cost Estimates for Design Storm Improvements: Northridge Heights
Option
Kalispell's Portion
New Pipe &
Engineering Design
Total Estimated
of Meridian Road
Pipe Size Upgrades
& Inspection
Cost
Reconstruction*
Glenwood
$87,400
$226,550
$47,093
$361,043
Drive
Meridian
$87,400
$289,225
$56,494
$433,119
Road
Itemized Costs
Meridian Road Reconstruction*
• New 24-inch RCP on Two Mile (Glenwood to Meridian) $24,000.00
• Upgrade to 42-inch RCP on Meridian (Two Mile to Idaho) $52,000.00
Total: $76,000.00
Total Plus Contingencies (15%): $87,400.00
Glenwood Drive Option
• Additional 30-inch RCP at Three Mile Road $ 6,000.00
• New 36-inch RCP from Three Mile to Glenwood $42,500.00
• New 42-inch RCP from Glenwood to Liberty $80,000.00
• Upgrade to 42-inch RCP on Two Mile (Glenwood To Meridian) $36,000.00
• Additional Upgrade to 48-inch RCP on Meridian (Two Mile to Idaho) $32,500.00
Total: $197,000.00
Total Plus Contingencies (15%): $226,550.00
Meridian Road Option
• New 36-inch RCP on Three Mile (Northwest Lane to Meridian) $ 51,000.00
• Upgrade to 42-inch on Meridian (Three Mile to Two Mile) $168,000.00
• Additional Upgrade to 48-inch RCP on Meridian (Two Mile to Idaho) $ 32,500.00
Total: $251,500.00
Total Plus Contingencies (15%): $289,225.00
* A proposed Montana Department of Transportation reconstruction project along Meridian Road includes storm drain
pipe installation. The City of Kalispell is responsible for the costs of upgrading the pipe sizes to handle existing
stormwater runoff onto Meridian Road from outside the construction boundaries.
Robert Peccia & Associates 14
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
One drawback of the Glenwood Drive option is that during a ten-year
storm a substantial amount of water (about 54 cfs) will run down the
roadside ditch along Glenwood (from Liberty to Two Mile Drive).
This would perpetuate the localized surcharge problems at culverts in
the Glenwood ditch and at Glenwood and Two Mile Drive. With the
Meridian Road alternative, almost all storm drain runoff south of
Three Mile Drive would travel below ground.
Another factor is that the pipe installation from Three Mile Drive
south to Liberty would occur through yards in a residential area which
would require extra planning and coordination during construction.
New storm drain pipes along Meridian Road are already planned for
(their sizes would need to be increased) with the roadway
reconstruction project.
In short, the Glenwood Drive option is slightly less expensive but the
Meridian Road alternative is cleaner, safer, and easier to construct.
All things considered, we recommend the Meridian Road option.
Lower Spring Creek
As shown in Figure 3, the 48-inch culvert running under Idaho Street
that carries Spring Creek runoff and the 60-inch pipe that intercepts
the Spring Creek flow near Meridian Road can easily handle the ten-
year storm. In spite of this, the lower stretches of Spring Creek have
a history of flooding that suggest a ten-year storm would cause
problems in the area.
It appears that the topography along lower Spring Creek may prevent
the water from reaching the intercept pipe in an efficient manner. We
recommend that a channel be designed and built along the banks of
Spring Creek from the culvert at Idaho Street to the intercept pipe
near Meridian Road. The channel would serve two purposes. First, it
would contain the stream as it passes through the region and direct the
water toward the intercept pipe and second, it would allow the water
to pool to the top of the pipe without spilling its banks and flooding
the surrounding area.
A topographic survey and hydraulic analysis should be conducted
before the channel's design is finalized. Since this area lies in a
Robert Peccia & Associates 15
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
delineated flood plain, a flood study and the filing of appropriate
permits may also be required.
City personnel have indicated that some obstructions (possibly tree
roots) in a large storm drain pipe downstream from Spring Creek may
inhibit flows during high runoff periods. There are currently plans to
investigate and remove any objects that are found.
Robert Peccia & Associates 16
.. .. I A i Tore
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
Because they happen so rarely, it is not economically feasible to
design storm drain networks to carry the runoff caused by
catastrophic events. A catastrophic event may include, among other
things, an extremely heavy rainstorm or a rapid snow melt over
frozen ground, or some combination of the two.
This does not mean, however, that a community is left helpless if one
of these events occurs. With strategically placed and well designed
detention basins, the tremendous amounts of runoff generated can be
diverted into the basin, stored, and slowly released back into the
system. The flows through the storm drain network therefore remain
at or below design storm levels.
The purpose of a detention basin is to make sure that no part of a
storm drain system encounters flows above the design storm
downstream from the basin. For this reason, detention basins are
placed at the upper end of a given drainage, usually above critical
features in the system.
Since they are almost always dry, detention basins are often located
in public parks. They are usually landscaped and used for recreation.
A common practice is to construct a football or soccer field
surrounded by a berm for spectators. During a severe storm the field
is transformed into a temporary pond and the berm forms its banks.
Northridge Heights
An ideal place for a detention basin in the Northridge Heights
drainage is in Sunset park located west of Garland Street and south of
Sunset Court. The storage capacity of the basin should be such that
ten-year flows (or less) are maintained at the point where the drainage
intersects Three Mile Drive near Northern Lane. Flow verses time
graphs (hydrographs) were generated at this location for the design
storm, the frozen ground conditions, and the 100-year storm (see
Figure 6). The shaded areas represent the volumes of water that must
be detained in each case (detention volumes are usually specified in
"acre-feet", which is one acre of area with a water depth of one foot).
Robert Peccia & Associates 17
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
As shown in Figure 6, a relatively small amount of storage (three
acre-feet) is required to handle the frozen -ground runoff while a
much larger basin (16 acre-feet) is needed for the 100-year storm. A
two -tiered basin that floods only a small area of the park during minor
events but would encompass a larger area during major storms could
be designed.
Some -localized ponding occurs just upstream from the proposed basin
site at the intersection of Sherry Lane and Ramsgate Drive. Storm
drain inlets and short lengths of pipe could be installed here to collect
this water and carry it to the nearby drainage channel. Some minor
modifications to the drainage channel would also be required at
various places in the Northridge Heights area.
Upper Spring Creek
The 48-inch culvert that crosses Idaho Street is the point where
Spring Creek runoff begins to impact Kalispell's storm drain system.
In order to prevent lower Spring Creek from reaching flows much
above design storm levels it is desirable to detain as much runoff as
possible upstream from this culvert. As shown in Figure 7, this
location encounters tremendous runoffs during a 100-year storm. The
10-year frozen -ground flows are less drastic.
It is unrealistic to try and detain the entire 100-year flow. Protection
from smaller events, like a rapid spring thaw can, however, be
achieved with reasonable effort. A modest detention basin of five or
six acre-feet could be constructed for a relatively small cost. To
maximize its benefits, this basin should be located as far upstream as
possible (at least above Two Mile Drive).
The area along Spring Creek between Two Mile Drive and Idaho
Street is mostly open field, though future development is certain to
occur. This provides the City of Kalispell (and Flathead County) with
an opportunity to incorporate a strategy that requires on site detention
to be a part of development plans. Installing a series of smaller basins
(to be funded by the developer) is more practical and economical than
having the city construct a second, large detention structure in this
area.
Robert Peccia & Associates 18
Flow (CFS)
75
6111
25
200
Flow (CFS)
150
2 3 4
Time (Hours)
10 Year: 3.1 Acre -Feet
(Frozen Ground)
1.1
10 Year: 4.5 Acre -Feet
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
In summary, we recommend two detention strategies for the Spring
Creek drainage, a City -constructed small basin (five or six acre-feet)
located above Two Mile Drive and on -site detention, built by the land
owner, as individual properties are developed.
Robert Peccia & Associates 20
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Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
Impacts of Development
Residential subdivisions, commercial districts, new streets and
parking areas, and virtually all other forms of land development
increase storm water runoff. This is because concrete, pavement,
roofs, and the other things typically associated with development are
smooth and impervious to water. Rainfall that lands on these surfaces
immediately runs off and storm water that passes across them does so
quickly. For these reasons most communities require property owners
to make provisions for storm water runoff when they plan a
development.
Northwest Kalispell is an active and economically vibrant area. A fair
amount of commercial and residential development is anticipated.
Much of the new growth will occur outside the city limits in the upper
reaches of the Spring Creek drainage. This will, of course, result in
increased runoff from these areas. The runoff should be comparable
to the ten-year frozen ground flows shown in Figures 3, 4, and 5.
Careful coordination between the City of Kalispell and Flathead
County, with respect to the impacts of new development, should
occur.
The improvements to the lower Spring Creek channel (recommended
earlier) should allow some of this additional runoff to be handled.
The rest will be taken care of by the detention strategies
recommended in the previous section.
Policies
The City of Kalispell levies an annual storm drain assessment that
generates approximately $235,000 per year. This appears to be an
appropriate level of funding and should allow the City to steadily
improve its system. Larger projects can be funded with bonds and the
debt could be retired with some of the annual assessments. Many
smaller improvements could be paid for outright.
The City is encouraged to adopt a policy requiring on -site detention
Robert Peccia & Associates 21
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
affecting Kalispell would come from outside the city limits, Flathead
County should be strongly encouraged to adopt a similar policy. A
common technique used to determine detention requirements is to
calculate the 100-year runoff from a given piece of property in its
undeveloped and developed states. The difference between the two is
the required storage volume. This strategy should be incorporated
into the Kalispell standards for design and construction.
It would be beneficial for Kalispell to produce (or update) a Storm
Drain Master Plan. This will allow the City to prioritize
improvements and more effectively allocate storm drain funds. It will
also provide guidance to the City for evaluating the property
developers' on -site detention plans.
Robert Peccia & Associates 22
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
CHAPTER 7: PROJECT SUMMARY
The improvements to northwest Kalispell's storm drain system
recommended in this report are intended to accomplish two primary
objectives. The first is to upgrade the system to carry design storm
flows. The second is to allow the system, by using detentions basins,
to handle frozen -ground runoffs, and 100-year storms.
Table 2 lists, in priority order, the recommended improvements and
their costs. The first two projects upgrade the system in both
drainages (Northridge Heights and Spring Creek) for design storm
flows. The next three involve detention -related activities for the same
drainages.
The project descriptions contained in this report are conceptual in
nature and are intended for planning purposes only. Preliminary
designs, conducted at the outset of each project, would determine
specific scopes of work and provide more detailed cost estimates.
Robert Peccia & Associates 23
Spring Creek - Northridge Heights - Meridian Road
Storm Drainage Analysis
Table 2
Storm Drain Improvements - Costs & Priorities
1.
Design Storm Improvements: Northridge Heights Drainage
Glenwood Drive Option
or
Meridian Road Option
$361 000
or
$433 000
2.
Design Storm Improvements: Lower Spring Creek Channel
$150,000
3.
Sunset Park Detention Basin & Related Improvements
$200,000
4.
Upper Spring Creek Detention - City Built
$100,000
5.
Upper Spring Creek Detention- By Developers
As Developed
Robert Peccia & Associates 24