Drainage ReportDRAINAGEREPORT
TETON TERRACEDEVELOPMENT
FLATHEAD COUNTY
GENERAL:
The proposed cluster development subdivision is located in the NE '/a of Section 12, Township
28 North, Range 22 West. The site is currently undeveloped land within the city limits of
Kalispell. The land is bordered on three sides by multi -family residential housing and suburban
agricultural land on the fourth side.
Teton Terrace Subdivision is approximately 4.2 acres and will consist of seven duplex and four
triplex units. Two asphalt paved access roads with concrete curbs, gutters and sidewalks are
planned. Individual driveways and walkways will also be concrete or asphalt. Topography of
the site slopes approximately one or two percent downward towards the southwest corner.
Minimal site grading will be required for drainage purposes.
SOILS:
The soils of the proposed site were analyzed based on findings of three exploratory test pits. In
general, the soil conditions encountered in the test pits consist of approximately 0.8 to 1.4 feet
of topsoil and organic material underlain by 1.8 to 2.1 feet of silty sand and sandy silt. The
sand and silt were underlain by lean clay which extends down to a depth of 7.3 to 8.2 feet. Silty
sand was encountered below the lean clay in Test Pit TP-1 (see attached test pit logs). No
groundwater was encountered in the test pits which were excavated to a maximum depth of 9.0
feet.
Infiltration is expected to be moderate to slow in the topsoil and silty sand and nonexistent in the
lean clay. In general, runoff will be directed to the road gutter system or drainage swales and
will flow to the southwest corner of the site. At the southwest corner, the drainage swale will
connect to an existing grassy drainage ditch. The existing drainage ditch flows south to Two
Mile drive then west approximately '/a mile where it empties into Spring Creek.
Rational Formula:
Storm runoff is determined using the following rational formula:
Q = C*I*A
where: Q = Runoff in cfs
C = Runoff Coefficient
Unimproved land - 0.1-0.3
Lawns (silty soil)
up to 2 % slope - 0.13-0.17
Asphalt - 0.7-0.95
Concrete - 0.8-0.9
Roofs - 0.75-0.95
(Runoff coefficient values from: Lindeburg,
C.E. Reference Manual, pg.6-24)
I = Rainfall Intensity in inches/hour
A = Drainage area in acres
DRAINAGE AREAS AND CALCULATIONS:
The 4.2 acre site is divided into 4 drainage zones for storm runoff calculations (see Plan Sheet
No. 5). Storm runoff is calculated for Zones I -IV for both pre -development and post -
development.
The undeveloped site was formerly farmland, and currently consists of grasses and weeds at
slopes generally less than 2%. Drainage area for the developed site is divided into
street/driveway/sidewalk, roof, and lawn areas.
Assumptions:
37' wide paved road surface, including curb & gutter
5' wide sidewalks
12' wide driveways, except drive for units 24, 25 & 26, which is
as shown on the plans.
Roof areas are the full building envelope as shown on the enclosed
Site Plan (dashed line).
All areas not included above will be lawn.
The rainfall intensity for the 2 year - 6 hour design storm is 0.11 inches per hour (see Rainfall
Intensity -Duration -Frequency Curve, attached) .
The calculations for each drainage zone are attached in spreadsheet format. Runoff from Zones
I, III & IV will be directed to the road gutter system or drainage swales. Zone III will flow to
the existing gutter in Hawthorne Avenue and will then flow south to the existing drainage ditch
along Two Mile Drive. Zones I & IV flow to the southwest corner of the site, where the
existing drainage ditch flows south to Two Mile Drive then west approximately 1/ mile where
it empties into Spring Creek. Zone II runoff is minimal and will be retained on -site in a small
retention/infiltration area.
The Developed Site runoff for each zone will be collected as noted above and transported via
the existing grassy drainage ditch system to Spring Creek. Erosion is not expected to be a
problem due to the flat site conditions.
Ian J. Baile 4P. 11137
Thomas, Dean & Hoskins, I c.
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Teton Terrace - STORM DRAINAGE CALCULATIONS - Jan., 1996
lJob #: K94-46-5 lComputed
by: IJB
DRAINAGE ZONE: III
DESCRIPTION
AREA
(acres)
RUNOFF
COEFF.
(C)
RAINFALL
INTENSITY
(in./hr.)
RUNOFF
(cfs)
UNDEVELOPED SITE:
0-
0
0,11
0.000
Unimproved
0.268
0.2
0.17
0.009
0
0
0.17
0.000
j
0
0
0.17
0.000
TOTAL UNDEVELOPED SITE RUNOFF ----------------------------------
0.009
DEVELOPED SITE:
0
0
0.17
0.000
Lawn/Pasture
0.076
0.25
0.17
0.003
Roof/Decks, etc.
0.111
0.9
0.17
0.017
Street/Driveway/Pkg.
0.081 1
0.9-
0.17
0.012
1
0
0.17
0.000
TOTAL DEVELOPED SITE RUNOFF ---------------------------------------
0.033
NET RUNOFF (Developed Site - Undeveloped Site) ----------------
0.023
DRAINAGE ZONE: IV
DESCRIPTION
AREA
(acres)
RUNOFF
COEFF.
(C)
RAINFALL
INTENSITY
(in./hr.)
RUNOFF
(cfs)
UNDEVELOPED SITE:
Unimproved
0.687
0.2
0.17
0.023
0
0.25
0.17
0.000
0
0.9
0.17
0.000
0
0.9
0.17
0.000
I
TOTAL UNDEVELOPED SITE RUNOFF ----------------------------------
0.023
DEVELOPED SITE:
0
0
0.17
0.000
Lawn/Pasture
0.404
0.25
0.17
0.017
Roof/Decks, etc,
0.253
0.9
0.17
0.039
Street/Driveway
0.030
0.9
0.17
0.005
TOTAL DEVELOPED SITE RUNOFF -------------------------------------
0.061
-
NET RUNOFF (Developed Site - Undeveloped Site) --------------0.037
Teton Terrace - STORM DRAINAGE CALCULATIONS - Jan., 1996
Job #: K94-46-5 lComputed
by: IJB
DRAINAGE ZONE: 1 11
DESCRIPTION
AREA
(acres)
RUNOFF
COEFF.
(C)
RAINFALL
INTENSITY
(in./hr.)
RUNOFF
(cfs)
UNDEVELOPED SITE:
0
0
0.17
0.000
Unimproved
3.136
0.2
0.17
0.107
0
0
0.17
0.000
0
0
0.17
0.000
TOTAL UNDEVELOPED SITE RUNOFF ----------------------------------
0.107
DEVELOPED SITE:
0
0
0.17
0.000
Lawn/Pasture
1.443
0.25
0.17
0.061
Roof/Decks, etc.
0.678
0.9
0.17
0.104
Street/Driveway/Pkg.
1.015
0.9
0.17
0.155
0 1
0
0.17
0.000
TOTAL DEVELOPED SITE RUNOFF -------------------------------------
0.320
NET RUNOFF (Developed Site - Undeveloped Site) ---------------
0.214
DRAINAGE ZONE: 11
DESCRIPTION
AREA
(acres)
RUNOFF
COEFF.
(C)
RAINFALL
INTENSITY
(in./hr.)
RUNOFF
(cfs)
UNDEVELOPED SITE:
Unimproved
0.109
0.2
0.17
0.004
0
0
0.17
0.000
0
0
0.17
0.000
0
0
0.17
0.000
TOTAL UNDEVELOPED SITE RUNOFF -------- ----------------- ----
0.004
DEVELOPED SITE:
0
0
0.17
0.000
Lawn/Pasture
0.109
0.25
0.17
0.005
Roof/Decks, etc.
0
0.9
0.17
0.000
Street/Driveway
0
0.9
0.17
0.000
TOTAL DEVELOPED SITE RUNOFF ------------------------------- ---
0.005
NET RUNOFF (Developed Site - Undeveloped Site) ---------------
0.001
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SURfACL: Native Grasses and Ver on
APPROX SURFACE LLEVATION 25, .,.3
SOIL_ ULSCRIIJTION
_ o 1:NFTRATION RESISTANCE/101STURE CONiCNi
2 L,,J n- n_ A = Riows per toot
UJ ® = Moisture content
o °= 0 10 20 30
TOPSOUORGANICS
1.4
Silty SAND, moist, brown
2 25
Sandy SILT, slightly moist, light brown
/ 3.5
Lean CLAY, varved, moist, variegated reddish,
greenish to yellowish brown
7.3
Bottom of Test Pit: 7.3 Feet
Started: 12/08/95
Completed: 12/08/95
Logged by: Terry Stonehocker
Excavated by: Nupac
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LUG—LN D NOTE The st,216cotton t,ne5 represent opproumate bounCoaes bctaecn
A, SI'1 tllows iao!
Atter ber Luni_,5_Plostw- Limit sal tyxs. ACto01 boundorics moy be grodua1 or t*onvGoccl
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0 Field ma1;11Jr^ �arltnnl Y �_-_--- -- water Content Log of Test Pit TP-3
v_ Or")und titer jjg:nd Llm'l Proposed Housing Project
Ic `no nplc Teton Terrace
I_ i SIB mf h -O sornpler
1L 2 Inct, I t) sornpler Kalispell, Montana
IIIk 2 1/2- Imh I.O. ring sampler December, 1995 K94-46-7
jfjr 3--inch ID thin--vtolled sompler THOMAS, DEAN & HOSKINS Figure 4
a No sample recovery & Engineering Consultants Sheet 1 of 1
9 SURi-ACC: Native Grasses and Vegf+ Tn = o r
a c3 APPROX SUR ACr LLCVAIION: 29; n_ ;
0 0.
SOIL nCSCRIf'TION o cr:
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TOPSOILIORGANICS
0.8
Sandy SILT, moist, dark brown to light brown
2.9
Lean CLAY, varved, moist, variegated reddish,
greenish to yellowish brown
- thin layer of white silt with sand at 3.4 feet
Bottom of Test Pit: 7.3 Feet
Started: 12/08/95
Completed: 12/08/95
Logged by: Terry Stonehocker
Excavated by: Nupac
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NO[[ The slvotificot;on lines represent eppr irnote boundortes between
50d types. ACt J01 bo.nd.—, n,0y 0C 9r0tiv'Ci of Ir0nsd.01101.
Log of Test Pit TP-2
Proposed Housing Project
Teton Terrace
Kalispell, Montana
December, 1995 K94-46-7
THOMAS, DEAN & HOSKINS Figure 3
Engineering Consultants Sheet 1 of 1
2
NW
SURI ACF._ Native Grasses and Vege+p ion
AI'PROX sur;r ACF 1-1 F vnuON: 2P
sou OtSCRI111ION
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ni N; AI10N PISISTANCE/MOISluR CO -IfN1
A. - flows nc, too,
® A90'siu,(r "o"Ic"t
0 10 20 30 40 S
0
;
\
TOPSOIL/ORGANICS
� i
�.
1 0
,
Silty SAND, moist, reddish brown
2.0
Sandy SILT• slightly moist, light reddish brown
�d = 97.0 pcf at w 7"4%
—ry-- - -
2.8
Ic
Lean CLAY, varved, moist, variegated reddish
to yellowish brown
1dry = 115.3 pcf at w = 14.8%
- - - • - ;
8.2
I f.
L ,_ _J J
j
Silty SAND, wet, yellowish brown
9.0
Bottom of Test Pit: 9.0 Feet
I • . • . .
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r 1 -1 _ T 1 - -1- I- r i _, -,-- Ir 1 _ -I- _ 1 -1 -,-
Started: 12/08195
_ _ r .. _. :. _ . .
Completed: 12/08/95
; -; - ,
Logged by: Terry Stonehocker
r ; _; I ; _; ; , ,-' -;_
Excavated by: Nupac
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-
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UoonOrr�es
0 SPT blows per loot /Utcr,Jc:g L,((1,t_loss c _ m+I sod types. ACI-i noy be
® ^:d rno,slure content ; Y_- ____Wote: c:11 Log of Test Pit TP-1
Y �'r:)und Wol'!r level
Proposed Housing Project
�: ,:�:)b/cI1(n:)asite snrnplc �,
Teton Terrace
I s�H : , Lu snrnplcr
�I ,n n I i, sample, Kalispell, Montana
Illk 2 -1 /2 inch I D ring sampler December, 1995 K94-46-7
IRT S-inch 1 D. thin -walled sampler
THOMAS, DEAN & HOSKINS Figure 2
No recovery & 1 1
s sample Engineering Consultants Sheet of