HomeMy WebLinkAboutCOUNTRY CLUB RESERVE - FDP180030 - - STORMWATER-RELATED DOCUMENTSNovember 25, 2017
Bud Curtiss
Northern Engineering Services, Inc.
301 N. Howes Street, Suite 100
Fort Collins, CO 80521
RE: Country Club Reserve – Subsurface Water Investigation and Drain Evaluation Report
Dear Mr. Curtiss:
Miller Groundwater Engineering has been evaluating subsurface water conditions and a proposed
subsurface drain network for the planned residential development called Country Club Reserve. The
subject site was formerly known as Boma Farm and is approximately 80 acres located immediately
southwest of the intersection of Douglas Road (County Road 54) and Turnberry Road (County Road 11)
in Fort Collins, Colorado. The site is the north half of the northeast quarter of Section 30, Township 8N,
Range 68W.
This letter report presents our work completed to date.
Information Referenced or Relied Upon
Our evaluation has relied primarily on information from the following sources:
Preliminary Geotechnical Subsurface Exploration report for Boma Farm Development by Earth
Engineering Consultants (EEC); EEC Project #1062013; February 22, 2006.
Supplemental Subsurface Exploration Report for Country Club Reserve (AKA) Boma Farm
Development, “Evaluation of Groundwater/Subsurface Conditions”; EEC Project No. 1172035;
June 15, 2017.
Topographic maps, site plans, and proposed-grade elevation data provided by Northern
Engineering.
A proposed subsurface drain network provided by Northern Engineering (updated Nov. 21,
2017).
Geologic Map of the Lower Cache La Poudre River Basin, North-Central Colorado by L.A. Hershey
and P.A. Schneider (1972), plus general area maps, and Google Earth aerial imagery.
Information reported for nearby permitted wells as found in the Division of Water Resources
(State of Colorado) well permit database.
Field observations and water level measurements made during several site visits in Summer
2017.
Updated Ecological Characterization Study (ECS) Letter Report for the Country Club Reserve
Development Parcel from Cedar Creek Associates (CCA), Inc. April 19, 2016.
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Site Investigation Overview
Eight exploratory geotechnical borings with groundwater level observations were provided by EEC in
February 2006 (Figure 1). Nine more borings were provided by EEC in May 2017 (Figure 1 and
Attachment A). To allow for follow-up water level measurements and some basic field tests, two of the
new borings were completed as screened monitoring wells (MW) and the remaining seven as temporary
piezometers (PZ). EEC measured water levels at the nine new locations in early and late May 2017.
Miller Groundwater measured the water levels again on June 7, 2017, and performed slug tests on the
two monitoring wells on July 1, 2017. Northern Engineering provided a professional survey of the
location and elevation of the new borings.
Subsurface Conditions
Lithology. Below the topsoil and vegetation layer, the site subsurface generally consists of 14 to 35 ft of
cohesive subsoils classified as lean clay with sand and sandy lean clay that transitions to clayey sand
with depth (EEC 2006; EEC 2017). These soils overlie a bedrock layer that is a claystone/siltstone with
sandstone lenses (EEC 2017). This bedrock layer is presumably the middle zone of the Pierre Shale
which has been reported in the area as sandy shale with sandstone members (Hershey and Schneider
1972). We have assumed that this bedrock has very low permeability at the site.
The 2006 borings terminated at 15 feet below ground surface (bgs) and most did not reach bedrock,
while the 2017 borings were typically terminated at bedrock. The bedrock was encountered at about 14
ft bgs in the northwest portion of the site in two of the 2006 borings, at about 17 to 24 ft bgs in the
western portion of the 2017 borings, and from 23 to 34 ft in the middle portion of the site. Bedrock
was not reached (>35 ft bgs) at MW-2 in the northeast corner and PZ-7 in the south-center of the site.
Groundwater Levels. Measured water level elevations, along with the corresponding water depth from
the existing natural grade, are noted in Figure 2 at each 2017 boring location. The contour lines in
Figure 2 indicate interpolated and extrapolated water levels from the June 2017 data. The general
groundwater flow direction is toward the east (Figure 2).
Subsurface Permeability. We briefly pumped and surged MW-1 and MW-2 as part of well development,
and also conducted basic slug tests on those two wells. We found that MW-1 could sustain a pumping
rate of about 1.0 to 1.3 gpm for at least three hours, and likely longer. We found that MW-2 could not
sustain a pumping rate of 0.7 to 1.0 gpm. It was pumped dry at those rates and we did not have means
(or need) to pump it at a lower rate. Our slug testing suggests a hydraulic conductivity (K) at MW-1
around 1 to 4 ft/day, and around 0.2 to 0.4 ft/day at MW-2. Those slug tests results are consistent with
our short-term pumping observations, and that range of results is consistent with the subsoils observed
at the site during drilling.
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BUD CURTISS
NOVEMBER 25, 2017
Sources of Subsurface Water
It is our current opinion and assumption that some minor portion of the site groundwater may originate
as precipitation both onsite and offsite. Due to the low permeability of the subsoils, we expect that the
net infiltration rate from precipitation is very small fraction of annual precipitation (i.e., that most
precipitation runs off as surface water or is held in shallow soils until evaporated). We note, however,
due to the lower permeability of the soils, that even minor rates of infiltration from the topsoil zone
could lead to elevated groundwater levels.
It is also our opinion and assumption that a significant portion of the site groundwater originates from
the storage reservoirs located to the north and northwest, and from the irrigation canal that is located
about about 1,500 to 2000 ft north of the site. That canal appears to be the Larimer County Canal. The
leakage rate from the canal or reservoirs does not have to be large to lead to elevated groundwater
levels in this area. We don’t know how much groundwater may originate from the neighborhood uphill
and to the west, but further delineation of water sources does not appear to be necessary.
Site groundwater appears to flow generally from the north and northwest and then toward the east
(Figure 2) where a portion of it very likely discharges to a deeply cut canal located immediately east of
Turnberry Road. The upper portion of that canal is at, and slightly below, the existing site grade around
the northeast corner of the site (about 5073 ft), but the canal then passes through a significant drop
structure, also located adjacent to the site, to an elevation of 5034 ft. That canal appears to be the
Number 8 Outlet Ditch coming from Windsor Reservoir No. 8 and Elder Reservoir (Google Maps and
Google Earth).
We observed this outlet ditch several times from November 2016 to July 1, 2017. During that period,
we did not observe high flows which we’d expect with active reservoir releases. Rather, we saw very
low but consistent steady flow in the canal both above and below the drop structure. We have never
observed this canal to be dry in our site visits or in Google Earth imagery taken on a variety of dates and
years. We expect the water in this canal above its drop structure is either from slow reservoir leakage or
slow groundwater leakage into the canal, or both. Below the drop structure, based on our water level
mapping (Figure 2) the canal surely receives additional groundwater discharge.
Pond and Wetlands
First, please note that Miller Groundwater does not have specific expertise in wetland delineation and
classification. But, with that caveat stated, we can offer comments based on our groundwater
observations that appear to relate to prior observations by other parties (CCA 2016) about ponds and
wetlands on the site.
The narrow pond in the south-central portion of the site continuously has open water, but was not
classified as a wetland (CCA 2016). Our groundwater observations clearly support the suggestion by
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BUD CURTISS
NOVEMBER 25, 2017
CCA (2016) that that existence of this pond is due to groundwater. The site survey found groundwater
in the pond’s vicinity to be at nearly the same elevation of the pond surface. It is also our understanding
that this pond was an excavation intended to expose groundwater for cattle use and would be
considered by the State of Colorado to be an unpermitted groundwater “well”. We expect it would be
the position of the State of Colorado that this pond is an unpermitted exposure of groundwater that
should be closed.
A surface depression described as a wetland area (CCA 2016) was noted along the center portion of the
north edge of the property, roughly 250 ft northeast of MW-1 and west of PZ-3 (Figure 1). CCA noted
that depression does not have standing open water, and we have been told the ground there is dry
much of the time. Those surface observations are consistent with our groundwater observations which
project (from nearby observation wells) that groundwater is probably about 3 feet below the lower part
of that depression (with some uncertainty noted due to observation locations). The groundwater depth
in that area is consistent with the depression being wet after precipitation events but not maintaining
water between precipitation events.
Water Rights and Permitted Wells
Impacts to other wells. The proposed drains at this site will be lowering the water table in a thin and
shallow zone of low permeability materials. One would not expect neighboring wells to be relying on
this groundwater zone for a water supply. Indeed, our review of the State of Colorado’s Permitted Well
database found no wells using water from this surface zone. For example, a permitted well we found
nearby in the Serromonte neighborhood to the south (Permit #212346-A) is reported to be 350 ft deep
and to obtain its water from a sand layer (presumably sandstone) which is located from 300 to 350 ft
below the surface, with multiple thick siltstone and shale layers (which have low permeability) reported
above that water-bearing sand layer. It is our current opinion that the subsurface drain system planned
for the CCR development would have no measureable effect on that neighboring well due to that well’s
much greater depth and it obtaining water from a different and separate hydrogeologic unit. We
expect that well’s pumped zone to have little to no hydraulic connection to the shallow groundwater
where the drains are located.
Drain Permit. Due to some initially open questions about the nature of the planned discharge location
for the drain system, a state permit was requested for the planned drains in order to evaluate any
potential water rights requirements. Based on an evaluation and site inspection performed first by
Miller Groundwater and then also by the Division of Water Resources (aka State Engineer’s Office), we
have received verbal approval from DWR for the drain permit. Formal permit approval is pending the
site owner obtaining written permission from the owner of the Number 8 Ditch to discharge the drain
water to that ditch. The State’s verbal approval for the permit was based on the observation that the
drain water will simply be groundwater that was already discharging (eventually) to the No. 8 Ditch,
therefore the drain discharge will not be a new exposure of groundwater.
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BUD CURTISS
NOVEMBER 25, 2017
Drain Evaluation Approach
The subsurface drain evaluation was performed primarily through mapping site surfaces (existing grade,
proposed grade, water table, and bedrock) combined with numerical groundwater flow simulations.
Flow simulation was performed using the USGS modeling code MODFLOW-2000. A MODFLOW model is
useful in this case because it helps organize and compare the topographic data, such as to account for
the spatially variable and potentially intersecting slopes of the ground surface, water table surface,
bedrock surface, and drain paths.
To keep this report brief, the modeling work will be summarized and only notable features and
observations will be highlighted. Additional model details can be provided upon request.
The model was constructed with a single layer that has spatially variable top and bottom elevations
representing the zone from ground surface to top of bedrock. The ground surface is based on detailed
maps of the existing surface and the proposed grading. The bedrock surface is inferred, interpolated,
and extrapolated from the boring logs. In the two deep borings where bedrock was not found, we
assumed bedrock was 1 ft below the bottom of the boring.
For this evaluation, we have assumed a homogenous hydraulic conductivity (K) equal to 1.0 ft/day. We
may consider alternative scenarios and different assumptions at a later time (such as spatial variability
and uncertainty in a representative K value), but for now this is a simplifying assumption and a middle-
range estimate based on the slug tests, boring observations, and experience in similar settings.
The relatively low permeability of the site subsoils and modest saturated thickness (saturated thickness
meaning that interval between the water table and the bedrock) suggests that volumetric groundwater
flow through the site is low. That in turn suggests low net recharge to groundwater from on-site
precipitation. These opinions are supported by model calibration results: While adjusting the simulated
net-recharge parameter and while holding permeability constant at K = 1 ft/day, calibrating the model
to observed water levels was possible only by using a negligible amount of deep net recharge. And,
calibration was sensitive to the net recharge rate.
Simulation Results
Figure 3 is a map of the simulated water table elevations under current (pre-drain) conditions. The
locations of observations used for water level calibration are shown.
Figure 4 is a calibration plot comparing the simulated groundwater levels to observed levels. The scaled
root-mean-square error (RMSE) of this calibration is 9.5%. There is no one-size-fits-all goal for the RMSE,
but less than 10% is considered adequate-to-good in common practice.
Figure 5 is a map of simulated depth-to-water (DTW) from finished grade before adding the proposed
drains to the model. Note that this detailed-looking map is created by subtracting a “non-detailed”
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BUD CURTISS
NOVEMBER 25, 2017
(approximated and smooth) water surface (Figure 3) from a detailed map of the proposed-grade
surface. The 13-ft DTW contour is highlighted in red in this and the following figure since that is the
selected target depth for groundwater at finished lot grades.
Figure 6 is a map of simulated DTW from the finished grade after adding the subsurface drain network
proposed by Northern Engineering (drain version 11/21/2017).
Figure 7 is a map of the simulated water table elevations across the site, after drainage.
Figure 8 is a map of the simulated change in groundwater level elevation (aka drawdown or cone of
depression) caused by the drain system. We note again that we expect these changes to occur only in
the surficial layer and not in the deeper bedrock zones.
Results and Comments
Comparing Figures 5 and 6 illustrates the simulated impact of the drain system on groundwater depth.
Most lots are projected to meet the 13 ft DTW goal. A few lots are projected to have drained
groundwater levels at about 11 to 12 feet below grade.
The long-term average discharge rate from this drain system is expected to be less than 10 gpm. (To be
“precise”, this particular simulation computed 2.9 gpm.) Initial flow rates will likely be higher during
initial drainage, but a low rate is expected once groundwater levels have re-equilibrated to lower levels.
Transient model simulations have not been conducted to estimate the duration of the initial drainage
period. Given the low permeability of the site subsoils, it could take many weeks or even a few months
for the initial dewatering period to approach final results. We recommend routine monitoring be
conducted to document water level changes before, during, and after the drains are installed.
Uncertainty about the projected long-term average drain flow rate is easily a factor of four (thus actual
rates from less than 1 gpm to 12 gpm) and, possibly a factor of ten (thus up to 30 gpm). The drain
discharge rate may also show seasonal variations. Regardless, the currently projected rate of 3 to 10
gpm is squarely consistent with the discharge we have observed from other subdrains in sites with
similar subsoils and groundwater levels. In our opinion, it is unlikely that the long-term average
drainage rate will be greater than 10 gpm.
Where a steep change in DTW is shown across a given individual lot (such as the northeast corner lot in
Figures 5 and 6), note that this arises mostly from surface grading and not the topography of the
groundwater surface. In other words, the water table surface itself is relatively smooth (Figures 2 and
3), but the lot grade may drop several feet from the front or center of the lot (foundation areas) toward
the back side of the lot (backyard). A steep change in depth to water is also seen in Figures 5 and 6
along the drainage swale and pond edges for the same reason, that reason being a steep change in
ground surface elevation.
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BUD CURTISS
NOVEMBER 25, 2017
Additional Considerations
Basement Perimeter Drains. This model does not include individual basement perimeter drains. Those
drains are considered a backup system and not part of the planned site-wide drainage. However,
individual basement drains are strongly recommended to guard against unforeseen local variations in
subsurface conditions and against occasional stray water (such as from lawn irrigation, poor surface
grading, or even domestic leaks) that could collect locally at basement excavations in this low-
permeability subsurface setting.
Trench Fill and Drain Bedding. The proposed drain depths do not pass below the top of the modeled
bedrock surface at this site. However, that bedrock surface is an estimated surface. During trench
construction, we strongly recommend that the trenching be periodically observed by qualified personnel
to determine if drain will be placed below the top of bedrock. Anyplace that the drain depth is observed
(or suspected) of being below the top of bedrock, then it is imperative that the permeable
bedding/trench-fill around the perforated drain be extended upward to at least 2 feet above the top of
the observed bedrock surface.
Similarly, if permeable layers (such as a sandy layer or sand seam) are found in the trench wall above the
drain depth, while the drain is positioned in a low-permeability layer, then it is strongly recommended
that extra permeable fill be added to the trench so that the permeable fill reaches from the drain up to
and through those observed permeable layers or seams.
Given the low permeability of this site, it would likely be beneficial to construct a “taller” than standard
interval of permeable fill before backfilling the trench with native materials or other low-permeability
materials. And, if this trench with extra bedding/fill is below a street or other structure, then a
geotechnical engineer may need to review the trench fill material and the overall trench completion.
Simulation and Evaluation Limitations
The scope of this evaluation considered drain function only with respect to the perforated drain pipe
depths and locations. This scope has not included reviewing drain pipe specs, bedding materials, or
other trench details. Our simulations assume the drains are constructed and installed properly and that
their function and ability to take in and remove groundwater to the drain invert level is well-maintained
over time. Periodic clean-out maintenance of the drains may be necessary.
Standard Technical Limitations in Subsurface Evaluations
Subsurface data is always limited in its spatial coverage and subsurface hydraulic testing produces only
approximate results. And, numerical models are greatly simplified approximations of a likely complex
subsurface. Therefore, estimates and projections about groundwater and subsurface drain behavior
have inherent and unavoidable uncertainties. No one can provide certainty. This is particularly true for
potential local-scale (i.e., lot-scale) variations in bedrock depth and subsurface permeability. By using
12MW-2PZ-4PZ-5PZ-6PZ-7PZ-3MW-1PZ-2PZ-1Figure 2: Boring Location DiagramCountry Club Reserve - Boma Farm DevelopmentFort Collins, ColoradoEEC Project #: 1172035 Date: June 2017EARTH ENGINEERING CONSULTANTS, LLCApproximate BoringLocations1LegendSite Photos(Photos taken in approximatelocation, in direction of arrow)Attachment A (from EEC report)
Miller Groundwater Engineering, LLCProject: Country Club Reserve
/BomaFarm/reporting/WaterTables.srf 8/2/17506650685070507150725073507450755076V.P.
V.P.V.P.V.P.
A.R.V.GAS
GAS
V.P.V.P.
WV
WVWV
WV
WVWV
V.P.
WV
H Y DWV
WV
MFES
MM
F ESFES
M M
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F.O.TELEFES
MGAS
F ES
M
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FES
M F ES
WSO
V.P.
VAULTCABLE
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GASGASV.P.
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B
B
B
B
B
B
B
B
MW-1
MW-2
oldP-3
PZ-1
PZ-2
PZ-3
PZ-4
PZ-5
PZ-6
PZ-7
Pond
5076.3
DTW: 6.4
5063
DTW: 7.25076.5
DTW: 9.7
5075.4
DTW: 13.2
5075.2
DTW: 3.8
5073.1
DTW: 6.8
5068.9
DTW: 16.3
5070.3
DTW: 4.6
5070.4
DTW: 2.8
202600 202800 203000 203200 203400 203600 203800 204000 204200 204400 204600 204800 205000 205200 205400 205600
Easting (feet)
148600
148800
149000
149200
149400
149600
149800
150000
150200
Northing (feet)E. Douglass Rd
Turnberry Rd(old P-3/B-6)
Figure 2. Groundwater elevations as interpolated from measurement points.
8/3/2017 Miller Groundwater Engineering, LLC
5060
5061
5062
5063
5064
5065
5066
5067
5068
5069
5070
5071
5072
5073
5074
5075
5076
5077
5078
5079
5080
5060 5061 5062 5063 5064 5065 5066 5067 5068 5069 5070 5071 5072 5073 5074 5075 5076 5077 5078 5079 5080 Simulated Level (feet) Observed Level (feet)
Figure 4. Model simulation of observed groundwater levels (simulation ID: CCR4).
MW-2
PZ-5
PZ-6
PZ-7
PZ-4
old P-3 (B-6)
pond
PZ-3
MW-1
PZ-2
PZ-1
Miller Groundwater Engineering, LLCProject: Country Club Reserve
/BomaFarm/.../DTWmaps-D3.srf 11/22/174
44
44
4
4
4
4
4
4
8
8
8
8
88
8
88
8
8
8
8 88
121
2
121212
12
12
1
21313 1313
13
13
202600 202800 203000 203200 203400 203600 203800 204000 204200 204400 204600 204800 205000 205200 205400 205600
Easting (feet)
148600
148800
149000
149200
149400
149600
149800
150000
150200
Northing (feet)E. Douglass Rd
Turnberry Rd-40.1481112131415
Depth to Water (feet)
<1>15
Figure 5. Simulated depth to water from planned graded surface, before drains.
Miller Groundwater Engineering, LLCProject: Country Club Reserve
/BomaFarm/.../DTWmaps-D3.srf 11/22/17444
8 8
88 888
8
8
11
11
11
11
11
1
1
11
111
1
11 1112
12
1
2 1212
121212
12
12
1212
13
13
13
13
13
13
1
3
13
1313
1
3
13
13
13 14
15
202600 202800 203000 203200 203400 203600 203800 204000 204200 204400 204600 204800 205000 205200 205400 205600
Easting (feet)
148600
148800
149000
149200
149400
149600
149800
150000
150200
Northing (feet)E. Douglass Rd
Turnberry Rd-40.1481112131415
Depth to Water (feet)
<1>15
Figure 6. Simulated depth to water from planned graded surface, with drains.
perforated drains
(drain version 11/21/2017)
Miller Groundwater Engineering, LLCProject: Country Club Reserve
/BomaFarm/.../Figs7&8s-D3.srf 11/22/1750505052
5054
5056
5058
5060
50605062
50625064
50645066
50665068
506850705070507250725074507450765076507850785080508050825082202600 202800 203000 203200 203400 203600 203800 204000 204200 204400 204600 204800 205000 205200 205400 205600
Easting (feet)
148600
148800
149000
149200
149400
149600
149800
150000
150200
Northing (feet)E. Douglass Rd
Turnberry RdFigure 7. Simulated groundwater elevations, with drains.
perforated drains
(drain version 11/21/2017)
Miller Groundwater Engineering, LLCProject: Country Club Reserve
/BomaFarm/.../Figs7&8s-D3.srf 11/22/171122 2
33 3
344 4
5
5
5
6
6
7
7
8
9
202600 202800 203000 203200 203400 203600 203800 204000 204200 204400 204600 204800 205000 205200 205400 205600
Easting (feet)
148600
148800
149000
149200
149400
149600
149800
150000
150200
Northing (feet)E. Douglass Rd
Turnberry RdFigure 8. Simulated change in water levels created by sub-drains (feet).
perforated drains
(drain version 11/21/2017)