HomeMy WebLinkAboutTHE GROVE AT FORT COLLINS - FDP - FDP110015 - REPORTS - DRAINAGE REPORT (6)Prepared for:
Campus Crest at Fort Collins, LLC
2100 Rexford Road, Suite 414
Charlotte, NC 28211
Supplemental Subsurface Water
Investigation
The Grove at Fort Collins
Larimer County, Colorado
The Grove at Fort Collins
Prepared by:
Water Resource Advisors for the West
1499 W. 120th Ave., Suite 200
Denver, CO 80234
Phone: 303-452-6611
Fax: 303-452-2759
www.applegategroup.com
November 16, 2011
AG File No. 10-132
TABLE OF CONTENTS
Introduction .................................................................................................................................................. 1
Site Investigation ........................................................................................................................................... 1
Groundwater Model ..................................................................................................................................... 2
Canal Monitoring .......................................................................................................................................... 2
Water Rights and Wells ................................................................................................................................. 3
Underdrain Design ........................................................................................................................................ 3
Water Quality ................................................................................................................................................ 4
FIGURES
Figure 1 ....................................................................................................................................... Vicinity Map
Figure 2 ......................................................................................................... Monitoring Well Location Map
Figure 3 .................................................................................................................... Groundwater Flow Map
Figure 4 .....................................................................................................Pre-development Depth to Water
Figure 5 ...................................................................................... Depth to Water with Grading and No Drain
Figure 6 ........................................................................................... Depth to Water with Grading and Drain
Figure 7 ............................................................................................................................... Area of Influence
Figure 8 ............................................................................... Cross Section of Canal and Groundwater Levels
APPENDICES
Appendix A ................................................................ Subsurface Report From Monitoring Well Installation
Appendix B ...........................................................................................................Subdrain Flow Calculations
Appendix C ........................................................................................ Description Of Groundwater Modeling
Appendix D ............................................................................................................ Water Quality Lab Results
The Grove at Ft. Collins, Subsurface Investigation Report | Introduction 1
INTRODUCTION
The Grove at Fort Collins is a proposed multi-family project located west of Centre Avenue
between West Prospect Road and West Drake Road, see Figure 1. The project area is
bordered by Centre Avenue to the east, Larimer #2 Ditch to the south, Windtrail P.U.D.
outfall swale to the north and an existing residential development to the west. The site
drains from the southwest to the northeast and has historically been used for agricultural
purposes. There is also an existing wetland which has been identified along the northern
property boundary. The site encompasses approximately 31 acres and the project is
proposed as residential student housing.
Earth Engineering Company, Inc. (EEC) conducted a geotechnical investigation of the site in
2009. This investigation included 13 test hole borings. Their report of July 10, 2009
describes the subsurface and groundwater conditions at the site. The borings encountered
sandy lean clay soils and sand and gravel underlain by claystone bedrock. This report is
intended to supplement the EEC report, specifically as applying to the groundwater
conditions.
Applegate Group was retained by Campus Crest at Fort Collins, LLC to design an
underdrain system to lower the groundwater in accordance with Section 5.6 of the Larimer
County Urban Street Standards, Subsurface Water Investigation. This included drilling of
12 monitor wells, preparation of a groundwater flow model, determination of the impact to
the water table, design flows from the underdrain system, engineering details and
recommended maintenance requirements.
SITE INVESTIGATION
During the drilling conducted by EEC in 2009, groundwater was encountered in 10 of the
13 borings, ranging in depth from 2 to 12 feet below surface on May 11, 2009. On July 1,
2009, approximately 6 weeks following the drilling the water levels in all the borings
ranged from 0 (surface) to 5.5 feet below surface.
EEC and Applegate constructed 12 monitor wells on August 27, 2010. The locations of the
wells are shown on Figure 2. The purpose of these monitor wells is to determine the
present, and where feasible, the future water levels. Soils samples were collected and
analyzed.
Previous water levels were taken by EEC in May and July of 2009. The water levels taken
on August 30, 2010, although at different locations than the previous borings, show depth
to water varying from 2 to 14 feet below surface. This is similar to the May 2009 water
levels, indicating that the groundwater in the area rises during the period the ditch is
flowing. The water levels are influenced by seasonal precipitation and the water flow in
the Larimer #2 Ditch. Interviews with ditch company personnel indicate that the ditch
comes on mid-May and off by mid-July. The water levels taken on July 10, 2009 reflect a
rise from the previous levels taken in May 2009. This would indicate that water levels are
The Grove at Ft. Collins, Subsurface Investigation Report | Groundwater Model 2
generally highest during the period that the ditch is running. The ditch flow influences the
groundwater table under the site for approximately two to three months a year. Inflow
from the drainage to the west is also a factor. Groundwater flow is to the north and
northeast as shown on Figure 3.
Subsurface samples were logged during the monitor well installation. The logs are
contained in Appendix A. The subsurface characteristics show that the predominant soil
type is sandy lean clay. All but wells 11 and 12 contain sand and gravel ranging from clay
with gravel to gravel lenses of 4-5 feet in thickness. These gravels are the predominant
water bearing strata.
GROUNDWATER MODEL
The site data was used to develop a groundwater flow model (MODFLOW). This model is
used to determine the effectiveness of the underdrain to maintain groundwater levels a
minimum of three feet below subgrade. A description of the modeling procedure is
included in Appendix C. Figure 4 shows the pre-development water level surface in July
2011. This surface is considered to be a representative estimate of high groundwater table
conditions. Figure 5 shows the same water level after the proposed grading and the
relocation and lining of a segment of the irrigation ditch. Figure 6 shows the depth to the
groundwater after grading, after the relocation and lining of the ditch, and with the
underdrain effects. As shown on Figure 7, drawdown in the aquifer below the wetlands is
indicated to be approximately one to three feet. This projected drawdown does not
directly translate to a corresponding reduction in the surface water within the wetland.
Effects on the surface water may be less and will be monitored.
CANAL MONITORING
In the spring of 2011 a plan was designed to monitor the groundwater levels on the site
prior to and during the period the Larimer #2 Ditch ran water. Monitoring wells were
installed on the north and south bank of the ditch (Figure 2). In addition to the wells
installed in 2010, six additional wells were installed (nos. 13, 14, 15, 16, 17, and18) at
varying depths to determine the influence of the canal seepage on groundwater levels.
Figure 8 is a cross section of the wells showing the groundwater levels at various times. As
shown, the groundwater levels rise once the ditch is running water. Additionally a water
measurement device was installed in the ditch to correlate the level in the ditch to the
groundwater. A good correlation is seen with changes in the ditch and changes in the
groundwater levels.
The groundwater model was updated to incorporate the results of the canal monitoring
data. The results of the model show that the proposed subsurface drains are properly
designed and will be effective in decreasing the groundwater levels to City standards.
The Grove at Ft. Collins, Subsurface Investigation Report | Water Rights and Wells 3
WATER RIGHTS AND WELLS
The site has been previously irrigated; however discussions with the ditch company
representatives revealed that there are no water rights from the Larimer #2 Ditch now
used on the property.
We also researched the well permit file at the State Engineer’s Office for wells in the area.
No well permits were found within 600 feet of the site. The nearest permitted well is
located approximately 1,200 feet east of the site. This well is registered to Colorado State
University with Permit No. 15871 for irrigation use.
UNDERDRAIN DESIGN
The intent of the underdrain design is to lower the site groundwater level to meet the
Larimer County Urban Area Street Standards requirement for a three foot difference
between street subgrade elevations and the maximum predicted water table. In order to
have a drain system which is accessible for cleanout and maintenance, 8” slotted PVC pipe
will be used throughout the design. Cleanouts will be located at the upstream end of each
drain and at each bend exceeding 45 degrees. Groundwater calculations estimate the
maximum groundwater flow rate that the drain will need to intercept is 15 gallons per
minute (0.033 cubic feet per second). In addition, the underdrain will be intercepting flows
from proposed rain gardens located along the north and south sides of Rolland Moore
Drive that will infiltrate rainwater during storm events. The flows that the underdrain can
expect to during a storm event are 0.209 cfs and 0.453 cfs for the north and south sides of
the street, respectively. The proposed 8” PVC pipe has a capacity of approximately 0.090
cfs and a normal depth of 0.54’ with the largest anticipated flow of 0.695 cfs at a minimum
slope of 0.5%; this is more than sufficient to carry both the anticipated maximum
groundwater flows combined with the rain garden flows. The 8” slotted PVC pipe should
be bedded in CDOT Class A Filter Material. Flow calculations can be found in Appendix B.
Multiple underdrain main lines are proposed to be located on site. There will be drain
segments located at the base of the slope on the south side of the site between some of the
proposed buildings and the existing ditch. Other drainlines will be located along the
centerline of the Rolland Moore Drive alignment and along part of the curblines of Native
Plant Way and Perennial Lane. The Northern Engineering Final utility plan indicates the
horizontal and vertical design of the subdrain. Figure 6 of this report shows that the
proposed grading and underdrain should prove sufficient to reduce peak seasonal
groundwater to a minimum depth of three feet below subgrade at the required locations.
The CAT-22 channel is an appropriate stormwater outfall and it currently serves to relieve
groundwater in the area as well. A perennial baseflow can be observed trickling in the
bottom of this channel even in the dead of winter. The underdrain system to be installed
with the Grove will also outfall directly into this channel rather than routing through the
The Grove at Ft. Collins, Subsurface Investigation Report | Water Quality 4
Windtrail wetland drainage. The underdrain outfall will connect into the outlet structure
of the water quality pond, downstream of the orifice plate.
The underdrain outfall will also have an invert elevation higher than the triple culverts
draining the outlet structure. These measures will help minimize the potential for
stormwater to backup into the underdrain system. The peak discharge of the underdrain
outfall is only 0.695 cfs (15 gpm, or 0.033 cfs, from groundwater and 0.662 cfs from the
rain gardens), and has a negligible impact on the outfall channel. The floor of the concrete
outlet structure will also be designed to prohibit groundwater discharge from trickling to
the south and saturating the area in front of the water quality plate.
WATER QUALITY
The groundwater quality was sampled in monitor Well #11 (see Figure 2). The objective
was to screen for “contamination or undesirable characteristics” per Section 5.6.2. Since
the range of water constituents is wide ranging, we chose to test for the EPA Primary
Standards as a screen for any unusual indicators. The lab testing results, shown in
Appendix D show the list of the Primary parameters.
Of the 16 parameters tested, 12 were below detectable limits. Four parameters were
detected as follows:
TDS = 410
Fluoride = 0.594
Nitrate = 0.08
Barium = 0.125
These four parameters are well below the EPA limits, indicating that the groundwater does
not have any unusual characteristics that would indicate further investigation.
FIGURES
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Project Site
The Grove Vicinity at Fort Map Collins Figure:
Water 1499 Denver, www.West ApplegateGroup.Resource CO 120th 80234-Ave.2728 Advisors , com Ste 200 for e-mail: the info@West Phone:KCD Fax: applegategroup.((303) 303) 452-452-6611 2759
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Figure 2
Monitoring Well Location Map
Figure 3
Ground Water Flow Map
(contours in feet)
Figure 4
Pre-development Depth to Water
July 2011
(contours in feet)
Figure 5
Depth to Water with Grading and No Drain
(contours in feet)
Figure 6
Depth to Water, with Grading and Drain
(contours in feet)
Figure 7
Area of Influence
(contours in feet)
Figure 8
Cross Section Showing Canal
and Ground Water Levels
APPENDICES
APPENDIX A
SUBSURFACE REPORT FROM MONITORING WELL INSTALLATION
APPENDIX B
SUBDRAIN FLOW CALCULATIONS
APPENDIX C
DESCRIPTION OF GROUNDWATER MODELING
A Brief Description of groundwater modeling performed for The Grove
Groundwater modeling is a useful tool for organizing and understanding field data and, in
turn, guiding designs and expectations of groundwater flow behavior. The groundwater
model constructed for this evaluation follows standard assumptions and methods and is
reasonably consistent with the field data available to date. However, it must be
recognized that there is inherent uncertainty in quantitative assessments and projections
of subsurface conditions. The results presented herein should be viewed as a reasonable
guide, but with detail and accuracy limited by project scope, available data, and by the
standard simplifying assumptions relied on in model construction.
A preliminary groundwater model of the property was constructed in 2010 (using
MODFLOW-2000) to help assess drainage requirements and evaluate drain placement.
A single-layer model with variable grid elevation and variable thickness was used. The
model used a 10 ft grid spacing in the north-south direction and 20 ft in the east-west
direction, with the grid spacing gradually increasing outside the property boundaries out
to the model’s boundaries. The model boundaries were loosely defined by Spring Creek
and wetlands to the north and the irrigation ditch to the south. The east and west model
boundaries were established at a distance from the development area, at locations
corresponding to likely flow lines.
The bottom of the model grid was based on the depth from ground surface to the
claystone “bedrock” as indicated by drilling logs from 13 exploratory soil borings and 12
monitoring well borings. (The locations and elevations of these 25 borings were
approximate at the time of the initial model construction.) It is possible that the shallow
portion of this claystone bedrock is weathered and has some non-zero permeability, but
for this model the bedrock was assumed to be impermeable, as is a reasonable and
common modeling assumption. For reference (i.e., depth to water from ground surface,
location of potential seeps, etc.) the model was constructed with variable top elevations
closely corresponding to the existing grade (in the first model) and then corresponding to
the proposed grade in the later model.
Initially, no site-specific estimates were available for the hydraulic conductivity of the
site soils (sandy clays with a gravel intervals observed in many borings) or for the
leakage rate of the irrigation ditch. Subsequent to the initial modeling, nine water-
injection “slug tests” were performed on six monitoring wells. The test results indicated
transmissivity on the order of 10 to 40 ft
2
/day. Tests conducted in wells screened mostly
in the sandy-clay or mixed zones yielded hydraulic conductivity (K) estimates in the
range of 0.5 to 1.5 ft/day. For wells screened across sand intervals, the K estimates were
in the range of 2 to 6 ft/day.
Based on the overall slug tests results and initial model recalibration simulations, a
hydraulic conductivity value of 2 ft/day was selected and assigned uniformly to the
model. This value is assumed to be reasonably representative for this single-layer
homogenous model. The model was well calibrated to the July 2011 water level data (at
14 of 15 monitoring wells) with this K value along with the prior model assumptions
(e.g., net recharge of 1 inch/year, ET rates of 36 inches per year where the water table is
very near the surface.) The modeled bedrock depth was adjusted to be slightly lower
around the MW-13 and MW-14 area based on information from those new borings.
Bedrock was slightly revised overall with the new well location and elevation survey data
now available.
Quantifying absolute rates of canal leakage is difficult to do with high precision. Plus,
the leakage rates likely vary spatially and vary seasonally. (Still, the canal monitoring
program would have allowed us to confidently quantify the relative increase in the
leakage rate since the gradient near the canal was directly monitored and since the change
in gradient could be projected.) While keeping that caveat on mind, we did develop
rough estimates of canal leakage. By combining the slug test permeability estimates with
the hydraulic gradient observed near the canal in June and July 2011, this would estimate
the July 2011 leakage rate from the segment of the canal across the CSURF property (a
1600-foot section of the canal located on a hillside) to be in the range of 0.04 CFS ± 0.03
CFS. For simplification, this was modeled as a specified flux in the current-conditions
model. This current-conditions model was calibrated to match the observed July 2011
water levels (i.e., high water conditions).
Next, the model was used to project future conditions. The proposed development’s
ground surface grade was assigned to the model, drains were added according to the
current drain plan, and the middle portion of the ditch was moved. The proposed
subsurface drains were simulated using MODFLOW’s “Drain Package” with the
specified drainage threshold heads set at the proposed elevations of the subsurface drains.
In this future-conditions model, the middle 1200-ft section of the canal was relocated
approximately 80 ft to the south. Since the new canal section will be lined with
compacted clay, the leakage rate assigned to this section was reduced by a factor of 50x
in the model. The model also assumed that a 200 ft section of the remaining channel on
the west end will be lined.
With the canal relocated and considering the grade-fill areas (which puts some segments
of the drains at a relatively high elevation compared to historical water levels), the model
indicated the drain system to intercept approximately 1 to 5 gpm. Note this estimate
does not include potential additional water from landscape irrigation or planned “rain
garden” infiltration galleries.
APPENDIX D
WATER QUALITY LAB RESULTS
TECHNOLOGY LABORATORY, INC.
CENTRE PROFESSIONAL PARK
1012 Centre Avenue
Fort Collins, Colorado 80526
(970) 490-1414
CERTIFICATE OF ANALYSIS
Applegate Group, Inc.
1499 West 120th Ave
Denver, CO 80234
Sampled: 10/01/10
Received: 10/01/10
Laboratory ID: A2601-01
Sample ID: Well # 11 Project No.: 10-132
Matrix: Water
Number Parameter Result Units Method Analyzed
CAS Date
Total Dissolved Solids
(TDS)
410 mg/L EPA-160.1 10/01/10
7439-97-6 Dissolved Mercury < 0.0002 mg/L EPA-245.1 10/07/10
16984-48-8 Fluoride 0.594 mg/L EPA-300.1 10/01/10
84145-82-4 Nitrate-N 0.08 mg/L EPA-300.1 10/01/10
14797-65-0 Nitrite-N < 0.05 mg/L EPA-300.1 10/01/10
57-12-5 Cyanide <0.02 mg/L EPA-335.2 10/13/10
7440-36-0 Dissolved Antimony < 0.006 mg/L EPA-6010B 10/12/10
7440-38-2 Dissolved Arsenic < 0.005 mg/L EPA-6010B 10/12/10
7440-39-3 Dissolved Barium 0.125 mg/L EPA-6010B 10/12/10
7440-41-7 Dissolved Beryllium < 0.0003 mg/L EPA-6010B 10/12/10
7440-43-9 Dissolved Cadmium < 0.001 mg/L EPA-6010B 10/12/10
7440-47-3 Dissolved Chromium < 0.004 mg/L EPA-6010B 10/12/10
7440-50-8 Dissolved Copper < 0.005 mg/L EPA-6010B 10/12/10
7439-92-1 Dissolved Lead < 0.003 mg/L EPA-6010B 10/12/10
7782-49-2 Dissolved Selenium <0.005 mg/L EPA-6010B 10/12/10
7440-28-0 Dissolved Thallium < 0.0005 mg/L EPA-6010B 10/12/10
The results contained in this report
Page 1 of 2 relate only to those items tested.
TECHNOLOGY LABORATORY, INC.
CENTRE PROFESSIONAL PARK
1012 Centre Avenue
Fort Collins, Colorado 80526
(970) 490-1414
CERTIFICATE OF ANALYSIS
Applegate Group, Inc.
1499 West 120th Ave
Denver, CO 80234
Sampled: 10/01/10
Received: 10/01/10
Laboratory ID: A2601-02
Sample ID: 150' From Parking Project No.: 10-132
Matrix: Water
Number Parameter Result Units Method Analyzed
CAS Date
Total Dissolved Solids
(TDS)
380 mg/L EPA-160.1 10/01/10
7439-97-6 Dissolved Mercury < 0.0002 mg/L EPA-245.1 10/07/10
16984-48-8 Fluoride 0.434 mg/L EPA-300.1 10/01/10
84145-82-4 Nitrate-N < 0.05 mg/L EPA-300.1 10/01/10
14797-65-0 Nitrite-N < 0.05 mg/L EPA-300.1 10/01/10
57-12-5 Cyanide <0.02 mg/L EPA-335.2 10/13/10
7440-36-0 Dissolved Antimony < 0.006 mg/L EPA-6010B 10/12/10
7440-38-2 Dissolved Arsenic < 0.005 mg/L EPA-6010B 10/12/10
7440-39-3 Dissolved Barium 0.138 mg/L EPA-6010B 10/12/10
7440-41-7 Dissolved Beryllium < 0.0003 mg/L EPA-6010B 10/12/10
7440-43-9 Dissolved Cadmium < 0.001 mg/L EPA-6010B 10/12/10
7440-47-3 Dissolved Chromium < 0.004 mg/L EPA-6010B 10/12/10
7440-50-8 Dissolved Copper < 0.005 mg/L EPA-6010B 10/12/10
7439-92-1 Dissolved Lead < 0.003 mg/L EPA-6010B 10/12/10
7782-49-2 Dissolved Selenium <0.005 mg/L EPA-6010B 10/12/10
7440-28-0 Dissolved Thallium < 0.0005 mg/L EPA-6010B 10/12/10
The results contained in this report
Page 2 of 2 relate only to those items tested.