HomeMy WebLinkAbout1.04 Grading and Drainage Plan7. Grading and Drainage Plan (pursuant to Section 4-203.E)
Site Map
A map of Lot 1 and the surrounding area has been prepared by Western Slope Consulting, LLC and is
provided on the following page. This map includes: (1) existing improvements and structures; (2) existing
hydraulic features; (3) topography; (4) property boundaries; and, (5) information regarding water bodies
receiving stormwater runoff. It will be the lot owner(s) responsibility to provide any additional site
information to the County at the time of building permit.
Grading and Drainage Plans
The applicant is not proposing to alter existing grades or drainage patterns in conjunction with this
application. Future residential development on Lot 1 is anticipated to have minor impacts on the
property's existing native areas. Therefore, it is expected that development on Lot 1 will maintain historic
drainage patterns.
The lot owner(s) will be responsible for identifying a building site on Lot 1 and for preparing and
submitting the plans, documents and specifications necessary for a County building permit. Given that
detailed information for development on Lot 1 is not available at this time, a development scenario has
been prepared in order to provide approximations for the potential impact that future residential
development might have on stormwater runoff. The Rational Method was used to calculate stormwater
runoff quantities for existing and estimated post -development conditions. For more information refer to
the "Calculations" included on pages 35-36 of this application and Appendix B.
The lot owner(s) will be responsible for preparing grading and drainage plans specific to the
improvements to be constructed on Lot 1. These plans will be prepared by a qualified licensed
professional engineer on behalf of the lot owner(s) and provided to the County as required.
Soil Stockpile and Snow Storage Areas
The applicant is not proposing to stockpile soil or store snow on Lot 1.
There is adequate space on the proposed 10 -acre lot to store soil and snow without impacting adjacent
properties or public roads. It is anticipated that the lot owner(s) will only stockpile soil on a temporary
basis in conjunction with construction activities on Lot 1. The lot owner(s) will be responsible for
providing the County with the information necessary for stockpiling soil on Lot 1 at time of building
permit.
It is anticipated that the only snow to be stored by the lot owner(s) will be that pushed aside during
plowing in the winter. Pursuant to Section 7-305, residential uses are exempt from the County's snow
storage standards.
Equipment Storage Areas
The applicant is not proposing to store equipment, fuel, lubricants, chemicals and/or waste on the
proposed lot.
If the lot owner(s) choose to store equipment, fuels, lubricants, chemicals and/or waste on Lot 1, they will
be responsible for doing so in compliance with the applicable Garfield County regulations.
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Temporary Roads
No temporary roads are anticipated in conjunction with residential development on Lot 1. The existing
access road and primitive driveway (refer to images 2 and 3 below) will be used for access during
construction. It will be the lot owner(s) responsibility to improve the primitive driveway in accordance
with the applicable County regulations.
Image 2. Existing Graveled Access Road Image 3.Existing Primitive Driveway
Areas of Steep Slope (slopes greater than 20%)
1 -meter elevation data from the USGS 3D Elevation Program and ArcGIS analytical tools were used to
analyze slopes on the proposed lot. Areas on Lot 1 with slopes greater than 20% (approximately) are
identified on the Site Plan included in this application.
The analysis of slopes on Lot 1 identified roughly 2.65 -acres that: (1) contain slopes less than 20%; (2) are
located within the required setbacks for the Rural (R) Zone District; and, (3) are outside of the 100 -foot
buffer required for the on-site well. Therefore, there are approximately 3 -acres on Lot 1 that could be
well-suited for construction of a residence.
Construction Schedule
The lot owner(s) will be responsible for developing a schedule for constructing improvements on Lot 1
and submitting this to the County at time of building permit. The construction schedule will indicate the
anticipated start and completion time periods of site grading and/or construction sequence, including the
installation and removal of erosion and sediment control measures, and the estimated duration of
exposure of each area prior to the completion of temporary erosion and sediment control measures.
Permanent Stabilization
The lot owner(s) will be responsible for providing the County with the information required for post -
construction stabilization of areas disturbed by construction activity on Lot 1. This will be addressed at
time of building permit.
Erosion Control Measures
The lot owner(s) will be responsible for providing the County with an erosion control plan that includes
plan view drawings of all erosion and sediment control measures showing approximate locations and site
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drainage patterns for construction phases and final design elements, in accordance with Section 4-
203.E.12. This will be addressed at time of building permit.
Estimated Costs
The lot owner(s) will be responsible for ensuring that estimated total cost, including installation and
maintenance, of required temporary soil erosion and sediment control measures on the proposed lot is
submitted to the County at time of building permit.
Calculations
As previously noted in this application, stormwater runoff quantities have been calculated for existing
conditions and for a development scenario on Lot 1. The Rational Method was used to make these
calculations. Plans, documents and specifications for residential development on Lot 1 are to be prepared
by the lot owner(s) at the time of building permit. Therefore, it is not possible to run stormwater
calculations for the proposed developed conditions on Lot 1 at this time.
A single distinct drainage basin exists on Lot 1. The basin generally drains from the north/northwest to
the southeast. Refer to the Drainage Map included in this application The basin has the following
characteristics under existing conditions and under the development scenario conditions:
Time of Concentration
The Velocity Method, commonly used by the National Resource Conservation Service (NRCS), was used to
calculate Time of Concentration (Tc) for the drainage basin on Lot 1. Refer to the calculations included as
Appendix B. The -recalculated for the basin is 29.036 minutes.
Changes in Peak Discharge
Satellite imagery was used to approximate the acreages for existing vegetation and the graveled access
road on the lot. Refer to Appendix B. Satellite imagery was also used to approximate square footages for
single dwelling units and landscape areas on properties neighboring Lot 1. Average square footages were
calculated from this analysis and used to approximate the area of Lot 1 that might be altered by future
residential development and landscaping. The average square footages calculated are provided in Table 3
below.
Table 3. Average Dwelling Unit and Landscape Coverage on Neighboring Properties
Type of Site Coverage on Neighboring Properties
Average Area
(square feet)
Average Area
(acres)
Dwelling Units
3,500
0.08
Landscape Areas
5,000
0.11
The area for a graveled driveway on Lot 1 was approximated by: (1) identifying a logical building site in
the northwest quadrant of the proposed lot; and, (2) calculating the square footage of a 12 -foot wide
driveway providing access across Lot 1 to the building site. Refer to Appendix B for more information. The
square footage calculated for the graveled driveway was approximately 8,500 square -feet (.20 -acres). In
addition, it was approximated that there would be 1,500 square -feet (0.03 -acres) of graveled parking area
associated with a single dwelling unit on the lot.
The approximate areas for residential buildings, landscaping, a graveled driveway and graveled parking
area were used to estimate the impact of these improvements on Lot l's historic drainage. The results of
this are included on the following page.
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Table 4. Existing Conditions
Type of Site Coverage
Area
(square feet)
Area
(acres)
Runoff Coefficient1
Weighted Runoff
Coefficient
Gambel Oak Mixed Montane Shrublandl
157,687.00
3.62
0.5
0.18100
Mountain Big Sagebrush Dominated
Shrublandl
271 814.00
6.24
0.42
0.26208
Existing Graveled Access Roadl
6,098.00
0.14
0.8
0.01120
Graveled Driveway/Parking Area
0.45428
1Runoff coefficients were obtained from the NRCS's Hydrology Training Series - Peak Discharge (Other Methods) Study Guide
included in Appendix B.
Table 5. Development Scenario Conditions
Type of Site Coverage
Area
(square feet)
Area
(acres)
Runoff Coefficient)
Weighted Runoff
Coefficient
Gambel Oak Mixed Montane Shrubland
157,687.00
3.62
0.5
0.18100
Mountain Big Sagebrush Dominated
Shrubland
253,314.00
5.83
0.42
0.24501
Existing Graveled Access Road
6,098.00
0.14
0.8
0.01120
Graveled Driveway/Parking Area
9,200.00
0.21
0.8
0.01690
Single Dwelling Unit
3,500.00
0.08
0.5
0.00402
Landscape Area
5,000.00
0.11
0.25
0.00287
Conditions
0.46100
1Runoff coefficients were obtained from the NRCS's Hydrology Training Series - Peak Discharge (Other Methods) Study Guide
included in Appendix B.
Table 6. Rainfall Intensity at T,1
2 -YR Runoff
(inches/hour)
5 -YR Runoff
(inches/hour)
10 -YR Runoff
(inches/hour)
25 -YR Runoff
(inches/hour)
50 -YR Runoff
(inches/hour)
100 -YR Runoff
(inches/hour)
0.804
1.18
1.47
1.85
2.12
2.38
1Point precipitation frequency estimates for the lot were obtained from NOAA's National Weather Service - Hydrometeorological
Design Studies Center. These are included in Appendix B.
Table 7. Change in Drainage Basin Runoff
)cfs = cubic feet per second
Overall, the calculated value for peak runoff from the approximated development areas on Lot 1 show an
increase of 1.48% (ranging from an additional 0.054 cfs - 0.160 cfs depending on the storm event) a
nominal increase in measurable stormwater runoff. Therefore, it is anticipated that drainage on Lot 1 will
accommodate historic drainage patterns and on-site improvements in a manner that conform with
Garfield County's regulations.
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2 -YR
Runoff
(cfs')
5 -YR
Runoff
(cfs1)
10 -YR
Runoff
(cfs))
25 -YR
Runoff
(cfs')
50 -YR
Runoff
(cfs1)
100 -YR
Runoff
(cfs')
Existing
3.652
5.361
6.678
8.404
9.631
10.812
Conditions
Development
Scenario
3.706
5.440
6.777
8.528
9.773
10.972
Conditions
Difference
0.054
0.079
0.099
0.124
0.142
0.160
(1.48%)
(1.48%)
(1.48%)
(1.48%)
(1.48%)
(1.48%)
)cfs = cubic feet per second
Overall, the calculated value for peak runoff from the approximated development areas on Lot 1 show an
increase of 1.48% (ranging from an additional 0.054 cfs - 0.160 cfs depending on the storm event) a
nominal increase in measurable stormwater runoff. Therefore, it is anticipated that drainage on Lot 1 will
accommodate historic drainage patterns and on-site improvements in a manner that conform with
Garfield County's regulations.
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Vegetated swales are a drainage and water quality feature that the lot owner(s) could employ to maintain
historic drainage conditions of Lot 1. It is not anticipated that stormwater retention or detention facilities
will be necessary however, that determination will need to be made by a qualified licensed professional
engineer on behalf of the lot owner(s). The lot owner(s) will be responsible for ensuring that stormwater
calculations specific to the improvements on Lot 1 are prepared and submitted to the County at time of
building permit.
Neighboring Areas
Pursuant to Section 7-203.E.15, the lot owner(s) will be responsible for including a description of
neighboring areas with regard to land use and existing pertinent features such as lakes, streams,
structures, roads, etc. with the grading, drainage and erosion control plans and documents submitted to
the County at time of building permit.
Stormwater Management
The lot owner(s) will be responsible for the preparation and submittal of a description of the stormwater
planning concept for Lot 1, in accordance with Section 4-203.E.16, at the time of building permit.
Stormwater Management Plan
No stormwater management plan application is being submitted to the Colorado Department of Public
Health and Environment (CDPHE) in conjunction with this application.
Reclamation, Revegetation and Soil Plan
The applicant is not proposing any improvements, as part of this application, that necessitate the
preparation of a Reclamation, Revegetation and Soil Plan.
The lot owner(s) will be responsible for siting, designing and constructing improvements on Lot 1 and will
be responsible for providing the County with a Reclamation, Revegetation and Soil Plan that conforms
with the applicable requirements of Section 4-203.E.18 and Section 7-208. This will be addressed at time
of building permit.
Hydraulic Calculations
It is not possible to prepare hydraulic calculations at this time. The extent of development on the lot will
be determined by the lot owner(s) and will dictate the sizing and design of drainage facilities and/or
structural best management practices.
Maintenance Requirements
The lot owner(s) will be responsible for the maintenance of all best management practices used on Lot 1.
The lot owner(s) will be responsible for providing the County with the information required under Section
4-203.E.20 at the time of building permit.
Spill Prevention Control and Countermeasures Plan
Section 4-203.E.21 is not applicable to this application.
It is not anticipated that the lot owner(s) will construct improvements on Lot 1 that trigger the
requirements of Section 4-203.E.21. However, in the event this does occur, the lot owner(s) will be
responsible for providing the County with the required spill prevention control and countermeasures
plan, pursuant to Section 4-203.E.21.
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