HomeMy WebLinkAbout18 - Design Report - The Crossing 2 at Baxter Meadows Ph 4G & 4H - StormJuly, 2018
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The Crossing 2 at Baxter Meadows Subdivision – Phase 4G & 4H
Stormwater Report
Bozeman, MT
Introduction
This report provides a design basis and hydraulic calculations for sizing stormwater
infrastructure for The Crossing 2 at Baxter Meadows Subdivision – Phase 4G & 4H. The
project in total is 5.8 acres of residential development and is located between the
intersection of Vaquero Parkway and Baxter Lane to Vaquero Parkway and Davis Lane.
Phases 4A through 4F of The Crossing have already been constructed. Stormwater
calculations for the entire project (including phase 4G and 4H) were submitted in The
Engineering Design Report for The Crossing at Baxter Meadows (March 2013).
Generally the stormwater system and calculations have stayed consistent with what was
originally submitted, but this report provides updated and additional information including
inlet calculations, culvert capacity calculations and pond sizing. Much of this report will
reference the Engineering Design Report for The Crossing at Baxter Meadows (March
2013).
The City of Bozeman Design Standards and Specifications (March 2004) was used as
the primary guideline for this stormwater drainage design along with The 2002 Edition of
the Circular 8 of Montana Department of Environmental Quality (MDEQ).
General Design
Phase 4G and 4H will be a combination of roadway and residential lots. Vaquero
Parkway has been designed with curb and gutter as originally designed in 2013.
Stormwater runoff generated from the roadway and residential lots is designed to be
collected by the curb and gutter and then directed toward the storm drain inlets. The
runoff will then move through the storm sewer system pipes to the detention ponds. The
detention ponds have been designed with outlet structures to limit stormwater runoff
from the developed site to the pre-developed runoff rates.
Inlets & Storm Sewer
Using the Modified Rational Method, a contributing flow to each inlet in the system was
determined. The inlet grates were designed to accommodate the 25-year storm event
without overtopping a depth 0.15’ below the top of curb or spreading greater than 9 feet.
Inlets at sag points were modeled using 50% clogging. Inlets and manholes will have a
9” sump for sediment collection.
To avoid crossing Spring Ditch with the storm sewer, the two northern more inlets (SDI-
100 and SDI-101) were moved south from their original 2013 design location. This
change created a larger drainage area that flows north to Existing Pond 7. The additional
flowrate and volume was modeled to verify that Pond 7 could handle the added area
without exceeding the pre-development flow rate.
Storm pipes were sized to accommodate the 100-year storm event and were modeled in
Storm and Sanitary Analysis. Pipes are sloped to maintain a minimum velocity of 3 ft/s
when flowing full to prevent sediment deposit. The Hydraulic Grade line was plotted
throughout the system and pipes were sized to keep the HGL within the pipe during the
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25 yr storm and 100 yr storm. A manning’s N of 0.009 was used for the A-2000 Storm
pipe within the model per the manufactures specifications.
Stormwater Treatment
The City of Bozeman requires infiltration, evapotranspiration or capture for reuse of the
runoff generated from the first 0.5 inches of rainfall from a 24 hour storm. A common
methodology for estimating the volume of direct runoff from a drainage basin is the SCS
method. This method requires basic data similar to the Rational Method: drainage area,
a runoff factor, time of concentration, and rainfall. However, the SCS approach is more
sophisticated in that it also considers the time distribution of the rainfall and an infiltration
rate that decreases during the course of a storm. The SCS method takes into account
the Initial Abstraction (Ia) which is all losses before runoff begins. This includes surface
depressions, water intercepted by vegetation, evapotranspiration and infiltration. Initial
abstraction generally correlates with soil and cover parameters. Through studies of
many small watersheds, Ia was found to be approximated by the following empirical
equation:
Ia = 0.2 x S [eq. 2-2 TR-55]
Where S = (1000/CN) – 10 [eq. 2-4 TR-55]
According to USDA’s Urban Hydrology for Small Watersheds, the Curve Number
associated with soil type B and ¼ acre residential lots is 80 (Table 2-2a). Using the
standard SCS method,
S = (1000/80) – 10 = 2.5 inches
Ia = 0.2 * 2.5 inches = 0.5 inches
Using a 10-year 24-hour rain event in the City of Bozeman the total precipitation (P) is
1.88 inches of which 0.5 inches is lost due to initial abstraction according to USDA’s
SCS method. Therefore this drainage plan includes provision for the first 0.5 inches of
rainfall from the 24-hour storm to be retained in surface depressions, intercepted by
vegetation, or evapotranspired or infiltrated
Stormwater Detention Ponds
Pond 5 and Pond 1A were designed in the original 2013 design report. Pond 1A will
remain the same as originally designed but updated calculations are provided for clarity
and it has been renamed Pond 1B. Pond 5 is similar to the original design but it has
been oversized to account for future development along Vaquero Parkway. This Pond
has been renamed Pond 5B and new calculations have been provided. The ponds are
designed with outlet structures to limit stormwater runoff from the developed site to the
pre-developed runoff rates for the 10-year storm and to not overtop during the 100-year
storm.
With this phase, additional drainage area will be added to the Existing Pond 7 basin.
Pond 7 was originally slightly oversized allowing for this extra volume to be attenuated
without exceeding the predeveloped rate. Revised pond calculations modeling the
existing pond volume and existing outlet structure can be found in the appendices
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Groundwater
Groundwater is known to be high in this area. A geotechnical report was done in 2001
that included borings near both pond location. ST-29 near Pond 5B showed depth to
groundwater at 3 feet below existing ground and ST-32 near Pond 1B showed depth to
groundwater at 9 feet below existing ground. Due to the high groundwater the maximum
pond excavation depth for Pond 5B is 2 feet.
Culverts
Both culverts under Vaquero Parkway were designed in the 2013 Design Report and
modeled in the flood plain study conducted in 2007. Inverts and length changed slightly
from the original design so new calculations are provided. Both culverts were modeled in
HY-8 for the 25-year storm, 100-year storm, and emergency overflow events.
According to the City of Bozeman standards the emergency overflow capacity shall be
100-percent of the whole culvert for the major storm. Both culverts meet this requirement
by providing the entire emergency overflow capacity within the culvert and without
overtopping the roadway. The headwater depth for the 25-year storm is provided in the
culverts calculations and does not exceed 1.5 times the culvert depth.
Maintenance
Regular maintenance of stormwater facilities is necessary for proper function of the
drainage system. In general, regular mowing of detention pond areas and removal of
debris from the outlet structure will be required to maintain functionality of the system.
Sediment removal from the ponds may be required over time to restore the detention
pond volume. Additional maintenance items include removing debris from inlet grates
and culverts, cleaning and flushing pipes, cleaning manhole sumps, and establishing
ground cover after construction. All of the detention ponds will be maintained by the
HOA (as is with the other existing ponds) after the phases have been incorporated into
the HOA.
Appendices
L. Stormwater Drainage Basins
M. Sub-Basin Runoff Calculations & Inlet Capacities
N. Pipe Profiles with HGL
O. Detention Pond Sizing and Outlet Structure Calculations
P. Culvert Sizing