HomeMy WebLinkAboutDRAINAGE REPORT 6.27.22 - 22-00303 - Pebble Creek - Parcel adjacent to 2332 W 2000 S - PlatConcordia Development Drainage Report
Pebble Creek Townhomes Project No. 01-21-0042
2022
bcrowther@civilize.design
3853 W. Mountain View Drive
Rexburg, ID 83440
208-351-2824
6/22/2022
Pebble Creek
Stormwater Drainage Report
Concordia Development Drainage Report
Pebble Creek Townhomes Project No. 01-21-0042
Table of Contents
1.0 Introduction ............................................................................................................................................. 0
1.1 Project Identification ........................................................................................................................... 0
1.2 Property Description ........................................................................................................................... 0
Property Location .................................................................................................................................. 0
Zoning ................................................................................................................................................... 1
Size ........................................................................................................................................................ 1
Development ......................................................................................................................................... 1
2.0 Requirements .......................................................................................................................................... 1
2.1 Stormwater Management in Idaho ...................................................................................................... 2
Federal Government .............................................................................................................................. 2
State Government .................................................................................................................................. 2
Local Government ................................................................................................................................ 3
2.2 Storm Water Quantity ......................................................................................................................... 3
Snowmelt .............................................................................................................................................. 3
3.0 Stormwater Runoff .................................................................................................................................. 4
3.1 Method ................................................................................................................................................ 4
Rational Method .................................................................................................................................... 4
Modified Rational Method .................................................................................................................... 5
3.2 Development of Parameters ................................................................................................................ 5
Rainfall Intensity ................................................................................................................................... 7
The Time of Concentration ................................................................................................................... 8
3.3 Storm Volume ..................................................................................................................................... 8
25-Year, 1-Hour Storm ......................................................................................................................... 8
100-Year, 1-Hour Storm ....................................................................................................................... 8
3.4 Procedure ............................................................................................................................................ 8
Applicability of Method ........................................................................................................................ 9
3.5 Resultant Runoff from Existing Developed Condition ....................................................................... 9
3.6 Resultant Runoff from Proposed Developed Condition ..................................................................... 9
5.0 Stormwater Conveyance ....................................................................................................................... 10
6.0 Stormwater Retention ........................................................................................................................... 10
6.1 Water Quality .................................................................................................................................... 10
6.2 Water Quantity .................................................................................................................................. 10
6.3 Retention Provided ............................................................................................................................ 10
Brent E Crowther
6/24/22
Concordia Development Drainage Report
Pebble Creek Townhomes Project No. 01-21-0042
Pebble Creek Townhomes
Stormwater Drainage Report
1.0 Introduction
The following document is a drainage report prepared for the Pebble Creek project located on 2000 South
in Rexburg, Idaho. The Drainage Report serves to locate the property, identify the design storm and
associated precipitation, determine the volume of water generated by the storm on the subject property,
calculate the pre-development and post-development run-off amount, and size conveyance facilities for
managing the storm.
1.1 Project Identification
The following table lists important project identification information and contact information for the
project.
Table 1 - Project Information
Project Name Mack’s Inn – Springhill Suites
Owner Concordia Development
Owner Contact Person Brandt Monette, President
Owner Address 940 South 5th West, Apt 12308
Rexburg, ID 83440
Owner Telephone Number 775-830-6526
Owner Email brandt@concordiadev.com
Engineer Civilize, PLLC
Engineer Contact Person Brent E. “Husk” Crowther, P.E.
Engineer Address 3853 W. Mountain View Dr.
Rexburg, ID 83440
Engineer Project Number 01-17-0027
Engineer Telephone Number 208-351-2824
Engineer Email bcrowther@civilize.design
1.2 Property Description
Property Location
Concordia Development has purchased portions of a parcel of property on the northwest corner of 2000
South and 12th West within the city limits of the City of Rexburg, Idaho. With respect to the public land
survey system, the property is located in SW 1/4, SECTION 35, TWP. 6 N, RANGE 39 E, B.M. The
parcel is located on the north side of 2000 South approximately 800 feet from the intersection of W 2000
South and 12th West. It is situated immediately east and across Smith Farm Road from an existing LDS
church.
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Zoning
The property falls within the Medium Density Residential 2 (MDR2) zone, although an application has
been submitted to change the zone to Low Density Residential 3 (LDR3).
Size
According to the preliminary plat, the parcel is 20.66 acres.
Development
The proposed project consists of 142 townhomes planned for construction in three phases.
2.0 Requirements
Stormwater is a potential pollutant of the Waters of the United States of America and as such is regulated
by Federal Code. Quoting from the Idaho Department of Environmental Quality (DEQ) website:
Stormwater is rain or melting snow that does not immediately soak into the ground. Stormwater
runs off of land and hard surfaces such as streets, parking lots, and rooftops, and picks up
pollutants, such as fertilizers, dirt, pesticides, and oil and grease. Eventually, stormwater soaks
into the ground or discharges to surface water (usually through storm drains), bringing the
pollutants with it.
PROPOSED
PROJECT
Figure 1: Vicinity Map (Madison County GIS, 2022)
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Construction activities that disturb 1 acre or more of land, including clearing, grading, and
excavation activities; industrial activities specifically listed by the US Environmental Protection
Agency (EPA); and municipal separate storm sewer systems (MS4), which are a city's or town's
storm drains are considered point sources of pollution (i.e., a source of pollution that comes from
a discrete pipe or other point) and require coverage by a National Pollutant Discharge Elimination
System (NPDES) stormwater permit.
2.1 Stormwater Management in Idaho
Federal, state, and local government agencies; business and industry; and individual land owners all share
responsibility for stormwater management in Idaho. Three separate state agencies have jurisdiction over
various aspects of stormwater management within the boundaries of the state.
Federal Government
EPA, Region 10, is the NPDES permitting authority for Idaho and is responsible for issuing NPDES
stormwater permits.
State Government
Idaho Department of Environmental Quality:
From the DEQ website:
DEQ provides technical assistance and support for controlling stormwater in Idaho. DEQ's
Catalog of Stormwater Best Management Practices for Cities and Counties includes site-design
techniques for controlling stormwater runoff associated with land development activities. DEQ
also provides plan and specification review for facilities that control, treat, or dispose of
stormwater if requested by the developer or design engineer.
Idaho Transportation Department:
From the DEQ website:
The Idaho Transportation Department maintains the storm drain system that lies within the state
highway right-of-way and incorporates erosion and sediment controls into its construction projects
to keep sediment out of stormwater. The Idaho Transportation Department also periodically
conducts erosion and sedimentation control workshops.
Idaho Department of Water Resources:
From the DEQ website:
The Idaho Department of Water Resources administers the Idaho waste disposal and injection well
program and the stream channel alteration program. Injection wells can be used for stormwater
runoff disposal; stream channel alteration permits are required when construction activities impact
a stream below the mean high water mark. This includes constructing a stormwater outfall along
a river, stream, or lake.
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Local Government
The applicable sections of Idaho Code Title 67, Chapter 65 – Local Land Use Planning, as set forth by
the Idaho legislature, require compliance with the City of Rexurg Comprehensive Plan, dated March 25,
2008 and the City of Rexburg Municipal Code, including Title 16-Zoning. Design criteria and regulations
for stormwater are outlined in Resolution 2016-15 Engineering Standards and Resolution 2022-07
Engineering Standards Amendment to Provide for Fiber Optic Infrastructure, which further describe that
all development within their jurisdiction is subject to the provisions of those documents. Specifically, the
narrative in that section states:
Storm Runoff
a. Storm drainage rainfall values and run off coefficients shall be as established in accordance
with State of Idaho Catalog of Storm Water Best Management Practices.
b. The peak flow rate and maximum water surface elevations must be calculated for the 100-
year/1-hour storm event.
c. The overflow route shall direct the 100-year/1-hour post-development flow safely towards
the downstream conveyance system. Facilities that do not have an adequate overflow location or
bypass path must be sized to fully infiltrate/drain the 100-year/1-hour event.
d. The City of Rexburg uses the 25-year/1-hour event for sizing of on-site runoff storage facility
if it can be shown that downstream facilities can safely accommodate flows in excess of the 25-
year/1-hour event.
e. Discharge into existing facilities must be restricted to the pre-development level unless
otherwise approved by City Engineer.
f. Catch Basins must be designed to accept peak runoff flow rate.
2.2 Storm Water Quantity
The City of Rexburg Engineering Standards address the requirements for water quantity by providing the
design storm parameters as well as the criteria for conveyance structures:
• Peak Flow Rate ................................................................................. 100-Year, 1-Hour Storm Event
• Overflow Route ................................................................................. 100-Year, 1-Hour Storm Event
• Retention Facility ................................................................................ 25-Year, 1-Hour Storm Event
The method for calculating those values is not specified. Methods appropriate for the drainage area size
and type of drainage will be utilized to calculate the volume and rate of stormwater flows. Various
references suggest the Rational Method, and consequently the Modified Rational Method, is applicable
for basins with a maximum area of 5 acres to 200 acres. The proposed project is 20 acres with numerous
isolated subbasins of less than 5 acres, each with total retention of stormwater. The City does not have
storm drain facilities in this sector of the city and development must employ 100% retention of
stormwater with infiltration and evaporation as the only means for eliminating the accumulated
stormwater. Fortunately, the soils in the area exhibit good drainage characteristics and infiltration into the
groundwater is a common method for disposing of stormwater. Therefore, the Modified Rational
Method, or a simplified application of that method, is identified as an appropriate approach for use in
determining peak flow rates and storm water volumes from the design storms.
Snowmelt
Snowmelt is not specifically addressed in the City of Rexburg Municipal Code or the City of Rexburg
Engineering Standards other than providing a percentage of the development for landscaping and a
portion of the parking lot for snow storage.
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3.0 Stormwater Runoff
3.1 Method
Not all the water that descends as precipitation results in runoff. The ground slope, cover, vegetation, soil
type, rainfall intensity and other abstractions affect the percentage of precipitation that does not percolate
into the soil. Various hydrologic methods are used to determine runoff, each accounts for those
abstractions in various ways. The method selected for this analysis is the Modified Rational Method.
Rational Method
The rational method was developed about 100 years ago for the purpose of predicting peak flow rates for
small, urban watersheds. It does not provide any information pertaining to the runoff hydrograph shape.
The rational method is a valid hydrologic design tool for predicting peak flow rates from urban
watersheds up to 50 acres. The rational method of predicting a design peak runoff rate is expressed by the
equation:
𝑃=𝐶�ℎ𝐴
Where Q = the design peak runoff rate in cfs,
C = runoff coefficient, dimensionless - defined as the ratio of the peak runoff rate to the
rainfall intensity,
i = rainfall intensity in in/hr, for the design recurrence interval and for duration equal to
the time of concentration (To) of the watershed, and
A = watershed area in acres
The equation, Q = CiA, may not appear to be dimensionally correct. Although "i" is specified in inches
per hour, one inch per hour is 1.008 cfs per acre, and, in using the equation, the two are taken to be
numerically equal.
The Rational Method generates a triangular shaped hydrograph where the rainfall intensity is assumed
constant through the duration of a storm assumed to be equivalent to the length of time it takes runoff to
flow from the most distant part of the
catchment to the outlet of the catchment. If
more than one catchment is defined for the
parcel of interest, the magnitude of the
triangular hydrograph increases as the design
storm duration lengthens as a result of
summing the various successive times of
concentration of each catchment until all are
contributing to the runoff. With the traditional
Rational Method, the peak flow of each
drainage area (catchment) is based on its
individual time of concentration and by extension, each catchment experiences a different storm, which
isn’t typically logical in a small area. It is only applicable to a peak flow/steady state condition
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Modified Rational Method
The Modified Rational Method is a somewhat recent adaptation of the Rational Method that can be used
to not only compute peak runoff rates, but also to estimate runoff volumes and hydrographs. This method
uses the same input data and coefficients as the Rational Method along with the further assumption that,
for the selected storm frequency, the duration of peak-producing rainfall is also the entire storm duration.
With the Modified Rational method, there is a single "storm duration" (and thus intensity) applied to all
drainage areas. Therefore, all routed hydrographs are based on the same storm. Since the global storm
duration needs to be greater than the highest
catchment Tc, it's not possible for all
catchments to be at their peak flow.
Since, theoretically, there are an infinite number
of rainfall intensities and associated durations
with the same frequency or probability, the
Modified Rational Method requires that several
of these events be analyzed in the method to
determine the most severe.
3.2 Development of Parameters
Runoff Coefficient “C”
The runoff coefficient “C” predicts the fraction of water that falls on a particular surface and results in
runoff based on soil, topography, vegetation, and use. Many attempts have been made to refine these
values. However, variations can still remain quite large (see Table 1).
When using the Rational Method for
small development projects, it is
typical to develop a composite runoff
coefficient that utilizes a weighted
average derived from the proportional
area for various types of surfaces
encountered in the development
project. In many cases, particularly
for individual site development
projects, the composite value is
derived from three main surface types;
rooftop, pavement including
sidewalks, and landscaping. The slope
also has an impact on the runoff
coefficient.
For the drainage management area in
question, the property is currently
undeveloped, thus, the runoff
coefficient is based on developed
property. By comparison, the
proposed project is classified as
developed and adds impervious
surface to the existing undeveloped
property. By summing the areas of
each type of surface for both the existing and the proposed development, and then computing a weighted
average for the runoff coefficient, we arrive at a composite value for the runoff coefficient.
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Pre-Development
The table below presents the calculation for the composite drainage area for the existing development
condition.
Post Development
The table below presents the calculation for the composite drainage area for the post-development
condition.
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Rainfall Intensity
Values for the rainfall intensity can be obtained from rainfall intensity curves in publication such as
NOAA Atlas II, Precipitation-Frequency Atlas of the Western United States, Volume V-Idaho. Figure 30
of that volume shows the isopluvials for the 100-Year, 24-Hour storm in tenths of an inch. The Atlas
provides similar figures for various return periods (2, 5, 10, 25, 50, and 100 years) for the 6-hour and 24-
hour storms.
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The Atlas provides interpolation procedures for other return periods and storm durations using regression
type equations. For the case of Rexburg, we need the precipitation for the 25-Year, 1-Hour storm and the
100-Year, 1-Hour storm events. Using the procedures in the Atlas, the estimated precipitation for those
two storms is:
• 25-Year, 1-Hour ............................................................................................................... 0.85 inches
• 100-Year, 1-Hour`............................................................................................................ 1.05 inches
The Time of Concentration
The time of concentration of a watershed is the time required for a particle of water to flow from the
hydraulically most distant point on a watershed to the outlet. It is assumed that, when the duration of a
storm equals the time of concentration, all parts of the watershed are contributing simultaneously to the
peak discharge at the outlet.
3.3 Storm Volume
The volume of water generated from the 100-Year, 1-Hour storm is determined by multiplying the
precipitation by the surface area of the subject property, which in this case is 20.66 acres.
25-Year, 1-Hour Storm
The City of Rexburg regulations stipulate using the 25-Year, 1-Hour storm event for calculation of
retention volume, which as a precipitation depth of 0.85 inches.
The volume of water generated by the 25-Year, 1-Hour storm event is 25,878 cubic feet:
𝑉=𝑃× 𝐴=(0.85 �ℎ𝑛𝑐�𝑒𝑞× 1 𝑒𝑛𝑛𝑞
12 �ℎ𝑛𝑐�𝑒𝑞)× (20.66 𝑎𝑐𝑞𝑒𝑞× 43,560 𝑞𝑞𝑞𝑎𝑞𝑒 𝑒𝑒𝑒𝑞
1 𝑎𝑐𝑞𝑒)=63,749 𝑐𝑒
100-Year, 1-Hour Storm
A more conservative approach is using the 100-Year, 24-Hour storm event for calculation of retention
volume, which as a precipitation depth of 2.8 inches.
The volume of water generated by the 25-Year, 1-Hour storm event is 25,878 cubic feet:
𝑉=𝑃× 𝐴=(2.8 �ℎ𝑛𝑐�𝑒𝑞× 1 𝑒𝑛𝑛𝑞
12 �ℎ𝑛𝑐�𝑒𝑞)× (20.66 𝑎𝑐𝑞𝑒𝑞× 43,560 𝑞𝑞𝑞𝑎𝑞𝑒 𝑒𝑒𝑒𝑞
1 𝑎𝑐𝑞𝑒)=209,998 𝑐𝑒
3.4 Procedure
In this case, there is limited routing of the storm flow in a pipe network and there is not a municipal storm
drain system to accept any runoff. The existing developed condition allows all stormwater and snowmelt
to drain naturally with a combination of infiltration and overland flow to the waterways. The regulations
for the City of Rexburg require retention of the additional runoff generated by the proposed development.
Therefore, all of the runoff must be managed on site and the difference in runoff volume between the
existing developed condition and the proposed developed condition will be retained.
Typically, when implementing the Rational Method or the Modified Rational Method, the time of
concentration is estimated and the rainfall intensity corresponding to the time of concentration is picked
from a intensity-duration-frequency curve, and a peak flow determined from the equation that is
subsequently used for routing the flow. In this case, the subcatchments are relatively small, there are
retention facilities provided for each subcatchment, and there is no storm routing through a pipe network
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connecting the subcatchments. Thus, the time of concentration is an extraneous calculation. Rather, the
total volume required for retention is more germane.
Applicability of Method
The subject stormwater management is in an urban area and is less than 20 acres, the time of
concentration for the drainage area is less than the duration of peak rainfall intensity, and the precipitation
is uniform across the site.
3.5 Resultant Runoff from Existing Developed Condition
The existing developed condition of the property includes impermeable surfaces including roofs,
pavement, and gravel parking areas along with some natural landscaped areas. The slope across the
ranges from 1% to 10% from south to north. To determine the runoff from the existing developed
condition, we multiply the storm volume by a composite runoff coefficient. By applying the composite
runoff coefficient to the volume of the calculated 100-Year, 1-Hour storm, we compute a runoff volume
estimate for the existing developed condition of 46,620 cubic feet as shown in the following table.
3.6 Resultant Runoff from Proposed Developed Condition
The post-development condition of the property would be considered residential and the slope across the
site remains similar to the existing developed condition. By applying the composite runoff coefficient to
the volume of the calculated 100-Year, 1-Hour storm, we compute a runoff volume estimate for the
proposed developed condition of 131,795 cubic feet as shown in the following table.
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5.0 Stormwater Conveyance
The proposed site does not have any conveyance structures other than overland flow across the property
from south to north. Therefore, there are no calculations associated with conveyance structures using the
25-Year, 1-Hour storm event.
6.0 Stormwater Retention
The retention facilities for the site include retention ponds, drainage swales, and French drains, more
properly labeled shallow injection wells in Idaho State Statute. Each drainage subbasin features some
combination of the three methods of retention.
6.1 Water Quality
The infiltration galleries will provide filtration of the stormwater as it works through the gravel fill in the
trenches and through the natural soil strata.
6.2 Water Quantity
Using the 100-Year, 24-Hour storm event, the difference between the existing developed condition runoff
and the proposed development condition runoff, taking into account the initial abstractions inherent with
the specific site surface characteristics such as slope, plant growth, roughness, etc, is 85,175 cubic feet.
In this case, the Owner prefers to provide additional retention beyond that required (63,749 cf) in an effort
to better manage the runoff from the proposed development
6.3 Retention Provided
By subtracting the infiltration provided via shallow injection wells with an assumed infiltration rate of 0.6
in-hour appropriate for a loamy-sandy-gravel over the 24-hour storm event, and summing the retention
provided through a combination of drainage swales and retention ponds for the 26 subbasins identified for
the project, we arrive at an aggregate retention volume provided of 96,860 cubic feet, which exceed the
required amount.
The appendix contains a more granular evaluation of each retention basin documenting the precipitation,
runoff volume, infiltration through shallow injection wells if applicable, retention volume in the swales
and retention basins as appropriate, and the balance for each subbasin. The swale system, designed as a
dry creek bed, is interconnected for many of the subbasins allowing water to flow between the retention
facilities and thus better balancing the system.
.
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Exhibit A
Calculation of Runoff
Rational Method
Project Number:01-21-0042 Date: 6/23/2022
Project Name:Pebble Creek By: BEC
Client:Concordia Development
Method and Coefficients
KEY: Yellow highlighted areas show required inputs
Blue highlighted areas show outputs calculated by the spreadsheet
Green highlighted areas show fixed constants—DO NOT CHANGE
Table of Runoff Coefficients for Site Conditions RATIONAL METHOD DESCRIPTION
Surface Type Runoff Coefficient
Pavement, Asphalt 0.95
Pavement, Concrete 0.95Pavement, Brick 0.85
Pavement, Gravel 0.75Roofs, Conventional 0.95Roof, Garden Roof (< 4 in) 0.5
Roof, Garden Roof (4 - 8 in) 0.3
Roof, Garden Roof (9 - 20 in)0.2Roof, Garden Roof (> 20 in) 0.1Turf, Flat (0 - 1% slope) 0.25
Turf, Average (1 - 3% slope) 0.35Turf, Hilly (3 - 10% slope) 0.4Turf, Steep (> 10% slope) 0.45
Vegetation, Flat (0 - 1% slope) 0.1
Vegetation, Average (1 - 3% slope) 0.2Vegetation, Hilly (3 - 10% slope) 0.25Vegetation, Steep (> 10% slope) 0.3
None 0
Instructions:
Site Surface Base Data - Calculate Composite Runoff Coefficient
EXISTING SURFACE BASE DATA PROPOSED SURFACE BASE DATA
Runoff Coefficient (from table above)Area (s.f.)Impervious Area (square feet)Runoff Coefficient (from table above)Area (s.f.)Impervious Area (square feet)
0.95 26,404 25,084 0.95 190,429 180,908
0.95 0 0 0.95 75,825 72,034
0.75 0 0 0.75 0 0
0.95 0 0 0.95 219,230 208,269
0.25 0 0 0.25 414,508 103,627
0.20 873,588 174,718 0.20 0 0
899,992 899,992
20.66 20.66
199,801 564,837
0.22 0.63
Calculate Total Volume of Runoff (using assumptions of the rational method)
Storm Water Depth (from NOAA Atlas) Storm Water Depth (from NOAA Atlas)
Location: Island Park, Idaho Location: Island Park, Idaho
Design Storm Depth (inches)Design Storm Depth (inches)
95th Percentile 0.68 95th Percentile 0.68
10 Year, 24-Hour 1.8 10 Year, 24-Hour 1.8
2.45 25-Year, 1-Hour 0.85 25-Year, 1-Hour 0.85
100-Year, 1-Hour 1.05 100-Year, 1-Hour 1.05
100 Year, 24-Hour 2.8 100 Year, 24-Hour 2.8
EXISTING SURFACE BASE DATA PROPOSED SURFACE BASE DATA
(assuming runoff coefficent applies to total storm precipitation) (assuming runoff coefficent applies to total storm precipitation)
V - CiA V - CiA
Runoff Storm Depth Site Area Volume Runoff Storm Depth Site Area Volume
Design Storm Coefficient (inches)(square feet)(cubic feet)Design Storm Coefficient (inches)(square feet)(cubic feet)
95th Percentile 0.22 0.68 899,992 11,322 95th Percentile 0.63 0.68 899,992 32,007
10 Year, 24-Hour 0.22 1.80 899,992 29,970 10 Year, 24-Hour 0.63 1.80 899,992 84,726
25-Year, 1-Hour 0.22 0.85 899,992 14,153 25-Year, 1-Hour 0.63 0.85 899,992 40,009100-Year, 1-Hour 0.22 1.05 899,992 17,483 100-Year, 1-Hour 0.63 1.05 899,992 49,423100 Year, 24-Hour 0.22 2.80 899,992 46,620 100 Year, 24-Hour 0.63 2.80 899,992 131,795
Calculate Difference Between Pre-Development and Post Development Runoff Design Storm Runoff Volume
95th Percentile 20,68510 Year, 24-Hour 54,75525-Year, 1-Hour 25,857
100-Year, 1-Hour 31,941
100 Year, 24-Hour 85,175
PROJECT ANALYSIS WORKSHEET
STORM DRAINAGE
RETENTION/DETENTION VOLUME
STORAGE VOLUME REQUIRED BASED ON RATIONAL METHOD
COMPOSITE RUNOFF COEFFICIENT
TOTAL IMPERVIOUS AREA
Pavement, Gravel
Roofs, Conventional
Turf, Flat (0 - 1% slope)
Vegetation, Average (1 - 3% slope)
Surface Type
Pavement, Asphalt
Pavement, Concrete
TOTAL IMPERVIOUS AREA
Pavement, Gravel
Roofs, Conventional
Turf, Flat (0 - 1% slope)
Vegetation, Average (1 - 3% slope)
TOTAL AREA (square feet)
TOTAL AREA (acres)
1) Calculate projected runoff for the baseline case design for your project using information from the table above (Runoff Coefficients), or, if not in the table, then from the manufacturer's specified material, product, or system. 2) Then, using the information from the table above, information from manufacturer of alternative materials, products or systems, calculate projected runoff for your design case using the equations noted (1 & 2). Indicated
improvements in runoff to meet requirements of guidelines.
3) SCS (NRCS) Methods may be used for calculating composite CN for subject site
. .
Surface Type
Pavement, Asphalt
Pavement, Concrete
TOTAL AREA (square feet)
TOTAL AREA (acres)
COMPOSITE RUNOFF COEFFICIENT
Phases:
AP Agency PlanningPP Predesign-ProgrammingPS Predesign-Site SelectionSD Schematic DesignDD Design DevelopmentCD Construction DocumentsCA Construction AdministrationCN ConstructionCP Correction PeriodOO Ongoing OccupancyNU Next Use
Not all the water that descends as precipitation results in runoff. The ground slope, cover, vegetation, soil type, rainfall intensity and other abstractions affect the percentage of precipitation that does not percolate into the soil. Various hydrologic methods are used to determine runoff, each accounts for those abstractions in various ways. The method selected for this analysis is the Rational Method.
The rational method was developed about 100 years ago for the purpose of predicting peak flow rates for small, urban watersheds. It does not provide any information pertaining to the runoff hydrograph shape. The rational
method is a valid hydrologic design tool for predicting peak flow rates from urban watersheds up to 50 acres. The rational method of predicting a design peak runoff rate is expressed by the equation:
𝑄=𝐶𝑖𝐴Where Q= the design peak runoff rate in cfs,C= runoff coefficient, dimensionless - defined as the ratio of the peak runoff rate to the rainfall intensity,i = rainfall intensity in in/hr, for the design recurrence interval and for duration equal to the time of concentration (To) of the watershed, andA = watershed area in acres
The equation, Q = CiA, may not appear to be dimensionally correct. Although "i" is specified in inches per hour, one inch per hour is 1.008 cfs per acre, and, in using the equation, the two are taken to be numerically equal.
PROJECT ANALYSIS WORKSHEET
STORM DRAINAGE
RETENTION/DETENTION VOLUME
STORAGE VOLUME REQUIRED BASED ON RATIONAL METHOD
Project Number:01-21-0042 Date: 6/22/2018
Project Name:Pebble Creek By: BEC
Client:Concordia Development
VARIABLES
Total Land Area (including roads)20.66 acres
Predevelopment Composite Runoff Coefficient 0.22
Postdevelopment Composite Runoff Coefficient 0.63
100-Year, 24-Hour Storm Precipitation 2.8 inches
Duration of Design Storm 24 hours
Surface Area of 4' Dia. Shallow Injection Well 100.48 sq ft.bottom + 8 foot barrel
Infiltration Rate 0.6 sandy, loamy, gravel
RETENTION
NO.
DRAINAGE
SUBBASIN ROOF ASPHALT CONCRETE LANDSCAPE TOTAL
PRECIPITATION
DEPTH PREDEVELOP
POST-
DEVELOP
VOLUME FOR
DETENTION/
RETENTION
INFILTRATION
AREA INFILTRATION
VOLUME
REQUIRED
VOLUME
SWALE
VOLUME
POND
TOTAL
VOLUME BALANCE
(ft^2)(ft^2)(ft^2)(ft^2)(ft^2)(cubic feet)(cubic feet)(cubic feet)(cubic feet)(square feet)(cubic feet)(cubic feet)(cubic feet)(cubic feet)(cubic feet)(cubic feet)
DRAINAGE SUBBASINS
1 A1 11,660 5,400 990 3,050 21,100 4,923 1,083 3,102 2,019 100 121 1,898 2,400 0 2,400 502
2 A2 7,260 2,320 970 13,650 24,200 5,647 1,242 3,557 2,315 0 0 2,315 3,420 0 3,420 1,105
3 A3 7,260 2,150 1,220 10,470 21,100 4,923 1,083 3,102 2,019 0 0 2,019 3,480 0 3,480 1,461
4 A4 6,570 2,270 340 14,420 23,600 5,507 1,211 3,469 2,258 100 121 2,137 1,200 1,200 2,400 263
5 B1 2,230 0 0 2,970 5,200 1,213 267 764 497 0 0 497 600 0 600 103
6 B2 2,230 0 0 2,170 4,400 1,027 226 647 421 0 0 421 540 0 540 119
7 B3 4,450 0 0 4,150 8,600 2,007 441 1,264 823 0 0 823 1,080 0 1,080 257
8 B4 2,970 0 0 2,430 5,400 1,260 277 794 517 0 0 517 690 0 690 173
9 B5 1,480 0 0 2,320 3,800 887 195 559 364 0 0 364 480 0 480 116
10 C1 22,150 9,590 5,490 17,970 55,200 12,880 2,834 8,114 5,281 100 121 5,160 1,140 4,875 6,015 855
11 C2 6,530 9,960 3,800 32,910 53,200 12,413 2,731 7,820 5,089 100 121 4,969 1,800 3,900 5,700 731
12 C3 5,810 11,150 2,580 24,360 43,900 10,243 2,254 6,453 4,200 100 121 4,079 540 4,500 5,040 961
13 C4 15,750 8,120 2,260 7,970 34,100 7,957 1,750 5,013 3,262 100 121 3,142 1,800 3,200 5,000 1,858
14 D1 14,450 0 0 78,150 92,600 21,607 4,753 13,612 8,859 100 121 8,738 6,600 2,875 9,475 737
15 E1 16,040 6,850 3,980 21,330 48,200 11,247 2,474 7,085 4,611 100 121 4,491 3,780 6,900 10,680 6,189
16 F1 5,840 2,310 230 11,020 19,400 4,527 996 2,852 1,856 0 0 1,856 2,640 0 2,640 784
17 G1 19,870 4,460 3,620 23,650 51,600 12,040 2,649 7,585 4,936 0 0 4,936 0 12,000 12,000 7,064
18 G2 26,440 0 6,340 65,420 98,200 22,913 5,041 14,435 9,394 100 121 9,274 5,100 0 5,100 -4,174
19 G3 0 9,740 3,050 10,610 23,400 5,460 1,201 3,440 2,239 100 121 2,118 1,600 1,800 3,400 1,282
20 H1 9,320 0 1,190 9,590 20,100 4,690 1,032 2,955 1,923 0 0 1,923 3,120 6,000 9,120 7,197
21 H2 11,030 6,180 2,540 5,450 25,200 5,880 1,294 3,704 2,411 100 121 2,290 0 1,600 1,600 -690
22 H3 6,530 2,950 1,030 9,290 19,800 4,620 1,016 2,911 1,894 100 121 1,774 2,760 0 2,760 986
23 I1 4,450 0 0 5,650 10,100 2,357 518 1,485 966 0 0 966 960 0 960 -6
24 I2 4,450 0 0 5,050 9,500 2,217 488 1,397 909 0 0 909 1,080 0 1,080 171
25 I3 2,230 0 0 2,170 4,400 1,027 226 647 421 0 0 421 540 0 540 119
26 I4 2,230 0 0 2,870 5,100 1,190 262 750 488 0 0 488 480 0 480 -8
Subtotal 219,230 83,450 39,630 389,090 731,400 170,660 37,545 107,516 69,971 1,206 1,447 68,524 47,830 48,850 96,680 28,156
Subtotal (acres)5.03 1.92 0.91 8.93 16.79
CITY STREETS
Smith Farm Road 0 36464 12720 8,480 57,664 13,455 2,960 8,477 5,517 1,809 2,170 3,346 0 0 0 -3,346
Pebble Creek Road 0 53449 18645 12,430 84,524 19,722 4,339 12,425 8,086 3,215 3,858 4,228 0 0 0 -4,228
2000 South 0 17066 4830 4,508 26,404 6,161 1,355 3,881 2,526 1,608 1,929 597 0 0 0 -597
Subtotal 0 106,979 36,195 25,418 168,592 39,338 8,654 24,783 16,129 6,632 7,958 8,171 0 0 0 -8,171
Subtotal (acres)0.00 2.46 0.83 0.58 3.87
TOTAL 219,230 190,429 75,825 414,508 899,992 209,998 46,200 132,299 86,099 7,837 9,405 76,694 47,830 48,850 96,680 19,986
TOTAL (acres)5.03 4.37 1.74 9.52 20.66
SURFACE AREA RUNOFF VOLUME INFILTRATION VOLUME RETENTION PROVIDED
Concordia Development Drainage Report
Pebble Creek Townhomes Project No. 01-21-0042
Civilize, PLLC 1 | P a g e
D:\OneDrive - Civilize, PLLC\Civilize\Proj\Concordia\01-21-0042 Pebble Creek\Design Concordia\400 Prelim\1000 Civil\Drainage\DR -
Rexburg West.docx
Exhibit B
Drainage and Grading Plan