Greater Detail: GRADING and STORMWATER Flashcards
Line type used for existing contours
dashed
interval labeled on uphill side
Line type used for proposed contours
solid
interval labeled on uphill side
FFE
TW/BW
TC/BC
TS/BS
Finished floor elevation
Top of Wall / Bottom of Wall
Top of Curb / Bottom of Curb
Top of Stair / Bottom of Stair
*all will include a spot elevation
BF
HP/LP
TF or RE
INV. EL
Bottom of Footing
High Point / Low Point
Top of Frame or Rim Elevation
Invert Elevation
*all will include a spot elevation
CB
DI
MH
AD
Catch Basin
Drain Inlet
Manhole
Area Drain
*all will include top of frame and invert elevation (except for area drains, which only requires top of frame)
CIP RCP CMP VCP PVC
Cast Iron Pipe Reinforced Concrete Pipe Corrugated Metal Pipe Vitrified Clay Pipe Polyvinyl Chloride Pipe
*all will include a pipe size
PL
CLL
CL
Property Line ____ __ __ ____
Contract Limit Line ____ __ ____
Center Line ____ _ ____
- center lines will include a flow direction
CF to CY
divide by 27
SF to SY
divide by 9
Cut / fill calc. that is best for linear construction (roads, pathways, utility trenching)
METHOD:
Take cross sections at a certain interval, calculate areas of cut and fill for each section, average the areas of all sections taken and multiply that area by the length
Average End Area
Cut / fill calc. that is best for large but relatively simple grading plans
May be used to calculate volumes of water in a pond / lake
Establish no cut / no fill line, measure SF area of cut and fill for each contour within the no cut / no fill line, calc. total cut and fill, multiply by the contour interval
Contour Area Method
Cut / fill calc. that is best for complex grading projects and urban conditions
Overlay a grid over the area to be regraded, calculate the average change in cut / fill per cell by determining the average difference in elevation of all 4 corners, add average cut and fill together and multiply by the area of one grid cell
Borrow Pit Method (aka Grid Method)
The final grade after all landscape development has been completed; top surface of planted areas, pavements, etc. Normally designated by contours and spot elevations on a grading plan
Finished Grade
Top of the material on which the surface material (topsoil, pavement + base layers, etc) is placed. Subgrade is represented by the top of a fill situation and the bottom of a cut excavation.
Subgrade
Subgrade that must attain a specified density
Compacted Subgrade
Indicates a soil that has not been excavated or changed in any way
Undisturbed Subgrade
Imported material placed beneath pavements (usually course or fine aggregate)
Base / Subbase
Usually the elevation of the first floor of a structure; may also be used to designate the elevation of any floor
Finished Floor Elevation
The process of removing soil
Proposed contours extend across existing contours in the UPHILL direction
Cut / Cutting
The process of adding soil
Proposed contours extend across existing contours in the DOWNHILL direction
Fill / Filling
Fill material that is imported to the site
Borrow
The densification of soil under controlled conditions, particularly a specified moisture content
Compaction
Normally the top layer of a soil profile; may range in thickness from 1” - 1’-0” or greater.
Has a high organic content and is therefore subject to decomposition and is not an appropriate subgrade material for structures
Topsoil
Problem soils / conditions as it relates to construction (6)
- Loose silts
- Soft clay
- Fine water-bearing sands
- Soils with high organic content (e.g. peat)
- High water table
- Bedrock
When soils become unconsolidated due to excavation, scraping, removal of vegetation etc
Erosion
Most effective strategy for minimizing erosion
Reduce area to be disturbed
TESC Plans
Temporary Erosion and Sedimentation Control Plans
Soil erosion and sedimentation control plans may be required by code to be submitted prior to start of construction
Methods of erosion control (2)
- Strip and stockpile existing topsoil to avoid unnecessary loss (to be replaced after regrading is complete)
- Grading should be informed by the erosion tendency of the site’s soil (increasing length and degree of slope will increase erosion potential)
If balance of cut and fill cannot be met is it better to have more cut or more fill
CUT
- less costly to export soil than to purchase and import soil
- cut subgrade / finished grade is more stable and erosion resistant than filled subgrade / finished grade
RIDGES (and roadway crowns) Point ___
Contour signature
Downhill
VALLEYS (and swales) Point ___
Contour signature
Uphill
Concave line spacing ___
Contour signature
Increasing distance in the downhill direction
Convex line spacing ___
Contour signature
Increasing distance in the uphill direction
Most expensive method of accommodating grade change:
Retaining wall
Max slope of mowed lawn
3:1 (33.33%)
Max slope of planted area
2:1 (50%)
Min slope to be visually significant
5:1 (20%)
Purpose of a crown in roadway design (2)
- Positive Drainage
2. Visually separate opposing traffic lanes
Parabolic Crown Section (roadways)
Rounded crown; common for asphalt; contours are rounded
Tangential Crown Section (roadways)
Pointed crown; common for concrete; contours are V shaped
Reverse Crown (roadways)
Typically used where it is not desirable to direct stormwater runoff to the edge of the road or in restricted road conditions (e.g. urban alleyway)
Curb Heights
6” typically
min 2”, max 8”
A relative high point
Example:
A high point in a swale that is higher in the longitudinal direction but lower in the perpendicular direction / than the edge if swale
Saddle
3 reasons to pitch water away from buildings
- Water penetration potential
- Reduce bearing capacity of soils
- Moisture on building materials
Delineates the limit of grading work
Limit line
Grading Plan Elements (4)
- All existing and proposed features
- Proposed contours are solid
- Existing contours are dashed
- Spot elevations (corners of features, top and bottom of vertical elements, FFEs, HP and LP, top of frame / rim, inverts
Notes on a grading plan (4)
- General / explanatory info
- Description of unique conditions
- Source of existing condition info
- Benchmarks and reference datum
Max. proposed slope depends on:
- shear strength of exposed soil
- maintenance considerations
- stabilization techniques
Grading sequence in construction (4)
- Site preparation
- Bulk excavation
- Backfilling and fine grading
- Finish surfacing
The time needed for water to flow from the most remote point in a watershed to the watershed outlet. It is a function of the topography, geology, and land use within the watershed.
Time of Concentration
Construction or natural drainage channel used to direct surface flow
Parabolic, trapezoidal, or triangular cross sections
Used to divert water away from an object in the landscape in order to protect it from flooding or to keep the subsoil under a structure dry.
Swale
Structure that allows water course to flow beneath o road, walk, or highway
Culvert
Conc. structure 2.5-4ft dia used to collect and divert surface runoff to an underground conduction system. Includes a sump or sediment bowl at base to trap debris
Catch Basin
1 catch basin per ____ paved area (typically)
10,000 SF
Structure that directs water directly into a drain pipe (does not have a sump)
Drain Inlet
Prefabricated structure that collects runoff from paved areas. May include a sediment bucket.
Area Drain
1 area drain per ____ paved area (typically)
1,000 - 2,000 SF
Linear Inlet structure used to collect sheet flow runoff in paved areas
Trench Drain
Structure usually 4FT dia made of conc., brick, or fiberglass rings that allows a person to enter a space below ground.
Used where there is a change in the size, slope, or direction of underground pipes
Round or rectangular
Often combined with a catch basin
Manhole
Limitations to infiltration facilities (3)
Soil permeability rates
Potential reduction of permeability over time
Potential for groundwater contamination
Conditions to be studied for any infiltration facility (6)
Depth to groundwater
Seasonal variation in groundwater level
Slope and direction of groundwater flow
Soil permeability
Vegetative cover
Quality of runoff
q = CIA
The Rational Method
Used to compute peak rate of runoff for a SMALL drainage area (e.g. less than 100 acres); measured in cubic feet per second. Used for the design of a range of drainage structures (length of sewer, inlets, detention ponds, etc)
C = runoff coefficient (0-1); depends on land use, soils, and slope.
I = rainfall intensity in in/hr for the design storm frequency (e.g. 10-year storm) and for the time of concentration of the drainage area
A = area of drainage area (acres)
C = 0
q = CIA
Completely pervious surface
C = 1
q = CIA
Completely impervious surface
Design Storm
A storm with a frequency and duration for which a management system is designed.
Nomograph
Used to estimate time of concentration / overland flow time
Take 2 given values and locate them on the nomograph; draw a straight line between them to solve for other values
Modified Rational Method
Accounts for a ‘antecedent precipitation’ factor (q = CCaIA)
Where Ca = multiplier for storm event (e.g. 2-10 yr = 1.0; 25 yr = 1.1; 50 yr = 1.2, 100 yr = 1.25)
Accounts for the fact that in this more infrequent major storm events that the soil will already be saturated / have a substantially reduced capacity to infiltrate
Used for the design of storage ponds and reservoirs (inflow and outflow volumes)
Plot of flow rate (q) over time (T)
A graph showing, for a given point on a channel, the discharge, stage, velocity, or other property of water with respect to time
Hydrograph
Open Drainage Systems
- define
- provide example
- biggest concerns
All surface runoff is collected and conveyed on the open ground
E.g. swales, gutters, channels, culverts, detention, retention, sediment basin, infiltration basins
Erosion and sediment are biggest concerns
Closed Drainage Systems
- define
- provide example
- advantages
- disadvantages
All surface runoff is collected at surface inlets and conveyed by underground pipes to an outlet
E.g. catch basins, drain inlets, area drains, trench drains, manholes, piping
Advantage: runoff may be intercepted before volume and velocity contribute to erosion
Disadvantages: costly, sediment is not filtered out (inherently), erosion may still occur at discharge points, reduced opportunity for infiltration
Combination System
- define
- advantages
Combines open and closed drainage systems
Advantages: reduced costs, lower potential for soil erosion bc surface runoff volumes are reduced, lower potential for erosion at discharge point bc volume is reduced
(3) considerations for pipe design
- pipes should increase in size toward outlet
- typically straight (less clogging, easier to clean)
- must pitch; pitch should flow with surface slope +/- to reduce excavation
Velocity of water in a swale is modified by (2)
- slope of swale
- vegetation (type and condition, particularly length)
Where excessive velocity cannot be avoided structural linings should be used (rip rap, gabion, concrete)
Min parameters of a swale design
Peak flow for a 10-year storm
2 equations used to determine dimensions of open channels (e.g. swales)
Manning’s Equation
Continuity Equation
A formula for calculating the velocity of flow in a channel as a function of relative roughness, cross-sectional configuration, and gradient
Manning’s Equation
A formula expressing the principle of conservation of mass as applied to the flow of water. It states that the product of cross-section of flow and velocity at any point in a channel is a constant
Continuity Equation
The velocity at which unstable flow conditions begin to occur
Critical Velocity
First step in designing any drainage system
Determine availability of an adequate outlet
Pipe design: min depth
3FT
Minimizes damage from traffic and frost
Pipe design: constraint when locating collection points
Locate away from trees, main walks, buildings as occasional clogging may cause flooding
Pipe design: what is needed where pipes join
Manhole, catch basin, or junction box
Pipe design: size may be determined by
Manning’s Equation and Continuity Equation
Principle of Subsurface Drainage and methods
Excess water removed by gravity; capillary action / surface tension retains enough water to support plants
Methods: Clay tile (segmented)
Perforated porous pipe (holes placed downward)
What is the min. depth of a swale
6”
source: LARE app
What is an acceptable range for longitudinal swale slope
2-4%
source: LARE app
Min. amount of slope allowable on a playing field
- 5%
source: LARE app
An accessible route with a running (longitudinal) slope GREATER THAN ___ is a ramp
1:20
Or 5%
Purpose of cross slope:
To create positive drainage
GRADING SEQUENCE 2:
Rough grading phase; major earthmoving and shaping for major earth forms and providing footing / foundation excavation for all structures
Bulk Excavation
GRADING SEQUENCE 3:
Undertaken after all structures have been built
Backfill must be compacted to minimize future settlement (compaction should not damage new / existing subgrade structures)
Fine grading ensures that forms and surfaces meet the desired grades
Backfilling and Fine Grading
GRADING SEQUENCE 4:
Paved / hard surfaces first, then topsoil is placed (salvaged or new)
Subsoil is typically roughened / scarified to allow topsoil to bond better and to promote root growth between soil layers
Elevations of topsoil and final surfaces must meet the elevations of the grading plan
Finish Surfacing
Uptake and release from trees, plants and the ground
Evapotranspiration and Bioretention
Best plant for bioretention
Broadleaf evergreens (evergreens transpire year-round; broadleaved biomass > needled evergreen)
The temporary capture and slow release of water to stormwater systems / infrastructure
Detention
The slowing down of stormwater to permit it to soak into soils and ultimately groundwater systems; water table height and soil type are critical
Infiltration
Stormwater Management System Functions (6)
- Evapotranspiration and Bioretention
- Evaporation
- Detention
- Infiltration
- Capture
- Treatment
The process of settling out particulate matter from runoff; when water is slowed down, sediment is allowed to sink
Sedimentation Management
Occurs as runoff flows through plant material and soil; sediment is physically strained out of runoff; sandy soils are ideal
Other benefits of runoff carried through plant material: plant resistance (slows water velocity)
Filtration
Runoff from impervious surfaces picks up added heat
Thermal Attenuation
The process of removing soluble nutrients, metals, and organics by binding ions and molecules to other particles or organic matter or clay
Adsorption
Benefits of Infiltration Systems
1 Primary, 4 Secondary
Primary:
1. Reduce surface runoff while recharging groundwater
Secondary:
1. Reduce downstream peak flows
- Reduce subsidence related to groundwater depletion
- Preserve existing vegetation
- Lowers development costs
Limitations of Infiltration Systems (4)
- Soil permeability rates
- Potential reduction of permeability rates over time
- Potential for groundwater contamination
- Infiltration systems improve water quality but should not be used to remove sediment / other particulate matter; any infiltration system must include filter strips or sediment traps before it enters the infiltration device
Temporarily stores runoff for a certain design storm or specific volume and allows water to infiltrate (e.g. rain garden)
May be a combined infiltration + detention system
Drawbacks: may require a large area; not adaptable to multiple uses (e.g. recreation); high rate of failure due to improper maintenance and / or installation
Infiltration Basins
An excavation backfilled w/ coarse aggregate stone
Voids bw stone provide volume for temporary water storage
Surface of facility may be planted w/ inlets or is highly porous: sand, stone, gravel
Best for relatively small drainage areas
Observation wells should be installed vertically in the facility to periodically monitor change in infiltration rate
Infiltration Trenches
Similar to infiltration trench but storage is oriented vertically (into the ground) rather than laterally
Variation w.o stone: prefabricated structures that can carry heavier loads and do not need stone backfill = greater volume of water stored in smaller footprint
Dry Well
Suitable for low-volume roads and where subgrade soil conditions provide sufficient permeability, depth to groundwater and where contamination will not occur from degraded stormwater quality
Porous Pavement
Structured or landform impoundments constructed to collect runoff for the purpose of reducing peak flow and controlling the rate of flow
Detention Systems
Basins w/ a permanent pool of water
Benefits: increased property values (when well designed), recreational opportunities, habitat creation
Disadvantages: safety problems, algal blooms, odors, mosquitoes, need for maintenance (e.g. sediment removal)
Retention Basin
Dry pond / dry basin
Control peak discharge rates through temporary storage of storm runoff where outflow rates are set at or below pre-development rates
Does not enhance stormwater quality (
May be doubled w/ recreation use but must include low-flow channels to provide positive drainage toward outlet and / or subsurface drainage (min 2% toward outlet)
Elongated forms preferable : elegonates flow from inlet to outlet
Side slopes of basin should not exceed 3:1
Maintenance access way min 10ft wide with max slope of 5:1 should be provided
Detention Basin
May be included as part of a detention / retention basin design
Provides additional volume for settling of sediment (retention-type)
Outlet orifice is small, which slows outward flow and provides more time for contaminants to settle out (detention-type)
Water Quality Basin
An impoundment area / structure that slows the velocity of runoff in order to allow sediment particles to settle out
Sediment Basin
Pipe and control structure are sized to slow stormwater release to a specified rate
Must include an overflow bypass should the pipe fill up
Detention Pipes and Vaults
Drainage of water from the roof is slowed by flow control device
Requires additional waterproofing membranes, scuppers, or overflew drains to control a max. depth of water ponding on the roof (typically limited to 4” max)
Structural engineers should be consulted to ensure that the roof can support additional loading of water
Rooftop Detention / Blueroof
All surface runoff is collected and conveyed on the open ground
E.g. swales, gutters, channels, culverts, detention, retention, sediment basin, infiltration basins
Open Drainage System
Considerations:
- Volume and velocity of runoff (prevent erosion; If necessary, flow energy should be dissipated)
- Collecting sediment at discharge points
All surface runoff is collected at surface inlets and conveyed by underground pipes to an outlet
E.g. catch basins, drain inlets, area drains, trench drains, manholes, piping
Closed Drainage System
Closed Drainage Sys. Advantages (1) / Disadvantages (4):
Advantage:
1. Runoff may be intercepted before volume and velocity contribute to erosion
Disadvantages:
- Costly
- Sediment is not filtered out (inherently)
- Erosion may still occur at discharge points
- Reduced opportunity for infiltration
(3) Basic Functions of any storm drainage system:
- Collect
- Conduct
- Dispose
Major concerns of swale design (2)
Erosion and sedimentation
An area bounded by ridges having a single outlet from which water can flow
Watershed
Spot elevations on hardscape should be calculated to within
0.01 ft
Spot elevations on softscape should be calculated to within
0.1 ft
In open landscapes, slopes of less than __% appear flat to the human eye
2%
Absolute min % for draining water across a very smooth, ridgid material (e.g. storm drain pipes, conc. gutters, conc. slabs).
Not recommended unless its impossible to avoid or you are given explicit permission to do so in the problem statement
0.5%
A workable minimum for shedding surface water across paving materials.
Ok for a cross slope.
1%
The generally recommended gradient for most any surface. Moves water well on most surfaces.
Recommended as the cross slope on sidewalks and the crown cross slope on roads.
Maximum allowable cross slope under ADA guidelines
2%
Gradient at which slope becomes obvious to the human eye
Usually the maximum gradient accepted for road intersections.
3%
Maximum longitudinal slope allowed for accessible ramps, including curb ramps
- 33%
12: 1
Rational Method Runoff Coefficient LOW values denote
Less runoff
Rational Method Runoff Coefficient HIGH values denote
More runoff
How many SF in an acre
43,560 SF
Lowest elevation inside a pipe; essentially the flow line of water
INVERT
Solve for invert
= Finished grade - required cover over pipe - pipe diameter
on LARE you can assume pipe thickness is zero, unless otherwise specified
Occurs whenever the water level immediately downstream of a pipe outlet rises above the top of the pipe.
Surcharge
Soil depth from finished grade to top of pipe
Cover
Min cover to maintain over pipes for LARE (unless otherwise specified)
12 inches
To reduce clogging, pipe sizes should
Never be reduced in a downstream direction
Spot elevations on hardscape should be calculated to within
0.01ft
unless otherwise indicated
Spot elevations on softscape should be calculated to within
0.1ft
unless otherwise indicated
As a general rule, keep the FFE of a building __ inches above adjacent grade
6 inches
Max swale side slope
unless otherwise stated
3:1 ( 33.3%)
Ideal swale longitudinal slope
unless otherwise stated
2%
