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Advanced Topics:Solutions for Drainage Problems
Gregory Rusciano
Program Associate, Water Resources
Rutgers Cooperative Extension
New Jersey Sea Grant Extension Program
Advanced Design Topics
Topics
• Dealing with clay soils
• Dealing with compacted soils
• Integrating rain gardens with sloped drainage channels and existing infrastructure
Advanced Design Topics
Dealing with clay soils…
1.Sizing considerations
2.Soil amendments
3.Under-drain systems
4.Overflow devices
Advanced Design Topics
Walk the property
What to look for…
• Identify areas of impervious cover
– Buildings/rooftops
– Paved areas
• Parking lots
• Driveways
• Sidewalks
• Identify semi-impervious areas
– Known areas of limited permeability
• Compacted soil
• High clay soils
• Identify sloped lawn areas
Advanced Design Topics
25
10
15
50
25
50
50' x 15' = 750 square feet
25' x 10' = 250 square feet
Total Area = 1,000 square feet
Driveway Area
One-Quarter of the Roof
25' x 12.5' = 312.5 square feet
HouseDriveway
Advanced Design Topics
Example in Sizing
Problem:
How big does a rain garden need to be to treat the
stormwater runoff from my driveway?
Advanced Design Topics
Example in Sizing
• Drainage Area = 1,000 square feet
• 1.25 inches of rain = 0.1 feet of rain
• 1,000 sq. ft. x 0.1 ft. = 100 cubic feet of water for the
design storm
• Let’s design a rain garden that is 6 inches deep
Answer:
10 ft wide x 20 ft long = 200 square feet
Check: 200 square feet x 0.5 ft deep = 100 cubic feet of
storage volume
Advanced Design Topics
Rain Garden Sizing Table
for NJ’s Water Quality Design StormArea of Impervious
Surface to be Treated (ft2)
Size of 6‖ deep Rain Garden
(ft2) or [w x d]
Size of 12‖ deep Rain Garden
(ft2) or [w x d]
500 100 or 10’x10’ 50 or 10’x5’
750 150 or 15’x10’ 75 or 10’x7½’
1,000 200 or 20’x10’ 100 or 10’x10’
1,500 300 or 30’x10’ 150 or 15’x10’
2,000 400 or 20’x20’ 200 or 20’x10’
Advanced Design Topics
Sizing for Clay Soils
• Decrease the depth and increase the surface area
– (3—4‖ maximum depth)
– Provides more exposure to sunlight for evaporation
– Provides more pore spaces available for infiltration
– Provides more contact with mulch and plants (uptake)
• Depends upon available space on the property
• This will drive up your budget (additional plants and mulch)
• May lead to more maintenance (additional pruning weeding, etc.)
Advanced Design Topics
Water should be completely
drained within 24 hours.
Percolation Test
Ruler
1st Hour2nd Hour3rd Hour4th Hour
Advanced Design Topics
24 Hour Method
• Perform your percolation test
– 1.5 inches/hour is ideal (sandy/sandy loam)
• If water percolates completely within 24 hours…
– Still acceptable but decreasing the depth is advised
• If water does not percolate completely within 24 hours…
– Determine the 24-hour rate
– Use it as your maximum depth
Advanced Design Topics
Soil Test
• Sample the soil and send to the Rutgers Soil
Testing Lab for:
• Nutrient analysis/ recommendations
• pH analysis/ recommendations
• Percent sand/ silt/ clay
or textural class (then refer to fact sheet)
• Soil Texture Test
Roll soil into a ball in hand and see how it forms
• Hard ball – Clay/Silt soil
• Soft ball – Loamy soil
• No ball – Sandy soil
But, don’t worry – clay/silt and
sandy soils can be amended to get
the preferred loamy soil texture
Gloucester County 4-H Fairgrounds
Advanced Design Topics
Adding Amendments
• Compost and/or sand should be used
• Requires amending at least an additional six inches of the existing soil
• The clay soil that you remove can be reused for the berm
Advanced Design Topics
Adding Amendments
• Replace existing soil with sandy loam top soil or add a sand/compost mixture with a rototiller
(obtain bank run or concrete sand only)
• It will drive up the price
• You want the final soil mixture to be at least 50% sand content (refer to soil textural class triangle chart on Cornell fact sheet)
Advanced Design Topics
General Soil Amendments Amounts for a
100 sq ft Rain Garden (six inch depth)
Soil Amendment Amount for 100 sq ft Rain Garden
Sand 1 cubic yard
Compost 1 cubic yard
Fertilizer Follow Soil Test Result Recommendations
Lime Follow Soil Test Result Recommendations
Gloucester County 4-H Fairgrounds
Advanced Design Topics
• Groundwater Recharge: Infiltrated water that is not
evapotranspired
• Groundwater shall not be recharged in areas with high
pollutant loading
• Waiver available for urban redevelopment
• Groundwater Criteria:
– Maintain existing (100%) average annual groundwater recharge
OR
– Infiltrate the increase in runoff volume for the 2-year storm
event
• No more than one foot between the rain garden
bottom and seasonably-high GW table
Groundwater Recharge
Advanced Design Topics
Cross-Section of Rain Garden - Bioretention Basin
BASIN SLOPE
4 IN. MULCH LAYER
2.5 – 3 FT. PLANTING
SOIL BED (MIN.
PERMEABILITY 0.5 IN.
PER HOUR)
12 IN. SAND BED
12 IN. GRAVEL BED
4 IN. PERF. PVC PIPE
FILTER FABRIC
NATIVE PLANTINGS
Advanced Design Topics
Cross-Section of Rain Garden - Bioretention Basin
BASIN SLOPE
3 IN. MULCH LAYER
1 – 3 FT. PLANTING
SOIL BED (MIN.
PERMEABILITY 1 IN.
PER HOUR)
12 IN. GRAVEL BED
4 IN. PERF. PVC PIPE
FILTER FABRIC
NATIVE PLANTINGS
Advanced Design Topics
12” ponding depth
0 Hours
9” ponding depth
6 Hours
3” infiltration
6” ponding depth
12 Hours
6” infiltration
Bioretention Basin – Expected Infiltration
Advanced Design Topics
1 Day
2 Days
24” infiltration
3 Days
36” infiltration
No Standing Water
12” infiltration No Standing Water
No Standing Water
Bioretention Basin – Expected Infiltration
Advanced Design Topics
5-10 ft.
Outflow
Underdrain Pipe Schematic (plan view)
PVC Cap
PVC Cap
PVC Cap
Elbow
Elbow
Coupler
Pea gravel
Pea gravel
Pea gravel 5-10 ft.
Advanced Design Topics
Underdrain Cross Section
Discharge
point
Stone bed
Berm
Gradual pipe angle
6-inch deep ponding area
Existing slope
Underdrain pipe
can also
discharge to
existing
infrastructure
•Curb and gutter
•Catch basins
•Underground storm sewer pipe
Advanced Design Topics
Berm Option
• Plant the berm with new plantings of grass plugs and mulch as necessary
• Reuse grass clumps or use sod or grass seed
• Use erosion control mesh blanket or burlap
• Overflow pathways
(depends on preference and erosion potential)– Sheet flow
– Pipe
– Berm notch lined with stone
Advanced Design Topics
Gloucester County 4-H Fairgrounds
Leonard Park, Morris County
Overflow
Installation Steps
Advanced Design Topics
Catch Basin Riser (cross section)
Six Inches
Rain garden
ponding areaRain garden
ponding area
Overflow
Advanced Design TopicsPlanning Steps
Site Visit
Determine Current Stormwater Flow
Union County Vocational School
Flow
Advanced Design Topics
Compacted Soils
• Rototill before planting
• Remove and replace with sandy loam mixture
• Only use heavy equipment around the perimeter of the rain garden
Advanced Design Topics
Walk the property
Stormwater conveyance…
• Rooftop gutters and downspouts (if any)
– Do they discharge above ground?
– Are they directly connected to the road?
– Are they directly connected to the underground storm sewer
• Existing stormwater infrastructure
– Curb/gutter in the parking lot or driveway
– Catch basins and storm sewers
• Look into the catch basins
• What is the direction of pipe flow?
– Open channel conveyance swales/ditches
– Detention basins
• Topography – flat vs. sloped
• Type of existing vegetation (if any)
Advanced Design Topics
Determine the various pathways of the stormwater runoff on the property
• Which of the pathways are ―impervious pathways?‖
• Where does the water ultimately go?
– A detention basin
– Nearby stream or lake
• Other signs that identify the pathways
– Erosion
– Patches of dead grass
– Areas of sedimentation
Ultimate goal: You need to determine a good place to “disconnect” the impervious pathway
Advanced Design Topics
Disconnecting the Impervious Pathway
• Try to install the rain garden as close to the source as possible
• Can the pathway be disconnected above ground?
• Does a ―cut‖ need to be made?
– Underground downspouts can be ―day-lighted‖ before discharging to the curb/gutter
– Curb cut before water enters the catch basin
Advanced Design Topics
Rain Garden Sizing Table
for NJ’s Water Quality Design StormArea of Impervious
Surface to be Treated (ft2)
Size of 6‖ deep Rain Garden
(ft2) or [w x d]
Size of 12‖ deep Rain Garden
(ft2) or [w x d]
500 100 or 10’x10’ 50 or 10’x5’
750 150 or 15’x10’ 75 or 10’x7½’
1,000 200 or 20’x10’ 100 or 10’x10’
1,500 300 or 30’x10’ 150 or 15’x10’
2,000 400 or 20’x20’ 200 or 20’x10’
Advanced Design Topics
Rooftop scenario
• Easiest scenario for estimating the drainage area
• Calculate the volume of water that discharges to the downspout
• If no gutters, follow the drip line with a ditch that conveys water into the rain garden
What if the gutters connect directly to the storm sewer system underground?
Advanced Design Topics
Hockman Farm, Winchester, Virginia
Roof
W
L1 L2
Surface Area = (L1xW) + (L2xW)
= (15’x20’) + (10’x20’)
= (300’) + (200’)
= 500 square feet
Roof
3. Measure Impervious Surface Area
Planning Steps
Site Visit
Advanced Design Topics
Rooftop scenario (cont.)
• Try to disconnect the downspouts before they enter the underground pipes
• Divert the water to an above ground rain garden using PVC pipe and fittings and/or corrugated plastic pipe
• If you cannot disconnect at the downspout try ―day-lighting‖ (if the underground pipe if it is near the surface of the lawn)
– Only attempt this if you are able to design the rain garden for the entire drainage area
– Be careful of more than one downspout connecting to the same underground pipe
Advanced Design Topics
Diverting the pathway
• You are changing the direction of flow
• Rain gardens are designed for 1.25 inches of rain
• Determine where the overflow would go for a storm larger than 1.25 inches
– Would diverting the water into a rain garden ADD to the problem?
– Take note of the down-slope infrastructure—can it handle the overflow in the event of a storm greater than 1.25 inches?
Next scenario…
Advanced Design Topics
Parking lot and driveways
• Estimating the drainage area can be difficult
• We often use surveying equipment for this
• What can you do?
– Obtain site plans and/or speak with the building manager
– Use the ―get wet‖ method—go out there in the rain.
• Consider…
– Parking lots and driveways are often pitched to convey water to one side or both sides (look for a slight ridge in the center)
– Sediment deposits along curbs are evidence of flow direction
– If there is not curb, dead grass patches and erosion are evidence of where the water leaves the asphalt
But even if you can accurately estimate the D.A. you might not be able to handle such a large rain garden project
Advanced Design TopicsPlanning Steps
Site Visit
1. Determine Current Stormwater Flow
Union County Vocational School
Flow
Advanced Design Topics
Dealing with curb cuts
• The goal is to divert the water into a rain garden before it enters the catch basin
• Make use of the existing infrastructure in the cases of overflow (above 1.25 inches of rain)
– The back end of the rain garden berm must not be lower then the elevation of the parking lot asphalt
– Reinforce the berm by making it wider/taller, if necessary.
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