III. Runoff Quantity Reduction and Quality Improvement

 

III. Runoff Quantity Reduction and Quality Improvement

In many cases, alternative stormwater management techniques will contribute to both runoff quantity reduction and runoff quality improvement. This section covers a number of these measures.

  • Soil Amendments
    • Soil amendments include fertilizing, composting, aerating, and the overall conditioning of soil. Soil amendments promote both a reduction in the quantity of stormwater runoff through infiltration and a degree of water treatment through sediment fallout and biological treatment.

Compost box

  • Benefits:
    • Reduces the amount of runoff draining from a property
      • Reduces the need for chemical fertilizers and herbicides
      • Increases the space between soil particles to allow infiltration of stormwater and non‐stormwater runoff, thereby promoting groundwater recharge
      • Studies have shown a 65% increase in soil moisture storage capacity through practicing soil amendments
      • Effective means to filtering out stormwater pollutants through sorption, precipitation, filtering, and bacteriological and chemical degradation
      • Reduces erosion and sedimentation
      • Reverses the negative impacts of soil compaction due to development
      • Adds organic bulk and humus for growing desired plants
      • Suppresses plant disease
      • Restores soil structure after damage caused by fertilizers


  • Soil Amendments Continued
    • Installation:
      • Composting:

• Rodent Resistant Bins

♦Composed of a container with a lid and a bottom and no openings greater than ¼ inch

♦Best used for composting fruit scraps and vegetable and yard trimmings

♦Collect yard trimmings and chop into small (6 inch) pieces

♦Balance browns and greens and keep fruit towards the middle of the bin

♦Water as necessary to keep as moist as a wrung‐out sponge and mix about once a week

♦Ready to use in 3‐8 months

• Open Piles and Simple Bins

♦Used for yard trimmings only

♦Chop trimmings to 6 inch pieces and mix into pile

♦Balance greens with browns and water as necessary to keep as moist as a wrung‐out sponge

♦Mix often to prevent rodents from nesting

♦Cover with tarp to keep moisture in

♦Ready to harvest in 3‐8 months

• Worm Composting

♦For composting fruit and vegetable scraps only

♦Buy or build a bin with dimensions of approximately 16 inches deep and 10 inches by 10 inches in length and width (must have a tight fitting lid and not holes greater than ¼ inch to keep rodents out)

♦Place bin in a dry and cool location


  • Soil Amendments Continued

♦Make worm bed with thin strips of non‐glossy newspaper fluffed and moistened but not dripping; add sawdust, leaves, compost, and/or straw

♦Add a few handfuls of soil

♦Acquire “red” worms from an old compost pile, a friend, or a worm farm (2 handfuls of worms is sufficient)

♦Feed the worms a meal of about a quart of fruit and vegetable trimmings and let them be for a couple weeks

♦Feed the worms about a pound per square foot of surface area in the bin per week (and always bury new scraps under the bedding)

♦Add fresh bedding at a depth of 3 inches to the worm farm every 1‐3 months

♦Maintain moisture in the bin

♦Begin harvesting worm compost 2‐3 months after initiating the worm bin and harvest at least once a year

♦Harvest by scraping out the rich black compost and use about 1 inch at the surface of topsoil

• Compost should be applied based on the existing amount of organic material in the compacted soil and as a general rule a 2:1 ratio of soil to compost can be used

• For existing gardens spread about 1‐6 inch of compost on top of soil as mulch or ½ inch of fine sifted compost for top dressing turf or gardens

  • Other Soil Amendments:

• Physical aeration and composting

• Addition of mulch, lime, and gypsum for soils with low pH (acidic)

• Soil analysis of the native area can be conducted to determine the appropriate mixture and quantity of each component


  • Soil Amendments Continued
  • Nutrients and lime should be added in soils with a pH below 6 in ratios of ½ pound of domelite per square foot of soil and 0.002 pounds of nitrogen per square foot of soil
  • Gypsum may be added to soil to increase soil structure of clay soils, and to increase sulfur and calcium content without changing the pH


  • Beneficial Landscaping
    • Homeowners can use landscaping techniques which create an environment that preserves the moisture of the landscape. This kind of landscaping acts as a stormwater management tool to reduce runoff quantity and to improve the quality of runoff by sediment fall‐out and biological degradation. It also has the added benefits associated with IPM.

  • Benefits:
    • Improves runoff infiltration
      • Improves watering efficiency and reduces landscape’s need for water
      • Filters out pollutants
      • Preserves moisture in landscape
      • Promotes improved plant growth, disease resistance, and aesthetic value of the landscape
      • Reduces pesticide and fertilizer needs


  • Beneficial Landscaping Continued

  • Implementation/characteristics:
    • Install porous surfaces in unplanted areas and disconnect pervious areas
      • Flagstone, interlocking pavers, and pervious concrete should be used for walkways and patios
      • Interlocking pavers can replace driveways
      • Create an interceptor drain at the base of a drive to divert to landscape
      • Lay gravel and/or mulch in unplanted areas to allow water to filter into surrounding soils
      • Install gravel sumps or percolation areas to keep water from collecting in unwanted spaces
      • Choose plants that conserve water, buffer runoff, and are locally adapted
      • Install and properly operate irrigation systems including low flow rotor heads, smart irrigation controllers, drip systems, soaker hoses for trees, shrubs, and groundcover
      • Use rocks and “dry creek beds” made of rocks to slow down water flow for sediment fallout (can also be used for diverting runoff to other landscape features)
      • Minimum of 10% organic matter by dry weight in all planted areas
      • Planting beds should be mulched with 2‐3 inches of organic material
      • Subsoils should be scarified 4 inches deep before incorporating topsoil


  • Beneficial Landscaping Continued


  • Rain Gardens
    • Rain gardens serve as colorful stormwater detention areas, which help to reduce the quantity of runoff during peak flow events and treat infiltrating water.

  • Benefits:
    • Reduces the volume of runoff from a single‐family property where space is limited but runoff management is desired
      • Provides some water treatment by removing suspended solids, metals, and nutrients
      • Provides an aesthetic amenity to a property
      • Supports groundwater recharge
      • Flowering plants and grasses appeal to beneficial insects such as bees and butterflies
      • An effective and less costly means to stormwater management compared to some of the more sophisticated devices


  • Rain Gardens Continued

  • Installation/ characteristics:
    • Determine the size and shape of the garden based on the surface area of impervious surface draining to the garden (for gardens within 30 feet of a building, most runoff will be coming from downspouts; each downspout contributes about 25% of a buildings runoff; calculate the draining surface area by finding the length and width of the bottom of the house to approximate the roof area and then multiply this number by 25% to find the volume of runoff coming from one downspout)
      • The slope of the lawn or ground on which the garden is to be built should be between 3‐10%
      • For clay soils which drain slowly, the area of the garden should be 60% of the draining surface area (surface area of the roof)
      • For sandy soils which drain quickly, the area of the garden should be closer to 20% of the draining surface area
      • For loamy soils which have a medium drainage rate, the area of the garden can range between 20‐60% of the draining surface area
      • Test the drainage of the soil before building a rain garden by digging a 6‐8 inch hole in the desired area and fill with water; if any water remains in the hole after 12 hours then the soil/site is not suitable
      • If the drainage surface area is very large you can construct multiple rain gardens
      • The rain garden should be curvy in shape with the longest length downhill of the drainage
      • Create an outline of the garden with a rope


  • Rain Gardens Continued

  • Dig the garden 8‐10 inches deep and level at the bottom
    • Excess soil can be used to create a low berm on the downhill edge of the garden
      • Amend the soil by mixing 2‐4 inches of compost into the top layer
      • If the soil has a high pH add lime to neutralize
      • If the soil is high in clay you may remove 1‐2 feet of soil and replace with soil composed of sand(50%), topsoil(30%), and compost(20%)
      • Create a shallow swale or “dry creek” composed of rocks leading from a downspout to the garden (gutter can also be used)
      • Establish sturdy ground cover and or rock cluster at drainage entrance to garden to prevent erosion
      • Once planted with drought and wet tolerant plants, cover garden with a few inches of dense mulch
      • Create and overflow area composed of rock cluster where water can leave the garden in a heavy storm



  • Tree Box Filters
    • Tree box filters are a soft‐engineered stormwater management approach designed to collect, treat, and percolate stormwater runoff. The simple design includes a tree planted in an open‐bottomed basin filled with mulch, a sandy media, and a layer of gravel at the bottom. The top of the basin is structured by a concrete or granite frame, covered with pervious pavers, brick, or an artistic metal grate. The tree box filter is ideal for tight urban areas, where traditional stormwater conveyance systems negatively impact the natural hydrologic cycle.

  • Benefits:
    • Utilizes physical, chemical, and evapotranspiration processes to keep stormwater runoff onsite, and to treat polluted runoff
      • Open‐bottomed catch basin allows stormwater percolation into the natural groundwater table, and provides tree roots extra room to grow
      • Adds an aesthetic element to stormwater management


  • Tree Box Filter Continued

  • Installation:
    • Design initiation and construction assistance is provided for free from AutoCAD
      • See resource for direction to assistance
    • Resources:


  • Vegetated Swales
    • Vegetated swales are open, sloping channels covered with vegetation, which convey stormwater runoff in a manner that also promotes infiltration and sediment fallout. Vegetated swales are good for lining the interface between the property and the street as an alternative to a curb or gutter stormwater conveyance.

  • Benefits:
    • Reduces runoff volume
      • Increases detention time of stormwater runoff allowing for greater rates of sediment fallout
      • Slows runoff flow, forcing pollutants and nutrients to settle out
      • Allows greater percolation to recharge groundwater aquifers
      • Controls peak discharge by reducing runoff velocity and allowing infiltration
      • Good option for retrofitting existing drainage ditches
      • Aesthetic benefits for the property


  • Vegetated Swales Continued

  • Installation:
    • Swales should be at least 5 feet from a property line and 10 ft from a building foundation
      • Should be constructed on permeable surfaces (not compacted)
      • The shape of the cross‐section of the swale can be parabolic or trapezoidal
      • The longitudinal slope of the swale should be as low as possible (no more than 4% and no less than 0.5%)
      • Swales should follow the natural topography of the landscape as much as possible
      • Work best in sandy loams and where soil is very dense a 2‐3 foot deep bed of loamy sand can be laid
      • For denser soils, a perforated underdrain may be laid below bed of soil to facilitate drainage (geotextile fabric should cover the gravel surrounding the underdrain pipe)
      • Check dams or blocks can be set to create temporary ponding for infiltration in low longitudinal sloped sites
      • The side slopes should be approximately 3:1 to avoid erosion
      • Swale bottoms should be between 8‐10 feet wide
      • Gravel diaphragms can be created for lateral inflows


  • Vegetated Swales Continued

 

  • Stones or riprap should be used at any outflow area from the swale to prevent scouring
    • Components of the swale can range from mown grass to varieties of dense‐rooted grasses, shrubs, groundcover, and trees


  • Dry Wells
    • Dry wells are simple excavated pits on the sides of houses which act as infiltration devices. They collect runoff from downspouts and other sources, allowing the runoff to percolate into the groundwater system.

 

  • Benefits:
    • Reduces the quantity of stormwater leaving a property
      • Encourages groundwater recharge via percolation through the well walls
      • Keeps runoff temperature cool to protect local stream life
      • Provides some stormwater runoff treatment via passing through aggregate fill
      • Protects against erosion due to runoff moving over a property’s landscape
      • Protects against wear on a house by preventing backsplash from runoff coming off of a roof


  • Dry Wells Continued
    • Installation:
      • Dry wells should be constructed in sandy or gravel‐like soils if possible
      • Choose a location below a downspout and dig a 3 foot cubed hole, sloping the bottom of the dry well away from the house to protect the foundation from the infiltrating water
      • Line the hole with a non‐woven geotextile fabric
      • Backfill the hole with ½ inch to 1 ½ inch crushed stone to within 3 inches of the surface of the hole, then fold the sides of the geotextile over the crushed stone
      • Fill in the remaining space with crushed stone
    • Considerations:
      • Should not be used for homes with improperly sealed foundations due to concerns of flooding
      • If flooding is a concern construct the dry well at least 6 feet from the wall of the home
      • Debris will occasionally have to be cleaned from the surface of the dry well
      • Geotextile fabric and stones may have to be removed and washed to dispose of accumulated sediment
    • Resources:


  • French Drains/ Infiltration Trenches
    • The French drain/infiltration device is a linear facility designed to catch and infiltrate stormwater runoff from various areas of a property by leading the runoff down a slope into the drain/trench. They are similar to dry wells in design and function; however, instead of being constructed below downspouts, they are built at low points of a property. The drain or trench holds the water for a period of days while the water percolates into the surrounding soil.


  • Benefits:
    • Reduces runoff flow rate, volume, and temperature
      • If medium is fine, may provide some water treatment function by filtering sediment, nutrients, and pollutants
      • Increases groundwater recharge
      • Appropriate for small properties because of the narrow design


  • French Drains/Infiltration Trenches Continued

  • Installation:
    • Suitable for sites with gentle slopes, permeable soils, and deep bedrock and groundwater tables
      • The figure above shows a deep trench; however, drains/trenches can be much shallower ranging from a single foot to several feet deep
      • For a smaller French drain, 1 foot by two feet is an appropriate cross sectional area
      • The length and width of the trench is determined by the nature of the influent runoff; if the runoff flows in a uniform sheet, then the length of the trench perpendicular to the flow should be maximized; if the runoff flows in a channelized manner, the length of the trench parallel to the flow of water should be maximized
      • Dig the trench and line with geotextile fabric, using overlap when necessary
      • Backfill the trench with ½ inch washed stone aggregate
      • Pea gravel can be substituted for the top few inches of backfill to promote better filtration of sediment and pollutants (and when clogging occurs, cleaning can be done be simply replacing the pea gravel layer)
      • A vertical observation well can be constructed with a 4‐6” diameter perforated PVC pipe to view and monitor the infiltration efficiency of the French drain (the pipe can be anchored with a metal rebar at the bottom of the drain; should have a cap at top)


  • French Drains/ Infiltration Trenches Continued


  • Drip‐line Trenches
    • Drip‐line trenches are simple excavated trenches constructed along the drip‐line of a roof to allow infiltration of stormwater runoff where a gutter and downspout system is absent. The drip‐line trench design is very similar to the French drain/infiltration trench design.

  • Benefits:
    • Reduces the quantity of stormwater leaving a property
      • Encourages groundwater recharge via percolation through the well walls
      • Keeps runoff temperature cool to protect local stream life
      • Provides some stormwater runoff treatment via passing through aggregate fill
      • Protects against erosion due to runoff moving over a property’s landscape
      • Protects against wear on a house by preventing backsplash from runoff coming off of a roof


  • Drip‐line Trenches Continued

  • Installation:
    • Dig a trench that is approximately 1 foot by 2 feet along the drip‐line of the roof with the bottom sloping slightly away from the house to protect the foundation
      • To extend the life of the trench, line the bottom and sides with non‐woven geotextile fabric
      • Backfill the trench with washed, clean ½ inch stone to within 3 inches of the top of the trench, fold over the geotextile fabric and then fill the remaining space with stone
      • May use larger stone or pressure treated wood to line the boundary of the trench and to keep the media in place



  • Soakaway Pits
    • Soakaway pits are excavated pits which detain and infiltrate runoff. They are almost identical to dry wells, with the exception that the stormwater runoff is led into the soakaway pit from below the surface.

  • Benefits:
    • Reduces the quantity of runoff leaving a property
      • Infiltrates runoff into the groundwater system
      • Provides some treatment for stormwater runoff
      • Keeps runoff temperature low in consideration of stream‐life
      • Keeps a portion of runoff off of the surface of the properties landscape


  • Soakaway Pits Continued
    • Installation:
      • Small scale infiltration basins designed to collect runoff from individual roof leaders
      • Excavated pits are lined with filtration paper or geotextile fabric and back‐filled with aggregate stones
      • Limited by soil type, depth of bedrock and water table
      • Should be at least 10 feet from a building
      • The buried roof leader entering the pit should extend across the length of the pit, should be perforated and near the surface of the soil (3‐6 inches)
      • The roof leader should have an overflow extension with a leaf screen above the section leading below the surface
      • Clean‐washed stone should be used for the media
      • Pits should be dug close to the surface less than five feet deep
      • Rectangular pits will prevent artificial raising of the water table
    • Resources:


  • Permeable Pavers
    • Permeable pavers are a broad group of pervious pavements which reduce stormwater runoff quantity and improve the quality of runoff that percolates into the soil below. Permeable pavers are a good alternative to impervious surfaces commonly used for roads, parking spaces, plaza areas, driveways, or other weight‐bearing areas.

  • Benefits:
    • Increase the pervious surface area of a property
      • Allows infiltration of runoff on generally one of the largest impervious surface areas on a property while maintaining the function of load‐bearing capacity
      • Can use small areas of permeable pavers to disconnect impervious areas without having to redo the entire driveway
      • Provides some effective treatment of filtering pollutants, nutrients, and sediments
      • A good aesthetic alternative to the otherwise unattractive appearance of impervious concrete or asphalt


  • Permeable Pavers Continued
    • Installation:
      • Permeable pavers are good for pedestrian walkways or driveways
      • Choose a location where the seasonally high groundwater table is at least 2 feet below the level of where the crushed stone reservoir will be
      • Ensure the area surrounding the permeable pavers is fully stabilized
      • Check the soil for good infiltration capacity (choose sandy or gravely soil)
      • Soil drainage should be no less than 0.5 inches per hour
      • Permeable Pavement:

• Permeable pavement is a form of pavement similar to impervious asphalt or concrete in appearance; however, it is composed of courser media, which allows runoff to soak through the pavement into the reservoir or soil below

• Excavate and grade the subsoil; scarify a minimum of 6 inches to provide adequate infiltration

• Layer the subsoil with non‐woven geotextile fabric so that several feet of fabric extends out the sides of the excavation for folding over the base layer during construction (overlapping areas should overlap by 24 inches)

• Install clean, washed 1.5‐2.5 inch stone aggregate for a base layer (a minimum depth of 6 inches of base aggregate)

• Install a 1‐2 inch choker layer of finer aggregate

• The bottom of the aggregate stone layer should be flat with no more than a 5% slope

• Fold the extended geotextile fabric over the center of base layer

• Install permeable asphalt at a thickness of 2‐4 inches

• Cut excess fabric from edges of paved area


  • Permeable Pavers Continued

  • Permeable Pavers:

• Permeable Pavers include concrete grid and grass pavers, interlocking pavers, and concrete modules, which overly a base of crushed stone, acting like a reservoir and detention space for runoff before it is percolated back into the soil and groundwater

• Excavate and grade subsoil; compact to a certain density depending on oil characteristics (see resources and consider contracting an engineer)

• Install non‐woven geotextile fabric along bottom and sides of excavated area (overlapping should extend 24 inches; extend fabric several feet beyond the edge of the excavation for later folding over base aggregate during construction)

• Install No. 57 aggregate in 4‐6 inch lifts

• Compact moist aggregate with several passes of a 10‐ton steel roller

• Install 3 inches of No. 8 aggregate and compact with 10‐ton steel roller

• Install pavers immediately after preparing the base layer

• Place pavers by hand and compact with a 5000 lb, 75‐90 Hz plate compactor


  • Permeable Pavers Continued

• Fill voids with No. 8 aggregate and compact again

• Cast‐in‐place concrete can be used for restraining edges (6” wide X 12” high)

  • Permeable Pavement:
    • Use clean, washed stone aggregate for a base layer before applying pervious surfaces
      • The bottom of the aggregate stone layer should be flat with no more than a 5% slope
      • Aggregate or plastic pavers can be filled with soil, gravel, or seeded with grass
      • When grass is used to fill the pavers, use deep rooted grass to allow roots to grow into base aggregate level


  • Green Roofs
    • Green roofs are light‐weight vegetated roofs comprised of waterproof lining, growing media, and short durable plants. Green roofs are alternatives to conventional roof systems, acting as stormwater management facilities by reducing runoff volumes and providing water treatment through sediment fallout, filtration, and biodegradation.

  • Benefits:
    • Reduces stormwater runoff volume by absorption and evapotranspiration
      • Increases contact time of stormwater runoff to improve water quality
      • Filters and bioremediates pollutants
      • Removes carbon dioxide from the atmosphere
      • Provides housing insulation, reducing costs of cooling and heating
      • Provides sound‐absorption, making a home a quieter place
      • Increased lifespan of the roof
      • Filters out dust particles and cools surrounding air, increasing nearby air quality
      • Increases bird and invertebrate habitat, providing some compensation for habitat loss during development
      • Creates a more aesthetic home


  • Green Roofs Continued

  • Installation:
    • Waterproofing the Roof‐Deck:

• A monolithic membrane (single layer of stone) composed of rubberized asphalt is applied as a hot liquid, creating the waterproof layer of the green roof (applied directly to the roof‐decking and so existing roofing must be completely removed)

• Bituminous protective sheet is placed over the monolithic membrane

• Thermoplastic sheet membranes are act as a vapor barrier and insulation layer

• High density polyethylene is placed over the thermoplastic sheet membrane

• A thin plastic sheet is placed over the entire waterproofing layer to protect it against disturbance from plant roots

  • Moisture Retention and Drainage:

• Recycled polyethylene components (they look like egg cartons) create drainage space for surplus stormwater and provide water for plants during dry periods

• Depth of the retention and drainage layer depends on the average quantity of stormwater entering the green roof and the load‐bearing capacity of the roof‐decking

• A geosynthetic mat is applied to protect the drainage/retention layer from soil


  • Green Roofs Continued
    • Soil:

• Green roof soil mixes are lighter than normal planting soil mix

• Usually consist of 75% mineral and 25% organic material

• Must provide appropriate oxygen, nutrient, pH, and moisture levels for plants

• Soil layer will range from 2‐4 inches

  • Plants:

• Plant choices for green roofs are restricted by the roof microclimate including likelihood of high winds, low temperatures (due to a lack of ambient temperature provided by the earth), and drought conditions

• The plants chosen should be characteristic of subalpine to alpine biomes including perennial grasses, wild flowers, and sedums, all of which have shallow root systems and do not grow over a foot high

  • Considerations:
    • Load‐bearing capacity of the roof is extremely important; consider consulting a structural engineer before deciding to install a green roof
      • Flat roofs or those with a slope less than 1.5% are easiest to install and less complex
      • Roofs with steeper slopes usually require cross‐battens to hold down drainage/retention system in place, as well as more erosion control
      • The maximum slope of a green roof is about 25%
      • Consider erosion mats to protect from wind uplift around corners and perimeters of roof
      • Non‐vegetated materials such as pavers or stone must be placed around all roof openings to protect against fire hazards
      • Careful waterproofing is essential
      • Outline roof outlets before installing the protection layer so that the root barrier and protection can be cut to match the outlets
      • Weed twice annually and remove any plants taller than a foot


  • Green Roofs Continued



  • Artful Stormwater Management
    • Artful stormwater management emphasizes the integration of artistic design into the creation of stormwater management facilities. The concept is to create stormwater management facilities that are also places where people can enjoy and appreciate part of the hydrologic cycle.

  • Benefits:
    • Meets the objectives of conventional stormwater management including both reduction of stormwater runoff volume and improvement of runoff water quality, along with the promotion of groundwater recharge
      • Provides an amenity to a property (aesthetic, educational, functional)
      • Increases the value of a property and the value of the human experience


  • Artful Stormwater Management Continued