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Engineering Aspects of LID Design
Mark Peterson, PE | Montana Stormwater Conference | May 2018
Overview
• LID Concept
• LID Examples
• Hydraulic Design
• How water enters the feature
• How water is stored
• How water leaves the feature
• LID Impacts on Total Runoff
• LID Impacts on Peak Runoff
Overview of LID
• LID Design:
• Stormwater viewed as a resource to be conserved or used rather than a waste product
• Promotes infiltration, to reduce runoff volume (primary purpose) and runoff peak flows (secondary purpose)
• LID features infiltrate large amounts of runoff, in total, but may not be significant during large rainfall events
• Reduces volume of pollutants that run off any site
LID Practices
• LID Design:
• Stormwater viewed as a resource to be conserved or used rather than a waste product
• Promotes infiltration, to reduce runoff volume (primary purpose) and runoff peak flows (secondary purpose)
• LID features infiltrate large amounts of runoff, in total, but may not be significant during large rainfall events
• Reduces volume of pollutants that run off any site
LID Examples
Bio-Retention Cell Along Street
• Many LID features use curb opening to direct water
off paved areas
Bio-Retention Cell Along Street
• In a residential area, they can be incorporated with
traffic calming feature (narrowing streets)
Bio-Retention Cell in Parking Lot
• In a parking lot, they can be used in median areas
Pervious Grid Pavers
Pervious Grid Pavers
• Allowing silts and clays, or vegetation in the spaces
could reduce infiltration substantially, and that surface
becomes the limiting layer.
Hydraulic Design
Hydraulic Design
• LID Features are hydraulically a storm water detention pond – water enters, water is stored and water exits.
• In order to determine the impact of an LID feature, it needs to be modeled, just like any other drainage feature
• How water enters an LID Feature
• Curb Openings (bio-retention cells)
• Need to be properly designed. Curb openings are very inefficient on continuous grades.
• Direct Surface Runoff (pervious pavement, parking lots without curbs, green roofs)
• Roof downspouts (rain barrels, rain gardens)
Hydraulic Design
• How water is stored in an LID Feature
• Surface Storage (bio-retention cells or rain barrels) – generally shallow depths (6-12 inches)
• Storage in the pore spaces of the soil/aggregate
• Normal gravels and normal compaction effort during construction substantially reduces the pore spaces
Hydraulic Design
• How water exits an LID Feature
• Infiltration into the soils below – most desirable
• Underdrain System – if subsurface soils are not very permeable – this can still provide some treatment
• Overflow – if all runoff is directed to the LID feature, excess water has to have a way to leave the feature.
• LID Features can be modeled just like a detention pond. If the feature is intended to reduce peak flows, it should be incorporated into the overall storm water runoff model.
Hydraulic Design
• The key component to a properly functioning LID feature is usually infiltration. Normal (or unintended) compaction can have major impacts.
• Field measurements of the actual infiltration are generally considered crucial (just like a drainfield).
LID Example for Peak Flow Reduction
Rainfall Event
• Basic concept of LID features – store and infiltrate the 90% event
• 90% event includes 90% of all rainfall events (usually considered to be daily rainfall events)
• 90% rainfall event is calculated differently from events like the 100-year event.
City 90% Event Years of Record
Helena 0.31” 79
Kalispell 0.31” 118
Billings 0.37” 70
Great Falls 0.38” 81
Bozeman 0.33” 77
Missoula 0.28” 70
Rainfall
• More data is generally available, ranging from 70 to 118 years in the six major cities.
• 90% event ranges from 0.28 inches to 0.38 inches of rain in 24 hours
• MS4 permit requires retention of an event of 0.50 inches in 24 hours
• Based on Helena’s design standards, the 10-year rainfall event is 1.9 inches in 24 hours. 100-year event is 2.9 inches in 24 hours. The 90% event is 0.31 inches in 24 hours.
Peak Flow Reduction
• To illustrate the impact of a bioretention cell on peak flow reduction:
• LID feature for a 0.25-acre drainage basin in Helena, 100% impervious
• In order to store/infiltrate the 0.5-inch event (NRCS Type 2 distribution), a bioretention cell 5 feet wide and 60 feet long is required (based on an assumed infiltration rate of 6 inches per hour). This is about 3% of the total drainage area.
• LID feature will store/infiltrate the entire 0.5-inch event – about 420 cubic feet of runoff in this case
Peak Flow Reduction
• At the water quality event (0.5 inches in 24 hours), the peak flow is 0.13 cfs.
• At the 10-year event (1.9 inches in 24 hours), the peak flow is 0.53 cfs.
0
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Time, hour
Runoff
0.5 inch inflow
10-year inflow
Peak Flow Reduction
• 10-year peak flow that bypasses the bioretention cell is ~ 0.43 cfs, assuming all water gets into LID feature
• LID feature reduces the peak flow from 0.53 cfs to 0.43 cfs, or about 0.10 cfs (~19%)
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Ove
rflo
w, c
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Time, hours
Overflow
0.5 inch inflow
10-year inflow
10-year Overflow
Volume Reduction
• LID feature will store/infiltrate the entire 0.5-inch event –about 420 cubic feet of runoff in this case
• Total infiltration into the bioretention cell during the 10-year event is ~ 1220 cubic feet (almost triple the infiltration from the 0.5-inch event)
• Bypass volume during the 10-year event is ~ 460 cubic feet – about 29% of the total volume.
• LID feature reduces the 10-year peak flow by about 19%, but reduces the 10-year total volume by about 71%.
Flow Reduction
• This analysis is a single location with specific parameters. Every site is going to be unique. The only way to determine the impacts on peak flow and volume is to model the system.
• Modeling is easily done using stormwater models such as SWMM.
• An LID feature that is designed for the 90% event does NOT reduce the peak flow by 90% (not even close).
• An LID feature that will reduce peak flows by an amount similar to a normal detention pond needs a volume similar to a normal detention pond.
LID Pollutant Removal
Pollutant Removal
• One of the advantages of LID features is that they can provide pollutant removal.
• Pollutants of concern include:
• Suspended solids (sediment)
• Heavy metals (lead, zinc, copper, cadmium)
• Nutrients (nitrogen and phosphorous)
• Organics (oil and grease, hydrocarbons, pesticides)
• Floatable trash and debris
Pollutant Removal
• LID features have varying degrees of success with removals of these pollutants
• Numerous studies have been done on LID features. They tend to be very site specific, and going from these studies to detailed design guidance is a big step.
• General observations about removal can provide some insight.
Pollutant Removal
• Infiltration will generally reduce velocities enough to promote a high degree of settling of solids
• Because heavy metals are often attached to the solids, infiltration also promotes a high degree of removal
• Removal by settling could be only temporary if particles can be re-suspended or if no maintenance is performed.
• Removal of nutrients and organics can be much more challenging. Usually requires a reaction with soils, and removal rates can be quite variable.
• Removal of trash and debris pretty straightforward.
• In general, the more water we can infiltrate, the less pollutants we discharge, either to receiving streams or to groundwater.
Reducing CSO
• In many cities in the US the sanitary sewer system and the storm sewer system are the same system. These are called combined sewers.
• In these systems, when there is significant rain, the sewer treatment system is overwhelmed, and raw or partially treated sewage is discharged to a natural water body. These are called Combined Sewer Overflows.
• Use of LID features can reduce the frequency of CSOs.
• In these situations, LID features can be very cost effective.
• When reviewing studies that tout the cost effectiveness of LID features, reducing CSOs is often a large benefit.
Montana Conclusions
• Most LID features hydraulically function as detention ponds.
• LID features can (and should) be modeled to determine hydraulic impacts.
• Significant reduction in total volume discharged can be achieved.
• Reduction in peak flows are often relatively small.
