Engineering Aspects of LID Design€¦ · Mark Peterson, PE | Montana Stormwater Conference | May 2018 . Overview •LID Concept •LID Examples •Hydraulic Design •How water enters

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.

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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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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.

QUESTIONS?

Mark Peterson, PE | [email protected]

Montana Stormwater Conference | May 2018