Green Infrastructure Infiltration Increases Urban Flood Risks? GI Benefits in New Development Can Worsen Existing Infrastructure Stresses in Old Developments.

Cincinnati Bioswale
There is no doubt that Green Infrastructure (GI) or Low Impact Development (LID) measures are an essential tool in a water manager or municipal engineer's modern tool kit. In new developments, distributed green infrastructure, and even more centralized features, including rain gardens, bioswales, perforated pipes or pavers and infiltration galleries can help manage typical water balance impacts of urbanization by reducing downstream erosion stresses, sustaining natural heritage features like wetlands and streams that are often home to sensitive aquatic and terrestrial species. GI and LID measures do this largely by infiltrating urban runoff from hardened impermeable surfaces like rooftops and roadways or parking lots into the ground (see Ontario Ministry of Environment and Climate Change Interpretation Bulletin, February 2015). In some areas of the province that rely on groundwater for municipal water supply, these measures may also help sustain source water quantity and make aquifers more resilient to climate variability, especially long term droughts (e.g., see page 159, Policy ID REC-1 in APPROVED SOURCE PROTECTION PLAN: CTC Source Protection Region, 2015).


In existing development areas, especially those built before the 1980's, infiltrating water into the ground as part of a GI or LID retrofit can have a significant downside for old muncipal infrastructure and private properties. That is because older infrastructure is sensitive to groundwater levels that drives infiltrated runoff into utility trenches, thorough cracks between joints of municipal sanitary sewers and service laterals, and into 1000's of kilometers of foundation drains surrounding properties with basements - drains that in many older areas connect to the sanitary sewer and ultimately drain to the municipal wastewater treatment plant.

This post explores how GI and LID infiltration stresses in old development areas can affect flooding risks, sewer overflow risks and watewater treatment costs in Ontario cities.

The risks from infiltration on wastewater systems is well-known. The Ontario Municipal Knowledge Network (OMKN) has highlighted the cost of treating infiltrated water and inflows in its 2008 General Inflow & Infiltration Management Practices - Best Practices Summary Report, also noting the first challenge related to Inflow and Infiltration (I&I or I/I) management is: "Protecting customers from basement flooding". Other concerns are "Increased flow to wastewater treatment plants and increased operating costs at the plants due to the excess volume of water requiring treatment". The OMNK's Best Practices Report Inflow and Infiltration - Increasing System Knowledge Through Flow Monitoring notes the drivers for managing I&I in Peel Region:

"There have been two catalysts for the Region’s I/I programs and studies: exceedance of system and plant capacities leading to sewer overflow and basement flooding occurrences"

More recently, infiltration has been noted as a concern by the insurance industry in terms of the lost wastewater system capacity and the impacts on basement flooding and insurance losses / damages. The Institute for Catastrophic Loss Reduction's investigations have recently indicated that " I/I directly contributes to flooding by filling up pipes with water, using up capacity that could convey larger storms".

And the potential impacts of GI and LID measures on wastewater system infiltration in Ontario was recognized ages ago -the former Ministry of the Environment conducted a Workshop on Stormwater Quality Best Management Practices in 1992 following the introduction of on-site infiltration source control LIDs called Best Management Practices (BMPs). The comprehensive Workshop Summary prepared by Marshall Macklin Monaghan Limited identifies concerns with on-site infiltration measures and these included:
“- basement leakage problems related to infiltration near housing
- surcharging of sanitary sewers by short circuiting of infiltrated water”

Lets look at quantifiable impact of groundwater infiltration on wastewater systems. For background, let's notes that groundwater infiltration is classified into two types in the sanitary sewer design profession:
  • Groundwater Infiltration (GWI)
  • Rainfall Dependent Inflow and Infiltration (RDII)
Green Infrastructure GI Low Impact Development LID Urban Basement Flooding Sewer Back-up Flood Risk Infiltration Stress
Groundwater Infiltration , Rainfall Dependent Inflow and Infiltration,
and other wastewater flow components to assess GI and LID Impacts.
From "An Approach for Estimating Groundwater Infiltration Rates
into Wastewater Collection Systems under Typical Year Conditions",
Zhang et. al. 2013
GWI is considered one component of Dry Weather Flow (DWF) which also includes Base Wastewater Flow (BWF). The hydrographs at right show these components of wet weather flow and are from An Approach for Estimating Groundwater Infiltration Rates into
under Typical Year Conditions by Li Zhang, Fang Cheng, Robert Herr, Gregory Barden, Hunter Kelly and Edward Burgess in " Journal of Water Management Modeling, R246-21. doi: 10.14796/JWMM.R246-21

Many GI and LID features are designed to get rain runoff or snow melt into the ground. The benefit in new development areas is that these practices sustain aquifer levels that in turn preserve baseflow rates to creeks (i.e., 'environmental flows') that sustain natural features. But the disbenefit in old areas is that raising groundwater levels raises GWI, using up sanitary sewer capacity that is no longer available during a big storm. As a result, when GWI goes up, so does basement flooding / sewer back-up risk because the previous wastewater system conveyance capacity is no longer available.

Green Infrastructure Basement Flooding
Groundwater Infiltration GWI Increases with Higher Precipitation.
From "An Approach for Estimating Groundwater Infiltration Rates
into Wastewater Collection Systems under Typical Year Conditions",
Zhang et. al. 2013
Can we quantify how infiltration affects GWI? Yes - using climate records and monitored wastewater flow rates, studies have correlated GWI rates to the amount of precipitation. At right, GWI in Cincinnati shows a strong correlation to precipitation on an annual basis. More precipitation means more infiltration, meaning higher GWI.

Low Impact Development Groundwater and Flooding Impacts
March Groundwater Infiltration GWI Increases Correlated
with Previous 15 Day Precipitation Total.
From "An Approach for Estimating Groundwater Infiltration Rates 
into Wastewater Collection Systems under Typical Year Conditions",
Zhang et. al. 2013
Seasonal influences of precipitation on GWI have also been found, with the highest spring GWI rates affected by precipitation over the previous 15 days. Zhang et. al's analysis in Cincinnati shows that if the previous 15 days were dry (no rain) the GWI flow at the plant was below 60 MGD, while with 2 inches of precipitation (about 50 mm), the GWI rate increased to about 90 MGD, a 50% increase.

Zhang et. al concluded "Significant positive
linear relationships were found between GWI and precipitation both annually
and monthly. The annual relationship showed that the adjusted R2 of the
regression result is 0.78, indicating that 78% of the varation of the yearly average
WWTP GWI can be explained by the annual precipitation."

LID and GI infiltration impacts wastewater systems and basement back-up risk
LID Runoff Reduction Benefits Can Lead to Groundwater Infiltration Stresses
In old developments with no GI or LID, limited precipitation infiltrates. It generally just runs off, collected in storm sewer systems. So only a small fraction of runoff is infiltrated. Monitoring by Credit Valley Conservation of the Elm Drive bioswale LID show the change in runoff when LIDs are retrofitted, indicating "69% of all rainfall is detained and infiltrated" in one study. The graph at right illustrates the runoff reduction benefit. Some detained runoff is evaporated or transpired by soil and vegetation in the LID feature, but some is infiltrated into the ground where it contributes to GWI and a portion of RDII. 

If half the runoff captured in the LID infiltrates, that would be equivalent to 35 % of rain infiltrating, as opposed to about 10% or less without the LID measure (i.e., 25% more infiltration). In the Toronto area, considering about 700 mm of rainfall a year, that would be like adding 25% x 700 mm = 175 mm of water into the ground each year (about 7 inches). In Cincinnati, 7 inches of annual precipitation increased GWI by 20% - that is before any runoff transformation or evaporation losses. So adding 7 inches of water directly into the ground with an LID would have an even more pronounced impact on GWI, with potentially more than a 20% increase. Added to this long term, slow GWI response impact, would also be short duration RDII increases that even further reduce wastewater system capacity as the extreme events infiltrate into LID measures as well. Practitioners know that even fully separated wastewater systems where inflow sources have been addressed can have large RDII components, suggesting that infiltration can have a fast response as well, taking away peak flow capacity. 

The take-away is that what is a benefit to new developments is not typically one in old developments, across cities with existing GWI and RDII stresses and resulting basement flooding risks and treatment costs that can be made worse by infiltrating rain the previously was runoff.

Any positive impacts to wastewater systems with GI and LID measures? Yes, potentially a few in some isolated areas.

Consider existing infrastructure
impacts when evaluating green
infrastructure and low impact
development measures effects on
groundwater, wastewater
system capacity and operating costs.
There area where GI and LID may contribute to positive wastewater system performance is in combined sewer areas. In those areas, holding back runoff in GI and LID measures that would enter combined sewers could reduce combined sewer overflows (CSO's) - but not all Ontario cities have combined sewers. In Toronto, only 23% of the systems are combined. Also, GI can be expensive compared to other technologies (about $400,000 per hectare in capital costs, or about $1.3M-1.5M per kilometre of retrofitted roadway). An often cities have F-5-5 control strategies and operation improvements that make GI and LID implementation redundant. For example in Toronto, wastewater and stormwater runoff from combined sewer areas will collected and fully treated as part of projects needed for operational improvements (e.g., to regularly maintain and bypass the main Coxwell Ave. sewer) - so adding LID and GI measures on top of other infrastructure projects that will already virtually eliminate CSO's would appear to be redundant, with high incremental added costs and limited marginal benefits.

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