Written collaboratively by John McMaine and Farhana Akhter.
Hydrology is the study of the movement, distribution, quantity, and quality of water. Hydrology considers every form of water (liquid, solid and gaseous) and their movement through earth’s surface and atmosphere.
The water movement on and below the earth surface and in the earth atmosphere can be explained with a cycle called hydrologic cycle (Figure 1). Water evaporates from a water body (changes from liquid to gas due to solar energy), condenses into clouds (changes from gas back into a liquid), and then returns to the earth as precipitation (e.g. rain, snow, sleet). In an undisturbed system, about 40% of precipitation becomes evapotranspiration, 50% of precipitation is infiltrated, and 10% of precipitation becomes runoff (Figure 2).
Evapotranspiration is made up of two components, evaporation and transpiration, water taken up by plants and released to the atmosphere. Infiltrated water moves both downward and laterally to either recharge aquifers or return to a surface water body. Excess precipitation that flows across the earth surface is called surface runoff.
Figure 1. Hydrologic cycle.
When urban areas expand, it significantly alters the landscape and consequently the ydrologic cycle. Urban expansion increases the area occupied by impervious surfaces such as building roofs, and paved surfaces such as parking lots, sidewalks, and roads. Impervious surfaces do not allow water to infiltrate into the earth’s surface, which shifts the percentage of precipitation that becomes runoff from about 10% in an undisturbed setting to around 55% in a fully urbanized setting (Figure 2). Since impervious surfaces reduce infiltration from 50% in an undisturbed setting to only about 15% in an urban system (Figure 2), it also reduces the groundwater recharge and thus lowers the groundwater table. Due to the increase in surface runoff, frequency of flooding increases and excessive streambank erosion occurs (Figure 3). Land use conversion from vegetation to infrastructure can also decrease transpiration. New pollutants are introduced in urban systems including nutrients from fertilizers, sediments from construction sites and stream bank erosion, metals from rooftops and roadways, pesticides from lawns and recreational area, and pathogens from leaky sewers and pet waste.
Figure 3. Stream bank erosion and channel incision are common in urbanizing landscapes. (Adapted from “Managing Stormwater Using Low Impact Development (LID) Techniques” by C.T. Agouridis and J.T.McMaine)
To address issues related to runoff, various forms of stormwater management are used. Conventional stormwater management is focused on flood control through conveyance and peak flow reduction through detention and controlled release. Detention and retention basins hold runoff and release water at a controlled rate that is less than or equal to the pre-development peak flow. Curb and gutter systems are used to collect runoff from parking lots, streets, and then convey flow to a storm sewer. The storm sewer conveys the untreated stormwater runoff to a stream or other water body. While conventional stormwater management using detention and retention does decrease peak flows at specific points in the watershed, total runoff volume is not decreased and streams receiving the excess volume undergo streambank erosion, downcutting, and habitat alteration.
Figure 4. Effect of Urbanization on the hydrograph. (Adapted from Design of Urban Stormwater Controls (p.15) by Design of Urban Stormwater Controls Task Force of the WEF, ASCE and EWRI).
Low Impact Development
Low impact development (LID), also referred to as Green Infrastructure (GI) is a stormwater management philosophy that reduces runoff volume, peak flow, and pollutants by increasing infiltration to with the overall goal of restoring pre-development hydrology. LID achieves this through structural and non-structural best management practices (BMPs). Non-structural BMPs include minimizing and disconnecting impervious areas and preserving hydrologically sensitive areas e.g. riparian buffers, wetlands and highly permeable soils. Structural BMPs include bioretention (also called rain gardens), pervious pavement, green roofs, and rainwater harvesting. Structural BMPs are spread throughout the watershed to manage runoff close to where it is generated.
As urban areas continue to expand all over the world, stormwater management is a critical issue. As impervious area increases across an urban or urbanizing watershed, infiltration and evapotranspiration decreases which significantly alters streamflow and has negative environmental consequences. Conventional stormwater management using conveyance, detention, and controlled release emphasizes flood management and peak flow reduction, however total water volume is not decreased, and water quality is negatively impacted. LID manages runoff through increasing infiltration, reducing runoff volume, and increasing evapotranspiration by restoring pre-development hydrology. LID techniques can reduce the effects of urbanization on water resources and can minimize the burden on existing stormwater infrastructure.