Stormwater Thermal Mitigation

Overview

Streams draining urban areas are often much warmer than those draining natural ones due to changes in surface cover and hydrology. Urbanization increases stream temperatures by decreasing riparian shading and replacing natural landscapes with hard, dark-coloured pavements and roofs that absorb and store heat from the sun. The added impervious cover increases the volume of heated runoff while at the same time reducing discharge of cool groundwater to streams. This heating effect is further exacerbated as runoff flows through stormwater management ponds or other impoundments, where detained water is exposed to solar warming for extended time periods between rain events. This page explores different techniques for mitigating the effects of urbanization on the stream thermal regime.

Thermal Load

Since stream warming is influenced by the runoff temperature and volume of runoff draining to streams, impacts are best assessed through an evaluation of thermal loads both in the stream and in runoff discharged to streams. The thermal load is a function of the flow rate, water temperature, water density and heat capacity of water (or the energy required to increase a kg of water by 1 degree C).

Thermal Load = Q x ρ x T x C

Where:

• Q = flow rate (m³/s)
• ρ = water density (1000kg/m³)
• T = water temperature (°C)
• C = heat capacity of water (4187J/kg°C)

Since urban runoff volumes often increase by 2 to 5 times after development, and stormwater pond effluent temperatures are between 4 and 11°C warmer than pond influent temperatures in the summer, the overall thermal load increases to streams can be very significant (Van Seters et al. 2019).

Temperature Target Selection

Setting a stream temperature threshold for the protection of aquatic life can be based on:

1. Matching the water temperature regime of either a known pre-development thermal condition or the thermal condition of an undisturbed stream with similar characteristics; and,

1. Meeting the upper tolerance of one or more target aquatic species.

The changing of land cover and land uses from rural to urban; the growth of urban settlements.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The changing of land cover and land uses from rural to urban; the growth of urban settlements.

A body of water smaller than a lake, often artificially formed.

refers to the characteristics and functions of a system prior to urban development.

A body of water smaller than a lake, often artificially formed.