Assessing thermal impacts of stormwater BMPs

W.R. Herb and O. Mohseni

While stormwater BMPs are designed to remove pollutants from stormwater runoff, they may increase the runoff temperature and adversely affect receiving cold water streams and the habitats of cold water fish species, such as trout. Wet ponds are an example of stormwater BMPs with the potential to thermally impact nearby trout streams. To assess the thermal impact of stormwater BMPs on stormwater runoff, the temperature of influent into and effluent from the stormwater BMP should be monitored.

The impact of a stormwater discharge on stream temperature (Figure 1) depends on both discharge rate and temperature of the runoff:

where:

• ΔT is the change in stream temperature due to the stormwater inflow
• Qs = stream discharge upstream of stormwater BMP outfall
• Tsu = stream temperature upstream of stormwater BMP outfall
• Tsd = stream temperature upstream of stormwater BMP outfall
• Qd = runoff discharge
• Td = temperature of runoff

There are a number of strategies for assessing the temperature impact of a stormwater BMP, three of which are listed below. The temperature parameters discussed below are shown in Figure 1.

Method 1) Measure the temperature of the BMP discharge (Td), and determine if the discharge is exceeding an effluent temperature standard. Although this is the simplest approach, measuring Td alone is not sufficient to determine the stream temperature impact (ΔT) of the effluent.

Method 2) Measure the temperature of the stream upstream and downstream of the stormwater outfall, and find the difference in temperature ΔT=(T_sd–T_su). A temperature probe downstream of the outfall should be placed downstream of the mixing zone. The length of the mixing zone depends upon the base flow and width of the stream, as well as the stormwater BMP effluent discharge (Gulliver 2007). Assessing thermal impacts of a stormwater BMP using in-stream measurements alone is only recommended for creeks and small streams, because estimating the mixing zone length for large streams during all flow conditions may be challenging.

Method 3) Measure the stormwater discharge rate and temperature in and out of the BMP (Q_u, T_u, Q_d, T_d), and the stream discharge and temperature upstream of the outfall (T_su, Q_su). With this information, the impact of the stormwater on stream temperature can be calculated with the BMP in place (ΔT₁) using Equation E2. In addition, the impact the stormwater would have on stream temperature without the BMP (ΔT₂) can also be calculated (Equation E3). By comparing ΔT₁ and ΔT₂, it can be determined if the BMP increases or decreases stream temperature impact.

General Considerations for Temperature Measurements

Prior to monitoring, the temperature probes need to be calibrated against a NIST (National Institute of Standards and Technology) traceable thermometer, or against 0 °C (32°F) temperature by placing the probe in a mix of ice and water. Probes should be placed in shaded areas of the sewer pipes or channels wherever possible, to avoid solar heating of the probe. It is recommended the probe be placed inside a PVC pipe anchored to the sewer to protect it from debris, as shown in figure 2.

To measure the water temperature in a stream or creek, the probe should be installed at least a few inches above the streambed and attached to stakes that are inserted securely into the streambed. It is not recommended to install the probe directly on, or buried in, the sediment bed, because the sediment is often a different temperature than the stream water due to groundwater inputs. Most streams are well-mixed water bodies, and the temperature near or above the sediment surface is representative of the entire water column temperature. For shallow streams (less than 8 inches deep), it is recommended that the temperature probe be installed in a shaded area of the stream channel to avoid direct solar radiation affecting the temperature measurement of the probe.

A measured difference in stream temperature (T_sd - T_su) may be due to atmospheric heating or surface inflow. During hot summer days, solar radiation can heat the stream such that the water temperature at the downstream point (T_sd) becomes warmer than at the upstream point (T_su). The temperature difference varies diurnally and depends upon the solar radiation received, the distance between the upstream and downstream measurement locations, stream discharge, and stream geometry. During storm events and for several hours after, inflow of surface runoff directly into the stream may have a significant impact on the temperature difference (T_sd- T_su). The thermal impact of surface inflow may be identified as transient change in the temperature difference (see figure 3).

General Considerations for Discharge Measurements

Flow rates in a sewer pipe (e.g. Q_u, Q_d) can be measured using pressure transducers, as discussed in Water Budget Measurement. Alternatively, inflows and outflow rates to a pond can be estimated by continuously measuring the pond level, and relating the rate of change of pond level to flow using the pond bathymetry (Herb et al. 2006).

The temperature and discharge data collection should be conducted near-continuously during the warmer months of the year; e.g., from the end of May until the end of August in the northern USA. The recording time steps should be selected based on the capacity of the data storage system and the frequency of data retrieval. It is recommended that a small time step (e.g., one measurement per 5 or 10 minutes) is used to measure stormwater BMP influent. A longer time step (e.g., one measurement per 30 minutes) can be used for measuring temperature and discharge downstream of the stormwater BMP because discharge from wet ponds, infiltration ponds, and constructed wetlands often occurs over longer time periods. One measurement per 15-30 minutes can also be used for stream temperature measurement because changes in stream temperature often occur over longer time periods.

Temperature Impacts of Infiltration Ponds

Assessing the impacts of an infiltration pond constructed near a cold water stream can be a challenging task, and in many cases, inconclusive. In general, infiltration ponds with reasonable setback distances from the stream channel, e.g. at least 30 m, would not be likely to measurably impact stream temperature. An exception to this may be ponds associated with gravel mines, where 1) large quantities of water infiltrate and 2) subsurface flow velocities are high.

References

Gulliver, J.S. 2007. Introduction to Chemical Transport in the Environment. Cambridge University Press, Cambridge, UK.

Herb, W.R., Weiss, M., Mohseni, O., and H.G. Stefan, 2006. Hydrothermal Simulation of a Storm Water Detention Pond or Infiltration Basin, St. Anthony Falls Laboratory Report 479, 34 pp.

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