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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />EVENT PROBABILITIES: Unknown. Further research is needed to determine trends and probabilities of <br />future deep soil frost events in Hennepin County. <br />4.1.3.9. Indications and Forecasting <br />Additional study is needed to develop deep soil frost event models and forecasts for Hennepin County. <br />Adequate weather forecasting already exists and would certainly be a major factor in any future soil frost <br />forecasts. Better data on the behavior of frost in local soils under various temperature, surface material, <br />soil moisture and snow cover conditions is required to develop models and forecasts. Hennepin -West <br />Mesonet data will provide much of the needed information. <br />4.1.3.10. Detection & Warning <br />In 2015, following the disruptive winter of 2013-2014 when hundreds of water service lines were frozen, <br />Hennepin County Emergency Management (HCEM) began to install a network of manually read frost tubes <br />at locations around Hennepin County. When possible, two frost tubes were installed at the same site. <br />One tube was for measuring frost depth under sod, and the other for frost depth under pavement because <br />of the significant differences between the two. Frost tubes are usually located near a Hennepin -West <br />Mesonet sensor station so that weather factors can be compared to the frost depth at the site. The <br />measurements, taken at least weekly, can provide indications that the frost is pushing deeper than normal <br />and is beginning to threaten water and sewer services, fire protection capabilities, and other vital services. <br />When appropriate, HCEM will send out alerts to public works officials that frost may threaten their water <br />and sewer infrastructure. <br />4.1.3.11. Critical Values and Thresholds <br />4.1.3.11.1. Air temperature: Air temperatures below freezing (32F/OC) are required to initiate <br />soil frost formation. A freezing index based on degree-days of freezing may be used to roughly <br />estimate frost depth potential in an area. <br />4.1.3.11.2. Pavement. Human -made surfaces, such as concrete or asphalt roadways create ideal <br />conditions for exceptionally deep frost penetration into soil. The differences between frost depth <br />under paved roads and frost depth under natural sod is large enough to produce a few feet of <br />difference at the same site. Therefore, measurements should specify of they are taken under <br />pavement or under sod. Factors such as the thermal conductivity of pavement and the removal <br />of snow cover combine to push frost deep into the underlying soils. This is important because a <br />lot of buried infrastructure is underneath immediately adjacent to roadways, increasing their <br />vulnerability to frost. <br />4.1.3.11.3. Surface albedo: Surface albedo is the ratio of irradiance of solar energy reflected to <br />the irradiance of solar energy absorbed by a surface. Asphalt, dark soils, turf grasses and forests <br />have low albedo. Snow cover, sand, and winter prairie grasses have higher albedo. The albedo <br />of the primary surface is important because it influences the snow cover characteristics of the <br />site. Snow cover is a central factor is controlling frost depth. <br />4.1.3.11.4. Soil type: Different soil types freeze at different rates. Frost tends to penetrate less in <br />clay (heavy textured) soils and more deeply in silty or sandy (lighter textured) soils. Inorganic soils <br />26 <br />