The city is built on dust. During the last ice age, glaciers ground rock into a fine flour, and wind lifted that silt off mud flats left by glacial meltwater and piled it along the bluffs, more than 90 feet deep in places in Iowa's Loess Hills. The Omaha Regional Stormwater Design Manual says soils within the city are predominantly the Monona-Ida association, and both of those series formed in loess. Most driveways in town sit on it.
Loess makes an odd foundation. It stands in steep banks for years when dry, yet people in the Loess Hills call it sugar clay because it loses its cohesion when soaked. It drains well, but it erodes readily, and it is the textbook frost-heaving soil. This guide follows water through a driveway on that ground: where it gets in, what it does to silt, why the river bottoms behave differently, and what a property owner can do about it.
What loess is, and how much of the city sits on it
The USDA's official series descriptions are specific. Monona soils are very deep and well drained, formed in loess on the ridges and side slopes of loess hills, on slopes from nearly flat to 40 percent, with 20 to 27 percent clay and less than 10 percent sand. Ida soils formed in calcareous loess on slopes from 2 to 60 percent, with carbonates within 10 inches of the surface and 18 to 25 percent clay. Both were first described in Monona County, Iowa, in the 1950s, and both occur in eastern Nebraska along the Missouri.
The particles explain the behavior. Loess grains typically measure 20 to 50 micrometers, silt-sized and angular, and a deposit is laced with fine vertical channels that let it fracture into bluffs. Angular grains lock together well enough that a cut in loess can stand in a steep bank for years without slumping. That same open, silty structure is what lets water move through it easily, and what makes it vulnerable once water is moving across it or collecting in it.
The manual places the Monona-Ida association in hydrologic soil group B, one of the four groups, A through D, used in its runoff tables. Pavement overrides that rating. The same manual assigns driveways, roofs and paved parking lots a runoff curve number of 98 in every soil group, the value for surfaces that shed nearly everything, so each paved square foot in the city sends its rain onto something else, usually the soil at its edge.
Firm when dry, weak when wet, and quick to heave
Water is what changes loess. The Loess Hills nickname, sugar clay, describes ground that is extremely hard when dry and loses all cohesion when wet, and the hills' cat-step terraces are the visible result of slumping and shearing. Under a driveway the same thing happens on a small scale: base soil that has been dry and firm for years can soften along an edge, or around a crack, in one wet spring.
Freezing adds a second failure. Frost heave needs three things, a frost-susceptible soil, a supply of water and freezing temperatures that reach into the ground, and silt is the classic frost-susceptible soil because its fine particles draw water toward the freezing front. Ice lenses grow, lift the pavement unevenly and crack it, which is how the spring pothole season starts. Eppley Airfield's normals give that process 96.7 freeze-thaw days a year, 50.8 of them in December through February, plus 62.5 nights at 20 degrees or colder.
The two effects compound. Water that enters in the fall freezes, lifts the pavement and cracks it; the thaw leaves the silt below wet and weak just as spring traffic and spring rain arrive. That is why damage that started months earlier tends to show up in March and April as a sunken edge, a new crack across the driveway or a pothole.
Where water gets under a driveway here
Runoff is the delivery system. The design manual sizes local drainage for 3.0 inches of rain in 24 hours as the two-year storm, 4.6 inches as the 10-year storm and 6.7 inches as the 100-year storm. Eppley averages 7.2 days a year with an inch or more of rain, and 4.4 of those fall between May and August. With a curve number of 98, almost all of that water leaves the driveway somewhere, and on loess the somewhere matters.
Three places deserve the closest look. Edges on sloped lots, where water runs along the side of the asphalt and carries silt away until the edge has nothing under it. Downspouts that discharge onto or beside the driveway, concentrating a whole roof's runoff on one patch of soil. And open cracks, which SealMaster describes as the path by which water erodes base material until potholes form. Monona soils occur on slopes up to 40 percent and Ida on slopes up to 60, so sloped driveways are common.
Winter adds a slower version of the same problem. Snow piled at the low end or along the edge of a driveway melts on sunny afternoons and refreezes at night, feeding water into the same edges and cracks through the cold months. With 26.4 inches of normal snowfall and 8.8 days a year with an inch or more, that cycle repeats every winter at Eppley, and it is one more reason to pile snow on the lawn side rather than against the pavement.
River bottoms and old channels behave differently
Not all of the metro is upland loess. Eppley Airfield itself sits on ground the Missouri abandoned: the river cut off an oxbow in an 1877 flood and left Carter Lake in its old course. Floodplain soils formed in water-laid alluvium rather than windblown silt. The Sarpy series, named for Sarpy County, is excessively drained sand with less than 10 percent silt and clay, rated rarely to frequently flooded between November and June. The Haynie series, calcareous alluvium, holds a saturated zone four to six feet down from April to June.
On those soils the concern shifts from surface erosion to groundwater and flooding. The 2019 Missouri River flood destroyed about 200 homes in Bellevue, and on soils like Haynie a saturated zone rises to within four to six feet of the surface in spring. Asphalt over sand drains quickly after a storm, but a base that has been under floodwater or soaked by high groundwater should be checked for soft spots before anyone seals over it.
What a property owner can do about it
Most of the fixes are about water. Extend or redirect downspouts so they discharge onto grass well away from the driveway edge. Keep a strip of turf along sloped edges so runoff spreads out instead of cutting a channel. Fill cracks every spring or fall, when air temperatures sit between 45 and 65 degrees and the cracks are open. Seal every two to three years so the surface itself sheds water. The crack filling page covers materials and timing, and the cost page covers prices.
Keep the soil on the property, too. Chapter 32 of the Omaha Municipal Code, the city's stormwater ordinance, requires an erosion control permit once ground disturbance passes one acre and expects smaller sites to minimize sediment loss; a driveway replacement falls in the second group. Storm inlets carry runoff untreated to the Papillion Creek system and the Missouri and Elkhorn rivers, so silt swept or hosed into the street ends up in those streams. The city also asks residents not to de-ice driveways with salt or sand.
When the base has failed, the answer is excavation, not sealer. A new driveway over loess should start with a compacted aggregate base, which HomeGuide puts at 6 to 8 inches under 2 to 3 inches of asphalt for residential use, with water directed off the downhill end. Nebraska requires one-call notice at least two full business days before digging. The asphalt paving page walks through the base, the permits and the wait before the first seal.
Describe the driveway and where its water goes, and independent Omaha contractors can price the fix.
Quick Answers
Why does loess erode so quickly next to a driveway?
Because the driveway concentrates water on it. A paved surface sheds nearly all of its rain, with a runoff curve number of 98 in the city's design manual, and that water leaves along the edges or the low end. Loess is silt that stands firm when dry but loses its cohesion when soaked and erodes readily, so a steady sheet of runoff along an edge carries it away faster than the same rain falling on turf.
Is loess the reason the city has so many potholes?
It is one of the ingredients. Frost heave needs a frost-susceptible soil, a supply of water and freezing temperatures, and silty loess is the textbook frost-susceptible soil. Eppley Airfield's normals add 96.7 freeze-thaw days a year. Put water into that silt through a crack and each cycle can lift and crack the pavement a little more, which is how spring potholes form. Keeping water out of the base breaks the chain.
Should downspouts ever drain onto a driveway?
It is better if they do not. A downspout concentrates a roof's runoff on one small area, and on loess that area is where erosion and softening start. Extensions that carry the water onto turf well away from the pavement edge, or a splash block that turns it toward the lawn, keep the base under the asphalt drier. Check them after heavy storms, since Eppley averages 7.2 days a year with an inch or more of rain.
Do floodplain lots need a different approach from upland lots?
The priorities shift. On upland loess, the main threats are surface runoff and erosion along edges. On river-bottom sand such as the Sarpy series, water drains through quickly, but the ground can flood or hold a high water table in spring, so the concern is a base that has been saturated from below. After high water, walk the driveway for soft or sunken spots and have them repaired before any sealing.
Free Omaha Quote
Free quote