Saturday, October 17, 2026
The DownspoutSonoma CountySubscribe

Stormwater and drainageGuide

How to stop erosion on a slope before the winter rains: a Sonoma County guide

By The Downspout · Posted Oct 17, 2026 · 13 min read

In Santa Rosa, 99.5% of the 2024 to 2025 water year's rain fell from October to April. Before the storms, cover bare soil with 2 inches of straw or wood chips, lay wattles on the contour, and protect downspouts. Find your slope with the calculator below.

Listen to this article

Read aloud by an AI voice.

An independent guide. Not a government agency. For permits, call Permit Sonoma or your city.

The rains are coming, and bare ground on a slope is the first thing they take.

You know the spot. The patch where nothing grew this summer. The corner where the downspout dumps.

Most of the fix is low-tech. Start by finding how steep your slope is.

Lay a straight board level on the slope, then measure the gap from its downhill end to the ground.

Ranges from Sonoma RCD and UC Cooperative Extension. Wattle spacing from Caltrans. Straw from NRCS, one 75 lb bale per 800 sq ft. Information only.

The short answer

Why now

How erosion starts

A sense of scale: one inch of rain on 1,000 square feet of slope is about 623 gallons. A 2 inch storm is about 1,250 gallons, all looking for the easiest way down.

First, measure your slope

Slope in percent is rise over run, times 100.

Our method: lay a straight 4 foot board on the slope, pointing downhill. Put a level on it and lift the downhill end until it reads level. Measure the gap under that end in inches. Enter both numbers in the calculator at the top.

A 16 inch gap under a 48 inch board is 33%. Engineers write that as 3:1, three across for one down.

Which erosion fixes suit which slopeRange bars on a slope scale from 0 to 70 percent, with marks at 10, 25, 33 and 50 percent. Mulch alone suits slopes under 33 percent. Fiber rolls, or wattles, suit slopes under 40 percent. Plants suit slopes under 50 percent. Over 50 percent calls for walls, terraces or a licensed professional. Sources: Sonoma Resource Conservation District and UC Cooperative Extension.What works on how steep a slopeSlope in percent: rise over run x 1000%10%25%33%50%Mulch aloneUnder 33%Fiber rolls (wattles)Under 40%PlantsUnder 50%Walls, terraces or a proOver 50%Sources: Sonoma RCD (mulch, plants, pros);UC Cooperative Extension (fiber rolls).

Step 1: Cover bare soil

Mulch is the first move. It protects soil from the direct hit of rain and slows runoff across the surface.[1] UC Cooperative Extension calls mulching the most effective post-fire technique on hillsides, and straw mulch low-cost.[2]

How much straw? NRCS figures one 75 pound bale covers about 800 square feet.[9] Permit Sonoma's hydroseeding spec, 4,000 pounds of straw an acre, works out to almost the same: about 92 pounds per 1,000 square feet.[6]

Bare area75 lb bales
200 sq ft1 (you will have extra)
500 sq ft1
1,000 sq ft2
2,000 sq ft3
5,000 sq ft7

Our calculation: area divided by 800 square feet per bale, rounded up. Bales vary in weight, so check the tag.

A fresh pile of wood chip mulch on bare dirt
Skip fine shredded bark. It can smolder in a fire.

Step 2: Break up the slope with wattles

Fiber rolls, often called straw wattles, are tubes of straw or other fiber, roughly 9 inches thick and up to 25 feet long.[2] Laid across a slope, they shorten it, slow the water and catch soil.[4]

How a fiber roll, or wattle, is set on a slopeTwo views. From above: a roll laid level across the slope with both ends hooked uphill, staked at each end and no more than 4 feet apart. From the side: the roll sits in a shallow trench a quarter to a third of its thickness deep, with a stake through it, and runoff from uphill meets it. Source: Napa County fiber roll guidance.Setting a wattleFrom above, then from the sideUphillDownhillLevel across the slope. Ends hooked uphill.Stakes at each end, no more than 4 ft apartSet in a trench 1/4 to 1/3of the roll's thickness deepSource: Napa County fiber roll guidance.

How far apart? The guidelines do not agree, which is worth knowing before you buy.

Wattle row spacing by slope, two guidelinesPaired bars, feet between fiber roll rows. At a 15 percent slope: Caltrans and Napa County 20 feet, the CNPS rule 67 feet. At 20 percent: 20 and 50 feet. At 30 percent: 15 and 33 feet. At 40 percent: 15 and 25 feet. Caltrans figures are for construction sites; the CNPS rule, 1,000 divided by slope percent, is for burned wildland.How far apart to put wattle rowsFeet between rows, by slopeCaltrans and Napa CountyCNPS rule15% (about 7:1)20 ft67 ft20% (5:1)20 ft50 ft30% (about 3:1)15 ft33 ft40% (2.5:1)15 ft25 ftCaltrans: construction sites. CNPS: 1,000 / slope %,for burned wildland. Closer rows catch more.

Caltrans and Napa County use 20 feet on slopes flatter than 4:1.[4][13]

Both use 15 feet between 4:1 and 2:1, and 10 feet on anything steeper.[4][13]

The CNPS rule of thumb, 1,000 divided by the slope percent, spaces rows much farther apart.[10]

Caltrans writes for construction sites. CNPS writes for burned wildland. Napa notes that closer spacing is more effective.[13]

Our worked example. A bare bank 20 feet wide and 30 feet long at 25% is 600 square feet. That is one bale of straw. Caltrans also puts rolls at the top and toe of a slope.[4] At its 15 foot spacing, that is three rows.

Watch the netting. Plastic netting on rolls and blankets can trap snakes, lizards, frogs, birds and small mammals, sometimes for years after plants grow through it.[14] The Coastal Commission recommends plastic-free netting of jute, sisal or coir, with a loose weave whose joints can move. It counts "degradable" plastic as plastic.[14] Rolls with plastic netting should come out once the slope is stable.[13][10]

Step 3: Tame the downspouts

A downspout turns a whole roof into one fast stream. On bare soil, that is a gully waiting to happen.

How to protect the ground at a downspout outletSide view. A downspout comes down a house wall and turns out at the ground. One, a splash guard takes the first force out of the water. Two, a pad of rock slows it. Three, plants and mulch beyond soak it up. The ground falls at least 5 percent away from the house for at least 10 feet. Based on the Sonoma RCD guide Slow it, Spread it, Sink it, Store it.A downspout that does not digSide view, not to scaleHouse wall1. Splash guard takes the first force out of the water2. Rock pad slows it3. Plants and mulch soak it upGround falls at least 5% away for 10 ft.

Better still, catch some of it. See whether rain barrels are legal in California and how to slow it, spread it and sink it. Not sure whether to keep the water or send it away? Read sink it or shed it.

Step 4: Plant for the long run

Mulch and wattles buy time. Plants are the long-term fix. Their roots knit together and hold soil in place.[1]

A small green plant growing out of brown soil
Roots: the slow, permanent fix.

Seed or no seed?

Scattering grass seed sounds like the obvious answer. The experts split, and for different reasons.

The case forThe case against
Quick cover on bare, freshly graded soil.[6]Usually not cost-effective after wildfire, CNPS says research agrees.[10]
UC calls seeding alone largely ineffective, but says pairing it with mulch may make it work better.[2]Seed-mix grasses tend to be short-lived, shallow rooted and flammable by next summer.[10]
Permit Sonoma's spec lists a mix for permitted slopes.[6]Does not work on slopes over 35%, and unseeded ground often regrows as well.[10]

The case for, in detail. For permitted grading, Permit Sonoma's notes call for temporary revegetation before October 1. They give a hydroseed mix of blando brome and rose clover, with fertilizer and straw mulch.[6]

The case against, in detail. In one Forest Service study CNPS cites, plots seeded with Italian ryegrass grew more invasive mustard.[10] CNPS does not rule out all seeding. If you do seed, it says, use locally sourced native perennial plants.[10]

Our take. These are two different situations. A freshly graded building pad is not a burned oak hillside. On a home slope, we would mulch first and plant natives for keeps. If you are under a permit, follow the permit.

Steep slopes and warning signs

Where the soil goes

Everything outside drains to creeks, the county reminds owners. On burned lots, it asks them not to use hoses or leaf blowers to clear ash and debris.[11] See only rain down the storm drain.

In creeks, mud is a fish problem. Writing about the Gualala River, the North Coast Water Board says elevated sediment levels impair spawning habitat, reduce oxygen exchange and alter stream structure.[15] Our story on fine sediment and salmon eggs goes deeper.

Our view: soil that stays on your slope keeps growing plants, and it never becomes mud in the creek. That is a good trade for a few bales of straw.

Who can help

What it means for you

Our view:

Next: decide where the water that runs off should go with sink it or shed it, then read swales: what they do and when not to build one.

Sources

Checked 2026-10-17. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.

Related reads

Grasses growing along a rocky stream's edgeStormwater and drainageSlow it, spread it, sink it: keep rain on your landSlow the rain down, spread it out, and let it soak in. The Sonoma Resource Conservation District says this protects your property and refills groundwater.Updated Oct 11, 2026A creek flowing through a culvert under a rural roadStormwater and drainageWhen you need a county permit to change drainageIn unincorporated Sonoma County, a drainage permit is required before any drainage improvement. Grading over 50 cubic yards needs a grading permit.Updated Oct 11, 2026Rain falling over green rice paddies with misty hills behindStormwater and drainageSink it or shed it? When to keep rain on your land and when to drain itMost properties need both. Sink clean water where the soil can take it and nothing will be harmed. Shed water, slowly and safely, where soaking it in would threaten a foundation, a slope, a septic system or a neighbor. The craft is deciding which water goes where.Posted Oct 12, 2026

Something wrong on this page?