Altadena Hillsides: Drainage And Erosion 101

Altadena hillside living is beautiful, but winter storms can turn a peaceful slope into a fast-moving water path in minutes. If you own a home on or near a slope, you know the stress that comes with the first heavy rain, especially after a dry summer or a nearby wildfire. The good news: with simple maintenance and a clear understanding of County rules, you can protect your property and plan improvements with confidence. This guide walks you through swale care, slope planting, safe access, permit basics, and when to bring in a professional. Let’s dive in.

Why Altadena slopes need a plan

Altadena sits on the lower flanks of the San Gabriel Mountains, where many streets and backyards sit on or below steep terrain. Our Mediterranean climate brings dry summers, then concentrated winter rain. After wildfires, soils can turn hydrophobic, vegetation is lost, and runoff accelerates.

The most common hillside risks include:

• Surface erosion that strips topsoil and undercuts paths or plantings.
• Concentrated flow in swales and drains that can turn into gullies when blocked.
• Slope instability from saturated soils, steep cuts, or poor drainage.
• Debris flows during high-intensity storms and in burn-scarred areas.
• Extra runoff from driveways and patios upslope that pushes water faster downslope.

Many private drainage paths eventually reach roadside swales or the County’s flood control system. Anything that changes how water leaves your property may require County review, so plan ahead.

Know the rules in unincorporated LA County

Altadena is unincorporated, which means LA County agencies oversee grading, drainage, and structures. Before you move earth or connect to public drainage, confirm requirements with the County planning or public works counter.

Common approvals and when they apply:

• Grading permits: often required if you change grade or move soil in a way that alters drainage. Small routine maintenance that does not change patterns may be allowed without a permit. Always check current thresholds with LA County Building and Safety.
• Encroachment permits: required for work in the public right-of-way, such as roadside swales, gutters, or curb/driveway cuts.
• Flood control review: modifications to channels, pipes, or any tie-in to the Los Angeles County Flood Control system typically need approval.
• Construction stormwater requirements: projects that disturb soil or change drainage may need best management practices and, in some cases, a construction stormwater permit under State and Regional water quality rules.
• Retaining walls and structures: walls above certain heights or carrying surcharge loads often require permits and a geotechnical report.
• Planning/design review: hillside grading, changes to lot coverage, or work in sensitive areas can trigger Planning review.

Bottom line: routine cleaning is usually fine, but changes to shape, grade, or any connection to public systems can trigger permits. Confirm first to save time and money later.

Inspect and maintain swales

Swales are shallow, gently sloped channels that guide water. On hillside lots, they reduce erosion by spreading and slowing flow. A few hours of upkeep can prevent costly damage during storms.

When to inspect

• Before the rainy season.
• After the first heavy storm of the season.
• After any major storm.
• After wildfire or nearby brush clearance.

What to check and clean

• Remove leaves, sediment, and trash from swales, inlet grates, and culverts.
• Keep gutters and downspouts clear. Direct downspouts to pervious areas or approved collection points.
• Make sure water follows the intended path and does not jump the swale or cut a new channel.

Small fixes you can do

• Light regrading to restore a shallow swale profile so water flows evenly.
• Reset small rock at concentrated flow points to break up velocity. Choose rock sized to stay put in expected flows.
• Install fiber rolls, straw wattles, or jute blankets on disturbed soil to slow runoff and protect bare areas while plants establish.
• Use sandbags for short-term protection during a storm, not as a permanent fix.

What to avoid

• Do not alter a swale’s geometry in ways that send water onto a neighbor or the street.
• Do not place rock or structures in regulated channels without County approval.
• Do not discharge roof drains in a way that overloads roadside swales.

Plant slopes for stability

Smart planting helps slow water, increase infiltration, and bind soil with roots. In Altadena’s climate, choose species and spacing that fit the slope exposure and soil.

Goals of planting

• Slow sheet flow across the surface.
• Anchor soil with deep, fibrous root systems.
• Increase infiltration without overwatering.

Smart plant choices

• Favor drought-tolerant, deep-rooting native species suited to local conditions. Common California-friendly slope plants include coyote brush, California buckwheat, toyon, and manzanita. Select based on your microclimate, soil, and fire-safety needs.
• Use a mix of groundcovers and shrubs to avoid monocultures and improve resilience.
• Avoid shallow-root ornamentals or turf on steep slopes where roots may not stabilize soil.

Groundcover and mulch

• Protect exposed soil with mulch or hydromulch while plants establish.
• Use erosion-control blankets on steeper or newly disturbed sections.
• Refresh mulch as needed to maintain coverage without piling it against structures.

Irrigation tips

• Irrigate to establish plants, then scale back to avoid oversaturation.
• Avoid over-irrigation, which can destabilize slopes and increase runoff.
• Check emitters and lines regularly for leaks.

Manage access safely

Getting people or equipment onto a slope should not create a new erosion path. Plan access that respects the hillside.

People paths

• Use stepping stones or gravel paths that include small cross drains or breaks so water does not run straight down the path.
• Avoid long, continuous fall lines that channel water.
• Keep paths clear of debris so water can disperse.

Vehicles and equipment

• Limit heavy equipment on slopes. Compaction and ruts can damage soil structure and redirect water.
• If equipment is necessary, use temporary mats or tracks and contractors experienced with hillside work.

Neighbor and street coordination

• Do not redirect water onto neighboring parcels without consent.
• Coordinate with neighbors if swales cross property lines.
• Work in the County right-of-way usually needs an encroachment permit. Contact Public Works before starting.

Low-impact controls that work

You can reduce runoff velocity and protect soil with simple, proven measures.

Temporary protection

• Fiber rolls, straw wattles, and jute blankets slow water and trap sediment on disturbed areas.
• Sandbags and small check dams help for short-term storm protection but are not long-term stabilization.

Capture and infiltration

• Small rain gardens, infiltration trenches, or dry wells can help capture runoff if placed with proper setbacks and clearances.
• Do not create concentrated flow onto the street or neighbor properties.
• Larger or near-foundation systems may require County review. Confirm before you install.

Energy dissipation

• Place appropriately sized rock at downspout outlets or grade breaks to reduce scour.
• Do not place rock in regulated channels or public systems without approval.

When to call a pro

Some conditions point to deeper issues. Bringing in the right expert early can prevent bigger problems and help you secure permits.

Civil engineer

• You plan grading or earthwork that may change drainage patterns.
• You need design for swales, catchment improvements, pipe connections, or underdrain systems.
• You need drainage and grading plans for County submittal.

Geotechnical engineer

• You see cracks in structures, tilted fences, ground slumping, bulging at the slope toe, or persistent seepage.
• You plan significant grading or new retaining walls on steep slopes.
• You live near a recent burn area and need stabilization guidance.

Structural engineer

• You are designing retaining walls over local height limits or with surcharge loads.
• Existing walls show distress or movement.

Landscape architect or erosion-control contractor

• You need a planting plan for difficult slopes and guidance on soil amendments and irrigation that will not destabilize the slope.

Set expectations: most consultants start with a site visit and written scope. Costs vary with slope complexity, public system connections, and post-fire or landslide conditions.

Working with the County

Knowing who to call and what to bring will save you time at the counter.

Who to contact

• Los Angeles County Department of Public Works for grading guidance, encroachment in the right-of-way, and drainage connections.
• Los Angeles County Department of Regional Planning for hillside development and discretionary review questions.
• LA County Building and Safety for building and structural permit thresholds.
• Los Angeles County Flood Control District for any work affecting channels or tie-ins.
• State water boards for construction stormwater requirements.
• CAL FIRE or local fire authorities for post-fire erosion guidance.

What to bring

• Your parcel number and property address.
• Site photos showing slopes, swales, public connections, and any damage.
• A plot plan or simple site plan with lot lines, structures, and existing drainage.
• Notes on past permits or improvements and any neighbor drainage issues.
• Observations on where water enters and leaves the lot.

What to expect

• Staff may advise whether a permit is needed and what plans or reports to submit.
• You may be asked for civil and geotechnical input for complex sites.
• Requirements and thresholds change. Confirm current rules before you design.

Quick homeowner checklist

• Inspect and photograph swales, inlets, and slopes before and after the rainy season.
• Keep gutters and downspouts clear. Direct water to pervious areas or approved collection points.
• Remove debris from swales and inlet grates. Regrade shallow swales so water follows the intended path.
• Use temporary controls like fiber rolls, mulch, and erosion blankets on exposed soils while vegetation establishes.
• Avoid placing fill or new hardscape that concentrates runoff without a drainage plan.
• Before grading, altering drainage, or working in the County right-of-way, contact Public Works, Planning, or Building and Safety for parcel-specific guidance.
• If you see signs of slope movement or structural distress, stop work and call a geotechnical or civil engineer.

Taking time now to tune up your swales, plant smart, and confirm requirements will pay off when the rains arrive. If you are planning to sell or just want clarity on which improvements add value, let a trusted local advisor help you think through timing and priorities. To discuss your property goals or get a data-driven read on value, connect with Unknown Company to Request a Free Home Valuation.

FAQs

Do Altadena homeowners need a permit to clean a roadside swale?

• Routine clearing of trash and small sediment is usually allowed, but major regrading or work in the public right-of-way generally requires an encroachment permit. Confirm with Los Angeles County Public Works before you start.

When do retaining walls on hillside lots require permits in LA County?

• Many walls over specific heights or carrying surcharge loads require permits, and some will need a geotechnical report. Check current thresholds with LA County Building and Safety and consult an engineer for larger or steep-slope walls.

Are rain barrels, rain gardens, or dry wells allowed in Altadena?

• Stormwater capture is encouraged if it meets setbacks and does not create concentrated flows onto neighbors or the street. Larger or near-foundation infiltration systems may require County review. Confirm before installation.

What should I do after a wildfire above my property?

• Use temporary measures like mulch, fiber rolls, seeding or hydromulch, and small diversion features to slow runoff. Seek guidance from County agencies and hire specialists for larger or high-risk sites.

How do I document a drainage or erosion issue before calling the County?

• Take photos, note dates and storm intensity, sketch flow paths, and gather prior permits or reports. Include upslope sources and any impacts to neighbors to help staff or consultants assess quickly.