Stormwater Fixes for Sammamish Lots That Stay Dry

Rainy months in Sammamish can test even a well‑built home. If you see water tracking along your driveway, pooling by your foundation, or carving channels through a wooded yard, you are not alone. Many Sammamish lots sit on slopes with glacial soils that drain unevenly, and winter storms often deliver long, soaking rains. The good news is you can keep your lot dry with smart, low‑impact fixes that work with our climate and meet local rules.

In this guide, you will learn why Sammamish properties get soggy, which solutions fit sloped and wooded sites, when permits may apply, and how to protect your foundation and landscaping without creating new problems. Let’s dive in.

Why Sammamish lots get soggy

Sammamish has a maritime climate with wet, cool winters and drier summers. Most rainfall arrives from October through March, so your drainage must handle long, steady storms and occasional high‑intensity bursts.

Many lots are wooded and sit on glacial soils. Some areas have fast‑draining sands and gravels, while others have compact till or shallow bedrock that slows infiltration. Clearing trees or compacting soil during construction can increase runoff and erosion, which pushes more water toward foundations or down slopes.

Common issues include runoff racing down driveways, damp crawlspaces or basements from poor grading, and neighbor‑to‑neighbor conflicts when natural flow paths are blocked. A site‑specific approach usually performs best here.

Permits and rules to check first

Before you dig, confirm local requirements. In Sammamish, stormwater and grading are regulated by the City’s Public Works and the Sammamish Municipal Code. Design guidance commonly references the King County Surface Water Design Manual and the Washington State Department of Ecology’s Stormwater Management Manual for Western Washington.

• Small maintenance like cleaning gutters usually does not need a permit.
• New infiltration systems, substantial grading, work near steep slopes, wetlands, or streams, or adding significant impervious area can trigger permits and reviews.
• Projects near steep slopes or where water affects a foundation often need a geotechnical evaluation.
• Infiltration practices must meet setbacks from structures, wells, septic systems, and property lines, and maintain vertical separation from seasonal high groundwater or bedrock.

Because thresholds change, contact City of Sammamish Public Works to confirm current triggers, setbacks, and submittal needs before you proceed.

Low‑impact fixes that work here

Below are proven practices for Sammamish’s sloped and wooded settings. Combine methods to spread risk, reduce peak flows, and build in safe overflow paths.

Gutters and downspout routing

Your roof collects thousands of gallons each season. Continuous, well‑sized gutters with secure downspouts are the first line of defense. Route downspouts away from foundations and steep slopes to a safe receiving area like a rain garden, dispersion trench, or drywell if soils allow.

• Provide a clear overflow path for big storms.
• Avoid sending concentrated flow to the street or a neighbor’s property.
• Inspect outlets and splash blocks and keep them free of leaves and roots.

Rain barrels and cisterns

Rain barrels and medium‑sized cisterns capture rooftop runoff for later irrigation or slow release. They reduce peak flow and offer water for summer plants.

• Set tanks on a flat, stable pad.
• Include screened inlets and tight lids to prevent mosquitoes.
• Always plan a stable overflow that routes away from your foundation and steep slopes.

Rain gardens and bioretention

Rain gardens are shallow, planted basins that capture and filter runoff through engineered soil. They look great, support native plants, and reduce runoff volume while improving water quality.

• Test soils and confirm vertical separation to groundwater or bedrock per accepted manuals before you rely on infiltration.
• In wooded yards, limit removal of organic duff to the work area and use pretreatment, such as a rock pad or filter strip, to keep sediment out.
• On slopes, keep the basin bottom level, stabilize berms, and consider an underdrain if infiltration is limited.
• Expect more frequent weeding and mulch touch‑ups in the first 1 to 3 years while plants establish.

Drywells and infiltration trenches

Drywells and trenches can be effective when native soils drain well and space is tight. They receive concentrated roof runoff and infiltrate it below ground.

• Verify infiltration rates and maintain minimum separations to groundwater, bedrock, and structures.
• Provide pretreatment to keep organic debris and fine sediment from clogging the system.
• Plan for periodic inspection and cleaning of inlets and sediment traps. Some systems need eventual replacement if infiltration slows.

Permeable paving and stabilized gravel

Permeable pavers, porous concrete or asphalt, and stabilized gravel allow water to pass through rather than run off.

• Match the system to your soils and include edge restraints.
• Maintain with sweeping or vacuuming to remove leaves and sediment. Avoid sand application.
• In shaded or heavily wooded areas, expect more frequent maintenance to prevent clogging.

Vegetated swales, dispersion, and level spreaders

Shallow swales and level spreaders slow and spread flows across stable vegetated areas so water can settle and infiltrate.

• Do not concentrate water into a single point on steep slopes.
• Use turf reinforcement or native plantings to harden swales against erosion.
• Ensure a stable outlet and enough vegetated length for filtration.

Grading, small walls, and drains

Thoughtful grading provides gentle overland flow paths that move water away from structures. Small berms can divert water around a house. Retaining walls with engineered drains can relieve perched water.

• On sloped lots, consider slope stability. Large cut or fill often requires permits and geotechnical input.
• Subsurface collection behind walls should be engineered so it does not destabilize a slope.
• Inspect outlets and joints after big storms.

Vegetation and forest management

Healthy canopy and deep‑rooted native plants slow rainfall, stabilize soil, and promote infiltration.

• Preserve existing trees where possible.
• Minimize root‑zone disturbance and avoid soil compaction during any work.
• Choose native, drought‑tolerant species that fit Sammamish conditions.

Plan your site: assessment first

Start with a simple site map. Walk your lot during a storm to trace where water starts, concentrates, and exits. Note seasonal seeps, soggy corners, and any areas where water touches your foundation or undercuts slopes.

• Map wells, septic components, property lines, and critical areas like streams or slope toes.
• Identify good infiltration zones and areas to avoid.
• If you plan to rely on infiltration or make grading changes near steep slopes or foundations, schedule soils testing and a shallow geotechnical review.

Always keep a safe overflow path

Every practice needs a backup plan for extreme storms. A stable, visible overflow path protects your home and your neighbor’s property.

• Route overflow away from foundations, septic systems, and slope edges.
• Keep overland flow paths clear when you regrade or build.
• Armor outlets with rock where needed to prevent scour.

Protect the site during installation

Construction can create more runoff if you disturb soil without controls.

• Use silt fences, stabilized access, and prompt reseeding or mulching during work.
• Keep organic debris and sediment out of inlets and infiltration media.
• Stage materials and equipment to avoid compacting future planting areas.

When to call a professional

Bring in the right help when:

• Water is affecting a foundation or crawlspace.
• You plan significant grading or work near steep slopes or streams.
• You want to install drywells, permeable pavement, or large rain gardens that rely on infiltration.

A geotechnical engineer can evaluate slope stability and setbacks. A civil or stormwater engineer and a landscape architect with low‑impact development experience can help size systems and design overflow routes. For small projects like gutters and barrels, a contractor familiar with local best practices may be sufficient.

Simple maintenance that prevents big problems

Build a quick maintenance plan into your routine. Many failures come from clogged inlets, buried outlets, or overwhelmed overflows.

• Gutters and downspouts: Clean seasonally and after major storms.
• Rain barrels/cisterns: Check screens, lids, and overflow paths.
• Rain gardens: Weed and refresh mulch 1 to 3 times per year, and clear inlets and outlets.
• Permeable paving: Sweep or vacuum several times per year, more if shaded or near trees.
• Drywells/trenches: Inspect inlet screens, clean sediment traps as needed, and monitor performance.

Buying or selling in Sammamish: why drainage matters

Good drainage protects your investment and shows well during inspections and winter showings. Clean gutters, clear downspouts, visible overflow paths, and a tidy rain garden or swale signal care. If you have engineered systems, gather documentation, permits, and maintenance records for buyers.

If you are evaluating a home, walk the exterior after rain if possible. Look for water staining, eroded soil, puddling near foundations, or blocked culverts. Ask about soil conditions and any past geotechnical or stormwater reports. A dry, well‑managed lot adds long‑term value and peace of mind.

Ready to talk through which fixes fit your property and how drainage can impact resale? Let’s connect and map a practical plan that supports your goals in every season. Reach out to Stephen Snee.

FAQs

Will infiltration near a foundation cause seepage?

• Not if it is properly sited and designed with required setbacks and vertical separation from groundwater or bedrock. A geotechnical review is often recommended when infiltration is near structures.

Do I need a permit for a Sammamish rain garden or drywell?

• It depends on size and location. Small, backyard rain gardens may not trigger major permits, but drywells and significant grading often do. Confirm with City of Sammamish Public Works before starting.

Will a rain garden breed mosquitoes on my property?

• A properly designed rain garden drains within a short period, so it does not hold standing water long enough to breed mosquitoes.

Can rain gardens and permeable paving work on wooded or shaded lots?

• Yes, but they need more pretreatment and maintenance because leaves and needles can clog surfaces and soils faster than in open, sunny sites.

Where can I get technical or financial help for drainage projects?

• Check with King Conservation District and King County for technical assistance, planting guidance, and occasional incentives. City of Sammamish Public Works can direct you to current local programs.