Steps To Build A Raingarden For Kansas Stormwater

This article gives a practical, step-by-step guide to designing, sizing, building, planting, and maintaining a raingarden that captures and treats stormwater in Kansas. The guidance is tailored for Kansas climate, soils, and common site conditions, and focuses on durable, low-maintenance solutions that protect foundations, reduce runoff, and support native prairie biodiversity.

Why a raingarden in Kansas?

Kansas has variable precipitation: intense spring and summer storms, dry stretches in summer, and cold winters. Many urban and suburban sites have increased runoff from roofs, driveways, and compacted lawns. A raingarden intercepts and infiltrates runoff on site, reducing erosion, sewer overflows, and downstream flooding while recharging groundwater and creating habitat.
A Kansas-appropriate raingarden must:

• Accept high-intensity short storms but also survive drought and freeze.
• Work on a range of soils, including clay-packed soils common in parts of Kansas.
• Use native or adapted plants with deep roots and seasonal interest.

Overview of the process

1. Pick a proper site and check setbacks and utilities.
2. Size the raingarden for the drainage area and desired capture volume.
3. Test infiltration and design soil mix and possible underdrain.
4. Excavate and shape a shallow depression with berms and an overflow.
5. Install amended soil, mulch, and plant palette with erosion protection.
6. Establish and maintain the garden for the first 2-3 years and then manage long term.

Each step below unpacks practical details and concrete takeaways.

Site selection and initial checks

Choose a site that receives runoff from the catchment you want to treat (roof, driveway, or lawn). Good sites have:

• At least 10 feet distance from building foundations; 10 to 20 feet is preferable for homes with basements.
• A gentle slope (up to about 10 percent) — raingardens work on slopes but may need terracing.
• Visibility and accessibility for maintenance.

Before you dig:

• Call the local utility locating service to mark underground lines.
• Check local ordinances and neighborhood covenants for restrictions or permits.
• Note the existing grading and where overflow will go in an extreme storm — provide a safe overflow route to a swale or storm sewer.

Sizing the raingarden

Designers commonly size raingardens to capture a specified rainfall depth (often 1 inch) from the drainage area for water-quality treatment. Use this simplified approach to get a practical size fast.
Basic calculation:

• Runoff volume (cubic feet) = Rainfall depth (feet) x Drainage area (sq ft) x Runoff coefficient.
• For a 1-inch storm, rainfall depth = 1/12 foot.

Typical runoff coefficients:

• Roof: 0.9
• Driveway/impervious: 0.95
• Lawns/vegetated: 0.2-0.4 (use 0.3 as a typical value)

Example:

• Roof area = 500 sq ft, runoff coefficient = 0.9.
• Volume = (1/12) x 500 x 0.9 = 37.5 cubic feet.

Decide a ponding depth (vertical depth of surface storage) — commonly 4 to 8 inches (0.33 to 0.67 ft). Then:

• Required surface area (sq ft) = Volume (cubic feet) / Ponding depth (feet).

Continuing example with 6-inch ponding (0.5 ft):

• Surface area = 37.5 / 0.5 = 75 sq ft.

Practical takeaways:

• For many suburban downspouts, raingardens around 50 to 150 sq ft handle typical 1-inch events.
• Increase size if you want to capture larger storms or accept slower drawdown.
• Provide an overflow sized and sloped to carry excess water away safely for rare large storms.

Percolation testing and underdrain decision

Before deciding soil depth or underdrain, test infiltration where you will place the garden.
Simple percolation test:

• Dig a hole about 6 to 12 inches deep and 6 inches wide in several locations.
• Pre-soak the hole, then fill with water and measure the drop in water level over a set time (e.g., drop in inches per hour).
• Desired performance: the raingarden should drain within 24 to 48 hours after a storm. If you see a drainage rate equivalent to at least 0.5 inch per hour, infiltration is generally acceptable; if much slower or ponded beyond 48 hours, plan an underdrain or locate elsewhere.

If your soil is slow (heavy clay) or the site is near structures, include a perforated underdrain wrapped in filter fabric and running to an approved outlet.

Soil media and depths

A good filtration medium encourages infiltration without clogging and supports plants.
Recommended engineered soil mix:

• 50 to 60 percent coarse sand.
• 20 to 30 percent compost.
• 20 to 30 percent topsoil or native soil.

Target depths:

• Planting/filtration layer: 18 to 24 inches over native soil.
• Ponding depth: 4 to 8 inches typically; up to 12 inches for larger systems.
• If using an underdrain, place it near the bottom of the filtration layer with a stone envelope so draining water passes through the media.

Practical takeaways:

• Use coarser sand, not fine beach sand; avoid adding too much clay or silt.
• Avoid ripping or compacting the existing subsoil during excavation — you want infiltration, so loosen the bottom slightly.
• If you cannot obtain engineered mix, use the best available topsoil amended with coarse sand and compost and accept the need for possible underdrain.

Excavation, shaping, and overflow design

Steps to construct:
1. Mark the outline and stakes for elevations of the bottom, rim, and overflow.
2. Excavate to the designed depth considering the planned soil media depth and ponding depth.
3. Shape the basin with gentle slopes (3:1 or flatter) on sidewalls for stability and ease of planting.
4. Construct a berm or compacted edge on the downstream side to hold ponding and direct overflow. Stabilize berms with sod, stones, or erosion control matting while vegetation establishes.
5. Design a permanent overflow riser or notch set 1 to 2 inches above the design ponding depth. Ensure the overflow route leads to a safe discharge location.
Practical tips:

• Keep excavated soil nearby for berms; do not place soils on the bottom of the basin.
• Access should allow equipment to enter if needed; small excavators speed up work.

Plant selection and layout for Kansas

Choose native or well-adapted species that tolerate both flooding and drought. Use a mix of sedges, grasses, perennials, and shrubs for structure and year-round function.
Suggested Kansas-appropriate plants:

• Switchgrass (Panicum virgatum) – deep-rooted prairie grass.
• Little bluestem (Schizachyrium scoparium) – drought tolerant.
• Pennsylvania sedge or Carex species – tolerate varying moisture.
• Swamp milkweed (Asclepias incarnata) – wet tolerant native for pollinators.
• Purple coneflower (Echinacea purpurea) – prairie perennial with summer interest.
• Black-eyed Susan (Rudbeckia hirta) – durable and attractive.
• Blue lobelia (Lobelia siphilitica) – prefers moist conditions, good near inlet.
• Joe-Pye weed (Eutrochium purpureum) – for larger gardens with wet pockets.
• Asters and goldenrod for late-season pollen sources.

Planting layout:

• Place moisture-loving species near the inlet and lowest part of the basin.
• Use more drought-tolerant species on upper slopes and edges.
• Group plants in colonies rather than single specimens for easier maintenance and stronger visual impact.

Mulch, erosion control, and initial care

After planting:

• Apply a 2 to 3 inch layer of shredded hardwood mulch on bare soil to reduce erosion and help retain moisture. Leave crowns of perennials exposed.
• Install erosion control blankets on steep berms or concentrated inflow paths until vegetation establishes.
• Protect the inlet with rock or a level spreader to diffuse high velocity flows.

Establishment care:

• Water new plants during extended dry spells for the first season; do not overwater the basin once established.
• Weed frequently in year one to prevent invasive species from establishing.
• Remove sediment buildup at the inlet after major storms; replace or refresh mulch as needed.

Maintenance schedule and long-term management

Year 1:

• Weekly inspections for the first two months after a big storm season, then monthly.
• Weed, water as needed, and check for standing water lasting more than 48 hours.

Years 2-3:

• Expect reduced weeding; plants should fill in and require selective thinning.
• Replenish mulch annually or every other year to maintain 2-3 inch depth.

Ongoing:

• Remove accumulated sediment and debris from the inlet and overflow annually or after heavy storms.
• Trim and divide clumps every 3-5 years to maintain vigor.
• Do not apply fertilizer — the goal is nutrient capture, not ornamentals.

Costs and phasing

Costs vary with size, soil replacement, whether you hire contractors, and plant selection. Typical ranges:

• DIY small raingarden (50-150 sq ft): $300 to $2,000 depending on soil, plants, and tools.
• Contractor-built: $1,500 to $10,000+ depending on excavation, engineered soil, underdrain, and plantings.

Phased approach:

• Start with a smaller pilot raingarden under a single downspout and observe performance for a season.
• Scale up later if the first installation performs well.

Permits, incentives, and community considerations in Kansas

Always verify local permit requirements and incentives. Many Kansas cities and counties have stormwater or conservation programs that encourage on-site stormwater management and may offer design assistance, rebates, or credits. Check with utilities or county conservation districts about rebates for disconnecting downspouts or installing raingardens.
Coordinate with neighbors if runoff is shared or if overflow may enter adjacent properties. Design overflow to avoid causing nuisances.

Troubleshooting common problems

Standing water beyond 48 hours:

• Check for clogged inlet or overflow.
• Re-evaluate infiltration rate — consider an underdrain or deepening the basin.

Erosion at inlet or outlet:

• Install rock splash pads, check dams, or a level spreader to dissipate energy.
• Soften concentrated flow with graded steps or stone.

Poor plant survival:

• Check soil mix and moisture exposure of planted zones.
• Replace inappropriate species with better-adapted natives.

Mosquito concerns:

• Properly designed raingardens drain within 24-48 hours and do not breed mosquitoes. Stagnant, long-persisting water is the issue–address infiltration or underdrain.

Final practical checklist

• Mark utilities and obtain necessary permits.
• Locate raingarden at least 10 feet from foundations; provide safe overflow route.
• Calculate capture volume for a 1-inch event and size surface area based on a 4-8 inch ponding depth.
• Perform percolation tests; plan underdrain if infiltration is too slow.
• Use an engineered soil mix (sand, compost, topsoil) and 18-24 inch planting depth.
• Install a stabilized inlet, compacted berm, and a defined overflow.
• Plant native Kansas species according to moisture gradient.
• Mulch, stabilize slopes, and maintain with weeding and sediment removal early on.
• Monitor drawdown time and adjust if water lingers.

A well-built raingarden in Kansas reduces stormwater impacts, supports native plants and pollinators, and adds attractive low-maintenance landscaping. With careful site assessment, proper soil media, and the right plants, you can build a resilient system that performs well through Kansas storms, droughts, and winters.