Slope Stabilization and Planting Plan
A residential client has a 1,200 sq ft backyard with a 25% slope (12.5 ft rise over 50 ft run) that is eroding and difficult to maintain. The soil is a clay loam with moderate drainage. The client wants a low-maintenance landscape that stabilizes the slope, provides seasonal interest, and includes a small patio area at the base. The budget is $15,000. The site receives full sun and is in USDA Zone 7. The client is concerned about water runoff onto a neighbor's property.
1. What design and planting strategy would you recommend to stabilize the slope and address the client's concerns? Include specific plant types and construction techniques.
Read the worked answer
To stabilize a 25% slope with clay loam, use a combination of structural and vegetative measures. First, regrade the slope to a maximum 3:1 (33%) where possible, but since it's already 25%, focus on terracing with retaining walls at intervals to reduce slope length and manage water. Use a geotextile fabric and a deep-rooted groundcover mix. Plant selection should include native grasses (e.g., switchgrass, little bluestem) and shrubs with fibrous roots (e.g., red osier dogwood, viburnum) for erosion control. Incorporate a dry creek bed or French drain at the base to redirect runoff. For seasonal interest, add ornamental grasses and perennials like coneflower and black-eyed Susan. The patio should be set into the slope with a small retaining wall and gravel base. Budget: allocate ~40% to hardscape (walls, patio), 30% to plants, 30% to drainage and soil prep.
2. How would you calculate the volume of runoff to design the drainage system, and what factors would you consider?
Read the worked answer
Use the Rational Method: Q = CiA, where Q is peak runoff (cfs), C is runoff coefficient (0.3-0.5 for lawns/landscaping on clay loam), i is rainfall intensity (in/hr for design storm, e.g., 2 in/hr for a 10-year storm), and A is area in acres (1200 sq ft = 0.0276 acres). For C=0.4, i=2, Q = 0.420.0276 = 0.022 cfs. This is low, but the slope increases velocity. Consider time of concentration, soil infiltration rate, and the need for a vegetated swale or dry creek to convey flow to a safe outlet. Also, check local regulations for stormwater management. Design the drainage to handle at least the 10-year storm, and include overflow provisions.