Cost-benefit considerations for stormwater wetlands

Cost factors for stormwater management ponds are extremely sensitive to site conditions. Availability of materials for embankment construction, outlet protection, cost of excavation, liner materials, and land costs are significant factors. Maintenance and inspection costs for mowing and periodic dredging are post-development factors. Other technologies such as infiltration trenches may be more cost-effective in smaller drainage areas due to construction and long-term maintenance costs (Young et al.,1996). Costs for ponds typically include costs for embankment, riser and spillway structures, outfall protection, vegetative stabilization, excavation, and grading. Additional costs for site preparation can include soil amendments, precision grading, plant materials and creation of occluding layers in coarse-textured soil types if wetlands systems must be created on upland sites due to project constraints. Project costs can be lowered if existing pre-construction site conditions are carefully considered and isolated areas with hydric (wet) soils contained within the footprint of the project are utilized as stormwater management facilities.

Additional maintenance costs may be incurred until the establishment of the wetland ecosystem. Invasive plants must be culled and dead plants replaced. The outlet structure may have to be adjusted, based on seasonal observations, to achieve the proper water surface in the pond. (FHWA, 1997).

Detailed Cost Estimate

The most appropriate technique for determining the cost to construct and maintain a specific BMP will be to apply unit costs to each component of construction, operation and/or maintenance. The following table shows the typical components for stormwater wetlands and presents those components of a construction project that are unique to this best management practice. Costs that would be associated with all aspects of a construction site, such as erosion and sediment control, mobilization, or traffic control, are not presented as unique costs. Designers are encouraged to use a cost worksheet to estimate their BMP construction cost.

Primary cost components for stormwater wetlands
Link to this table

Implementation stage	Primary cost components	Basic cost estimate	Other considerations
Site preparation	Tree and plant protection	Protection cost ($/acre) * affected area (acre)	Removal of existing structures, topsoil removal and stockpiling
	Topsoil salvage	Salvage cost ($/acre) * affected area (acre)
	Clearing and grubbing	Clearing cost ($/acre) * affected area
Site formation	Excavation and grading	4-foot depth excavation cost ($/acre) * area (acre)	Soil and rock fill material, tunneling
	Hauling material off-site	Excavation cost * (% of material to be hauled away)
Structural components	Inlet structure	($/structure)	Pipes, catch basins, manholes, valves
	Outlet structure	($/structure)
Site restoration	Soil preparation	Soil cost ($/acre) * seeding/planting area (1 foot average depth per acre)	Tree protection, soil amendments, seedbed prepparation, trails
	Seeding (or sodding)	Seeding cost ($/acre) * seeded area (acre)
	Planting/transplanting	Planting cost ($/acre) * planted area (acre)
Annual operation, maintenance, and inspection	Debris removal	Removal cost ($/acre) * area (acre) * frequency (2 per year)	Vegetation maintenance, cleaning of structures
	Invasive plant removal	Labor cost ($/hr) * time * frequency
	Sediment removal	Removal cost ($/acre) * area (acre) * frequency (1 per 5 years)
	Erosion repair	Repair cost ($/acre) * area affected
	Gate/valve operation	Operation cost ($) * operation frequency (2 per year)
	Inspection	Inspection cost ($) * inspection frequency (2 per year)
	Mowing	Mowing cost ($/acre) * mowing frequency (4 per year)