Eye of the Landholder
As beauty is in the eye of the beholder, expenses are different for every landholder. Land development engineering expenses vary for every landowner. The least expensive option is a detention facility draining via gravity to a public system, like a drainage ditch or storm sewer. In Harris County, detention rates depend on the outfall type. For an 8-foot outfall, 12-15% of the land is used for detention, while a 4-foot outfall requires 25-30%. Excavated material is often used to raise project elevations, including the detention facility's high bank. Adding one foot of detention can reduce land use by over 10%, a neat little trick for maximizing developable area.
For projects where land use is too costly, a deeper facility with pumping is an alternative. Harris County requires additional detention for pumped systems, increasing excavation. Pumping costs can be high and require analysis of excavation costs and benefits. Pumps cost approximately $250,000 for 10-15 cfs, with larger systems more. Energy expenses are around $2/cfs/storm, and maintenance requires less than two days of labor annually.
In certain areas, infiltration systems might be appropriate instead of pumps. These systems allow stormwater from the smaller events to infiltrate into the soil matrix and require hydrologic soil group B or A soils in order to allow sufficient infiltration to make them worthwhile. For example, when infiltration is above 0.1 inches per hour, over a 24 hour period, the bottom 2.5 inches of stormwater can infiltrate. This may not seem like much, but it could reducing pumping from every storm. Vegetation must be maintained. Based on research by James J. Houle, annual maintenance costs should be about $1,000 per acre of infiltration system which is comparable to the price for maintaining conventional dry or wet detention facilities. Advanced infiltrations systems in which the bottom of the detention facility is specifically designed to support infiltration rather than detention can increase this infiltration rate to above 50 inches per hour. These require designed soils, but they can potentially reduce the pumping costs by a half or more.Finally, underground detention, while effective, is usually a last-resort method due to cost and complexity. Pervious pavers with detention stone can reduce land needs and work well in Houston if drained properly. They include concrete, asphalt, or pavers over 57 stone storage and need periodic vacuum sweeping. A new Heights project recently designed by my staff at Tetra proved ideal during rain events, preventing puddles in parking areas and draining as designed. Other forms of underground detention include concrete vaults and plastic chambers and can reduce the amount of land required for detention while being constructed under pervious pavements. Note that underground facilities are difficult to maintain, and sedimentation filters and trash systems should be installed at easy access points such as inlets or storm sewers. Emptying these chambers is an annual expense of approximately $500/ chamber so the goal would be to reduce the number as much as possible.
Dr. Culp is the most senior hydrologist at Tetra Land Services and has three decades of civil engineering experience. His Ph.D. scholarship studied the effectiveness of structural BMP for the control of storm water pollution in Harris County while performing water quality monitoring and modeling upon selected ponds for the county. Dr. Culp also co-authored the City of Houston stormwater quality management plan. He is one of Texas’s original Certified Floodplain Managers. Recently, Dr. Culp and his staff have developed a series of drainage studies for Industrial and Oil Majors along the Texas Gulf Coast. Dr. Culp is married with two children, and lives on his farm in Southwest Houston.