Green Solutions: A Guide to Low Impact Development
Low Impact Development (LID) attempts to mimic existing conditions in the developed world with regards to rainfall and stormwater runoff related design. LID techniques include: reducing pavement widths; reducing the amount of pavement in parking systems; increasing flow path lengths; adding filtration systems to the green areas; not directing stormwater to storm sewers; adding trees with large canopies to intercept stormwater and to enhance evapotranspiration; adding vegetation in unusual areas such as green roofs. Each of these systems has advantages and disadvantages, which we will review in brief.
Note that LID often conflicts with legacy development codes designed to enhance traffic flow and parking availability. Codes model the busiest day as the design standard, hence, many spots are rarely used. Either way, storm sewers direct all runoff to the detention facility. Alternatively, LID would use bioswales with low growing vegetation between parking areas. Drainage from these bioswales infiltrates, and when infiltration capacity is exceeded, outflows toward a connective storm sewer and finally to a much smaller detention pond. Simultaneously, pervious pavements could be implemented for areas that rarely receive parking resulting in a reduction of pavement, storm sewers, and formal detention as stormwater run-off has been redistributed into the parking lot.
Soil preparation remaining the same, grass pave systems can save 25% over traditional concrete for a significant portion of parking. Decreased storm sewer infrastructure costs necessarily follow from good LID design. Detention storage is easier to implement if every group of parking has four feet of bioswale where two rows of parking come together. Combine that with eighteen feet of pervious pavement through the use of vegetated stone storage parking spots or pervious concrete and you have reduced detention with no adverse impact to the footprint in any way.
Pervious pavements include pervious concrete, concrete blocks, pervious asphalt or non-pavement materials such as vegetated parking. Pervious pavements which appear to be solid require annual cleaning with a suction type vacuum system to remove any accumulated fines. This cleaning will also remove oils and other materials that often adhere to the fines. Non-pavement materials will require regular mowing by a mulching style mower. If designed beneath the pervious pavers, this detention system would require no maintenance.
Raingardens and bioswales, while technically not the same are very similar. Both require measurement of base soil infiltration potential. Regionally, infiltration rates above 0.5 in/hr are considered good candidates for infiltration systems, however, Houston soils generally require help in the form of a graded drainage system under the raingarden or bioswale. System maintenance requires removal of any debris and upkeep of vegetation. Selection of vegetation that prefers being dry, but tolerates saturation for short periods of time will ensure that occasional watering is required the first year, but never thereafter.
The city has a program that provides funding for Green Development of which LID is a component. Visit www.houstontx.gov/igd/ or contact me at Tetra Land Services to learn more.
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.