in a Post-Harvey World
Low-Impact Development (LID), a landscape-based infrastructure design strategy, typically focuses on water quality management, although the storm water management benefits can be significant if the right strategies are applied and maintained. As LID gains prevalence, it plays an increasing role in resilient design — particularly in the wake of extreme natural events such as Hurricane Harvey.
The driving concept behind LID is maintaining the pre-development hydrology of a site through a combination of conserved pervious areas, distributed detention/retention, and engineered landscape/soil.
The driving concept behind LID is maintaining the pre-development hydrology of a site through a combination of conserved pervious areas, distributed detention/retention, and engineered landscape/soil. In the context of urban development, LID mitigates the impacts of impervious surfaces, restoring pre-development runoff rates.
Generally, flood mitigation calls for large, end-of-system detention facilities to mitigate storm water impacts. LID features instead provide distributed detention to store water in the upstream regions of a development, which impacts the timing and volume of runoff before it gets to the end of the system. Through a combination of distributed surface and subsurface storage, LID can been used to significantly reduce the size of required storm water detention systems.
Given the small scale and distributed nature of LID, expectations for storm water management should be grounded in the principal goal: pre-development hydrology. LID was never intended as a flood control strategy for extreme events like Hurricane Harvey, which brought unprecedented rainfall. No amount of LID-prescribed soil amendments or increase in pervious surfaces will prevent such extensive flooding; however, LID can help manage street-level flooding and allow for a quick rebound after extreme events. This was recently evidenced at the Bagby Street mixed-use district, located in the heavy commercial area of Midtown Houston, in which the rain gardens helped keep Bagby Street (a major thoroughfare) accessible while the adjacent Brazos Street (conventional drainage) languished under almost three feet of water.
Additionally, LID features are typically located in the upstream reaches of a development, allowing them to absorb and store water even as floodwaters continue to rise. LID furthermore creates disconnection of impervious surfaces, slowing the flow of runoff as it travels toward the floodplain. LID provides a cost-effective method for managing storm water to provide both on-site and downstream drainage benefits.
A major component of LID is appropriately chosen vegetation — often native or adaptive — which tolerates long periods of inundation or drought. Over time, native plants help restore the long-term permeability of soil. If properly maintained, healthy plant roots will also improve soil permeability, absorbing more moisture and further reducing runoff rates. Thus in addition to providing water quality benefits, these plants enhance the storm water management attributes of LID.
While LID does not on its own “solve” flooding, it significantly advances floodplain management by preserving pre-development conditions. In a post-Harvey world, our approach to urban design must evolve to focus on resiliency and conservation of natural, hydrologically sensitive landscapes. Properly implemented, LID strategies can substantially reduce risk and mitigate damages associated with extreme, water-related weather events — both surplus and drought.