Hydrological Connections between Low-Impact Development, Watershed Best Management Practices, and Sustainable Development

Low-impact development (LID) is a relatively new concept in storm water management, which has been promoted by civil engineering, architecture design, and landscape ecology communities in recent years. Unlike traditional “conservation design criteria” that are mainly prepared to protect natural flow paths and existing vegetative features in large areas adjacent to the development site, LID principles emphasize the direct treatment of storm water at the source. It aims to develop a plethora of site design strategies with a goal of maintaining or replicating the predevelopment hydrologic regime through the use of design techniques (USEPA 2005). Hence, LID principles adopt the concept of decentralized multifunctional site designs and incorporate on-site storm water management practices, such as functional landscape, as opposed to conventional storm water management approaches that typically convey storm water runoff into large centralized facilities for treatment. LID “functional landscape” is thus designed to mimic the predevelopment hydrological conditions through runoff volume control, peak runoff rate control, flow frequency/duration control, and water quality control (U.S. EPA 2005). The current spectrum of LID technologies spans from storm water retention and detention ponds, to impervious pavements, to open bioswales, to flatter grades, to green roofs, to rain barrels and rain gardens, to level spreader and vegetated filter strips, and to some local erosion-control measures. Since the water volume and frequency of discharges can be maintained with more natural methods, an extended vision may be geared toward water quality degradation and ecosystem integrity issues.

LID technologies enhance the lengthening of flow paths and runoff time to increase the infiltration potential, and such prolonged hydraulic residence time at the surface of the soil offers opportunities for pollutant removal. Pollutants of major concern include nitrogen species, phosphorus species, heavy metals, and pathogens. Nutrients, such as nitrite, nitrate, and phosphorus, have become common contaminants in many aquatic systems in the United States. For example, the Upper Floridan aquifer is particularly vulnerable to impacts from land-use activities in karst/high recharge areas, where the aquifer is only thinly confined. Nitrate concentrations have increased in many Upper Floridan aquifer springs since the 1950s, exceeding $1\mathrm{mg}{\mathrm{L}}^{-1}$ in recent years at some springs in Lake, Marion, Orange, Seminole, and Volusia Counties (Phelps 2004; Walsh et al. 2006; St. Johns River Water Management District 2008). In public health, nitrate $\left(\mathrm{N}{\mathrm{O}}_3^{-}\right)$ may be toxic and can cause human health problems such as methemoglobinemia, liver damage, and even cancers (Cabel et al. 1982; Huang et al. 1998). Nitrate has a regulatory health limit in the United States of maximum contamination level (MCL) of $10\mathrm{mg}-\mathrm{N}{\mathrm{L}}^{-1}$ . On the other hand, phosphorus (P) may trigger the eutrophication issues in fresh water bodies, which could result in toxic algae and eventually endanger the source of drinking waters (Howarth et al. 2000). High nitrogen and phosphorus content in the water body has impeded water reuse potential and impacted ecosystem integrity and human health. These nutrients can actually be reduced or even mostly removed by using some engineered or functionalized filter media mixture through the applications of LID technologies and the watershed best management practices (BMPs) (Kim et al. 2000; Hsieh and Davis 2005; Birch et al. 2005; Seelsaen et al. 2006; Hossain et al. 2009; Ryan et al. 2009). These sorption media, such as sand, expanded clay, tree bark, wood chips, sawdust, tire crumb, alfalfa, mulch, cotton, wheat straw, and sulfur/limestone, can be applied to empower microscale LID controls that may be distributed throughout the site to effectively remove pollutants that can have detrimental effects on ecosystems.

Integrated sorption media mixtures and distributed microscale LID technologies offer both ecological and environmental benefits with different scales. At the household level, the implementation of rain gutter disconnects along with sorption media-based grass swales, bioretention systems, and other functional landscape devices may redirect rooftop runoff out of storm sewers and treat runoff on site. It reduces the burden of combined sewer overflow (CSO). LID may be expanded to fit into a suite of macroscale ecological and geoenvironmental engineering approaches, such as constructed wetland, to retrofit the hydrological cycle at existing highly urbanized areas with potential for pollution controls. They can be deemed as an integral part of green infrastructures in eco-cities. When dealing with integrated watershed management, they can also be put into practices with flexibility in connection with a variety of watershed BMPs. Examples may include, but are not limited to, riparian buffers (i.e., equivalent to large-scale open bioswales or vegetated filter strips), constructed wetlands (i.e., equivalent to large-scale bioretention ponds), conservation farms (i.e., equivalent to large-scale flatter grades), etc. Fig. 1 delineates the simplified schematic that presents the synergistic deployment of both LID and watershed BMPs simultaneously in a watershed. Substantial cost savings are anticipated with regard to life cycle cost considerations (U.S. EPA 2005).

LID technologies are linked with climate change impacts as well. Climate change will impact civil infrastructures’ management with serious ramifications. Sea-level rise will highly likely flood the coastal margin, especially the low-lying areas, and disrupt the coastal hydrological cycle. Inland precipitation patterns and associated summer extreme flow and winter snowstorm regimes will undergo change. At the watershed scale, standard design of water storage and drainage capacity may not have enough resilience to regulate the hydrological cycle and mitigate the impacts during intermittent floods and droughts. Larger hurricanes and floods could overwhelm coastal watersheds and devastate inland flood control and drainage structures. At the urban scale, there will be more swings between floods and droughts in association with uncertain variations of temperature change. It is likely that the synergistic effect of gradual sea level rise, unusual temperature variations, storm surges, and intermittent floods and droughts will collectively alter the planning, design, and operation of civil infrastructures in the next few decades.

To provide smart and sustainable civil infrastructures in concert with LID technologies, there is a need to advance our understanding of the emergence of urban landscape patterns driven simultaneously by socioeconomic and ecohydrological factors under climate change. The reciprocal linkage between both factors could in turn affect the urban hydrological cycle as well as transport and fate of nitrogen and phosphorus concentrations. As our society feverishly tackles the climate change impacts, LID technologies may be viewed as adaptive management strategies at the regional scale within complex interdependent civil and environmental engineering systems. With this understanding, the hydrological connections among LID, watershed BMPs, and sustainable development have blazed a new pathway for “sustainable hydrology.” It lies in the nexus of climate change, geomorphology and land use dynamics, sustainability sciences, social sciences, and traditional water resources engineering regimes, and deserves more research. With sustainability implications, it is believed that long-term development of sustainable hydrology may affect many interdisciplinary areas, such as heat island effect, green buildings and infrastructures, urban landscape ecology, ecosystem service design, storm water BMPs, watershed total maximum daily loads (TMDLs), urban regeneration, and carbon sequestration.