Eco-Hhydrological Impacts Of Urbanization And Low Impace Development

By changing regional surface, urbanization has long been recognized as affecting hydrological characteristics of streams and rivers and consequently as one important factor in deterioration of riverine ecosystem. Low Impact Development (LID) is a site design strategy with a goal of maintaining or replicating the predevelopment hydrologic regime through the use of design techniques to create a functionally equivalent hydrologic landscape and finally achieves the goal of protecting or improving eco-environment. Study on alteration of urbanization on hydrological regime and effect of LID on mitigation of urbanization and downstream eco-environment are of importance in ecological utilization and management of urban rainfall and promote a positive hydrological process.Taking one area going through rapid development in Millcreek watershed, Lenexa city, an eco-hydrological indicator system was developed, which was then used to examine the effects of urbanization and performances of bioretention, one of the most popular LID on mitigation of urban hydrological effect and improvement in down stream ecology. Factors influencing bioretention performances and a global sensitivity analysis of bioretention design elements were then discussed. At last, hydrological effects of four stromwater management practices of detention pond, pervious pavement, infiltration trench and bioretention were examined to compare the differences of LID and traditional runoff controls. Main conclusions were drawn based on the study presented herein:(1) Five fundamental flow characteristics of magnitude, frequency, duration, timing and rate of change determine the numerous potential eco-related hydrological indicators. An indicators system was developed based on the riverin improvement by hydrological control and intension and goals of LID, composed of indirect and direct eco-hydrological indicators to evaluate and predict down stream biodiversity and ecology. Indirect indicators include flow duration curve, peak flow discharge exceedance curve and runoff coefficients, while direct indicator is T0.5.(2) By changing imperviousness, depression depth on the pervious area and Maning’s n, SWMM models were developed to simulate runoff under different land use development. Storms with different return periods were used to evaluate effect of urbanization on the hydrological effects and the results showed that the higher imperviousness, the steeper hydrographs, lower flow duration time, greater variation in flow magnitude and greater runoff coefficient. Flow duration curve, peak flow discharge exceedance curve, runoff coefficients and T0.5 were used to examine effect of imperviousness on the magnitude, frequency, and duration of instantaneous runoff , peak discharge and T0.5. Results showed that the higher imperviousness, the greater peak discharge, frequency and duration. T0.5 showed a decreasing tendency indicating a negative effect of urbanization on the riverin ecology.(3) A paired long-term SWMM and RECARGA based model was used to simulate conditions both before and after development and water movement in bioretention. Flow duration curves demonstrated that after bioretention controlled, frequency, magnitude and duration of small flows that occur more than 90% of the total time closely matches those of undeveloped condition across a full spectrum. Peak flow descreased significantly under bioretention controlled condition. Peak flow frequency exceedance curve showed that bioretention had great performance for the high frenecy rainfall events, for the low frequency rainfall events, however, performances control is not obvious. Runoff coefficient after bioretention controlled is 0.165, which is still larger than the value under pre-development of 0.081, but still showed significant improvement than the developed uncontrolled. T0.5 value after bioretention controlled showed significant improvement compared with the value of“developed uncontrolled”condition, which was close to the pre-development T0.5 of 1.34%. This indicated the significant performance of bioretention in improving downstream ecology. Flow duration curve, peak flow exceedance curve and T0.5 can be effective indicators to evaluate LID performances in mimicking pre-development hydrology and improvement of riverin ecology.(4) Runoff coefficient, which considered runoff both from impervious and pervious area and also considered specific topography like slope and subcatchement width, can be used as important factor for bioretention design. The underdrain is not important for the water quality target designed biorention, however, it proved to be an important factor if runoff volume control is one of the design objectives.(5)Study of global sensitivity analysis of bioretention design elements on its hydrologic performances showed that bioretention surface area is the most sensitive design element to most of the hydrologic performance metrics for both groups of without underdrain and with underdrain while the depth of the gravel is the least sensitive design element. When the bioretention are installed without underdrain, sensitivity of design elements ranked in the order of: surface area>saturated infiltration rate of planting soil>depth of planting soil>saturated infiltration When the bioretention are installed with underdrain, sensitivity of design elements ranked in the order of: surface area> saturated infiltration rate of native soil>underdrain size. The design elements with underdrain are more sensitive to the hydrologic performances metrics than without underdrain.(6) Hydrological and eco-logical performances of the best management practices(BMP) and LID were analyzed and compared using an indicator system composed of flow duration curve, peak flow frequency exceedance curve and runoff coefficient to cover a full hydrologic regime based on the long-term simulation. Results indicated detention had a better performance for the large rainfall events, but is limited in recharge to groundwater; infiltration trench was better than biorention, but it is hard to incooperate into local plan because it has no water pollutant removal efficiency and asthetic function. Porous pavement has the best performances in imitating the“undeveloped”flow duration curve, peak flow exceedance curve and runoff coefficient, however, it tends to have“over control”phenomenon, which is limited due to the expensive porous materials. Biorention, which had great hydrological performances for the small rainfall events and water quality improvement and ashthetic function, is the most promising LID practice. It is suggested that combination of both traditional and LID stormwater practices will achieve the best performances in copying the“undeveloped”hydrology.As a conclusion, by studying advanced management strategy and storm water management practices, ecological effects of LID and urbanization were studied, which will offer some advice to solve urbanization problems and improve environment quality.