Effects of Urbanization on the Water Cycle and Aquatic Ecosystems III Posters
Presiding: C Welty, University of Maryland Baltimore County; C M Swan, University of Maryland Baltimore County
NB33O-01 1330h
Hydraulic Erosion of Cohesive Riverbanks in Response to Urban Runoff
Urbanized rivers experience increased peak discharges due to increases in non-permeable surface area within the watershed, most likely leading to higher and more frequent excess shear stresses (τ - τc), where τ is applied shear stress by channel flow and τc is critical shear stress for entrainment of boundary material. Typically, higher excess shear stress leads to increased bank erosion, but due to the complexities of cohesive soil detachment, the relation between excess shear stress and cohesive bank erosion has not been established. Cohesive bank material is detached by two processes: (1) hydraulic erosion - the lift and drag imposed by channel flow, and (2) subaerial erosion - the weakening and weathering of bank material imposed by dynamic soil moisture conditions. In this study, we analyzed hydraulic erosion of cohesive riverbanks by measuring discharge and bank erosion rates at three transects along a 600 m segment of Sand River, an urban ephemeral stream located in Aiken, SC. We evaluated magnitude, duration, event peak, and variability (number of peaks) of calculated excess shear stress distributions, and correlated the values of these four independent variables to bank erosion measurements at the three transects. Stepwise regression, in conjunction with rank correlation, found that the explanatory variable for amount of cohesive bank erosion was variability of excess shear stress at the transect with the lowest critical shear stress and event peak of excess shear stress at the two transects with higher critical shear stresses. Based on these observations we propose that (1) the event peak of excess shear stress dictates the amount of hydraulic erosion of cohesive riverbanks with moderate critical shear stresses, and (2) the variability of excess shear stress dictates the amount of hydraulic erosion of cohesive riverbanks with low critical shear stresses.
NB33O-02 1330h
Simulated Impacts of Small-scale Spatial Distribution of Impervious Area on Runoff Response of Field-scale Catchments
Impervious surface is known to negatively affect catchment hydrology through both its extent and spatial distribution. In this study, we empirically quantify via model simulations the impacts of different configurations of impervious surface on watershed response to rainfall. An ensemble of spatial distributions of 2-m impervious elements are generated for two small, headwater catchments (ca. 0.5 ha) in the USDA - Agricultural Research Service North Appalachian Experimental Watershed (Coshocton, OH). The runoff response of these watersheds under each of the simulated impervious surface scenarios is examined through the simulation of 41 historical rainfall-runoff events over the period of 1975-2003 with the Gridded Surface-Subsurface Hydrologic Analysis (GSSHA) model. The differential dependence of the hydrologic response on fractional coverage, drainage distance to outlet, and hydrologic connectivity of impervious elements is characterized by the distribution of simulated runoff peaks and volumes under the assumption of Hortonian runoff generation. The uncertainties associated with model predicted runoff with the distribution of impervious area within a watershed are further discussed on the basis of the analysis.
NB33O-03 1330h
Quantifying Baseflow Declines due to Increased Imperviousness in Urbanizing Watersheds
Land development and the associated increase in imperviousness are generally expected to cause the following two effects on streamflow: 1) increasing storm runoff volumes and peak flows; and 2) decreasing baseflow volumes and low flows. It has been suggested that the second effect is a necessary result of the first. This paper describes several case studies of baseflow trends in urbanizing watersheds based on long-term streamgage data from the northeast U.S. Hydrograph separation is used to determine annual baseflow volumes and trends are analyzed using nonparametric methods. Results show that urbanization has a variety of possible effects on baseflow. Instances of rapid declines in baseflow are due to anthropogenic water-balance alterations, such as groundwater use coupled with wastewater export from the watershed, not simply a result of increasing imperviousness. Such cases show a characteristic decline in annual total water yield (Q/P) and should not be used to infer the effect of imperviousness. The results also show that in the absence of significant interbasin transfers, decreases in baseflow are generally lower in magnitude than increases in storm runoff caused by increased imperviousness. It is suggested that this is due to: 1) reduced evapotranspiration losses; 2) leakage from water and sewer infrastructure; 3) reinfiltration of runoff from upland areas of the watershed; and 4) land development in groundwater discharge zones where there is little effect on watershed recharge.
NB33O-04 1330h
Diel Patterns in Discharge and Water Chemistry in an Urban River.
Water chemistry variables were measured during base-flow conditions at hourly intervals over 24 h, once in July and once in August 2003, in the East Branch of the DuPage River (EBDP). The EBDP flows for its entire 56 km length through one of the most heavily urbanized areas in Illinois and receives effluent from 8 wastewater treatment plants (WWTP). Discharge in EBDP varied by an average of 45% during a 24 h period, with minimum discharge occurring around 12:00 h and maximum around 21:00 h. Water chemistry variables tended to follow the diel pattern in stream discharge. Diel differences between minimum and maximum values for some water chemistry variables were substantial, e.g., SRP varied by 1.1 mg/L and NO3 varied by 3.5 mg/L, and maximum values occurred between 23:00 and 3:00 h. The influence of benthic metabolism and sediment adsorption-desorption on diel patterns in SRP and NO3 in EBDP are under investigation. The results of this study suggest that the diel pattern of discharge related to WWTP operation must be considered to accurately monitor water quality variables in urban rivers.
NB33O-05 1330h
Impacts of Urbanization on the Biodiversity of the Imperiled Snail Fauna (Gastropoda: Prosobranchia: Pleuroceridae) of the Cahaba River, Alabama, USA.
The conservation of the Pleuroceridae is of concern in the Mobile River Basin (MRB) since 31 of 38 extinct gastropod species from this drainage are pleurocerids. The fauna of the Cahaba River, as a global hot-spot for pleurocerid diversity, is of particular concern. Its flow is not regulated by dams, so its fauna has not been affected by habitat alteration due to impoundment like other major MRB tributaries. Ongoing urbanization within its watershed, however, is expected to have consequences for its snail fauna. A 1993 survey documented 23 pleurocerid species among 109 sites in the drainage. Changes in land cover from 1992 to 2004 were quantified for 10 of these. Five sites showed a 13-36% increase in urban land-cover; five showed no change. Elimia cahawbensis, E. carinifera, E. carinocostata, E. clara and Pleurocera vestita occurred among these sites during 1993 (S= 0-4 spp./site). A comparison of species occurrence in 1992 and 2005 showed no change in S at 6 sites, while three lost 1 species, and one lost 3 species. Reductions in S were not correlated with changes in land cover, however. Factors not directly correlated with urbanization may thus be contributing to losses of snail diversity in the MRB.
NB33O-06 1330h
Effects of Urbanization and Colonization of Hydrilla verticillata on the Macroinvertebrate Community Structure in Tributaries to the Lower St. Johns River, Florida
We are investigating relationships between urbanization and macroinvertebrate communities in 20 tributaries of the St. John's River, Florida. Preliminary data are available for three streams. The first is unimpacted (=reference), the second is urbanized but has a forested riparian zone (=urban), while the third is urbanized but lacks a forested riparian zone (=Hydrilla). Due to the lack of shading this third stream has been colonized by the invasive macrophyte Hydrilla verticillata. Macroinvertebrate taxa richness was 25 for the reference, 15 for the urban, and 45 for the Hydrilla stream. Densities for the reference, urban, and Hydrilla streams were 2,546, 7,026, and 24,451 ind/m2, respectively. The increase in diversity and density in the Hydrilla stream may be a result of a shift from a lotic to a more lentic environment. Functional feeding-group composition varied between streams. The reference had the greatest proportion of filter-feeders (26% vs 5-6% of ind/m2). The Hydrilla stream had the greatest proportion of predators (9% vs 2-4%) and scrapers (12% vs 1%). The urban stream had the greatest proportion of collector-gatherers (90% vs 66-71%). These preliminary data indicate that quantitative and qualitative differences in land-use/land-cover can have substantial influences on community functional attributes of small subtropical streams.
NB33O-07 1330h
Comparison of Community Metabolism in two Streams With Different Landuse
Gross primary production (GPP) and community respiration (CR) were measured in a forested (Panther Creek) and an agricultural stream (Ledbetter Creek) using the single-station, open-system method. Reaeration coefficients were estimated using the energy-dissipation model. Both GPP and CR were consistently higher in the forested stream than the agricultural stream. P/R ratio ranged from 0.09 to 0.23 showing the heterotrophic nature in both streams. In Panther Creek, mean daily GPP (1.58 gO2/m2) and CR (15.82 gO2/m2) were significantly higher during fall than summer (GPP=1.17 gO2, CR=5.02 gO2/m2). However, the seasonal differences in daily GPP (0.45 gO2 in summer and 0.33 gO2/m2 in fall) and CR (2.39 gO2 in summer and 3.53 gO2/m2 in fall) were not significant in Ledbetter Creek. Higher GPP and CR in the forested stream can be attributed to more stable hydrology and sediment composition compare to the agricultural stream that is subject to frequent spates and sediment movements. The higher metabolic activities observed in Panther Creek during fall compare to summer is a result of combined effect of higher light intensity reaching to benthic community and increased organic matter post litterfall.
NB33O-08 1330h
The Urban Ecology Institute's field studies program: utilizing urban areas for experiential learning and ecological research
The Urban Ecology Institute (UEI) promotes the stewardship of healthy urban ecosystems by improving science and civic education for middle and high school youth and by working with urban communities to protect and transform natural resources. Established in 1999, UEI's field studies program engages over 1000 youth in the greater Boston area. A substantial component of this program involves water quality monitoring. We have recently adapted protocols from published leaf breakdown studies for incorporation into the UEI water quality curriculum. A 2004 pilot study of these leaf breakdown activities, conducted at four sites, compared rates of red maple breakdown to those of Norway maple, a potentially invasive urban street tree. Preliminary data from this successful pilot study suggest that leaf litter inputs from the two different tree species have varying effects on stream ecosystem function. We present this study as an example of how urban areas can be utilized for both ecological research and inclusive experiential learning through which science and mathematic knowledge can be effectively communicated.
http://www.bc.edu/bc_org/research/urbaneco/program/fieldStudies_home.html
NB33O-09 1330h
Land use and Water Quality Changes of the Nakkhu Khola River, Kathmandu, Nepal
The Nakkhu Khola river, which is one of tributaries of the River Bagmati flowing through the heart of Kathmandu valley, was studied to evaluate the effect of land-use changes on the river water quality and public perception. The land-use patterns show increasing urbanization in the order of up>mid>downstream. We analyzed selected physico-chemical parameters including temperature, transparency, velocity, pH, specific conductivity, total alkalinity, nitrate-nitrogen, total ammonia, total phosphate, chloride, Biological Oxygen Demand and Chemical Oxygen Demand at up, mid and downstream stations during monsoon and post-monsoon. The result showed decreasing water quality conditions downstream. Community surveys at residents from corresponding river sites indicate no use of river water for drinking and contrasting perceptions along the river on causes of degradation. Physico-chemical studies integrated with biological criteria are needed to assess further water quality changes in Nepal rivers associated with anthropogenic development.