Improving Process Understanding of Urban Bottomland Hardwood Forests, and Contemporary Floodplain Management

Background/Question/Methods

Contemporary urban floodplain management practices are under increasing scrutiny, particularly in regions where climate change predictions include increased precipitation. Bottomland hardwood forest (BHF) conversion in the 19^th and 20^th centuries included installation of drainage and flood control structures coupled to changes in vegetation and soils, which drastically altered the hydrology of streams, floodplains, and remnant BHF. With urbanization often comes increased frequency of peak flows, altered microclimates and reduced biodiversity. Stream channels typically broaden and deepen in response to increased volume, velocity and frequency of peak flows, leading to increased channel instability, accelerated erosion, loss of floodplain function and habitat. Instrumentation was installed in lower reaches of the Hinkson Creek Watershed (230km²) located in the city of Columbia, Mo, in the spring of 2010 in a case study comparing a remnant BHF and an agricultural floodplain site. Instrumentation includes two 80 m² grids of eighteen equally spaced piezometers and volumetric water content sensor profiles. Grids were enlarged to 120 m² to measure leaf area index (LAI, BHF=3.1), surface infiltration capacity, and soil characteristics by the soil core method at depths of 0, 15, 30, 50, 75 and 100 cm (n = 302).

Results/Conclusions

Climate over the period of study was typical for central Missouri. 372 mm (14.6 inches) of precipitation fell between July 1 through August 31 2010 in the city of Columbia, which  received more than 1346 mm (53 inches) of precipitation in 2010. Results indicate average dry bulk density (n=150) was 1.10 (SD = 0.10) and 1.03 (SD = 0.10) in agricultural and BHF sites, respectively. Soil porosity (n=150) was 0.56 (SD = 0.04) and 0.59 (SD = 0.04) in agricultural and BHF sites, respectively. Average infiltration capacity was 44 cm/hr (SD = 38 cm/hr) and 59 cm/hr (SD = 54 cm/hr) in agricultural and BHF sites, respectively. Depth integrated calculations of equivalent depth of soil water (EDSW) was 33.3 cm/m (SD = 2.24 cm/m) and 36.9 cm/m (SD = 2.68 cm/m) in the agricultural and BHF sites respectively. A two-sample t-test indicated that EDSW is statistically different between the sites at the 99% confidence level. These results have important implications for current BHF floodplain management practices. Future research and data analyses will yield recommendations to best restore the hydrologic regime of urban forested floodplains and the services provided, including flow attenuation, water quality, habitat, and stream channel stability.