COOL CREEK WATERSHED MANAGEMENT PLAN MODELING AND PLANNING APPROACHES TO MITIGATE EFFECTS OF URBANIZATION ON DOWNSTREAM CHANNEL REACHES

Cool Creek, a 14.5-mile long watercourse with a watershed area of 24 square miles, drains parts of the City of Carmel, the Town of Westfield, and unincorporated areas of Hamilton County, Indiana, immediately north of Indianapolis. The lower portion of the watershed, in the City of Carmel, is fully developed with residential areas located adjacent to a relatively undisturbed riparian zone. The upper watershed, draining the Town of Westfield and unincorporated areas of Hamilton County, has experienced one of the highest population growth rates in the State of Indiana during the past 10 years (2000 Census Data), as the suburban sprawl of Indianapolis spreads into areas north of the City. Concerns about the sensitivity of Cool Creek to rapid changes in land use have been raised in local communities. These communities have developed a jointly funded, comprehensive investigation of existing and future stormwater management issues in the Cool Creek Watershed. Of particular concern were the increase in frequency of bank full events and the degradation of the main channel (Copeland, et. al., 1994). Urban runoff quality was also a key concern given the rapid growth in the headwaters of the watershed. Stream water quality sampling conducted during the study confirmed urban runoff quality concerns, showing elevated concentrations of total suspended solids and nutrients in several grab samples obtained during storm events.Multiple hydrologic/hydraulic modeling techniques were used to analyze Cool Creek and its watershed under existing and future land use conditions. Available GIS data allowed for an accurate physical representation of the watershed. The hydrologic modeling effort included the study of cumulative effects of future development in the watershed and the appropriateness of current local stormwater detention design standards. The HEC-RAS (steady-state hydraulic) model was used to compute flood profiles and cross-sectional velocity distributions for stream reaches experiencing streambank erosion. The velocity distributions were examined carefully to identify appropriate streambank stabilization techniques. The HEC-RAS model results were also used to remap floodplains on GIS maps to better define the riparian corridor and to establish protection areas.In addition to steady-state hydraulic modeling, the unsteady flow model XP-SWMM was used to analyze potential off-line regional detention basins designed to control the magnitude and duration of more frequent (channel forming) storm events, enhance water quality, and reduce streambank erosion. XP-SWMM was used to more accurately represent unsteady flow characteristics associated with the dynamic interaction between the stream channel, lateral diversion structures, and discharge structures.The modeling efforts revealed that existing stormwater detention standards (as enforced locally) were effective in controlling peak flow rates for larger (i.e. 50-year and 100-year) storm events, but ineffective in controlling peak flow rates and flow duration for smaller (i.e. 1-year and 2-year) storm events that lead to streambank erosion in downstream channel reaches. The results of these analyses were used to develop a comprehensive set of capital improvement and watershed-wide stormwater management policy recommendations to more effectively protect downstream channel reaches as the watershed continues to develop.