Since the mid‐1930s a variety of soil conservation practices have been applied to agricultural lands throughout the United States. While intended to reduce soil erosion, if effective, these practices should alter the hydrology of streams which drain the treated lands. This hypothesis was explored for the East Branch of the Pecatonica River, a gaged 221 square mile agricultural catchment in southwestern Wisconsin. On the basis of the analysis of peak and daily flow data there has been a decrease in flood peaks and in winter/spring flood volumes and an increase in hydrologic rise times and in the contribution of winter/spring snowmelt events to base flow. These changes do not appear to be due to climatic variations, reservoir construction, or major land use changes. Instead, they appear to have resulted from the adoption of various soil conservation practices, particularly those involving the treatment of gullies and the adoption of conservation tillage.

As watersheds are urbanized, their surfaces are made less pervious and more channelized, which reduces infiltration and speeds up the removal of excess runoff. Traditional storm water management seeks to remove runoff as quickly as possible, gathering excess runoff in detention basins for peak reduction where necessary. In contrast, more recently developed “low impact” alternatives manage rainfall where it falls, through a combination of enhancing infiltration properties of pervious areas and rerouting impervious runoff across pervious areas to allow an opportunity for infiltration. In this paper, we investigate the potential for reducing the hydrologic impacts of urbanization by using infiltration based, low impact storm water management. We describe a group of preliminary experiments using relatively simple engineering tools to compare three basic scenarios of development: an undeveloped landscape; a fully developed landscape using traditional, high impact storm water management; and a fully developed landscape using infiltration based, low impact design. Based on these experiments, it appears that by manipulating the layout of urbanized landscapes, it is possible to reduce impacts on hydrology relative to traditional, fully connected storm water systems. However, the amount of reduction in impact is sensitive to both rainfall event size and soil texture, with greatest reductions being possible for small, relatively frequent rainfall events and more pervious soil textures. Thus, low impact techniques appear to provide a valuable tool for reducing runoff for the events that see the greatest relative increases from urbanization: those generated by the small, relatively frequent rainfall events that are small enough to produce little or no runoff from pervious surfaces, but produce runoff from impervious areas. However, it is clear that there still needs to be measures in place for flood management for larger, more intense, and relatively rarer storm events, which are capable of producing significant runoff even for undeveloped basins.

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Flood peak attenuation caused by storage of flood water on overbank surfaces effectively reduces the magnitude of peak discharges in some, but not all watersheds. Several geomorphic factors that affect the storage and conveyance of flood water were investigated to assess their quantitative influence on downstream peak discharges. The MIKE11 rainfall-runoff and hydrodynamic models were calibrated for the Grant River watershed, southwestern Wisconsin. Alternative geomorphic conditions were modeled and compared to the original case. Results indicate that channel-fioodplain-terrace morphology, valley width, stream slope, and hydraulic roughness each influence peak discharges, especially for moderate magnitude (5-to 50year recurrence interval) floods. Peak discharges varied by as much as 49% between simulations depending on geomorphic conditions. Watersheds that effectively attenuate produce peak discharges that are strongly correlated with total runoff. Watersheds that attenuate little produce peak discharges that exhibit greater variance due to storm intensity and duration. floodplain flow velocities are typically much lower than channel flow velocities, and the floodplain acts as a storage reservoir [Bhowmik and Stall, 1979; Gendel'man, 198!; Wong and Laurenson, 1983; Bates and Pilgrim, 1983]. This storage and later release of a. portion of the total flood volume produces flood hydrographs that are low and broad compared to those produced in similar watersheds that lack floodplain storage (e.g., gullies or mountain streams). For high-magnitude, low-frequency floods, peak discharges are only minimally attenuated as the total discharge is dominated by overbank flow, with the entire floodplain delivering flood water much like a channel. This general explanation is well understood. Some research has considered the attenuation process in more detail, including map and field analysis of the hydraulic geometry and carrying capacity of floodplains [Bhowmik, 1984; Bhowmik and Demissie, !982; Bhowmik and Stall, 1979], geomorprologic studies of floodplain inundation and drainage [Zwolinski, 1992; Hughes, 1980; Lewin and Manton, 1975], and flume studies of the interaction of channel and floodplain flows [Smith, 1978; Rajaratnam and Ahmadi, 1979; Knight and Demetriou, 1983; Knight, 1989]. Quantitative data on flood peak attenuation are limited to comparison of upstream and downstream measurements of discharge and relatively simple computer modeling studies. Archer [1989] cites gage data that show up to 30% attenua-Paper number 94WR00323. 0043-1397/94/94WR-00323 $05.00 tion of peak discharges between two gages separated by 34.6 km on the River Tees, England. Diehi [1990], using a diffusive wave model of Black Earth Creek, Wisconsin, found that an increase in floodplain roughness from 0.053 to 0.100 (Manning n) can reduce peak discharges of moderate floods (4-to 50-year recurrence interval) by up to 27%. Wolff's [1983] computer simulations of an idealized system consisting of a prismatic channel and simplified floodplain produced...

Development type has emerged as an important focal point for addressing a wide range of social, cultural, and environmental concerns related to urban growth. Low impact development techniques that rely heavily on infiltration practices are increasingly being used to manage storm water. In this study, four development types (conventional curvilinear, urban cluster, coving, and new urbanism) were modeled both with and without infiltration practices to determine their relative effects on urban runoff. Modeling was performed with a modified version of the Natural Resources Conservation Service (NRCS) runoff method that enables evaluation of infiltration practices. Model results indicate that urban cluster developments produce the smallest volume of runoff due to the large portion of land kept in a natural condition. Infiltration practices are most effective for small storms and in developments with Hydrologic Group A soils. Significant reductions in runoff can be achieved in all four development types if infiltration practices treat many impervious surfaces. As more infiltration practices are implemented, the differences in runoff among development types diminish. With a strategic combination of site layout and infiltration design, any development type can reduce hydrologic impacts, allowing developers to consider other factors, such as convenience, marketability, community needs, and aesthetics. (KEY TERMS: low impact development (LID); urban planning; hydrologic modeling; bioretention; storm water management; land use.)