The allocation of surgeries to operating rooms (ORs) is a challenging combinatorial optimization problem. There is also significant uncertainty in the duration of surgical procedures, which further complicates assignment decisions. In this article, we present stochastic optimization models for the assignment of surgeries to ORs on a given day of surgery. The objective includes a fixed cost of opening ORs and a variable cost of overtime relative to a fixed length-of-day. We describe two types of models. The first is a two-stage stochastic linear program with binary decisions in the first-stage and simple recourse in the second stage. The second is its robust counter-part, in which the objective is to minimize the maximum cost associated with an uncertainty set for surgery durations. We describe the mathematical models, bounds on the optimal solution, and solution methodologies, including an easy-to-implement heuristic. Numerical experiments based on real data from a large health care provider are used to contrast the results for the two models, and illustrate the potential for impact in practice. Based on our numerical experimentation we find that a fast and easy-toimplement heuristic works fairly well on average across many instances. We also find that the robust method performs approximately as well as the heuristic, is much faster than solving than the stochastic recourse model, and has the benefit of limiting the worst-case outcome of the recourse problem.

This paper compares hydrologic records and geomorphic effects of several historic floods in the central Appalachian region of the eastern United States. The most recent of these, occurring in November 1985, was the largest ever recorded in West Virginia, with peak discharges exceeding the estimated 500-year discharge at eight of eleven stations in the South Branch Potomac River and Cheat River basins. Geomorphic effects on valley floors included some of the most severe and widespread floodplain erosion ever documented and exceeded anything seen in previous floods, even though comparable or greater rainfall and unit discharge have been observed several times in the region over the past 50 years. Comparison of discharge-drainage area plots suggests that the intensity and spatial scale of the November 1985 flood were optimal for erosion of valley floors along the three forks of the South Branch Potomac River. However, when a larger geographic area is considered, rainfall totals and discharge-drainage area relationships are insufficient predictors of geomorphic effectiveness for valley floors at drainage areas of 250 to 2500 km2.Unit stream power was calculated for the largest recorded flood discharge at 46 stations in the central Appalachians. Maximum values of unit stream power are developed in bedrock canyons, where the boundaries are resistant to erosion and the flow cross-section cannot adjust its width to accommodate extreme discharges. The largest value was 2570 W m-2; record discharge at most stations was associated with unit stream power values less than 300 Wm-2, but more stations exceeded this value in the November 1985 flood than in the other floods that were analysed. Unit stream power at indirect discharge measurement sites near areas experiencing severe erosion in this and other central Appalachian floods generally exceeded 300 W m-'; reach-average values of 200-500 W m-' were calculated for valleys where erosion damage was most widespread. Despite these general trends, unit stream power is not a reliable predictor of geomorphic change for individual sites. Improved understanding of flood impacts will require more detailed investigation of interactions between local site characteristics and patterns of flood flow over the valley floor.

Dead Run is a 14.3 km2 urban drainage basin, which is a tributary to the Gwynns Falls, the principal study watershed of the Baltimore Ecosystem Study. Hydrologic response in urban watersheds is examined through analyses of rainfall and discharge observations from the Dead Run watershed during a 6 month period beginning in June of 2003. Rainfall variability for flash flood–producing storms in Dead Run can be quite large when viewed from a Euclidean perspective. When viewed from the perspective of a distance metric imposed by the drainage network of Dead Run, however, the spatial variability of rainfall is small. The drainage network structure diminishes the effects of spatial rainfall variability for storm event hydrologic response, resulting in Dead Run exhibiting striking uniformity of response to storms with contrasting spatial distribution of rainfall. There is large storm‐to‐storm variation in the event water balance of Dead Run. Variation is linked to antecedent soil moisture (from the pervious portion of the watershed underlain by urban soils), rainfall variability, and spatial heterogeneity of runoff production.

A storm system near the Blue Ridge Mountains of Virginia produced peak rainfall accumulations exceeding 600 mm in a 6‐hour period during the morning and early afternoon of June 27, 1995. The peak flood discharge of 3,000 m3 s−1 on the Rapidan River at a drainage area of 295 km2 places this event on the envelope curve of flood discharge for the United States east of the Mississippi River. Observations of radar reflectivity factor and Doppler velocity made by the Sterling, Virginia, WSR‐88D (Weather Surveillance Radar–1988 Doppler) are used for analyses of the storm. The temporal and spatial variability of rainfall are examined on a 1‐km grid scale and 6‐min timescale. Like many heavy rainfall events, storm motion played a key role in the production of heavy rainfall for the Rapidan storm. Storm motion and storm evolution for the Rapidan storm were closely linked to topographic features at the scale of the ridges which extend southward from the Blue Ridge and delineate the Rapidan basin. Key elements of the storm environment included strong boundary layer winds directed upslope toward the Blue Ridge, weak upper level winds, high precipitable water values, and a near‐saturated atmospheric column up to 6 km. An important element of storm structure was the low‐reflectivity centroid of the storm. This feature of the storm was related both to the exceptional rainfall rates of the storm and to the underestimation of storm total rainfall by the operational WS‐88D precipitation products. Components of the atmospheric and land surface water budgets are derived. The cumulative discharge from the Rapidan River was 0.87×108 m3 (296 mm over the 295‐km2 catchment). The storm total precipitation for the Rapidan basin was 1.01×108 m3 (344 mm over the catchment). The precipitation efficiency of the storm, that is, the ratio of storm total rainfall to atmospheric water vapor inflow, was approximately 90%.