WATER QUALITY TRAP EFFICIENCY OF STORM WATER MANAGEMENT BASINS1

ABSTRACT: While the quality of rivers has received much attention, the degradation of small streams in upland areas of watersheds has only recently been recognized as a major problem. A major cause of the problem is increases in nonpoint source pollution that accompany urban expansion. A case study is used to examine the potential for storm water detention as a means of controlling water quality in streams of small watersheds. The storm water management basin, which is frequently used to control increases in discharge rates, can also be used to reduce the level of pollutants in inflow to receiving streams. Data collected on a 148-acre site in Maryland shows that a detention basin can trap as much as 98 percent of the pollutant in the inflow. For the 11 water quality parameters, most showed reductions of at least 60 percent, depending on storm characteristics.     

THROUGHFALL IN PLANTED STANDS OF FOUR SOUTHERN PINE SPECIES IN EAST TEXAS1

Throughfall was measured during 1978–79 beneath the canopies of adjacent stands of four major southern pine species, all on identical soil type and topography in the Stephen F. Austin Experimental Forest. Observations from 44 storms in a randomized network of 15, 5.08 cm PVC gages in a 0.4 ha plot of each species showed that throughfall expressed as percent of storm precipitation, is greatest under longleaf pine and least under loblolly pine; throughfall under shortleaf and slash pine did not differ significantly. Generally, through-fall decreased with storm size and intensity, with distance from the nearest tree stem, and is greater during summer half-year (May–October). Canopy drips, apparently accounting for the greater throughfall for the gage position closer to the stems, were more numerous than reported elsewhere. The 5.08 cm PVC gages proved to be acceptable substitutes for standard nonrecording gages in measuring throughfall. A network of 15 such gages was sufficient to sample throughfall data with 90 percent accuracy in each of the four southern pine plantations.     

RELIABILITY OF THE DESIGN STORM CONCEPT IN EVALUATING RUNOFF PEAK FLOW1

A comparative study was undertaken to evaluate peak runoff flow rates using (1) a continuous series of actual rainfall events and (2) design storms. The ILLUDAS computer model was used to simulate runoff over a catchment within the city of Montreal, Canada. A ten-year period, five-minute increment rainfall data base was used to derive peak flow frequency curves. Two types of design storms were analyzed: one derived from intensity duration frequency curves (Chicago type), the other from averaging actual rainfall patterns (Huff type). Antecedent soil moisture conditions were considered in the analyses. It was found that the probability distribution of runoff peak flow was sensitive to the choice of design storm pattern and to the antecedent soil moisture condition. A symmetrical, Chicago-type design storm with antecedent dry soil moisture produced a flow frequency curve similar to the one obtained from a series of historical rainfall events.     

OPTIMUM ALTERNATIVES FOR CONTROLLING COMBINED SEWAGE OVERFLOWS - A CASE STUDY1

ABSTRACT: Frequent high quantity overflows of combined sewage entering the Mississippi River near the city of Red Wing, Minnesota, have degraded water quality and caused concern among federal and state environmental agencies. The city of Red Wing was required to conduct a comprehensive waste control study, as part of the sewer system Construction Grant (Section 201 of PL 92–500), to identify alternative waste control and treatment measures and to recommend the optimum combination of alternatives in terms of both cost and waste control effectiveness. The study involved these basic steps: determination of present and future (year 2020) sanitary flow rates and volumes, storm runoff discharges, frequencies and volumes, and combined sewage bypass volumes; identification of alternative waste control measures; elimination of unfeasible alternatives; detailed analysis of the hydrologic, economic, and waste control feasibility of the promising alternatives; selection of the optimum combination of alternative waste control measures to satisfy the study objectives, and determination of construction priorities for the optimum control measures. Because of an uncertain budget and undetermined conditions of state and federal assistance, the city has not yet selected the optimum waste control measure for its needs. When the decisionmaking process between representatives of the city and the state commences, the optimum combination of waste control alternatives can be easily identified using the results of this study.     

THE EFFECT OF MAINTENANCE ON STORM WATER DETENTION BASIN EFFICIENCY1

ABSTRACT: Storm water detention is an effective and popular method for controlling the effects of increased urbanization and development. Detention basins are used to control both increases in flow rates and sedimentation. While numerous storm water management policies have been proposed, they most often fail to give adequate consideration to maintenance of the basin. Sediment accumulation with time and the growth of grass and weeds in the emergency spillway are two maintenance problems. A model that was calibrated with data from a storm water detention basin in Montgomery County, Maryland, is used to evaluate the effect of maintenance on the efficiency of the detention basin. Sediment accumulation in the basin caused the peak reduction factor to decrease while it increased as vegetation growth in the emergency spillway increased. Thus, the detention basin will not function as intended in the design when the basin is not properly maintained. Thus, maintenance of detention basins should be one component of a comprehensive storm water management policy.     

PROBLEMS IN WEIGHTING OF HYETOGRAPHS1

ABSTRACT: It was found that the conventional weighting factor application to hyetograph ordinates results in artificially attenuated storm patterns. A modified weighting procedure is suggested which allows adjustments in the storm timing, peak intensity, and volume but conserves the storm pattern observed at the raingage nearest to the watershed point of interest. The systematic underestimation of peak flood flows, which result from conventional hyetograph weighting, can be avoided by conserving the hyetograph shape from the raingage nearest to any subarea of a modeled watershed and merely applying weighting factors to the rainfall volumes and temporal center of gravity of several hyetographs.     

DETERMINING CRITICAL WATER QUALITY CONDITIONS FOR INORGANIC NITROGEN IN DRY,SEMI‐URBANIZED WATERSHEDS1

ABSTRACT: Traditional approaches to establishing critical water quality conditions, based on statistical analysis of low flow conditions and expressed as a recurrence interval for low flow conditions (e.g., 7Q10), may be inappropriate for drier watersheds. The use of 7Q10 as a standard design flow assumes year‐round flow, but in these watersheds, 7Q10 is zero or very small. In addition, the increasing use of multiple year dynamic water quality models at daily time steps can supercede the use of steady state approaches. Many of these watersheds are also under increasing urbanization pressure, which accentuates the flashiness of runoff and the episodic nature of critical water quality conditions. To illustrate, the conditions in the Santa Clara River, California, are considered. A statistical analysis indicates that higher inorganic nitrogen concentrations correlate strongly with low flow. However, peaks in concentrations can occur during the first storms, particularly where nonpoint source contribution is significant. Critical conditions can thus occur at different flow regimes depending on the relative magnitude of flow and pollutant contributions from various sources. The use of steady state models for these dry semi‐urbanized watersheds based on 7Q10 flows is thus unlikely to accurately simulate the potential for exceeding water quality objectives. Dynamic simulation of water quality is necessary, and as the recent intense storm event sampling data indicate, the models should be formulated to consider even smaller time steps. This places increasing demand on computational resources and datasets to accurately calibrate the models at this temporal resolution.     

山东沿海的9216号台风暴潮灾害

在１９９２年第１６号强热带风暴和天文大潮的作用下，山东沿海发生了特大风暴潮灾害，直接经济损失４３．５亿元，死亡７６人；砂质海岸遭受强烈侵蚀，损失土地约１４５．３ｈａ。     

REGIONALIZATION OF STORM HYETOGRAPHS1

ABSTRACT: Regionalization of design storms can enhance their utility. Otherwise they have to be separately developed for different regions. Huff curves developed from point rainfall data collected at Coshocton, Ohio, and Chicago, Illinois, and from area-averaged Illinois and Texas precipitation data, are compared. The curves are similar in shape and position, with some visual differences depending on quartile. Kolmogorov-Smirnov tests showed no significant differences in moat of the comparisons. Where significant differences existed, they may not represent real differences due to the small number of storms sampled. Consequently, regionalization of Huff curves from Ohio to Illinois to Texas may be appropriate. The comparison of Huff curves is affected to an unknown degree both by the effects of area averaging of data and by basis. of-development differences. The effects of observed differences in Huff curves on watershed response variables (e.g., peak flow) requires further study.     

REGIONAL RAINFALL INTENSITY-DURATION-FREQUENCY CURVES FOR PENNSYLVANIA1

ABSTRACT: A statistical analysis of all available continuous hourly and 15-minute duration rainfall records for Pennsylvania was performed to develop an updated procedure to estimate design storms. As a resuit of this study, Pennsylvania was divided into five homogeneous rainfall regions and a set of rainfall intensity-duration curves developed for each region, for return periods of 1 to 100 years and durations ranging from 5 minutes to 24 hours. The PDT-IDF curves were judged to be a better representation of Pennsylvania rainfall than the nationwide TP-40 maps, particularly for storm events of 10-years and lower return periods. The average time distribution of 24-hour storms in Pennsylvania was found to be well represented by the SCS Type II distribution. The Corps of Engineers SPS 24-hour distribution was found to differ appreciably from both the SCS Type H and the Pennsylvania 24-hour storm distribution. For storm durations between 15 and 90 minutes the standard Yarnell intensity-duration curves closely resemble Pennsylvania storm distributions.