Evaluation of the urban design storm concept

This report describes an evaluation of the assumption commonly employed in drainage design that the return period of the rainfall used to design a system is the same as the peak flow produced by that rainfall. Specifically, the sensitivity of the frequency response of four catchments to design storm parameters is examined. Parameters include, hyetograph shape, antecedent soil moisture and rainfall duration. A continuous simulation model is used to compute simulated historical frequency responses for three different long term rainfall records. Design storms are also developed from depth-duration-frequency analyses of the rainfall data. Comparisons are made on frequency graphs. It is concluded that an appropriate choice of design storm parameters can produce a design which yields peak flows of the desired return period.U.S. Department of the InteriorU.S. Geological SurveyOpe

Effect of raindrop impact and surface roughness on sheet flow

An experimental and analytical study was conducted to investigate the mechanics of sheet flow as it is affected by rainfall. Water surface profile data were taken in a laboratory flume using artificially generated rainfall and a hydraulically smooth surface. The one-dimensional spatially varied flow equation as developed from the momentum approach was then used to compute the boundary shear stress and subsequently a Weisbach type friction factor. It was found that the results below a Reynolds number of approximately 1000 could be expressed as f = C/NR where C increases with increasing rainfall intensity and surface slope. Velocity profile studies show that velocity in the surface region is retarded by the rainfall. Turbulence intensity measurements indicate that turbulence is generated at the surface due to the rainfall and also at the boundary for flow which would normally be laminar without rainfall. Spectral analysis of the turbulent measurements indicates that the rainfall shifts the turbulent energy to higher frequencies than would be the case without rainfall. Analysis of flow over rough surfaces taken by the Corps of Engineers shows that rainfall has little effect on resistance beyond the transition region and the transition point may be lowered by the presence of rainfall. A separate study of a single drop striking a stagnant water layer shows that the velocity and pressure field can be computed using a SyntheticCell-Fluid scheme to solve the Navier-Stokes equations for this case. A dimensionless maximum impact pressure model was developed and the velocity field and free surface configuration were studied. It was found that surface tension is significant, the diameter of the region of disturbance was approximately one inch, and that locally high shear stress are generated. These stresses could easily cause soil erosion.U.S. Department of the InteriorU.S. Geological SurveyOpe

Mechanics of a drop after striking a stagnant water layer

This report represents a first step in the theoretical analysis of sheet flow with rainfall impinging on the free surface. This is a common occurrence in urban areas and many design procedures do not recognize the added resistance due to the rainfall. The analytical approach consists of the numerical solution of the Navier-Stokes equations, including surface tension as a boundary condition, using a Synthetic-Cell-Fluid scheme which rigorously conserves mass and momentum. The results include a maximum impact pressure model and a quantitative discussion of pressure distribution, boundary shear, the effect of surface tension, the free surface configuration, and the various forms of energy and its transformation during the impact process. Experiments were performed consisting of drop impact pressure measurements using various drop sizes, impact velocities and water layer depths. These data were used to successfully verify the theoretical work.U.S. Department of the InteriorU.S. Geological SurveyOpe

Incorporation of uncertainties in real-time catchment flood forecasting

Floods have become the most prevalent and costly natural hazards in the U.S. When preparing real-time flood forecasts for a catchment flood warning and preparedness system, consideration must be given to four sources of uncertainty -- natural, data, model parameters, and model structure. A general procedure has been developed for applying reliability analysis to evaluate the effects of the various sources of uncertainty on hydrologic models used for forecasting and prediction of catchment floods. Three reliability analysis methods -- Monte Carlo simulation, mean value and advanced first-order second moment analyses (MVFOSM and AFOSM, respectively) - - were applied to the rainfall -runoff modeling reliability problem. Comparison of these methods indicates that the AFOSM method is probably best suited to the rainfall-runoff modeling reliability problem with the MVFOSM showing some promise. The feasibility and utility of the reliability analysis procedure are shown for a case study employing as an example the HEC-1 and RORB rainfall-runoff watershed models to forecast flood events on the Vermilion River watershed at Pontiac, Illinois. The utility of the reliability analysis approach is demonstrated for four important hydrologic problems: 1) determination of forecast (or prediction) reliability, 2) determination of the flood level exceedance probability due to a current storm and development of "rules of thumb" for flood warning decision making considering this probabilistic information, 3) determination of the key sources of uncertainty influencing model forecast reliability, 4) selection of hydrologic models based on comparison of model forecast reliability. Central to this demonstration is the reliability analysis methods' ability to estimate the exceedance probability for any hydrologic target level of interest and, hence, to produce forecast cumulative density functions and probability distribution functions. For typical hydrologic modeling cases, reduction of the underlying modeling uncertainties is the key to obtaining useful, reliable forecasts. Furthermore, determination of the rainfall excess is the primary source of uncertainty, especially in the estimation of the temporal and areal rainfall distributions.U.S. Department of the InteriorU.S. Geological SurveyOpe

Advanced methodology for storm sewer design—phase II

This report describes further development of computer models for determining the diameter, slope and elevations of each pipe in a storm drainage system in which the layout and manhole locations are specified. The design procedure is based on a least-cost criterion and utilizes discrete differential dynamic programming as the search technique. In this phase of the study a detention storage capability has been added to the model using two approaches. The first approach requires the specification of a maximum allowable outflow and computes the required storage. The second approach determines the storage volume such that the sum of the storage and pipe system costs is a minimum. The procedure for computation of expected damage costs has been changed to reflect the variation of flood damage with flood volume. Also a surface runoff component has been added. This option uses the hydrograph generation portion of the Illinois Urbana Drainage Area Simulator model. Improved cost specification methods as well as flexible pipe elevation constraint capabilities have been added. The new developments are illustrated using two example basins.U.S. Department of the InteriorU.S. Geological SurveyOpe

Flood plain management through allocation of land uses–a dynamic programming model

Despite heroic structural measures, flood damages continue to rise. This research develops a means for identifying more nearly optimal patterns of land use with particular reference to timing, depth, and duration of flooding. The major premise is that flood plain management is best viewed as a problem of allocating land uses to land parcels. A dynamic programming model is developed to determine what combination of downstream uses, which require flood protection, and upstream uses, which may increase runoff or provide protection through longer water retention, should be encouraged. The dynamic programming model and an associated simplified routing technique are demonstrated on a real watershed. Desirable extensions of the model are identified. One major result of the project is the realization of a need to classify watersheds by the degree of effective interdependence among land use decisions so as to determine the most appropriate types of analytical model s and public sector interventions for particular cases. Thinking about flood management as a problem of land use allocation is shown to be a fruitful conceptualization for exploring the issues, for developing models, and for identifying appropriate public sector interventions.U.S. Geological SurveyU.S. Department of the InteriorOpe

Advanced methodologies for design of storm sewer systems

This report describes the development of a series of computer models capable of determining the diameter, slope and crown elevations of each sewer in a storm drainage system in which the layout and manhole locations are predetermined. The criterion for design decisions is the generation of a least-cost system. The basis for all of the models is the application of discrete differential dynamic programing (DDDP) as the optimization tool. Two important concepts are introduced as optimal model components: hydrograph routing and risks and uncertainties in designs. Three routing procedures are adopted, each with its own advantages. Expected flood damage costs are evaluated through the analysis of numerous risks and uncertainties associated with the design. This analysis permits the estimation of the probability of exceeding the capacity and the corresponding expected assessed damage of any sewer in the system. The expected damage cost is added to the installation cost to obtain the total cost which is then minimized in the DDDP procedure. Two example sewer systems are used as a basis for illustrating different aspects of the various least-cost design models and developing user guidelines.U.S. Department of the InteriorU.S. Geological SurveyOpe

Model for floodplain management in urbanizing areas

A target land use pattern found using a dynamic programming model is shown to be a useful reference for comparing the success of floodplain management policies. At least in the test case, there is interdependence in the land use allocation for floodplain management--that is, a good solution includes some reduction of current land use in the floodplain and some provision of detention storage. For the test case, current floodplain management policies are not sufficient; some of the existing floodplain use should be removed. Although specific land use patterns are in part sensitive to potential error in land value data and to inaccuracy in the routing model, the general conclusion that some existing use must be removed is stable within the range of likely error. Trend surface analysis is shown to be a potentially useful way of generating bid price data for use in land use allocation models. Sensitivity analysis of the dynamic programming model with respect to routing of hydrographs is conducted through simulation based on expected distributions of error.U.S. Geological SurveyU.S. Department of the InteriorOpe

Illinois least-cost sewer system design model: ILSD-1 & 2 user’s guide

ILSD models are sewer system models for least-cost optimal design of the entire system. ILSD-1 designs for a specified layout the size and slope of the sewers with or without detention storages with user supplied rainfall and/or inlet hydrographs. ILSD-2 is similar to ILSD-1 but also with risk consideration; i.e., with the risk damage cost included in the optimization procedure and a risk equation supplied by the user. The user may choose either ILSD-1 or 2 as he (she) wishes and according to the available data. This user's guide provides the necessary information to use the computer program. Data preparation for various options to fit different engineering situations is presented.U.S. Department of the InteriorU.S. Geological SurveyOpe

Calibration Procedure and Improvements in Multsed (HES 38)

U.S. Army Construction Engineering Lab. Contract no. DACW88-84-D-0003-35unpublishednot peer reviewe