Journal of Irrigation and Drainage Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Construction operations are susceptible to high rates of rainfall-induced soil erosion due to the earth-disturbing nature of land-grading activities. Silt fence sediment barriers are a temporary sediment control practice comprising geotextile material supported by posts. Silt fence sediment barriers create containment systems to remove suspended soil particles by intercepting stormwater runoff and forming temporary impoundments. Most existing silt fence sediment barrier design guidance primarily relies on state design standards based on rules of thumb that lack scientific justification. Proper design and implementation of silt fence sediment barriers is critical and must consider site-specific factors such as topography, precipitation, soil type, and other site parameters. This technical paper describes a silt fence sediment barrier design and installation standard developed through a literature review of current state agency guidance and performance-based research. This study proposes a hydrologic design approach using local site conditions while accounting for detention volumes provided by silt fence sediment barrier installations. A streamlined hydrologic design approach and volumetric storage relationships were developed for three standard silt fence sediment barrier installation configurations (linear, J-hook, and C-shape). In addition, a user-friendly spreadsheet-based tool was developed to assist designers in calculating hydrologic and volumetric parameters, properly sizing silt fence sediment barrier segments, and estimating maintenance needs. A case study and recommendations for state agency implementation are included to demonstrate the application of the developed design approach.

The Natural Resources Conservation Service (NRCS) abstraction method is based on two main assumptions. The first is that the ratio of actual water retention to maximum potential retention is equal to the ratio of actual surface runoff to potential surface runoff. The second assumption is that the initial abstraction for the watershed is 20% of the maximum potential retention. This study shows that both assumptions violate continuity principles and proposes a modification that renders an elementary relationship accounting for all abstraction forms by dividing them into a variable and constant components. Consequently, the surface runoff computation becomes dependent on the soil initial moisture content and implicitly influenced by the initial abstraction, while retaining the advantage of the subjective selection of curve number from an extensive database from which the NRCS method has gained popularity. A time of concentration model is proposed to extend the computation for flood hydrograph generation. A numerical example is provided to demonstrate the computation and results and to show how the surface runoff variation pattern behaves for the NRCS and modified methods when either a variable or constant abstraction component dominates.

Because drainage network data is unavailable in many urban areas, the accuracy of urban flood simulation may be reduced. A linear reservoir method is proposed to characterize the drainage capacity of a pipe network, and a high-resolution hydrodynamic model of an urban surface and drainage pipe network is developed. Remote sensing satellites, design specifications, and other methods were combined with actual surveys to identify storm wells and generalize underground drainage systems. A high-resolution digital elevation model (DEM) that can represent the microscopic characteristics of the urban surface was used as an input. The process of water accumulation was investigated in the study area, and the results were validated with measured data. The results show that the simulated data agree well with the measured data. Overall, the model and method can quickly and accurately simulate large-scale high-resolution urban rainfall-runoff processes.

Crop evapotranspiration under deficit soil water conditions must be quantified to accurately manage deficit irrigation and crop water stress and achieve targeted water savings. A stress coefficient is commonly used to quantify the effect of inadequate soil water on crop evapotranspiration. A 6-year deficit irrigation field trial of maize in northeastern Colorado was used to derive the stress coefficient for maize. Results showed that crop evapotranspiration was affected by both the effect of current soil water deficit on water uptake and stomatal resistance and the impact of prior water stress on plant growth. Measured evapotranspiration was less than potential crop evapotranspiration when the soil water deficit exceeded 25% of the total plant available water. A curvilinear relationship modeled the measured stress coefficient better than the commonly used linear relationship. Prior water stress resulted in a reduction in canopy ground cover, which was linearly related to the basal crop coefficient. These combined effects, when incorporated into the crop coefficient times reference evapotranspiration model, were able to accurately estimate crop evapotranspiration with deficit irrigation.

In drainage and sewerage systems, vertical dropshafts are commonly used to convey water flow from high to low elevations. When the underground tunnel is surcharged, the lower part of the shaft is submerged. The impact of falling water on the free surface in the dropshaft entrains a vast amount of air, leading to the transport or entrapment of air in the tunnel, causing undesirable effects such as the reduction in flow conveyance capacity or air blowback. In this study, a comprehensive series of experiments was performed to study the influence of flow variables on the flow regimes in a submerged dropshaft connected to a horizontal tunnel. The air-water flow in the dropshaft was recorded using high-speed imaging, with air concentration and high-frequency pressure measurements. The experiments showed that the flow in the dropshaft-tunnel system can be classified into four regimes: (I) bubbly column flow, (II) rising slugs, (III) instability and surging, and (IV) net air transport in the tunnel, based on the dropshaft Froude number and the ratio of dropshaft diameter to tunnel diameter.

An accurate prediction of scour depth around bridge piers is crucial for economical and safe design of bridge pier foundations. The main objective of the present study is to identify the influencing cohesive parameters and their effects on the local scour processes around bridge piers, depending on various proportions of clay–sand–gravel mixtures. Twenty experimental tests were performed in a channel 25 m long and 1.0 m wide for this purpose. Runs lasted from 16 to 40 h. It was noted from the experimental work that an increment of clay fraction significantly reduces the scour depth around bridge piers. It was also observed that the initiation of scour occurred at the sides of the pier where separation of flow occurred. Typically, the maximum scour depth at the equilibrium stage was still observed at the sides of the pier. A dimensional analysis was used to propose mathematical relationships assessing the temporal scour depth variation at the wake and sides of the pier. The developed relationships yielded reasonable results with maximum error of two folds for 95.22% of total data sets for scour depth at the wake and 92.57% of the total data sets for scour depth at the sides of the pier.

The curve number (CN) method is the most widely used method to estimate the depth of runoff resulting from a given depth of rainfall. However, there are still doubts about the effect of rainfall characteristics and soil moisture conditions on the method’s parameters (CN, and the standard initial abstraction ratio, $\lambda$). In this study, design rainfall distributions were used along with Hortonian infiltration characteristics to calculate CN and $\lambda$ values for a highly clayey soil ($>80\%$). Prerainfall infiltration conditions were characterized by the measured initial soil moisture content, and Hortonian infiltration model was obtained from experimental infiltration test measurements. The main results show that there is no influence of initial infiltration rates on CN values, and rainfall duration generates substantial influence on CN. The CN is inversely related to rainfall depth, and $\lambda$ is dependent on the initial soil moisture. Further, neither tabulated CN nor standard $\lambda$ corresponds to the simulated values obtained in this case. Therefore, these findings suggest that the method’s parameters should be specifically adapted to local hydrological conditions.

The records of about 8,500 USGS streamflow measuring stations in the US provide an immense amount of runoff data over most of the country that would be useful for calibration of the curve number if the usual component of baseflow could be removed. A streamflow partitioning method that separates the flow into surface runoff and baseflow components works directly on a daily streamflow record with a commonly available spreadsheet program. The method is simple enough for manual calculation if only a few runoff events need to be partitioned, as in some flood studies, and is easy to use when many years of data are involved. The method uses a single parameter to calibrate to a streamflow record, and the calculated surface runoff and calibrated curve number have low sensitivity to changes in the value of the parameter. The method offers opportunities for improvement of the curve number because of the large number of calibrated values that could become available.

Installation of shallow drainage systems (mole drainage, gravel mole drainage, and subsoiling connected to an underlying tile drainage system) is common in fine-textured grassland soils. The performance and efficiency of these systems are known to vary, and a greater understanding of these criteria is needed. This study analyzed shallow drainage systems installed on four sites in southwest Ireland, namely Doonbeg (DG), Athea (AA), Castleisland 1 (C1), and Castleisland 2 (C2), across two high-rainfall periods (8 months in 2015–2016 and 5 months in 2017). Discharge characteristics in terms of peak flow rate, total discharge, and discharge hydrographs were analyzed. All drainage systems were responsive to rainfall events, but efficiency varied both temporally and between systems.

The flow separation zone (FSZ) is one of the main flow characteristics of the combining open channel junction and is associated with energy dissipation. Bed shear stress in the contracted flow region increases as the channel’s effective width is reduced. Reduction of the FSZ at the junction to improve efficiency is desirable. The present numerical study focuses on the effect of continuous suction and blowing perturbations in reducing the FSZ at a right-angled open channel junction. The flow field is simulated using computational fluid dynamics (CFD) software. The numerical model is validated by comparing the simulated velocity field, water surface elevation, and energy loss for the unperturbed junction flow with corresponding experimental results available in the literature. The continuous suction and blowing perturbation is generated by a sinusoidal function with zero net discharge and is applied through a rectangular slit. Three different slit locations are considered downstream of the junction. The simulated results (time-averaged) show that the sinusoidal perturbation is effective in reducing the dimensions of the FSZ and, consequently, the energy loss and bed shear stress. The results demonstrate the enhanced effectiveness in reducing the energy loss when the slit is more proximate to the junction for the chosen flow configuration and the sinusoidal perturbation characteristics. The results of the time-averaged depth ratios show only a marginal reduction due to the flow perturbation.