Rollin H. Hotchkiss

Development of a snowfall-snowmelt routine for mountainous terrain for the soil water assessment tool (SWAT)

The soil water assessment tool (SWAT) is a hydrologic model originally developed to evaluate water resources in large agricultural basins. SWAT was not designed to model heterogeneous mountain basins typical of the western United States, and as a result, has performed poorly when applied to mountainous locations. The intent of this study was to increase the versatility of SWAT by developing the capability to simulate hydrology of a non-agricultural mountainous region with a large snowmelt component. A western Wyoming basin, representative of Rocky Mountain basins, was selected to evaluate model performance, identify governing hydrologic processes, and improve the snowmelt routine. An initial evaluation of SWAT performance indicated an inability of the model to represent snowmelt processes. Based on simulation results and field observations, algorithms were developed which use elevation bands to distribute temperature and precipitation with elevation. Additional routines which control snowpack temperature, meltwater production, and areal snow coverage were designed to simulate the influence of season and elevation on the evolution of basin snowpack. The development of the new snowmelt algorithms improved the average annual Nash–Sutcliffe R2 correlation between simulated and observed Wind River streamflow from an initial value of −0.70 to +0.86.

Sustainable sediment management in reservoirs and regulated rivers: Experiences from five continents

By trapping sediment in reservoirs, dams interrupt the continuity of sediment transport through rivers, resulting in loss of reservoir storage and reduced usable life, and depriving downstream reaches of sediments essential for channel form and aquatic habitats. With the acceleration of new dam construction globally, these impacts are increasingly widespread. There are proven techniques to pass sediment through or around reservoirs, to preserve reservoir capacity and to minimize downstream impacts, but they are not applied in many situations where they would be effective. This paper summarizes collective experience from five continents in managing reservoir sediments and mitigating downstream sediment starvation. Where geometry is favorable it is often possible to bypass sediment around the reservoir, which avoids reservoir sedimentation and supplies sediment to downstream reaches with rates and timing similar to pre-dam conditions. Sluicing (or drawdown routing) permits sediment to be transported through the reservoir rapidly to avoid sedimentation during high flows; it requires relatively large capacity outlets. Drawdown flushing involves scouring and re-suspending sediment deposited in the reservoir and transporting it downstream through low-level gates in the dam; it works best in narrow reservoirs with steep longitudinal gradients and with flow velocities maintained above the threshold to transport sediment. Turbidity currents can often be vented through the dam, with the advantage that the reservoir need not be drawn down to pass sediment. In planning dams, we recommend that these sediment management approaches be utilized where possible to sustain reservoir capacity and minimize environmental impacts of dams.

Water yield responses to high and low spatial resolution climate change scenarios in the Missouri River Basin

River Basin in Colorado. However, differences in water yields calculated from GCM and regional climate model scenarios have not been compared for a major river basin. [3] The uncertainty in hydrologic impacts due to the spatial scale of climate scenarios needs further exploration. Given the computer and human resources needed to generate higher resolution scenarios, it is important to determine the added value and/or further uncertainty in impacts assessment that result from their use. Moreover, the differences in hydrological impact could have implications for water resource planning in the long-term future. [4] In this project, two climate change scenarios were developed for the Missouri River Basin: one from a coarse grid GCM and one from a fine scale Regional Climate Model (RCM). The GCM provided the initial and lateral boundary conditions for driving the RCM. Impacts of these scenarios on water yields were analyzed using the Soil and Water Assessment Tool (SWAT) hydrologic model.

Evaluating velocity measurement techniques in shallow streams

Accurate flow field measurements in shallow rivers are necessary for many applications including biological investigations and numerical model development. Unfortunately, river velocity data is difficult to obtain due to the limitations of traditional velocity meters. Acoustic Doppler Current Profilers (ADCP) provide a potential alternative to traditional point-velocity measurements. However, these instruments have not been thoroughly tested against accepted techniques in natural streams. The objectives of this research were to evaluate the adequacy of ADCP instruments for conducting velocity measurements in quasi-wadeable streams and to provide instrument selection guidance for similar flow environments. These objectives were met by conducting ADCP, Acoustic Doppler Velocimeter (ADV), and Price current meter measurements at nine coinciding verticals in two rivers. The performance of each instrument was evaluated with regards to data accuracy, desired parameters, and required sampling time. ADCP measurements compared favorably with ADV and Price meter data for velocity profiles and depth-averaged velocities. The ADCP also showed a significant improvement in estimates of local bed shear stress when compared to global estimates determined from the water surface profile. However, excessive noise reduced the effectiveness of ADCP measurements of velocity standard deviation and velocity components. The results are discussed in the context of instrument selection for parameterization of models

Step-pool morphology in high-gradient countersunk culverts

Bed stability and morphology in countersunk culverts on steep slopes were examined to improve understanding of the parameters governing sediment flow characteristics in mountain streams. The knowledge gained was used to provide preliminary construction guidelines for the stream simulation approach of countersunk culverts, an approach that is under consideration in Washington State. Prototype conditions were evaluated in a flume for a 30 percent countersunk culvert with slopes ranging from 3 percent to 7 percent and particle relative submergence varying from 0.5 to 2.0 for three bed size distributions. The experiments were designed to satisfy the conditions of dynamic similarity, and they are preferred to field measurements because they allow a high degree of control over testing conditions. It was found that step-pool bedforms are the most ubiquitous features along the culvert gravel bed. A new formula was developed that correlates step height with the gravel-bed size distribution, relative submergence of the particles, and the Froude number. The step spacing was found to be related to step height and streambed longitudinal slope. This information was combined into a generic design method for streambed simulation of high-gradient countersunk culverts. An example case that illustrates the application of the newly derived formulas to the construction of a gravel bed for a countersunk culvert is provided.

Culvert Hydraulics: Comparison of Current Computer Models and Recommended Improvements

Several computer programs are available to analyze culvert hydraulics, including HY-8, FishXing, Broken-back Culvert Analysis Program (BCAP), Hydraflow Express, CulvertMaster, Culvert, and Hydrologic Engineering Center River Analysis System (HEC-RAS). The use of slightly different methodologies to complete the analysis results in different estimates for headwater depth, flow control, and outlet velocity. This paper compares the available hydraulic features and tests the accuracy of these seven programs as part of an effort to update the HY-8 program. Four test cases were developed. The flow controls, headwater depths, and outlet velocities were compared with values obtained through calculations based on best practice as outlined in the FHWA publication, Hydraulic Design Series No. 5. Several limitations were identified. BCAP has limited ability to analyze straight barrel culverts operating under outlet control with high tailwater, CulvertMaster reports outlet control for some low discharges as a surrogate for what is referred to as “entrance control,” Culvert misidentifies the location of hydraulic control for lower discharges and overestimates headwater elevations, and HydraFlow Express incorrectly uses critical depth at the outlet for inlet control calculations. With the test cases used here, HY-8, HEC-RAS, and FishXing most consistently agreed with accepted empirical results. It is recommended that program providers improve their products to replicate better the hydraulic conditions simulated in this paper and to extend program capabilities to include more environmentally sensitive design considerations such as fish passage.

Reservoir Sustainability and Sediment Management

AbstractDespite mounting demand for a more sustainable worldwide water supply system, available reservoir capacity is relentlessly diminishing due to sedimentation. Neither sustainable reservoir life spans nor intergenerational equity is achieved through conventional cost-benefit analyses (CBAs), which render all benefits and costs projected to occur more than several decades into a project as negligible. Consequently, future costs, including dam decommissioning or retrofitting with sediment management facilities, would be regarded as nonfactors in an analysis. CBAs have also historically failed to account for infrastructure and environmental impacts of sedimentation over time. Alternatives to the traditional application of the CBA do exist, however, such as dam owners instituting retirement funds or insurance policies, beneficiaries paying for rehabilitation or maintenance, and economists incorporating infrastructure damages and potentially logistic discount rates into their analyses. A brief case study ...

HYDRAULIC JUMPS IN BROKEN-BACK CULVERTS

A broken-back culvert has one or more changes in grade within the prismatic barrel profile. One section of a broken-back culvert is usually steep, with the steep slope contributing to high outlet velocities unless a hydraulic jump forms upstream from the culvert outlet. Predictive equations have been published for a wide range of experimental conditions but still do not describe the hydraulics of a jump as it traverses the steeply sloped section and the most downstream section of a broken-back culvert. A computer program, the Broken-Back Culvert Analysis Program (BCAP), analyzes the hydraulics of circular or box-shaped broken-back culverts and provides a comprehensive design tool for engineers. It predicts whether a hydraulic jump will occur and, if so, where it begins and ends. Analyses are performed for 10 discharges. Outputs include rating curves for headwater and outlet depths and outlet velocity and tabulations of hydraulic parameters for each discharge. The water surface profiles throughout the culvert, except through the hydraulic jump, are computed and are plotted on the screen. Experiments were performed with model culverts to assess the accuracy of the predictive equations in BCAP for headwater depth, the location of a hydraulic jump, and the hydraulic jump length. The predictions for the headwater depth at the culvert inlet matched the experimental observations well, but predictions for hydraulic jump location and length were less satisfactory. BCAP has been used for hundreds of design projects in Nebraska and has been downloaded almost 300 times from at least 22 different states. It is recommended that BCAP be improved as better experimental data become available.

Predicting fish passage design flows at ungaged streams in eastern Washington

Abstract The Washington Department of Fish and Wildlife recently established statewide guidelines for incorporating adult fish migration into culvert design. One of the design options requires an estimate of fish passage design flow for ungaged culvert sites. A new model is presented for estimating fish passage design flows at ungaged sites in Eastern Washington. The model is founded on a unique definition of fish passage design flow and an area-based approach for estimating this flow at ungaged sites. The fish passage design flow was developed by combining the concepts of allowable fish delay, established by fisheries experts to be three days, with a consecutive day analysis. This design flow ensures that fish are not delayed for more than three consecutive days during a water year. A fish passage design flow per unit area is assigned to previously delineated subwatersheds in Eastern Washington using similarity relationships derived from basin parameters and US Geological Survey gaging stations. The percent standard error for this model is 36%, a significant improvement from the 75% standard error calculated for the model previously developed for fish passage design flows in Eastern Washington.

Simple Methods for Energy Dissipation at Culvert Outlets

Experiments were performed to investigate the energy dissipation realized at the outlet of a culvert using two simple design alternatives: (1) Design I, a simple weir near the culvert outlet, and (2) Design II, a weir with a drop upstream in the culvert outlet barrel. The two designs are intended to reduce the flow energy at the outlet by inducing a hydraulic jump within the culvert barrel, without the aid of tailwater. This paper examines the jump geometry, the effectiveness of each jump type, and proposes a design procedure for practicing engineers. The design procedure is applicable to culverts with approach Froude numbers from 2.6–6.0. Both designs are effective in reducing outlet velocity 0.7 to 8.5 ft/s (0.21–2.59 m/s), momentum 10–48%, and energy 6–71%. The design layouts allow easy access for maintenance activities.