Measured Scour Depth Research Articles

A Scour Depth Approach for Deriving Erodibility Parameters from Jet Erosion Tests

Abstract. Typically the erosion rate of cohesive soils is modeled using the excess shear stress equation, which includes two soil parameters: the erodibility coefficient (k d ) and the critical shear stress (I„ c ). A jet erosion test (JET) is a standardized method available for deriving the erodibility of cohesive soils. The JET data are typically analyzed using a Blaisdell solution approach. A second solution approach based on direct parameter optimization to the measured scour depth data has recently been proposed but with limited evaluation. Therefore, the objectives of this research were to: (1) develop a new spreadsheet tool that simultaneously solves for the erodibility parameters using both solution approaches, (2) evaluate the solutions in terms of their ability to predict the observed scour depth data, and (3) quantify differences in the predicted erodibility parameters from the two approaches. A series of JETs conducted across the Illinois River watershed in eastern Oklahoma were used to evaluate the performance of the spreadsheet and the solution methodologies. The new scour depth solution provided improved fits to the original scour depth data along with being more stable in converging to a solution as a function of the initial parameter estimates. The automated spreadsheet provides an easy-to-use tool for deriving erodibility parameters from JETs.

RSS Based Bridge Scour Measurement Using Underwater Acoustic Sensor Networks

Bridge Scour is one of the major causes of bridge failures all around the world and there have been significant efforts for its detection and measurement using different acoustic approaches. In this paper, we propose and investigate an effective method to utilize Received Signal Strength (RSS) for measuring scour depth where acoustic sensors are deployed. We also extend a statistical testing to determine the difference in signal levels at the sensor nodes prior to and after scour formation and subsequently determine the actual depth of scour. Additionally, we make an attempt to evaluate underwater distance and depth using signal strength perceived at the receiver which makes it free from the requirement of accurate receiver-sender synchronization in contrast to Time of Flight (ToF) or Time of Arrival (ToA) techniques. The scour depths are eventually compared for the conditions when the bottom is composed of a single or multiple layers. The simulation results clearly show that different depths are calculated for the case of multilayered bottom (0.8 to 3.9 meters for instance) as compared to a constant depth of 2 meters for the case of a single layered bottom.

Scour depths near coastal structures due to the 2011 Tohoku Tsunami

Herein, scour depth measurements are compared with theoretical and empirical estimates obtained at four sites in Chiba and Fukushima Prefectures impacted by the 2011 Tohoku Tsunami. At sites where scour was caused by either floodwall overtopping or acceleration of flow around building foundation footings, results of published scour experiments are compared with measured scour depths.

Observation Method for Estimating Future Scour Depth at Existing Bridges

Bridge scour can cause damage to bridge foundations and abutments. Bridges with foundations that are unstable for calculated and/or observed scour conditions are termed scour-critical bridges. There are approximately 17,000 scour-critical bridges in the United States. This designation comes in part from the use of overly conservative methods that predict excessive scour depths in erosion-resistant materials. Other methods capable of overcoming this overconservatism are relatively uneconomical because they require site-specific erosion testing. This paper proposes a new bridge scour assessment method. The new method, termed the observation method for scour (OMS), was developed for the Texas Department of Transportation’s statewide bridge scour assessment program. The proposed method does not require site-specific erosion testing and accounts for time-dependent scour in erosion-resistant materials. OMS was developed for use as a first-order assessment in combination with a routine bridge inspection program. OMS uses charts that extrapolate or interpolate measured scour depths at the bridge to obtain the scour depth corresponding to a specified future flood event. The scour vulnerability depends on the comparison between the predicted and allowable scour depths. This paper also includes a new hydraulic-hydrologic analysis procedure for the determination of flow parameters required in OMS. This procedure was developed specifically for the state of Texas. The new hydraulic-hydrologic analysis procedure could possibly be applied to other regions that have sufficient flow gauges. The nine case histories used to validate OMS showed good agreement between predicted and measured values. OMS was then applied to 16 bridges, 10 of which were scour-critical bridges that had sufficient information for OMS to be carried out. Six out of these 10 bridges were found to be stable according to OMS.

Local scour estimation at check dams in torrential streams in south east spain

.For several decades, the check dam building has been one of the most commonly used structural measures in the Forestry Hydrological Restoration Programs and Plans in the torrential basins of south east Spain. Two examples are the catchments studied here, drained by the Cárcavo and Torrecilla torrential streams belonging to the Segura Basin. Obtaining a more stable bed and a less powerful flow requires transitory hydro‐morphological stages such as check dam filling upstream or local erosion downstream. The main aims of this paper are to estimate local scouring induced by check dams downstream, and to find evolution stages of scouring depth, the response time of the local erosion point and the local maximum depth to reach a stable bed situation. To do this, the results obtained by different methods were compared with measured scour depths for various hydraulic radii. From this comparison, performed in non‐uniform sediment size beds and in clear‐water conditions, the Fahlbusch's formula gives a better fit, while the Jaeger's and Bormann and Julien's methods tend to underestimate the results. These may be used to calculate local instantaneous scouring in flows less than bankfull, but give a worse estimate for maximum equilibrium scouring. Under active bed conditions, the temporal response of the cross‐sections experiencing local scour processes below these structures reflects the variations in hydraulic regime during flooding. In general, a rapid evolution phase is followed by a slow stage, which in the most torrential sectors has still not reached the equilibrium threshold.

Measurement of scour in cohesive soils around a vertical pile-simplified instrumentation and regression analysis

The flow-structure interaction in an ocean environment, for a pile placed in an erodible bed where currents and waves are of prime concern, changes the bed elevation due to scour in the vicinity of the obstruction. There are certain difficulties in predicting the scour depth, as the particulate movement of sediments due to scour is time-dependent. Considering the complexity of the problem, an instrumentation system has been developed for measuring scour depth with time. This paper outlines the experimental laboratory techniques used to measure scour around pile foundations in silty-clay sediments. Detailed laboratory testing on model piles having a diameter of 50-110 mm was carried out in a wave flume having a length of 30 m and a width of 2.0 m. The motivation of these studies is to obtain observational experience of scour rate in silty-clay soil. Scour depths were monitored continuously for different combinations of wave characteristics and current velocities, and scour-time history is measured. Based on the measurements, a relationship for the scour depth in terms of duration of flow, soil properties, model characteristics, and fluid parameters is presented.

Local scouring in low and high gradient streams at bed sills

The main characteristics of local scouring downstream of bed sills, forming a staircase-like system in high-gradient streams with non-uniform alluvium, have been investigated through 13 clear-water laboratory runs. Three initial longitudinal slopes and different flow rates were considered, keeping the same distance between the baffles. The grain size distribution of the sediment is that of a real alpine torrent scaled to the model dimensions. The measured scour depth, length and shape are compared to previous results concerning low gradient and uniform sediment gradings. A dimensional analysis approach appears to remain valid; nevertheless some simplifications cannot be made, since the jet regime plays an important role both for the depth and the length of the scour, and consequently affects the scour shape. Two equations are proposed for the estimation of the maximum scour depth and length. The equations are from previous data sets on low-gradient tests and a new one of experimental results on high-gradient cases.

Case study: An analysis of pier scour using the SRICOS method

In the process of the bridge design, especially the determination of width and length of pier foundations, the scour depth around the bridge could be very important factor for the economic and safe design of the bridge. The Woodrow Wilson bridge across the Potomac River in Washington D.C. is a six lane bridge which is being replaced by a twelve lane bridge due to the rapid growth of traffic. In this study, the scour depths are calculated for the existing bridge by using the SRICOS method, which has been developed to predict the evolution of the local scour depth at a bridge pier founded in soil or soft rock and compared with the measured scour depths. The scour depths of the replacement bridge are also calculated and discussed for the design purposes. The soil samples are taken from the bridge site by using thin wall Shelby tubes and tested in the EFA (Erosion Function Apparatus) to obtain the erosion function. The discharge hydrograph for the entire bridge life is obtained from the USGS website. Using these hydrologic and geotechnical data, the scour analyses were performed for the selected bridge piers. The E-SRICOS method and the S-SRICOS method give reasonable predictions.

IMPINGING JET CALIBRATION OF EXCESS SHEAR SEDIMENT DETACHMENT PARAMETERS

An excess shear sediment detachment relation is commonly used in erosion prediction models. Traditionalfield methods for calibration of soil-dependent empirical constants are both expensive and conceptually suspect. The subprocessesof sediment detachment and transport cannot be physically separated, therefore they cannot be calibratedindependently. Impingement of a clear-water jet on a soil bed produces detachment without transport at the point ofmaximum scour. In this article, a mathematical description of the impinging jet scour process is used to calibrate anexcess shear detachment relation. Four calibration models are developed based on variants of the jet-scour rateequations (differential and integral form, each evaluated using arithmetic and logarithmic values for dependentvariables). All models assume detachment is linearly related to excess shear.<br><br>The validity of this calibration technique has been tested in the laboratory by measuring scour depth versus time forsix different soils, all evaluated in a disturbed, saturated condition. A total of 14 replicates on the soils were conductedwhere hydraulic inputs were varied with replicate. Statistical analyses were used to place confidence limits about theoptimal sediment detachment values for each of the four models. Only the arithmetic differential model produced resultsconsiderably different than the other three. The integrated log model produced the tightest confidence intervals and this issuggested for further use. The size of the confidence intervals demonstrates the replicability of the technique. Thetechnique produced detachment rate constants that are similar to field calibration studies for soils of similar properties.Critical shear values are an approximate order of magnitude lower, but all soils were evaluated only in the highly erosive,disturbed, unconsolidated, and saturated condition. The technique can be applied to soils in other conditions and it ispossible that this discrepancy is due to different soil conditions.