Online hydraulic design and calculations

The advance in mobile apps and internet accessibility changed the way we socialize and the way we work: Sharing information, instant communications, cloud computing and online calculations are just few examples of the internet in our daily life. One advantage of mobile apps and online calculations is that they liberate us from the desktop and allows us performing engineering calculations on site, as soon as we need or as soon as we collect new data.

Fig1. Welcome screen OTOSHEE

Some years ago, I created a website for performing some calculation that I used quite often. Over the years I’ve been including new equations, but the interface was not so friendly. I decided to improve the interface and make it friendlier, so that it can be used by others. Try the online hydraulic design website.

Fig 2. Screen OTOSHEE

Available options

The web site http://otoshee.eu.pn/ contains sections for:

• Time of concentration. 4 time of concentration equation are available.
• Rip rap. Rip rap sizing for piers and abutments.
• Weirs & gutters. Discharge estimation for weirs and gutters.
• Pipes. Head losses and minimum pipe diameter estimation.
• Scour. Hydraulic scour estimation for groynes, weirs and pipes.
• IDF precipitation. An online interactive GIS map with IDF equation for different locations.

Fig 3. IDF precipitation screen

Precipitation intensity for highway & bridge drainage design

Drainage projects usually consider a precipitation intensity based on the time of concentration and the return period. However, when designing drainage systems for bridges and highways it is important to verify the compatibility between the precipitation intensity and the driver's safety. Thus, two additional criterion should be considered: Hydroplaning and visibility.

Hydroplaning

Hydroplaning is the situation when when a layer of water is built between the wheels of a vehicle and the road surface. Hydroplaning leads to a loss of traction. The driver  of the vehicle looses control and the vehicle begins to slide. The hydroplaning of a vehicle is a function of the speed, the water depth and the inflation pressure of the wheels.

Figure 1. Hydroplaning concept (Source: Tirebuyer)

The hydroplaning based intensity is a new approach to drainage design. It seeks that rainfall that is just sufficient to cause a water depth of sheet flow at the edge of the traveled way that will cause hydroplaning. The design concept is that removal and control of flooding, caused by rainfall in excess of rainfall that will cause hydroplaning, is overdesign from a vehicle safety standpoint.

The limit speed is defined by the standards of the highways or bridge deck. Then, the speed that initiates hydroplaning is used for defining the water depth D when hydroplaning occurs. The most popular studies about hydroplaning speed are the studies performed by NASA and Gallaway et al., (1979). Then, it is assumed that water flows in sheet with depth D across the surface to the edge of the gutter. Combining the depth D with the rational formula and the Manning equation it is possible to define the hydroplaning design rainfall intensity.

Figure 2. Hydroplaning speed equation reported by Gallawat et al., (1979).

Visibility

The rainfall and the windshield wipers reduce the visibility of the driver. Such visibility reduction also reduces the safe stopping distance.

Figure 3. Driver visibility during rain (Source: Earth magazine)

The visibility design concept is that drainage removal or control of flooding caused by rain in excess of that rain, which will cause driver vision impairment, is overdesign from a vehicle safety standpoint (more drainage capacity than required).

The safe stopping distance is a function of the vehicle speed and the mechanical characteristics of the vehicle (break time and deceleration). Thus, this design criterion uses the rain intensity that allows a visibility equal to the safe stopping distance. Several studies analyzed the visibility considering the effect of different rain intensities and windshield wipers. One of the first methods for estimating the driver's visibility under different rainfall intensities and vehicle speeds is based on the experiments from Ivey et al., (1975).

We can see that there is no single method for estimating the rainfall intensity. Thus, the selection of the highway-bridge rainfall intensity is a multi-criteria process. It is suggested to consider all the criterion; the two criterion from this post and the traditional IDF - rational criterion. Then, engineering criterion should decide the optimum one.

If you require advice for design of highway/bridge drainage or doubts about the rainfall intensity, feel free to contact us.

Figure 4. Driver visibility suggested by Ivey et al., (1975)

Bridge collapse (Peru) & stream bank erosion

Last days a video of a bridge collapse in Lima (Peru) became viral. The video shows the moment when the scour of a bridge gets scoured and the bridge collapses. Discussions about the collapse of this bridge became a trending topic in several social networks, especially in Peru. Some discussions tried to find someone or something to blame and others focussed on whether the bridge was properly designed or not. In this post we will not discuss such topics. Whether there is someone to blame or not; whether it was properly designed or not, the fact is that the video a good example of stream bed erosion. Due to space limitation, this post will be an introduction to stream bank erosion. In a future post we may simulate in detail the erosion process of the video and analyse whether a properly designed rip rap would have prevented the erosion (and collapse) or not.

Fig 1. Bridge collapse due to stream bank erosion in Peru (Source youtube)

Rivers are dynamic systems that change over time. One of the processes defining such change is stream bank erosion. All rivers have stream bank erosion. Even the so-called stable rivers have eroding banks. Certain events such a flooding usually trigger stream bank erosion

Stream bank erosion becomes even more complicated when the steam has bends inducing secondary flows. Even small bends create a vertical velocities profile and a helicoidally flow known as centre region cell. Besides this helicoidally flow, the outer bank of the river also experiences a so called outer bank cell flow. This outer bank cell flow is a small helicoidally flow with a direction that opposes the main helicoidally flow. Although the outer bank cell is small and weak, it plays an important role in stream bank erosion.

Fig 2. Secondary flows (Source: Blanckaert and Vriend 2004)

The full solution of the stream bank erosion is quite complicated. Thus, simplified approaches were developed based on the main mechanisms controlling stream bank erosion. The mechanisms controlling stream bank erosion may be divided in two groups: Scour and Mass failure

• Mass failure is the process when large chunks of bank material collapse into the river. Sometimes mass failure is a consequence of local scour removal.
• Scour failure is the direct removal of bank material

There are two main modelling approaches for analysing bank erosion due to the mentioned processes:

• Bank failure. Bank failure is a geotechnical based model that evaluates the bank stability and its critical failure plane. Once the soil gets saturated due to the flood, several theoretical failure planes are analysed considering its resisting forces and the driving forces, in order to define the one with the lowest safety factor. If the critical safety factor is lower than one, then all the soil above the plane will be eroded by mass failure.

Fig 3. Mass failure (Source: Carey B 2014)

• Toe scour. Toe scour considers the bank material removed by the flow. The shear stress between the flow and the bank toe is evaluated considering the critical share stress and the erodibility of the bank. If the shear stress is higher than the critical shear stress, a portion of the bank will be eroded. After the toe gets eroded, a second failure mechanism may occur. Due to the toe erosion, the soil will look like a cantilever. Depending on the size of toe scour and the volume of soil above the scour whole, a cantilever shear failure may occur.

Fig 4. Toe scour (Source: Carey B 2014)

If we get more information about the physical characteristics of the bridge and the river, in a further post we may simulate the scour process that collapsed this bridge, and analyze in more detail this case.