Introduction

Hydrology is an important discipline in civil engineering , it deals with the elements of the hydrologic cycle which includes processes like: precipitation, infiltration, evapotranspiration and surface runoff, with respect to water resources, it deals with the flow and storage of water in rivers, lakes and reservoirs, in addition to subsurface water resources like ground water. However, one of the essential principles in hydrology is the hydrograph routing; which is a technique used by hydrologists to study the changes in the shape of hydrograph (flow vs. time) as water travels through a section of river channel or reservoir, such analysis are very useful in flood forecasting and design of reservoirs and flood protection schemes. The aim of this report is to discuss a set of lab experiment that is performed to illustrate the use of reservoirs for flood and storage control.

Aim

Two sets of experiments are carried out in the laboratory using reservoirs to obtain a better understanding of storage and flood control. Experiment 1 has a constant inflow and experiment 2 has a varying inflow to the top reservoir. Various graphs like inflow and outflow hydrographs (graph of flow vs time) are compared to get a better understanding on the effect of different inflow on outflow. In experiment 2, level pool routing is used to compare with the outflow from experimental values.

Theory

Hydrograph is known as the graph of flow vs time for a wave travelling downstream

past a specific point. The resulting shape of a hydrograph depends on various factors

including but not limited to shape of the river basin, land use, addition of tributary

flows etc. Figures 1 & 2 below describe how peak discharge is affected by the shape

of the reservoir/channel. The round, lager basin takes longer time for the water to

reach downstream than the narrower one. (Gupta, 2008)

Hydrographs are very important input to a technique called routing. Routing is

described as the process of obtaining a downstream hydrograph based on the values of

an upstream hydrograph. The resulting hydrograph has a lower peak; it has been

attenuated: that is the graph has been lowered (reduced peak). Figure 2 above shows

how the reduced peak changes as the water flows downstream, it is worth noting that

the overall volume of the water doesn’t change. The hydrograph spreads itself over

time, so the time taken to peak increases; known as translation.

The basis on which reservoir routing lies is the continuity equation which assumes

that no water is lost; that is the mass that enters the reservoirs is equal to the mass that

leaves the reservoir. (Gribbin, 2006). The continuity equation is expressed as:

(1)

Where,

: mean inflow into reservoir for time interval,

: mean outflow from the reservoir for time interval,

: change in reservoir storage for time interval

: time interval between two values

The equation keeps account of the water entering, stored or leaving a reservoir. The

continuity equation can be re-written in finite different forms; rewriting equation 1

gives,

(2)

Equation 2 can now be re- arranged to give,

(3)

In equation 3, at the start all the terms on the LHS are known, can be

calculated from the inflow hydrograph, is calculated based on the fact that at H = 0, both the storage and outflow are zero. The parameter is the storage in the

lower reservoir i.e. amount of water discharged from the upper reservoir. Hence depth

values from upper reservoir are used, first, to calculate the storage and then to

calculate the volume (for flow). Therefore, the RHS can be calculated and a graph of

O(outflow) against is plotted. The graph is generally a curve and an equation

is generated to back calculate the outflow values. This outflow obtained is the reduced

peak outflow also known as attenuation as mentioned earlier.

The routing method is based on several assumptions:

o The water surface in the reservoir is horizontal.

o Outflow and storage are a function of height.

o Outflow is directly proportional to height. (Gribbin, 2006)

One of the main characteristics of Inflow and Outflow Hydrograph is the peak

outflow value- which should intersect with the inflow curve. At that point inflow=

outflow and temporary storage is a maximum, see Fig 3. At this point, the

corresponding depth should be found out and be used as the design height for the

storage reservoir. (Gribbin, 2006).

Apparatus

· Two identical reservoirs (900mm wide x 900mm long x 300mm deep) which are mounted on top of each other with a vertical distance of 600mm

· Sharp edged weir located in the upper reservoir to give outflow to the bottom reservoir.

· Scale on each reservoir to measure depth (in cm)

· Rotameter which is used to control , and measure inflow rate of water pumped to the reservoir (in liters per minute)

· Stopwatch to record time

· Mirror to check the level water present in the upper reservoir

Experimental Procedure:

-The lab experiment consists of two tests that aims to confirm the validity of the measured data by the apparatus like flow rate and water depths in the reservoirs, then these data are used to plot both the inflow and outflow hydrographs in order to illustrate the attenuation of peak flow between the two hydrographs.

1-In the first test: the water level was checked initially to be at the same level of the weir crest, in addition the lower reservoir was checked to be empty of water and plugged tightly to prevent any losses in water storage during the experiment, after that the data recording software was launched to measure the voltage from transducers approximately every one second.

-The pump was then turned on , and the release valve was opened slowly to obtain constant flow rate of approximately 40 l/min on the rotameter for two minutes, then the valve was closed and pump is turned off, however the weir in the upper reservoir is still delivering outflow to the lower reservoir, once it stops overflowing the data recording software is stopped, and the water in the lower reservoir is drained back again to the storage tank.

2-In the second test: the same steps are followed as in the first test ,but in this case the flow rate provided is not constant, instead it increases gradually up to the peak inflow and in the same manner also decreases from the peak inflow to zero. -The pump was turned on initially, then the release valve was opened to obtain 10l/min flow rate for 40 seconds, after that it is increased by 10 l/min for another 40 seconds and the process continues until reaching maximum discharge of 50 l/min, then it is reduced back again by 10 l/min steps for 40 seconds until reaching zero and the pump is then turned off, the weir is kept to overflow water to the lower reservoir, once it’s stopped , the acquisition software is stopped and data is saved in tabular excel format.

Sources of error:

The following sources of errors might have affected the accuracy of experiment results:

-The fluctuation of the rotameter reading at start and end of experiment

-The air trapped in pump pipe which was released when opening the inflow valve.

-The noise in transducers which resulted in large amount of fluctuating and negative values of flow

-The time interval of the acquisition software was not equal throughout the experiment, thus a large amount of approximation and statistical analysis were required for the data in order to get useful output. .

-The human error in fixing the rotameter reading flow to the desired value, and the timing of the inflow steps in the experiment.

Conclusion:

In conclusion, it can be inferred that the hydrograph routing analysis is very critical and important process in order to predict the characteristics of downstream outflow, this plays an important role in solving reservoir storage problems as well as the design against river floods. However in this lab experiment, two tests were performed to study visualize this concept in small laboratory scale ,in the first test, the total inflow to the reservoir system was determined using constant flow rate , while in the second test the outflow hydrograph was determined theoretically and experimentally for varying flow rate in order to study the attenuation and translation of flow downstream which represent the flood waves in real life analysis.