Flood Routing

Question 1

What are the major differences between hydrologic and hydraulic routing

Answer 1

The procedures for describing the propagation of flood waves - or flood routing - aim at the definition of the characteristics of a flood wave, the rise and fall in streamflow during a storm (flow, flow velocity and/or water level as a function of time) based on known characteristics in an upstream location. There are two types of methods used to do this, hydraulic methods, which are based on the equations of Barre de Saint Venant for gradually varied 1d flow and hydrologic methods, to express a relation between inflow and outflow.

The choice of method depends on the characteristics of the problem. Hydraulic methods are generally more accurate and more applicable, however they require more complex solution techniques and numerical simulators. Furthermore, hydraulic methods produce flow, flow velocity and water depth as a function of time along the river. On the other hand hydrologic models have limited applicability and often provide more unreliable results for situations with backwater effects and if the geometry of the system changes due to flooding (or dynamic control structures). Hydrologic methods also only produce the hydrograph at the end of the reach. Despite all this, the method is simpler and offers faster solutions,

Question 2

Derive Muskingum

Answer 2

Refer to notes

Question 3

Describe the semi-graphical method for calibrating the k and x parameters of the Muskingum method

Answer 3

The parameter k represents the travel time of the flood wave

The value x depends on the shape of the wedge. The minimal value for x is 0 and occurs for a horizontal water profile (big reservoirs). On physical grounds, it can be shown that the maximal value is 0.5 for which the attenuation of the flood peak is minimal, while it is maximal for x = 0

These parameters can be determined by means of optimisation techniques or semi-graphical method

Based on the equation S = kq+kx(i-q) it can be deduced that there is a linear relation between S and q + x(i-q). So the two can be graphed (with S on the y-axis) and the k value can be discerned from the slope of the line. The value for x with the minimal hysteresis represents the optimal x value.

Question 4

What is the meaning of k and x

Answer 4

The parameter k represents the travel time of the flood wave

The value x depends on the shape of the wedge. The minimal value for x is 0 and occurs for a horizontal water profile (big reservoirs). On physical grounds, it can be shown that the maximal value is 0.5 for which the attenuation of the flood peak is minimal, while it is maximal for x = 0

Question 5

Difference between river and reservoir routing

Answer 5

Reservoir Routing:
- When there is a unique, invariable relation between the outflow discharge and storage or with the water level upstream of the outlet
- The maximal q occurs at the intersection of the inflow and outflow hydrographs.
- This is the case for big reservoirs or lakes with a large width or depth. Such reservoirs have a horizontal water surface and low flow velocities.
- An additional condition is the existence of a unique, invariable stage-discharge relation at the outlet. Reservoirs with control structures that regulate the outflow at the outlet are thus excluded.

River Routing
- In long, small reservoirs and in rivers the storage flow is characterised by hysteresis
- When following the hysteresis curve, as flow is increasing the point where storage is maximal and thus the point for where i=q will be reached first. The point for which q is maximal will only be reached later