Floods and Flood Routing

Outline Flood Estimation size of Floods Flood Frequency Flood Routing Flood Control Flood Forecasting

Introduction Flood is an unusual high stage of a river due to runoff from rainfall and/or melting of snow in quantities too great to be confined in the normal water surface elevations of the river or stream

Flood Estimation Design Flood is The maximum flood that any structure can safely pass and is selected after consideration of economic and hydrologic factors. When the structure is designed for a flood less than the maximum probable, there exists a certain amount of flood risk to the structure, nor is it economical to design for 100% flood protection. Protection against the highest rare floods is uneconomical because of the large investment and infrequent flood occurrence.

Flood Estimation The maximum flood discharge (peak flood) in a river may be determined by the following methods: 1- Rational Methods 2- Flood Frequency Studies

Flood Estimation 1- Rational Methods Q= C I A Where: Q: the peak discharge I: rainfall Intensity A: Catchment Area C: Coefficient of Runoff = (R/P)

Flood Estimation Ex 1: For an area of 20 hectares of 20 minutes concentration time, determine the peak discharge corresponding to a storm of 25-year recurrence interval. Assume a runoff coefficient of 0.6. From intensity-duration-frequency curves for the area, for T = 25-yr, t = 20 min, i = 12 cm/hr.

Flood Estimation If the IDF curves are not available for the catchment and the maximum precipitation of P cm occurs during a storm period tR hours then the design intensity can be abstained from the following equation:

Flood Estimation Ex2:Determine the design flood discharge (allowing an increase of one-third) for a bridge site with the following data: Catchment area = 2 × 105 ha Duration of storm = 8 hours Storm precipitation = 3 cm Time of concentration = 2 hr Gauged discharge for a past flood with average maximum daily rainfall of 18 cm was 3400 cumec.

Flood Estimation Ex3: A 45 min storm produces 30 mm of rain over a catchment of 120 ha. If the time of concentration is 30 min and runoff coefficient is 0.3, estimate the resulting peak flow rate

Flood Estimation Ex 4: A basin is divided by 1-h isochrones into four sub-areas of size 200,250,350 and 170 hectares from the upstream end of the outlet respectively. A rainfall event of 5-h duration with intensities of 1.7 cm/hr for the first two hours and 1.25 cm/hr for the next 3 hours occurs uniformly above the basin. Assuming a constant runoff coefficient of 0.5, estimate the peak rate of runoff.

Introduction Flow Routing is a procedure to estimate downstream hydrograph from upstream hydrograph (Figure 1). Since flow routing has been widely used in flood estimations, flow routing is usually called flood routing.

Introduction The routed hydrograph is delayed by a time lag (translation) and is attenuated. Flow routing is divided into river flow routing and reservoir flow routing.

Basic Equations From the conservation of mass, water balance for a system in Figure 2 can be expressed as where I is upstream inflow , O is downstream outflow, S is the storage (reservoir or a river reach).

In practical calculation, it is more convenient to use a finite difference form of Eq(1) for a Δt duration. The mean values for the inflow and outflow are used instead of the instantaneous value. To estimate the downstream outflow, it is also necessary to get the storage function that links the input and output. S= f(I,O) (3) It is then possible to solve the outflow from Eq (2) and (3).

Flood Routing River Flow Routing (The Muskingum Method) The storage function in a river reach is linked with both inflow and outflow. where K is the storage time constant for the reach, X is a weighing factor ( between 0~ 0.5, usually around 0.2).

Flood Routing From the water balance equation Simplify it to get the Muskingum equation

Flood Routing Where: It is important to check if If not, some adjustments to the parameters are needed. If there are rounding errors, adjust the largest C value first

Flood Routing Ex6: Estimate the downstream hydrograph using the Muskingum method with K=3hr and X=0.3. The time interval is 3 hours. The upstream hydrograph is as follows:

Flood Routing Ex7: A stream has a uniform flow of 10 m3/s. a flood in which the discharge increases linearly from 10 m3/s to a peak of 70 m3/s in 6 hours and then decreases linearly to a value of 10 m3/s in 24 hours from the peak arrives at a reach. Route the flood through the reach in which k=8 h, and x=0. estimate the peak flow at the town and the time at which the peak flow will occur.

Flood Routing Reservoir Flow Routing For a flood going through a reservoir, the water level in the reservoir is assumed as horizontal. The storage function would be linked with the reservoir water level.

Flood Routing Since the storage below the spillway crest plays no role in the flow routing process, only the storage above the crest is considered.

Flood Routing The discharge over the spillway crest is a function of h. where C is the discharge coefficient, b is the width of the spillway crest.

Ex8:A triangular-shaped inflow hydrograph is routed through a reservoir assuming it is completely full at the beginning of the storm. The spillway crest is 20 m wide and has a coefficient of 2.7. The reservoir area is 0.5 km2 and has vertical sides. What are the discharge over the spillway crest and estimate the Δt ?

FLOOD CONTROL In a flood control project, the degree of flood protection (i.e., to the required stage) should be justified by an economical analysis of the costs involved in raising the structure to the required heights (say, the height of spillway)

FLOOD CONTROL The flood control costs include: (i) Capital costs involved in the construction of the structure to the required flood height (i.e., to offer the required degree of flood protection, say, by a combination of dam spillway, levees and channel improvement) (ii) Interest cost on capital expenditure (iii) Sinking fund, depreciation and taxes (iv) Operational expenses and maintenance cost

FLOOD CONTROL An assessment of the damages caused by floods can be made when the data is collected and presented in the following form: (i) Area affected (km2) (ii) Population affected (iii) Crops affected (a) in hectares (b) value in local currency

FLOOD CONTROL (iv) Damage to property (a) number of houses damaged (b) cost of replacing or repairing (v) Loss of livestock (a) category number (b) value (vi) Human lives lost (vii) Damage to public works