CHAPTER 8

HYDROLOGICAL FORECASTING(Flood/Flow Estimation)

COURSE OUTCOME: Estimate peak discharge and propose urban drainage dimensions using MSMA (Urban Stormwater Management Manual for Malaysia) and Probability Distribution.

Lesson Outcomes:• Estimate the peak discharge using MSMA• Propose dimension of drainage system• Calculate and estimate the peak discharge

using probability distribution

Introduction Hydrological forecasting is important to

estimate and manage all event related to flood.

Flood forecasting is the use of real time precipitation and streamflow data in rainfall-runoff and streamflow routing models to forecast flow rates and water levels for periods ranging from a few hours to days ahead, depending on the size of the watershed or river basin.

Its can forecast the effects of urbanization on runoff from undeveloped watershed.

Effects of Urbanization

The urbanization effects are increased total runoff volume and peak flow rates.

The major changes in flow rates in urban watersheds are due to the followings;1. The volume of water available for runoff

increases because of the increased impervious cover provided by parking lots, streets and roofs cover which reduce the amount of the infiltration.

2. Changes in hydraulics efficiency associated with artificial channels, curbing, gutters and storm drainage collection systems increase the velocity of the flow and the magnitude of flood peaks.

Flood Control Structural Methods

– Flood Control Reservoir• Detention/Retention Pond• Dam

– Channel Modification & Environmental Impacts• Swales

– Diversion Channels, Levees & By Pass Channels– Increased Infiltration

• To increase the amount of previous area wherever possible

• Example; Porous parking lots through the use of concrete block or similar shapes laid such as water can infiltrate trough the soil-filled center.

Detention Pond

Retention Pond

Flood Control Structure •Placing control structures in a river system is a management technique used to mitigate flooding associated with periods of heavy rainfall.  •While this approach helps to reduce flooding in urban and agricultural areas, flow patterns in the vicinity of a control structure can jeopardize its stability.  •In the example to the right, local flow patterns have caused scour to occur in the channel used to convey flood flows.

Dam

Figure 4.9: Schematic of a Wet Swale

Swale

Nonstructural Methods– Flood proofing– Flood warning mechanisms– Land use controls such as zoning &

development ordinances– Flood insurance programs– Flood preparedness activities– Public awareness & education programs

Methods

There are several methods to estimate

flood or flow rate such as; Empirical Formula Rational Method Frequency Analysis

– Normal Distribution– Extreme Gumbel– Log Pearson Type III

Empirical Formula

Q = CAn

Q = Flood discharge (m3/s)A = Area of catchment (km2)n = Index (0.5 – 1.25)C = Coefficient (wheather and catchment)

log Q = log C + n log Aneed many catchment

Rational MethodCalculate Peak Flow

Qp = CIA

Qp = the peak runoff rate

C = the runoff coefficient (assumed to dimensionless) I = The average rainfall intensity for a storm with a duration equal to a critical period

of the time tc

A = the size of the drainage area t

c = the time of concentration

Rational Method

Useful for small, usually urban, watersheds (<10acres, but DOT says <200acres)

Q = CIA (english) or Q = 0.0028CIA (metric)Q = runoff (ft3/sec) or (m3/sec)C = coefficient representing ratio or runoff to

rainfallI = intensity of rainfall (in/hour or mm/hour)A = drainage area (acres or hectares)

Runoff Coefficient

• Coefficient that represents the fraction of runoff to rainfall

• Depends on type of surface

Iowa DOT Design Manual, Chapter 4, The Rational Method

Runoff Coefficient

Iowa DOT Design Manual, Chapter 4, The Rational Method

Runoff Coefficient

Iowa DOT Design Manual, Chapter 4, The Rational Method

Runoff Coefficient

When a drainage area has distinct parts with different C

Used weighted average

C = C1A1 + C2A2 + ….. + CnAn

ΣAi

Average Recurrence Interval, ARI

(Design Event)● 2 ARI -- Design of intakes and spread of

water on pavement for primary highways and city streets

● 10 ARI -- Design of intakes and spread of water on pavement for freeways and interstate highways

● 50 ARI -- Design of subways (underpasses) and sag vertical curves where storm sewer pipe is the only outlet

● 100 ARI -- Major storm check on all projects

Intensity

Average intensity for a selected frequency and duration

Based on “design” event (i.e. 50-year storm)Overdesign is costlyUnderdesign may be inadequate

Duration

Time of Concentration (tc)

● Time for water to flow from hydraulically most distance point on the watershed to the point of interest

tc = to + td (MASMA)

to = time of overland flowtd = time of flow in drain

Time of Concentration (tc)

Depends on:– Size and shape of drainage area– Type of surface– Slope of drainage area– Rainfall intensity– Whether flow is entirely overland or

whether some is channelized

Rational Method using MASMA

One of the most frequently used urban hydrology methods in Malaysia (simplicity)

It gives satisfactory results for small catchment (up to 80 hectares)

Qy = C yIt A

360 Q

y= y year ARI peak flow (m3/s)

C = dimensionless runoff coefficient yI

t = y year ARI average rainfall intensity over time

of concentration, tc (mm/hr)

A = drainage area (ha)

Assumption

1. The peak flow occurs when the entire catchment is contributing to the flow

2. The rainfall intensity is the same over the entire catchment area3. The rainfall intensity is uniform over a time

duration equal to the time of concentration, tc 4. The ARI of the computed peak flow is the same as that of the

rainfall intensity i.e. A 5 year ARI rainfall intensity will produce a 5 year ARI peak flow

Not recommended

the catchment area is greater than 80 hectares

ponding of stormwater in the catchment might affect peak discharge

the design and operation of large (and hence more costly) drainage facilities is to be undertaken, particularly if they involve storage

Procedure for Estimating Peak Flow using the Rational Method

1. Select design ARI

- select design ARI for both minor and major drainage systems

- Table 4.1 Design Storm ARIs for Urban Stormwater Systems

- ARI, Average Recurrences Interval (Return Period)

- the average length of time between events that have the same magnitude or volume and duration

2. Discretise sub catchment

- divide sub catchment into segments of homogeneous land use or surface slope

- determine the surface slope and land use of the catchment

3. Estimate time of concentration tc

- estimate overland flow

- estimate flow times for all other flow components within the sub catchments such as pipe, kerb gutter and channel etc

- tc =

the flow travel time from the most hydraulically

remote point in the contributing catchments area to the point under study

- Refer Design Chart 14.1

4. Determine average rainfall intensity,yIt

- calculate yIt for design ARI of y years and

duration t equal to the time of concentration,

from IDF data for area of interest

- Refer equation 13.1and 13.3, Table 13.A1and

13.3, Figure 13.3

5. Estimate runoff coefficients

- estimate C values for each segment if there are

different land covers

- Design Chart 14.3 and 14.4

6. Calculate average runoff coefficient

Cavg

= ∑CiA

i

∑Ai

7. Calculate peak flow rate from equation

Qy = C yI

t A

360

Example 1

Determine the design peak for flow generated from a minor drainage of medium density residential area of 10 hectares in Kuala Lumpur. Assume 80 m of overland flow followed by 400 m of flow in an open drain. Catchment area average slope = 0.5%. The catchment is shown.

Catchment area 10 ha

Main drain

River

Solution

Step 1: Determine tc

Step 2: Determine I and C

ln(I) = a + b ln t + c (ln t)2 + d (ln t)3 Eqn. 13.3

Pd = P30 – FD (P60 - P30) Eqn. 13.3

I = Pd / d Eqn. 13.4

Step 3: Determine Qp

Exercise 1

The catchments area in Melaka Town has two different characteristics shown in figure. Determine the total design peak flow generated from minor drainage for the whole catchments. Average velocity in open drain is 1.0 m/s and average slope is 1.0%.

Exercise 2

The catchments in rural area at Kuala Pilah, Negeri Sembilan has two different areas shown in figure. Determine the design peak flow generated from minor drainage for Area A and major drainage for Area B. The average velocity in open drain is 1.0 m/s and the average slope is 0.5%.

Data for whole catchment

Overland flow = 100 mFlow in Drain = 450 mLow Density ResidentialSurface: Poorly grassed

Area A = 5 haMedium Soil - Forest

Area B = 10 haSandy Soil- Forest

Propose dimension

Q = AV

Most effective cross section From the Manning and Chezy equation, it is

clear that the conveyance of a channel increases as the hydraulic radius increases or as the wetted perimeter decreases.

Thus, from the viewpoint of hydraulics, there is among all channel cross sections of a specified geometric shape and area an optimum set of dimensions for that shape.

Most effective cross section The one that will have the greatest

capacity for a given slope, area and roughness.

If these parameter constant, velocity will be greatest when the wetted perimeter is smallest.

The most efficient (effective) is the most economical.

Semicircular – smallest wetted perimeter

Most effective cross section The most efficient cross section happened

when:– Flow rate (Q) is maximum– Slope (S) constant so hydraulics radius (R) is

maximum and wetted perimeter (P) is minimum

– dP = 0

dy

Example 2

Using the design peak discharge from Example 1, propose the dimensions of rectangular drainage with freeboard which is 5% from water depth.

Exercise 3

Using the design peak discharge from Exercise 1, propose the dimensions of rectangular drainage with freeboard which is 5% from water depth.