Lecture 6 - Flood Control Flashcards Preview

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Flashcards in Lecture 6 - Flood Control Deck (42):

is the result of runoff from rainfall in quantities to great to be conveyed or confined in the low-water channels of streams.



Disaster risk is a complex interplay of several factors…

Hazard (strength, frequency)

Population (exposed population density)

Vulnerability (structural, physical, socio-economic profile)


Commonly Accepted Methods for Reducing Floods

Reduction of peak flow by reservoirs

Confinement of the flow within a predetermined channel by levees, flood walls, or a closed conduit

Reduction of peak stage by increased velocities resulting from channel improvement

Diversion of floodwaters through a flood bypass, which may return the water to the same channel at a point downstream or deliver it to another channel or different watershed

Floodproofing of specific properties

Reduction of flood runoff by land management

Temporary evacuation of flood threatened areas on the basis of flood warnings

Flood plain management


is the normally dry land area adjoining rivers, streams, lakes, bays, or oceans that is inundated during flood events.



can be defined as natural geologic features or from a regulatory perspective.



Hydrologic and hydraulic analyses of floods are:

a. Floodplain information studies
b. Evaluation of future land-use alternatives
c. Evaluation of flood-loss reduction measures
d. Design studies
e. Operation studies


Hydrologic analysis entails:

rainfall-runoff analysis or a flood-flow frequency analysis


Hydraulic analysis involves the determination of water-surface elevations using

steady or non-steady state


Effective Flood Control Operating System (EFCOS)

Aims to achieve an effective flood control operation for Pasig-Marikina- Laguna Lake Complex through Real time rainfall and water level data collection at the Rosario Master Control Station via telemetry system; The effective use of the warning system along Mangahan Floodway; and The multiplex communication system among Rosario MCS, Napindan HCS, DPWH Central Office and PAGASA Data Information Center.


The flood-prone area in Metro Manila

a) Coastal lowlands alongside Manila Bay
b) Marikina flood plain
c) Laguna Lake plain
d) Valley plains cutting through the central plateau


Pressing Flood Hazard Issues in Metro Manila

1. Watershed Deforestation
2. Watershed Land-use change
3. Improper and unregulated land-use and urban planning
4. Inefficient flood control infrastructure system
5. Lack of flood early warning system
6. Lack of maintenance of rivers and drains
7. Impact of global environmental change
8. Fragmented institutional flood management arrangements


is the operation of an overall program of corrective and preventive measures for reducing flood damage

Floodplain management


Flood Protection in Urban Flooding

a. Increase bankfull capacity
• Modification of channels and banks
• Artificial channel linings
• Flood relief channels
b. Decrease flood peaks and increase flood flow duration
• Upstream water flow regulation
• Upstream sediment flow regulation
• Spreading ground through floodways
c. Increase channel gradient
• Straightening of sinuous rivers


Flood abatement in Urban Flooding

a. Upstream afforestation or reforestation
b. Flood control effective farming practices
c. Comprehensive forest and vegetation protection programs


Countermeasures to flooding

Behavioral Adaptation
a. Monitoring and prediction
b. Floodplain zoning
c. Flood proofing
d. Public relief funds
e. Flood insurance


Two types of floodplain inundation maps

flood-prone area and flood-hazard maps.


One aspect of floodplain management

balancing the economic gain from floodplain development against the resulting increase in flood hazard.


The 100-year flood area can be divided into

floodway and a floodway fringe


The objective of flood control

to reduce or alleviate the negative consequences of flooding.


Alternative measures that modify the flood runoff are referred to

structural measures


modify the damage susceptibility of floodplains are referred to

non-structural measures


The basic functional relationships required to assess the value of flood damage reduction alternatives are

stage-damage, stage-discharge, and flood-flow frequency relationships


Two types of flood mitigation infrastructures

storage reservoirs and retarding basins


Main function of flood mitigation infrastructures

is to store a portion of the flood flow so as to minimize the flood peak at the point to be protected.


One criterion for evaluating FMR is the

percentage of the drainage area controlled by total the reservoirs.


Site selection should be based on the yield of

highest economic return.


A second criterion for evaluating flood-mitigation reservoirs is

storage capacity


are necessary in planning reservoir operations for flood mitigation.

Streamflow forecasts


The outlet of retarding basins consists of

large spillway or one ungated or more sluiceways


The discharge capacity works should the

of outlet equal flow the D/S ca pacity of channel.


The planning of a system of retarding basins must prevent the

synchronization of flows.


One of the oldest and widely used methods of protecting land from floodwater is to

erect a barrier preventing overflow.


Most levees are

homogenous embankments.


Floodwalls are constructed to

withstand hydrostatic pressures


The cost of a levee system consists of the

cost of the land for levees and channel plus the cost of levee construction.


Two options for levees crossing tributary channels.

a) Carry the levee U/S along the tributaries;
b) Block the channel and create an interior drainage problem


The best solution for the problem depends on the

local the topography and stream/coast characteristics.


in effect, an artesian spring in the aquifer under the levee, with velocity sufficient to move the foundation material.

sand boil


If the possibility of failure is imminent, a ________may be constructed.

setback levee


is created by diversion works and topography that permits excess water in a river to be diverted.

flood bypass


is considerably more complicated than a simple determination of the amount of flow to be diverted.

hydraulic design of the diversion works for a bypass


Admission of water can be through a gap in

levee line, fuseplug levee, spillway section or use of weirs.