Lecture 6 - Flood Control Flashcards
is the result of runoff from rainfall in quantities to great to be conveyed or confined in the low-water channels of streams.
flood
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.
floodplain
can be defined as natural geologic features or from a regulatory perspective.
Floodplains
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.
Water
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
- Watershed Deforestation
- Watershed Land-use change
- Improper and unregulated land-use and urban planning
- Inefficient flood control infrastructure system
- Lack of flood early warning system
- Lack of maintenance of rivers and drains
- Impact of global environmental change
- 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
maximum
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.
