• FLOODWAYS• FLOOD-PLAIN ZONING•SPREADING GROUNDS•SOIL CONSERVATION METHODS

EVANGELISTA, Ken Joshua GReporter:

FLOODWAYSFloodways serve two functions in flood abatement;

• by providing an additional channel storage for storinga portion of the flood water

• by opening up a new channel to carry a part of themain stream flow.

FLOODWAYS

FLOODWAYS

• The land in the floodways may be used for agriculturebut extensive or costly development should never beencouraged.

FLOODWAYS

• Weirs and gated spillways provide better control over the flow but are costlyand their construction can be justified only when overflow is expected tooccur quite frequently and when neighbouring important cities are to beprotected.

FLOOD-PLAIN ZONING• Flood-plain zoning reduces the risk and cost of damages

flooding causes. Local governments may pass flood zonelaws that restrict land use. These laws prevent or limitbuilding and development in flood zones to lessen therisks associated with flooding.

FLOOD-PLAIN ZONING

FLOOD-PLAIN ZONING

SPREADING GROUNDS• Flood flows are sometimes diverted into large flat land

areas that are capable of absorbing water at relativelyhigher rates.

SPREADING GROUNDS

• Spreading grounds serve to reduce downstream flood peaks and replenishunderground reservoirs which provide water by pumping for irrigationpurpose or other water supply uses.

SOIL CONSERVATION METHODS• Improved vegetable cover has a significant influence on the

reduction of floods of small magnitude.

Problems:

A stream channel whose shape can be approximated as a rectangle of base width100 m carries a discharge of 450 m3/s. Its bed slope has been found to be 18 × 10–4

and its roughness in terms of Manning’s n equals 0.02. The stream is a tributary to ariver where the existing flood stage is 3 m above the normal depth in the stream.Find the approximate backwater curve in the tributary.

Solution:

Given

Q = 450 m3/s, Sb = 0.0018

n = 0.02, B = 100 m

For a wide rectangular channel hydraulic radius can be approximated by the depthin the calculation. Now equilibrium depth in the tributary can be calculated from

Problems:

The following data concerning flow in a semicircular bend are given

r = 490 m, C = 40 m1/2/s, h = 5 m

ux = 1.1 m/s

Find the deviation in the path of a particle starting along the centre line at thebeginning of the curve and finishing at its end.

Hence the deviation from the centreline at the end of the curveequals

0.0416 × 1440 = 59.91 m.

The practical significance of it will be apparent when one considers aship travelling around such a curve. The ship must steer itself towards thecentre to avoid being moved towards the outerbank.