This document discusses several methods for flood abatement: - Floodways provide additional channel storage and carry part of the main stream flow to reduce flooding. Agriculture is allowed but not extensive development. Weirs and gated spillways provide better control but are costly. - Flood-plain zoning restricts development in flood zones to reduce risks and costs of flooding. - Spreading grounds divert flood flows into flat lands that can absorb water, reducing downstream flooding while replenishing groundwater supplies. - Improved vegetation cover significantly reduces small floods.

1. • FLOODWAYS
• FLOOD-PLAIN ZONING
• SPREADING GROUNDS
• SOIL CONSERVATION METHODS
EVANGELISTA, Ken Joshua G
Reporter:

2. FLOODWAYS
Floodways serve two functions in flood abatement;
• by providing an additional channel storage for storing
a portion of the flood water
• by opening up a new channel to carry a part of the
main stream flow.

3. FLOODWAYS

4. FLOODWAYS
• The land in the floodways may be used for agriculture
but extensive or costly development should never be
encouraged.

5. FLOODWAYS
• Weirs and gated spillways provide better control over the flow but are costly
and their construction can be justified only when overflow is expected to
occur quite frequently and when neighbouring important cities are to be
protected.

6. FLOOD-PLAIN ZONING
• Flood-plain zoning reduces the risk and cost of damages
flooding causes. Local governments may pass flood zone
laws that restrict land use. These laws prevent or limit
building and development in flood zones to lessen the
risks associated with flooding.

7. FLOOD-PLAIN ZONING

8. FLOOD-PLAIN ZONING

9. SPREADING GROUNDS
• Flood flows are sometimes diverted into large flat land
areas that are capable of absorbing water at relatively
higher rates.

10. SPREADING GROUNDS
• Spreading grounds serve to reduce downstream flood peaks and replenish
underground reservoirs which provide water by pumping for irrigation
purpose or other water supply uses.

11. SOIL CONSERVATION METHODS
• Improved vegetable cover has a significant influence on the
reduction of floods of small magnitude.

13. Problems:
A stream channel whose shape can be approximated as a rectangle of base width
100 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 a
river 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 depth
in the calculation. Now equilibrium depth in the tributary can be calculated from

16. 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 the
beginning of the curve and finishing at its end.

17. Hence the deviation from the centreline at the end of the curve
equals
0.0416 × 1440 = 59.91 m.
The practical significance of it will be apparent when one considers a
ship travelling around such a curve. The ship must steer itself towards the
centre to avoid being moved towards the outerbank.