1) The document discusses sustainable flood risk management (SFRM) and the use of detention ponds to reduce flood risk. 2) It provides a case study of using detention ponds in Guwahati City, India to reduce flood depth and inundated area based on land use changes between 2006 and 2011. 3) The study found that a detention pond was able to reduce the maximum flood depth by 46.5% and flood inundated area by 43% for the 2011 land use, demonstrating the effectiveness of detention ponds for urban flood mitigation.

1. SUSTAINABLE FLOOD RISK
MANAGEMENT (SFRM)

2. AN OVERVIEW
12/23/20202
• Why this topic?
• SFRM- What and Why
• Flood Control Measures
• Flood Hazard Mapping
• Detention ponds for SFRM
• Detention ponds – flow simulation
• Case Study - Guwahati City
• Summary
• Future Scope

3. WHY THIS TOPIC?
3
• Flood fatalities increase from year to year
• Kerala floods (2018 and 2019)
• Gujarat floods (2017)
• Assam floods (2016)
• Higher population density + under developed standards
=> greater damage
• In urban areas, extremely
high land use
12/23/2020
fig 1: Image of flooded land
(source: https://unsplash.com)

4. SFRM – What and Why
12/23/20204
• Appropriate use of structural and non structural
measures
• Aims at,
• Reduce life- safety risks
• Wise use of flood plain lands
• Economic growth
• Environmental enhancement
• Parameters
VulnerabilityHazard
Infrastructure
Performance
Exposure

5. FLOOD CONTROL MEASURES
12/23/20205
Structural
Measures
Detention
Reservoirs
Levees
Floodways
Channel
improvement
Non-structural
Measures
Flood plain
zoning
Flood
forecasting
Evacuation and
relocation
Flood insurance

6. FLOOD HAZARD MAPPING
12/23/20206
Fig 2: Flood Mapping Diagram
(Source: Journal of Environmental Planning and
Management, 2015)
COMPUTATION

7. COMPUTATION-3D MATRIX MODEL
12/23/20207
1) Find weighted scores for hazard parameters like,
a) Flood depth
b) Flood inundated area
c) Flood volume
d) Population density
e) Road networks
eg: W = aA + bB + cC + dD
Where,
a, b, c, d – Weight for flood damage (0 to 10)
A, B, C, D – percent occupied by parameters
2) Individual Hazard Ranking – Linear Interpolation(0–100)
3) Individual Hazard Mapping

8. 12/23/20208
4) Overall Hazard Ranking - 3D Matrix model
X
Parameter 2
Z
Parameter 1
Y
Parameter 3
HR 1 2 3
1 1 2 3
2 2 4 6
3 3 6 9
X – Parameter 1
Y
Parameter2
HR 1 2 3 4 6 9
1 1 2 3 4 6 9
2 2 4 6 8 12 18
3 3 6 9 12 18 27
XY
Parameter 1 * Parameter 2
Z
Parameter
Source: (Islam and Sado
2000; Islam and Sado 2002
ASCE)

9. DETENTION PONDS FOR SFRM
12/23/20209
Detention Ponds
• Basins that receive and hold runoff for release at a
predetermined rate
• Also known as retarding basins
• To reduce the peak runoff to streams and storm sewers
(Especially in urban catchments)
• Generally earthen structures
• Known as retention basins – Permanently retain water
• Known as dry ponds – Temporarily retain water

10. 12/23/202010
Main Function:
• Takes large amount
of water
• Limits the outflow
by having a small
opening at the
lowest point
Other Functions:
• Reduce stream bank erosion downstream
• Enhances water quality
Fig 3: detention pond cross section
(Source: https://en.mwikipedia.org)
DETENTION POND FOR SFRM….

11. 12/23/202011
• To obtain side slopes, area, depth
• WL at anytime – Continuity equation
ΔV = (Qin – Qout)Δt
Where,
ΔV = change in storage in Δt time
Inflow, Qin = Ʃqi + (Aр * r)
Outflow, Qout = Cd*ap*√(2g*Hd) if, HPL>TWL else, Qout = 0
HPL - water level in the detention pond
TWL - Tail water level
ILH - invert level of the outlet of the detention pond
• Modeling – software like MATLAB
DETENTION PONDS – FLOW SIMULATION

12. 12/23/202012
1) Provides negligible groundwater recharge
2) Frequently clogs at inlets and outlets - affects
retention times and pollutant removal efficiency
3) Requires large land area
4) Cannot be used in watersheds with cold- water
fisheries.
LIMITATIONS OF DETENTION PONDS

13. 12/23/202013
Fig 3 Guwahati city – water sheds
(Source : Detention Ponds for Managing Flood Risk due to Increased
Imperviousness: Case Study in an Urbanizing Catchment of India ASCE)
CASE STUDY – DETENTION POND FOR
FLOOD MITIGATION IN URBAN CATCHMENT

14. 12/23/202014
1) Development of FIMs :
Peak rainfall intensity of a 100-year return period
(Intensity-Duration-Frequency (IDF) )relationship
• SWMM- Flood parameters
2) Development of Individual HR maps:(2006&2011)
on the basis of,
• Flood Depth and Inundated Area
• Land use
• Population density & road networks
METHODOLOGY

15. METHODOLOGY…
12/23/202015
3) Development of an Overall HR map: (2006&2011)
3D matrix model
4) Implementation of Detention pond:
• The Wetland, Borosila Bil (Natural lake), near to the
outlet of water shed 3
• Capacity – Continuity Equation
• Depth assumed
• Surface area – ArcGIS
Fig 4: Location of detention pond in the
study area
(Source : Detention Ponds for
Managing Flood Risk due to
Increased Imperviousness:
Case Study in an Urbanizing
Catchment of India ASCE)

16. 12/23/202016
Fig 5
Overall HR map of 2006 before
implementation of detention pond
METHODOLOGY…
Analysis of maps:
(source: Detention Ponds
for Managing Flood Risk
due to Increased
Imperviousness: Case
Study in an Urbanizing
Catchment of India ASCE)

17. METHODOLOGY….
12/23/202017
Fig 6
Overall HR map of 2011 before
implementation of detention pond
Analysis of maps:
(source: Detention Ponds
for Managing Flood Risk
due to Increased
Imperviousness: Case
Study in an Urbanizing
Catchment of India ASCE)

18. 12/23/202018
Fig 7
Overall HR map of 2011 after
implementation of detention pond
METHODOLOGY…
Analysis of maps:
(source: Detention Ponds
for Managing Flood Risk
due to Increased
Imperviousness: Case
Study in an Urbanizing
Catchment of India ASCE)

19. FINDINGS
12/23/202019
• Max. flood depth decreased from 0.947 m to 0.506
(46.5%)
• The inundated area - from 63.1 to 35.9 km² (43%)
for the land use of 2011
• To achieve this, the total area of the lake has to be
maintained with a depth of 1.34 m
• Assume, the lake to be rectangular

20. SUMMARY
12/23/202020
1) SFRM and its relevance
2) Flood management measures
3) Analysis of impact of flood
4) Detention ponds and it’s effectiveness in urban
flood mitigation
5) A real life scenario

21. FUTURE SCOPE
12/23/202021
1) Explore the utilities of detention ponds for managing
urban water problems
2) Application of this methodology in the urban
scenario of Kerala
(Similar geographical properties with Guwahati)
3) Modification in the implementation method to yield
greater results

22. REFERENCES:
12/23/202022
1) “Engineering Hydrology” – K Subramanya (4th edition, 2013,
McGraw Hill Education)
2) S.N., Sahoo. and P, Sreeja. (2018.) “Detention Ponds for Managing
Flood Risk due to Increased Imperviousness: Case Study in an
Urbanizing Catchment of India”
3) S. N., Sahoo. and P, Sreeja. (2017.) “Development of flood
inundation maps (FIM) and quantification of flood risk in an Urban
catchment of Brahmaputra river”
4) E., Ahmadisharaf., M., Tajrishy., and N., Alamdari (2016.)
“Integrating flood hazard into site selection of detention basins
using spatial multi criteria decision making”
5) Islam., and Sado., (2000.); Islam., and Sado., (2002.) “Development
Priority Map for Flood Countermeasures by Remote Sensing Data
with Geographic Information System”
THANK YOU

23. DEVELOPMENT OF FLOOD
INUNDATION MAPS
12/23/202023
1. Find peak rainfall intensity of a 100-year return period
using Intensity-Duration-Frequency (IDF) relationship
i = KTª/ (t+b) ͫ
where,
i = rainfall intensity in cm/h
T = return period in years
t = storm duration in hours
K =7.206 , a = 0.156 , b= 0.75 , m= 0.94
(depends on location)

24. DEVELOPMENT OF OVERALL
HR MAP
12/23/202024
X
Flood depth
and flooded
area
Y
Land use
Z
Population
and flood
affected
roads
3D Matrix Model:
Source: (Islam and Sado 2000; Islam
and Sado 2002 ASCE)
HR 1 2 3
1 1 2 3
2 2 4 6
3 3 6 9
X
Flood depth and flooded area
Y
Landuse

25. 12/23/202025
HR 1 2 3 4 6 9
1 1 2 3 4 6 9
2 2 4 6 8 12 18
3 3 6 9 12 18 27
XY
Flood depth and flooded area – Land use
Z
PopulationandFlood
affectedroad Overall HR map :
HR Flood risk
zone
1
Low
2
3
4
Medium
6
8
9
High12
18

26. 12/23/202026
Where,
Ʃqi - Sum of all the discharges connected to any
detention pond
Ap - Area of the detention pond(ArcGIS, GRASS
GIS etc)
r - Rainfall intensity (IDF relationship)
Cd - Coefficient of discharge for the outlet of the
detention pond
ap - Area of outlet pipe
Hd - Head in the detention pond
Hd = HPL - TWL if TWL > ILH and HPL > TWL
Hd = HPL – ILH if TWL < ILH