1. Love Field Modernization Program
2D Hydrodynamic Analysis of Love Field Drainage System Improvements
Association of State Flood Plain Managers

2. Outline Project Background Evolution of Flood Prevention Practices
Evolution of H&H Methods
Love Field Drainage System – Existing Conditions
Evaluating Solution Alternatives
Conclusions

3. Background
Existing storm drainage system undersized, designed for 5-yr storm
No floodplain management practices used in the past
The City requires new drainage systems to be designed for the 100-yr storm
Client (SWA) only responsible for drainage improvements associated with LFMP
National Flood Insurance Act (1968) established the NFIP – Move people out of floodplain
1973 Flood Disaster Protection Act - Required mandatory purchase of flood insurance for mortgage loans
Defined “floodplain” as the area flooded during the 100-yr storm

4. Progression of Flood Prevention Practices
Underground Storm Drainage
(prior to 1968)
Preserve draining capacity of natural creeks

5. Progression of Flood Prevention Practices
Floodplain Management Practices
(1968)
Move people out of floodplains

6. Progression of Flood Prevention Practices
Drainage systems have traditionally been designed to carry peak discharge of design storm. Rational formula or unit hydrograph based method used to determine peak discharge.
Floodplains are delineated using traditional H&H methods such as HEC-HMS and HEC-RAS computer programs
Today urban watersheds comprise complex drainage systems with a combination of underground drainage systems and above ground hydraulic structures.

7. Advancement of H&H Methods
How to define the design storm event
Frequency of event
Depth over contributing drainage area (inches)
Duration of storm (hours)
Rainfall intensity (in/hour)
Time Distribution (intensity vs. time)
Spatial Distribution (intensity/time/location)
Travel pattern (speed and direction)

8. Dallas Area Rainfall Data
Duration
Frequency
Annual Chance
Depth (in)
Intensity (in/hr)
24 hr
5yr
20%
5.4
0.22
10yr
10%
6.5
0.28
25yr 4%
7.6
0.31
100yr 1%
9.7
0.42
12 hr
4.8
0.39
5.6
0.45
0.53
8.5
0.72
6 hr
4.0
0.64
4.7
0.74
5.5
0.90
7.0
1.30
3 hr
3.4
1.10
3.9
1.20
5.3
1.60
5.8
2.00
1 hr
2.5
2.45
2.9
2.90
3.40
4.4
4.40
TP40 Curves
IDF Curves

9. Types of Rainfall Distributions in the US

10. Love Field Design Storm (100yr, 24hr – 9.7”)
Type II
Type III

11. What is the chance that a given storm happens in a given period of time?

12. What is the chance that a given storm happens in a given period of time?

13. Watershed Definition Its size and shape
Its topography (flat, hills, etc)
Its geography (soil, vegetation, creeks, etc)
Its type and extent of land development (rural, urban, etc)
Its level of infrastructure (paved gutters, storm sewers, etc)
Other hydraulic structures (diversion channels, gates, etc)

14. Watershed Response Runoff (volume) Discharge rate (flow)
Extent of Flooding (area)
Flood depth
Flood duration
Extent of damaged property

15. Dallas Love Field - Outfall 16 Drainage Area
340 acres
Outfall 16 drainage area = 340 acres

16. Dallas Love Field - Outfall 18 Drainage Area
510 acres
Outfall 18 drainage area = 510 acres

17. Dallas Love Field - Offsite Drainage Area
660 acres
Offsite area = 660 acres

18. Outfall 18 - Total Contributing Area
1,170 acres
Total Outfall 18 drainage area = 1170 acres
FEMA 100-yr Peak Discharge = 4,000 cfs

19. Hydrologic & Hydraulic Analysis
Rational methods compute peak discharge based primarily on size of drainage area and peak rainfall intensity.
HEC-HMS computer program uses unit hydrograph method to determine runoff (volume) and peak discharge.
HEC-RAS computer program computes hydraulic routing of discharges through a river system to determine water elevations. Typically peak discharges are routed and maximum flood elevations are computed.
InfoWorks offers a variety of runoff models (SCS, CN, Fixed, GreenAmpt) and routing models (SWMM, Snyder, SCS, Desbordes) and allows to divide the watershed into small subcatchments

20. Traditional Hydrologic & Hydraulic Analysis
HEC-HMS Model

21. FEMA HMS Model Hydrograph
Peak Q = 3,950 cfs

22. What if the watershed is more complex?

23. Love Field Existing Storm Drainage System
Over 550 conduits

24. Typical Cross Section of Circular Pipe
18 miles of circular pipes (6-78-in diameter
Over 20 miles of underground storm conduits.
18 miles of circular pipes varying from 6” to 78” in diameter.
Manning, Colebrook-White, Hazen Williams, etc.

25. Typical Cross Section of RCBs
1.9 miles of RCBs (W = 3-18 ft; H = 4-6 ft)
1.9 miles of culverts, varying in widths from 3 to 18 ft and in height from 4 to 6 ft

26. Cross Section of Horseshoe Tunnel
0.5 miles of tunnel (W = 6ft; H = 8ft)

27. Cross Section of Braniff Channel
1,200 ft of concrete open channel

28. Model Network of Existing System
600 conduits and 600 nodes

29. Outfall 16 Drainage Area
340 acres

30. Contributing sub-catchments can be assigned to each inlet in the entire system

31. Contributing sub-catchments assigned to each inlet in the system are individually defined

32. Sub-catchment Definition

33. Individual hydrograph are then computed for each sub-catchment in the entire system

34. Total contributing area at Outfall 16

35. Flow vs. time at Outfall 16
Total Volume = 300 MG

36. 2-D Simulation of Existing Conditions - Outfall 16

37. And what happens to the water on the surface?

38. 3D View of Terminal “bowl”

39. 2D Simulation Polygon Definition

40. Triangular mesh for 2D analysis
Total 2D simulation area = 1,320 acres
Approximately 100,00 triangles
Maximum and minimum triangle area = 1000 and 250 ft2 respectively

41. Close up of triangular mesh near tower

42. 2D Simulation of Existing Conditions

43. Existing Conditions Inundation Map

44. 2D Proposed Conditions: new apron grading and flood wall at the control tower

45. Proposed Conditions Inundation Map

46. Proposed Apron Grading (w/Drainage Improvements)

47. Comparison of Existing and Proposed Conditions
Existing Conditions – Outfall 18
Proposed Conditions – Outfall 18

48. Conclusions:
Upgrading undersized systems may require a combination of detention, retention, flow diversion, new lines and upsizing existing storm sewers
Traditional H-H methods relying on static analysis are limited in evaluating existing drainage systems in highly urbanized watersheds
Evaluating complex storm drainage systems requires more sophisticated analysis (i.e. 2D- hydrodynamic models) in order to gain the necessary insight to identify more sound and economic solutions

49. Questions?