TWCA 2017 Fall Conference

1. 1
Christopher Herrington, P.E.
512-974-2840
Chris.Herrington@AustinTexas.gov
Addressing Hydrology at the Watershed Scale
A Novel Approach to Stormwater Management
with a Water Supply Nexus

2. 1. Humans alter hydrology, and hydrology is king
2. Storage is the solution to altered hydrology
3. Distributed small-scale stormwater controls can
be a meaningful solution to altered hydrology
(with benefits)
2
Executive Summary
Urban Hydrology Restoration, Proof of Concept Modeling report
austintexas.gov/watershed_protection/publications/document.cfm?id=268805

3. More people means…
3
• More wastewater to manage
• Land becomes more expensive
• Increased demand for potable water
• More impervious cover = degraded stream function

4. Humans change Hydrology
4
“The Urban Stream Syndrome” – Walsh et al. 2005

5. 5

6. 6austintexas.gov/watershed_protection/publications/document.cfm?id=196394
%
Impervious
Cover
Fraction of
Rainfall that
becomes
runoff
# Runoff
Events per
Year
Annual
Runoff
(inch)
0 0.064 46.0 1.18
100 0.781 73.4 22.91

7. 7

8. 8
Erosion
Water Pollution
Flooding

9. The (hydrology) Problem
Anthropogenic disturbance (eg, impervious cover):
• reduces rainfall infiltration
• decreases creek baseflow
• increases stormwater runoff, flooding
• increases erosion, pollutant loading

10. Stream Function Pyramid
10Harman et al. 2012

11. How do we fix hydrology*?
STORAGE
11
*in a cost-effective way

12. Conventional Stormwater Management
12

13. When it stops raining…
13
24%
9,429,728,998 gal/year

14. Annual average water demand-Austin
Citywide
• Outdoor uses = 24% of total citywide demand
Residential indoor non-potable uses
(toilets, clothes washers)
• Single-family = 27% of total sector demand
• Multi-family = 32% of total sector demand
1441.5%

15. 15

16. One Water:
Integrated Water Resource Management
Integrated water resources management is
based on the equitable and efficient
management and sustainable use of water and
recognizes that water is an integral part of the
ecosystem, a natural resource, and a social and
economic good, whose quantity and quality
determine the nature of its utilization.
-Global Water Partnership 16

17. Green Stormwater Infrastructure
17
Centralized, Regional, Community
Decentralized, Building

18. Fixing today’s Austin flood, erosion and water quality
problems
Can I borrow
$2,263,142,800?
(let’s talk about
maintenance later)
(p.s., it will take 87 years
just to fix today’s problems)
Austin Watershed Protection Master Plan

19. Centralized, Regional GSI
Significant economy of scale, but:
• Opportunities for construction are limited
– Land increasingly expensive
• Each project increases maintenance burden
• Construction and O&M paid entirely by utility
• Failures are “big”
• Does not offset demands on potable water
19

20. Decentralized, small-scale GSI
• Constructed on private land
– High degree of opportunity in urban watersheds
• Reduced burden on utility
• Failure impacts smaller and distributed
• Can offset demands on potable water
But, is there any hydrologic benefit?
20

21. Proof of Concept Modeling
Can distributed,
small-scale
stormwater controls
provide a meaningful
intervention in urban
hydrology?

22. Proof of Concept Modeling
Existing Regional Controls Distributed + Regional Controls

23. Distributed Control Scenarios
23
Land Use Category
Total
Paved
IC
Total
Roof
IC
IC area treated
Total #
ParcelsMax High Low
Single Family 18.1 61.1 79.2 45.8 15.3 1,257
Multifamily 8.0 6.9 14.8 9.1 3.1 40
Commercial 37.3 25.6 62.9 37.9 12.7 88
Office 9.2 5.7 14.9 8.9 3.0 22
Industrial 18.6 15.9 34.5 21.2 7.1 40
Civic 32.8 11.6 44.4 25.1 8.5 13
% of IC treated 71.4 % 42.2 % 14.2 %
% of surface area treated 26.4% 16.0% 3.4%

24. WLR3: Rain Cistern Sizes
24
Land Use Category
Average roof
area
(acres)
Cistern
volume
(gallons)
Single Family and Duplex 2,150 2,500
Multifamily 6,933 8,000
Commercial 10,522 12,500
Office 10,427 12,500
Industrial 16,688 20,000
Civic 13,605 15,500
All roof surfaces treated, capturing up to 2 inch rainfall

25. 25
Cistern
Raingarden,
Rainscape

26. SWAT Model
26
Topography + Soils + Land Use = Hydrology

27. Average Peak Flow

28. Flow Standard Deviation

29. Ratio of Baseflow to Total Flow

30. Ecologically Significant?
At low rain cistern saturation, change in
hydrologic metrics correspond to 20%
improvement in Austin aquatic life multi-
metric index scores
30

31. Water Supply Benefits?
• 27% reduction in “water stress days”
• 11% increase in evapotranspiration
• Potential to offset demand on potable supply
31

32. Lessons Learned
• Hydrology shifts proportional to density of
treatments
• Hydrologic improvement even without treating
transportation land use
• Equivalent to a potential reduction in of 25%
effective impervious cover
32

33. 33
Rainwater Harvesting is Real!

34. Implementation Testing

35. 1. Humans alter hydrology, and hydrology is king
2. Storage is the solution to altered hydrology
3. Distributed small-scale stormwater controls can
be a meaningful solution to altered hydrology
(with benefits)
35
Executive Summary
Urban Hydrology Restoration, Proof of Concept Modeling report
austintexas.gov/watershed_protection/publications/document.cfm?id=268805

36. Texas Water Research Network
esi.utexas.edu/research/texas-water-research-network/
Save the Date!
January 11-12, 2018
Austin, Texas