christie beeman and jeff haltiner philip williams & associates hydrograph modification: an...
DESCRIPTION
What’s the problem? Altered hydrology can cause channel erosion. Higher, more erosive peak flows Longer duration of lower, but still erosive, flowsTRANSCRIPT
Christie Beeman and Jeff HaltinerPhilip Williams & Associates
[email protected]; [email protected]
Hydrograph Modification:An Introduction and Overview
• Flood Management– Large, infrequent events (quantity)
• Stormwater Quality– Small, frequent events (quality)
Stormwater Regulation
• Hydrograph Modification Management
– Small, frequent events (quantity --> quality)
What’s the problem? Altered hydrology
can cause channel erosion.
• Higher, more erosive peak flows
• Longer duration of lower, but still erosive, flows
from turning this…
Why regulate Hydro Mod?
To prevent this…
into this.
What’s the problem?
• Channel morphology is a response to the watershed delivery of water and sediment
• In a stable creek channel, water and
sediment are in balance: – no net erosion or deposition over time
• Changes in watershed hydrology & sediment supply can upset the balance
• Watershed impacts of development tend to cause channel erosion/degradation
EROSION DEPOSITION
What’s the problem?
What’s the problem?Channel erosion can cause: • Reduced water quality (sediment load,
turbidity) = regulatory “hook”
• Damage to adjacent property & infrastructure
• Loss of riparian habitat
• Loss of aquatic habitat
• Downstream sediment delivery/deposition
runoff
infiltration
precipitation
How does it happen?
How does it happen?
Time
Run
off
Pre-Development
Urbanization tends to increase stormwater runoff: peak flows volume frequency
Post-Development
How does it happen?
Hydrograph Modification Management
Post-project runoff peaks and durations must not exceed pre-project levels if an increase could cause erosion or other significant effects on beneficial uses.
Bay Area standard:
How do you measure it?
• Quantifying potential hydrograph modification impact (and mitigation) is a challenge
• Analysis requires – Rainfall-runoff modeling
– Comparison of pre- and post-project conditions
How do you measure it?
• Single event “design storm” models (e.g. Q100):– Common tools for flood analysis, but
– Not effective for analyzing smaller, more frequent events
Time
Run
off
Pre-Development
Urbanization tends to increase stormwater runoff: peak flows volume frequency
Post-Development
How do you measure it?
• Event-based models predict runoff response for a particular storm event
• Don’t reflect cumulative runoff response over time
How do you measure it?
• Continuous hydrologic models: – Can evaluate flow peak and duration over
full range of flows, but
– Require specialized expertise, onerous for smaller projects
How do you measure it?• Continuous simulation models use a long-term
rainfall record (30+years)
• Statistical analysis of runoff response to all events
• Don’t reflect cumulative effect of runoff response over time
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1 2 3 4 5 6 7 8 9 10Recurrence Interval (years)
Peak
Flo
w (c
fs)
Impervious Drainage Area Discharge
Type D Soil, Scrub Cover
Type A Soil, Scrub Cover
Flow
(cfs
/acr
e)
Recurrence interval (years)
How do you measure it?
Peak Flow
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009 0.0010
% Time Exceeded
Flow
(cfs
)
Impervious Drainage Area Discharge
Type D Soil, Scrub Cover
Type A Soil, Scrub Cover
flow
(cfs
/acr
e)
time exceeded (hours)0 100 400300200 500
How do you measure it?
Flow duration
What’s the result? Altered hydrology
can cause channel erosion.
• Higher, more erosive peak flows
• Longer duration of lower, but still erosive, flows
Urbanization also tends to reduce the natural sediment supply: land development detention basins
Post-Development
EROSION DEPOSITION
Channel Response
Response of the stream is complex, depends on channel and watershed characteristics …
Channel Response… but we have simple models to predict potential impacts from development.
Restoration often seeks to accelerate this natural process to achieve new dynamic equilibrium.
After Schumm, and Simon & HuppChannel Response
Stable Channel
Channel Incision
Channel Incision
Bank Erosion / Collapse
Bank Erosion / Collapse
Channel Widening
Channel Widening
New dynamic equilibrium
Example: Rifle Range Creek, Oakland
higher creek flows
+ lower sediment supply
erosion
Example: Rifle Range Creek, Oakland
Summary
• In a stable creek channel, water and sediment are in balance: – no net erosion or deposition over time
• Watershed impacts of development tend to cause channel degradation
• Specific channel response depends on complex interaction of watershed and channel characteristics
Conclusion
• The goal of hydrograph modification regulation is to manage water quantity to preserve water quality and stream function
• Challenge is to develop a regulatory scheme that is simple enough to apply but sophisticated enough to be effective
Questions?