Christie Beeman and Jeff HaltinerPhilip Williams & Associates

c.beeman@pwa-ltd.com; j.haltiner@pwa-ltd.com

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?