Stephen Blanton, PEMay 20, 2008

Addressing Stormwater Impacts on Hawaiian Streams

Mr. Stephen Blanton, PE14 years experience focusing on water resources, geomorphology, environmental science, civil engineering and planning.

Currently working for ENTRIX, Inc, conducting stream restoration projects aimed at restoring natural function to impacted river/stream systems.

Hawaii work included; TMDL for Nawiliwili, Pearl Harbor, and Kaukonahua. Dam failure analysis for Reservoir 24, Maui. Maui County hydrologic study (West Maui Mountains), and various water quality studies on Maui and Oahu.

Natural Function of a Healthy Stream-

A natural functioning stream has the ability to dissipate energy from normal high flow event without excessive erosion, sediment transport, and channel adjustment. To achieve this, a stream must have functioning riparian area and floodplain.

Functioning Floodplain

Not so much

Healthy Riparian Area and Floodplain

Improves water qualityfilters sediment

Controls rates and volume of erosionattenuates flood peakstreambank stability

Aquifer Rechargedry season streamflow

The key is to keep the water on the land

Typical Stream Profile

Stormwater Impacts A healthy naturally functioning stream has developed a dynamic equilibrium based on the hydrologic characteristic of the watershed.

Changes to the drainage area’s hydrology due to land use changes alter this equilibrium and the stream then starts to adjust to accommodate the conditions.

Typical land use changes are: residential/commercial/industrial development, timber harvest, agriculture, fire, and diversions.

Residential Development

Agricultural Development

Typical Impacts to a Watershed

Increased runoff volumes

Increased peak runoff rates

Increased flow velocities

More frequent channel forming flows

Lower baseflow

Less groundwater recharge

Water Quality

Hydrologic Analysis

North slope of Haleakala

Mostly Agricultural land – Sugar

Small Residential area in upper region

Impacts of Impervious Area

0

20

40

60

80

100

120

140

160

3/1/19960:00

3/1/199612:00

3/2/19960:00

3/2/199612:00

3/3/19960:00

3/3/199612:00

3/4/19960:00

3/4/199612:00

3/5/19960:00

3/5/199612:00

3/6/19960:00

Date

Flow

Rat

e, c

fs

Residetial 38% ImperviousAgriculture 7% Impervious

Runoff Increase

0

2000000

4000000

6000000

8000000

10000000

12000000

14000000

12/17/1995 0:00 2/5/1996 0:00 3/26/1996 0:00 5/15/1996 0:00 7/4/1996 0:00 8/23/1996 0:00

Date

Acc

umul

ated

Run

off V

olum

e, ft

3

Agriculture- 7% Impervious

Residential- 38% Impervious

Water Quality Issues

Increase in:

Nutrients- Nitrogen, Phosphorus.

Sediment load- TSS

Herbicides & Pesticides

Metals- copper, zinc, lead, chromium

Temperature

Adverse Impacts

Native Species of Fish, Shrimp, and Snails

Habitat Impacts Native Survival and Allows Invasive (more tolerant) Species to gain presence and displace natives.

Concrete Channels lack of riparian vegetation create barriers for the amphidromous species to complete natural life cycle.

Lack of natural stream functions impacts coral reef health.

Stream Health Related to Impervious Area

Non-Supporting Urban Drainages

Change so all zones are sources………

Typical Impacted Stream

Extreme Event

Stormwater Regulations in The State of Hawaii

Counties address stormwater from development using Counties address stormwater from development using design standards. design standards.

The State addresses stormwater quality through the The State addresses stormwater quality through the NPDES and TMDL programs associated with the Clean NPDES and TMDL programs associated with the Clean Water Act.Water Act.

Future of Stormwater and Stream Health

Low Impact DevelopmentLow Impact Development-- The goal is to maintain The goal is to maintain historic/natural hydrologic conditions.historic/natural hydrologic conditions.

Restoration OpportunityRestoration Opportunity-- Add natural function back to impacted Add natural function back to impacted streams.streams.

Mother Nature Conducting Stream Restoration