Reservoir Sedimentation and Wildfire in the Sierra Nevada Mountains, CA

J. Toby Minear and Scott WrightCalifornia Water Science CenterUnited States Geological Survey

Outline• Introduction

– What is reservoir sedimentation?– How does fire affect sedimentation? – ‘3W’ model for reservoir sedimentation in California

• Sedimentation study with Sierra Nevada Conservancy

• How we could use your help…

• Reservoir blocks downstream transport of sediment• Sediment accumulates in the reservoir = reservoir

sedimentation• Reduced sediment supply to downstream areas• “Hungry water” = downstream erosion of bed and banks•

Reservoir sedimentation

Erosion of bed and banks from “Hungry water”

Reservoir Sedimentation

Sediment

Sediment

DamWater

Effects of Fire on Sediment Dynamics• Fires in S. CA and Coast Ranges:

– Fires can increase sediment loads 5-120+ times the pre-fire level

– In particular, the ‘fire-flood’ scenario leads to highest sediment rates– Typically 5+ years before rates return to pre-fire levels

• Effects are not as well known for the Sierras

CalFire fire history database

Modeling Reservoir Sedimentation in the Sierras

• Mutiple dams in the same watershed and changes to trap efficiency with time are major issues

• Minear and Kondolf (2009) came up with a method to address this issue: ‘3W’ model– Estimates long-term sediment yields from reservoir sedimentation

records; applies these yields to unmeasured reservoirs– Accounts for multiple dams in the same watershed and changes in

trap efficiency– User-chosen time-step (yearly time-step for 2009 paper) and number

of sub-regions (geomorphic regions for 2009 paper)

Reservoir Sedimentation in California: ‘3W’ Model

- 12 geomorphic regions

- 1,391 dams

- 70 dams with measured sedimentation rates

3W model: statistical approach; based on geomorphic regionto estimate median sediment yield

Minear and Kondolf, 2009, WRR

Black dots = measured reservoirs

White dots = unmeasured reservoirs

3W Model Results

Sediment yield rates by geomorphic region:

Minear and Kondolf, 2009, WRR

3W Model Results

Estimated reservoir capacity remaining in 2008 (as percent of

original)

Minear and Kondolf, 2009, WRR

3W Model Results

Estimated reservoir sedimentation in

acre-feet

Minear and Kondolf, 2009, WRR

Limitations of the 3W modelReservoir B,Bulit 1957

Reservoir C,Bulit 1933

Reservoir A,Bulit 1949

Reservoir F,Bulit 1947

Reservoir E,Bulit 1964

Reservoir D,Bulit 1984

1. Not GIS friendly

2. Difficult to include GIS attributes / modeling - e.g watershed parameters,

fires, soils, climate, climate change, etc.

time

hydr

olog

y

3. No hydrology

Objectives of this study

1. Determine better reservoir sedimentation rates in the Sierra, particularly related to fire

2. Compile existing reservoir sedimentation records into a single, publicly accessible database

**We could use help from local partners to help obtain the sedimentation and capacity data

Improved Reservoir Sedimentation Model

Dam data(size, date constructed,sedimentation, operation)

Gage data (hydrology + sediment)

+ +

GIS data: watershed factors(fire history, slope, aspect, watershed size, mainstem length, etc.)

Addressing sediment concerns and long-term reservoir storage in the Sierras (both in the reservoir and downstream)

Publicly Accessible Database

Interagency project to update nation-wide reservoir sedimentation database: http://ida.water.usgs.gov/ressed/

The RESSED database:

Future Steps

Next Phase: Field studies of individual reservoirs– Determine sedimentation rates related to fire– Additional bathymetric mapping, coring, sediment

mapping– Will depend on finding interested partners

Questions?

jminear@usgs.gov