Santa Rosa CreekHydrology and Hydraulics

Study

FMA Annual Conference2016

Overview• Tour of the SRC Watershed • Project Background

– Reservoir Hydraulics– HEC-HMS Modeling– 2-D HEC-RAS Modeling

• Calibration Challenges & Other Considerations

Photo provided by Water Agency; Spring Lake emergency overflow, NYE 2005 storm

Photo provided by Water Agency; Matanzas Reservoir, NYE 2005 storm

Photo taken by Betty Andrews, Santa Rosa Ave. culverts, Dec. 11, 2014 storm (~2-5 yr event)

Calibration Points of

Reference

Santa Rosa Creek Watershed

• 80 sq. mi. • El. ranges from

40’-2700’• 14 major creeks &

tributaries• 4 flood control

reservoirs• 2 diversion

structures• 4 rainfall gages & 3

stream gages

The Flood Management System

• Flood management systems designed in 1950s by the Water Agency, NRCS

– Two major diversion systems– Four reservoirs

Santa Rosa Creek

• System characteristics– Reservoirs built by SCWA &

SCS in 1960s– Santa Rosa Creek diversion

(and Spring Creek)– Downtown culverts (~1,500 feet

long) beneath City Hall– Prince Memorial Greenway

• FEMA FIS – Profile for lower 8,000’ of

channel only• USACE

– Prior work on H&H to support ecosystem and flood management projects

Photos taken by Betty Andrews, Dec. 11, 2014 storm (~2-5 yr event)

Santa Rosa Creek Diversion• Hans Albert Einstein,

UC Berkeley– Built physical model

at UCB in 1960

• Prior analysis done in HEC-RAS

– >50% flow diverted to Spring Lake for Qs < ~10,000 cfs

• Focused RAS model developed to inform HEC-HMS curves

• Upstream extent of RAS model

Matanzas Reservoir

Flood Management Reservoirs

• Very little info available

– Prior stage-storage-discharge curves from record drawings

• Circa-1960s NRCS designs

• As-builtsprovided by SCWA

Updated Reservoir Hydraulics

• Updated principal spillway and emergency spillway hydraulics

• Developed new stage-storage-discharge curves

HEC-HMS Model Development

• Refined subbasin delineations

• Updated Rainfall– 2 calibration events

(NYE 2005 & Jan 2010)

– Design Rainfall• Hydrology model

updates– Loss rates– Transformation– Routing– Baseflow– Reservoirs and

Diversions

Hydrological methods developed to adhere to SCWA’s FCDC Update.

HEC-HMS Calibration Brush Creek Gage

Calibration Challenges Matanzas Creek Gage

Rain stops here

Switch from barrel to orifice flow –700 cfs vs 1200 cfs

Matanzas Creek HEC-HMS calibration off by ~500 cfs

HEC-HMS Calibration Santa Rosa Creek Gage

HEC-HMS 100-yr Discharge Comparison

Location Drainage Area (Sq. Mi.) NRCS 1965 USACE

2010ESA 2015 AMCII CN

ESA 2015 Calibrated CN

Santa Rosa Creekabove Diversion 20.9 7,280 8,240 9,290 11,164below Spring Lake Outlet 22.2 1,930 4,301 5,036 6,629above Matanzas Creek 31.4 6,400 8,595 9,723 12,003above Piner Creek 57.4 11,100 14,942 14,869 19,629at Mouth 75.2 16,500 19,550 19,544 23,786Spring Creekabove Diversion 2 970 1,046 703 1,233below Diversion 2 0 111 162 211above Matanzas Creek 5 1,380 1,293 1,217 1,436Brush CreekMiddle Fork above Dam 1.2 930 934 573 659Middle Fork below Dam 1.2 295 719 245 252Ducker Cr above Austin 1.5 683 914 685 732Austin Cr above Brush Cr 2.3 1,170 1,435 1,871 2,051Brush Cr ab Santa Rosa Cr 8.0 3,970 5,333 4,228 4,698Matanzas Creekabove Matanzas Cr Dam 11.5 5,000 4,472 6,572 7,338below Matanzas Cr Dam 11.5 1,000 3,716 4,847 6,431above Santa Rosa Cr 22.8 4,250 5,780 6,496 8,840Piner CreekPaulin Creek above Dam 1.9 900 953 944 1,108Paulin Creek below Dam 1.9 300 600 392 650Piner Cr above Paulin Cr 5.2 1,620 2,081 2,253 2,633Piner Cr below Paulin Cr 9.8 2,830 3,750 3,773 4,371

Piner Cr above Santa Rosa Cr 11.8 3,480 4,153 4,222 4,872

~20% increase

Slide 14

BA1 compared to which??Betty Andrews, 2/10/2016

HEC-RAS 2D for Santa Rosa Creek

• Model updates– Steady state to

unsteady flow – 1D to 2D

• RAS Sensitivity Analyses

– Lateral link weir coefficients

– Time step discretization

• Calibration • Design events

NYE2005 1st Calibration Run on Brush Creek

NYE2005 1st Calibration Run on Matanzas

• RAS flows are 412 cfs < HMS flows due to

– Out of bank flooding– Hydraulic routing &

attenuation

• HMS CNs approaching AMCIII conditions

• Insufficient flow BCs with initial setup

NYE2005 1st Calibration Run on SR Creek

NYE2005 Calibration Adding Lornadell Creek

Lornadell Creek Dynamics

NYE2005 Calibration With Lornadell Creek

NYE2005 Calibration With Lornadell Creek

NYE2005 Calibration at SRC

SR culverts entranceFulton Rd Prince Memorial Greenway

NYE2005 Calibration at SRC

Prior Study ‘n’ values

Adjusting for Prince Memorial Greenway Project

‘n’ values x 120%

Observed WSE at USGS Gage

Design Storm Flood Dynamics – Floodplain & urban storage are important!!!– Unsteady 2D representation improves physical

representation of storage

Hydraulic vs Hydrologic Routing

RAS2D Results – NYE 2005

RAS2D Results – 10-yr Flood

RAS2D Results – 100-yr Flood

100-yr Downtown Flooding

Take-Home Considerations

• Hydraulic routing and floodplain/urban storage play an important role

• Calibrating hydrology in the absence of hydraulics may be problematic for some flood events

• USGS discharge values as opposed to matching stage?

• Lack of storm drain system contributions also likely playing large role

• Impacts on Flood Frequency Analyses?

Questions???

City of Santa Rosa Engineering