Final_Hydraulics_Memorandum_-_Sir_Francis_Drake.pdf

Sir Francis Drake Boulevard

CA FLAP CR109(1)

Final Hydraulics Recommendations Memorandum

Marin County, California

Prepared for:

U.S. Department of Transportation Federal Highway Administration

Central Federal Lands Highway Division IDIQ Contract No. DTFH68-10-D-00002

Task Order No. T-14-080

September 2015

Prepared by:

707 17th Street, Suite 2400 Denver, Colorado 80202

Project No. WVXW3080

TABLE OF CONTENTS

Page No.

I. Project Description and Location

A. Description B. Location

II. Drainage Basins

A. Major Basins B. Minor Basins

III. Drainage Design Criteria

A. General Criteria B. Hydrologic Criteria C. Hydraulic Criteria

IV. Recommended Drainage Improvements

A. Roadway Elevation B. Minor Crossings C. Major Crossings

V. Erosion Control and Water Quality

VI. Conclusion

VII. References

Appendix Drainage Basin Maps Culvert Summary Tables

Regression Method and Equations Regression & Precipitation Maps Regression Analysis

NOAA Atlas 14 Precipitation Table and Map NRCS Soils Map TR-55 Input Data & Analysis

HY-8 Analysis – Existing Conditions HY-8 Analysis – Proposed Conditions

Flood-Prone Area Culvert Locations & Cross Sections East Schooner Creek Plan & Profile Sheet HEC-RAS Data Summary Output Tables, Plan and Cross Sections

Schooner Creek Culvert Preliminary Hydraulic Assessment (Report)

Schooner Creek Hydrologic & Hydraulic Analysis Basin Map, Hydrologic Analysis (Regression, TR-55, & HEC-HMS) HEC-RAS Data Summary Output Tables, Plan and Cross Sections Scour and Riprap Calculations

Final Hydraulics Recommendations Memorandum Sir Francis Drake Blvd

I. PROJECT DESCRIPTION AND LOCATION

A. DESCRIPTION

This project consists of improvements to the California County Road (CR) 109, Sir Francis Drake Boulevard. The Central Federal Lands Highway Division (CFLHD) in cooperation with the County of Marin proposes to rehabilitate, restore, and resurface (3R) approximately 12.1 miles of roadway within the Point Reyes National Seashore (PORE). Four short segments and one half-mile stretch along this roadway will be completely reconstructed (4R) to provide wider lanes, improve sight distance, and improve drainage conditions. This portion of the route is in a county easement, and is maintained by the county. The project limits are from MP 30.79 to 42.93 and are shown on the following page.

Existing roadway paved width averages 18’ to 19’ from MP 38.62 to 42.93 and approximately 23’ wide on many of the remaining 7.83 miles of the project. One goal of the project is to widen the existing roadway to a consistent 24’ with 11’ travel lanes and 1’ paved shoulders. This would complete the development of a uniform typical section for the 12.1 continuous miles of roadway.

A scoping and site review meeting was conducted in August 2012 and a Scoping Report was completed in September 2013 (Reference 1). A field review meeting was held in June 2014 following the 15% plan submittal. Additional field review meetings were held in October 2014 following the 30% plan submittal and in April 2015 following the 70% plan submittal. The purpose of these meetings was to review the plans, add site features, review existing culverts, document and review parking pullouts, review guardrail height and end terminals, document signs, utilities, and discuss design requirements with Marin County staff. Part of the site investigation was to determine existing drainage conditions within the project area and to make recommendations for improvements.

During the reviews, the team examined the major drainage crossings and visible culverts, noted size and condition of the pipes and needs for replacement of pipes, end sections, and/or headwalls. The purpose of this memorandum is to document the findings of the various field reviews, to define criteria and computational methods used for the hydrologic and hydraulic analysis, and to document the recommendations for culvert replacements for the project.

LOCATION MAP

B. LOCATION

CR 109 improvements begin at the intersection of Sir Francis Drake Boulevard and Chimney Rock and Lighthouse Roads on the southwestern tip of the Point Reyes National Seashore and continue northeast along Sir Francis Drake Boulevard to the intersection with Pierce Point Road. The route provides access to the Elephant Seal Overlook, the Historic Point Reyes Lifeboat Station, Drakes Beach, and numerous historic beef and dairy ranches. The route also provides access for school buses and delivery trucks to and from these ranches. Work consists of pulverization of existing pavement, new asphalt pavement, new signs and pavement markings, culvert and cattle guard replacements, and drainage improvements.

Begin Project

End Project

II. DRAINAGE BASINS

A. MAJOR BASINS

CR 109 crosses two main drainage features including Schooner Creek and a main tributary to Schooner Creek known as East Schooner Creek. This tributary parallels the upper portion of Sir Frances Drake Boulevard and crosses under the road just east of Schooner Creek in a 60” arch culvert at MP 32.93. This crossing is upstream of the flood-prone area. The tributary joins with Schooner Creek a short distance downstream. Schooner Creek passes under Sir Francis Drake Boulevard from the north to the south in a double 84” CMP culvert before outfalling into Schooner Bay then Drakes Estero, then Drakes Bay and eventually the Pacific Ocean.

The southwesterly end of Sir Francis Drake Boulevard is within a watershed that is flat, narrow and absent of major drainage features.

B. MINOR BASINS

In addition to the major drainage crossings, there are approximately 62 existing minor culvert crossings within the project area. This includes cross culverts for minor basins and ditch relief culverts. Pipe sizes for these basins range from 12” to 48” in diameter.

Existing culverts were noted during the scoping review in August 2012 and were further assessed during the field reviews in June and October of 2014 and April 2015. The general approach being taken for minor culvert replacement is as follows:

Existing 12"-18” culverts will generally be upsized to a minimum of 24” where there is enough grade and cover. A minimum of 18” will be used if the cover is not available. At two locations a 15” pipe will be used due to minimal cover at low points in the roadway.

24" and larger culverts that appear to be hydraulically adequate and in good condition are to remain. However, if the culvert is too short as a result of the proposed pavement widening, the culvert will be replaced rather than extended.

Marin County has requested that all existing culverts being replaced be replaced with HDPE pipes.

III. DRAINAGE DESIGN CRITERIA

A. GENERAL CRITERIA

Hydrology and hydraulics criteria for the County of Marin 3R project is generally based on the guidelines outlined in Chapter 7 of the Federal Lands Highway Project Development and Design Manual (PDDM) (Reference 2).

B. HYDROLOGIC CRITERIA

Hydrologic analysis for the project was done only in specific locations. These locations were identified to be in larger flow areas (major basins), or areas where further analysis was required to make proper recommendations. Locations were chosen based on field observations of active flow or evidence of erosion at the inlet or outlet of the culvert. The basins in the flood-prone area were evaluated to verify the adequacy of existing culvert sizes. This is a 4R section and the roadway is being raised approximately three feet so the culverts have been upsized to accommodate 100-year flows. Two different hydrologic methodologies were used to evaluate runoff from the ten basins evaluated, depending on their size.

The USGS National Streamflow Statistics (NSS) program uses current statewide regression equations to estimate flood magnitude and frequency on ungaged watersheds. The NSS recently replaced the National Flood Frequency (NFF) program. A recent update of the regression equations and hydraulic regions for the state of California was released in a July 2012 report (Reference 3) where 30 years of additional flood gage data have been analyzed. The software program developed by NSS to estimate flood magnitude and frequency would normally be used in this instance (Reference 4), but the software does not include this latest change.

Instead, the regression equations from the July 2012 report were used to calculate design flows for the project area. California is divided into six hydrologic regions, each with their own regression equations. The project area is within the North Coast Region. Variables used in the equations are basin area and mean annual precipitation. This methodology recommends a minimum basin area of 0.04 square miles (26 acres). Basins 1.0, 5.0, and 8.0 meet this criterion for minimum basin area.

However, the regression equations were used only for the largest basin (Basin 8.0), which is the East Schooner Creek culvert crossing. Generally the regression equations predicted flow rates lower than the TR-55 method explained below.

For the remaining basins analyzed, the NRCS TR-55 graphical peak method (Reference 5) was used. This method relies on basin parameters such as soil cover, hydrologic soils group, runoff curve number, area, length, slope, and time of concentration. Rainfall precipitation was based on NOAA (Atlas 14) Point Precipitation Frequency Estimates for the 24-hour storm duration.

These two approaches are consistent with the recommended hydrology guidelines presented in the PDDM. For the purpose of comparison to regression analysis, the TR-55 method was also used for the basin tributary to the East Schooner Creek culvert crossing (Basin 8.0) which resulted in higher flow rates. Yet another source of comparison came from the Culvert Replacement Conceptual Design report (Reference 10) for the improvements at Estero Road, approximately 2000 feet upstream. The 100-year flow rate of 500 cfs stated in the report is consistent with the results from the regression analysis.

The minimum design storm frequency is based on the roadway classification. For this route, based on the design speed (< 45 mph), current and future Average Daily Traffic (< 1500), and not being designated as critical access road; this route is classified as a low-standard road. Therefore culvert replacements for the majority of the project area are recommended to be designed for the 25-year storm event, in accordance with Table 7.1-A of the PDDM. The flood-prone area is located in an unregulated floodplain and it is recommended to design cross culverts in this area for the 100-year storm event, per the Scoping Report (Reference 1).

C. HYDRAULIC CRITERIA

Culverts The minimum culvert size for replacements will typically be 24”. Most of the existing culverts smaller than 24” inches are to be replaced. In some cases where very little cover is available, they are to be replaced with the largest pipe that will fit, and a design exception will be required. End treatments typically consist of end sections at the outlet and headwalls at the inlet. Culvert outlets include rock riprap outlet protection where needed. Many of the culverts are within heavily vegetated areas with little erosion potential and riprap will not be required. Culvert hydraulic design is based on the PDDM, with both capacity design and stability design considered.

Headwater to depth ratios (HW/D) up to 1.5 are allowed for culverts less than 48” in diameter. Object markers are also provided at the end of each culvert (existing and proposed).

Riprap Local scour at culvert entrances and outlets was reported at only two locations. An additional location was identified where East Schooner Creek is very close to the road and is undermining the roadway. The need for riprap protection at the culvert inlets and outlets and stream channels has been evaluated and designed based on the guidance in HEC-14 (Reference 6). In general, the existing culvert entrances and outlets are heavily vegetated with minimal channel slope. Local scour is not expected in these areas once vegetation is reestablished, and therefore, riprap is not required.

Inlets Drop inlets are used in cut sections of the roadway with narrow ditch widths where the back slope is steep and a headwall will not fit. Curb inlets are used in the paved ditch sections. In general, Marin County prefers headwalls (with a curb cut in the paved ditch) to drop inlets for maintenance purposes. Where headwalls cannot be accommodated due to steep back slopes, curb inlets are provided. There is one sump location in the flooded area where additional inlets were added to minimize flow spread width in larger storm events.

Ditches Roadside ditch sections are utilized in conjunction with the roadway sections.

Roadside ditches are either grass lined or riprap lined, depending on grade and velocity, and have been designed based on the guidance in HEC-15 (Reference 7).

Paved ditch sections are also utilized in locations to minimize environmental impacts and to reduce construction limits. Ditches will be protected during construction with erosion control devices.

Floodplains According to the effective Flood Insurance Rate Maps (FIRMs), the project area is designated as Zone D which states that flood hazards are undetermined, but possible. The project area is not in a FEMA regulated floodplain.

Aquatic Organism Passage In general, culvert crossings on live (perennial) streams should be considered to accommodate aquatic organism passage. There is one live stream crossing of East Schooner Creek that was designed to accommodate this. The proposed concrete box culvert will have a natural stream bottom, so the streambed through the culvert will mimic the natural stream conditions. Upstream portions of this stream have had recent improvements to restore habitat for federally threatened steelhead trout and endangered coho salmon (Reference 11).

IV. RECOMMENDED DRAINAGE IMPROVEMENTS

A. ROADWAY ELEVATION

The flood-prone area was analyzed to determine how high the roadway should be raised to eliminate overtopping in the 100-year storm event. Cross sections were generated along East Schooner Creek downstream of the culvert crossing from approximate Stations 500+00 to 522+00 and modeled using the HEC-RAS software program (Reference 12). Results show that raising the road approximately three feet from the existing elevation within the flood-prone area will prevent overtopping.

In addition to raising the roadway profile, existing culverts in this area will also be upsized to convey the 100-year tributary flows.

B. MINOR CROSSINGS

During the scoping field review in August 2012, an inventory was taken of the existing drainage culverts along a portion of the project. The existing culverts were assessed during the field reviews and recommendations have been made.

In general, the minor culverts were structurally intact but require cleaning to remove sediment and debris. Undersized culverts need to be upsized to 24” if the roadway cover allows for it. Some of the culverts require end section improvements as well as regrading to repair the effects of scour. The culverts that are damaged beyond repair should be completely replaced.

During the 15% field review in June 2014, the 30% field review in October 2014, and the 70% field review in April 2015 there was further assessment of many of the culverts on a location by location basis and further discussion with Marin County staff. Existing culverts were inspected for condition, hydraulic adequacy, and erosion problems. Roadside ditches were also visually evaluated for capacity and erosion problems. In some of the narrower areas, a paved ditch and curbed section has been provided to reduce the construction limits.

The existing culverts within the flood-prone area (Basins 3.0 through 6.0, MP 33.16 to MP 33.34), have been evaluated for hydraulic capacity. The raised roadway profile shifted low points and the proposed culverts have been relocated and upsized to carry the 100-year storm event. Culverts outside the flood-prone area will maintain 25-year capacity.

See tables below for comparison between existing and proposed culvert characteristics.

Existing Culvert Crossing Characteristics

Basin Station Existing Crossing

Design Discharge*

(cfs) Roadway Elevation

Inlet Invert Elevation

Outlet Invert

Elevation HW/D Ratio

1.0 500+75 24" CMP 30.43 11.8 8.2 6.6 1.85

2.0 503+75 15" CMP 2.32 13.8 10.4 8.0 0.74

3.0 505+46 18" RCP 1.07 12.9 9.8 9.0 0.35

4.0 508+06 21" CMP 21.62 11.6 8.9 7.7 1.59

5.0 516+22 24" RCP 43.24 18.0 15.3 14.5 1.44

6.0 517+85 18" RCP 10.51 19.5 17.1 16.6 1.43

7.0 522+53 18" RCP 3.03 24.2 21.8 21.4 0.62

8.0 529+37 Approx. 60"

Concrete Arch 563.07 30.4 25.4 25.0 1.36

*Bold indicates 100-year discharge used for design rather than 25-year.

Proposed Culvert Crossing Characteristics

Basin Station Proposed Crossing

Design Discharge*

(cfs) Roadway Elevation

Inlet Invert Elevation

Outlet Invert

Elevation HW/D Ratio

1.0 500+75 36" HDPE 29.91 13.9 9.34 8.86 0.89

2.0 503+75 24" HDPE 2.33 14.9 10.66 9.91 0.36

3.0 508+06 24" HDPE 14.79 14.1 9.21 8.77 1.11

3.1 509+10 24" HDPE 1.83 13.7 10.63 9.21 N/A

4.0 511+00 18" HDPE 4.95 15.2 12.27 12.03 0.86

5.0 516+22 2 - 24" HDPE 44.01 20.9 16.96 16.45 1.64

6.0 517+85 18" HDPE 10.91 20.9 17.61 17.22 1.67

7.0 521+00 18" HDPE 2.17 23.3 19.74 19.50 0.51

7.1 523+38 18" HDPE 1.39 26.6 23.32 23.11 0.41

8.0 529+37 12' x 6' CBC 562.61 33.3 25.53 24.26 1.31

*Bold indicates 100-year discharge used for design rather than 25-year.

C. MAJOR CROSSINGS

The major crossings have been evaluated based on observations made during field reviews, discussions with Marin County maintenance staff, and additional hydrologic & hydraulic analysis. Careful consideration was given to the potential for future corrosion and sediment deposition at the major crossing locations.

Recommendations for replacement have been made based on reducing this potential and designing for a 50-year service life.

An initial condition assessment has been provided in the scoping report from information gathered during the scoping meeting in August 2012 as well as further observations from the field reviews, and each of the locations is discussed below.

MP 33.67 (Station 490+80) The existing crossing is a double 84” CMP that conveys Schooner Creek under Sir Francis Drake Boulevard. The pipes are functioning adequately and appear to be in good condition. This culvert crossing will be replaced to provide better capacity for tidal flows and therefore promoting a more natural habitat for aquatic organisms. A separate study was performed to evaluate the culvert crossing and the impacts on Schooner Creek compared with the natural conditions of this tidally-influenced waterway (Reference 13), and is provided in the Appendix of this report. The study also provides recommendations for upgrades of the structure. Out of the options evaluated, the preferred option is a precast concrete arch. It was determined that a size of 28’ span x 7’ rise would closely replicate natural conditions of the tidal flows.

As the design progressed other factors such as construction phasing, environmental impacts, and geometric constraints with the existing roadway alignment were also considered. The final configuration is a 32’ span x 8’ rise concrete arch. This size of structure allows for slightly raising the grade of the roadway to provide additional hydraulic capacity, while still maintaining the existing alignment to avoid further impacts to the sensitive surrounding environment. The arch can be constructed in phases while still maintaining one lane of traffic during construction. This span also allows for the existing 84” CMP pipes to remain in place as the arch is being constructed so that a separate temporary drainage bypass structure is not needed.

A hydraulic analysis has been performed to evaluate the recommended 32-foot span concrete arch to convey Schooner Creek under Sir Francis Drake Boulevard. Cross sections were generated along Schooner Creek both upstream and downstream of the roadway crossing and modeled using the HEC-RAS software program (Reference 12). Boundary conditions were set at the downstream extent of the model to account for both high and low tidal levels (elevations 7’ and 2’ respectively). Results show the proposed concrete arch will pass the 100-year flood event without overtopping the road with approximately one foot of freeboard.

Scour calculations were performed using the guidance in HEC-18 (Reference 8), considering the methods for both open-bottom culverts (Section 6.9) and the NCHRP 24-20 Abutment Scour Approach (Section 8.6.3). Geotechnical data shows the existing channel bed material is gravelly sand, and an approximate D50 particle size of 1mm was used in the clear-water scour analysis. Results for the open bottom culvert analysis show the scour depths are 21.2 feet below the channel bottom for the 500-year event and 17.0 feet for the 100-year event. Results for the NCHRP 24-20 Abutment Scour predicted a much higher scour depth of 33.57 feet for the 100-year. This value was deemed impractical and a 500-year scour depth was not calculated using this approach. The structural design accounted for scour depths calculated using the open-bottom culvert analysis. As shown in the plans, the 32-foot concrete arch will rest on buried footings and drilled shafts will extend below the scour depths.

Riprap calculations were performed using the guidance for bottomless culverts in HEC-23 (Reference 9, Design Guideline 18). Extents of the riprap layout were modified to better meet field conditions, and to achieve a balance between protecting the footings and minimizing impacts within the stream channel.

MP 32.93 (Station 529+37) The existing crossing of East Schooner Creek is an approximate 60” arch CMP that is about 2/3 filled with sediment that currently does not have capacity to carry large storm flows. This culvert was evaluated for capacity and aquatic organism passage since it is downstream of other improvements completed in 2008 as part of the Drakes Estero Coastal Watershed Restoration Project (Reference 9). A 12’ x 6’ box culvert is recommended with the bottom buried one foot to allow for a natural channel bottom for aquatic organism passage. This box size will accommodate the 100-year flow rate and will not overtop the road as long as the culvert is maintained and sediment is not allowed to accumulate.

Final culvert profiles were designed using the guidelines in the Guide for Designing and Staking Culvert in the Field (Reference 14). Culvert end treatments, in the form of headwalls or end sections are provided at all new culvert installations and are shown on the plans. A separate detail sheet for the box culvert is provided in the Appendix.

V. EROSION CONTROL AND WATER QUALITY

An erosion and sediment control plan was developed for the project during final design, which consists of measures that serve as Best Management Practices (BMP’s) for water quality during construction, as well as permanent measures. The BMP’s for erosion and sediment control focus primarily toward protecting receiving waters and water sources in areas of construction activity. They include controls such as silt fence, sediment logs, inlet protection, and riprap outlet protection at culverts. Roadside ditches will also be protected during construction.

VI. CONCLUSION

The Federal Lands Highway Project Development and Design Manual (PDDM) was used for drainage criteria, with a 100-year recurrence interval used for the analysis of the cross culverts in the flood-prone area. The current recommendations for each minor culvert location have been assessed during the field review, and final recommendations for major culverts are based on consideration of all pertinent criteria and constraints, with input and acceptance by Marin County and CFLHD personnel. The two hydrology methods used, regression analysis and TR-55, are consistent with the PDDM.

VII. REFERENCES

1. Point Reyes National Seashore CA FLAP CR109(1) Project Delivery Plan and National Park Scoping Report, prepared for the USDOT Central Federal Lands Highway Division by Jacobs, issued September 2013.

2. Federal Lands Highway Project Development and Design Manual (PDDM), prepared by the Federal Highway Administration (FHWA-USDT), February 2011.

3. Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006, prepared in cooperation with FEMA, 2012.

4. National Flood Frequency Computer Program, Version 3.2, developed by Kernell G.

Ries III and Michele Y. Crouse, U.S. Geological Survey, updated October 16, 2002.

5. WinTR-55 Computer Program, Urban Hydrology for Small Watersheds, Version 1.00.10, developed by the Natural Resources Conservation Service, dated April 1, 2011.

6. FHWA HEC-14, Hydraulic Design of Energy Dissipators for Culverts and Channels, prepared by the Federal Highway Administration (NHI-06-086), 2006.

7. FHWA HEC-15, Design of Roadside Channels with Flexible Linings, prepared by the Federal Highway Administration (NHI-05-114), 2005.

8. FHWA HEC-18, Evaluating Sour at Bridges, Fifth Edition, prepared by the Federal

Highway Administration (HIF-12-003), 2012.

9. FHWA HEC-23, Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition, Volume 2, prepared by the Federal Highway Administration (NHI-09-112), 2009.

10. Point Reyes National Seashore Coastal Watershed Restoration Project – Culvert

Replacement Conceptual Design Final Report, prepared for the National Park

Service – Point Reyes National Seashore by Northwest Hydraulic Consultants and Jones & Stokes Assoc., October 2002.

11. Restoring Salmon and Trout Habitat at East Schooner Creek, Point Reyes National

Seashore, National Park Service, Pacific Coast Science and Learning Center, San Francisco Bay Area Network, Resource Brief May 2009.

12. HEC-RAS, River Analysis System Computer Program, Version 4.1.0, Hydraulic

Engineering Center – U.S. Army Corps of Engineers, January 2010.

13. Sir Francis Drake Boulevard Upgrade – Schooner Creek Culvert Preliminary Hydraulic Assessment, prepared for Central Federal Lands Highway Division by Jacobs, February 2015.

14. Guide for Designing and Staking Culvert in the Field, prepared by the Central

Federal Lands Highway Division, dated January 9, 1996.

J:\_Transportation\WVXW3080_SirFrancisDrake\700FHWA\T-14-080\Hydraulics\9_Final-Report\Final Hydraulics Memorandum - Sir

Francis Drake.doc

APPENDIX

Basin Station Existing Crossing Design

Discharge (cfs) Roadway Elevation

Inlet Invert Elevation

Outlet Invert Elevation HW/D Ratio

1.0 500+75 24" CMP 30.43 11.8 8.2 6.6 1.85

2.0 503+75 15" CMP 2.32 13.8 10.4 8.0 0.74

3.0 505+46 18" RCP 1.07 12.9 9.8 9.0 0.35

4.0 508+06 21" CMP 21.62 11.6 8.9 7.7 1.59

5.0 516+22 24" RCP 43.24 18.0 15.3 14.5 1.44

6.0 517+85 18" RCP 10.51 19.5 17.1 16.6 1.43

7.0 522+53 18" RCP 3.03 24.2 21.8 21.4 0.62

8.0 529+37 Approx. 60"

Concrete Arch

563.07 30.4 25.4 25.0 1.36

*Discharge shown in bold is for 100-year design flow, otherwise 25-year design flow.

CR109(1) Sir Francis Drake Blvd

Existing Culvert Summary Table

Basin Mile Post Station Existing Crossing 25-year Flow

(cfs) 100-year Flow

(cfs) Hydrology

Method Proposed Crossing Remarks

1.0 33.48 500+75 24" CMP 29.91 42.14 TR-55 36" HDPE Replace old culvert

2.0 33.43 503+75 15" CMP 2.33 3.27 TR-55 24" HDPE Replace old culvert

3.0 33.34 508+06 21" CMP 10.51 14.79 TR-55 24" HDPE Replace old culvert

3.1 33.32 509+10 N/A 1.30 1.83 TR-56 24" HDPE Inlet in paved ditch at low point

4.0 33.28 511+00 N/A 3.51 4.95 TR-55 18" HDPE New culvert

5.0 33.19 516+22 24" RCP 31.23 44.01 TR-55 2 - 24" HDPE Replace old culvert

6.0 33.16 517+85 18" RCP 7.75 10.91 TR-55 18" HDPE Replace old culvert

7.0 33.10 521+00 N/A 2.17 3.04 TR-55 18" HDPE New culvert

7.1 33.04 523+38 18" RCP 1.39 1.96 TR-55 18" HDPE Replace old culvert at Station 522+53

8.0 32.93 529+37 Approx. 60" Concrete

Arch

389.37 562.61 Regression 12' x 6' CBC

Aquatic Organism Passage Bottom buried with 1 foot native material

Proposed Culvert Summary Table

CR109(1) Sir Francis Drake Blvd

Prepared in cooperation with the Federal Emergency Management Agency

Methods for Determining Magnitude and

Frequency of Floods in California, Based on Data through Water Year 2006

By Anthony J. Gotvald, Nancy A. Barth, Andrea G. Veilleux, and Charles Parrett

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Scientific-Investigations Report 2012–5113

ABSTRACT

Methods for estimating the magnitude and frequency of floods in

California that are not substantially affected by regulation or diversions have been updated.

Annual peak-flow data through water year 2006 were analyzed for streamflow-gaging stations

(streamgages) in California having 10 or more years of data. Flood-frequency estimates were computed for the streamgages by using the expected moments algorithm to fit a Pearson Type III distribution to logarithms of annual peak flows for each streamgage. Low-outlier and historic information were incorporated into the flood-frequency analysis, and a generalized Grubbs-Beck test was used

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First posted July 3, 2012

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Sacramento, CA 95819 http://ca.water.usgs.gov/

Page 1 of 3USGS Scientific-Investigations Report 2012–5113: Methods for Determining Magnitude ...

10/18/2012http://pubs.usgs.gov/sir/2012/5113/ to detect multiple potentially influential low outliers. Special methods for fitting the distribution were developed for streamgages in the desert region in southeastern California.

Additionally, basin characteristics for the streamgages were computed by using a geographical information system.

Regional regression analysis, using generalized least squares regression, was used to develop a set of equations for estimating flows with 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities for ungaged basins in California that are outside of the southeastern desert region. Flood-frequency estimates and basin characteristics for 630 streamgages were combined to form the final database used in the regional regression analysis. Five hydrologic regions were developed for the area of California outside of the desert region. The final regional regression equations are functions of drainage area and mean annual precipitation for four of the five regions. In one region, the Sierra Nevada region, the final equations are functions of drainage area, mean basin elevation, and mean annual precipitation. Average standard errors of prediction for the regression equations in all five regions range from 42.7 to 161.9 percent.

For the desert region of California, an analysis of 33 streamgages was used to develop regional estimates of all three parameters

(mean, standard deviation, and skew) of the log-Pearson Type III distribution. The regional estimates were then used to develop a set of equations for estimating flows with 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities for ungaged basins. The final regional regression equations are functions of drainage area. Average standard errors of prediction for these regression equations range from 214.2 to 856.2 percent.

Annual peak-flow data through water year 2006 were analyzed for eight streamgages in California having 10 or more years of data considered to be affected by urbanization. Flood-frequency estimates were computed for the urban streamgages by fitting a

Pearson Type III distribution to logarithms of annual peak flows for each streamgage. Regression analysis could not be used to develop flood-frequency estimation equations for urban streams because of the limited number of sites. Flood-frequency estimates for the eight urban sites were graphically compared to flood-frequency estimates for 630 non-urban sites.

The regression equations developed from this study will be incorporated into the U.S. Geological Survey (USGS) StreamStats program. The StreamStats program is a Web-based application that provides streamflow statistics and basin characteristics for

USGS streamgages and ungaged sites of interest. StreamStats can also compute basin characteristics and provide estimates of streamflow statistics for ungaged sites when users select the location of a site along any stream in California.

Page 2 of 3USGS Scientific-Investigations Report 2012–5113: Methods for Determining Magnitude ...

Suggested citation:

Gotvald, A.J., Barth, N.A., Veilleux, A.G., and Parrett, Charles, 2012, Methods for determining magnitude and frequency of floods in California, based on data through water year 2006: U.S.

Geological Survey Scientific Investigations Report 2012–5113, 38 p., 1 pl., available online only at http://pubs.usgs.gov/sir/2012/5113/.

Contents

Abstract

Introduction

Purpose and Scope

Previous Studies

Description of the Study Area

Data Compilation

Peak-Flow Data

Physical and Climatic Basin Characteristics

Flood Magnitude and Frequency at Streamgages

General Log-Pearson Type III Frequency Analysis

Expected Moments Algorithm (EMA)

Multiple Grubbs-Beck Test for Detecting Low Outliers

Parameter Estimation Method for Frequency Analysis in the Desert Region

Trial Mixed-Population Frequency Analysis

Estimation of Flood Magnitude and Frequency at Ungaged Sites

Regression Analysis

Regionalization of Flood-Frequency Estimates

Regional Regression Equations

Accuracy and Limitations

Application of Methods

Estimation for a Streamgage

Estimation for an Ungaged Site Near a Streamgage

Estimation for an Ungaged Site Draining More Than One Hydrologic Region

Effects of Urbanization on Floods

StreamStats

Summary and Conclusions

References Cited

Appendix. Parameter Estimation Method for the Desert Region of California

Regional Skew Model

Regional Regression Model for Standard Deviation

Regional Regression Model for Mean

Equations for Estimating Flood Frequency at Ungaged Sites

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Page 3 of 3USGS Scientific-Investigations Report 2012–5113: Methods for Determining Magnitude ...

Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006

By Anthony J. Gotvald, Nancy A. Barth, Andrea G. Veilleux, and Charles Parrett

Abstract Methods for estimating the magnitude and frequency

by regulation or diversions have been updated. Annual

having 10 or more years of data. Flood-frequency estimates

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computed by using a geographical information system.

6���� ��� ����� ,���������������4���������������+����� �� �����!��� ������������7��,����,����,�8�,���,���,��)7�,�� ��

region. Flood-frequency estimates and basin characteristics ��9����

used in the regional regression analysis. Five hydrologic

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are functions of drainage area and mean annual precipitation ��������%4��

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standard errors of prediction for the regression equations in all %4��

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regional regression equations are functions of drainage area.

Average standard errors of prediction for these regression equations range from 214.2 to 856.2 percent.

5 �������������������� �����4� ������

6�� ����� �� ������������� ����������������4���������� frequency estimation equations for urban streams because of the limited number of sites. Flood-frequency estimates for frequency estimates for 630 non-urban sites.

����� ��+����� ����4�������� ������ �������D)3)�/����������3� 4���"D3/3#� 3� ��!3������ �!)�'���3� ��!3������ �!������E��� �4������� �D3/3��� interest. StreamStats can also compute basin characteristics �4��������!����������

4 Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006

120°122°124°

42°

40°

38°

36°

34°

COAST RAN

GES

COAST RANGES

KLAM

ATH

M

OUNTAINS

SIERRA

NEVADA

TRANSVERSE RANGE

Subtropical Pacific moisture

Pacific moisture

SAN GABRIEL/SAN BERNARDINO

RANGES

CASCADE

RAN

GE

Physiographic Region

Hydrologic Region—Desert

EXPLANATION

California Coast Ranges

California Trough

Great Basin

Klamath Mountains

Los Angeles Mountains

Mexican Highlands

Salton Trough

Sierra Nevada

Sonoran Desert

Middle Cascade Mountains

Lower Californian Province

0 75 150 MILES

0 75 150 KILOMETERS

CAL I FORN I A

OREGON

N E V A D A

A R I Z O N A

PACIFIC OCEAN

Base modified from U.S. Geological Survey (USGS) 1:100,000-scale digital data Shaded relief from USGS National Elevation Dataset (NED)

Figure 1. Physiographic regions in California (Fenneman and Johnson, 1946).

OrtizVT Text Box Project Location

OrtizVT Line

Data Compilation 5

Table 1. Basin characteristics considered for use in regional regression analysis for California.

Name Description Unit Data source

���"��" Drainage area of the basin Square miles $% ��������������� http://www.horizon-systems.com/NHDPlus/

("�������� Perimeter of the basin Miles $% ��������������� http://www.horizon-systems.com/NHDPlus/ minimum elevations in the basin

Feet $% ��������������� http://www.horizon-systems.com/NHDPlus/

http://www.horizon-systems.com/NHDPlus/

�)�/��� Minimum elevation in the basin Feet $% ��������������� http://www.horizon-systems.com/NHDPlus/

)"1�"��" Percentage of basin area covered by lakes and ponds

Percent 2%%3���� ��� �)����4���������,����5�)4�67 )����4�����http://www.mrlc.gov/nlcd2001.php

�)8%%% Percentage of basin area above ���� ����+�8�%%%�+���

Percent $% ��������������� http://www.horizon-systems.com/NHDPlus/

9:;)�;�)�/ Elevation at the outlet of the basin Feet $% ��������������� http://www.horizon-systems.com/NHDPlus/

basin perimeter

Feet per mile $% ��������������� http://www.horizon-systems.com/NHDPlus/

���;249"�; Distance from basin centroid to coast along a line perpendicular �����������4�

Miles $% ��������������� http://www.horizon-systems.com/NHDPlus/

�)�/ Average basin elevation Feet $% ��������������� http://www.horizon-systems.com/NHDPlus/

(�)���$%� Average basin slope Percent $% ��������������� http://www.horizon-systems.com/NHDPlus/

*9���; Percentage of basin area covered by forest

Percent 2%%3���� ��� �)����4���������,����5�)4�67 ���'����4������http://www.mrlc.gov/ nlcd2001.php

�����/ Percentage of basin area covered by impervious surface

Percent 2%%3���� ��� �)����4���������,����5�)4�67 Percent Impervious http://www.mrlc.gov/ nlcd2001.php

���4�� Mean annual precipitation Inches <%% ���������� �� ���������3=>3?2%%%����� http://www.prism.oregonstate.edu/products/

@"��"0 "���� temperature in the basin

Degrees Fahrenheit <%% ���������� �� ���������3=>3?2%%%����� http://www.prism.oregonstate.edu/products/ temperature in the basin

Degrees Fahrenheit <%% ���������� �� ���������3=>3?2%%%����� http://www.prism.oregonstate.edu/products/

)9�B&4��; )�� ������+�����,�� ��'����� � Degrees $% ��������������� http://www.horizon-systems.com/NHDPlus/

)";&4��; )�� ������+�����,�� ��'����� � Degrees $% ��������������� http://www.horizon-systems.com/NHDPlus/

6 Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006 was found to have errors in the stream network that resulted in drainage basin errors in portions of the upper Sacramento River Basin, including some parts of the Pit and Feather River drainages. The basin characteristics computed using a geographic information system (GIS) were used for all streamgages in the study, but some streamgages in the upper Sacramento River Basin were eliminated from further analysis because of potential inaccuracies in the computed basin char-acteristics. Streamgages were eliminated from further analysis if the GIS-calculated drainage areas differed substantially from those published in the NWIS peak-flow database. Differences in drainage area were considered substantial if they exceeded 50 percent for drainage basins smaller than 0.1 square mile, 20 percent for drainage basins between 0.1 and 1 square mile in size, and 10 percent for basins greater than 1 square mile in size. These criteria resulted in 771 streamgages that were considered for further use in the study (pl. 1; table 2).

Streamgages that were excluded from analysis because they did not meet the criteria for drainage area differences included 23 streamgages for which flood-frequency characteristics were previously reported by Parrett and others (2011). Because of the limitations of the NHDPlus dataset, users are cautioned to verify basin boundaries and drainage areas computed at ungaged sites to ensure that results are reasonable.

Flood Magnitude and Frequency at Streamgages

A frequency analysis of annual peak-flow data collected at a streamgage provides an estimate of the flood magnitude and frequency at that specific stream site. Flood-frequency flows were described in previous USGS reports as T-year floods based on the recurrence interval for the flood quantile (for example, the “100-year flood”). The use of recurrence-interval terminology is now discouraged because it can be confusing to the general public. The term has been interpreted to imply a set time interval between floods of a particular magnitude, when in fact floods are random processes that are best understood by using probabilistic terms. While the T-year recurrence interval flood is statistically expected to occur or be exceeded, on average, once during the T-year period, it may be equaled or exceeded multiple times during the period or not at all.

Terminology associated with flood-frequency estimates is shifting away from the T-year recurrence interval flood to the P-percent annual exceedance probability (AEP) flood. The use of percent AEP flood is now preferred because it conveys the probability, or odds, of a flood of a given magnitude being equaled or exceeded in any given year. For example, a 1-percent AEP flood (formerly known as the “100-year flood”) corresponds to the flow magnitude that has a 0.01 probability of being equaled or exceeded in any given year. The P-percent is computed as the reciprocal of the recurrence interval “T” multiplied by 100 (for example, 1/100 × 100 = 1 percent).

T-year recurrence intervals with corresponding percent AEPs are listed in table 3 (Gotvald and others, 2009).

Table 3. T-year recurrence intervals with corresponding P-percent annual exceedance probabilities for flood-frequency flow estimates.

T-year recurrence interval

P-percent annual exceedance probability

2 50

5 20

10 10

25 4

50 2

100 1

200 0.5

500 0.2

Table 2. Summary of streamgages in California that were considered for use in the regional regression analysis, 2006.

[Table 2 is available in a Microsoft© Excel spreadsheet and can be accessed and downloaded at http://pubs.usgs.gov/sir/2012/5113/]

Table 4. Flood-frequency statistics for streamgages in California that were considered for use in the regression equations, 2006.

[Table 4 is available in a Microsoft© Excel spreadsheet and can be accessed and downloaded at http://pubs.usgs.gov/sir/2012/5113/]

Flood-frequency estimates for streamgages are computed by fitting a known statistical distribution to the series of annual peak flows. The statistical distribution commonly used in the United States is the log-Pearson Type III distribution (hereafter referred to as the LP3 distribution). Guidelines and computa-tional methods for using the LP3 distribution are described in Bulletin 17B of the Hydrology Subcommittee of the Interagency Advisory Committee on Water Data (1982). General procedures for fitting the LP3 distribution, the expected moments algorithm (EMA), a new method for statistically detecting multiple potentially influential low outliers when fitting the LP3 distribution, a special application of the LP3 method developed for the desert region of California, and a trial application of a mixed-population flood-frequency analysis for high-elevation streamgages in the California mountains are described in the following sections of the report. The final flood-frequency estimates from the LP3 analysis for the 771 streamgages in California considered in this study are given in table 4.

Flood-frequency estimates could not be computed for station 11067000 Day Creek near Etiwanda, Calif. (map identification number 122), because of the uncertainty of debris flow effects on some of the higher annual peak flows. Also, estimates could not be computed for station 11142500 Arroyo De La Cruz near San Simeon, Calif. (map identification number 219), because of the uncertainty of the indirect-discharge measurements used to determine the larger annual peak flows.

http://pubs.usgs.gov/sir/2012/5113/ http://pubs.usgs.gov/sir/2012/5113/

12 Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006

Estimation of Flood Magnitude and Frequency at Ungaged Sites set of equations for estimating the magnitude and frequency

measured basin characteristics of the associated drainage use in the regional regression analysis (table 4).

Regression Analysis

candidate regression models for all combinations of basin characteristics and the development of hydrologic regions #�������#���6�#

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(basin characteristics) and response variables (P-percent �$%�����3���

��� ���8����5 arithmetic values of basin drainage area and P-percent AEP �9����������������� the logarithms of basin drainage area and the logarithms of �0 constant variance in the response variable over the range of the �5��

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more accurate and provide a better estimate of the regression

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regional regression analysis for the desert region are provided

Regression analysis requires that data be as spatially

same hydrologic response to a given storm and thus represent only one spatial observation. A statistical analysis using redundant streamgages misrepresents the information in the ����0D��(��� ��2���� ��������<3��B ����������

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665 streamgages for further regional analysis.

Regionalization of Flood-Frequency Estimates

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���������0!++)3��� �/12���������� � performed on the 632 streamgages outside of the desert region

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(base 10) prior to the regression analyses to (1) obtain linear �������5��

Estimation of Flood Magnitude and Frequency at Ungaged Sites 13

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variables ranged from 86.0 to 99.3 percent for the 1-percent

outside the desert region. Regression residuals for the

tive drainage basin in order to determine geographical patterns ���1

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patterns of regression residuals to develop the hydrologic

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