location hydraulic study

Location Hydraulic Study Alisal Creek Bridge

at Hartnell Road State Bridge Number 44C0110

County Bridge Number 209

Prepared for

Monterey County RMA

February 2017

10680 White Rock Road, Suite 100 Rancho Cordova, CA 95670-6165

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Table of Contents 1.0 Introduction .......................................................................................................................... 1

1.1 Project Description ........................................................................................................... 1

1.2 Project Setting .................................................................................................................. 1

1.3 Existing Bridge ................................................................................................................. 1

1.4 Proposed Bridge ............................................................................................................... 1

1.5 Purpose of this Report ...................................................................................................... 2

1.6 Vertical Datum ................................................................................................................. 2

2.0 Regulatory Setting ............................................................................................................... 3

2.1 National Flood Insurance Program .................................................................................. 3

2.2 Executive Order 11988..................................................................................................... 5

3.0 Project Assessment............................................................................................................... 5

3.1 Hydrologic Assessment .................................................................................................... 5

3.2 Hydraulic Modeling ......................................................................................................... 7

3.3 Hydraulic Assessment ...................................................................................................... 8

4.0 Conclusion ......................................................................................................................... 10

5.0 References ......................................................................................................................... 12

Appendix A Location Hydraulic Study Form .......................................................................... 13

Appendix B Summary Floodplain Encroachment Report ...................................................... 15

Appendix C Bridge General Plan .............................................................................................. 16

This Location Hydraulic Study was prepared by or under the direction of the following

Registered Person:

________________________________________ Mark J. Christensen, P.E.; Registration Expires 12/31/18

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Table of Figures Figure No. Page No.

Figure 1 – Project Location Map .................................................................................................... 2

Figure 2 – Project Vicinity Map ..................................................................................................... 3

Figure 3 – National Flood Insurance Rate Map (FEMA 2009) ...................................................... 4

Figure 4 – Hartnell Road Drainage Area ........................................................................................ 5

Figure 5 – Downstream Tributary................................................................................................... 7

Figure 6 - Flood Limits Based on HEC-RAS Analysis .................................................................. 9

In accordance with Executive Order 11988 (E.O. 11988) – Floodplain Management and the

regulations promulgated by the Federal Highway Administration in its Federal-Aid Policy Guide

(FAPG) directive, as revised in Title 23 - Code of Federal Regulations (CFR), Chapter I,

Subchapter G, Part 650, Subpart A, updated June 10, 2016, and in general conformance with the

guidance from the State of California, Department of Transportation (Caltrans), Standard

Environmental Reference (SER), Environmental Handbook, Volume 1: Guidance for Compliance

(EH Vol 1), Chapter 17, "Floodplains," (downloaded from the web) last updated June 21, 2016

11:09 AM; and Caltrans Local Assistance Procedures Manual (LAPM), Chapter 6,

"Environmental Procedures," dated March 14, 2013, the following documentation is provided.

Location Hydraulic Study

Alisal Creek Bridge at Hartnell Road 226646

February 1, 2017 1

1.0 Introduction 1.1 Project Description

The County of Monterey has secured preliminary engineering funds through the Highway

Bridge Program for the replacement of the Alisal Creek Bridge on Hartnell Road (County

Bridge #209 , State Bridge No. 44C0110). Preparation of this Location Hydraulic Study

is required to demonstrate that an adequate bridge waterway opening is provided so as to

not exacerbate the impacts of flooding.

1.2 Project Setting

The study area is located on Alisal Creek near the city of Salinas, California. Alisal Creek

originates at the northwestern slope of the Gabilan Ranges and flows in a southeasterly

direction at the Hartnell Road bridge crossing. Downstream of the bridge, Alisal Creek

flows in a northwesterly direction until it reaches Carr Lake in Salinas. Carr Lake, although

drained in the 1920s to make room for farmland, does become inundated during heavy

rainy seasons. At Carr Lake, Alisal Creek joins with Gabilan Creek and Natividad Creek,

and outlets into Reclamation Ditch. Reclamation Ditch continues in a northwesterly

direction and later joins with the Salinas River and ultimately outfalls into the Pacific

Ocean near Moss Landing, approximately 15 miles downstream (northwest) of the project

site (see Figures 1 and 2).

The project site is located within a regulatory floodway, as shown on the FEMA FIRM

map provided in Figure 3. Federal regulations state that it is not permissible to increase

the flood height with any encroachment into the floodway; therefore, flood neutrality must

be achieved in the design of the new bridge.

The bridge is located in a rural area of the County, surrounded by agricultural lands with

residential/agricultural structures nearby (see Figure 2). The closest residences are located

approximately 200 feet northeast of the bridge.

1.3 Existing Bridge

The bridge is located over Alisal Creek on Hartnell Road; 0.15 miles south of Alisal Road,

southeast of the city of Salinas in Monterey County. The existing bridge has two spans of

different vintage. The northern span, built in 1945, is a reinforced concrete box culvert

with unknown foundation. Sometime after its construction in 1945, the northern span was

extended and the southern span was added. The culvert was extended and widened using

steel railroad rail frames topped with a reinforced concrete deck.

The existing bridge is approximately 21 feet long and 25.5 feet wide, carrying two

vehicular lanes with no shoulders. According to Monterey County Public Works Annual

Average Daily Traffic (AADT Book 2013), Hartnell Road carries 1900 ADT and over

2000 ADT is expected in 20 years. Based on the Caltrans Structure Inventory and

Appraisal Report dated August 5, 2013, the bridge has a sufficiency rating of 45.3 and is

structurally deficient.

1.4 Proposed Bridge The bridge will be replaced with a reinforced concrete double box culvert that will

accommodate two 12 foot lanes with 8 foot shoulders and Type 732 barriers at each edge

of deck. The box culvert will be placed at an approximate 45 degree skew to the roadway



February 1, 2017 2

and the channel will be regraded at each end to remove the 90 degree turns at both ends of

the bridge. The “S” configuration of the channel at the existing bridge has led to scour

problems in the southern span and a significant amount of aggradation in the northern span.

In addition to improving the channel transition underneath the roadway, the roadway

profile will be raised approximately 2.5 feet at the structure to accommodate a taller culvert

with a greater flow capacity.

1.5 Purpose of this Report The purpose of the Location Hydraulic Study is to examine the existing floodplain within

the project limits, document the existing 100-year water surface elevation (WSE),

determine the post project WSE at the project site, and ascertain the impact of the proposed

action on the base (100-year) flood.

1.6 Vertical Datum All elevations in this report are based on the National Geodetic Vertical Datum, 1988

(NGVD 88).

Figure 1 – Project Location Map

Carr Lake



February 1, 2017 3

Figure 2 – Project Vicinity Map

2.0 Regulatory Setting 2.1 National Flood Insurance Program

The Federal Emergency Management Agency (FEMA) guidelines are employed for the

design, construction and regulation to reduce flood loss and to protect environmental and

cultural resources.

Per FEMA, the National Flood Insurance Program aims to reduce the impact of flooding

on private and public structures. It does so by providing affordable insurance to property

owners and by encouraging communities to adopt and enforce floodplain management

regulations. These efforts help mitigate the effects of flooding on new and improved

structures. Overall, the program reduces the socio-economic impact of disasters by

promoting the purchase and retention of general risk insurance, but also of flood

insurance, specifically.

FEMA has prepared Flood Insurance Rate Maps (FIRMs) which are the official maps of

communities on which FEMA has delineated both the special hazard areas and the risk

premium zones applicable to the communities. The applicable FIRM (see Figure 3) covers

Alisal Creek Bridge at Hartnell Road, and shows the bridge in Zone A. Zone A is defined

by FEMA as an area subject to inundation by the 1-percent-annual-chance flood event

generally determined using approximate methodologies. Because detailed hydraulic

analyses have not been performed, no Base Flood Elevations (BFEs) or flood depths are

shown. Mandatory flood insurance purchase requirements and floodplain management

standards apply.



February 1, 2017 4

Figure 3 – National Flood Insurance Rate Map (FEMA 2009)

BRIDGE LOCATION



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2.2 Executive Order 11988 Executive Order 11988 (E.O. 11988) requires federal agencies to avoid to the extent

possible the long and short-term adverse impacts associated with the occupancy and

modification of flood plains and to avoid direct and indirect support of floodplain

development wherever there is a practicable alternative. Requirements for compliance

with E.O. 11988 are provided in Title 23 - CFR, Part 650, Subpart A - Location and

Hydraulic Design of Encroachments on Flood Plains, which prescribes “Federal Highway

Administration (FHWA) policies and procedures for the location and hydraulic design of

highway encroachments on flood plains, including direct Federal highway projects

administered by the FHWA.” This report follows the requirements presented therein.

3.0 Project Assessment 3.1 Hydrologic Assessment

Alisal Creek drains a watershed of approximately 26 square miles at the Hartnell Road

Bridge crossing (see Figure 4 below). The Federal Emergency Management Agency

(FEMA) Flood Insurance Study (FIS) for Monterey County dated April 2, 2009 does not

include hydrologic information for Alisal Creek. It does include design discharges at

various locations along Reclamation Ditch; however, these values are not appropriate for

use in this study because they occur quite a ways downstream of our project and reflect

the storage capacity of Carr Lake.

Figure 4 – Hartnell Road Drainage Area



February 1, 2017 6

Since this is a federally funded project administered through Caltrans, the hydrology for

the design of the replacement bridge has been developed in accordance with the

recommendations given in the Caltrans Highway Design Manual (HDM) and Caltrans

Local Assistance Procedures Manual (LAPM). Flow rates for the design and base flood

events have been determined using the regional flood-frequency equations for the Central

Coast (Region 4) provided in Caltrans HDM Table 819.2C (July 1, 2015). Per Chapter

11 of the LAPM, culverts should be designed to convey the 10-year flood with no

freeboard, and the 100-year flood should not cause damage to the facility or adjacent

property.

Hydraulic analysis shows that the 10-year event passes through the proposed structure

under pressure flow. Therefore, a design exception has been processed with Caltrans to

allow the return period on the design flood event to be reduced. Per the hydraulic analysis,

the new structure can convey slightly more than 380 cfs, which corresponds to a 5-year

return period. Therefore, the 5-year flood event was adopted as the design flood for the

hydraulic study. Note that although the proposed structure does not satisfy Caltrans’

requirements for the 10-year flood event, the proposed structure does have a greater

hydraulic capacity than the existing structure; therefore, hydraulic conditions are

improved by the project.

A summary of the flow rates obtained from the Caltrans regional flood-frequency

equations are provided in Table 1. The average annual precipitation used in the calculation

of the Caltrans flow rates was obtained from the California Data Exchange Center "Salinas

No 2 (SAP)" precipitation sensor, which is located approximately 5.3 miles away from

the project site, within the same drainage basin. The mean annual precipitation recorded

by this gage is 16 inches.

TABLE 1

ADOPTED FLOWS

Flood Event Return Period

(years) Alisal Creek at

Hartnell Road (cfs)

Design Flood 5 380

Base Flood 100 2,700

The adopted flows given in Table 1 represent the flows in the channel at the culvert

crossing. There is a significant tributary (10.2 sq. mi.) located just downstream of the

bridge (see Figure 5), which adds approximately 170 cfs to the channel in a design flood

event and 1,220 cfs in a base flood event. The flow from this tributary was added into the

hydraulic model at the downstream location to capture its effects on the water surface

elevations at the bridge.



February 1, 2017 7

Figure 5 – Downstream Tributary

3.2 Hydraulic Modeling A hydraulic analysis was performed using HEC-RAS software to determine the effect of

replacing the Hartnell Road Bridge with a new double box culvert (see Appendix C for

General Plan). Two hydraulic models were created to analyze the existing and proposed

conditions for the adopted flows given in Table 1.

Manning’s “n” values were used to model the channel friction. A value of 0.03 was

assumed for the main channel to represent the fairly clean and uniform earth-lined channel.

The “n” value was reduced to 0.013 through the proposed culvert opening to account for

the concrete invert slab. An “n” value of 0.050 was assumed for the channel overbanks.

Table 2 provides the water surface elevations at the upstream face of the bridge for both

the existing and proposed conditions. Table 3 summarizes the available freeboard on the

design and base floods for the existing and proposed conditions. The analysis shows that

the design flood is contained within the channel banks and passes through the existing

structure under pressure flow. The design flood passes through the proposed culvert with

approximately 0.5’ of freeboard. Therefore, the proposed culvert conveys the design flood

without creating backwater effects.



February 1, 2017 8

The base flood overflows the channel banks and spreads out over a wide floodplain. In

the existing condition, the base flood sheet flows across the roadway at the structure;

however, due to a raise in roadway profile in the proposed condition, sheet flow is blocked

at the structure. As a result, a slight (1”) increase in the base flood water surface elevation

was observed in the proposed condition. This small rise in the base water surface elevation

at the culvert crossing is expected to have an insignificant impact on the plan limits of the

100-yr floodplain (see Figure 6). Additionally, the roadway profile has been raised in order

to accommodate more flow through the structure, as the use of a lower magnitude design

flood is not desired.

TABLE 2

WATER SURFACE ELEVATIONS

Flow Discharge,

Q (cfs)

Existing Condition

(ft)

Proposed Condition

(ft)

Change in WSE (ft)

Design Flood 380 66.46 66.10 -0.36

Base Flood 2,700 68.32 68.40 +0.o8

TABLE 3

AVAILABLE FREEBOARD

Flow Structure Min. Soffit Elevation

(ft)

Water Surface Elevation

(ft)

Freeboard (ft)

Design Flood Existing 65.12 66.46 Pressure Flow

Base Flood Existing 65.12 68.32 Overtops

Design Flood Proposed 66.53 66.10 0.43

Base Flood Proposed 66.53 68.40 Pressure Flow

3.3 Hydraulic Assessment The proposed design improves hydraulic conditions and lowers the design water surface

elevation. By raising the roadway profile approximately 2.5 ft. and increasing the flow

area inside the structure, the 5-yr flood event passes through the structure with

approximately 0.5’ of freeboard. In addition, realigning the channel to pass underneath the

roadway at a 45 degree angle as opposed to a 90 degree angle and providing warped

wingwalls at each end of the culvert minimizes hydraulic losses and improves scour



February 1, 2017 9

conditions. A design exception has been processed by the County with Caltrans to allow

for the reduced design flood event of 5 years.

Figure 6 - Flood Limits Based on HEC-RAS Analysis



February 1, 2017 10

4.0 Conclusion Per FAPG 23 CFR 650A, Subchapter G - Engineering and Traffic Operations, Part 650 -

Bridges, Structures and Hydraulics, Subpart A - Location and Hydraulic Design of

Encroachments on Floodplains, Section 650.111 - Location Hydraulic Studies, Paragraph

(c) and (d), the Location Hydraulic Study shall include discussion of the following items.

Commensurate with the significance of the risk or environmental impact of the proposed

action, these items are evaluated as:

4.1 Sec. 650.111(c)(1) - The risks associated with implementation of the action

Per FHWA’s definition, "risk" shall mean the consequences associated with the probability

of flooding attributable to an encroachment. It shall include the potential for property loss

and hazard to life during the service life of the bridge and roadway. The proposed action

lowers the design water surface elevation and has an insignificant impact on the base water

surface elevation. The proposed action allows a greater flood event to be conveyed

underneath the roadway thereby reducing the occurrence of overtopping and providing a

safer crossing for the traveling public. Therefore, the proposed action reduces the risk to

property and life. Thus, the risk associated with the encroachment resulting from the

proposed action with regard to the base flood is not significant.

4.2 Sec. 650.111(c)(2) - The impacts on natural and beneficial floodplain values

Natural and beneficial floodplain values include, but are not limited to: fish, wildlife,

plants, open space, natural beauty, scientific study, outdoor recreation, agriculture,

aquaculture, forestry, natural moderation of floods, water quality maintenance, and

groundwater recharge. The proposed action does not raise the backwater surface elevation,

thus the risks associated with the implementation of the proposed action with regard to the

impact on natural and beneficial floodplain values is not significant.

4.3 Sec. 650.111(c)(3) - The support of probable incompatible floodplain development

The proposed action does not increase the vehicular capacity of the crossing and thus the

risk associated with the implementation of the proposed action with regard to probable

incompatible floodplain development is not significant.

4.4 Sec. 650.111(c)(4) - The measures to minimize floodplain impacts associated with the action

The structure, in its existing condition, is in pressure flow during the design flood event

and is overtopped during the base flood event. By raising the roadway at the crossing, a

larger flow area is provided through the structure and the design water surface elevation is

lowered. The base flood will not overtop the proposed structure, but it will overtop the

roadway on either side of the structure. Thus the proposed action does not create a dam to

impede the base flow, and the floodplain as shown on the FEMA Flood Insurance Rate

Maps will remain unchanged. Therefore, the risks associated with the implementation of

the proposed action with regard to the base flood are not significant.



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4.5 Sec. 650.111(c)(5) - The measures to restore and preserve the natural and beneficial floodplain values impacted by the action

The proposed action does not result in any identified significant permanent impacts to the

natural and beneficial floodplain values, and mitigation measures are not required. All

environmental impacts will be a result of construction activities and will be mitigated with

standard measures such as revegetation, best management practices, and the requirements

that are part of the project permit conditions. Thus the risks associated with the

implementation of the proposed action with regard to the base flood are not significant.

4.6 Sec. 650.111(d) - The practicability of alternatives to any significant encroachment

The proposed action replaces a substandard structure with a culvert capable of conveying

the design flood in accordance with code. This action does not result in new floodplain

encroachment, thus the proposed action represents no significant encroachment to the

base floodplain and alternatives to the action were not considered nor practical.

This study concludes that the work does not support probable incompatible floodplain

development, thus does not represent a significant impact. A Location Hydraulic Study

form and Summary Floodplain Evaluation Report have been completed and are attached,

see Appendix A and Appendix B.



February 1, 2017 12

5.0 References

1. U.S. Department of Homeland Security, Federal Emergency Management Agency

(FEMA), (April 2, 2009). Flood Insurance Study, Monterey County, California and

Incorporated Areas (Flood Insurance Study Numbers 06053CV001A, 06053CV001B,

and 06053CV001C )

2. Design Hydraulic Study, "Alisal Creek Bridge at Hartnell Road," TRC Engineers, Inc.,

February 2016.

3. Federal Highway Administration, Federal Aid Policy Guide, Title 23 - Code of Federal

Regulations, Subchapter G - Engineering and Traffic Operations, Part 650 - Bridges,

Structures, and Hydraulics, Subpart A - Location Hydraulic Design of Encroachments on

Flood Plains



February 1, 2017 13

Appendix A Location Hydraulic Study Form

Dist. 05 Co. MON Rte Hartnell Road P.M. E.A. . Bridge # 44C0110

Federal-Aid Project Number BRLO-5944(103) Project Name: Alisal Creek Bridge at Hartnell Road

Floodplain Description: The project is located on FEMA Flood Insurance Rate Map (FIRM) panel

06053C0240G for Monterey County and incorporated areas, effective April 2, 2009.

1. Description of Proposal (include any physical barriers i.e. concrete barriers, soundwalls, etc. and design elements to minimize floodplain

impacts)

The Project proposes to replace the existing structure (44C0110) on Hartnell Road over Alisal Creek and

construct the minimum approach work to accommodate the project. This will consist of approximately

470 feet of roadway reconstruction at the south end and 370 feet of roadway reconstruction at the north

end of the bridge to raise the vertical alignment of the bridge. The new structure is anticipated to be a

reinforced double concrete box culvert.

2. ADT: Current 1,900 Projected 2,000+

3. Hydraulic Data: Base Flood Q100= 2,700 cfs WSE100= 68.40’

The flood of record, if greater than Q100: Q= NA cfs WSE= NA

Overtopping flood Q= >520 cfs WSE= >67.08’

Are NFIP maps and studies available? NO YES X

4. Is the highway location alternative within a regulatory floodway? NO X YES

5. Attach map with flood limits outlined showing all buildings or other improvements within the base floodplain.

Potential Q100 backwater damages:

A. Residences? NO YES X*

B. Other Bldgs? NO YES X*

C. Crops? NO YES X*

D. Natural and beneficial Floodplain values? NO X YES ”Natural and beneficial flood-plain values" shall include but are not limited to fish, wildlife, plants, open space, natural beauty, scientific study, outdoor

recreation, agriculture, aquaculture, forestry, natural moderation of floods, water quality maintenance, and groundwater recharge. * See Figure 3 of LHS report: No increase from existing condition.

6. Type of Traffic:

A. Emergency supply or evacuation route? NO YES X

B. Emergency vehicle access? NO__________YES X

C. Practicable detour available? NO YES X

D. School bus or mail route? NO YES X

7. Estimated duration of traffic interruption for 100-year event hours: Same as existing

8. Estimated value of Q100 flood damages (if any) – moderate risk level.

A. Roadway $ NA

B Property $ NA

Total $ NA

9. Assessment of Level of Risk Low X , compared to existing condition

Moderate

High

For High Risk projects, during design phase, additional Design Study Risk Analysis may be necessary to determine design alternative.



February 1, 2017 16

Appendix C Bridge General Plan

location hydraulic study

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