App_B-Conn_Dam_Spillway_Hydraulic_Analysis_28Mar2018-Final_(1).pdf

Napa City (0283001.00) 1 Woodard & Curran

Conn Dam Spillway Analysis February 2018

TECHNICAL MEMORANDUM

SUBJECT: Napa City Conn Dam Probable Maximum Flood Analysis

PREPARED FOR: Joy Eldredge, City of Napa

PREPARED BY: Benjamin Bass, Ph.D. Woodard & Curran

Chris van Lienden, P.E. Woodard & Curran

REVIEWED BY: Paul Porada, P.E. Woodard & Curran

DATE: March 28, 2018

REFERENCE: 0283001.00

1. INTRODUCTION

City of Napa (“City”) requested that Woodard & Curran evaluate the capacity of the

Conn Dam spillway under a Probable Maximum Flood (PMF) event. Conn Dam is a zoned embankment dam constructed in 1945-1946 with a maximum height of about

125 feet (elevation about 330 feet NGVD29) and a crest length of about 700 feet. The dam’s reservoir (Lake Hennesey) has a capacity of about 30,000 acre-feet and serves as the primary source of municipal water supply for the City of Napa.

A reinforced concrete, uncontrolled spillway, located approximately 3,000 feet west of the dam conveys overflows from Lake Hennessey to Conn Creek. The spillway is about

800 feet long and consists of a converging inlet section, a trapezoidal chute, and a flip bucket and stilling basin. The spillway crest is 315 feet (NGVD29), which is 15 feet below the crest of the dam.

The Lake Hennessey study area, located in Napa County, CA, is shown in Figure 1. The primary location of interest is the Conn Dam spillway; however, to determine water levels and possible overtopping of the spillway, water flowing from the Conn Dam’s upstream drainage area is required. This upstream drainage area, which is referred to as Lake Hennessey watershed, has been split into five subbasins that discharge to Lake

Hennessey and subsequently into the Conn Dam spillway under significant flood events. Subbasin delineation is described further in section 3.2.3.1 To perform analysis of the PMF along the spillway, the following primary items were developed:

i) Probable Maximum Precipitation (PMP): This rainfall event represents the greatest depth of precipitation for a given duration that is physically possible over the study area’s drainage area

MOORE CR

CONN CR

CHILES C R

SAGE CR

CONN CR

Study Location Figure #1

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DOC: 1_Fig_Hennessey_Wshed.mxdSCALE: 1 " = 8,575 '

City of Napa Dam Spillway Capacity

F 0 1.5 30.75 Miles

Lake Hennessey

Conn Dam Spillway

Rivers / Creeks

Subbasins

Study Area

Napa City (0283001.00) 3 Woodard & Curran

Conn Dam Spillway Analysis March 2018

ii) Hydrologic Model: A hydrologic model was developed to convert rainfall into runoff and determine the resulting Probable Maximum Flow (PMF) at the outlet of Lake Hennessey due to the PMP

iii) Hydraulic Model: Finally, a hydraulic model was developed to evaluate water levels along the Conn Dam spillway resulting from the PMF

2. PROBABLE MAXIMUM PRECIPITATION

The PMP represents the greatest depth of precipitation for a given duration that is physically possible over the study area’s drainage area. This study utilized the most up-to-date data available and federally approved PMP methodology.

2.1 Data Sources

The National Oceanic and Atmospheric Administration’s (NOAA) National Weather

Service (NWS) has developed guidance and data for developing PMP’s since the

1940’s. The latest PMP update was provided in 1999 with NOAA’s

Hydrometeorological Report No. 59 (HMR-59). This study utilized the HMR-59 report’s findings given that a more up-to-date, federally approved resource for determining

PMP’s throughout the state of California does not exist.

In HMR-59, the PMP was determined across the state of California for a 24-hour duration and a 10 mi2 drainage area. This PMP is referred to as the PMP index and must be adjusted based on a study area’s location in the state of California and the size of the study area’s drainage area. The PMP index was developed based on modifications made to the most severe historical storms observed throughout the state of California. The most severe observed storms represent a lower bound for the

PMP index and the upper-limit or actual PMP index was determined via three tools referred to as: moisture maximization, storm transposition, and envelopment that are discussed in detail in HMR-59. Note that the PMP index calculated using the procedures outlined in HMR-59 can vary significantly from the PMP index calculated using the older HMR-36, reprinted 1969. For the Lake Hennessey Watershed, the PMP index is about 30 percent greater using the HMR-59 procedures.

The following section discusses how the PMP index (24-hour, 10 mi2) values were adjusted for this study’s watershed.

2.2 Computational Methods

HMR-59 provides a shapefile of contours referred to as the PMP index, which represents the PMP for a duration of 24 hours and an area of 10 mi2. PMP index contours within the vicinity of the study area are shown in Figure 2. The first step in determining the PMP requires averaging these contours across the study area, which results in a PMP index of 27.1 inches for Lake Hennessey watershed.

27 24

PMP Index Contours Figure #2

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City of Napa Dam Spillway Capacity

F 0 1 20.5 Miles

Lake Hennessey Watershed HMR-59 PMP Index Contours

Lake Hennessey

Napa City (0283001.00) 5 Woodard & Curran

However, this value needs to be adjusted based on the study area’s location and its drainage area of 82.8 mi2, since a larger drainage area can receive a greater amount of rainfall per HMR-59. Lake Hennessey watershed falls within the Midcoastal region, which is defined by HMR-59 to be representative of “low coastal mountains running along the Californian coast between the Central Valley and the Pacific Ocean.” The PMP index of 27.1 inches was multiplied by the depth-area relations defined in Table 8.1 of

HMR-59 for the Midcoastal region and Table 8.2 of HMR-59 for a drainage area size of 82.8 mi2 to determine PMP rainfall intensities at 6-hour intervals. Table 1, shows the resulting cumulative and incremental 6-hour PMP values determined for the study area over a 72-hour duration.

Table 1: Calculated PMP for Lake Hennessey Watershed

Duration (Hours) Cumulative Rainfall

(inches)

Incremental Rainfall

(inches)

0 0 0

6 10.4 10.4

12 17.5 7.0

18 21.4 4.0

24 24.0 2.6

30 26.8 2.8

36 29.8 3.0

42 32.7 2.9

48 35.3 2.6

54 37.5 2.2

60 39.4 1.9

66 40.9 1.5

72 42.0 1.1

The PMP was determined for the entire watershed rather than each subbasin of the watershed since the entire watershed falls within the Midcoastal region and variations in the PMP index were minor across the study area (the average PMP for the 5 subbasins differs from the average for each of the subbasins by less than 4%). The PMP can be determined for durations from 1 to 72 hours. Since this study was performed for spillway failure analysis (i.e. overtopping), a duration of 72 hours was utilized for the rainfall event based on the general-storm PMP methodology. Also, the all season or annual maxima PMP was utilized, which occurs during winter months (November –

February). Hence, seasonal ratios or reductions from the all season PMP were not

Napa City (0283001.00) 6 Woodard & Curran evaluated since this study investigated a worst-case scenario for the entire year rather than a worst-case scenario for a given month.

HMR-59 also provides optional guidelines for evaluating different temporal and spatial adjustments to the PMP as well as snow contributions if appropriate for the study area.

In this study, sensitivity analysis or modifications to the general-storm PMP were made based on variations in the temporal distribution of rainfall. HMR-59 suggests modifying the temporal distribution of the PMP by grouping the four most intense 6-hour rainfall values together and placing this most intense 24 hours of rainfall at different points in time throughout the 72-hour duration of rainfall. According to HMR-

59, the remainder of the 6-hour rainfall values can be placed at any location throughout the 72-hour event. To achieve a smooth transition in rainfall values, the remaining 6-hour intervals were placed in ascending and then descending order before and after the peak 24-hour rainfall interval. Front-, middle-, and end-loaded rainfall events were subsequently developed based on this approach to determine sensitivity in the resulting PMF. Figure 3 demonstrates the rainfall intensity associated with each of these temporal distributions. Finally, the PMP values were interpolated from a 6-hour to 1-hour interval based on fitting a quadratic curve to the cumulative rainfall for the front-, middle-, and end-loaded rainfall distributions and extracting hourly rainfall intensity values.

Figure 3: PMP Temporal Distributions

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3. HYDROLOGIC MODEL DEVELOPMENT

A hydrologic model of the study area was developed to convert the PMP rainfall into the reservoir’s PMF. Specifically runoff (flow) and the reservoir water level at the outlet of Lake Hennessey were required to be used as boundary conditions for the hydraulic model of the spillway. The Hydrologic Engineering Center’s Hydrologic Modeling

System (HEC-HMS) version 4.2.1 was employed to develop a hydrologic model of the

Lake Hennessey watershed. As streamflow gauges are not available along the watershed’s streams, the hydrologic model was developed using best practices for an ungauged watershed (i.e. calibration to observed streamflow was not performed). The

HEC’s software Geospatial Hydrologic Modeling Extension (HEC-GeoHMS), which makes use of ESRI Arc-GIS capabilities, was used to delineate subbasins and determine their properties for the hydrologic model.

3.1 Data Sources

Data sources for the HEC-HMS model include elevation data in the form of a Digital

Elevation Model (DEM) to delineate subbasins and determine basic basin properties such as subbasin area, subbasin slope, longest flowpath, and subbasin centroid. A

1/3rd arc second (approximately 10 meter) DEM, which is the highest resolution DEM publicly available for the study area, was obtained from the United States Geological

Survey (USGS) for this purpose.

Soil and land use data which were used to determine hydrologic parameters within each subbasin were obtained via the U.S. Department of Agriculture’s Geospatial

Gateway from the Natural Resources Conservation Service’s (NRCS) U.S. General Soil

Map (STATSGO2) and the 2011 National Land Cover Dataset (NLCD), respectively.

Table 2 summarizes the methods and data sources used for each hydrologic model process. The processes are discussed further in the next sections.

Napa City (0283001.00) 8 Woodard & Curran

Table 2: Methods and Data Utilized in Geo-HMS and HEC-HMS

Process Method Data Sources

Basin Delineation Geo-HMS Tools 10m (1/3 arc-second) DEM:

USGS (2017)

Loss (infiltration) SCS Curve Numbers

Soil Types:

NRCS STATSGO2 (2015)

Land Use / Imperviousness:

NLCD (2011)

Transform Snyder Unit

Hydrograph

10m (1/3 arc-second) DEM:

USGS (2017)

Baseflow No baseflow assumed N/A

River Routing Not applicable for watershed N/A

3.2 Subbasin Delineation

Prior to delineating the sub-basins, terrain preprocessing on the DEM was performed per Geo-HMS recommendations. Any sinks or isolated grid cell’s with relatively very low elevations in the DEM were filled based on surrounding elevations. Following preprocessing of the DEM, subbasins were delineated based on natural elevation divides and flow directions using the Geo-HMS software. The number of subbasins delineated can vary depending on user specifications. For this study, five primary subbasins were delineated which contain major creeks of the Lake Hennessey watershed (as shown in Figure 1). In addition to the five primary subbasins, direct rainfall and storage in Lake Hennessey was included in the HEC-HMS model. A graphical representation of the hydrologic model is shown in Figure 4.

Napa City (0283001.00) 9 Woodard & Curran

Figure 4: Graphical Representation of the HEC-HMS model

3.3 Loss (infiltration) Method

To determine infiltration losses, the SCS Curve Number method was utilized. This method empirically determines precipitation excess (total precipitation not lost to infiltration) by considering precipitation, soil cover, land use, and antecedent moisture.

For each rainfall time-step, precipitation excess is calculated using the following equation:

𝑃𝑒 = (𝑃 − 𝐼𝑎)2

𝑃 − 𝐼𝑎 + 𝑆 where 𝑃𝑒 = accumulated precipitation excess at time t, P = accumulated rainfall depth at time t, 𝐼𝑎 = the initial abstraction (initial loss), and S = potential maximum retention which is a measure of the ability of a watershed to abstract and retain storm precipitation. For the PMF analysis the soil is considered saturated, so the initial abstraction was set to zero. The maximum retention (S) is related to subbasin characteristics based on the Curve Number:

𝑆 =

(1000 − 10𝐶𝑁)

𝐶𝑁

where CN ranges from 100 (for water bodies or areas with no infiltration) to roughly

30 for soils with high infiltration rates as shown in Table 3. Since each subbasin is made

Napa City (0283001.00) 10 Woodard & Curran up of several hydrologic soil groups and land uses, a composite CN was determined using the following equation:

𝐶𝑁𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 =

∑ 𝐴𝑖𝐶𝑁𝑖

∑ 𝐴 where 𝑖 represents a subdivision within the subbasin, which was based on each raster grid cell of soil and land use data, and A represents the area of that subdivision. The total area considered in this equation does not include impervious areas which allow for direct runoff (no infiltration). The soil and land use conditions throughout Lake

Hennessey watershed are shown in Figure 5. The spatial variation in the CN’s throughout the subbasins are shown in Figure 6, and Table 4 summarizes the final composite CN and impervious values used for each subbasin in HEC-HMS. Note that rainfall across Lake Hennessey was represented with a curve number of 100 for water.

Table 3: Curve Numbers based on Land Use and Soil Group Combinations

Hydrologic Soil Group and Corresponding

Curve Numbers based on Land Use

Source1 Land Use Description A B C D

USGS2 & HMS3 Water, Woody Wetlands, and

Emergent Herbaceous Wetlands 100 100 100 100

USGS Urban Developed 54 70 80 85

USGS Barren Land 32 58 72 79

USGS Forest (Deciduous, Evergreen Mixed) 30 55 70 77

HMS Shrub/Scrub 55 72 81 86

HMS Herbaceous 61 71 81 89

USGS Cultivated Crops 67 78 85 89 1To allow for detailed descriptions of soil and land use combinations that relate to curve numbers both USGS and HEC-HMS resources were utilized

2USGS (2011)

3HEC-HMS Technical Reference Manual

Hydrologic Soil Group and Land Use Land Cover Figure #5

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DOC: 5_Fig_Soil_LULC.mxdSCALE: 1 " = 12,279 '

City of Napa Dam Spillway Capacity

F 0 2 41 Miles

Land Use Land Cover Barren Land Forest Herbaceous Shrub Urban Developed Water, Wetlands

Hydrologic Soil Group B: Moderately Low Runoff Potential C: Moderately High Runoff Potential D: High Runoff Potential Lake Hennessey

Lake Hennessey Watershed Curve Number Variation Figure #6

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DOC: 6_Fig_CurveNumber.mxdSCALE: 1 " = 6,699 '

City of Napa Dam Spillway Capacity

F 0 1 20.5 Miles

Curve Number

Napa City (0283001.00) 13 Woodard & Curran

Table 4: Composite Curve Numbers and Imperviousness used in HEC-

HMS

Subbasin Curve Number Imperviousness

1 (Conn Creek) 78 0.82

2 (Moore Creek) 80 0

3 (Chiles Creek) 80 0

4 (Sage Creek) 81 0

5 (Confluence) 81 0

3.4 Transform (subbasin routing) Method

The Snyder Unit Hydrograph (UH) Transform method was utilized to convert excess precipitation into runoff for each subbasin. This method was utilized since it allows for the derivation of a subbasin’s UH based on a subbasins characteristics and can be used in the absence of runoff data to derive a UH. The Snyder method requires the determination of the basin lag, 𝑡𝑝, which is based on subbasin characteristics and a peaking coefficient which can only be estimated for ungauged watersheds. The equation recommended for the study area (Linsley, 1975) was used to determine the basin lag:

𝑡𝑝 = 𝐶𝑡 (

𝐿𝐿𝑐

√𝑆

𝑁 where 𝑡𝑝 is the basin lag or time between the centroid of the rainfall hyetograph and peak flow, 𝐶𝑡 is a basin coefficient which ranges from 0.4 in mountainous areas to 8.0 along flat slopes such as the Gulf of Mexico, L is the length of the main stream from the outlet to the upstream end of the subbasin, 𝐿𝑐is the length along the main stream from the outlet to a point nearest the subbasin centroid, S is the overall slope of the basin from the point of concentration to the outlet of the basin, and N is an empirical constant. For this study a 𝐶𝑡 of 0.72, an N of 0.38, and a peaking coefficient of 0.5 were used based on recommendations from Linsley (1975), who modified the Snyder UH constants for application in the Sierra Nevada of California. Physical subbasin parameters used in the Snyder equation to determine basin lag were determined in

GIS and are shown in Table 5.

Napa City (0283001.00) 14 Woodard & Curran

Table 5: Subbasin Properties Used in Snyder Equation

Subbasin

L = Length of

Stream

(miles)

Lc = Length to Centroid of Subbasin

(miles)

S = slope of basin

(ft/ft) tp = basin lag

= calculated based on

Snyder equation

(hours)

1 (Conn Creek) 9.56 4.30 0.038 5.51

2 (Moore Creek) 8.13 3.12 0.033 4.70

3 (Chiles Creek) 4.66 2.49 0.024 3.71

4 (Sage Creek) 8.02 4.02 0.028 5.33

5 (Confluence) 1.80 0.35 0.049 1.07

3.5 Representation of Lake Hennessey

Thus far, the loss (SCS Curve Number) and transform (Snyder UH) methods used to calculate infiltration losses and route flow through each subbasin has been discussed.

However, as was shown in Figure 1 and Figure 4, flows from every subbasin discharges to Lake Hennessey, which then connects to the Conn Dam spillway. To accurately represent the translation (time shift) and attenuation (reduction in magnitude) of flow from the inlet of Lake Hennessey to the outlet where the spillway is located, properties of the reservoir were determined based on best data available. These properties include the relationship between the lake’s water levels and available storage volume

(elevation-volume), which was determined from the elevation-area relationship for the lake and the conic formula, and the relationship between the lake’s water levels and resulting discharge to the spillway (elevation-discharge) as shown in Figure 7. Rainfall across Lake Hennessey was additionally included by representing it as a subbasin with a curve number of 100 which results in direct runoff (i.e. rainfall intensity multiplied by the Lake’s area of 1.2 mi2).

The elevation-area relationship for the lake was obtained from detailed LiDAR analysis of the lake’s bathymetry performed by Limnotech (2017). However, this data was limited to a maximum elevation of 318 feet, which is slightly above the spillway crest of 315 ft. and below the dam crest of 330 ft. To estimate storage for elevations above the spillway crest, the elevation-area curve was extrapolated using the 1/3rd arc second (approximately 10 meter) USGS DEM. Note that the area calculated from the

DEM at 318 feet is somewhat higher than the area calculated from Limnotech’s survey;

the difference could be due to a more limited extent for the bathymetric survey into the upstream river, or the lower resolution of the USGS DEM. As the bathymetric survey is more recent and was performed at a higher resolution than the DEM, the bathymetric survey data was used where available.

The elevation-discharge relationship was estimated using the broad-crested weir flow equation:

Napa City (0283001.00) 15 Woodard & Curran

𝑄 = 𝐶𝐿𝐻3/2

where Q is the discharge (cfs), L is the length of the crest (ft) that water flows across, H is the water level above the crest (ft), and C is a coefficient of discharge. C was set equal to 3.09 based on typical values and with the assumption that contraction effects of the entrance walls are negligible (U.S. Bureau of Reclamation, 1977). The length of the spillway crest (L) is about 135 feet based on as-built dimensions (Napa City Water

Works, 1944).

An assumption has been made that discharge across the dam crest does not occur even if the water surface elevation reaches a high enough level for the dam to be overtopped. This assumption is conservative with respect to spillway capacity, since flow over the dam would reduce the flow through the spillway as well as reducing the lake water level at the top of the spillway during the PMF.

Figure 7: Elevation-Area and Elevation-Discharge Relationships used for

Lake Hennessey

Napa City (0283001.00) 16 Woodard & Curran

3.6 Model Results

Table 6 summarizes results from the HEC-HMS simulations using a front-, middle-, and end-loaded PMP. This analysis suggests that the end-loaded PMP results in the most conservative peak flow. While only peak flows and lake elevation were used to evaluate the steady state or maximum water levels along the Conn Dam spillway in the hydraulic analysis, a hydrograph of flow into Lake Hennessey and discharging into the spillway is included for reference in Figure 8 for the end-loaded precipitation event.

Figure 9 shows the elevation in Lake Hennessey throughout the HEC-HMS simulation of the end-loaded PMP.

The HEC-HMS simulation indicates that under the end-loaded event the water level in

Lake Hennessy would exceed the dam crest by several feet. This would likely result in dam failure. Modifications to the spillway entrance would be needed if a minimum freeboard of 1.5 feet would be required below the dam crest under the PMF.

Developing potential improvements is beyond the scope of this report.

Table 6: HEC-HMS Results

Peak Inflow to Lake Hennessey (cfs) Peak Discharge to Spillway (cfs)

Front-

Loaded

Middle-

Loaded

End-

Loaded

Front-

Loaded

Middle-

Loaded

End-

Loaded

38,500 43,200 42,800 33,300 38,000 38,900

Napa City (0283001.00) 17 Woodard & Curran

Figure 8: HEC-HMS Hydrograph Results for End-Loaded PMP

Figure 9: Lake Hennessey Water Level (ft) throughout HEC-HMS

Simulation

Napa City (0283001.00) 18 Woodard & Curran

4. HYDRAULIC MODEL DEVELOPMENT

A hydraulic model of the Conn Dam spillway was developed to evaluate the hydraulic performance of the spillway during the PMF. The Hydrologic Engineering Center’s

Riverine Analysis System (HEC-RAS) version 5.0.3 was employed to develop the hydraulic model. The HEC’s software Geospatial Hydraulic Modeling Extension (HEC-

GeoRAS), which makes use of ESRI Arc-GIS capabilities, was utilized to extract cross-sections and their properties for the hydraulic model.

4.1 Data sources and Model Development

As previously described, the Conn Dam spillway is reinforced concrete and consists of a converging inlet section, a trapezoidal chute, and a flip bucket and stilling basin. Two bridge crossings have also been constructed: a downstream bridge that crosses perpendicular to the spillway above the flip bucket, and an upstream bridge that crosses at an angle. Neither bridge has piers in the spillway.

A layout of the HEC-RAS model is shown in Figure 10; as shown in Figure 10, the model includes 17 cross sections and both bridges. The geometry for the hydraulic model was developed using a combination of as-built drawings for the spillway itself

(Napa City Water Works, 1944, included in Appendix A), and the 1/3 arc-second DEM outside of the spillway walls. Bridge geometry was extracted using Google Earth and supplemented by field survey from City of Napa staff (see Table 7). The HEC-RAS model uses the same vertical datum as the as-built spillway drawings and the lake bathymetric survey (NGVD29). The bottom bridge also has three attached pipe crossings; as these crossings are on the downstream side of the bridge and do not hang down significantly into the flow path they were not included in the HEC-RAS model. Cross-sections were extended far enough into the floodplain to capture the full extent of flooding; hence, some of the cross-sections are longer than others as shown.

Table 7: Bridge Dimensions

Upstream Bridge Downstream Bridge

Height of Deck bottom above spillway (ft) 19.5 18.7

Bridge thickness (ft) 8.7 8.7

Bridge width (ft) 30 8.8

Manning’s coefficients were uniformly set to 0.015 in the reinforced concrete section within the spillway walls, and the floodplain was set to a manning’s coefficient of 0.07 based on typical values for medium to dense brush in the winter (Chow, 1959).

HEC-RAS Model Layout Figure #10

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DOC: 10_Cross Section Overview.mxdSCALE: 1 " = 1,296 '

City of Napa Dam Spillway Capacity

F 0 0.2 0.40.1 Miles

Bridges Spillway Walls River Cross Sections

Napa City (0283001.00) 20 Woodard & Curran

Steady state HEC-RAS analysis was performed using an upstream boundary condition defined by the maximum water level and peak discharge from the lake based on the

PMP simulation in HEC-HMS (335.5 ft and 38,900 cfs, respectively). A downstream critical depth boundary condition was defined since a hydraulic jump occurs at the downstream end of the spillway where the flip bucket is located. For the upstream bridge, the energy equation was used since it is completely submerged during the

PMF. For the downstream bridge, which is only submerged on its headwater, the pressure/weir equation was used.

The downstream stilling basin and flip bucket are not designed for PMF flows and would likely be submerged during the event; the banks of the downstream Conn Creek would also likely be overtopped. These components were not included in the HEC-RAS model.

4.2 Model Results

The modeled water surface profile along the spillway during the PMP is shown in

Figure 11 (assuming no flow over the dam crest). An example cross-section is shown in Figure 12. As shown in Figure 11, the walls of the spillway are overtopped along the entire spillway during the probable maximum flood (PMF). Specifically, the height of water exceeding the banks (or walls) of the spillway ranges from 2 to 30 feet along the length of the spillway. Figure 13 shows a floodplain map of the PMF.

4.3 Conclusions

The spillway does not have sufficient capacity to convey the peak flow resulting from the PMP for the watershed. Improvements needed to address the spillway capacity deficiencies are outside the scope of this report and have not been investigated.

Napa City (0283001.00) 21 Woodard & Curran

Figure 11: PMF Water Surface Elevation Profile along Conn Dam Spillway

Napa City (0283001.00) 22 Woodard & Curran

Figure 12: Example Cross-Section (Cross-Section 6)

Figure 13: PMF Floodplain along Conn Dam Spillway

Napa City (0283001.00) 23 Woodard & Curran

5. REFERENCES

Kennedy/Jenks/Chilton (1986). Hydrologic Analysis Report: Conn Dam Watershed and

Spillway Napa County, CA.

Limnotech (2017). Survey of Lake Hennesey’s Bathymetry.

Linsley, R., M. Kohler, and J. Paulhus (1975). Hydrology for Engineers, McGraw-Hill.

Napa City Water Works (1944). Conn Creek Project, Drawing No. 20, “Reservoir

Topography,” A. Kempkey, Consulting Engineer.

National Oceanic and Atmospheric Administration (NOAA) National Weather Service

(NWS) Hydrometeorological Design Studies Center (1999). Hydrometeorological

Report No. 59 (HMR-59). Available online at http://nws.noaa.gov/oh/hdsc/studies/pmp.html Accessed [01/31/2018].

Natural Resources Conservation Service (NRCS), United States Department of

Agriculture Web Soil Survey (STATSGO2) (2015). Available online at http://websoilsurvey.nrcs.usda.gov/ Accessed [01/29/2018].

U.S. Department of the Interior, Bureau of Reclamation (1977). Design of Small Dams.

U.S. Government Printing Office, Washington, D.C.

U.S. Geological Survey, 20171122, USGS NED 1/3 arc-second n39w123 1 x 1 degree

ArcGrid 2017: U.S. Geological Survey. Available online at https://www.sciencebase.gov/catalog/item/5a552b42e4b01e7be242bc6e Accessed

[01/31/2018]

U.S. Geological Survey National Land Cover Dataset (NLCD) (2011). Land Cover.

Availabe online at https://viewer.nationalmap.gov/basic/ Accessed [01/31/2018]

Ven Te Chow (1959). Open-channel hydraulics. McGraw-Hill, p.g. 680.

http://nws.noaa.gov/oh/hdsc/studies/pmp.html http://websoilsurvey.nrcs.usda.gov/ https://www.sciencebase.gov/catalog/item/5a552b42e4b01e7be242bc6e https://viewer.nationalmap.gov/basic/

Napa City (0283001.00) Woodard & Curran

APPENDIX A: AS-BUILT DRAWINGS FOR

CONN DAM SPILLWAY