(25) Hydraulics and Sedimintation Study

0

Hydraulics and Sedimentation Study for the San Dieguito River Wetland Restoration Project

Prepared for Environmental Services Southern California Edison Company 2244 Walnut Grove Avenue P. O. Box 800, Rosemead, CA 91770

Prepared by Howard H. Chang, Ph.D., P.E. May 2004

Chang Consultants Hydraulic and Hydrologic Engineering Erosion and Sedimentation

P. O. Box 9492 Rancho Santa Fe, CA 92067 TEL.: (858) 756-9050 FAX: (858) 756-9460

1

TABLE OF CONTENTS

GLOSSARY OF TERMS…………………………………………………………………..1

EXECUTIVE SUMMARY …………………………………………………………………3

I. INTRODUCTION……………………………………………………………………… 5

II. BASIC HYDROLOGY……………………………………………………………….. 8

Floods……………………………………………………………………………… 8

Sediment Data……………………………………………………………………… 10

Effective Flow Area and Ineffective Flow Area…………………………………… 12

III. METHODS OF ANALYSIS………………………………………………………….. 13

Mathematical Model for General Scour ……………………………………………13

Selection of the Engelund-Hansen Formula………………………………………. 13

IV. HYDRAULIC DESIGN FOR A WEIR ON NORTHEASTERN BERM……………. 15

Rating Curves for Weir Flow……………………………………………………… 16

Modification of Flood Hydrograph………………………………………………… 19

Effects of Weir on Tidal Basin…………………………………………………….. 19

Summary and Conclusions………………………………………………………… 20

V. MODELING FOR EXISTING CONDITIONS AND PROPOSED CONDITIONS….. 21

Channel-Bed and Water-Surface Profiles Based on the FLUVIAL-12 Model …… 22

Spatial Variations in Velocity Along the River Channel………………………….. 26

Bridge Hydraulics…………………………………………………………………. 27

Simulated Results on Scour………………………………………………………. 27

VI. BEACH SAND SUPPLY……………………………………………………………… 41

Simulation of Sediment Delivery………………………………………………….. 42

Simulated Results on Sediment Delivery by the 100-yr Flood …………………… 44

Simulated Results on Long-Term Sediment Delivery………………………………46

VII. DESIGN FOR BERMS AND BANK PROTECTION………………………………..46

Top Elevation of Berms …………………………………………………………….46

Top Elevation and Toe-Down for Bank Protection ……………………………….. 47

REFERENCES…………………………………………………………………………… ..50

2

LIST OF FIGURES…………………………………………………………………………50

APPENDIX A. MAXIMUM SCOUR PROFILES AT BRIDGE CROSSINGS FOR 100-YR FLOOD

APPENDIX B. MAXIMUM SCOUR PROFILES AT BRIDGE CROSSINGS FOR 50-, 25- AND 10-

YR FLOODS

1

GLOSSARY OF TERMS

Aggradation: A rise in channel bed elevation, usually caused by sediment deposition. Alluvial: Relating to, composed of, or found in alluvium Bank protection: A structure placed on a riverbank to protect the bank against erosion. Such structures are usually made of riprap stones, revetments, dikes, etc. Bed load: That part of the sediment load that travels in contact with the bed by rolling, sliding and saltation. It is also the coarser portion of the sediment load. Channel reach: Any stretch of the channel. Channelization: To make a channel. Cross sections : Channel sections that are perpendicular to the flow direction that are used to define the river channel geometry for a river study. Degradation: A lowering of the channel-bed elevation usually caused by erosion. Drainage basin: A surface area from which rainfall drains toward a single point. Drop structure : A rigid structure erected across a river channel through which there is a drop in channel-bed elevation. Erodible boundary model: A model that considers the changes in channel boundary, including channel-bed scour and fill, changes in channel width and changes related to channel curvature. Erodible bed model: A model that only considers the changes in channel-bed level by assuming that channel width does not change. Field calibration: The correlation of modeling results using field data. It usually involves fine adjustments of certain parameters used in modeling to improve the correlation. Flood hydrograph: A relationship showing how the flood discharge varies with time during its occurrence. Fluvial processes: Processes that are caused by stream action, including sediment transport, flood flow, erosion, deposition, and river channel changes. Grade control structure : A rigid structure constructed across a river channel used to stabilize the bed elevation at the location. A drop structure is also a grade control structure.

2

Head cutting: Channel-bed erosion occurring upstream of a sand or gravel pit or any other depression. Model: For this study, a model is computer software developed to simulate the hydraulics of flow, sediment transport and river channel changes. Pit capture: A stream is diverted from its normal course into a pit of lower elevation Scour (general and local): Erosion or removal of material caused by stream action. General scour is caused by the imbalance (non-uniformity) in sediment transport along a river channel. Local scour is caused by any local obstruction to flow, such as bridge piers, abutments, tree trunks, etc. Sediment delivery: The cumulative amount of sediment that is delivered passing a river section in a specified period of time. Sediment transport/replenishment: Sediment transport is the movement of sediment by flow measured usually in volume or weight per unit time. Replenishment is sediment supply to make up any previous deficit. Study channel reach: A river channel reach that is covered in a study. Such a reach is defined by a series of cross sections taken along the channel. Suspended load: Sediment load that travels in suspension, consisting of the finer portion of the transported sediment. Tractive force: The force exerted by the flow on the channel boundary or on any object in the river channel, usually measured in force per unit surface area.

3

EXECUTIVE SUMMARY

A hydraulic and sedimentation study has been made for the San Dieguito River wetland restoration

project. This study uses the updated channel and sediment data for the final design phase of the project.

The purpose of the study is to provide the necessary information for the final design.

The inlet channel of the San Dieguito River undergoes episodes of opening and closure. Closure is

the effect of littoral sediment transport on the beach that blocks off the stream mouth. Opening is caused

primarily by storm flows. Changes in channel morphology related to sediment flushing and recharge can be

substantial, and they have important effects on the hydraulics of flow, sediment transport, and channel

boundary scour. Major issues for the project include flood control, river channel scour, and sediment

delivery to the beach. In order to assess the project impacts on these issues, it was necessary to apply a

sediment transport model for the study. The FLUVIAL-12 model was employed for this project. The

model was calibrated using 12 sets of field data, including two data sets from the San Dieguito River.

The project plan includes new tidal basins. Berms will be installed to separate the tidal basins from

the effective flow area of the river channel. Flood flow and sand flow will be conveyed through the effective

flow area. The tidal basins do not act as sediment traps to cause a deficit in sediment supply to the beach.

Project Impacts on Flood Level - The proposed project includes berms installed in the river

channel. The northeastern berm encroaches into the floodplain of the San Dieguito River and it may thus

cause backwater effects on the river channel. A scheme was developed to eliminate the backwater effects.

This scheme includes a weir installed near the upstream end of the northeastern berm. The weir has a crest

elevation that is about 7 feet lower than the top of the berm so that it admits a sufficient discharge into the

tidal basin to reduce the backwater effects in the river channel. The 100-yr water-surface elevations were

computed for the existing channel and for the proposed project. These two water-surface profiles are

closely similar with only minor differences. The proposed project will not raise the flood level; therefore, it

will have no flooding impacts.

4

Project Impacts on Flow Velocity - Flow velocities were computed for the existing and proposed

conditions. The proposed project will not raise the velocities at the bridge crossings during the peak 100-yr

flood. The velocity is also a criterion for assessing the need for bank protection.

Project Impacts on River Channel Scour - River channel changes were simulated for the

existing and proposed conditions. The inlet channel is subject to severe scour during the 100-yr flood. The

crossings of the Highway 101 Bridge, the railroad bridge, and the Jimmy Durante Bridge are prone to

severe scour that may lead to failure during the 100-yr flood. The El Camino Real Bridge has a stone

blanket on the channel bed that checks the scour development. The simulated results show that the

proposed project will not increase the maximum general scour at the bridge crossings. Minor increase in

scour depth is predicted at the river bend near river mile 0.7. Protection of the concave bank at the river

bend is a part of the project scope. The present bank protection at the location is inadequate for protection

against the 100-yr flood. The channel reach upstream from the inlet channel is simulated to undergo

changes. These changes consist of minor scour and fill of the channel boundary; they are not nearly as

substantial as those in the inlet channel.

Project Impacts on Beach Sand Supply - Sediment deliveries along the river channel were

simulated for the existing and proposed conditions. The net amount of sand delivered to the river mouth is

the sand supply to the beach. For the 30-yr time span of the project, this amount is 214,000 tons for the

existing conditions and 227,000 tons for the proposed conditions. The proposed project will slightly

increase the supply of beach sand by the San Dieguito River.

Recommendations for Top and Toe Elevations of Berms and Bank Protection - The top of

berms and bank protection should stay above the 100-yr flood level to avoid overtopping. According to

the standard of the U. S. Army Corps of Engineers, the top of berms should maintain a freeboard of three

feet above the design flood level. Computed water-surface profiles were used to determine the required top

elevations for the proposed berms and bank protection. The toe of bank protection must entrench beyond

the potential scour. For this project, the recommended toe elevations of bank protection are about 5 feet

below the potential scour. The 5-foot margin is a safety factor used to account for uncertainties, such as

5

erratic hydrologic phenomenon, bank settlement, local scour due to tree trunks, etc. The recommended

top and toe profiles are provided.

6

Hydraulics and Sedimentation Study

for the San Dieguito River Wetland Restoration Project

I. INTRODUCTION

This report presents a hydraulic and sedimentation study for the San Dieguito River wetland

restoration project. This project has undergone several technical reviews in the past. A recent topographic

survey of the river channel has been made and sediment samples have also been collected and analyzed.

This study uses the updated channel and sediment data for the final design phase of the project. The

purpose of the study is to provide the necessary information for the final design.

Fig. 1 is a map of the San Dieguito River west of I-5, and Fig. 2 shows the river channel east of I-5.

The channel geometry is defined at selected cross sections. Locations of cross sections used in the study

are shown in the work map; approximate cross section locations are shown in Figs. 1 and 2. Important

locations and their respective channel stations are listed in Table 1.

Table 1. List of important locations along the river channel

Points of interest Location

River miles

River mouth 0

Highway 101 0.087 - 0.107

Railroad Bridge 0.293 – 0.299

Jimmy Durante Bridge 0.570 - 0.581

River bend 0.706

Interstate 5 Bridge 1.355 – 1.391

El Camino Real 2.806 – 2.813

Upstream limit of study 5.839

7

Fig. 1. Map of the San Dieguito River west of I-5

Fig. 2. Map of the San Dieguito River east of I-5

8

Mouths of coastal streams in semi-arid regions, such as California, undergo episodes of opening and

closure. A sand bar typically blocks the stream mouth during the dry season while floods flush the river

mouth open. Closure is the effect of littoral sediment transport on the beach that blocks off the stream

mouth. Opening is caused primarily by storm flows. For the San Dieguito River, changes in channel

morphology related to sediment flushing and recharge can be substantial, and they have important effects on

the hydraulics of flow, sediment transport, and channel boundary scour.

The San Dieguito River was monitored by Coastal Environment, Inc. Cross-sectional surveys were

made periodically at selected stream gaging stations along the river channel. The 1993 flood, estimated to

be an 18-yr flood, occurred in March and it ended on March 15. Prior to the flood, the channel bed near

the river mouth had been silted up to a level above the mean sea level. Gaging station 0.13 is located 0.13

river mile from the mouth. Cross-sectional profiles at this gaging station surveyed at time intervals before

and after the 1993 flood are shown in Fig. 3. The flood caused scour affecting a width exceeding 200 feet

and a maximum depth over 7 feet at this location as depicted in the figure. The figure also shows that the

channel bed was gradually silted up after the flood. In general, channel changes due to sediment flushing

and recharge are more pronounced at the river mouth and they tend to diminish toward upstream away from

the coastal influence.

Fig. 3. Measured channel changes during the 1993 flood at a location 690 feet from the beach

9

Major issues for the project include flood control, channel boundary scour and sediment delivery to

the beach. In order to assess the project impacts on these issues, it was necessary to apply a sediment

transport model for the river study.

II. BASIC HYDROLOGY

Floods - The San Dieguito River has a total drainage area of 346.5 square miles, of which 303

square miles are above Lake Hodges. Since its completion in 1926, the dam has controlled 87.4 % of the

drainage basin. Lake Hodges cuts off the surface runoff of small storms to the lower reach. The reservoir

spills during larger storms. Significant spillage of the reservoir occurred in 26 years of the last 78 years

(1926-2003). For larger storms, the upper basin above Lake Hodges supplies the discharge in the Lower

San Dieguito River, but for smaller storms, the flow in the lower reach is only supplied by runoff from the

lower river basin below Lake Hodges. A summary of peak discharges for representative return periods is

given in Table 2. There is a lack of sufficient stream flow data for the river channel. The County of San

Diego used hydrological simulation to determine the flood discharges. Peak discharges of other floods may

be estimated based the assumption that the distribution of peak discharges follows a lognormal distribution.

Table 2. Flood discharges for the lower San Dieguito River

Flood event Peak Discharge, cfs

10-yr 5,700

50-yr 31,400

100-yr 41,800

In this study, the 100-yr flood was used together with a flood series representative of the long-term

flood flow. The 100-yr flood hydrograph is shown in Fig. 4. In the future, one should expect various flood

events. In the time span of 100 years, one may expect statistically one flood event exceeding the 100-year

flood, two events exceeding the 50-year flood, four events exceeding the 25-year flood, ten events

exceeding the 10-yr flood, etc. For this stream reach, most of the sediment transport occurs during major

floods. Those events less than the 10-yr flood have limited discharge and hence transport capacity;

10

therefore, only those events equal to or greater than the 10-yr flood were included in the flood series for

simulation. The series of flood events occur randomly. The sequence of occurrence of these floods is

beyond human prediction, but the particular order of flood events does not affect the results pertaining to the

long-term sediment delivery. The sequence of flood events as exemplified in Fig. 4 was employed to

represent the long-term flood flow. It is assumed in this study that the occurrence follows the following

order: 10-yr flood, 30-yr flood, 20-yr flood, 40-yr flood, 15-yr flood, 100-yr flood, 20-yr flood, 15-yr

flood, 70-yr flood, and 10-yr flood. It is recognized that flood sequences are almost infinite in possibility

and the sequence of events assumed in this study is only a special case. However, the objective of the

environmental impact study was to assess the impacts of the proposed project on sediment delivery by the

river channel. The same flood series was used in simulation for the pre-project and post-project conditions.

The results for these two conditions were compared. Since the pre-project and post-project conditions

were simulated based on the same flood series, the comparative results are useful for assessing the project

impacts on sediment delivery by the river.

11

Fig. 4. Hydrograph of the 100-yr flood

Fig. 5. Flood series for 100-yr time span

Sediment Data - The study reach of the San Dieguito River is a sand bed river. Sediment samples

were taken from the river bed at several locations. These samples were analyzed and their grain size

distributions are shown in Fig. 6. The bed material consists primarily of sand with small amounts of fines (silt

and clay) and gravel.

12

Fig. 6. Grain size distributions

Grain Size Distributions

0

10

20

30

40

50

60

70

80

90

100

0.010.1110100

Grain size, mm

Perc

ent f

iner

by

wei

ght

RM=0RM=0.62

RM=1.45RM=2.69

Grain Size Distributions

0

10

20

30

40

50

60

70

80

90

100

0.010.1110100

Grain size, mm

Perc

ent f

iner

by

wei

ght

RM=3.43RM=3.93RM=4.74RM=5.79

13

Effective Flow Area and Ineffective Flow Area - The lower San Dieguito River has a main

channel and a broad floodplain. Distribution of flow velocity in the floodplain is not uniform, as illustrated in

Fig. 7. This velocity distribution was simulated using the two-dimensional hydrodynamic model FESWNS

(Federal Highway Administration, 1992). Based on the velocity distribution, the broad floodplain can be

divided into an effective flow area and an ineffective flow area. The effective flow area has significant flow

velocities and it contributes to the conveyance of most of the flow discharge. The ineffective flow area, on

the other hand, has very small flow velocities and it does not contribute significantly to the conveyance of the

flow discharge. The effective flow area as delineated using the FESWNS model is shown in the figure to be

within the boundaries designated by dashed lines; it is along the main channel of the river and it passes

through the bridge openings. The ineffective flow area is outside the effective flow area.

Fig. 7. Sample two-dimensional velocity distribution in the floodplain

showing effective and ineffective flow areas (Q = 20,000 cfs)

14

III. METHODS OF ANALYSIS

Stream channel scour consists of general scour and local scour. General scour is related to the

sediment supplied to and transported out of a channel reach. Local scour is due to a local obstruction to

flow by a bridge pier/bent or abutment.

To determine general scour, it is necessary to consider the sediment supply by flow to the channel

reach and sediment removal out of the reach. Sediment delivery in the stream channel and supply to the

subject area is related to the flood hydrograph, channel geometry, and sediment characteristics, etc. To

account for these factors, it requires mathematical simulation of the hydraulics of stream flow, sediment

transport and stream channel changes.

Mathematical Model for General Scour - The FLUVIAL-12 model (Chang, 1988) is

employed for this project. For a given flood hydrograph, the FLUVIAL model simulates spatial and

temporal variations in water-surface elevation, sediment transport and channel geometry. Scour and fill of

the streambed are coupled with width variation in the prediction of river channel changes. Computations are

based on finite difference approximations to energy and mass conservation that are representative of open

channel flow.

The model simulates the inter-related changes in channel-bed profile and channel width, based upon

a stream's tendency to seek uniformities in sediment discharge and power expenditure. At each time step,

scour and fill of the channel bed are computed based on the spatial variation in sediment discharge along the

channel. Channel-bed corrections for scour and fill will reduce the non-uniformity in sediment discharge.

Width changes are also made at each time step, resulting in a movement toward uniformity in power

expenditure along the channel. Because the energy gradient is a measure of the power expenditure,

uniformity in power expenditure also means a uniform energy gradient or linear water surface profile. A

river channel may not have a uniform power expenditure or linear water-surface profile, but it is constantly

adjusting itself toward that direction. The model was calibrated using 12 sets of field data. Such calibration

15

studies are as listed in the Users Manual for FLUVIAL-12. Most of the calibration studies were peer-

reviewed.

Selection of the Engelund-Hansen Formula The Engelund-Hansen formula was selected for

the study for the following reasons:

(1) The selection was based on the most extensive evaluation of formulas made by Brownlie (see

Fig. 8); the Engelund-Hansen formula has the best correlation with field data.

(2) The Engelund-Hansen formula was used in many studies in this region. The results of these

studies were verified by field data.

(3) In a calibration study of the FLVUAIL-12 model, the results generated by the Engelund-

Hansen formula can be correlated with the measured channel changes in the San Dieguito River

during the 1993 flood.

Fig. 8. Evaluation of sediment transport formulas by Brownlie

Engelund-Hansen Formula - Engelund and Hansen (1967) applied Bagnold's stream power

concept and the similarity principle to obtain their sediment transport equation:

16

f' ϕ = 0.1 (τ* )5/2 (1) 2gRS with f' = -------- (2) U2 qs τo ϕ = ------------------, τ* = ------------- (3) γs [(s - 1) gd3 ] 1/2 (γs - γ) d

where f' is the friction factor, d is the median fall diameter of the bed material, ϕ is the dimensionless

sediment discharge, s is the specific gravity of sediment, and τ* is the dimensionless shear stress or the

Shields stress. Substituting Eqs. 2 and 3 into Eq. 1 yields

s US RS 1/2

Cs = 0.05 ----- ---------------- [------------] (4) s - 1 [(s - 1)gd] 1/2 (s - 1) d

where Cs (= Qs/Q) is the sediment concentration by weight. This equation relates sediment concentration to

the U-S product (which is the rate of energy expenditure per unit weight of water) and the R-S product

(which is the shear stress). Strictly speaking, the Engelund-Hansen formula should be applied to streams

with a dune bed in accordance with the similarity principle. However, it can be applied to upper flow regime

with particle size greater than 0.15 mm without serious error.

IV. HYDRAULIC DESIGN FOR A WEIR ON NORTHEASTERN BERM

The northeastern berm encroaches into the floodplain of the San Dieguito River and it may thus

cause backwater effects on the upstream river channel. The proposed project includes three berms and

dredging of the main channel. Preliminary FLUVIAL-12 results indicate that the proposed northeastern

berm will cause backwater effects on the river channel. The rises in water-surface elevation due to

backwater, based on the 100-yr flood, are small in magnitude, measured in about 2 inches.

17

A scheme has been developed to eliminate the backwater effects. This scheme includes a weir

installed near the upstream end of the northeastern berm at river mile 2.31. This weir is a cut in the berm; it

has a crest elevation that is about 7 feet lower than the top of the berm so that it may admit a part of the

flood flow into the northeastern tidal basin. In order to achieve the objective of reducing the backwater

effects in the river channel, a sufficient discharge must be admitted into the tidal basin. On the other hand,

the weir flow discharge must be small enough so that it will not cause damages to the tidal basin.

In order to develop the desired weir configuration, a weir with a vertical side slope (rectangular

weir) was first assumed and different sets of weir lengths and crest elevations were tested. Based on the

test results, the following geometric features for the weir were selected:

Weir length: 240 feet

Weir crest elevation: 13.8 feet

At the location of the weir, the berm has the top elevation of 20.5 feet. The performance and effectiveness

of the weir were analyzed as described below.

In subsequent development, weirs with a side slope were considered. Such weirs are trapezoidal in

cross-sectional shape and they may therefore be referred to as trapezoidal weirs. A flat side slope for the

weir will allow human passage along the top of the berm, thereby eliminating the need of a bridge over the

weir opening. Two different flat side slopes were considered; these are: (1) the 5 horizontal units to one

vertical unit side slope, and (2) the 12 horizontal units to one vertical unit side slope.

Rating Curves for Weir Flow - Weir flow occurs when the river stage exceeds the weir crest

elevation of 13.8 feet. Based on the results of FLUVIAL-12 modeling, the river stage at the weir location

reaches 13.8 feet at the discharge of 14,000 cfs, which is the peak discharge of the 25-yr flood. In other

words, weir flow occurs only during floods that are greater than the 25-yr flood.

The discharge overflowing the crest of a rectangular weir was computed based on the following

equation for broad-crested weirs (Chow, Open Channel Hydraulics, MacGraw-Hill, 1959):

18

Q = 3.09 H3/2 L (5)

In which, Q is the weir discharge in cfs; H is the total head above the weir crest; L is the weir length of 240

feet. Based on this equation, weir discharges at various stages were computed as summarized in Table 3.

The stage-discharge relation, or the rating curve, for the weir is also shown in Fig. 9.

The last column in Table 3 is based on results from the FLUVIAL-12 model. At the peak 100-yr

flood discharge of 41,800 cfs, the weir flow is 5,300 cfs, which is roughly 12.7 % of the peak flood flow.

The diverted discharge into the northeastern tidal basin is reintroduced into the main channel at river mile

1.45 through the downstream exit of the tidal basin; the combined flood flows continue downstream to the

ocean.

Table 3. Stage-discharge relations for the river and rectangular weir

River stage feet

Head over weir feet

Weir discharge cfs

River discharge cfs

13.8 0 0 14,000

14.0 0.2 66 15,000

14.5 0.7 434 18,000

15.0 1.2 975 21,000

15.5 1.7 1,644 24,000

16.0 2.2 2,410 26,500

16.5 2.7 3,290 29,000

17.0 3.2 4,245 32,000

17.5 3.7 5,300 42,000

17.8 4.0 5,933 48,000

For a weir with a trapezoidal cross section, the discharge of flow over the weir crest is given by the

equation (Chow, Open Channel Hydraulics, MacGraw-Hill, 1959):

19

5.671 [(L+zy)y]1.5 Q = ----------------------- (6) (L+ 2zy)0.5

In which, L is the weir length that is also the bed width of the trapezoidal channel; z is the side slope in

horizontal units for one vertical unit; and y is water depth over the weir crest. The total head H (or the river

stage) above the weir crest is computed from the equation

H = y + V2/2g (7)

In order to develop the trapezoidal weir configuration for the selected side slopes, different sets of

weir lengths and crest elevations were tested. The purpose of these tests was to match the performance of

the trapezoidal weir with that of the rectangular weir, which has a vertical side slope. Based on the test

results, it was concluded that the same weir crest elevation of 13.8 feet could be used for trapezoidal weirs.

The weir lengths were adjusted for the trapezoidal weirs; the selected weir lengths are summarized in Table

4. The rating curves for these three weir configurations are plotted as shown in Fig. 9 for comparison. It is

easy to see that these rating curves are closely similar. In other words, at a river stage, closely similar

discharges will be diverted into the northeastern tidal basin through these weirs.

20

Fig. 9. Rating curves for weirs (Series 1 is for rectangular weir with L=240 feet; Series 2 is for the

trapezoidal weir with L=225 feet and z = 5; and Series 3 is for the trapezoidal weir with L=215 feet and z =

12)

Rating Curves for Weirs

0

1000

2000

3000

4000

5000

6000

7000

13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5

River Stage, feet

Dis

char

ge, c

fs

Series1Series2Series3

21

Table 4. Summary of weir configurations

Side slope Crest elevation, ft Weir length, ft

0 (rectangular weir) 13.8 240

5 to 1 (trapezoidal weir) 13.8 225

12 to 1 (trapezoidal weir) 13.8 215

Modification of Flood Hydrograph - When a portion of the flood discharge is diverted into the

tidal basin through the weir. The flood discharge in the river channel adjacent to the berm (from Section

1.457 to Section 2.240) is reduced. The reduction of river discharge is equal to the weir discharge and it

varies in direct relation to the river stage. In the modeling study, the modified flood hydrograph was used

for the river reach adjacent to the northeastern berm from Section 1.457 to Section 2.240. The section

numbers are in river miles. At the confluence of the main channel and the northeastern tidal basin between

Section 1.402 and Section 1.457, the diverted flow is again reintroduced into the main channel and the

combined discharge was used in the modeling study.

Effects of Weir on Tidal Basin - Since a portion of the flood flow will be routed through the tidal

basin, the effects of this flow on the tidal basin need to be investigated. The flow through the tidal basin has

the maximum discharge of 5,300 cfs during the 100-yr flood. The flow velocity through the tidal basin

depends on the tidal basin configuration. For the proposed tidal basin design, the minimum cross-sectional

area of flow through the tidal basin is at the berm opening (mouth) to the river channel at river mile 1.45.

The flood stage at this section is computed to be 15.4 feet at the 100-yr peak discharge. The river stage

also controls the stage at the berm opening. At this stage, the cross-sectional area of flow at the berm

opening is about 2,000 square feet. With the discharge of 5,300 cfs, the velocity is computed to be 2.7 feet

per second. This velocity is very small and it is considered to be a non-scouring velocity. At other places in

the tidal basin, the velocities are lower than 2 feet per second. It may therefore be concluded that the flow

through the tidal basin will not cause scour damages. Of course, the weir structure and its adjacent areas

need to be hardened.

22

Another important consideration for the flow diversion scheme is whether the weir flow would cause

sediment deposition in the tidal basin. This potential impact is now analyzed as described below. The weir

has the crest elevation of 13.8 feet, which is well above the adjacent channel bed elevation of about 2-3

feet. For this reason, bed sediment will not be transported into the tidal basin. In other words, the potential

for siltation in the tidal basin due to bed sediment is insignificant.

Suspended sediment load and floating debris will be transported into the tidal basin during weir

flows. Suspended load and floating debris should not be significant maintenance problems for the tidal

basin for the following reasons.

(1) The occurrence of weir flow is very rare; its return period is 25 years.

(2) The duration of weir flow is much shorter than the flood duration. As shown in Fig. 4, the 100-yr

flood has the duration of roughly 50 hours, but the duration of weir flow is less than 20 hours. The

discharge of weir flow is very small during most of the flow period.

(3) Only the suspended load in the top layer of the river flow can be transported into the tidal basin. It

consists of the finest portion of the suspended load, or the wash load. In sedimentation engineering,

it is well known that the wash load has a very slow settling velocity and it tends to stay in suspension

for a long period of time. For this reason, much of the wash load, after entering the tidal basin, will

actually be transported out of the tidal basin to re-enter the downstream river channel.

(4) Floating debris is normally near the water surface but with most of its volume submerged in the

water. In order for the debris to be carried into the tidal basin, it will require a certain water depth

for the weir flow. The maximum water depth over the weir crest is about 3 feet. Because of the

limited water depth, large debris may not be carried into the basin. Smaller pieces that enter the

tidal basin may also be transported out of the basin.

Summary and Conclusions : A scheme has been developed to eliminate the backwater effects of

the northeastern berm on the upstream river channel. This scheme includes a weir installed near the

upstream end of the northeastern berm at river mile 2.31. A portion of the flood flow in the river channel

will be diverted into the tidal basin through the weir. For a rectangular weir, a weir length of 240 feet and

23

crest elevation of 13.8 feet have been selected based on performance and effectiveness, evaluated based on

hydraulic computations and FLUVIAL-12 modeling.

The 100-yr water-surface elevations for the river channel were simulated using the FLUVIAL-12

model for the existing conditions without the berms and the weir and proposed conditions with the berms

and weir. The comparison will be presented in a later section of the report; it shows that these two water-

surface profiles are closely similar with only minor differences. Water-surface elevations for the proposed

conditions are generally lower especially along the inlet channel. Upstream from the east end of the berm at

river mile 2.31, backwater effect due to proposed wetland project is totally eliminated.

With flow diversion through the weir, the flow velocity through the tidal basin varies with the tidal

basin configuration. For all the planned alternatives, the minimum cross-sectional area of flow through the

tidal basin is at the berm opening to the river channel at river mile 1.45. The maximum velocity through the

opening is computed to be 2.7 feet per second. This velocity is very small and it is considered to be a non-

scouring velocity. At other places in the tidal basin, the velocities are lower than 2 feet per second. It may

therefore be concluded that the flow through the tidal basin will not cause scour damages. Of course, the

weir structure and its adjacent local areas need to be hardened.

This potential impact of the weir flow on tidal basin sedimentation was also analyzed. The weir

crest elevation of 13.8 feet is well above the adjacent channel bed elevation of about 2-3 feet. For this

reason, bed sediment will not be transported into the tidal basin; the potential for siltation due to bed

sediment in the tidal basin is insignificant. Suspended sediment load and floating debris will be transported

into the tidal basin during weir flows. The effects of siltation due to suspended load and floating debris are

not significant maintenance problems for the tidal basin.

V. MODELING FOR EXISTING CONDITIONS AND PROPOSED CONDITIONS

The FLUVIAL-12 model was used to simulate the hydraulics of river flow, sediment transport,

together with potential changes in river channel geometry during the 100-yr flood. Modeling runs were

24

made for the river channel under existing conditions and with the proposed wetland restoration project. The

results for these two cases are compared in order to assess the project impacts. The modeled results

pertaining to water-surface profiles, flow velocities, bridge hydraulics, and river channel scour are presented

and described below:

Channel-Bed and Water-Surface Profiles Based on the FLUVIAL-12 Model - The channel

reach from the river mouth to river mile 0.6 is referred to as the inlet channel. This reach is subject to

significant scour during major floods; therefore, the water-surface profile is affected by the scour

development. Channel-bed scour has the direct effect of lowering the flood level. For this reason, a water-

surface profile computed using the FLUVIAL-12 model with scour consideration should be lower than the

corresponding profile obtained based on the HEC-2 model that assumes rigid channel boundary.

Figures 10 and 11 show the simulated water-surface profiles and channel-bed profile changes

during the 100-yr flood for the existing and proposed conditions, respectively. It can be seen that the

channel bed is susceptible to major scour along the inlet channel from the river mouth to river mile 0.6.

Such scour development is primarily because the flow is now constricted to a narrow channel. After scour

and sediment flushing by a flood, the inlet channel also undergoes refill by beach sand related to the littoral

process. Major channel bed scour is also predicted at channel bends. The scour depth at channel bends is

directly related to the flood discharge; it is the greatest near the peak flow and it tends to be refilled, at least

in part, during the falling flood discharge.

The computed water-surface elevations are based on the peak 100-yr flood discharge. Numerical

comparison of the water-surface elevations for these two conditions as given in Table 5 shows that these

two water-surface profiles are closely similar with only minor differences. Water-surface elevations for the

proposed conditions are generally lower especially along the inlet channel. Since a portion of the flow is

diverted into the northeastern tidal basin through a weir, backwater effects due to tidal basin is eliminated.

Since the proposed project will not cause increased flood level, the project will have no flooding impacts.

25

Fig. 10. Water-surface and channel-bed profile changes

Longitudinal Profiles During 100-yr Flood for Existing Conditions

-20

-15

-10

-5

0

5

10

15

20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Channel station, river miles

Ele

vatio

n, f

eet

Peak W. S.Initial bedBed at peak flowBed after flood

Interstate 5 -

River bend -

River m

outh

Highw

ay 101 -

R. R

. Bridge -

Durante B

r. -

Longitudinal Profiles During 100-yr Flood for Existing Conditions

-15

-10

-5

0

5

10

15

20

25

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3


Ele

vatio

n, f

eet


- El C

amino

Real

River bend -

26

during 100-yr flood for existing conditions

Fig. 11. Water-surface and channel-bed profile changes during 100-yr flood

Longitudinal Profiles During 100-yr Flood for Proposed Conditions

-15

-10

-5

0

5

10

15

20

25

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3


Ele

vatio

n, f

eet


- El C

amino R

eal

River bend -

River bend -

27

for proposed conditions

Longitudinal Profiles During 100-yr Flood for Proposed Conditions

-20

-15

-10

-5

0

5

10

15

20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5


Ele

vatio

n, f

eet


Interstate 5 -

River bend -

River m

outh

Highw

ay 101 -

R. R

. Bridge -

Durante B

r. -

28

Table 5. Comparison of computed for 100-yr flood elevations based on FLUVIAL-12

Computed water-surface elevation feet, NGVD

Section

river mile

Location Existing conditions Proposed plan

0.00 River mouth 0 0

0.107 Highway 101 Bridge 3.0 2.8

0.191 4.6 4.2

0.293 Railroad Bridge 6.7 6.2

0.374 7.6 7.2

0.493 9.1 8.6

0.581 Jimmy Durante Bridge 9.3 8.7

0.706 River bend 11.6 11.2

0.967 12.5 12.5

1.192 13.8 13.7

1.391 I-5 Bridge 14.9 14.9

1.596 16.0 16.0

2.062 17.1 16.9

2.183 17.3 17.1

2.311 East end of berm 17.5 17.4

2.479 17.8 17.7

2.688 18.6 18.6

2.806 El Camino Real 20.3 20.3

3.048 21.7 21.7

3.148 21.9 21.9

29

3.453 23.0 23.0

3.567 23.5 23.5

Spatial Variations in Velocity Along the River Channel - The flow velocity is often used as a

criterion to assess the potential for scour. The cross-sectionally averaged velocities are included in the

output of the FLUVIAL-12 model. Such velocities simulated at the peak 100-yr flood and their spatial

variations along the river channel are shown in Fig. 12 for the existing and proposed conditions. The spatial

variations in velocity are useful to characterize the flow along the river channel. The figure shows that the

river channel has very high velocities along the entire inlet channel. This is related to the concentration of

flow in the confined channel width. The high velocities are also responsible for the severe scour

development.

Fig. 12. Spatial variations of velocity along river channel for existing and proposed conditions

The velocities are higher than 6 feet per second along the entire inlet channel and at the I-5 Bridge

and near the El Camino Real Bridge crossings where the flow is also concentrated. For the rest of the river

channel shown in Fig. 12, the flow velocities are generally lower than 6 feet per second. When the

Spatial Variations of Flow Velocity During 100-yr Flood

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8


Vel

ocity

, fps

Existing conditions

Proposed conditions

- Durante B

ridge

Flow toward left

I-5 Bridge -

30

velocities for the existing and proposed conditions are compared, it can be seen that the proposed plan will

result in increases in flow velocity along the bermed reaches. Such increases are directly related to the

encroachment of flow by the berms. Since the velocities within the encroachment are still lower than 6 feet

per second, such encroachment is permissible under the local guidelines.

The velocity is also used as a criterion to assess the need for bank protection for the berms. For

berms that are made of cohesive materials, planted with vegetation, and parallel to flow, the permissible

velocity of 6 feet per second is normally used as the criterion for bank stability. In other words, a berm that

meets these conditions does not require additional protection. The south berms on both sides of the I-5

Bridge are parallel to the river flow and they will be constructed with cohesive materials and planted with

vegetation. For this reason, additional bank protection is not needed for these berms. The berm northeast

of the I-5 Bridge is not parallel to the river flow and therefore it will need bank protection.

Bridge Hydraulics - Flow velocities at the bridge crossings during the peak 100-yr flood for the

existing conditions and proposed plan are summarized in Table 6 for comparison. It can be seen that the

proposed project will not raise the velocities at the bridge locations.

Table 6. Comparison of bridge hydraulics Velocity, feet/sec Structure Location

river mile Existing conditions Proposed conditions

Highway 101 Br. 0.107 11.0 9.4

Railroad Bridge 0.293 8.3 8.3

Jimmy Durante Br. 0.581 10.8 10.8

I-5 Bridge 1.391 6.0 6.0

El Camino Real Br. 2.806 5.7 5.7

Simulated Results on Scour - The lower San Dieguito River is a sand bed river. Grain size

distributions of sediment samples are shown in Fig. 6. The 100-yr flood was used to simulate sediment

31

transport and stream channel changes for the lower San Dieguito River under the existing conditions as well

as under the proposed plan. Simulated results are presented in graphical forms, in Figs. 10, 11, 13, 14, 15,

16, 17, 18, 19, 20, 21, 22, 23 and 24. In each figure, the graphical results usually include those before the

flood, at the peak flow, and at the end of flood. These results are described below. Simulated results

pertaining to potential channel-bed scour during the 100-yr flood for the existing conditions are shown in

Fig. 10; those for the proposed plan are shown in Fig. 11. The cross-sectional changes as simulated

Changes at Sec. 0.107 (Highway 101 Bridg) for Existing Conditions

-20

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200

Station (looking downstream), ft

Ele

vatio

n, f

eet

InitialAt peak flowAt end of floodPeak W. S.

Changes at Sec. 0.107 (Highway 101 Bridg) for Proposed Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

Initial

At peak flowAt end of floodPeak W. S.

32

are shown in Figs.13 through 24.

Fig. 13. Simulated changes at Highway 101 Bridge crossing

for the existing and proposed conditions

Fig. 14. Simulated changes at the railroad bridge crossing

Changes at Sec. 0.293 (Railroad Bridge) for Proposed Conditions

-15

-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet


33


Changes at Sec. 0.293 (Railroad Bridge) for Existing Conditions

-15

-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet


34

Fig. 15. Simulated changes in the inlet channel for the existing and proposed conditions

Changes at Sec. 0.454 (Inlet Channel) for Existing Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250


Ele

vatio

n, f

eet


Changes at Sec. 0.454 (Inlet Channel) for Proposed Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250


Ele

vatio

n, f

eet


35

Fig. 16. Simulated changes at Jimmie Durante Bridge crossing

Changes at Sec. 0.581 (Durante Bridge) for Existing Conditions

-20

-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet


Changes at Sec. 0.581 (Durante Bridge) for Propsoed Conditions

-20

-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet


36


37

Fig. 17. Simulated changes at river bend for the existing and proposed conditions

Changes at Sec. 0.706 (River Bend) for Proposed Conditions

-20

-15

-10

-5

0

5

10

15

20

25

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900


Ele

vatio

n, f

eet


Changes at Sec. 0.706 (River Bend) for Existing Conditions

-20

-15

-10

-5

0

5

10

15

20

25

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900


Ele

vatio

n, f

eet


38

Fig. 18. Simulated changes at section 0.866 for the existing and proposed conditions

Changes at Sec. 0.866 for Existing Conditions

-10

-5

0

5

10

15

20

4300 4500 4700 4900 5100 5300 5500 5700


Ele

vatio

n, f

eet


Changes at Sec. 0.866 for Proposed Conditions

-10

-5

0

5

10

15

20

4500 4700 4900 5100 5300 5500 5700


Ele

vatio

n, f

eet


39

Fig. 19. Simulated changes at I-5 Bridge crossing for the existing and proposed conditions

Changes at Sec. 1.391 (I-5 Bridge) for Existing Conditions

-10

-5

0

5

10

15

20

25

30

4300 4400 4500 4600 4700 4800 4900 5000 5100


Ele

vatio

n, f

eet


Changes at Sec. 1.391 (I-5 Bridge) for Proposed Conditions

-10

-5

0

5

10

15

20

25

30

4300 4400 4500 4600 4700 4800 4900 5000 5100


Ele

vatio

n, f

eet


40

Changes at Sec. 1.596 for Existing Conditions

-5

0

5

10

15

20

25

4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 5400 5500 5600


Ele

vatio

n, f

eet


Changes at Sec. 1.596 for Proposed Conditions

-5

0

5

10

15

20

25

4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 5400


Ele

vatio

n, f

eet


41



-10

-5

0

5

10

15

20

25

4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 5400


Ele

vatio

n, f

eet


Berm


-15

-10

-5

0

5

10

15

20

25

4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200


Ele

vatio

n, f

eet


Berm

42


43



-10

-5

0

5

10

15

20

25

4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 5400


Ele

vatio

n, f

eet


Berm


-10

-5

0

5

10

15

20

25

4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200


Ele

vatio

n, f

eet


Berm

44



-15

-10

-5

0

5

10

15

20

25

4300 4400 4500 4600 4700 4800 4900 5000 5100 5200


Ele

vatio

n, f

eet


Berm


-10

-5

0

5

10

15

20

25

4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300


Ele

vatio

n, f

eet


Berm

45

Fig. 24. Simulated changes at Camino Real Bridge crossing

Changes at Sec. 2.806 (El Camino Real) for Existing Conditions

0

5

10

15

20

25

30

4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 5400 5500


Ele

vatio

n, f

eet


Changes at Sec. 2.806 (El Camino Real) for Proposed Conditions

0

5

10

15

20

25

30

4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 5400 5500


Ele

vatio

n, f

eet


46

It can be seen from the channel-bed changes that the inlet channel is subject to severe scour during

the 100-yr flood. The changes in bed elevation are especially pronounced at the bridge crossings. Since

the depths of general scour at the bridge crossings are so large, these bridges (including the Highway 101

Bridge, the railroad bridge, and the Jimmy Durante Bridge) are prone to severe scour damages that may

lead to failure during the 100-yr flood. The El Camino Real Bridge has a stone blanket on the channel bed

that checks the scour development. The stone blanket has a surface elevation of 3.5 feet.

Changes in the inlet channel are exemplified in Fig. 15 by the cross-sectional changes at Section

0.452. Such changes are characterized by channel-bed erosion and widening. Since the current bank

protection is not engineered; it is subject to significant damages during the flood. Changes at the river bend

are shown in Fig. 17 by the cross-sectional changes at Section 0.706. At this section, the scour is

concentrated near the concave bank. The present bank protection is not adequate for protection against the

100-yr flood. The maximum scour at this location would be slightly increased by the proposed project.

Protection of the concave bank at the river bend is a part of the project scope.

The channel reach upstream from the inlet channel is simulated to undergo changes as exemplified in

Figs. 18 through 23. These changes consist of minor scour and fill of the channel boundary; they are not

nearly as substantial as those in the inlet channel.

The maximum scour profiles at the bridge crossings for the existing and proposed conditions are

shown in the Appendix. The minimum bed elevations reached by scour at selected locations as simulated for

the existing and proposed conditions are summarized in Table 7 for comparison. The table shows that

general scour depths under the proposed plan are generally less than the corresponding ones for the existing

conditions. The proposed project will not increase the maximum scour at the bridge crossings. Minor

increases in scour depth are shown to occur at the river bend. The basic fact is that tidal basins are

designed to stay separate from the effective flow area of the river and therefore, they do not act as sediment

traps to cause a deficit in sediment supply to the river channel.

47

Table 7. Comparison of general scour during 100-yr flood

Minimum elevation reached by

general scour, ft (NGVD)

Structure Location

river mile Existing conditions Proposed plan

Highway 101 Br. 0.107 -14.0 -9.3

Railroad Bridge 0.293 -18.0 -16.8

Jimmy Durante Br. 0.581 -17.2 -16.5

River bend 0.706 -22.1 -23.0

I-5 Bridge 1.391 -5.1 -5.1

El Camino Real Br. 2.806 3.5 3.5

VI. BEACH SAND SUPPLY

Simulation of Sediment Delivery - Sediment delivery is defined as the cumulative amount of

sediment that has been delivered passing a certain channel section for a specified period of time, that is,

?

Y = ∫ Qs dt (5) T where Y is sediment delivery (yield); Qs is sediment discharge; t is time; and T is the duration. The sediment

discharge Qs pertains only to bed-material load of sand, gravel and cobble. Fine sediments of clay and silt

constituting the wash load may not be computed by a sediment transport formula. Sediment delivery is

widely employed by hydrologists for watershed management; it is used herein to keep track of sediment

supply and removal along the channel reach.

Spatial variations in sediment delivery are manifested as channel storage or depletion of sediment

associated stream channel changes since the sediment supply from upstream may be different from the

removal. The spatial variation of sediment delivery depicts the erosion and deposition along a stream reach.

48

A decreasing delivery in the downstream direction, i.e. negative gradient for the delivery-distance curve,

signifies that sediment load is partially stored in the channel to result in a net deposition. On the other hand,

an increasing delivery in the downstream direction (positive gradient for the delivery-distance curve)

indicates sediment removal from the channel boundary or net scour. A uniform sediment delivery along the

channel (horizontal curve) indicates sediment balance, i.e., zero storage or depletion. Channel reaches with

net sediment storage or depletion may be designated in each figure on the basis of the gradient. From the

engineering viewpoint, it is best to achieve a uniform delivery, the non-silt and non-scour condition, for

dynamic equilibrium.

Sediment deliveries were simulated based on the 100-yr flood as well as the 100-yr flood series.

The proposed wetland restoration project has a life span of 30 years. While major flood events are

responsible for most of the sediment delivery, the occurrence of floods in the next 30 years is clearly beyond

human prediction. For this reason, the long-term sediment delivery along the river channel is simulated using

the 100-yr flood series. The delivery in the 30-yr time span is prorated based on the total amount of

delivery for the 100-yr time span.

Simulated Results on Sediment Delivery by the 100-yr Flood - Spatial variations in sediment

delivery by the 100-yr flood along the river channel are shown in Fig. 25 for the existing and proposed

conditions, respectively. The spatial patterns are similar; they both show a general increasing trend of

delivery toward the downstream direction, depicting the general pattern of sediment removal, or scour, from

the channel boundary. Most significant scour is along the inlet channel from river mile 0 to 0.6 where the

stored sediment is simulated to be removed by the major flood event. Sediment removal from the inlet

channel does not contribute to a net supply of beach sand since the inlet channel is recharged by beach sand

during the dry season. The channel reach from river mile 0.6 to 1.8 is upstream from the inlet channel.

Along this reach, a mild increasing trend for sediment delivery is depicted for the both conditions. Total

deliveries of sediment at selected river locations for both cases are listed in Table 8 for comparison.

As shown in Fig. 25, there is more sediment inflow to the inlet channel measured by delivery at the

entrance (river mile 0.6) for the proposed conditions, but there is greater sediment outflow at the river

49

mouth for the existing conditions. In other words, dredging removal of material from the inlet channel under

the proposed plan would result in reduced deliveries. However, the reduction in delivery is less than the

dredged amount of 135,000 tons. The supply of beach sand would not be reduced by the proposed

dredging so long as the dredged sand is placed on the beach.

Fig. 25. Spatial variations of sediment delivery during the 100-yr flood for the existing and proposed

conditions

Table 8. Comparison of sediment deliveries by 100-yr flood

Total sediment delivery 1,000 tons

Structure Location river mile

Existing Proposed plan

River mouth 0 722 620

Highway 101 Bridge 0.107 634 551

Railroad Bridge 0.293 445 418

Jimmy Durante Bridge 0.581 226 273

Spatial Variations of Sediment Delivery During 100-yr Flood

0

100

200

300

400

500

600

700

800

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8


Sedi

men

t del

iver

y, 1

000

tons

Existing conditions

Proposed conditions

- Durante B

ridge

Flow toward left

I-5 Bridge -

Sediment recharge zone

50

I-5 Bridge 1.391 122 168

El Camino Real Br. 2.806 87 84

The above analysis applies to the short-term sand supply due to a single 100-yr flood. For the

proposed plan, sand dredged from the inlet channel will be placed on the beach and the inlet channel will be

maintained at the design bed level. Sand recharge to the inlet channel from the beach through littoral drift

will also be returned to the beach. Under these conditions, sand supply to the beach by the San Dieguito

River is controlled by the supply from the upstream river channel in the long term. As shown in Fig. 25,

sediment deliveries at upstream locations are closely similar for these two conditions. In other words, the

existing sand flow through the lagoon area would not be altered by the proposed plan.

Simulated Results on Long -Term Sediment Delivery – Long-term sediment deliveries were

simulated based on the 100-yr flood series. The proposed wetland restoration project has a life span of 30

years. The delivery in the 30-yr time span is prorated based on the total amount of delivery for the 100-yr

time span. Spatial variations in sediment delivery as shown in Fig. 26 depicts simulated results by

FLUVIAL-12. The constant delivery along the recharge zone of the inlet channel is used to depict that

there is no net sand supply from this channel reach. As it has been described previously that the inlet

channel is subject to flushing during floods and flushing is followed by sediment recharge from the beach

during the tidal exchange. Different delivery patterns for the two cases shown in the figure are described

below separately.

The spatial variations in sediment delivery as shown in Fig. 26 are described, reach by reach,

starting from upstream toward downstream. The first channel reach just upstream from El Camino Real at

river mile 2.806 is under the backwater influence from the bridge and its road embankment. The backwater

effects contribute to the low sediment delivery along the reach.

For the next reach from El Camino Real to river mile 2.2, the delivery pattern has a decreasing trend

toward downstream and it indicates that this channel reach is subject to sediment deposition. The next

channel reach from river mile 2.2 to 0.6 has an increasing trend for sediment delivery toward downstream.

51

This trend depicts general erosion of the channel boundary along the channel reach. Since this channel

reach is outside the recharge zone, sediment eroded from the boundary plus those transported through the

reach contribute to a net sand supply at the beach.

Fig. 26. Spatial variations of sediment delivery during 30-yr time span


Along the recharge zone from river mile 0.6 to the river mouth, the constant delivery is used to

depict no net contribution of beach sand from the reach. The net amount delivered to the river mouth is due

to sediment supplied from other upstream reaches; this amount is 214,000 tons for the existing conditions

and it is 227,000 tons for the proposed conditions in the 30-yr time span. The total amount of sand delivery

for the proposed conditions is slightly more than that for the existing conditions. This increase in sand

delivery is related to the encroachment of flow by the berms and the associated increase in flow velocity.

Spatial Variations of Sediment Delivery During 30-yr Time Span

0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8


Sedi

men

t del

iver

y, 1

000

tons

Existing conditions

Proposed conditions

- Durante B

ridge

Flow toward left

I-5 Bridge -

Sediment recharge zone

52

A summary of the spatial variations in sediment delivery for the existing and proposed cases is given

in Table 9. It can be seen from the table that the proposed plan will not decrease the supply of beach sand.

53

Table 9. Comparison of sediment deliveries in 30-yr time span

Total sediment delivery

1,000 tons

Structure Location

river mile Existing Proposed

River mouth 0 214 227

Highway 101 Bridge 0.107 214 227

Railroad Bridge 0.293 214 227

Jimmy Durante Bridge 0.581 214 227

I-5 Bridge 1.391 113 154

El Camino Real Br. 2.806 72 72

VII. DESIGN FOR BERMS AND BANK PROTECTION

Top Elevation of Berms - The top of berms and bank protection should stay above the 100-yr

flood level to avoid overtopping, which can cause erosional damages. According to the standard of the U.

S. Army Corps of Engineers, the top of levees should maintain a freeboard of three feet above the design

flood level. Computed water-surface profiles are used to determine the required top elevations for the

proposed berms and bank protection. The recommended top elevations for the berms as shown in Table

11 are based on water-surface elevations computed using the FLUVIAL-12 model, which considers

channel-bed scour.

Water-surface profiles of the river channel have been computed using two different computer

models: the HEC-2 model and the FLUVIAL-12 model. The former is a fixed boundary model that

assumes fixed channel boundary during a flood. The latter is an erodible boundary model that considers

channel boundary changes during a flood. Since the lower San Dieguito River is subject to significant

changes during floods, water-surface profiles obtained using these two models are therefore quite different.

Computed water-surface elevations together with the recommended top berm elevations are listed in Table

10.

54

Table 10. Computed water-surface elevations and recommended

top elevations for berms

Computed water-surface elevation feet River mile

per HEC-2 per FLUVIAL-12

Recommended

Top elevation feet

0.581 (J. Durante Br.) 15.8 8.7

0.830 (D/S end of berm) 16.9 11.8 16.0

1.045 17.0 13.0 16.3

1.192 17.1 13.7 16.6

1.343 (D/S face of I-5) 17.2 14.5 17.0

1.391 (U/S face of I-5) 17.2 14.9 17.5

1.457 18.3 15.3 18.5

1.674 18.4 16.4 19.0

1.805 18.6 16.7 19.5

2.122 18.8 17.1 19.8

2.311 18.9 17.5 20.0

Top Elevation and Toe-Down for Bank Protection – The top of bank protection should stay

above the 100-yr water surface. Recommended top elevations of bank protection as listed in Table 11

were selected based on computed water-surface profile at the peak discharge of the 100-yr flood plus a

freeboard.

The toe of bank protection must entrench beyond the potential scour. For this project, the

recommended toe elevations of bank protection are about 5 feet below the potential scour. The 5-foot

55

margin is a safety factor used to account for uncertainties, such as erratic hydrologic phenomenon, bank

settlement, local scour due to tree trunks, etc. The recommended top and toe profiles are also shown in

Figs. 27 and 28.

Table 11. Recommended elevations for berm and bank protection

Bank protection Section River mile

Top elevation of berm Feet Top elevation, feet Toe elevation, feet

Protection along river bend

0.640 11.7 -20.5

0.666 12.0 -25.0

0.706 12.0 -28.0

0.745 12.5 -16.0

Northeastern berm

1.457 18.5 17.0 -10.5

1.522 18.7 17.2 -11.0

1.596 18.8 17.3 -10.5

1.674 19.0 17.5 -10.5

1.737 19.3 17.8 -12.0

1.805 19.5 18.3 -22.5

1.847 19.8 18.3 -25.0

1.895 19.8 18.3 -17.5

1.979 19.8 18.3 -13.2

2.062 19.8 18.3 -8.5

2.122 19.8 18.3 -8.0

2.183 19.9 18.4 -7.0

56

2.240 19.9 18.4 -10.5

2.270 20.0 18.5 -17.0

2.311 20.0 18.5 -18.0

57

Fig. 27. Top and toe profiles of bank protection near the river bend

Fig. 28. Top and toe profiles of bank protection along the northeastern berm

Longitudinal Profiles for Northeastern Berm

-25

-20

-15

-10

-5

0

5

10

15

20

25

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4Channel station, river miles

Ele

vatio

n, fe

et

Top of bermExisting bed profileMaximum scour profileToe of bank protection

Upstream

end of berm -

Longitudinal Profiles for Bank Protection Near River Bend

-30

-25

-20

-15

-10

-5

0

5

10

15

20

0.5 0.55 0.6 0.65 0.7 0.75 0.8


Ele

vatio

n, fe

et

Top of bank protectionExisting bed profileMaximum scour profileToe of bank protection

River bend

D/S lim

it of protection

U/S lim

it of protection

Durante B

ridge

58

REFERENCES

Brownlie, W. R., "Prediction of Flow Depth and Sediment Discharge in Open Channels," Rept. No. KH-R-43A, W.M. Keck Laboratory of Hydraulics and Water Resources, California Institute of Technology, Pasadena, California, November 1981. Chang, H. H., Fluvial Processes in River Engineering, John Wiley & Sons, New York, NY, 1988, 432 pp. Vanoni, V., Sedimentation Engineering, ASCE Manual 54, 1975. LIST OF FIGURES

Fig. 1. Map of the San Dieguito River west of I-5

Fig. 2. Map of the San Dieguito River east of I-5

Fig. 3. Measured channel changes during the 1993 flood at a location 690 feet from the beach

Fig. 4. Hydrograph of the 100-yr flood

Fig. 5. Flood series for 100-yr time span

Fig. 6. Grain size distributions

Fig. 7. Sample two-dimensional velocity distribution in the floodplain showing effective and ineffective

flow areas (Q = 20,000 cfs)

Fig. 8. Evaluation of sediment transport formulas by Brownlie

Fig. 9. Rating curves for weirs (Series 1 is for rectangular weir with L=240 feet; Series 2 is for the

trapezoidal weir with L=225 feet and z = 5; and Series 3 is for the trapezoidal weir with L=215 feet and z =

12)

Fig. 10. Water-surface and channel-bed profile changes during 100-yr flood for existing conditions

Fig. 11. Water-surface and channel-bed profile changes during 100-yr flood for proposed conditions

Fig. 12. Spatial variations of velocity along river channel for existing and proposed conditions

Fig. 13. Simulated changes at Highway 101 Bridge crossing for the existing and proposed conditions

Fig. 14. Simulated changes at the railroad bridge crossing for the existing and proposed conditions

Fig. 15. Simulated changes in the inlet channel for the existing and proposed conditions

Fig. 16. Simulated changes at Jimmie Durante Bridge crossing for the existing and proposed conditions

59

Fig. 17. Simulated changes at river bend for the existing and proposed conditions


Fig. 19. Simulated changes at I-5 Bridge crossing for the existing and proposed conditions





Fig. 24. Simulated changes at Camino Real Bridge crossing for the existing and proposed conditions

Fig. 25. Spatial variations of sediment delivery during the 100-yr flood for the existing and proposed

conditions

Fig. 26. Spatial variations of sediment delivery during 30-yr time span for the existing and proposed

conditions

Fig. 27. Top and toe profiles of bank protection near the river bend

Fig. 28. Top and toe profiles of bank protection along the northeastern berm

60

APPENDIX A. MAXIMUM SCOUR PROFILES AT BRIDGE CROSSINGS DURING 100-YR

FLOOD

Maximum Scour Profile at Sec. 0.107 (Highway 101 Bridg) for Existing Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

Maximum Scour Profile at Sec. 0.107 (Highway 101 Bridg) for Proposed Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

61

Maximum Scour Profile at Sec. 0.293 (Railroad Bridge) for Existing Conditions

-20

-15

-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet

Maximum Scour Profile at Sec. 0.293 (Railroad Bridge) for Proposed Conditions

-20

-15

-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet

62

Maximum Scour Profile at Sec. 0.581 (Durante Bridge) for Proposed Conditions

-20

-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet

Maximum Scour Profile during 100-yr Floodat Sec. 0.581 (Durante Bridge) for Proposed Conditions

-20

-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet

63

Maximum Scour Profile at Sec. 1.391 (I-5 Bridge) for Existing Conditions

-10

-5

0

5

10

15

20

25

30

4300 4400 4500 4600 4700 4800 4900 5000 5100


Ele

vatio

n, f

eet

Maximum Scour Profile at Sec. 1.391 (I-5 Bridge) for Proposed Conditions

-10

-5

0

5

10

15

20

25

30

4300 4400 4500 4600 4700 4800 4900 5000 5100


Ele

vatio

n, f

eet

64

APPENDIX B. MAXIMUM SCOUR PROFILES AT BRIDGE CROSSINGS FOR 50-, 25- AND 10-

YR FLOODS

Maximum Scour Profile during 50-yr flood at Sec. 0.107 (Highway 101 Bridg) for Proposed Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

Maximum Scour Profile during 50-YR Floodat Sec. 0.107 (Highway 101 Bridg) for existing Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

65

Maximum Scour Profile during 50-yr Flood at Sec. 0.293 (Railroad Bridge) for Existing Conditions

-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet

Maximum Scour Profile during 50-yr Flood at Sec. 0.293 (Railroad Bridge) for Proposed Conditions

-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet

66

Maximum Scour Profile during 50-yr Floodat Sec. 0.581 (Durante Bridge) for Existing Conditions

-20

-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet

Maximum Scour Profile During 50-yr Flood at Sec. 0.581 (Durante Bridge) for Proposed Conditions

-20

-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet

67

Maximum Scour Profile during 25-yr Floodat Sec. 0.107 (Highway 101 Bridg) for Proposed Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

Maximum Scour Profile during 25-yr Floodat Sec. 0.107 (Highway 101 Bridg) for existing Conditions

-15

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

68


-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet


-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet

69


-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet


-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet

70

Maximum Scour Profile during 10-yr Floodat Sec. 0.107 (Highway 101 Bridg) for Proposed Conditions

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

Maximum Scour Profile during 10-yr Floodat Sec. 0.107 (Highway 101 Bridg) for Existing Conditions

-10

-5

0

5

10

15

20

4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300


Ele

vatio

n, f

eet

71


-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet


-10

-5

0

5

10

15

20

4900 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 6000


Ele

vatio

n, f

eet

72


-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet


-15

-10

-5

0

5

10

15

20

4800 4850 4900 4950 5000 5050 5100 5150


Ele

vatio

n, f

eet

Documents

(25) Hydraulics and Sedimintation Study