NORTHGATE CHANNEL REPORT final II 5 18 07

NORTHGATE DIVERSION CHANNEL DRAINAGE STUDY

Presented to

City of El Paso

Prepared for

City Engineering Department

DORADO ENGINEERING, INC. in association with Professor H. S. Oey P.E., RPLS and

Strategies & Plus

MAY 2007 FINAL REPORT

CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL

DORADO ENGINEERING, INC.

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TABLE OF CONTENTS

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

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

Project Scope………….…………………………………………….. 6

II. RESEARCH SECTION…………………………………………………… 8 Existing Drainage Problems………………………………………. 8 Proposed Plans……………………………………………………… 9 Hydrology and Hydraulic Studies…………………………….….. 16 Hydraulic Designs…………………………………………………… 25

III. FIELD DOCUMENTATION……………………………………………… 35

Description………………………………..………………………….. 35 Physical Characteristics…………………………………………… 36 Site Investigation……………………………………………………. 37 Soils Report………………………………………………………….. 38

IV. CONSCLUSION AND RECOMMENDATIONS………………………. 39

V. CONSTRUCTION COST ESTIMATES………………………………... 41

REFERENCES..………………………………………………………..……………. 44

APPENDICES………………………………………………………………………… 45

Appendix A – Flood Hydrograph Package (HEC-1)

Appendix B – Plates (1 through 14) Appendix C – Soil Report



EXECUTIVE

SUMMARY



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EXECUTIVE SUMMARY

In December of 2006, the City of El Paso retained the services of DORADO

ENGINEERING, INC. to develop a drainage study to resolve the flooding problems at

the Northgate Diversion Channel.

This document, Northgate Diversion Channel, examines and analyzes the drainage

problems in a specific region in Northeast El Paso. The study area is described as

bounded by the crest of the Franklin Mountains, Hondo Pass Drive, Galena Drive,

Moonlight Avenue and the Northgate Diversion Channel area. The Northgate Diversion

Channel is located adjacent to the intersection of Hondo Pass Avenue and Hondo Pass

Drive and is a dirt channel (arroyo) that suffered extensive soil erosion during the 2006

summer rains. The report investigates and assesses the most economical way to

reconstruct the existing channel and prepare the necessary engineering plans and

construction documents. Best management practices based on approved criteria is

delineated and structural and nonstructural improvements are recommended.

The report is comprised of the following major elements:

1. Executive Summary

2. Introduction and Project Scope

3. Hydrology and Hydraulic Studies

4. Field Documentation and Evaluation

5. Topographic Field Surveys

6. Conclusion and Recommendations

7. Construction Cost Estimates



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8. Study Support Appendices

The study focuses on the ability to reasonably protect the study area from damage

which might accrue due to a storm of the order of the Criteria Storm adopted. Our study

indicates that all or the major portions of the proposed plan can be justified by a detailed

cost-benefit analysis. The estimated cost of works provided for in the report outlines

three alternate solutions which are; 1) retention dam, 2) concrete open channel, and 3)

closed conduit. Line item listings of the three cost estimates for the reconstruction of

the existing channel including a 10% contingency amount are depicted in Section V.

During the design phase the cost estimates may vary somewhat due to the inflationary

bid prices currently prevailing in the construction industry.

After a very thorough analysis and evaluation of the existing conditions at Northgate

Diversion Channel, it was concluded that using two – 66 inch reinforced concrete pipe

will be the best approach in conveying the 100 year criteria storm flow from the slopes

of the Franklin Mountains down to the existing Northgate Dam Impounding Area. The

cost of the recommended option is $736,060.

The recommendation is based on the following observations:

1. The most efficient and cleanest method of conveying storm runoff

2. The most approachable method of working on the steep slopes of existing arroyo

3. The least time to construct

4. The least work to maintain

5. The most environmental friendly since the storm pipes are covered in the arroyo

6. Elimination of existing arroyo

7. The most economical



DRAINAGE

STUDY



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I. INTRODUCTION

The City of El Paso retained the services of Dorado Engineering, Inc. to perform the

necessary research in conjunction with the City Engineering Department, Street

Department, and the U. S. Corps of Engineers, to gather all available data needed to

design and reconstruct the existing channel in the Northeast Region of the City, also

know as the Northgate Diversion Channel.

The City of El Paso requested that prior

to the preparation of design plans and

specifications of the reconstruction

project, A Preliminary Design Phase

Report be submitted for approval. The

report focuses on the ability to

reasonably protect the study area from

damage which might accrue based on the 100 year storm. This report will propose

three plans to alleviate and abate the storm water flood problem. In general it provides

for short period detention, diversion, conveyance, through channels and ultimate

delivery to dry lake sumps which will necessarily have to be excavated. The water will

be disposed of by a combination of evaporation and infiltration or seepage. This plan is

the most economical method to accomplish the required purpose of adequate protection

from a flood which might arise with the occurrence of a 100 year storm. Site

improvements are specified and related estimated construction costs are identified.



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PROJECT SCOPE

Northgate Diversion Channel Scope of Work Revised 12-4-06

The Northgate Diversion Channel is located adjacent to the intersection of Hondo Pass

Ave. and Hondo Pass Drive and is a dirt channel (arroyo) that suffered extensive soil

erosion during the 2006 summer rains.

The project scope, as delineated by the City of El Paso, consists of the following:

1. Perform the necessary research with the City Engineering Department, Street

Department and the U. S. Corps of Engineers, to gather all available data that

will assist in the design and reconstruction of existing channel.

2. Perform the necessary field work to gather the required information to locate

channel set grade, set channel cross sections and estimate cuts and fills.

3. Determine watershed area of channel and perform the hydraulic computations,

based on the 100 year storm, to determine expected flows through the channel.

4. Based on computed hydraulic computations design channel for size and grade to

safely carry the expected 100 year flows.

5. Based on gathered information, determine most economical way to construct

channel such as, concrete, gabions, rock rip rap, dirt or a combination.

6. Coordinate project with all effected City Departments, Utilities, U. S. Corps and

all other effected agencies.

7. Provide soil investigation.

8. Prepare the necessary engineering plans and construction documents to

advertise project.

a. Design drawings

b. Specifications

c. Cost estimates

9. Acquire required grading permit.

10. Provide water pollution control plan.

11. Reproduction



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12. Assist owner in bidding process in determining the following:

a. Bid period and date

b. Respond to all questions from perspective bidding

c. Attend pre-bid conference

d. Prepare addenda

e. Evaluate bids

f. Provide recommendation on experience and acceptance of contractors

and sub contractors

g. Perform purchasing list

h. Issue Certificate of Substantial Completion

i. Provide As “Built” plans

j. Construction sequence plan

k. Construction schedule

13. Represent owner in all matters concerning the constructability of project such as:

a. Be in charge of construction management

b. Provide timely reports to owner

c. Respond to all matters concerning the project construction

d. Review and make recommendations to owners on

1. Submittals

2. Payment request

3. Change orders

e. Perform purchasing list

f. Issue Certificate of Substantial Completion

g. Provide As “Built” plans

h. Construction sequence plan

i. Construction schedule

Project Schedule *Preliminary Design Phase…..…………………………………………………45 Days

Final Design Phase…..………………………………………………………….15 Days * Provided that the City Engineering dept. has enough information on file, such as drawings and

topographic maps of the contributing water shed and existing drainage structures.



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II. RESEARCH SECTION

This study is made under the authority of a contract with the City of El Paso, Texas.

The City of El Paso of El Paso commissioned two previous drainage and flood control

studies for the Northeast Region of the city in 1960 and 1978. The purpose of the

previous studies was to develop a proposed master drainage plan and flood control

basin. Accordingly the city has taken some action in developing drainage and flood

control measures, however, drainage problems have continued.

EXISTING DRAINAGE PROBLEMS

Under existing conditions, flooding occurs at the intersection of

Hondo Pass Avenue and Hondo Pass Drive which are

adjacent to Mountain Hills Estates.

During the 2006 summer rains, the existing arroyo that carries

the flows from the Mountain Franklin slopes to the existing

Northgate Dam Impounding Area was over taxed causing the

rainfall water to go over the existing arroyo banks, resulting on

extensive erosion on the arroyo and the right-of-way of Hondo

Pass Drive. The arroyo overflows came very near to eroding

Hondo Pass Drive which eventually would have over spilled to

the existing residential subdivision. The existing condition of

the arroyo presents serious safety concerns to adjacent

properties if not corrected.



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DESCRIPTION OF PROPOSED PLANS

The plans herein being studied consist of detention dam, concrete open channel and a

closed concrete conduit. Each plan will be studied and together with the estimated cost

will be rated and recommendation made.

The detention dam will function by temporarily impounding the 100 year criteria storm

run-off in a reservoir and releasing them at maximum non-damaging rates of flow. Thus

reduce the extremely large and extensively damaging peak flows or discharge rate.

The concrete open channel and close conduits will function as control conveyances to

move the expected peak flows from the mountain alluvial washes to the existing

Northgate Dam impounding area. All flows will be contained within a proposed concrete

channel or closed concrete conduits to eliminate all erosion on existing soils. The

proposed channel and closed conduits are designed to convey the 100 year criteria

storm.

In determining the rate of run-off, several methods could be considered. Estimating the

time of concentration and use of the rational formula was considered, but was not used,

since the center of the mass of the storm, time wise, is in the late portion of the storm.

Instead through the use of HEC-1 computer program we were able to develop a

hygrograph of the actual storm on the area using the 100 year criteria storm. The storm

that was used in this study was the City of El Paso 100 year storm for contributing

watershed areas larger than 200 acres as shown on the City of El Paso Subdivision



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Improvements Design Standards Plate 2-9 “Rainfall Curves”. Based on Plate 2-9 the

100 year criteria storm for watershed area of 805 acres the intensity is 3.80 inches in

three hours. This information was inputted into the HEC-1 program that uses the United

States Soil Conservation Surfaces (U.S.S.C.S.) unit hydrograph to compute the

hydrographs used for this design. This is the storm that was used in computing the

hydrographs shown on pages 24 and 26.

Rainfall Criteria Used

The “criteria storm” of a return frequency of 100 years that was be used in the design of

the detention reservoir (dam) open channel and closed conduits. The storm duration is

three hours with a total rainfall of 3.80 inches in the eye of the storm, for the watershed

area of 805 acres. The rainfall sequence is as follows;

1st hour …………………….. 0.50 inch

2nd hour…………………….. 0.78 inch

3rd hour…………………….. 2.55 inch

The rainfall intensity in inches for the criteria storm versus the area in acres is shown on

the subdivision design standards plate 2-9 and was used in the design.

Detention Reservoir Capacity (Dam)

It is recommended and used as criteria that the storage capacity of the proposed dam

be designed to contain one hundred percent (100 %) of the runoff for the criteria storm

as calculated plus a 10 year silt volume.



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Spillway Capacity

It is recommended and used as criteria that the spillway be designed for the passage of

the maximum rate of flow for the 100 year criteria storm, assuming the reservoir full and

the outlet flow closed. A minimum of five feet of free board is used.

Outflow Pipe

The outflow pipe will be designed to empty the detention reservoir (dam) within 72 hours

maximum. The outflow pipe will also be designed to minimize erosion at the lower

reaches of the detention reservoir (dam).

Runoff Factor

Mountainous watersheds 85%

As shown on plate 2-10 of the subdivision standards and was used in the design.

Silt and Debris

Mountainous watersheds 0.003 ac. ft. / ac

As shown on the plate 2-5 of the subdivision standards and was used in the design.

Design Criteria Used

1. Drainage Area ………………………………………… 805 acres 2. Design Storm (100 Yr) ……………………………….. 3.80 inches in 3 hours

3. Runoff Factor.………………………………………….. 85 %

4. Total Calculated Rainfall Volume…………..………… 127 Ac. - Ft.



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5. Peak Calculated Runoff ………………………….…… 1400 c. f. s.

6. Required Silt Storage …………………………………. 3.0 Ac. - Ft.

7. Total Required Reservoir Capacity …………………. 130 Ac. - Ft.

8. Spillway Capacity ……………………………………… 1400 c. f. s.

9. Width of Spillway ……………………………………… 80.0 Feet 10. Depth of Flow at Spillway …………………………… 3.0 Feet 11. Spillway Elevation……………………………………. 4365 12. Empty Reservoir Time ………………………………. 72 Hours 13. Outflow Pipe………………………………………….. 24 Inch RCP 14. Top of Dam Elevation ……………………………….. 4370 15. Maximum Highwater Elevation at Dam ..…………. 4365



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Hydrologic Study Studies of the 100-year floods (according to the city of El Paso this corresponds to a

storm intensity of 3.80 inches over a period of 3 hours) for Northgate Drainage Area,

and the watersheds feeding them, are conducted using the U.S. Corps of Engineers

software HEC1 (Visual Hec) and available reports on the records. The study area is

located in North East El Paso. The subsoil in this study area is characterized by steep

soils that are shallow over the caliche, or are deep and gravelly and also belongs to the

Harkey-Glendale Association which is characterized by deep, nearly level loamy soils,

very fine sand to silty, clay loam. The presence of fissures and Karst formations are

also observed in the area. Watershed boundaries are identified from U.S. Geological

Survey (Topographic) Triangles. Characteristics of the dam and the watersheds in this

study are summarized in the following table.

The total calculated storage capacity of the dam without an active outflow pipe is 208

acre-feet. When a twenty four (24) inch outflow pipe is introduced into the hydrographs

and the inflow and outflow are simultaneously calculated the required capacity of the

dam is 126.70 acre feet as shown, on Table 22-1, page 22 and on the Outflow/Storage

Curves on Page 23.

Calculated storm runoff values in Karst formations are always higher than actual values

observed in the field. This is due to the cracks and fissures in the surface that enables

water to flow into the ground. Photographs taken of the terrain show the presence of

Karst formations and land fissures (openings) that are not taken into account when

hydrology and hydraulic calculation are performed. It is a known fact in this area that



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Karst and Fissures do exist in the Franklin Mountains and they account for some of the

storm runoff never reaching the lower portion of the Franklins because some of the

storm runoff flows disappears into underground cavitations (Karst and Fissures). These

underground flows that occurred in the Franklin Mountains served as an additional

factor of safety into the design of storm carrying structures. Silt produced by storm

runoffs in mountainous terrain is estimated at 0.003 ac-ft per acre of watershed area.

Table 17-1 on Page 17 shows the watershed area, maximum runoff volume, maximum

outflow volume, maximum storage capacity, volume of silt, peak stage, and time of peak

after the start of rain.

TABLE 17-1 Runoff simulation results are shown in Appendix A (from HEC-1 output).

The input data that was used to calculate the hydrograph shown on Page 24, which was

used in the design of the dam, and the hydrograph shown on Page 26, which was used

in the design of the concrete channel and concrete pipes, was seven (7) inches of

rainfall in twenty four (24) hours, this storm is equivalent to the City of El Paso 100 year

storm of 3.80 inches in three (3) hours. A six minute input interval was used to do the

storm simulation using the U.S.S.C.S unit hydrograph and curve number 65 in the HEC-

1 program number.

Based on the results of the simulated storm hydrograph the dam was designed to have

Dam Watershed Area, Acres

Concrete. Pipe dia. in inches

Flow capacity cfs

Max Out-Flow, cfs

Storm Runoff Storage, Ac-ft

Calculated10-year Volume of Silt, Ac-ft

Peak Stage, ft

Peak Time from start, hrs.

Dam 1 805 24 328 96 208 3.0 30 17



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a maximum storage capacity of approximately 208 ac-ft with no outflow and storing 100

percent of the storm. The peak ponding volume at the highest stage is 127 ac-ft with a

simultaneous maximum outflow of 96 cfs. The dam is designed to detain the runoff

water temporarily to reduce the downstream outflow through the community below. The

required ponding capacity for the dam is calculated according to the runoff volume that

has to be detained for this purpose. Using the topographic map provided to us the

reservoir capacities and rating curves are calculated and tabulated.

The following Table shows what the probabilities of a certain year storm occurrences

are.

The probability that an event of given recurrence interval will be equaled or exceeded

during periods of various lengths.

In the absence of dams and/or culverts, the maximum stage is not calculated but it is

estimated not to be significantly high. The max storm flow is calculated to be 1400 cfs

as shown on the hydrograph on Page 26. This is the max uninterrupted storm runoff

flow through the existing arroyos and berms.

Hydraulic Study

Storm

Recurrence -

Period Years

5 10 25 50 100

5 0.67 0.89 0.996 1.00 1.00

10 0.41 0.65 0.093 0.995 1.00

50 0.10 0.18 0.40 0.64 0.87

100 0.05 0.10 0.22 0.40 0.63



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Two methods that were used for conveying runoff flows through the open channel and

pipe or rectangular cross section closed conduits. The flow capacity of the conduits are

computed using Manning’s formula with roughness coefficient n = 0.013 as

recommended by the Texas Department of Transportation.

Q = 1.49 A R2/3 S1/2 / n

where n = 0.013 and S = 0.0025

A = cross-sectional area

P = wetted perimeter

R = hydraulic radius

For the submerged openings, the calculations are based on the following formula,

Q = C A (2gh)0.5

C is given the value 0.70

g = 32.2 ft/sec2

The outflow volumes of an 18 inch and 24 inch concrete pipes were calculated for

different stage at different storage volumes at Table 20-1. The relationship between

highwater elevation, volume of water stored and volume on outlet pipe is shown on

Table 22-1, page 23, on stage Outflow/Storage Curves.



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TABLE 20-1

In the hydrology simulations of the arroyos the stage elevation is taken as 0.00 (zero) at

the culvert invert, which is assumed 4 ft above the low point of the ponding area before

silting occurred. The highest water stage is approximately 30 ft above the culvert invert

elevation. The berm should allow at least a 5 ft freeboard.

Outflow capacity of an 18" & 24" concrete pipe opening, cfs

Depth Head,H

(ft) h=H-1.5/2 C A(18")

Q=CA (2gh)^0.5 A(24")

h=H-2/2

Q=CA (2gh)^0.5

3 2.25 0.7 1.766 14.88 3.14 2 24.95 4 3.25 0.7 1.766 17.88 3.14 3 30.55 5 4.25 0.7 1.766 20.45 3.14 4 35.28 6 5.25 0.7 1.766 22.73 3.14 5 39.44 7 6.25 0.7 1.766 24.80 3.14 6 43.21 8 7.25 0.7 1.766 26.71 3.14 7 46.67 9 8.25 0.7 1.766 28.49 3.14 8 49.89 10 9.25 0.7 1.766 30.17 3.14 9 52.92 11 10.25 0.7 1.766 31.76 3.14 10 55.78 12 11.25 0.7 1.766 33.27 3.14 11 58.50 13 12.25 0.7 1.766 34.72 3.14 12 61.10 14 13.25 0.7 1.766 36.11 3.14 13 63.60 15 14.25 0.7 1.766 37.45 3.14 14 66.00 16 15.25 0.7 1.766 38.74 3.14 15 68.32 17 16.25 0.7 1.766 39.99 3.14 16 70.56 18 17.25 0.7 1.766 41.20 3.14 17 72.73 19 18.25 0.7 1.766 42.38 3.14 18 74.84 20 19.25 0.7 1.766 43.53 3.14 19 76.89 21 20.25 0.7 1.766 44.64 3.14 20 78.88 22 21.25 0.7 1.766 45.73 3.14 21 80.83 23 22.25 0.7 1.766 46.79 3.14 22 82.73 24 23.25 0.7 1.766 47.83 3.14 23 84.59 25 24.25 0.7 1.766 48.85 3.14 24 86.41

30 29.25 0.7 1.766 53.65 3.14 29 94.99 35 34.25 0.7 1.766 58.06 3.14 34 102.85



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Volume of Silt Accumulation

Ten year silt volume is calculated in accordance with the City of El Paso ordinance, Title

19, “Subdivision Improvement Standards”. The area is mountainous, and the silt

volume is 0.003 ac-ft per acre of watershed area.

Recommendation

Results of the Hydrologic and Hydraulic studies lead us to the following

recommendations:

• A 35 ft dam should be built at the indicated location.

• An outlet tower should be provided at the toe of the upstream side of the dam

with a 24” concrete pipe thru the dam at an invert elevation of at least 2 ft above

the bottom of the dam.



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3 1 2

NORTHGATE DIVERSION DAM

Time

Elevation

Stage, ft

Storage, ac-ft

24” Pipe Outflow, cfs

0:00 4335 0 0 0

2 4336 11.6 12.1 58

4 4365 29.1 117.4 94

6 4366 30.2 126.7 96

8 4366 30.2 126.6 96

11 4365 29.4 119.9 94

13 4353 18.7 42.2 75

18 4340 5.3 1.6 37 20 4335 0.1 0 1

RE = Stage = Water Depth at Dam

TABLE 22-1



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3

2

1



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HYDRAULIC DESIGNS

1. Detention/Retention Basin (35 Ft High Dam) Plates 1 and 2

The proposed detention basin will be designed to impound the 100 year criteria storm

for an 805 acre watershed. As shown on the Northgate Diversion Table 22-1, Page 22

on the Outflow/Storage Curves on Page 23, calculations using the U.S.S.C.S.

hydrographs the maximum highwater mark (stage) will be 30.20 feet (4366 elevation)

from invert of dam with a maximum impounding (storage) of 126.70 acre feet and

maximum outflow of 96 cubic feet per second (cfs). The emergency spillway will be

designed to pass the 100 year criteria storm of 1400 cfs. As calculated by the

hydrograph shown on Page 21. The dam is an earthen dam with side slopes of one

vertical over three horizontal. A 20 feet wide road on top is provided for maintenance.

The intake for out-flow is placed 5 ft above the low point to allow the pond to store some

water before releasing it. The opening/outflow conduit is designed to allow the pond to

store water over a one week period and reduce the downstream flow. A spillway is

constructed for emergency flow if the water elevation in the pond rises above a level

providing freeboard to the dam. The outlet through the dam is a 24” concrete pipe

releasing water downstream. The road will be reconstructed to form an inverted

triangular cross-section capable of conveying the runoff overflow of the spillway.



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2. Rectangular Concrete Open Channel Plates 4 to 8 The second option to consider is a concrete open channel from the top of existing berm

to the bottom North of Hondo Pass. As shown on the hydrograph on Page 26, the

expected storm runoff is 1400 cfs as per the 100 year criteria storm and from the 802

acre watershed flows through the existing berms and into the existing Northgate Dam.

As shown on pages 28 and 29 the proposed rectangular channel will be Ten (10) feet

wide by five (5) feet high and will flow at approximately 2.50 feet in depth at the super

critical flow with a velocity of sixty (60) feet per second. In order to minimize the velocity

force and to slow down the flows three (3) energy decipitators were introduced. The

channel will be approximately 550 feet long. In order to minimize rocks and boulders

flowing down the channel and scouring and possibly chipping the concrete a desilting

basin will be constructed on the intake to catch all the boulders before entering the

channel.

A. Watershed Area……………………………………….. 805 AC

B. Runoff Coefficient……………………………………… 0.85

C. Peak Calculated Runoff (100yr)……………………… 1400 cfs

D. Bottom Width…………………………………………… 10 ft

E. Average Slope………………………………………….. 15%

F. Normal Depth…………………………………………… 2.30 ft

G. Velocity………………………………………………….. 60.23 ft/sec

H. Manning’s “n”…………………………………………… 0.013

Three energy dissipitators are proposed to alleviate the high velocity and to minimize

erosion. Manning’s formula was used to compute all flowing results.



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3. Closed Conduit – 2-66 Inch Reinforced Concrete Pipes Plates 9 thru 13

The third option that was considered is the placement of two (2) sixty six (66) inch

reinforced concrete pipes from the top of the existing berms to the bottom of Hondo

Pass Drive. The two pipes were be sized using the hydrograph on Page 26 that was

developed through the use of the United States Soil Conservation Surfaces

(U.S.S.C.S.) unit hydrograph, with an input of 3.80 inches in three (3) hours, 100 year

criteria storm and 802 acres of watershed area. As shown on Page 23 hydrograph, the

total calculated peak runoff is 1400 cfs and the two (2) sixty six (66) inch reinforced

concrete pipes were sized to handle the calculated peak runoff. The hydraulic

calculations for the two (2) sixty six (66) inch pipes are shown on pages 32, 33 and 34.

As shown on calculations sheet the expected velocity on the pipes carrying 1400 cfs at

a normal depth of 3.90 feet is thirty nine (39) feet per second which might be considered

slouring velocity. According to the pipe manufacture (Rinker Materials) the thirty nine

(39) feet per second velocity affects the durability of the pipe, what the effect the pipe is

the amount and size of mountain alluvian that may be carrying on the pipe. The alluvian

gravel may be scoured inside of the pipes. To minimize the amount and size of alluvian

gravel that may be flowing in the pipe, a desilting basin is recommended at the inlet

structure of the pipes. This will minimize scouring of the pipes. Also, in order to avoid

an erosion problem the outlet structure should be designed as an energy dissipitator.

A. Watershed Area……………………………………….. 805 AC

B. Runoff Coefficient……………………………………… 0.85

C. Peak Calculated Runoff (100yr)………………… 1400 cfs

D. Pipe Diameter………………………………………….. 5.50 FT



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E. Number of Pipes Proposed…..……………………….. 2.0

F. Design Slope……………………………………………. 6.0%

G. Velocity………………………………………………….. 38.87 FT/SEC

H. Manning’s “n”…………………………………………… 0.013

Manning’s formula was used to compute all flowing results.



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III. FIELD DOCUMENTATION

DESCRIPTION The study area is about 805 acres. The

entire area is rectangular in shape. The

western boundary is the crest of the

Franklin Mountains. The northern

boundary follows down alluvial slopes of

the Franklin Mountains and connects to

Hondo Pass Drive. The eastern boundary

is Galena Drive. The southerly boundary

is essentially Moonlight Avenue as it

conceptually extends to the crest of the

Franklin Mountains.

The topography ranges from the extremely steep

slopes of the Franklin Mountains through the

relatively moderate slopes of the alluvial terraces

and fans to the almost flat floor of the desert.

Vegetation is sparse desert type consisting of yucca,

grease wood, mesquite and desert weeds in the

lower and flat portions.

A large mass of stones and rocks has

accumulated at the eastern edge of the

channel (dirt arroyo).



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PHYSICAL CHARACTERISTICS Geologically the area is composed

of the Eastern slope of the Franklin

Mountains which is the escarpment

of a typical block fault. The

escarpment rises as much as 3000

feet above the lower portion of the

desert floor. The mountains

traversed by numerous cross faults,

along which some of the major

canyons have developed. The rocks

are principally sediments ranging from pre-Cambrian to Cretaceous, although there are

some igneous rocks ranging in age from pre-Cambrian to Tertiary.



37

SITE INVESTIGATION Site visits to the Northgate Diversion

Channel were conducted in December

2006. The objectives were to

investigate and verify the contributing

drainage areas, assess obstructions

within the waterways, document

structural distress to any structures and

determine the need for improvements

and construction.

Photo 2 – Erosion Photo 1 – Berm

Photo 3 – Erosion Photo 4 – Canyon Wall



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SOILS REPORT Pending (Appendix C)



RECOMMEN

DATIONS



39

IV. CONCLUSION AND RECOMMENDATIONS After a very thorough analysis and evaluation of the existing conditions at Northgate

Diversion Channel, it was concluded that:

A. Using two – 66 inch reinforced concrete pipe will be the best approach in

conveying the 100 year criteria storm flow from the slopes of the Franklin

Mountains down to the existing Northgate Dam Impounding Area.

Our recommendation is based on the following observations:

8. The most efficient and cleanest method of conveying storm runoff

9. The most approachable method of working on the steep slopes of existing arroyo

10. The least time to construct

11. The least work to maintain

12. The most environmental friendly since the storm pipes are cover in the arroyo

13. Elimination of existing arroyo

14. The most economical

B. Northgate Diversion Dam is our second recommendation. Our recommendation

is based on the following:

1. Will be able to detain all calculated maximum peak runoff.

2. Will have an uncontrolled outflow. Pipe capable of evacuating dam reservoir in

seventy two (72) hours.

3. Will require extensive excavation to create storage reservior and actual dam.

4. Difficult terrain to excavate- rock – mountain alluvian.

5. Diffucult site to approach by consruction equipment and contruction material.

6. More expensive to construct.



40

C. Concrete open channel is our third and last recommendation. Our

recommendation is based on the following:

1. High degree of diffuclty to construct form concrete (cast in place).

2. Extremely diffucult terrain to manuver with construction equipment.

3. The concrete open channek steepness may create a high safety risk due to its

high velocity.

4. Not environment friendly to have an exposed concrete open channel on the

slopes of the mountain.

5. More expensive to construct and maintain than closed conduits.



COST

ESTIMATES



41

V. CONSTRUCTION COST ESTIMATES Northgate Diversion Dam

March 21, 2007

COST ESTIMATE No.

DESCRIPTION QTY UNIT UNIT PRICE TOTAL

1. Clearing & Grading 15 A.C. 3,500.00 52,500.00

2. Grading temporary access roads 2300 F.T. 12.00 27,600.00

3. Excavation 90,680 L.Y. 9.00 816,120.00

4. Compacted dam embankment 58,807 C.Y. 10.50 617,474.00

5. Dam embankment loose rock 6450 TON 50.00 322,500.00

6. Dam embankment imperious soil material 17,559 C.Y. 30.00 526,770.00

7. Dam embankment 6 inch bedding material 1752 S.Y. 15.00 26,280.00

8. 24 inch reinforced concrete pipe – concrete encased 280 L.F. 300.00 84,000.00

9. Emergency concrete spillway 1100 C.Y. 450.00 495,000.00

10. Concrete outlet tower 1 E.A. 30,000 30,000.00

11. 46 inch pressured concrete manhole 1 E.A. 7,500.00 7,500.00

12. Gauge pole 1 E.A. 2,000.00 2,000.00

SUB TOTAL 3,007,744.00

Contingency (5%) 150,387.00

GRAND TOTAL 3,158,131.00



42

Northgate Diversion Channel March 21, 2007

COST ESTIMATE No.


1. Clearing & Grading 590 L.F. 25.00 14,750.00

2. Fill 1459 C.Y. 30.00 43,770.00

3. Cut 460 C.Y. 30.00 13,800.00

4. Concrete Channel 965 C.Y. 600.00 579,000.00

5. 2 – Headwalls 20 C.Y. 500.00 10,000.00

6. Safety Gate 1 E.A. 25,000.00 25,000.00

7. Temporary road grading 850 L.F. 25.00 21,250.00

8. Loose rock dissipitator 500 TON 50.00 25,000.00

SUB TOTAL 732,570.00


GRAND TOTAL 805,827.00



43

Northgate Diversion Pipe March 21, 2007

COST ESTIMATE No.


1. Clearing & Grading 590 L.F. 25.00 14,750.00

2. Fill 7498 C.Y. 30.00 224,940.00

3. Cut 2909 C.Y. 30.00 87,270.00

4. Bedding 805 C.Y. 15.00 12,075.00

5. Pipe – 66” 1086 L.F. 260.00 282,360.00

6. 2 – Headwalls 20 C.Y. 500.00 10,000.00

7. Safety Gate 1 E.A. 25,000.00 25,000.00

8. Temporary road grading 850 L.F. 15.00 12,750.00

SUB TOTAL 669,145.00


GRAND TOTAL 736,060.00



REFERENCES



44

V. REFERENCES

1. “Drainage & Flood Control Studies, Northeast Area of El Paso”, Lance

Engineers, Inc., June 1960, - for the City of El Paso, Texas.

2. “Preliminary Engineering Study for Northeast Drainage Improvements”, Parkhill,

Smith and Cooper, Inc., March 1978, - for the City of El Paso, Texas

3. Texas Water Code – Subsection 26.027, Texas Natural Resource Conservation

Commission.

4. Title 19 – Subdivision Ordinance: Subdivision Improvement Design Standards.

Retention Basin Design.

5. Storm Water Management Ordinance: Ordinance No. 013477, Revision 1-27-98.



HEC-1

PRINTOUTS



45

APPENDICES

Appendix A – Computed Hydrograph HEC-1

Appendix B – Plates

1. Dam (Plates 1,2,3) 2. Closed Conduit (Plates 4,5,6) 3. Open Channel (Plates 7,8,9) 4. Channel Cross Section (Plates 10,11) 5. Closed Conduit Cross Section (Plates 12,13) 6. Open Channel Dissipator (Plate 14)

Appendix C – Soil Report



PLATES

E A

R T

H E

N B

E R

M

HONDO PASS

RA

NC

ICH

GA

LEN

A

BE

LK

MA

GN

ETI

C D

R.

SANDS

VIR

GO

SWANSON

GABEL

CRAIGO

MOONLIGHT

MAR

BLE

GA

LEN

A

PROP. CHANNEL

PR

OP

. DA

M

PIPE

OUTLET PIPE

WA

TER

SH

ED

805

AC

RE

S

1 4

0 0

C F

S(E

XIST

ING

)

TYPICAL CHANNEL PLAN

Documents

NORTHGATE CHANNEL REPORT final II 5 18 07