RECEIVED STORMWATER DIVISION

FEB 1 4 2001 ENGINEERING REPORT

o n '

DRAINAGE CANAL CONNECTING

MYAKKA RIVER AND ROBERTS BAY

SARASOTA COUNTY, FLORIDA

for

A L B E R T E . BLACKBURN

Venice , F l o r i d a

M a r c h , 1959

. -

I' DE LEUW, CATHER & BRILL

E n g i n e e r s -Arch i t ec t s

New York, N. Y. Sanford , F la .

Brookl ine , Mass . Buffalo. N. Y .

APPROVED

b Leuw, Cather & c rill'

F J. Elmiger zzazz De Leuw, Cather & Bri l l

~ c d k p W . C . B l i s s . Jr.

PROJECT ENGINEER De Leuw. Cather & Bri l l

( 7 6 - d R. E. Landau

SOILS ENGINEER De Leuw, Cather & Bri l l

DE L E U W , C A T H E R & BRILL . E N G I N E E R S - A R C H I T E C T S 2 0 2 E A S T 4 4 t h S T R E E T . N E W Y O R K 1 7 , N . Y . . M U R R A Y H I L L 2 - 3 9 5 0 CMARltS t . D t L t U W L t ROY U. CATWEI C L l H l O N 8. f. BRII1 b t N J * M I N GRAY

ASSOCIAltS [ D C A I F. COPELL ROBERT I. RICHARDS March 18, 1959

Mr. Albert E. Blackburn P o s t Office Box 1448 Venice, Flor ida . .

RE: Blackburn Canal

Dear Mr. Blackburn:

We have completed our studies on the feasibility of constructing a canal connecting the Myakka River and Roberts Bay a t Venice, Flor ida and the benefits to be derived therefrom, and a r e presenting herewith our engineering repor t prepared in accordance with the t e r m s of our agreement dated Feb rua ry 4, 1959.

The conclusions and recommendations contained in this r epo r t a r e based on a careful analysis of rainfall and s t ream gaging records for the Myakka River Basin, actual determination of high water elevations in the immediate vicinity of the proposed canal and a t State Road 72, channel c r o s s -sections of the r iver above and below the canal, and a field survey along the route of the partially-completed canal.

As a r e su l t of the studies we have made and summarized in this report , it i s our conclusion that construction of the canal i s pract ical f rom an engineering standpoint, that definite relief from periodic flooding by the Myakka River will be obtained provided the additional construction and ex- cavation recommended herein i s performed and that there will be no adverse effects to the natural resources of the Myakka River o r Roberts Bay.

We appreciate the opportunity of undertaking this assignment for yau and look forward to ass is t ing you in the further development of this project.

n

TABLE O F CONTENTS

L E T T E R O F TRANSMITTAL

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

A . Purpose of Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 B . Descr ip t ion of A r e a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

C . Descr ip t ion of Exist ing Drainage Conditions . . . . . . . . . . . . 4 D . Scope of Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

ENGINEERING STUDIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 A . Geology and Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 B . B a s i c Invest igat ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0 C . Watershed A r e a and Runoff . . . . . . . . . . . . . . . . . . . . . . . 1 1 D . Design Flow i n the Myakka River . . . . . . . . . . . . . . . . . . . . 1 3

E . Canal Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONCLUSIONS 1 7

A . Hydraulic Effect of the Canal . . . . . . . . . . . . . . . . . . . 1 7 B . Cons t ruc t ion Work Remaining . . . . . . . . . . . . . . . . . . . . . 1 9 C . Controvers ia l F e a t u r e s . . . . . . . . . . . . . . . . . . . . . . . . . . 21

D . Benefi ts to the A r e a . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RECOMMENDATIONS 2 4

TABLE O F EXHIBITS

EXHIBIT I Drainage Bas ins . Myakka and P e a c e R i v e r s

EXHIBIT I1 Canal Location Map

EXHIBIT I11 Canal P l a n and Prof i l e s

EXHIBIT IV C r o s s Sections of Myakka River

EXHIBIT V Geologic Soils . Format ions

EXHIBIT VI P l a n of Sheet Pi l ing Bulkheads

EXHIBIT VII Deta i l s of Sheet Piling Bulkheads

INTRODUCTION

A . Purpose,of Study

The purpose of this study has been to investigate the Myakka River

drainage basin and i t s history so a s to determine whether the projected

canal will offer any re l ief f rom flooding conditions, what additional con-

struction in or adjacent to the canal might be needed to make it effective

and practical , and, if such a canal i s feasible, what i t s effect would be on

adjacent land a r e a s , including the benefits which might accrue to the s u r -

rounding te r r i to ry .

B . Description of Area

The Myakka River , fed by i t s t r ibutary s t r e a m s , flows in a south-

wester ly direction from headwaters in eas te rn Manatee County to a point

six miles nor theast of Venice. F r o m that point the r iver bends to the e a s t

and continues on in a southeasterly direction until i t empties into Charlotte

Harbor below the mouth of the Peace River a t Punta Gorda. The main strand

i s approximately 50 miles in length from head to mouth and drains an a r e a

of about 450 square miles in Manatee, Hardee, Sarasota and Charlotte

Counties. The combined Myakka and Peace Rivers a r e considered by the

U. S. Ge ological Survey to be one of the major drainage basins of Flor ida,

having a total a r e a of approximately 2500 square miles (Exhibit I ) .

The Myakka i s typical of the many low-gradient coastal s t r eams of

Florida. The mouth i s long, wide and shallow extending 12 miles inland

f rom Charlotte Harbor to a point north of U. S. 41 (Tamiami T r a i l ) , and

has ill-defined, marshy banks with the usual heavy growth of Mangrove

and marsh g ra s s . Beyond that point, the r iver i s meandering and slow,

comparatively narrow, and has well-defined channel and banks. Through

i t s lower reaches the r iver bottom is several feet deeper than a t i t s mouth

due to scouring action during periods of high run-off. The r iver bottom

opposite the north half of the Blackburn proper ty in Township 39 South,

Range 20 Eas t has been found to consist of hard rock, a factor which will

be discussed in more detail in a la ter section of this report .

Far ther to the north and lying wholly within the l imits of the Myakka

River State P a r k on ei ther side of State Road 72 connecting Sarasota and

Arcadia, the r iver widens out to form two small lakes. Lower Myakka

Lake, one mile south of the highway, i s approximately 213 of a square mile

in a r e a with an average depth of three to four feet. A shallow ea r th dam has

been constructed ac ros s i ts lower end to re ta in the r iver waters during ex -

t remely d ry periods but this dam i s a rb i t ra r i ly broken and kept open mos t

of the time to permit r iver flow. Jus t south of the dam and adjoining the

r iver i s a deep hole approximately 150 feet ac ros s and said to be 130 feet

deep, apparently formed in the same manner a s the two sal t springs in south-

eas te rn Sarasota County descr ibed in Bulletin 27 of the Flor ida Geological

Survey. There i s known to be a smal l flow from this hole a t t imes when the

r iver level i s below i ts r i m . Upper Myakka Lake i s 2 112 mi les north of

the highway and has an a rea of approximately 1112 square miles . It i s

i r regular in shape with a considerably greater storage capacity than the

lower lake, having an average depth of approximately 5 I/Z feet . The

lower end of the lake has a permanent concrete dam and spillway to control

the lake elevation to a minimum of 13.65 compared to the approximate e l e -

vation of 1C. OC. in the lower lake.

C. Description of Existing Drainage Conditions

As will be pointed out l a t e r , the flat t e r ra in and low porosity soils of

the Myakka basin lead quickly to surface flooding during periods of high

rainfall . This i s complicated in the a r ea between the Blackburn proper ty

and the Myakka Lakes by the fact that there a r e no well-defined drainage

courses . During periods of extended rainfall, such a s often accompany

hur r icanes , the already-taxed capacity of the Myakka i s soon exceeded when

i t receives the sheet of overland runoff from this region, leading to general

flooding of the lower middle r iver 'basin . This condition can be attr ibuted

further to the fact that the Myakka i s tidal in d ry weather a lmos t to the

north l imits of the Blackburn property and that f rom i t s nar rowes t c r o s s -

section opposite the center of the Blackburn property, there i s a r is ing

bot tom gradient to i ts mouth. This serves a s a natural inland bottleneck

to throttle the flow in the r i ve r .

Local reports indicate that there i s usually some flooding a t l eas t

once each year and that occasionally i t will occur two or three t imes in a

season. It i s further reported that rapid r i s e of the r iver to i t s flood peak

and re turn generally takes place within a 24- to 48-hour period. However,

full recession of the r iver to i ts normal level i s accomplished over a much

longer period and drying out of the flooded lands adjacent to i t may not take

place in less than two to three months, with consequent heavy loss to property

owners in use and productivity of the land. Apparently, a cer ta in amount

of runoff from Cow Pen Slough to the West reached the Myakka by overland

flow in ea r l i e r years since recent high water marks established by the Myakka

have not been a s high a s those recorded before construction of the canal

connecting the Cow Pen with Shakett Creek and Roberts Bay.

The Blackburn property e a s t of the Myakka River and occupying a l l of

Township 39 South, Range 20 Eas t north of Tamiami Tra i l has borne the

brunt of much of the frequent flooding. The land i s used pr imar i ly for

grazing and has been grubbed and planted with Pensacola Bahia g r a s s in

several a r eas . In an attempt to divert some of the overland flow from the

north, a shallow ditch and dike have been constructed along the western

two-thirds of the north property line. These serve the purpose until such

t ime a s the r iver floods and the dike height i s exceeded by flood waters .

Of course , there i s always a certain amount of d i rec t flooding f rom the

r iver when it exceeds i t s banks adjacent to the property.

To relieve the usual periodic flooding, Mr . Blackburn conceived the

idea and secured the rights-of-way for a canal connecting the Myakka River

and Roberts Bay along the route of an old drainage ditch that emptied into

Cur ry Creek and thence into the Bay. A substantial portion of the con-

struction of this canal was accomplished before the question a rose a s to

the actual resu l t s to be obtained from such a canal and the effects i t would

have on the surrounding a rea .

D . Scope of Investigations

Inasmuch a s there i s no local authority having d i rec t regulatory powers

over drainage construction in Sarasota County, a meeting of a l l in terested

local, state and federal officials was convened in Venice on January 7 , 1959

ta discuss the mat ter in more detail. It was determined a t that t ime that ,

p r ior to the completion of the canal, a comprehensive investigation and

repor t on the Myakka River problem should be made by a qualified f i rm of

engineering consultants. Subsequent to the meeting, Mr. Blackburn retained-

this f i r m to make such a study.

As a f i r s t s tep in evaluation of the problems involved, a field in-

spection of the site was made by engineers of this f i rm for familiarization

and orientation purposes (Exhibit 11). This included an inspection of the

canal route from i t s junction with the Myakka River to i t s ending a t Roberts

Bay. In addition, the Blackburn property e a s t of the r iver was inspected

and visual observations of the Myakka River a t several points bebveen U. S.

41 and State Road 72 were made.

Inquiries of people living or working along the r iver were made to

determine observed past flood conditions and the extent andduration of

severe floods and runoff. F r o m these inquiries, i t was possible to obtain

a general picture of the peak flood character is t ics of the r iver a t severa l

locations.

Field surveys were then made along the route of the canal and a t

several locations in the Myakka basin to obtain information needed in the

engineering analysis. A topographical survey of the canal was made between

the Venice By-Way and the Myakka River and cross-sect ions of the canal

were taken a t intervals of 500 feet. The elevations and location of a l l roads ,

br idges , ditches, and s t reams intersecting o r intercepted by the canal were

obtained (Exhibit 111). Prof i les a c r o s s the Myakka River drainage basin

f rom the western to eastern l imits were taken a t points 314-mile north of

the canal and 3 112 miles south of the canal. Cross-sect ions of the Myakka

River were obtained a t these two locations, a t the canal i tself , a t a point

one mile south of the canal, and a t a point ten mi les north a t State Road 72.

Soundings were taken in the r iver above and below the canal and in both the

Lower and Upper Myakka Lakes. Finally, high water elevations were

established a t the northern l imits of the Blackburn property and a t State

Road 72 (Exhibit IV).

Extensive r e sea rch into hydrological and geological data gathered by

various local , state, and governmental agencies was made to secure r e -

maining information needed. This included s t r eam gaging records for the

Myakka River compiled by the U. S. Geological Survey, rainfall records

of the U. S. Weather Bureau, geological data prepared by the Flor ida

Geological Survey, tide tables published by the U. S. Coast and Geodetic

Survey, and various other repor t s and bulletins including the very help-

ful r epo r t on Flor idh 's Water Resources prepare2 in 195i by the Fior ida

Water Resources Study Commission.

ENGINEERING STUDIES

The engineering studies undertaken during the preparation of this

repor t include a survey of the geology of the a r e a a s re la ted to svrface run-

off, basic investigations to determine alternate drainage schemes and,

finally, the design considerations for the recommended solution.

A . Geology and Soils

The soils in Sarasota County have recently been mapped by the

University of Flor ida Agricultural Experiment Station in cooperation with

the U. S. Department of Agriculture but the published repor t on this study

i s not yet available. Although there i s some disagreement among geolo-

gists a s to the classification of formations underlying the Pamlico sands,

our own r e sea rch indicates that in the vicinity of the Blackburn Canal the

general description i s a s shown in Exhibit V.

Geology for the a r ea is covered in some detail in Bulletins 27 and 29

of the F lor ida Geological Survey. Briefly, i t might be stated that the

Flor ida peninsula i s made up entirely of sedimentary deposits of a marine

o r estuarine nature laid up over the geologic ages by cyclical changes in

the elevation of the sea coupled, in la ter s tages , with some folding o r

tilting of the ea r th ' s surface. Surficial deposits (Exhibit V) in the lower

portion of the Myakka basin a r e generally Pamlico quartz sands of the late

Pleistocene Ser ies merging into Talbot sands of an ea r l i e r stage in the

upper reaches of the r i ve r . Between t h e coast and the r iver the sands

overlie an ea r l i e r Pleistocene deposit of shell m a r l f rom the Anastasia

formation possibly intermingled with deposits of the much older

Caloosahatchee Marl f rom the Pliocene s e r i e s known to ex is t ea s t of the

r iver . The central portion of the basin i s underlain by an exposed portion

of the Hawthorne formation of the Miocene Ser ies , a sandy phosphatic l ime-

stone, which i s only exceeded in geologic age in Flor ida by the Tampa and

Ocala limestone deposits. It i s believed that the rock bottom in the Myakka

mentioned ear l ie r i s a hard lens of this formation. The headwaters of the

Myakka, north of the lakes, drain a portion of a large a r e a of phosphatic

sand, clay and gravel attributed to the Bone Valley formation of the late

Pliocene Ser ies .

The t e r r a in in that portion of Sarasota County t raversed by the Mye.kka

does not lend itself readily to proper drainage. The land i s f lat to gently

sloping, extensively spotted with shallow depressions, ponds and sloughs

which quickly f i l l with water during heavy ra ins due to the lour porosity of

the underlying shell m a r l s , and i s variously overgrown with palmetto, pine,

cypress and oak except for those a r e a s west of the Myakka which have been

cleared for farming. There are a few s t r e a m s , pseudostreams and ditches

draining the southeastern p a r t of the county into the r iver in the vicinity

of the Tamiami Tra i l but well-defined drainage from eas t of Venice to the

Myakka Lakes i s non-existent. When a l l the ponds and depressions become

full, a r e a drainage i s achieved by overland flow. The wes te rn boundary of

the Myakka basin i s a line roughly one to two miles from and paralleling the

r iver f rom i t s mouth to State Road 72. F r o m that point, it f l a r e s 3 o r 4 more

miles away from the r iver to include the t r ibu ta ry s t reams draining the up-

Lands of eas te rn S l r a so t a and Manatee Counties. The eas te rn boundary i s

not quite so definite but generally spli ts the plateau between the Myakka and

Peace Rivers . North of State Road 72 it closely follows the e a s t Manatee

County line.

B. Basic Investigations

On f i r s t examination, two al ternates to the Blackburn Canal for r e -

lieving flood conditions on the Myakka River offer t hemselves.

F i r s t would be the general straightening and deepening of the Myakka

f rom i ts mouth to Lower Myakka Lake, and second would be the excavation

of a canal linking the Myakka and Cow Pen Slough northeast of Venice with

Shakett Creek and Dona Bay.

The f i r s t of these, though a good solution, has been discarded be-

cause of i t s obviously prohibitive cost , part icularly through sections where

rock would be encountered, and also because of the extensive damage which

would resu l t to the natural beauty of the r iver .

The second alternate i s a desirable solution, but would resu l t in high

maintenance costs because control s t ructures would be necessary to p r e -

vent overdrainage. The Blackburn Canal offers the possibility of flood

relief without expensive control s t ructures .

A11 information gathered ior this study indicates that the basic

problem to be considered a t this t ime i s the positive relief of excess run-

off into the Myakka River during periods of extreme rainfall. At some

future date the conservation of this runoff for human consumption by uti-

lizing the Myakka Lakes and the a r e a between them for reservoi r storage ma)

become necessary , thus adding greatly to the facilities for flood control.

Such a possibility, however, has not been taken into consideration in the

following hydraulic analysis.

In determining the amount of relief to be provided, i t has been con-

cluded that the maximum capacity of the existing Myakka Channel opposite

the Blackburn property should be utilized so a s to keep the canal construc-

tion costs to a minimum. This may mean some spot flooding of low bank

a r e a s along the r iver and a t t r ibutary entrances .

C. Watershed Area and Runoff

Stream gaging infcrmation has been obtained for many years a t a

station located just below the Upper Myakka Lake. This station records the

flow for a 235-square-mile section of the upper Myakka watershed. A

maximum flow of 6 , 6 2 0 c . f . s . was recorded in 1947 and the minimum and

average flow for the past 20 yea r s a r e 0 and 244 c.f . s . , respectively.

In comparison, the maximum flow in 1956 was 826 c . f . S.

The additional watershed a rea between the entrance to the Blackburn

Canal and the gaging station amounts to approximately 65 square miles for

a total watershed of 300 square miles. The equivalent discharge a t the

canal would then be 8,500 c. f . s. F o r design purposes, this has been

a rb i t ra r i ly reduced to 8,000 c. f . s. , taking into consideration the storage

capacity of Lower Myakka Lake and i t s probable leveling effect on the flow.

An even g rea t e r reduction could probably be expected but this was not con-

s idered advisable. In design of the canal proper , an additional 250 c. f . s .

has been anticipated from a r e a s adjoining the canal.

As a further check on the above, the following analysis of rainfall

records was made:

According to the Rainfall Intensity-Duration-Frequency Curves for

Tampa, F lor ida (U. S. Department of Commerce, Weather Bureau, Techni

cal Paper NO. 25), the maximum precipitation in 24 hours for a 25-year

s torm would be 0. 31 24 = 7 . 5 in . Using a runoff coefficient of n = 0.1 the

total runoff to reach the r iver a t the end of 24 hours would be

Although greater intensities of rainfall occur during shor t intervals

of t ime, the maximum intensity i s taken a s based upon a 24-hour s torm

duration, in o rde r to allow for t ime of concentration a t the r iver location.

Carrying this one step fur ther , the selected design discharge of 8,500 c . f . s .

is roughly equivalent to tha t which might be expected once in 50 y e a r s .

D.

Cross - sec t ions of the Myakka River channel w e r e taken a t five

locations (Exhibit IV) . At C r o s s i n g No. 1 and C r o s s i n g No. 2, high wa te r

elevations w e r e es tabl ished. Assuming a n ac tual channel length of 18 m i l e s

for the 9 m i l e s between these two points , t h e r e i s a bottom gradient of

Assuming that r ecorded high w a t e r s fo r these two points occur a t the

same t i m e , the wa te r su r face gradient for th is r e a c h would be

18.7 - 11. 6 = 16 = 0. 39 ft. / m i l e

The probabil i ty tha t such a simultaneous peak would o c c u r , however , s e e m s

unlikely.

The following ana lys i s of the Myakka channel a t each of the c ross ings

indicates the ac tual hydraul ic r e q u i r e m e n t s in o r d e r for the channel to handle

the des ign d i scharge .

(1) State Road 72 - A t high wa te r elevation of 18.7, the channel a r e a i s

2,100 sq. f t . A t the expected d i scharge of 6 ,620 c . f . s . the velocity would

be 3.13 f t . I sec .

(2 ) Blackburn Bridge (North p roper ty l ine) - A t an elevation of 10.0 (1. 6 f ee t

below known high wa te r l ine) when the r i v e r s t a r t s to leave i t s banks , the

channel c r o s s - s e c t i o n i s 2,100 sq. f t . F o r a d i scharge of 8 , 0 0 0 c . f . s . the

a v e r a g e velocity would be 3 .75 ft . / s e c . The requ i red energy gradient would

/ o r 1. 2 f t . /mi .

F o r uniform flow between the above two points , a s suming a constant channel

c r o s s -sec t ion and a n average discharge Q = 7 , 310 C. f . S. , the s lcpe of :he

r e q u i r e d e n e r g y gradient is

=( 7,310 (0.03) x 5 ,280 - 1.2 ft . / m i . 1.486 x 2,100 x

F r o m a compar ison of the computed bottom gradient and the r e q u i r e d

e n e r g y gradient , it i s apparent that a backwater condition e x i s t s f r o m the

Blackburn p roper ty nor th to Lower Myakka Lake.

The following s u m m a r i z e s the hydraulic conditions a t tlie canal mouth

and below

TABLE I

C r o s s i n g Bottom Assumed A r e a Q V S E l . Water El . Sq .F t . c . f . s . ft . / s e c . f t . / m i

(3) Blackburn Canal -6.7 8 . 0 1,400 8 ,000 5 .7 3. 76

(4) Venice Road -7 .0 8 . 0 1, 370 8 , 0 0 0 5 .7 3.76

(5) 4 Miles South of (3) -3. 0 3 .0 2 , 9 2 0 8 , 0 0 0 2 . 8 1. 6

I t i s evident that definite thrott l ing of the Myakka flow o c c u r s between the

Blackburn Canal and Venice Road. The elevation of 8 .00 a t the Blackburn

Canal i s the elevation which would be necessary to prevent flooding of the

adjacent land. Crossing No. ( 5 ) shows a definite change in flow conditions.

The r iver bed here widens out considerably and the water level i s not in-

fluenced so much by flood wa te r s and high discharge a s i t i s by tide action

from Charlotte Harbor. In o rde r to handle the design discharge a t normal

high tide conditions with the required energy gradient, the water elevation

a t this point should not exceed 4. 5 ft.

E . Canal Design

In order for the r i ve r to stay within i t s banks a t the entrance to the

Blackburn Canal, the known high water level mus t be reduced approximately

three feet. The proposed canal i s a t the las t strategic point for offering

this relief .

On the assumption that the r iver elevation should be reduced 3 feet , i t

was decided that a canal cross-sect ional a r e a should be selected that would

equal, a s near ly a s possible, the reduction in cross-sect ional a r e a of the

r iver without requiring any more additional canal excavation than necessary .

At the mouth of the cana l , such a r i ve r a r ea would be approximately 150 x 3 =

450 sq. f t . Limiting the canal entrance velocity to 1.75 it. Isec . indicates a

canal design discharge. Q = 800 c . f . s . Although this i s only 10% of the

maximum r iver discharge, i t will be an extremely important factor in

lowering the r iver level a t peak discharge. As will be shown l a t e r , the in-

c rease in discharge of the canal as the water level r i s e s i s a lmost in p ro -

portion to the third power of the depth.

F r o m the field survey notes a plan and profile for the six-mile length

of the canal were plotted (Exhibit 111). Canal c ross -sec t ions were plotted

and studied for the mos t economical section meeting the above c r i te r ia .

These sections were la ter used for computing final earthwork quantities.

The controlling water level a t Roberts Bay was taken to be 1 .6 , the

expected high tide. The effects of high flood o r hurr icane t ides were not

considered, a s this condition cannot be provided for in a feasible program.

The channel section a t the lower end i s controlled by the existing Venice

By-Way br idge, a p re -cas t reinforced concrete s t ructure of @-foot span,

having a p re -cas t pile center pier. By t r i a l , a section of 45'feet bottom

width, 2 on 1 side slopes, and a depth of 7 feet was selected a s being sa t i s -

factory. With a design discharge, Q, a t this point of 800 + 250 = 1,050 c. f . s .

the energy gradient by the Manning Formula , using n = 0.030 f o r large

canals with straight banks in fair condition, i s S : 0. 00025. The average

velocity would be V = 2. 50 ft. I sec . This would be within the range of

2.00 - 3.00 f t . I sec . established a s a permissable , non-scouring canal

velocity in non-colloidal, silty or sandy loams by the Special Committee

on Irrigation Research, Transactions, American Society of Civil Engineers,

1926. The shell m a r l generally encountered along the reach of the canal

could probably be compared to non-colloidal graded, loam to cobbles with

a maximum permissable velocity of 5. 00 ft. Isec .

The natural ground s t a r t s r is ing away f rom the Cur ry Creek flood

plain approximately one mile e a s t of the Venice By-Way. A t this point the

canal bottom width i s reduced from 45 feet to 30 feet so a s to achieve a

hydraulically more efficient section and also so a s to approximate the a l -

ready completed excavation. T r i a l solutions further indicate that such a

section approximates the required a r e a a t the Myakka River and gives the

desired gradient.

With the bottom elevation of the canal established a t each end, a grade

line was drawn, following closely the completed portion of the canal, and

computations for the backwater curve were made and plotted (Table 11)

1 486 Manning's formula Q = A x - n R ~ / ~ s"' transposed in the form a = ; D 6 / 3 s1I2 for trapezoidal open channels (King, Handbook of

Hydraulics) was used throughout. In the la t ter formula, K , taken from

D tables in King's handbook, i s a factor depending upon the - ra t io , where

b

D i s the depth of water and b the bottom width of the channel.

CONCLUSIONS

A . Hydraulic Effect of the Canal

The highest effect of the canal will be produced during the period when

the water level is r is ing and the slope consequently increases . It can be

assumed that during t imes of average flow in the r i ve r

244 x 300 = 2 3 5 : 312 c.f. s.,

the corresponding water level will be approximately,3.00 and the canal

TABLE I1

K D813 : S 0 - - 112. an - : n IS ASSUMED 0 . 0 3 0 : Q IS IN C.F S. n K O

. TAKEN FROM TABLES. HORACE W. KING HANDBOOK OF HYDRAULICS

BACKWATER CURVE COMPUTATIONS

DE LEUW. CATHER 6 BRILL

ENGINEERS - ARCHITECTS

discharge will not exceed about 80 c.f . a. At an elevation of 2.00 or l e s s

there will be no significant flow except that caused by tidal fluctuations.

As flood stage approaches and the elevation of 5. 00 i s reached, the d i s -

charge of the canal r i s e s quickly, almost direct ly in proportion to the

third power of the depth. When the r iver elevation reaches 8. 00 and the

canal discharge i s EOO c.f. s . . a certain balance should take place between

the approaching m a s s e s of flood water and the total discharges of the canal

and r iver .

Referr ing to Table11 the entrance velocity a t design discharge i s

1.9 ft. Isec . and the maximum velocity i n the canal i s 2 .8 ft. I s ec . Even

i f the r i ve r elevation should r i s e to 9. 00 with a resul tant canal discharge

of 1300 c.f. s . , the entrance velocity would only r e a c h 2 . 6 f t . I s e c . ,

offering no threat of dangerous erosion. The event of any extended discharge

at the la t ter r a t e s eems unlikely.

The canal discharge will empty into Roberts Bay approximately one-

half mile ea s t of the U. S. 41 bridge. Velocities which would cause damage

a r e not anticipated here under any conditions and, a s reported ea r l i e r f rom

local observations along the r i v e r , the peak discharge period should not

exceed 24 to 48 hours.

B. Construction Work Remaining

Between the Venice By-Way and Roberts Bay no excavation work has

been done. To ensure unimpeded flow for the l a s t one-third mile to the bay,

the existing Curry Creek channel should be widened and straightened to a

minimum bottom width cf 45 feet a t Venice By-Way and f lared gradually to

60 feet a t the Bay.

F r o m Venice By-Way to Station 50f00 the present approximate

bottom width of 30 feet should be widened to 45 feet to the profile shown

on Exhibit IV and the channel bottom cleaned out to the proper elevation.

F r o m Station 5 0 t 0 0 to Station 140+00 there i s some bottom excavation and

side shaping to be done. Between Station 140f 00 and Station 235s 00, where

excavation i s under way, there i s a considerable amount of work to be

done. Sxcavation i s complete f rom the la t ter point to the Myakka River.

There i s no need for concern about the over-excavation which has occurred

in some s t re tches a s gradual silting can be expected to fill in such holes

Following i s a tabulation of excavation work remaining:

TABLE 111

Station

Roberts Bay to 0+ 00

0 + 00 to 5W00

50 + 00 to121)rOO

12C+00 to 14Gs00

14C+00 to 21Gt00

210- 00 to 233t00 *

Amount, C. Y.

Not determined

* Havana Road ~ - -

TOTAL 249,300

The approximate amount of mater ia l a l ready excavated has been computed

to be 412,000 C. Y. This does not take into consideration the creek o r

ditch section already existing when excavation began and, therefore , i s

undoubtedly on the high side.

To protect and control the size of the canal entrance a t the Myakka

River , sheet piling should te installed on each side of the entrance a s

shown on Exhibit VI. An alternate design for e i ther steel or p re s t r e s sed

concrete sheeting is shown on Exhibit VII. F r o m knowledge of the under-

lying rock, it i s doubtful whether concrete piling could be used without p r e -

punching. Such an option could, however, be offered for bidding.

To prevent overdrainage of the a r e a s adjacent to the canal and to .

eliminate the probability of erosion a t low canal s tages , culver ts and flumes

of adequate size should be installed a t each ditch o r s t ream emptying into

the canal.

C . Controversial Fea tures

The question of diversion of f r e s h water to Roberts Bay has been

touched on briefly in an ea r l i e r section. I t was shown a t that t ime that the

velocity and amount of water to be diverted would peak over a relatively short

period of t ime. Inasmuch a s the average Myakka flow sends a reIatively

insignificant amount of water through the canal, the peaks which occur

once, and a t the mos t , hvo o r three t imes a year would not adversely affect

the Myakka River and Roberts Bay.

As previously indicated under the heading "Description of Area", the

Myakka River may run d ry during an average period of two months in a year.

Depending on the level of the t ides, water from the Gulf of Mexico will back

up into the r i ve r bed to a point which r a r e ly reaches a s f a r north a s the

location of the proposed canal. Surface runoff water which remains in the

r iver i s actually ponded by the denser salt water , with the resu l t that the

ent i re r iver becomes subject to tidal action. Consequently, the water in the

proposed canal would seldom be salty. In o rde r to insure the continued

satisfactory pe r fo rmance of potable well water supplies adjacent to the

canal, an investigation was made to determine the possibility of sa l t water

intrusion under the deliberately adverse assumption that the canal will

contain salt water over an extended period of t ime. A review of the existing

wells in the a r e a indicates that the preponderance of wells tap an a r t e s i an

aquifer located a t a depth in excess of 300 feet below ground. F r o m the

geology of the a r e a , i t i s evident that water in this aquifer i s in no way

rela ted to the surface water table, nor could there be any physical con-

nection by the very nature of the supply being ar tes ian. Thus, if the

ent i re sur face of the a r e a were flooded with sal t water , the quality of water

obtained from such a r t e s i an wel ls would in no way be affected. A r e l a -

tively smal l number of i r r igat ion wells tap an aquifer within the compara-

tively shallow phosphatic l imestone formation. Although this water i s non-

ar tes ian, the surface 'lAquaclude" layer , which i s impervious to water , p r e -

vents penetration of surface flows to the lower s t ra ta .

It may be stated therefore that there i s every reason to expect the

existing wells to function a s satisfactorily after the completion of the

canal a s they do a t the present t ime .

D. Benefits to the Area

It would be impossible to a s s e s s the monetary value accruing to the

south central portion of Sarasota County from the elimination of periodic

flooding along the Myakka River. A summation of the proper ty l o s s e s

resulting over the past decade would only present a smal l p a r t of the

picture a s many thousands of a c r e s have never been productively developed

to their full usefulness because of the lack of drainage and the th rea t of

annual overflow. Many hundreds of a c r e s of c rops have been lost in the pas t ,

due, i f not directly to flooding, to the poor runoff induced by the flooded

lowlands. With the completion of the Blackburn Canal and the construction

of a systematic pattern of drainage ditches in presently undrained sections,

a whole new a r e a will be available for development and existing developed

a r e a s can be better utilized. This will profit not only the proper ty owners

r ipar ian to the canal and r iver but the present and future populations of

Sarasota County a s wel l .

The canal will a l so open an exciting new avenue of in te res t to boatmen

along the Gulf Coast. With completion of an intercoastal waterway con-

necting Sarasota and Venice with Lemon Bay, a varied course of sal t water

fishing and cruising can be exchanged in a mat ter of minutes for a t r i p up

an extremely picturesque f r e sh water s t ream, and navigational potentiali-

t i es on the Myakka River will be improved with a corresponding increase

in property values.

An indirect benefit in constructlng the canal i s i t s usefulness a s p a r t

of a network in the possible future development of the r iver a s a reservoi r

for water from surface runoff. Being derived from rainfall , the surface

runoff water found in the Myakka River has a low minera l content. In-

stead of permitt ing such water to flow freely into the Gulf of Mexico, it

can be ponded by installing removable tide gates in the r i ve r just south of

the proposed canal and in the canal i tself , and providing suitable additional

r e se rvo i r capacity for control of overal l r iver discharge. Wlth a minimum

of controls, such a system will be able to handle the excess water available

during period of peak flow to prevent flooding of adjacent a r e a s . Because

of the large volumetric capacity of the r iver channel, and i t s associated

lakes and canals, i t s development a s a source of potable water will be of

future importance to the a r ea .

RECOMMENDATIONS

On the basis of the studies and conclusions reached in this repor t , it

i s recommended that:

1. The Blackburn Canal be constructed along the alignment and to the

profile and cross-sect ions shown on Exhibits I11 and IV.

2 . The Cur ry Creek channel between the Venice By-Way and Roberts

Bay should be widened and straightened to a bottom width of 45 feet a t

Venice By-Way and gradually flared to a width of 60 feet a t Roberts Bay.

3. Culverts of adequate size, capped with compacted ea r th backfill,

and concrete flumes o r spillways of proper shape extending to mean low

water level in the canal, should be installed a t each ditch o r s t r eam

emptying into the canal.

4. P r e s t r e s s e d concrete o r steel sheet piling a s shown on Exhibits

1'1 and VII should be installed a t the entrance to the canal f rom the Myakka

River.

5. Reinforced concrete bridges, similar to that a t Auburn Road, should

be constructed over the canal a t Havana and Jackson Road.

DRAINAGE BASINS - MYAKKA AND PEACE RIVERS w

SCALE OF MILES

D E LEUW, CATHER B BRILL ENGINEERS - ARCHFECTS

LOCATION MAP

"*W~"l" . , .o,.=.r.r,,

L.. - 1.11 OD' = ---. -

LI c " . T 1 B L C O F A R E I S LNOPCRIYETEn?)

C . O S W HU( *.It. L". .I.rt. O O l l E l "* .+., L..., 10,,r*r., 111, 21, ,,a 122 - -

I- "or* L"., a0 -- 3 4 3 0 0 .* loo - - . ,,I. 7 $50 110 0 110 - - - o I arm ,moo O S ~ O - - ,lb .

Q cmorsvwo ar vrwncr ROAD ~ c . M w . 8 C S E C T W E T I ~ O ~ I I m l c r l

I- I . .'X, S C C Z l D I T w O U IT.U)+OO TO IT.1M+5i

TYPICAL CANAL SECTIONS

rcrrr ran caosslNGs Qro @ CROSS SECTIONS OF MYAKKA RIVER

SCALE ron CnoSSlNG @

I" 2 0 6FET HRI m a 2 .-- --5

v ~ n r L.-..- i vrar o 10 PO FCET I0 20 FEET

NAME OF

FORMATION

PAMLICO

ANASTASIA

HAWTHORN

TAMPA L I M E S T O N E

OCALA L I M E S T O N E

G E O L O G I C A L S O I L F O R M A T I O N S

EXHIBIT XI

I I 2 I b

S C A L E : I = 20'

D E LEUW,CATHER,a B R I L L ENGINEERS-ARCHITECTS

EXHIBIT 3DI

1

STEEL SHEET P IL ING

SECTION C-C

1 2 - 7 / 1 6 " d S R Grodc Strands

F i l l with Cement Mwfor

SECTION B-8

r Precost Anchor Beam ,-Pile Cop Anchor Cop7 I

20'

Sheet ?;ling Min. 2 / Long-

* - 2 0 " O Prestressed Corcrete pi les - 12" 4 t o t

Note . 'Length of Anchor Pile Should InSw6 1 0 ' ~ o c k Penelrotion

PRESTRESSED CONCRETE SHEET P IL ING

DETAILS OF SHEET PILING BULKHEADS

DE LEUW, CATHER, B B R I L L ENGINEERS - ARCHITECTS I