Bread Truck Drainage

42.40 m

Elev.

5.05 m

6.55 m

4.09 m

Irregular

Channel Cross-section

Approximate

Channel Cross-section

Parachute Drainage

33.02 m

Elev

15.90 m

4.95 m

2.44 m

Approximate

Cross-section

is actively changing. The reasons for this activity

A

Pw

R

S

are not yet clear. Hydraulic and geotechnical

N

forces are clearly causing such changes, but their

Bread Truck drainage

86.7

43.8

1.98

0.0044 0.018

relationship to the dynamics of the entire system

Parachute drainage

40.3

33.4

1.21

0.0062 0.018

remains unknown. Basic parameters such as cur-

rent velocity, discharge and channel geometry

need to be measured accurately to define the gul-

Table 5 gives values of the above parameters, as

lies' roles in tidal flat hydrology.

well as area (*A*) and wetted perimeter (*P*w). We

To evaluate the potential magnitude and rela-

assumed that the depth of flow was at bankfull

tive importance of forces causing erosion and

stage and that the channel slope in the gully was

sediment transport within the gullies, empirical

in uniform flow. These calculations produced

equations were used to estimate hydraulic pa-

very high flow velocities (more than 4.5 m/s) and

rameters for peak flow during ebb. Gully pro-

Froude numbers in excess of 1.0. Since these val-

files, bank elevations and calculated slopes for

ues indicate supercritical flow, the flow must un-

the Bread Truck and Parachute drainages were

dergo a hydraulic jump to a greater depth, which

used to approximate the shape of the drainage

is not possible for the assumed bankfull condi-

cross section and estimate maximum velocities

tions. Thus, simplifying the hydraulic conditions

and discharge (Fig. 28 and 29).

to a uniform flow assumption is not feasible for

Sediment transport and gully erosion are pri-

currents within the gullies.

marily related to the flow velocity. Gully longitu-

Obviously, these flow velocity calculations are

dinal profiles and cross sections were measured

not representative of the actual situation and

for the Bread Truck and Parachute drainages in

more detailed information on the hydraulic con-

an attempt to determine flow velocities in these

trols (downstream and upstream) or actual veloc-

channels. Figures 28 and 29 are representative

ity measurements are needed. Our observations

sectional shapes that expand in each gully as one

suggest flow velocities are actually in the 13 m/s

proceeds downstream.

range during ebb.

The Manning equation was used to calculate

the average flow velocity *V *in the gully.

(1)

Erosion of gully headwalls is causing their pro-

where *N *= Manning coefficient (0.018 for clean

gressive elongation into the mudflats and pre-

earth)

sumably into the ponds. Rates of recession of the

gully headwall and adjacent lateral walls were

30