こうえいフォーラム第１０号／２００２. １

STUDY OF STORM WATER INTAKE STRUCTURE AND ACCUMULATED SOIL REMOVAL SYSTEMS OF DEEP UNDERGROUNDLARGE DIAMETER STORM WATER TUNNEL RESERVOIR
大深度・大口径雨水貯留管の取水と堆積物除去に関する実験検討松田 優*・木村 一政*・佐藤 誠**
Masaru MATSUDA, Kazumasa KIMURA and Makoto SATO

浸水の減少や合流式下水道による河川放流水質の改善を目的として建設される大深度大口径雨水貯留管は、取水時に落下する水の衝撃や流入する土砂の堆積などの問題に配慮して設計する必要がある。そこで雨水著留管の取水構造と排砂システムに関する研究を行った。研究は、取水構造設計の基礎資料となる水理模型実験と最適な貯留の断面や縦断勾配の設定の基礎資料となる排砂実験の二つの実験を基本に行った。
本論文は1999年 8 月にオーストラリアで開催された International Conference Urban
Storm Drainage に投稿した原稿の再掲である。
Key Words : deep underground large diametric storm water tunnel, flush water volume, inlet,
prang pool, sediment discharge, tractive flow discharge, tractive force, vortexflow drop shaft

１．PREFACE

storm water is taken in from existing sewers. The other is that it

IIn the urban areas of large cities, where there are few plots ofis difficult to collect soil and sand mechanically, because soil

vacant land available for public use and highly dense systems ofand sand coming in from the reserved storm water, which

life-supporting facilities are buried under the streets, it isincludes organic matter, builds up inside the storm water tunnel

difficult to find space for construction on the land surface or inalong its entire length. It is desirable, therefore, that soil andshallow strata beneath the streets. Therefore, facilities such as

sand be carried out and removed after they are collected bysewers, streets and rivers are being buried deeper underground.

tractive force and that a system be constructed to remove

The Municipality of Kawasaki City is now building theaccumulated soil and sand efficiently. This study deals with

"Egawa Deep-Underground Large-Diameter Storm Water

these two problems and explains the experimentation design

Tunnel", a storm water reservoir that uses the shield method tomethodology and its results.

reduce the load from overflow during flood mitigation. It alsoof 1,500m and 81,000m of water reservoir approximately 35m

２．STUDY OF WATER INTAKE AND HEAD
STRUCTURE

beneath the surface.

（1）THE STRUCTURE AND PROBLEMS OF A VORTEX-

includes a sewer with an inner diameter of 8.5m, a total length3

Building a deep-underground large-diameter storm water

FLOW DROP SHAFT

tunnel involves two main problems(one of them is how to select

There are three kinds of water intake and head structure, as

the water intake structure and design the fall so that fallingshown in Table 1. Since the construction area is small, and theenergy will dissipate itself) since falls become heavy when

design flow rate and head are too high for the area shown in
Table 2, we have decided to adopt a vortex-flow head structure＊
川崎市建設局 下水道部＊＊ 首都圏事業部 上下水道部

that enables us to reduce the structure for a large quantity ofwater intake. This structure will be in scale as shown in Figure

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Study of storm water intake structure and accumulated soil removal system of deep underground large diameter storm water tunnel reservoir

Table 1 Intake and Drop Structure

Equation 1
Table 2 Design condition

1, from the calculation using "Equation 1". In order to design
the structure, it is necessary to clarify a) the hydraulic regime
from water influx to tunnel reservoir and b) the shock absorption
effect and its applied pressure in the absorption tank. Since these
could not be clarified theoretically, however, we investigatedthem using the following hydraulic model experiment.
（2）HYDRAULIC MODEL EXPERIMENTThe hydraulic model of the vortex-flow drop shaft is designedon the flow in an open channel. Therefore, it is important tomaintain geometric similarity of the channel, and similarity of
the inertial force controlled by gravity. For this purpose, theFigure 1 Detail of Vortex-flow Dropmodel shown in Figure 2 was created including the distance of
37 meters between the upper water inflow channel and about
100 meters of the downstream tunnel reservoir in addition to thewhole vortex-flow drop shaft. The experiment was conducted inaccordance with the measurement method shown in Table 3.
The model of the vortex-flow drop shaft used in the experiment
was designed on the basis of the specifications obtained from theexperiment proposed by Kennedy and Jain (1980).
（3）RESULTS OF THE MODEL TEST
1）Hydraulic Regime of Influx
As for the hydraulic regime in the inlet tank shown in Figure
3, when flow rate is low (Q=4.512m3/s and Q=9.124m3/s), storm
water increases its flow speed immediately after the inlet pipe,runs in supercritical flow, and increases its depth by making

60

Figure 2 Model
Table 3 Measurement methodこうえいフォーラム第１０号／２００２. １

Figure 3 Intake Tank

Figure 4 Section of Inlet Tank

Figure 5 Concept Drawing of Tarap

hydraulic jump at the inlet immediately before the influx of the

As for shock absorption, experiments were conducted on thevortex-flow head. The probable reason for this is that the slope

plans of the prang pool shown in Figure 1 and the trap shown inof the water channel accelerates the flow to supercritical level at

Figure 5. In the case of the prang pool plan, a shock absorption

first, following which the reduction in the width of the water

effect that is compatible with the tunnel reservoir structure canchannel causes the flow to make a hydraulic jump near the inlet.

be attained even if the water inside the tunnel reservoir is not

If the flow rate is Q=18.048m 3 /s of design flow rate,

deep enough for the designed flow rate. In the case of the trap

supercritical flow is not observed in storm water, and storm

plan, however, the scatter condition starts at more than 8.0 m3/s

water runs towards the inlet in subcritical flow. The hydraulicof flow rate, and storm water is discharged into the tunnel

jump phenomenon, which was observed when a small quantity

reservoir without shock absorption. It is feared that this willof storm water flowed, is not observed. The results show that, if

affect the bottom of the tunnel reservoir at the maximum speed

a tangential inlet structure is properly designed, the vortex-flowof 15.0 m/s, since a supercritical flow area is generated along thecan secure a stable flow-down capacity. Furthermore, the results

long basin. As a result of these experiments, we have decided toof the experiment suggest that the shape of the tangential inlet

adopt the prang pool plan in our design that minimizes structural

structure will not be adversely affected by backwater on theeffect on the reservoir.

upper flow of the inlet pipe.

Figure 6 also shows the water pressure at which the base of2）Hydraulic Regime of the Vertical Shaft

the lower part of the vertical shaft is affected by the water vein

The hydraulic regime from the inlet to lower shock absorber,that comes down the vertical shaft. When storm water of theand the air-core of the horizontal section inside the inlet aredesigned flow rate flows in flowhead down, there is a resultant

shown in Figure 4. A water vein that influxes from the slit

water head of at most 8.5m directly at the bottom of the shaft.

clings to the wall of the inlet and rolls down by centrifugal force,

This value is approximately 4.0m higher than the 4.5m of static

forming a helical flow. The revolution of this helical flow ispressure in the center of the prang pool. Since this hydraulicvery sensitive to inlet speed from the slit and the higher the flowpressure is considered to come from the falling speed, it israte, the faster the revolution of the helical flow. Even at a very

estimated that the flow speed that affects the bottom of the prang

low flow rate, it forms a helical flow down the wall of the inlet.

pool is approximately 9.0m/s. From the study on the hydraulic

The minimum air-core area at the set flow rate takes up
approximately 37 % of the shaft cross section in section A. Thisvalue is greater than 25 % of the minimum ratio of air-core area
stipulated at the flood outlet of the water tunnel at a dam, which,generally speaking, flows at very high rate. It can be judged ,therefore, that safety for the downstream is sufficiently secured.
Therefore, the shaft diameter of 2,300mm (D) and the slit width(1/4 D) adopted in the experiment can be assumed to beappropriate for the flow-down of the designed flow rate.
3）Shock Absorption Effect in the Absorption Tank;
Operational Pressure

Figure 6 Pressure Distribution

61

Study of storm water intake structure and accumulated soil removal system of deep underground large diameter storm water tunnel reservoirpressure and the progress of abrasion of the concrete due to thehigh flow speed, we do not think that the concrete will be worn
out, judging from the values of the hydraulic pressure and flow
speed obtained from the results of the experiment.
（4）DETERMINATION OF WATER INTAKE AND HEAD
STRUCTURE
The vortex-flow head has almost never been used in Japan.
However, in this case, we believe that it is most suitable as thewater intake structure for storm-water tunnel reservoirs because
it requires a minimum of space, because it can attain the

Figure 8 Outline of Flushing System

necessary water conductivity, and because a stable water intake
hydraulic regime is secured by it. The structure of the vortex-

flush, collect and carry out the accumulated soil and grit with theflow head we have adopted is shown in Figure 1.

system shown in Figure 7.
1）Flush water

３．GRASPING OF TRACTION CHARACTERISTICS
FOR ACCUMULATED SOIL AND GRIT FLUSHING

ventilating station shown in Figure 8. We have decided to（1）ACCUMULATION REMOVAL SYSTEM

adopt a flushing tank, the capacity of which is approximately

Storm water reserved in the tunnel reservoir is drained by aFlush water will be provided by the flushing tank in the

600m3 due to the restrictions posed by the size of the ventilating

pump after the rain stops. The greater part of soil and grit

station and the condition of the site in this area.

containing organic matter, however, remains inside the tunnel

2）Flushreservoir. In order to prevent the sewage from giving off a bad

We have decided to adopt a combination of both a flushingsmell and to keep the inside of the tunnel reservoir in goodpipe and a portable flushing robot in order to remove thecondition, it is necessary to flush the inner tunnel reservoir.

extraneous matter at the side of or on the upper surface of theSince the "Egawa Storm-water Tunnel Reservoir," which has antunnel reservoir, and to carry soil and grit accumulated at theinternal diameter of 8.5m and a total length of 1,500m is abottom of the tunnel reservoirs all the way to the end. We havegigantic cylindrical shape, it is difficult to remove accumulated

also decided to adopt this combination for the bottom of thesoil and grit mechanically. For this purpose, we have decided toinside of the tunnel reservoir. This shape includes a sandremoval trench at the foot of its circular section, the shape
shown in Figure 9, which helps to flush and collect matter
effectively with as little water as possible. This was importantdue to the restricted quantity of flush water. Also, we have

Figure 7 Residue Water-supply System

62

Figure 9 Bottom of Reservoir Tunnelこうえいフォーラム第１０号／２００２. １

secured a flat platform measuring approximately 1.8 m-wide inwhich enough for the flushing robot to run at either side of theplatform safely and efficiently, and furthermore for the flushing（2）HYDRAULIC MODEL TEST FOR DETERMINING
THE TRACTION CHARACTERISTICS

robot to run. It has also been decided to set the longitudinal1）Explanation of the Experiments

slope of the trench at 1/200. This value was decided in

In the experiments, we observed sedimentation of sand byconsideration of the conditions for building, maintaining andallowing a fixed amount of sedimentation discharge after pilingmanaging a shield tunnel.

up sand in the sand removal trench, and also observed the sand3）Flushing pipe

removal area (the area in which sand flows out and a waterbed

A flushing pipe is a facility for flushing the soil and gritappears) developing in the upper stream. We extracted a 20m-

accumulated at the bottom of the pipe down to the end. To belong steel water channel (a partially glassed-in wall) withmore specific, it is used for conducting flushing water supplied

variable slopes as the experimental water channel as shown inby the flushing tank from an aqueduct buried under the platform

Figure 10. This was used as part of the real sand removal

on both sides of the trench into the tunnel reservoir, gushingtrench, which has a total length of 1,500m and a width of 0.6m.

water from a total of four flush valves placed every 375m andTable 4 shows the comparison between the real sand removal

washing away the accumulated soil and grit using the current, as

trench and the model.

shown in Figure 8. The flush point can be selected freely fromIn the experimental water channel, we used 4m of the upper

among the four flush valves.

part as the baffling area, and then covered the downstream4）Flushing robots

distance of 15m with 1-2cm thick sand to prepare for the initial

Flushing robots are used to flush the side or upper part of theconditions for sand accumulation. Two kinds of sand was used

inside of the tunnel reservoir which cannot be cleaned byflushing. To be more specific, a robot capable of inspecting andflushing is moved along the sand removal trench and used forinspecting residue inside the pipe, for spraying the flushingwater out of the nozzles, for flushing the inside of the pipe andfor removing the accumulated soil and grit by water impact.
Water is supplied to the robots through the flush valve used forflushing with a flushing pipe.
5）Carrying out Soil and Grit
The soil and grit carried out to the end of the tunnel reservoir
in accordance with Flush above are discharged outside thetunnel reservoir by a drainage pump after they are broken into

Figure 10 Hydraulic Model Experimentsmall pieces by the comminuting pump installed at the return
pumping station shown in Figure 8.
6）Problems related to the Accumulation Removal

Table 4 Profile of experiment Sands

SystemIn practical operation, only the bottom of the tunnel reservoir
is flushed using the flushing pipe after each rainfall as alreadyshown in Figure 7. In addition, the inside of the tunnel
reservoir is inspected and flushed regularly by the flushingrobot. Under this system, however, it is important to clarify
traction characteristics for flushing soil and grit accumulated atTable 5 Experiment model and real structure

the bottom of the pipe, but it is difficult to analyze the traction
characteristics theoretically. Therefore, we have conducted thefollowing hydraulic model experiments, with the purpose ofdetermining the traction characteristics.

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Study of storm water intake structure and accumulated soil removal system of deep underground large diameter storm water tunnel reservoir

in the experiment, an almost uniform sand (Sand No.1) with anaverage grain diameter of 0.17mm, and the other a mixed sand
(Sand No.2) with a maximum grain diameter of 5mm and an
average grain diameter of 0.71mm. Table 5 shows a conversionof the above into real sand accumulation. All the following
figures for sand accumulation conditions show converted to thereal structure values.
The experiment was conducted at five flow rates: 0.05, 0.10,
0.20, 0.30, and 0.40m3/s under the fixed flow condition. The
water flow experiment was standed by lowering downstream
gate after spreading sand and putting up the gate at the end of

Figure 11 Flushing area of sandthe area downstream of the water channel. A water depth
designated suitable to each flow rate was set by storing water soin Figure 11. The slower the flow rate, the sharper the wedge,that the sand would not move. In this case, the flow of the waterand the faster the flow rate, the duller the wedge. The top of thechannel without sand accumulation runs in supercritical flow

accumulated sand area (the boundary between water and sandsince the roughness coefficient of sand is n=0.013, while thethat are washed away by a current) moves downstream at a more

flow of the water channel with sand accumulation entersor less constant speed. Almost the same phenomenon occurs

subcritical flow as the roughness coefficient increases to n=0.02.

even when the sand grain diameter is different. Especially in the2）Results of the Experimentcase of mixed sand with an average grain diameter of 1.2 mm,a）Moving Configuration of Sand

the maximum diameter of a grain is approximately 8mm. Yet,With a grain diameter of 0.28mm, as shown in Table 6,

in no case did sand of the large diameter remain.

motion of sand was, generally, in a traction phenomenon at aflow rate of 0.05m3/s and 0.10m3/s. Ripples developed on the

Figure 12 shows the relationship between motion velocity

bed. When the flow rate exceeded 0.10m/sec., suspended flow

towards the downstream of the top of the accumulated sand area

developed and the bed became flat. With an average sand grain

and the flow rate.

diameter of 1.2mm, however, the motion of sand was almost incomparatively low, the motion velocity increases at an ever-traction up to the flow rate of 0.2m /s. There was a conspicuous

increasing speed in areas in which the flow rate exceeds its

suspended flow of sand when the flow rate exceeded 0.3m /s.

criticality (the flow rate that exceeds a critical tractive flow

With this sand grain diameter, ripples were not formed on the

discharge), and its velocity increases in proportion to the flow

bed even at on slow flow rate, and the bed was flat.

rate when it is higher than 0.2m3/s. The results show that theb）Motion Velocity of Sand

smaller in diameter the sand grain, the higher the motion

The flushing area of sand that moves from the upper stream

velocity, and that the motion velocity is not inconsistent with antakes the shape of a wedge that is wide fat in the center andactual physical phenomenon. The above are, however, thenarrow at both ends as a result of the velocity distribution shown

experimented results that we obtained by spreading sand3

3

In an area where the flow rate isTable 6 Moving configuration of sandFlow Discharge（m3/sec）
Figure 12 Flow Discharge and Motion

64こうえいフォーラム第１０号／２００２. １

Flow Discharge（m3/sec）

Flow Discharge（m3/sec）

Figure 13 Flow Discharge and Sediment discharge

Figure 14 Flow Discharge and Total Sediment Discharge

uniformly over the water channel, and it should be noted that acertain flushing flow discharge. When the relation between the

different phenomenon may be observed when soil and sand aretraction capacity and the size of the flush tank facility is

actually spread.

considered, it is found that the efficient flush water volumec）Sand Removal Capacity

suitable for traction is approximately 0.20 m3/s.

Figure 13 shows sediment discharge at each flow rate that

It has been confirmed from the above experiment that thewas obtained from the results of the traction experiments. This

flush water must be 15.3 cm deep in order to flow at on efficient

sediment discharge was obtained as per unit sediment discharge

flush water volume of 0.2 m3/sec, if the longitudinal slope isby multiplying the per unit time motion velocity of the top of

1/200 in the section of Figure 10, and that adequate safety canaccumulated sand area by the accumulation thickness, on the

be secured at 50cm of the flush water trench depth, which isassumption that the top of the accumulated sand area moved at a

obtained from the shape of the section for its maintenance.

nearly constant speed and that the accumulation thickness in theaccumulated sand area was kept constant. The sediment
discharge remains constant in relation to sand grain size, but
increases in proportion to the flow rate. However, the absolute

４．CONCLUSION
The conclusions we have drawn from the study made mainlyon the basis of the hydraulic model experiment are as follows:

volume of the sediment discharge for a grain diameter of- It has been confirmed that the vortex-flow fall construction

0.28mm is about three times the sediment discharge for a grain

is suitable as a water intake structure for deep-underground

diameter of 1.2 mm. From this result, it is possible to estimate

large-diameter storm water.

sediment discharge in relation to flush water volume.

- It has been confirmed that efficient flushing flow discharge
exists in the traction of soil and grit.

（3）EFFICIENT DETERMINATION OF FLUSHING FLOWDISCHARG

The "Egawa Storm-water Tunnel Reservoir" is now being
constructed, with a scheduled completion date in the year 2001.

Figure 14 shows total sedimentation discharge {= sediment

We would like to verify a hypothesis of the outcome of thisdischarge x (600÷flow rate)} at a flushing capacity ofexperiment since the Egawa Storm-water Tunnel Reservoir willaccumulated sand and grit with a flush water volume at 600m3.

function as a tunnel reservoir in the near future.

At a given flush water volume, the bigger the unit time flush
water volume, the larger the flushing capacity, but the increasein ratio is less pronounced at the border of a per unit time
flushing capacity of 0.20m3/s. In the case of the combine
particle size of 1.2mm, flushing capacity decreased slightly the

Reference
１）AIC Committee 210 (1987), Erosion of Concrete in Hydraulic Structures,
AIC Materials Journal, March-April, ACI Committee 210, pp. 136-157.
２）Iowa Institute of Hydraulic Research (1980). Vortex-Flow Drop Structures

for the Milwaukee Metroporitan Sewerage District Inline Storage System,
IIHR Report No. 264, The University of Iowa, Iowa.

flush water volume of 0.4m3/s. This suggests that even if the３）The American Society for Testing and Materials (1982). Abrasion-Erosion

tractive flow discharge increases, high flush efficiency inResistance of Fiber-Reinforced Concrete , Cement, Concrete, andAggregates, pp.3-100

proportion to increase in the discharge cannot necessarily beraised, and that total sedimentation discharge cannot be
increased according to the increase in the flow rate in excess of a65