VOL. 7, NO. 1 WATER RESOURCES BULLETIN FEBRUARY I97 1

TECHNICAL NOTES

A CRAZY IDEA ON URBAN WATER MANAGEMENT?’

Sie Ling Chiang’

ABSTRACT. High percentage of imperviousness in the city is the source of storm runoff. Roof area contributes significantly to the imperviousness. An attempt to make use of roofs as urban flood con- trol device and water conservation measure is advocated. Two different schemes, one for built-up industrial-commercial area, the other for residential area, are suggested. The former utilizes the roof as detention reservoir for flood control, the latter employs recharge pit to convert runoff into ground water resource. The proposed schemes are not only hydrologically, hydraulically and structurally sound but also economically feasible. It is worth considering in the future planning of urban renewal and urban development. (KEY WORDS: urban hydrology; water management; roof contribution; flood control; ground water recharge; water resources)

INTRODUCTION

As the population expands and more people concentrate in the city, the “water” problem will become extremely serious. One of the most interesting urban hydrologic problem is that the city suffers from the shortage of water supply on the one hand and flood damages on the other hand. Our engineering practice toward this problem so far has been passive. As the re- sult of it, the size and the length of both water supply line and sewer line are ever increasing. Is this an irreversible trend? Can we have any active device?

SOURCE OF FLOOD AND SUPPLY

High percentage of imperviousness in the city is definitely the source srea producing storm runoff. The major impervious area probably are roofs and paved streets including parking lots. Although these are source areas of flood, they could be sources of supply too, if properly managed. The best device is the one that converts the flood water into supply so that not only the flood damage is reduced but the dependable water supply is enhanced.

One of the well-known flood control methods is to build detention reservoirs upstream of a proposed flood protection area. Applying the same concept to the urban flood control problem, we may look for the control device way “up” where the raindrops first contact our

‘Paper No. (TN) 71017 of the Water Resources Bulletin (Journal of the American Water Resources As-

’Research Assistant, Department of Civil Engineering, The Pennsylvania State University, University sociation). Discussions are open until six months from date of publication.

Park, Pennsylvania 16802.

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172 Sie Ling Chiang

civilization-the “roof.” Instead of using roof as a drainage device as we are doing now, why can’t we employ it as a control and regulating device as well as a conservation measure.

QUALITY OF WATER FROM ROOFS AND STREETS

Roofs have been utilized as fresh water harvesting machine in the past and even now at some places in Formosa and other countries. Before the water supply system was available, rain from the roof was the water of the best quality farmers could obtain. Being little contam- inated and free of hardness, it is good for cooking, drinking, washing, etc.

Of course in the highly industrialized city, the air is polluted. The quality of rain water hence would be low. However, since we are determined to clean up our water and our air, the quality of rain water from the roof would not be any worse than that from lake or reser- voir and should be adequate for various purposes.

Streets, unlike roofs, are subject to littering. The runoff from streets usually is contami- nated, especially after being mixed with catch basin water. It has been shown that street- catchbasin runoff would have a shock load BOD as high as 50% of raw sanitary sewer. The quality of runoff from streets certainly is not comparable to that from roofs. They need separate treatment.

PROPOSED URBAN RAIN WATER MANAGEMENT SCHEMES

Two schemes, one for built-up industrial and commercial areas, the other for residential

(1) For industrial-commercial areas where multistory buildings generally are designed with areas, are proposed as follows:

flat roofs, the following provisions are suggested: (a) Every roof should be flat with gentle slope for drainage. (b) The roof should be designed to carry 1 .O foot of hydraulic head. Therefore every

(c) The size of down-spout should be such that it will drain 6 inches of rain water in at

(d) The roof drain may be either hooked into the storm sewer line or the water-harvest-

(e) Control valves may be provided for diverting the roof water for other users.

roof is a small detention reservoir.

least two days.

ing line for downstream users.

The above suggested scheme has several advantages: First of all, it substantially reduces the flood peak and will increase the sustained flow. Secondly the ponding water provides water conditioning for the upper most story. Thirdly the water with high head available is the source for watering grass, washing, fire protection. A skating rink may be provided if desired.

( 2 ) For residential areas where a flat roof is not practical, it is recommended that a small ground water recharge pit should be provided by each house owner. Wherever practica- ble the roof drain is to discharge into the pervious recharge pit. The size of the pit may be determined locally according to the climatic condition and the management scheme. The pit can be very economically constructed without jeopardizing the appearance of the back yard. By doing so the ground water is replenished by every residence, at the same time the storm runoff is reduced.

PERCENTAGE OF THE ROOF AREA

One cannot appreciate the above suggested approach without understanding the percentage

URBANWATERMANAGEMENT 173

of the roof in various urban subareas. Two of them are presented as follows: Figure 1 shows three downtown commercial blocks in the Borough of State College, Pa.

Practically they are completely paved. The shaded area represents the roof area, the other being paved streets and parking lots. The average percentage of the shaded area in this case is estimated to be about equal to that of the unshaded. It should be noted that the unshaded area is largely attributed to the Borough’s public parking lot and parking areas for the munici- pal employees and employees of a small factory. In the city where the land is more valuable and elevated parking lots are used, the percentage surface area for parking is reduced. There- fore we may safely conclude that in the built-up area in a city, more than half of the surface area is occupied by roofs.

P -1: 0 200 ft Scale - 0 1OOft Scale -

Figure 1. Three downtown commercial Figure 2. A block of typical middle-class blocks in the Borough of State College, Pennsylvania.

residential area.

Figure 2 shows a block of typical middle-class residential area. In addition to the roof area (shaded) and the paved driveway and street, there is a high percentage of grassed yard. The estimate shows 13% roof, 19% driveway and street and 68% yard. Depending upon the charac- teristics of locality and income level of the residence, the above relative percentages could vary from 10% to 25% for roof, from 15% to 25% for driveway and street, and from 75% to 50% for yard.

There are of course various land use classes in a city. Most of them would fall between the built-up commercial area and the residential area. The above examples provide the extent of variation of roof areas in an urban area.

174 Sie Ling Chiang

ENGINEERING AND ECONOMIC FEASIBILITY

The primary idea of this proposal is to spread the hydrologic loading over every building in the city, instead of concentrating it in the sewer. The reservoir so created requires little addi- tional cost, yet will substantially reduce the design peak discharge. The most critical element of urban flood is its relatively short time of concentration. The reservoir on the roof will have tremendous detention effect.

A 6 inch storm has a return period of approximately 100 years. This is longer than the current practice storm sewer design. The flood peak resulting from the 6 inch storm is so high that the cost of building such drainage system is normally beyond our financial capability. If the runoff from the roof area is retained and released over two to three days after the storm, the roof contribution to the flood peak becomes negligible. The size of the combined sewer therefore can be determined from domestic and street runoff alone. In the built-up area there is more roof area than area of parking lot and street, hence reduction on peak discharge could be 50% or more. In other words, same amount of money spent to drain a 20-year storm now would protect us against a 100-year storm after this measure is taken.

From engineering point of view, it is entirely feasible to design roofs to sustain one foot of static head. As a matter of fact a minimum live load of 40 pound per square foot which is equivalent to 8 inch of static head, has been commonly used in the roof design. It costs no more to pond 6 inch rain water. The suggested additional loading of 4 inch head is nothing but a factor of safety. For large building in the city, the cost of roof is of such a small fraction of the total cost that the economical feasibility is out of question.

CODE AND REGULATION

Although this approach is feasible from both engineering and economic points of view, its success can be guaranteed only when these required measures are written into codes or regula- tions and enforced. For example, the detention reservoir on the roof would not be effective without enforcing each building to reduce the size of the down spout.

SUMMARY

An attempt to make use of roofs as urban flood control device and water conservation measure is advocated. Two different schemes, one for built-up industrial-commercial area, the other for residential area, are suggested. The former utilizes the roof as detention reservoir for flood control, the latter employs recharge pit to convert runoff into ground water resource. The proposed schemes are not only hydrologically, hydraulically and structurally sound but also economically feasible. It is worth considering in the future planning of urban renewal and urban development.