Hydrology for- Urban Land Planning - 

A Guidebook on the Hydrologic 

Effects of Urban Land Use 

By Luna B. Leopold 

GEOLOGICAL SURVEY CIRCULAR 554 

Washington 1968 
United States Department of the Interior 

STEWART 1. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

Free on opplication to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Pam 

1 

3 

7 

11 

11 

16 

16 

18 

ILLUSTRATIONS 

FIGURES 1-8. Graphs showing: 

1. Hypothetical unit hydrographs relating runoff to rainfall, 

with definitions of significant parameters -_____-_-- 

2. Effect of urbanization on mean annual flood for a l-squaremile drainage area __________________-____________ 

3. Flood-frequency curve for partial-duration series, West 

Branch Brandyine Creek at Coatesville, Pa., based 

on data for 1942, 1944-51 _________-______--_______ 

4. Regional flood-frequency data for the Brandywine Creek 

basin, Pennsylvania ___--_________________________ 

5. Flood-frequency curves for a 1-square-mile basin in 

various states of urbanization ..................... 

6. Increase in number of flows per year equal to or exceeding 

original channel capacity (I-square-mile drainage 

area), as ratio to number of overbank flows before 

urbanization, for different degrees of urbanization -_- 

7. Annual sediment production per square mile for urbanized 

and natural areas _-_-_______________-____________ 

8. Relation of sediment yield and discharge for an urban and 

a rural or unurbanized area ____ ___________-_____-_ 

Page 

3 

6 

10 

12 

13 

TABLE 

Page 

TABLE 1. Increase in discharge as a result of urbanization in a 1-square-mile 

I11 
HYDROL.OGY FOR URBAN LAND PLANNING-A GUIDEBOOK ON THE HYDROLOGIC 

EFFECTS OF URBAN LAND USE 

By LUNA B. LEOPOLD 

This circular attempts to summarize existing 

knowledge of the effects of urbanization on 

hydrologic factors. It also attempts to express 

this knowledge in terms that the planner can 

use to test alternatives during the planning 

process. Because the available data used in 

this report are applied to a portion of the 

Brandywine Creek basin in Pennsylvania, this 

can be considered as a report on the basic hydrologic conditions of the Brandywine Creek 

basin prior to the beginning of major urbanization. Because the available data are not yet 

adequate, this report can be considered as a 

compilation of tentative suggestions in the 

form of an explanatory, not a definitive, handbook. 

The application of current knowledge of the 

hydrologic effects of urbanization to the 

Brandywine should be viewed as a forecast of 

conditions which may be expected as urbanization pro&eds. By making such forecasts in 

advance of aetual urban development, the 

methods can be tested, data can be extended, 

and procedures improved as verification becomes possible. 

PLANNING PROCEDURES AND HYDROLOGIC 

VARIABLES 

A planning document presented to a community for adoption must always be more suggestive than coercive. This is true not only 

because the planner is unable to foresee the 

innumerable complications of actual development, but also because there are many detailed 

alternatives which would accomplish generally 

similar results. The planner is particularly 

concerned with both the constraints and the 

opportunities offered by the principal physiographic characteristics of the area, especially 

the location of hillslopes, soils, and streams. 

The existing pattern of land use and the accompanying distribution of woods and agriculture are parameters which over a period of 

years may actually change, albeit slowly. Roads, 

villages, industries, and other manmade features are more or less permanent and exert 

their greatest influence in their effect on 

further development, especially through land 

values. 

Of particular concern to the planner are 

those alternatives that affect the hydrologic 

functioning of the basins. To be interpreted 

hydrologically, the details of the land-use pattern must be expressed in terms of hydrologic 

parameters which are affected by land use. 

These parameters in turn become hydrologic 

variables by which the effects of alternative 

planning patterns can be evaluated in hydrologic terms. 

effects of land-use changes on the hydrology of 

an area: changes in peak flow characteristics, 

changes in total runoff, changes in quality of 

water, and changes in the hydrologic amenities. 

The hydrologic amenities are what might be 

called the appearance or the impression which 

the river, its channel and its valleys, leaves with 

the observer. Of all land-use changes affecting 

the hydrology of an area, urbanization is by 

far the most forceful. 

Runoff, which spans the entire regimen of 

flow, can be measured by number and by characteristics of rise in streamflow. The many 

There are four interrelated but separable -' 

1 
rises in flow, along with concomitant sediment 

loads, control the stability of the stream channel. The two principal factors governing flow 

regimen are the percentage of area made impervious and the rate at which water is transmitted across the land to stream channels. The 

former is governed by the type of land use ; the 

latter is governed by the density, size, and characteristics of tributary channels and thus by the 

provision of storm sewerage. Stream channels 

form in response to the regimen of flow of the 

stream. Changes in the regimen of flow, 

whether through land use or other changes, 

cause adjustments in the stream channels to 

accommodate the flows. 

The volume of runoff is governed primarily 

by infiltration characteristics and is related to 

land slope and soil type as well as to the type 

of vegetative cover. It is thus directly related 

to the percentage of the area covered by roofs, 

streets, md other impervious surfaces at times 

of hydrograph rise during storms. 

A summary of some data on the percentage 

of land rendered impervious by different degrees of urbanization is presented by Lull and 

Sopper (1966). Antoine (1964) presents the 

following data, on the percentage of impervious 

surface area in residential properties : 

Id hizr of Impeiviotix 

risidpwtinl ai pa xurfacr area fS[l tti (iicrrtnt) 

6,000 ~ - ~- ~ 80 -_ 

6,000-15,000 ~ ~ 40 

15,000 ~~ 25 ~~ 

The percentage decreases markedly as size 

of lot increases. Felton and Lull (1963) estimate in the Philadelphia area that 32 percent 

of the surface area is impervious on lots averaging 0.2 acre in size, whereas only 8 percent 

of the surface area is impervious on lots averaging 1.8 acres. 

As volume of runoff from a storm increases, 

the size of flood peak also increases. Runoff 

volume also affects low flows because in any 

series of storms the larger the percentage of direct runoff, the smaller the amount of water 

available for soil moisture replenishment and 

for ground-water storage. An increase in total 

runoff from a given series of storms as a result 

of imperviousness results in decreased groundwater recharge and decreased low flows. Thus, 

increased imperviousness has the effect of increasing flood peaks during storm periods and 

decreasing low flows between storms. 

The principal effect of land use on sediment 

comes from the exposure of the soil to storm 

runoff. This occurs mainly when bare ground 

is exposed during construction. It is well 

known that sediment production is sensitive to 

land slope. Sediment yield from urban areas 

tends to be larger than in unurbanized areas 

even if there are only small and widely scattered units of unprotected soil in the urban 

area. In aggregate, these scattered bare areas 

are sufficient to yield considerable sediment. 

A major effect of urbanization is the introduction of effluent from sewage disposal plants, 

and often the introduction of raw sewage, into 

channels. Raw sewage obviously degrades 

water quality, but even treated effluent contains dissolved minerals not extracted by sewage treatment. These minerals act as nutrients 

and promote algae and plankton growth in a 

stream. This growth in turn alters the balance 

in the stream biota. 

Land use in all forms affects water quality. 

Agricultural use results in an increase of nutrients in stream water both from the excretion 

products of farm animals and from commercial fertilizers. A change from agricultural use 

to residential use, as in urbanization, tends to 

reduce these types of nutrients, but this tendency is counteracted by the widely scattered 

pollutants of the city, such as oil and gasoline 

products, which are carried through the storm 

sewers to the streams. The net result is generally aq adverse effect on water quality. This 

effect can be measured by the balance and 

variety of organic life in the stream, by the 

quantities of dissolved material, and by the 

bacterial level. Unfortunately data describing 

quality €actors in streams from urban versus 

unurbanized areas are particularly lacking. 

Finally, the amenity value of the hydrologic 

environment is especially affected by three 

factors. The first factor is the stability of the 

stream channel itself. A channel, which is 

gradually enlarged owing to increased floods 

caused by urbanization, tends to have unstable 

and unvegetated banks, scoured or muddy 

2 
channel beds, and unusual debris accumulations. These all tend to decrease the amenity 

value of a stream. 

The second factor is the accumulation of artifacts of civilization in the channel and on the 

flood plain : beer cans, oil drums, bits of lumber, 

concrete, wire-the whole gamut of rubbish of 

an urban area. Though this may not importantly sffect the hydrologic function of the 

channel, it becomes a detriment of what is here 

called the hydrologic amenity. 

I ?Lag time 

of stream flow 

Center of mass 

TIME, IN HOURSafter urbanization 

TIME, IN HOURSFigure 1.-Hypothetical unit hydrographs relating 

runoff to rainfall, with definitions of 

significant parameters. 

The third factor is the change brought on 

by the disruption of balance in the stream 

biota. The addition of nutrients promotes the 

growth of plankton and algae. A clear stream, 

then, may change to one in which rocks are 

covered with slime ; turbidity usually increases, 

and odors may develop. As a result of increased 

turbidity and reduced oxygen content desirable 

game fish give way to less desirable species. 

Although lack of quantitative objective data on 

the balance of stream biota is often a handicap 

to any meaningful and complete evaluation of 

the effects of urbanization, qualitative observations tend to confirm these conclusions. 

AVAI1,ABILITY OF DATA AND THE TECHNIQUE 

OF ANALYSIS 

Basic hydrologic data on both peak flow and 

volume of runoff may be expressed in terms of 

the Characteristics of the unit hydrograph, that 

is, the average time distribution graph of flow 

from a unit or standard storm. The unit hydrograph shows the percentage of the total 

storm runoff occurring in each successive unit 

of time. The standard storm may be, for example, a typical storm which produced 1 inch 

of runoff (fig. 1). Such data are derived from 

the study of individual storms and the associated runoff graphs measured at gaging 

stations. 

One factor stating the relation between the 

storm and the runoff is lag time. This is defined 

as the time interval between the center of mass 

of the storm precipitation and the center of 

mass of the resultant hydrograph. Lag time is 

a function of two basin parameters-the mean 

basin slope and the basin length. These factors 

empirically correlate with lag time if expressed 

in the fmrn of the basin ratio (basin length L 

divided by the square root of the mean basin 

gradient, s). This basin ratio is also related to 

drainago area. As drainage area increases, the 

basin length increases and the average value 

of slope generally decreases. Thus, natural 

basin characteristics can be translated into 

flood-flow characteristics. 

Lag time may be materially altered by the 

effects of urbanization on the basin surface. 

Water runs off faster from streets and roofs 

than from natural vegetated areas. This tends 

to decrease the lag time. The construction of 

artificial channels, especially storm sew,ers, 

also decreases lag time. As the time required 

for a given amount of water to run off shortens, 

the peak rate of runoff (flood peak) increases. 

In addition to the basin ratio and lag time, 

the regimen of a stream, however, can be described in many other ways, including flood 

3 
frequency, flow duration, mean annual flood, 

discharge at bankfull stage, and frequency of 

bankfull stage. This is evidenced in past studies 

of the effects of urbanization on the hydrology 

of an area. Many different techniques of relating rainfall to runoff have been used, along 

with various parameters to measure the degree 

of urbanization. In order to evaluate our present knowledge, it is necessary to express the 

results of these studies in some common denominator. 

Most reports on hydrologic effects of urbanization present the conclusions in a form which 

is more useful to the hydrologist than to the 

urban planner. This circular will attempt to 

interpret the hydrologic conclusions of these 

studies in terms that are meaningful to the 

planner. Two forms of presentation will be 

used. 

The first is a slight modification of a method 

previously used by several investigators, especially D. G. Anderson (1968) and L. D. 

James (1965). The percentage of an area 

sewered is plotted against the percentage of the 

area rendered impervious by urbanization ; 

isopleth lines (lines of equal value of the ratio) 

on the graph show the ratio of peak discharge 

under urbanized conditions to the peak discharge under rural or unurbanized conditions. 

Such a graph will be different for different 

drainage area sizes and for different flow frequencies. 

The second method utilizes a relationship between the degree of urbanization and the frequency at which the original channel capacity 

would be exceeded. 

Table 1 is an interpretation and summary of 

the effects of urbanization on peak discharges 

based on previous studies. Results of the 

studies were interpreted and extrapolated to a 

common denominator of 1 sq mi (square mile), 

a practical unit of size for planning. 

Carter (1961) developed a technique that 

followed the reasoning previously used by 

Snyder (1938) and that showed lag time as a 

function of basin characteristics. For 20 

streams in the vicinity of Washington, D.C., 

Carter developed this relation for natural 

basins, for partly sewered, and for completely 

sewered basins. As in most studies the difficulty comes in translating these descriptive 

terms to quantitative measures of urbanization. 

From data presented by Carter, values were 

read for a basin ratio of 0.12 representing a 

1-sq-mi area having an estimated length of 1.2 

miles and an average slope of 100 feet per mile. 

It was further assumed that in Carter’s study, 

“partly sewered” is equivalent to 50 percent 

sewered and 20 percent impervious. These 

conditions provide some of the data shown in 

table 1. 

Table l.-Increase in discharge as a result of urbanization in a 1-square-mile area 

[Discharxe is mean annual flood; recurrence interval is 2.3 years. Data are expressed as ratio of discharge after urhanization to discharge under previous conditions. Data from James (1965) have no suaerscriptl 

Percentage of area served by storm sewerage 

Percentage of area made impervious 

n 20 50 80 

1.0 11.2 11.8 12.2 

21.3 21.7 22.2 

1.3 1.6 2.0 

1.8 2.2 

1.4 

31.9 

1.3 42.1 13.2 l4.7 

12.8 2.0 2.5 

53.7 

62.0 2.5 34.2 

1.6 

1.6 1.9 - 3.2 

1.7 13.6 14.7 45.6 

2.0 2.8 16.0 - - 3.6 

- - 

0 

{ 20 

- - i - - 

50 

80 

100 

a Wiitala (1961). 

OEspey. Morgan. and Masch 

{ 

’ Anderson (1968). Martens (1966). 3 Wilson (1966). (1066). I Carter (1061). 

As an indication of the change in impervious 

area resulting from urbanization, Harris and 

Rantz (1964) showed that an area near Palto 

Alto, Calif., changed from 5.7 percent to 19.1 

percent impervious in a 10-year period. 

One of the most complete analyses of urbanization effects was made by D. G. Anderson 

(1968) in his study of the urbanization in 

Fairfax County, Va., near the metropolitan 

complex of the Nation’s Capital. Anderson’s 

analysis follows the procedure suggested earlier 

by Carter, but Anderson included a larger 

array of data from 64 gaging stations. Anderson closely confirmed the conclusions of Carter, 

but he carried the analysis further in a plot of 

4 
the ratio of peak discharge to the mean annual 

flood for different percentages of basin imperviousness and for flood flows exceeding the 

mean annual flood. For table 1, data from 

Anderson’s study were read directly from his 

graph at the 2.33-year recurrence interval and 

expressed two separate conditions of sewerage. 

The first condition was expressed as “main 

channels natural, upstream drainage sewered” ; 

this was assumed to be 50 percent sewered. The 

second condition was expressed as “completely 

sewered drainage basin” ; this was assumed to 

be 100 percent sewered. 

Wiitala (1961) presented data on urbanized 

versus rural conditions for a medium-sized 

watershed in Michigan. His data were translated into a ratio of peak discharges and it was 

assumed from his report that the urbanized 

condition represented 20 percent impervious 

area and 50 percent sewered area. 

Martens (1966) reported on three small 

drainage basins in and near Charlotte, N.C. 

Using flood-frequency curves from long-term 

records at gaging stations in the State, he 

constructed a graph similar to that of Anderson; that is, ratio to mean annual flood for 

various degrees of basin imperviousness. As 

before, the difficulty lies in ascertaining the 

relation of Martens’ urbanized condition to the 

degree sewered. In reading from Martens’ 

graph for recurrence interval 2.33 years, it is 

assumed that the conditions he discussed include no sewerage and represent changes in 

impervious area only. 

Wilson (1966) presented data on flood frequency for four drainage basins of 1.1 to 11.2 

sq mi near Jackson, Miss. He presented his 

analysis in the form of discharge of mean annual flood plotted against drainage area size, 

and he interpolated lines to represent the percentage of the basin having storm sewers and 

improved channels. It is assumed that his 

description “20 percent of basin with storm 

sewers and improved channels” would be 

equivalent to 20 percent impervious and 20 

Espey, Morgan, and Masch (1966) analyzed 

runoff data from urban and rural areas in 

Texas. To utilize this study, data were used 

corresponding to a basin length of 5,500 feet 

and a slope of 0.02. It was also assumed from 

his description of the area that “urban” could 

be expressed as 50 percent sewered and 20 

percent impervious. 

James (1965) analyzed runoff data from a 

44-sq-mi basin south of Sacramento, Calif., 

within which 12 sq mi had been urbanized. 

From the basic data on flow, he obtained empirical coefficients used to route a series of 

synthetic flows by using a mathematical model 

expressed as a digital computer program. The 

results were plotted in a series of curves which 

separated the effects of flood frequency, drainage area, and degree of urbanization. Though 

the derived curves do not present field data, 

they also were incorporated into table 1. 

Thus in table 1 are compiled, with certain 

necessary assumptions, the data for seven published and unpublished references which report measurements of the effect of urbanization 011 peak flow. Although -interpretations 

were necessary to express the degree of 

urbanization in quantitative terms, there is 

considerable agreement among the data. 

0 20 40 60 80 100 

PERCENTAGE OF AREA IMPERVIOUS percent sewered. Similarly, his value of 80 

percent was assumed to be 80 percent sewered 

and 80 percent impervious. 

Figure 2.-Effect of urbanization on mean annual 

Rood for a 1-square-mile drainage area. 

(Based on data from table 1.) 

5 
Data from table 1 have been transposed into 

the graph shown in figure 2. The ratios of peak 

discharge of urbanized to rural areas are presented for different percentages of sewerage 

and impervious area; lines of equal values of 

the ratio are drawn through the data. Briefly, 

these data show that for unsewered areas the 

differences between 0 and 100 percent impervious will increase peak discharge on the average 

2.5 times. For areas that are 100 percent 

sewered, peak discharge for 0 percent impervious will be about 1.7 times the mean 

annual food and the ratio increases to about 

eight for 100 percent impervious areas. Figure 

2, then, reduces the basic data to the same units 

applicable to a 1-sq-mi drainage basin and to 

the mean annual. flood. 

A basin produces big flows from large and 

intense storms and smaller flows from less intense but 

10,000 

n 

z 

0 

U 

w 

v, 

ai 

Lu 

CL 

ILu 

w 

LL 

2 5000 

m 

=> 

U 

z - 

. 

w 

c3 

ai 

I 

U 

v, 

a 

- 

n 

0 

more frequent storms. The great or 

catastrophic event is rare, and the storm of 

ordinary magnitude is frequent. These events 

can be arranged in order of magnitude and 

counted. For example, all discharge events exceeding 400 cfs (cubic feet per second) can be 

tabulated from the record at a stream-gaging 

station and arranged in order of magnitude; 

the val?xes in the array can be plotted as a 

discharge-frequency curve. This has been done 

for the gaging station on West Branch Brandywine Creek at Coatesville, Pa., for 9 years of 

record (fig. 3). The theory and practice of 

constructing such flow-frequency curves is well 

known. The plotting position or frequency 

often used is defined as 

R=- n+l 

rn 

where R is the recurrence interval in years, n 

is number of years of record, and m is the rank 

of the individual event in the array. 

1 

Q/ 

/ 

/ 

/ 

' 

/ 

0.1 0.2 0.3 0.5 1 23 5 10 20 

RECURRENCE INTERVAL, IN YEARS 

Figure 3.-Flood-frequency curve for partial-duration series, West Branch Brandywine Creek at Coatesville, Pa., 

based on data for 1942, 1944-51. 

6 
Note in figure 3 that the largest flow in the 

%year record was nearly 10,000 cfs. The number 50 printed on the graph means that there 

were 50 flows equal to or exceeding 500 cfs. 

Once a year, on the average, a discharge value 

of about 900 cfs will be equalled or exceeded. 

A slightly different result would be obtained 

if, instead of using the peak flow for each 

storm, only the largest flow in each year were 

included in the array. The principle involved 

is similar. The arithmetic mean of the peak 

flows for the nine annual events is the “average 

annual flood.” The statistics of this array are 

such that the recurrence interval of this average 

annual llood is the same regardless of the length 

of record, which specifically is 2.3 years. That 

is to say, a flood of that magnitude can be expected to be equaled or exceeded on an average 

of once in 2.3 years, or 10 times in 23 years. 

Studies of river channels have shown that 

rivers construct and maintain channels which 

will carry without overflow a discharge somewhat smaller than the average annual flood. In 

fact the recurrence interval of the bankfull 

stage in most rivers is a flow having a recurrence interval of about 1.5 to 2 years. 

Urbanization tends to increase the flood potential from a given basin. The channel then 

will receive flows which exceed its capacity not 

just once in 1.5 to 2 years on the average but 

more often. It is now proposed to estimate how 

much more often and to indicate the effect of 

this increased frequency on the channel itself. 

EFFECT OF URBANIZATION ON INCREASING 

FREQUENCY OF OVERBANK FLOW 

Taking the East Branch of Brandywine 

Creek as an example, the flow-frequency curve 

can be constructed for a typical subbasin having a 1-sq-mi drainage area. Figure 4A shows 

the relation of average annual flood to drainage 

area, and figure 4B shows the flood-frequency 

curve for annual peaks for basins in the 

Brandywine area. The diagrams shown in figure 4 are similar to those published in the 

nationwide series of flood reports, U.S. Geological Survey Water-Supply Papers 1671-1689. 

From these curves a discharge-frequency relationship is developed for a drainage area of 

1 sq mi. The average annual flood is read from 

the upper graph of figure 4 as 75 cfs, and the 

lower graph is used to construct the frequency 

curve in figure 5 pertaining to a 1-sq-mi basin 

marked “unurbanized.” 

The arithmetic for the construction of the 

curve is as follows: 

Recurrence &ti0 to Recurrence 

intervnl of inran ;unnu:il Ilischsuw intrrval duration nnnual flood ’ flood 2 (cfs) series I 

(years) (years ) 

1.1 0.55 41 0.4 

1.5 .75 56 .92 

2.0 .90 68 1.45 

2.3 1.0 75 1.78 

5 1.45 110 4.5 

10 1.9 145 9.5 

’ Only the hixhest floud each yesir. 

“From fisurc 4H. :’ Obtained by multirdying ratios by 75 cls from fixure 4rl fur R 

’ All peaks during the ye;ii. Thc values in this column are mathr- drninaae arc% of 1 sci mi. 

matieally rchited to those in thc first. 

The graph marked “unurbanized” in figure 

5 is constructed on semilogarithmic paper from 

the data listed in the third and fourth columns 

of the preceding table. The ordinate is the discharge, and the lower abscissa is the recurrence 

interval in the duration series. An auxiliary 

scale gives the average number of floods in a 

10-year period (calculated as 10 years divided 

by the recurrence interval). Thus, the flow 

expected to occur once in 10 years would be 

about 145 cfs and the fifth largest would be 75 

cfs. The latter would also be the average value 

of the largest flows each year during the 10- 

year record and thus would be the “average 

annual flood.” It would plot, therefore, at an 

abscissa position approximately at 2.3-year recurrence interval. 

The effect of urbanization on the average 

annual flood is shown in figure 2, which shows 

the increase in average annual flood for different degrees of urbanization as measured by the 

increase in percentages of impervious area and , 

area served by storm sewers. For convenience b‘ 

these are tabulated as follows: 

Percentage of area sewered 

0 

20 

40 

50 

80 

100 

Percentage Ratio to 

of area average inznervmts annual flood 

0 1 

20 1.5 

40 2.3 

50 2.7 

60 4.2 

60 4.4 

7 
W 

rn 

n 

c 

W 

W 

rn 

z 

n 

rn 

z 

-I 

rn 

W 

< 

> 

- 

7 

- 

z 

4 

rn 

> 

W 

v, 

RATIO TO AVERAGE ANNUAL FLOOD 

0 - - N N 

vr 0 cn 0 cn AVERAGE ANNUAL FLOOD, IN CUBIC FEET PER SECOND 

4 

vr 0 

0 0 

- VI 0 

0 0 

0 0 

cn 

0 

0 

0 

- 

b 
0 c" 

I 0 ?;\. 

0 

a 

0 

E 

1 0 

o> 

70 m 

vr 5 

P, 

v1 

d 

0 

5 

ul 

0 

DISCHARGE, IN CUBIC FEET PER SECOND 

d 

0 

0 

d 

cn 

0 

N 

0 

0 

h) 

cn 

0 

I 

w 

0 

0 
The average annual flood of 75 cfs was then 

multiplied by these ratios and plotted as shown 

in figure 5 at the 2.3-year interval. These 

values form the basis of a series of frequency 

curves for combinations of sewered area and 

impervious area. The shapes of the curves are 

guided by the principle that the most infrequent 

floods occur under conditions that are not appreciably affected by imperviousness of the 

basin. 

The most frequent flows are therefore increased by smaller ratios than would be the 

average annual flood. Also, the most frequent 

flows are decreased in number because low 

flows from an urbanized area are not sustained 

by ground water as in a natural basin. The 

curves representing urbanized conditions therefore converge at low flow values. 

Obviously the frequency curves in figure 5 

are extrapolations based on minimal data and 

require corroboration or revision as additional 

field data become available. 

The flood-frequency curve under original 

(unurbanized) condition passes through a value 

of 67 cfs at a recurrence interval of 1.5 years. 

At bankfull condition natural channels generally can carry the flow having that recurrence 

interval. If one assumes that this flow approximates the capacity of the natural channels, the 

intersection of the estimated curves for different degrees of urbanization with the discharge 

value of 67 cfs can be used to estimate the inPercentage sewered 20 40 50 80 100 

Percentage impervious 20 

I 

40 50 60 60 

I 

NONE MEASURE OF URBANIZATION COM PLE TE 

Figure 6.-Increase in number of flows per year equal to or exceeding original channel capacity (l-squaremile drainage area), as ratio to number of overbank flows before urbanization, for different degrees of 

urbanization. (Derived from figure 5.) 

10 
crease in number of flows equal to or exceeding 

natural channel capacity. An auxiliary scale is 

shown at the top of figure 5 to facilitate this. 

For example, under natural conditions it is 

expected that a 10-year record would show 

about seven flows equal to or exceeding 67 cfs, 

or channel capacity. But if the average annual 

flood were increased 1.5 times (from 75 to 112 

cfs) corresponding to 20 percent sewered and 

20 percent impervious, the new frequency curve 

indicates that 14 flows of 67 cfs or greater 

would occur in a 10-year period, or a twofold 

increase in number of flows. Similarly, the 

ratio of number of flows exceeding bankfull 

capacity was read from the intersection of the 

other curves in figure 5 with the ordinate value 

of 67 cfs to obtain the ratios plotted in figure 6. 

Figure 6 shows that with an area 50 percent 

sewered and 50 percent impervious, for example, the number of flows equal to or exceeding 

bankfull channel capacity would, over a period 

of years, be increased nearly fourfold. 

LOCAL STORAGE TO COMPENSATE FOR PEAK 

FLOW INCREASE 

3. Check dams, ungated, built in headwater 

swales. 

4. Storage volumes in basements of large 

buildings receiving water from roofs or 

gutters and emptying into natural 

streams or swales. 

5. Off channel storage volumes such as artificial ponds, fountains, or tanks. 

6, Small reservoirs in stream channels such as 

those built for farm ponds. 

Various types of storage volumes could be 

used simultaneously in various mixes. The 

effectiveness depends on the volume of storage 

relative to the volume of inflow during a storm 

peak period. Design criteria to guide city engineers and developers are needed. 

SEDIMENT PRODUCTION 

The basic data available for analyzing the 

effect of urbanization on sediment yield, though 

sparse, have been summarized to some extent 

in the literature. Especially valuable is the report by Wolman (1964) who summarized not 

only the data obtained from sediment sampling 

stations in streams in Eastern United States 

Urbanization tends to increase both the flood 2 but also studied the sediment yield from buildvolume and the flood peak. But the increase 

can be compensated so that the discharge 

through channels downstream is maintained to 

any degree desired within the range which existed prior to urbanization. It is obvious that 

reservoir storage is installed on a river in order 

to reduce the magnitude of peak discharge by 

spreading the flow over a longer time period. 

Channels themselves provide temporary storage 

and act as if they were small reservoirs. Overbank flooding on to the flat flood plain is a way 

that natural rivers provide for temporary storage and thus decrease flood peaks downstream. 

This effect of storage has been fully investigated and described (for example see Leopold 

and Maddock, 1954, especially p. 36-49). 

The provision of flood storage upstream, 

then, will decrease flood peaks and compensate 

for the increase caused by urbanization. This 

storage could take many forms including the 

following : 

1. Drop inlet boxes at street gutter inlets. 

2. Street-side swales instead of paved gutters 

and curbs. 

ing construction activities. Sediment yields ' 

from urbanized or developing areas ranged 

from 1000 to more than 100,000 tons per square 

mile per year. 

It should be recognized that sediment yield 

per square mile decreases with increasing 

drainage area, but nevertheless it is apparent 

that unurbanized drainage basins yield 200 to 

500 tons per square mile per year, on the average. These figures are slightly higher for the 

farmed Piedmont lands, which may be expected 

to produre sediment yield of 500 tons per square 

mile per year, such as the Watts Branch basin 

near Rockville, Md. 

The data on urbanized areas studied by 

Wolman are plotted in figure 7 together with 

data from suspended load sampling stations of 

the U.S. Geological Survey as summarized by 

Wark and Keller (1963). 

In the graph (fig. 7) three bands or zones are 

labeled A, C, and UC. Wolman and Schick 

(1967) differentiated the following types of 

activity : A, agricultural or natural ; C, underb' 

11 
200,000 

100.000 

E 

< w 

> 

CL 

w 

a 

w d 

- 

zi 

w 

E 

10,000 

0 

cn 

E 

w 

a 

Ln 

z 

E 

z 

z 

0 

F 1000 U 

3 

n 

0 

E 

p. 

+ 

z 

Ly 

I n 

w 

cn 

100 

50 

Urbanized (data from Wolman, 1964) 

X 

Mostly natural (U. S. Geological Survey data) 

I I 

0 

X 

X 

EXPLANATION 

0 I I 

Watts Branch . 

Y 

X 

0.01 0.1 1 IO * 100 

DRAINAGE AREA, IN SQUARE MILES 

Figure 7.-Annual sediment production per square mile for urbailized and natural areas. Zones: A, agricultural; C, under construction ; UC, under construction and undiluted. 

going building construction, but highly diluted 

before reaching channels ; and UC, undiluted 

sediment yields delivered to stream channels 

from construction sites. 

They found that when building sites are denuded €or construction, excavations are made, 

and dirt is piled without cover or protection 

near the site, the sediment movement in a rill 

or stream channel is very large in terms of tons 

per year immediately downhill from the construction site. If the channel contains little 

water except during storms (an ephemeral 

stream), there is no chance for dilution ; during 

storm flow the sediment movement is great. If 

the construction debris gets into perennial 

channels, or for other reasons is distributed 

along a channel or dispersed over a wide area, 

the dilution lowers the yield per square mile 

per year. Thus, Wolman and Schick drew the 

distinction between agricultural, construction, 

and construction-undiluted. 

For very small areas, Wolman (1964) said, 

“Because construction denudes the natural 

cover and exposes the soil beneath, the tonnage 

of sediment derived by erosion from an acre of 

ground under construction in developments and 

highways may exceed 20,000 to 40,000 times the 

amount eroded from farms and woodlands in an 

equivalent period of time.” 

Figure 7 shows the data as a relation between 

annual sediment yield per square mile and 

drainage basin size. The usual suspended load 

station is on a basin of more than 10 sq mi in 

area. Seldom is urbanization complete for 

basins of this size. ~ 

12 
10,000 

000 

100 

10 

EXPLANATION 

Little Falls Branch near 

Bethesda, Md. (urban) 

Rockville, Md. (rural) 

X- Watts Branch near 

x 

4 x 

10 

k 8 

0 

Figure 8.-Relation of sediment yield and discharge for an urban and a rural or unurbanized area. 

13 

100 1000 

DISCHARGE, IN CUBIC FEET PER SECOND 

10,000 
The data measured or estimated by Wolman 

(1964) in small urbanizing, developed or industrial areas show clearly that the sediment 

yield is larger by 10 to 100 times that of rural 

areas. Guy and Ferguson (1962) observed an 

increase of 250 times in an area near Kensington, Md. 

Keller (1962) compared the sediment rating 

curves for Northwest Branch of Anacostia 

River near Colesville, Md., a relatively unurbanized basin, and the Anacostia River basin 

near Hyattsville, Md., which is partly urbanized. He found the sediment production to be 

about four times greater in the urbanized area. 

Most sediment carried by a stream is moved 

by high flows. In Brandywine Creek, for example, about 54 percent of the total sediment 

transported annually by the river (drainage 

area 312 sq mi) is carried by flows that occur, 

on the average, about 3 days each year. 

To illustrate the difference in sediment 

samples obtained during storm flow, actual 

data for two stations are shown in figure 8. 

The sediment rating curve, which is a plot of 

the discharge at any moment in time against 

the concurrent rate of sediment transport, gives 

an indication of the order of magnitude of the 

increase in sediment production from developed, as against rural, areas. The sediment 

rating curves in figure 8 are for stations near 

Washington, D.C. Watts Branch drains an 

area prjmarily used for farming though urban 

influences have recently extended into the 

In the tabulation below, a comparison is 

made between sediment yield from Watts 

Branch, a rural landscape, and Little Falls 

Branch, an urban one. These basins are of the 

size and type represented in East Branch 

BrandyTvine Creek. 

Drainaoe Tone per Tons per basin. Little Falls Branch near Bethesda drains area year year per a nearly compeltely urbanized community, con- Watts Branch in (8s mi) 8q mi 

Md. Md. (rural) - ____ __~_ 3.7 1,910 516 

sisting af Bethesda and parts of Chevy Chase, Rockville, 

Note that the sediment rating curves tend to 

converge at high discharges. One might suppose that at those discharges the urbanized 

areas are actually contributing no more sediment than the unurbanized ones. This is not 

the case, however, owing to the fact that as a 

result of urbanization, the number of high flows 

increases materially. Because most of the sediment during the year is carried during periods 

of high flow, the result is that urbanized areas 

yield on the average larger sediment loads than 

the unurbanized ones. 

The difference in drainage basin size between 

Watts Branch (3.7 sq mi) and Little Falls 

Branch (4.1 sq mi) is not alone sufficient to 

explain the larger discharges in the latter basin. 

For about the same number of sample storms, 

note that Little Falls Branch data include discharges varying from 20 to 1500 cfs. In contrast, Watts Branch data (unurbanized) include flows ranging from 7 to 150 cfs. At least 

some of this difference is probably due to the 

effect of urbanization on increasing peak flow 

from a storm of given size, as discussed earlier. The two basins are only 10 miles apart 

and storms are comparable. 

Little Falls Branch 

near Bethesda, 

Md. (urban) ___ _~~_ 4.1 9,530 3,220 

Sediment production is importantly related 

to land slope. Using multiple correlation techniques for a large variety of data from experimental watersheds, Musgrave (1947) developed 

a multiple correlation in which the rate of 

erosion is found to be proportional to the 1.35 

power of land slope and to the 0.35 power of 

the slope length. The same conclusion had been ' 

derived theoretically by Horton (1945) and 

verified by comparison with the percentage of 

area eroded in the Boise River basin, Idaho. 

Sediment yield, therefore, is more highIy sensitive to land slope than to length of slope but is 

postively correlated with both. 

Some idea, however, can be obtained of the 

difficulty in keeping steep slopes stable after 

the original vegetation has been disturbed, particularly during construction. If, for example, 

land slopes of 5 and 10 percent are compared, 

the doubling of the slope would increase the 

erosion rate by 2.3 times. 

Increased slope length does not have such a 

large effect on erosion rate. Doubling slope 

14 
length would increase the erosion rate by only 

22 percent. 

Because a slope of 10 percent drops 10 feet 

in a 100-foot horizontal, temporary storage in 

the form of depressions which might hold silt 

would be nearly absent. For land slopes above 

10 percent, stream channels also would tend to 

be nearly devoid of areas or depressions which 

could hold up sediment during its passage 

downhill. From a practical standpoint, therefore, a figure of about 10 percent probably 

would be a physical and economic limit beyond 

which construction would be especially harmful 

insofar as sediment production is concerned. 

Any such limiting slope, however, would have 

to be determined by detailed economic studies. 

Wark and Keller (1963) related the average 

annual sediment discharge in the Potomac 

River basin to percentage of forest cover and, 

separately, to the percentage of land in crops. 

Average annual sediment yield increased from 

50 to 400 tons per square mile per year, or 

eightfold, as forest cover in the basin declined 

from 80 percent to 20 percent. Sediment yield 

increased from 70 to 300 tons per square mile 

per year, or fourfold, as land in crops increased 

from 10 to 50 percent. 

EFFECT OF INCREASED PEAK FLOWS ON 

SEDIMENT YIELD 

It has been pointed out in the comparison of 

sediment rating curves for urban versus rural 

areas that the rating curves do not appear to 

be as much different as the values of sediment 

yield on an annual yield basis. It has been 

mentioned that a slight increase of sediment 

concentration can make a large difference in 

total annual sediment yield owing to the fact 

that urban areas produce a larger number of 

high flows. If the number of flows above bankfull stage is increased by urbanization, the 

banks and bed of a channel in erodible material 

will not remain stable, but the channel will enlarge through erosion. Computation indicates 

the seriousness of this factor. 

For example, assume that a channel is capable of carrying 55 cfs at bankfull stage. In the 

Brandywine area this represents a channel 

draining a basin slightly less than 1 sq mi in 

area. The channel necessary to carry 55 cfs 

at bankfull stage would probably have a velocity of slightly less than 2.5 feet per second and 

would be about 2 feet deep and 11 feet wide. In 

figure 2, urbanization might cause a flow of this 

frequency to increase 2.7 times, or 150 cfs. If 

this channel had to adjust itself to carry a flood 

of 150 cfs at bankfull stage, it is estimated that 

the new velocity would be about 2.5 feet per 

second, and the necessary depth and width 

would have changed respectively to about 3 feet 

and 20 feet. In other words, this stream would 

deepen about 50 percent and increase in width a 

little less than twice its original size. If such 

erosion takes place through at least one-fourth 

mile of channel length in a drainage basin of 1 

sq mi, the amount of sediment produced by this 

erosion would be 50,000 cubic feet. At 100 

pounds per cubic foot, this amounts to 2,500 

tons. 

This amount can be compared with the meac 

annual sediment yield for Watts Branch, an 

unurbanized area near Rockville, Md. Annual 

sediment yield of Watts Branch is 516 tons per 

square mile. Thus, the channel erosion alone 

under. the assumptions made would produce as 

much sediment as 5 years’ usual production 

from an unurbanized area of the same size. 

Therefore, one can visualize that as urbanization proceeds, not only does construction activity have the potential of increasing sediment 

loads many thousands of times while construction is in progress, but also the result of the 

urbanization through its increase in peak flow 

would produce large amounts of sediment from 

channel enlargement as well. This emphasizes ~ 

the need to provide temporary storage far upstream to counteract the tendency of urbanization to increase the number and size of 

high flows. 

WATER QUALITY 

There is little doubt that as urbanization increases, particularly from industrial use of 

land and water, the quality of water decreases. 

However, quantitative data to support this observation are sparse. There are two principal 

effects of urbanization on water quality. First, 

the influx of waste materials tends to increase 

the dissolved-solids content and decrease the 

dissolved-oxygen content. Second, as flood peaks 

increase as a result of the increased area of 

imperviousness and decreased lag time, less 

15 
water is available for ground-water recharge. 

The stream becomes flashier in that flood peaks 

are higher and flows during nonstorm periods 

are lower. 

A recent study on the Passaic River at Little 

Falls, N.J., by Anderson and Faust (1965) 

provides quantihtive data on the effect of 

urbanization and industrialization on water 

quality. Seventeen years of data for the flow 

and chemical quality of the 760-sq-mi drainage 

basin were analyzed. During these 17 years, 

diversions of water for domestic and industrial 

supplies increased more than 30 percent between 1950 and 1963. Returns of waste waters 

into the basin became as much as 10 percent of 

the water withdrawn. Analysis of the data 

showed that at relatively low discharge the 

dissolved-solids content increased about 10 ppm 

(parts per million) between 1948 and 1955 but 

increased 75 ppm between 1955 and 1963. That 

is, during the period of greatest population 

growth the dissolved-solids content increased 

nearly 40 percent in a period of 8 years. 

A long-term change in the average content of 

dissolved oxygen was also noted. Between 1950 

and 1964 the dissolved-oxygen content dropped 

from an average of 78 percent of saturation to 

62 percent of saturation. Further, the analysis 

demonstrated that these average changes in 

water quality occurred in all seasons of the 

year. 

An aspect of population growth not generally 

appreciated is the large segment of population 

using septic tanks for disposal of sewage. In 

a given area this segment often becomes large 

before community water and sewerage systems 

are built. For the planner it should be important to know how septic-tank installations can 

affect water quality in streams and in the 

ground. In the upper East Branch of Brandywine Creek, a basin of 37 sq mi, the population 

in 1967 was 4,200. As of that date, there were 

no Community water or sewerage systems; all 

the population was served by individual wells 

and septic tanks. Population projections indicate that the basin will have 14,000 persons by 

the year 1990. During the initial part of this 

projected growth at Ieast, the number of wells 

and septic tanks can be expected to increase 

materially. 

The soil, containing as it does a flourishing 

fauna of micro-organisms, tends to destroy or 

adsorb pathogenic bacteria. Effluent draining 

from the seepage field of a septic tank tends 

therefore to be cleansed of its pathogens. 

McGauhey and Krone (1954) showed that the 

coliform count was reduced by three orders of 

magnitude in moving from an injection well a 

distance of 50 feet through sand and gravel. 

In 100 feet the count was reduced to a small 

number. As for rate of movement, Mallmann 

and Mack (1961) showed that bacteria introduced into a permeable soil by a septic-tank 

seepage field moved 10 feet in 2 days and 20 

feet in 3 days and appeared in a well 30 feet 

away after 10 days. 

Both the rate and effectiveness of the process of pathogen reduction depend on the type 

of soil as has been summarized by Olson 

(1964), who emphasized that position of the 

ground-water table is a critical factor in the 

transmission of pollutants. 

Studies by Wayman, Page, and Robertson 

(1965) of the changes in primary sewage 

effluent through natural materials in conditions 

of saturated flow showed that “most soils removed over 90 percent of the bacteria from 

sewage within a few feet of travel * * * [but 

there was] severe clogging in the finer-grained 

soils.” They found, however, that “dissolved 

solids moved through the columns [of soil] 

virtually unaffected * * *.” 

The Same authors report on infiltration of 

polluted river water through sandy loam. “ABS 

[synthetic detergent] and coliform bacteria are 

significantly reduced by infiltration through the 

unsaturated zone ; dissolved solids do not seem 

to be removed * * *. Once a pollutant gets 

into the ground water (saturated flow) little 

additional change in removal of ABS or dissolved solids, even for movement over extensive 

horizontal distances, is to be expected. This 

result is in agreement with the data * * * for 

flow of sewage effluent through various soil 

columns (saturated flow) .” 

The data are not definitive regarding the 

minimum distance a septic-tank seepage field 

should be separated from a stream channel, 

but the application of data cited above with 

16 
general principles does indicate some tentative 

rules of thumb which might be useful to the 

planner. A perennial stream represents the 

intersection of the saturated zone (water table) 

with the earth's surface. The observations indicate that, for soil cleansing to be effective, 

contaminated water must move through unsaturated soil at least 100 feet. Owing to the gentle 

gradient of the water table near the perennial 

stream and the fact that seepage water moves 

vertically as well as toward a nearby channel, 

it would seem prudent that no septic tank 

should be as close to a channel as about 300 

feet, if protection of the stream water quality 

is to be achieved. The distance should probably be greater from a perennial than from an 

ephemeral channel. (An ephemeral stream is 

one which contains flowing water only in storm 

thermal pattern. The more solar energy a 

stream absorbs, the greater its temperature 

variation diurnally as well as seasonally. By 

greatly increasing the surface area exposed to 

the sun's radiation, the construction of ponds 

and lakes has profoundly affected stream temperature regimen. On Long Island, Pluhowski 

found that ponds having mean depth of about 

2 feet or less substantially increase downstream 

diurnal temperature fluctuations whereas ponds 

deeper than 2 feet exhibit a dampening effect 

on daily temperatures. For example, during the 

period October 31 to November 2, 1967, the 

mean daily range of temperatures at Swan 

River, in south-central Long Island, varied 

from 9°F in a reach immediately below a 

shallow pond (mean depth, 0.5 foot) to 3°F 

below Swan Lake (mean depth, 3 feet). In 

periods.) In general, it might be advisable to J reaches unaffected by man's activities, the 

have no source of pollution such as a seepage 

field closer than 300 feet to a channel or 

watercourse. 

Even this minimum setback does not prevent 

the dissolved materials (nitrates, phosphates, 

chlorides) from enriching the stream water 

and thus potentially encouraging the proliferation of algae and otherwise creating a biotic 

imbalance. 

The only detailed study of the effect of urbanization on water temperature is that of E. J. 

Pluhowski (1968), some of whose results are 

summarized here. He chose five streams on 

Long Island for detailed analysis and found 

that streams most affected by man's activities 

exhibit temperatures in summer from 10" to 

15°F above those in an unurbanized control. 

Connetqiiot River, the control stream, flows 

through one of the few remaining undeveloped 

areas of central Long Island. Temperatures in 

reaches most affected by ponding, realinement, 

or clear cutting of trees are significantly higher 

in summer, but winter temperatures are 5" to 

10°F colder than those observed in reaches unaffected by man. 

Solar radiation is the predominant factor in 

the energy balance determining a stream's 

mean daily temperature fluctuation was about 

4°F. 

Under natural conditions, less than 5 percent 

of the streamflow on Long Island originates 

as direct surface runoff. With the conversion 

of large areas of western Long Island from 

farmland to suburban use during the last 20 

years, the proportion of streamflow originating 

as surface runoff has increased sharply. As a 

direct ons sequence, streams most affected by 

street runoff may exhibit temperature patterns 

that are markedly different from those observed 

in streams flowing through natural settings. 

During the period August 25 to 27, 1967, a 

series of heavy rainstorms overspread Long 

Island. Throughout this period, temperatures 

at each of the five observation sites on Connetquot River showed little day-to-day change. 

In contrast, temperatures in the upper reaches 

of East Meadow Brook, which drains highly 

urbanized central Nassau County, increased 

steadily in response to the relatively warm 

street runoff. Pluhowski found that by August 

27, water temperatures had risen 10" to 12°F 

above prestorm levels and were 15°F higher 

than concurrent temperatures in the control 

stream. 

17 
SELECTED REFERENCES 

Anderson, D. G., 1968, Effects of urban development 

on floods in northern Virginia: U.S. Geol. Survey 

open-file rept., 39 p., 5 figs. 

Anderson, Peter W., and Faust, Samuel D., 1965, 

Changes in quality of water in the Passaic River 

at Little Falls, New Jersey, as shown by long-term 

data, in Geological Survey research 1965: U.S. 

Geol. Survey Prof. Paper 525-D, p. D214-D218. 

Antoine, L. H., 1964, Drainage and best use of urban 

land: Public Works [New York], v. 95, p. 88-90. 

Carter, R. W., 1961, Magnitude and frequency of floods 

in suburban areas, in Short papers in the geologic 

and hydrologic sciences: U.S. Geol. Survey Prof. 

Paper 424-B, p. B9-Bll. 

Espey, W. H., Morgan, C. W., and Masch, F. D., 1966, 

Study of some effects of urbanization on storm 

runoff from a small watershed: Texas Water 

Devel. Board Rept. 23, 109 p. 

Felton, P. N., and Lull, H. W., 1963, Suburban hydrology can improve watershed conditions : Public 

Works, v. 94, p. 93-94. 

Guy, H. P., and Ferguson, G. E., 1962, Sediment in 

small reservoirs due to urbanization: Am. SOC. 

Civil Engineers Proc., HY 2, p. 27-37. 

Harris, E. E., and Rantz, S. E., 1964, Effect of urban 

growth on streamflow regimen of Permanente 

Creek, Santa Clara County, California: U.S. Geol. 

Survey Water-Supply Paper 1591-B, 18 p. 

Horton, R. E., 1945, Erosional development of streams 

and their drainage basins, hydrophysical approach 

to quantitative morphology : Geol. SOC. America 

Bull., v. 56, no. 3, p. 275-370. 

James, L. D., 1965, Using a computer to estimate the 

effects of urban development on flood peaks: Water 

Resources Research, v. 1, no. 2, p. 223-234. 

Keller, F. J., 1962, The effect of urban growth on sediment discharge, Northwest Branch Anacostia River 

basin, Maryland in Short papers in geology and 

hydrology: U.S. Geol. Survey Prof. Paper 450-C, 

Leopold, L. B., and Maddock, T., Jr., 1954, The flood 

control controversy: New York, The Ronald Press 

Company, 275 p. 

Leopold, L. B., Wolman, M. G., and Miller, J. P., 1964, 

Fluvial processes in geomorphology : San Francisco, Calif., W. H. Freeman and Co., 522 p. 

p. C129-C131. 

Lull, H. W., and Sopper, W. E., 1966, Hydrologic effects 

from urbanization of forested watersheds in the 

northeast : Upper Darby, Pa., Northeastern Forest 

Expt. Sta., 24 p. 

McGauhey, P. H., and Krone, R. B., 1954, Report on the 

investigation of travel of pollution : California 

State Water Pollution Control Board Pub. 11, 

218 p. 

Mallmann, W. L., and Mack, W. N., 1961, Biological 

contamination of ground water: Robert A. "aft 

Sanitary Eng. Center Tech. Rept. W61-5, p. 35-43. 

Martens, L. A., 1966, Flood inundation and effects of 

urbanization in metropolitan Charlotte [North 

Carolina]: U.S. Geol. Survey open-file rept., 54 p. 

Musgrave, G. W., 1947, Quantitative evaluation of 

factors in water erosion-First approximation : 

Jour. Soil and Water Conserv., v. 2, no. 3, p. 133- 

138. 

Olson, G. W., 1964, Application of soil survey to problems of health, sanitation, and engineering : 

Cornel1 Univ. Agr. Expt. Sta. Mem. 387, 77 p. 

Pluhowski, E. J., 1968, Urbanization and its effect on 

stream temperature: Baltimore, Md., Johns Hopkins Univ., Ph. D. dissert. (in preparation). 

Snyder, F. F., 1938, Synthetic unit hydrographs: Am. 

Geophys. Union Trans., v. 19, p. 447-454. 

Swenson, H. A., 1964, Sediment in streams: Jour. Soil 

and Water Conserv., v. 19, no. 6, p. 223-226. 

Wark, J. W., and Keller, F. J., 1963, Preliminary study 

of sediment sources and transport in the Potomac 

River Basin: Interstate Comm. on Potomac River 

Basin, Washington, D.C., Tech. Bull. 1963-11, 28 p. 

Wayman, C., Page, H. L., and Robertson, J. B., 1965, 

Behavior of surfactants and other detergent components in water and soil-water environments : 

Federal Housing Adm. Tech. Studies Pub. 532, 

136 D. 

Wiitala, S. W., 1961, Some aspects of the effect of urban 

and suburban development upon runoff: U.S. Geol. 

Survey open-file rept., 28 p. 

Wilson, K. V., 1966, Flood frequency of streams in 

Jackson, Mississippi : U.S. Geol. Survey open-file 

rept., 6 p. 

Wolman, M. G., 1964, Problems posed by sediment derived from construction activities in MarylandReport to the Maryland Water Pollution Control 

Commission : Annapolis, Md., 125 p. 

Wolman, M. G., and Schick, P. A., 1967, Effects of construction on fluvial sediment, urban and suburban 

areas of Maryland : Water Resources Research, 

v. 3, no. 2, p. 451-462. 

18 

ir U.S. GOVERNMENT PRINTING OFFICI; ' 1968 0 - 298-448