# 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 annualflood 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. Increasein 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 anddischargefor 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

I11HYDROL.OGY FOR URBAN LAND PLANNING-A 

### GUIDEBOOKON THEHYDROLOGIC
EFFECTS 

### OF URBAN LAND USE

By

### LUNA B. 

### LEOPOLD

Thiscircular attemptsto summarizeexisting 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 anexplanatory, 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 foradoption 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 mayactuallychange,albeitslowly. 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 hydrologicterms.

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, andchangesinthehydrologic amenities. The hydrologic amenities are what might be called the appearance or the impression which theriver,its channelanditsvalleys,leaveswith theobserver. 

### Of all land-use changes affecting the hydrology 

### of an area, urbanization 

### is by farthemost 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 -'

1rises 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 

### atwhich water is transmitted across the land tostream channels. The former 

### is governedby thetype of land use ;the latter isgovernedbythedensity,size,andcharacteristicsof tributary channelsandthusbythe 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 thepercentage of the area covered by roofs, streets, mdother impervious surfaces attimes 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 followingdata, on the percentage of impervious surface area in residential properties :

Id hizr of Impeiviotix risidpwtinl aipa xurfacrarea fS[ltti (iicrrtnt)

~ - ~-

~ 80 6,000 -_ 6,000-15,000

~ ~ 40

~~ 25 15,000

~~

The percentage decreases markedly as size of lot increases. Felton and Lull (1963) 

### estimate in the Philadelphia area that 32 percent ofthesurfaceareais imperviouson lotsaveraging 0.2 acre in size, whereas only 8 percent ofthe surfacearea isimperviouson lotsaveraging 1.8acres. As volume of runoff from a storm increases, the size of flood peak also increases. Runoff volume also affects low flows because in any seriesof stormsthelarger thepercentage 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 sufficientto yield considerable sediment. A major effect of urbanization is the introduction of effluent fromsewage disposalplants, 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 sewagetreatment. These minerals act asnutrients and promote algae and plankton growth in a stream. This growth 

### inturn alters thebalance in the stream biota. Land use in all forms affects water quality. Agricultural use results in an increase of nutrients in streamwater both fromthe excretion products of farm animals and from commercialfertilizers. 

### A changefromagricultural 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 arecarried 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



### 2channel beds, and unusual debris accumulations. These all tend to decrease the amenity value of a stream. Thesecondfactoristheaccumulation of artifacts of civilization in the channel and on the flood plain :beer cans,oil drums,bitsof lumber, concrete, wire-the whole gamut of rubbish of
an urban area. Though this may not importantly sffect the hydrologic function of the channel,itbecomes a detrimentof what ishere called the hydrologic amenity. 

# I

?Lag time

of streamflow

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, andodorsmay develop. 

### Asaresult 

### of increased turbidity and reduced oxygen content desirable game fish give way to less desirable species. Although lack of quantitative objective dataon thebalance 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 theCharacteristics of theunit 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 1inch 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 andtherunoff is lagtime. Thisis defined
as thetime intervalbetween 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 andthebasin length. These factors empirically correlate with lagtime 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 foragiven amountof waterto 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



### 3frequency, flow duration, mean annual flood, discharge at bankfull stage, and frequency of bankfull stage. This isevidenced inpaststudies of theeffects 

### of urbanization on the hydrology of an area. Many different techniques of 

### relating rainfall to runoff have been used, along with various parameters to measurethe degree of urbanization. In order to evaluate our present knowledge, it is necessary to express the results 

### of these studies in some common denominator.
Mostreports 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. Thefirst 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 isplotted againstthepercentage of the
area rendered impervious by urbanization ; isopleth lines (linesof equal value of theratio)
on the graph showthe 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.
Thesecondmethod utilizes a relationship between the degree of urbanization and the frequency 

### at which the original channel capacity would be exceeded. Table 1 

### is aninterpretation and summary of the effectsof urbanization on peak discharges based on previous studies. Results of the studies were interpreted and extrapolated to a
common denominator of 1sq mi (squaremile),
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 termstoquantitative measuresof urbanization. From data presented by Carter, values were read for 

### a basin ratio 

### of 

### 0.12 representing 

### a 1-sq-mi areahaving anestimated length of 1.2 miles andanaverage slopeof 100feetper 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 urbanizationin 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 anindication of thechange 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 followstheprocedure suggested earlier by Carter, but Anderson included 

### a larger array of data from 64 gaging stations. Anderson closely confirmedthe conclusions of Carter, but he carried the analysis furtherin 

### a plot of

4the ratio of peak dischargeto 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 channelsnatural, upstream drainage sewered” ; thiswas assumedtobe 50percent 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 aratio of peak discharges and itwas 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 

### tothe 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 1.1to 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 resultswereplotted in 

### a seriesof 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 1are 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.)
5Data from table 

### 1have been transposed into thegraphshown in figure

### 2. The ratiosof 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 100percent impervious will increasepeak dischargeon 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 eightfor 

### 100percent imperviousareas. Figure 2,then,reducesthebasic datato the sameunits 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 CLILu 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 (cubicfeet 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. Thishasbeen done for thegagingstation

### onWest Branch Brandywine Creek 

### at Coatesville, Pa., for 9 years of record (fig. 

### 3). The theory and practice 

### of constructingsuchflow-frequency curves iswell known. The plotting position or frequency often used is defined 

### as 

### R=-n+l
rn where 

### R is the recurrence interval in years, n isnumber of yearsof record,and mis therank 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 atCoatesville, Pa., based on data for 1942, 1944-51.
6Note in figure 3 that the largest flow in the %year record was nearly 10,000cfs. Thenumber 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 orexceeded. 

### 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 flowsforthe nineannualevents isthe“average annual flood.” The statistics of this array are suchthatthe recurrenceinterval ofthisaverage annualllood isthesameregardless of thelength of record, which specifically is 2.3years. That is to say, a flood of that magnitude can be expected tobe equaled orexceeded on anaverage of once in 2.3years, 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 theaverage annual flood. In fact the recurrence interval of the bankfull stage in most rivers is a flow having 

### a recurrence interval of about 1.5to 2 years. Urbanization tends to increase the flood potential from 

### a given basin. The channel then will receive flowswhich exceed itscapacity not just once in 1.5to 

### 2 years on the average but
moreoften. 

### It is nowproposed toestimatehow 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 asanexample, the flow-frequency curve can be constructed for 

### a typical subbasin having 

### a 1-sq-mi drainage area. Figure 4A shows therelation of averageannual floodtodrainage 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 1sqmi. Theaverage 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-mibasin marked “unurbanized.” The arithmetic for the construction of the
curve is asfollows:

Recurrence Recurrence &ti0 to intervnlof 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 5isconstructed on semilogarithmicpaper from the datalisted in thethird and fourth columns of the preceding table. Theordinate is the discharge,andthelower abscissa istherecurrence interval in the duration series. An auxiliary scale gives the average number of floods in 

### a 10-year period (calculated as 10years divided by the recurrence interval). Thus, the flow expected to occur once in 10 years would be about 145cfs and thefifth largest would be 

### 75 cfs. The latter would also be theaveragevalue 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 differentdegreesof urbanization asmeasured by the increase in percentages 

### of impervious area and

, area served by storm sewers. For convenience b‘ these are tabulated as follows:

Percentage

ofarea sewered 0 20 40 50 80 100 Percentage Ratio to ofarea average inznervmts annualflood 0 1 20 1.5 40 2.3 50 2.7 60 4.2 60 4.4
7W rn n c W W rn 

### z n rn
z -I rn W < 

### > 

# -

7 

# - z
4 rn 

### > W v, 

### RATIO 

### TO AVERAGE ANNUAL FLOOD



# - 

# - N N 0 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



## -

b0 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 0The 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 imperviousarea. The shapes of the curves are guidedby theprinciplethatthemostinfrequent 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 flowsfromanurbanized areaarenot sustained by ground water as in a natural basin. The curvesrepresentingurbanizedconditions

### 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) conditionpassesthrough

### avalue 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 assumesthatthis

### 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 estimatethe inPercentagesewered 

### 20 40 50 80 

### 100
Percentage impervious 

### 20 

### I 

### 40 50 

### 60 60 

## I NONE MEASURE OF URBANIZATION 

### COM

### PLETE
Figure 6.-Increase in number of flows per year equal toor exceeding original channel capacity (l-squaremile drainage area), as ratio to number of overbankflows before urbanization, for different degrees of urbanization. (Derived from figure 5.) 

### 10crease in number of flowsequal toorexceeding naturalchannel 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,
orchannel capacity. But if theaverage annual flood were increased 1.5times (from75 to 112 cfs) corresponding to 20 percent sewered and 20percentimpervious,thenew 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 5with theordinatevalue of 67 cfstoobtain theratiosplotted in figure 6. Figure 6 showsthat with anarea 

### 50 percent sewered and 

### 50 percent impervious, for example,thenumber of flowsequaltoorexceeding 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 

### tothe volume of inflow during a storm peak period. Design criteria toguide city engineers anddevelopersareneeded.
SEDIMENT PRODUCTION The basic data available for analyzing the effect 

### of urbanization on sedimentyield,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 tendstoincrease both theflood2 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 storageis installed on 

### ariver in order to reduce the magnitude of peak discharge by spreading the flow over 

### a longer time period. Channelsthemselvesprovidetemporary storage and act

### as if they were small reservoirs. Overbank flooding

### on tothe

### flat floodplain is 

### a way that natural riversprovide fortemporary storageandthusdecreaseflood 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 gutterinlets. 2. Street-side swales instead of paved gutters and curbs. ing construction activities. Sediment yields ' from urbanized or developing areas ranged from 1000to morethan 100,000tons per square mileper 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, ontheaverage. These figures are slightly higher for the farmed Piedmont lands,which maybe expected to produre sedimentyieldof 500tonsper 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 datafrom suspended load sampling stations of the U.S. Geological Survey as summarized by Wark and Keller (1963).
Inthegraph (fig.7)threebandsorzones are labeled A, C, and UC. Wolman and Schick (1967) differentiated the following types of activity :

### A,agricultural or natural ;

### C, underb'

11200,000

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(datafrom Wolman, 1964)

X Mostly natural (U. S. Geological Survey data)
I I 0

X

X EXPLANATION Watts Branch 

# .

0 

### I I
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
orstreamchannel isverylargeintermsof tons per year immediately downhill from the construction site. If the channel contains little water except during storms (an ephemeral stream),thereisnochancefordilution ;during storm flow the sedimentmovement isgreat. 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 exposesthesoilbeneath,thetonnage of sediment derivedby erosion from anacre

### of ground under constructionin developmentsand highwaysmayexceed20,000to40,000timesthe amount erodedfromfarmsandwoodlandsin an equivalentperiod of time.” Figure7showsthedata

### as

### arelation 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

x 4 

### 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 10,000The 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 100times that of rural
areas. Guy and Ferguson (1962) observed an increase of 250times in an areanear Kensington, Md. Keller (1962) compared the sediment rating
curves for Northwest Branch of Anacostia River near Colesville, Md., 

### a relatively unurbanized basin, andtheAnacostia Riverbasin near Hyattsville, Md., which is partly urbanized. 

### He found the sediment production to 

### be about fourtimes greater intheurbanized 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 flowsthat 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 theconcurrentrateof sedimenttransport, 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 arefor stations near Washington, D.C. Watts Branch drains an areaprjmarily used forfarmingthough 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 Toneper Tonsper basin. Little

### FallsBranchnearBethesda drains area year yearper 

### 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,

Notethat thesedimentrating curvestend 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,thenumber of high flows increases materially. Because most of the sediment during theyear is carried during periods of high 

### flow, the result is that urbanized areas yield onthe averagelarger sedimentloadsthan the unurbanized ones. Thedifferencein drainagebasin size between Watts Branch (3.7 sq mi) and Little Falls Branch (4.1 sq mi) is not alone sufficient to explainthelargerdischarges inthe latterbasin. 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 flowsranging from 7 to 150cfs. 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 largevariety of datafrom experimentalwatersheds,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 theslopelength. The sameconclusionhad been ' derived theoretically by Horton (1945) and verified by comparison with the percentage of
area eroded in the Boise River basin, Idaho. Sedimentyield, therefore, ismore highIy sensitive to land slopethan tolength of slope butis postively correlated with both. Some idea, however, can be obtained of the difficulty in keeping steep slopes stable after theoriginal vegetation hasbeen disturbed, particularly during construction. If, for example, land slopes of 5 and 10percent are compared, the doubling of the slope would increase the erosion rateby2.3times. Increased slope length does not have such a large effect on erosion 

### rate. Doubling slope



### 14length would increase the erosion rate by only 22 percent. Because 

### a slope of 10 percent drops 10feet in 

### a 100-foot horizontal, temporary storage in the form of depressions which might hold silt would be nearly absent. For land slopes above 10percent, stream channels also would tend to be nearly devoid of areasor 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 constructionwould be especiallyharmful insofar as sediment production is concerned. Any such limiting slope, however, would have tobe determined by detailed economic studies.
Wark and Keller (1963) related theaverage 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, asforest cover in the basin declined from 80 percent to 20 percent. Sediment yield increased from 

### 70 to 300 tons per square mile peryear, orfourfold,asland in cropsincreased from 10to 50 percent.
EFFECT OF INCREASED PEAK FLOWS ON SEDIMENT YIELD 

### It hasbeen 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 asthe 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 thenumber of flowsabove bankfull stage is increased by urbanization, the banks andbed of 

### a channel in erodible material will not remain stable,but the channel will enlarge through erosion. Computation indicates the seriousness of thisfactor.
For example, assume that a channel is capable of carrying 55cfs

### atbankfull stage. Inthe Brandywine area this represents a channel draining 

### a basin slightly less than 1

### sq mi in area. The channel necessary to carry 55 cfs atbankfull stage would probably have a velocity of slightly less than 2.5 feet per second and would be about2 feetdeepand 11feetwide. In figure2,urbanization mightcause a flow of this frequency to increase 2.7 times, or 150cfs. If thischannel had toadjust itself to carry

### a flood of 150cfsatbankfull 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 respectivelyto about3 feet and 20 feet. Inother words, this streamwould deepenabout 50percent andincrease 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 sqmi,theamount 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 canbe compared with themeac annual sediment yield for Watts Branch, an unurbanized area near Rockville, Md. Annual sedimentyield of Watts Branch is 516tons 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 havethepotential 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 amountsof 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 asurbanization increases, particularly from industrial use of land andwater, thequality 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-oxygencontent.Second,

### asfloodpeaks increase 

### as a result 

### of the increased area 

### of imperviousness and decreased lag time, less

15water is available 

### for ground-water recharge. The streambecomes flashierinthat flood peaks
arehigher and flows during nonstorm periods
are lower. 

### A recent studyonthePassaic River 

### atLittle 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-midrainage basin were analyzed. During these 17 years, diversionsof waterfor domestic and industrial supplies increased more than 30 percent between 1950 and 1963. Returns

### of waste waters intothebasin became 

### as much 

### as 10percent of the water withdrawn. Analysis 

### of the data showed that 

### at relatively low discharge the dissolved-solids content increased about 10ppm (partsper 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 

### 8years. 

### A long-term changeintheaveragecontent of dissolved oxygenwasalso noted. Between 1950 and 1964the dissolved-oxygen content dropped from anaverageof 78 percent of saturation to 62percent of saturation. Further, the analysis demonstrated that these average changes in water quality occurred in all seasons of the year. An aspectof population growthnot generally appreciated isthe 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
arebuilt. 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, thepopulation 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 indicatethat thebasin will have 14,000persons 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 proc

### ess 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 materialsin 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 thefiner-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 waterthrough sandyloam. “ABS [syntheticdetergent] and coliformbacteria are significantly reducedby infiltration throughthe 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,evenformovement 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 (saturatedflow)

### .”
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

16general principles does indicate some tentative rules of thumb which might be useful to the planner. A perennial stream represents the intersectionof the saturated zone (watertable) with the earth's surface. The observations indicate that, for soil cleansing to be effective, contaminatedwatermustmovethroughunsaturated soil atleast 100feet. Owingtothegentle 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
onewhich containsflowingwater 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 2feetorless substantiallyincrease downstream diurnaltemperaturefluctuationswhereasponds deeper than 2 feet exhibit 

### a dampening effect
ondailytemperatures. For example,duringthe 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 doesnot prevent the dissolved materials (nitrates, phosphates, chlorides) from enriching the stream water and thus potentially encouraging theproliferation of algae and otherwise creating 

### a biotic imbalance. Theonlydetailed study

### of theeffectof 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,
orclearcuttingof treesaresignificantly higher in summer, but winter temperatures are 5" to 10°Fcolder than those observed in reaches unaffected by man. Solar radiation is thepredominant factor in the energy balance determining 

### a stream's mean daily temperature fluctuation was about 

### 4°F. Under natural conditions, lessthan 

### 5percent 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,theproportion of streamflow originating


### as surface runoff has increased sharply. As 

### a direct 

### ons sequence, streams most affected by streetrunoff may exhibittemperaturepatterns thataremarkedly differentfromthose 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 theupper reaches 

### of East Meadow 

### Brook, which drains highly urbanized central Nassau County, increased steadily in response to the relatively warm street runoff. Pluhowski found thatby 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.

17SELECTED 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 Shortpapers 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,Hydrologiceffects 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. CenterTech. 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 aspectsof 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