Stormwater Management I N Urban Collector Streams H

STORMWATER MANAGEMENT IN

URBAN COLLECTOR STREAMS

H. Rooney Malcom, P. E. Associate Professor of Civil Engineering North Carolina State University

Marian E. Avera Research Assistant

Charles M. Bullard Research ~ssistant

and

Cynthia C. Lancaster Research Assistant

The work upon which this publication is based was supported by funds provided by the City of Charlotte, North Carolina, and the Water Resources Research Institute of The University of North Carolina.

project No. 86-07-70048

June 1986 DISCLAIMER STATEMENT Contents of this publication do not necessarily reflect the views and policies of the Water Resources Research Institute, nor does mention of trade names or commercial products constitute their endorsement or recommendation for use by the Institute or the State of North Carolina. ACKNOW LEDGM

The principal investigator wishes to thank several persons and organizations who have made special contributions to the project. Ms. Beth Avera, Mrs. Cindy Lancaster and Mr. Michael Bul lard, Research Assistants, were full participants in the research. They pursued several aspects with independence and vigor. Separate reports prepared by each have been condensed and incorporated here. They merit inclusion as authors. The staff of the Engineering Department of the City of Charlotte, particularly Mr. Jim Schumacher and Ms. Jane Suggs, provided considerable data resources and constructive criticism of the work.

The members of the Stormwater Management Advisory Committee, a group of citizens having professional and civic interest in the subject, provided extraordinarily helpful reactions to the research as it progressed. The members did not always agree with the recommendations of the project, nor did the principal investigator always accept the views of the members of the Committee, for the nature of stormwater management is that the facts discernible leave considerable room for reasonable people to disagree on a course of action. The Committee will issue a separate recommendation to the City Council. The members of the Committee are Mr. Jim Bogan, Mr. John Burmeister, Ms. Susan Foster, Mr. Bob Grimes, Mr. John Huson, Mr. Dave Lewis, Mr. Lee McLaren, Mr. Bill McCoy and Mr. Sam Smith. We note with sadness the passing of Mr. George Selden in December 1985. His contributions to this subject and to orderly progress in Charlotte will be missed.

One hesitates to mention the names of all who help in such a project for fear of omission, but certainly some should be thanked for special contribution. Mr. Harold Eddins and Mr. Macon Jackson of the US. Geological Survey provided much hard- to-f ind hydrologic data col lected in Mecklenburg County. The value of USGS data is largely underestimated. Only through the use of measured data can design procedures be evaluated. Mr. John Tucker of the C.E. Dept., NCSU, provided valuable assistance in construction cost estimating and microcomputer applications.

Finally, the principal investigator wishes to thank those who made the challenging project financially and admin~stratively possible. The Charlotte City Council authorized the bulk of the funds, and the Water Resources Research Institute provided the administrative support, particularly Dr. David Moreau and Ms. Linda Larnbert. ABSTRACT The City of Charlotte experiences considerable damage and inconvenience associated with flooding and streambank degradation in urban collector streams having watersheds of sizes ranging from 20 acres to about one square mile. The research is intended to provide information for City officials to use in revising ordinances, pol icies and procedures. The investigators evaluated specific instruments, chiefly the f lood-plain restriction of the subdivision ordinance, the detention provision of the zoning ordinance, the stormwater design manual and the storm-drainage repair pol icy. The f lood-plain restriction of the subdivision ordinance of the City of Charlotte prohibits construction below the elevation of the 20-year f 1006, plus two feet. The elevation of the 20- year flood, plus two feet, is comfortably close to and slightly above the elevation of the 100-year flood in all cases analyzed.

The design objective of the detention requirement is that the peak outflow from the site after development be no greater than the peak outflow from the site prior to development in the ten-year storm. It is recommended that the stormwater detention provision be revised such that a fee is charged toward which credit can be earned for effective instal led detention. provide for payment to be made by the City to developments in which effective detention can be supplied in excess of that nominally required by the design objective of the ordinance. Remove the exclusion of small sites from coverage by the ordinance. Extend coverage to single family developments. The Stormwater Impoundment Design Manual is in need of updating with respect to hydrograph formulation and hydrau 1ic analytical procedures.

A graduated set of levels of service was suggested for the management of urban collector streams. Descriptors suited for communicating performance objectives between designers and laypersons were formulated.

Cost estimates were made for implementing improvements to the stream system. The approaches were to continue the present responsive approach, to improve the level of service of streams, and to increase the standard design storm. To achieve high performance goals for urban collector streams requires comprehensive treatment. Consideration of administrative structure for efficient comprehensive treatment leads directly to the concept of the stormwater utility, application of which was explored. TABLE OF CONTENTS Page ACKNOWLEDGMENT ...... ii i

LIST OF TABLES ...... m.....m..m...... mm.mmix

RECOMMENDATIONS...... ~...... ~...... ~..~~~..m.~...... xvi

I . INTRODUCTION...... 1 Background ...... Objectives of the Research ...... ~escriptionof Effort ...... LAYPERSON'S DISCUSSION...... ^^^^....^...... ^..^.^ PROBLEM STATEMENT...... a.m...... m.m... Urban Collector Streams ~efined...... Historical Perspective of Watershed Development .. Time-Honored Strategies ...... The Two-Part Nature of the problem ...... EXISTING ORDINANCES, POLICIES AND PROCEDURES ...... Flood-Plain Restriction ...... m...... m.m..m. Detention provision ...... Stormwater Impoundment Design Manual...... Storm Drainage Repair Policy ...... LEVELS OF SERVICE ......

Flood-plain restriction...... ^...... Detention Provision ...... Stormwater ~mpoundment~esign Manual ...... Storm Drainage Repair PO~~CY...... TABLE OF CONTENTS (Continued)

Page LONG-RANGE ALTERNATIVES ...... Graded Responses under the Existing Approach ..... A Stormwater Utility...... A PERCEPTION ......

CHARACTERIZATION THE STREAM SYSTEM

Watersheds Studied ...... ~nalysisof the Data...... Findings...... m.~~.~...... ~....~...~...... SYSTEM COMPLAINTS

Categorie s of Complaints Implement. ed Solutions... ind dings ...... ANALYSIS OF ORDINANCES AND POLICIES ...... mm.... Floodway restriction...... ^.^....^.^.....^. ~etentionprovision ...... Findings...... STORMWATER IMPOUNDMENT DESIGN MANUAL ...... Current procedure ...... m...... mm... proposed procedure...... m...... mm.m...m.. proposed vs Current procedure ...... ~ustificationof proposed Procedure ...... STORM DRAINAGE REPAIR POLICY ...... ~..~~~.~~~.~~~.~ LEVELS OF SERVICE APPLIED TO URBAN STREAMS ...... TRANSPORTATION LEVEL OF SERVICE ...... Capacity Level of Service ...... Condition Level of service...... ^.....^.... Development of a LOS scheme...... ^....^ TABLE OF CONTENTS (Continued)

Page

APPLICABILITY OF LOS FOR URBAN STREAMS...... 85 The LOS Concept as a Nanagement ~ool...... 86 Current Management of Streams in Charlotte ...... 86 Role of LOS Measures in Future Management ...... 87 Comparison of Streams and Streets...... 88 DEVELOPMENT OF LOS FOR STREAMS ...... 89 Assessment of Urban Stream problems ...... 89 Flooding Problems in Charlotte ...... 91 Erosion problems in Urban Streams...... 92 Erosion problems in Charlotte Streams...... 93 Qualitative LOS Measures ...... 93 Quantitative LOS Measures ...... me.96

Sample Tributary Area ...... am..96 Sample of McMullen Creek Complaints ...... 99 FINDINGS ...... 103 VI . PHYSICAL TREATMENTS FOR URBAN STREAMS ...... 105 CONVENTIONAL TREATMENT ALTERNATIVES ...... 105 Pipe Installation...... e.m~...... ~...... m 105 Channel Improvements ...... m...... m...... 105 Stormwater Detention ...... 106 velocity Reduction structure^...... ^...... 107 Suggested Policy Changes Regarding Stream Trtmnts 107 THE LINEAR LAKE ALTERNATIVE ...... 109 Objective ...... m..mm.m.m...... m.. ~esults...... m...... m...... Recommendations .....m...... m ~ntroduction...... Case Study ...... ~ngineeringDesign consideration^.....^...^.^...^ Weir Design ...... m.m...... Aesthetics ...... HEC-2 Water Surface profile ~pplication...... ECO~O~~CS...... m.mm...m..m...... Examples of Other Linear Lakes ......

vii TABLE OF CONTENTS (Continued)

Page VII . FINANCIAL MANAGEMENT OF STORMWATER SYSTEMS ...... 125 Existing Developments ...... New ~evelopments...... Stormwater Utility ......

VIII. REFERENCES...... a*......

A. SELECTED CHARLOTTE STREAMFLOW DATA ......

LISTING COMPLAINT FILES STUDIED. ...oo.o..o.,,

C . RAINFALL TABLES COMPUTED FOR CHARLOTTE. NCo ..0..

viii LIST OF TABLES Page 11-1 Recommended Criteria for Levels of Service ...... 16 111-1 Gaging Stations Selected for Streamflow Study ...... 27 111-2 Sample Summary for McMullen Cr at Sharon View ...... 36 IV-1 Summary of Analyses of ~etentionponds on File ...... 53 IV-2 Sample of Fees vs Increased Runoff ...... 57 IV-3 Hydrographs for Site by Current and Proposed Proc ... 66 V- 1 Summary of Complaint Files...... 100

LIST OF FIGURES Page 11-1 Annual Floods on Little Sugar Cr at ~yvola/~rchdale. 11-2 Annual Floods on Irwin Cr at Wastewater Plant ...... 11-3 Annual Floods on McMullen Cr at Sharon View Rd ...... 111-1 Return periods of Annual Peaks vs Occurrence ...... 111-2 Return periods of Annual Peaks vs Year of Occurrence 111-3 Annual Floods on Long Cr with ~ndicatedTrend ...... 111-4 Annual Floods on Irwin Cr with ~ndicatedTrend ...... 111-5 Annual Floods on McMullen Cr with Indicated Trend ... 111-6 Annual Floods on McAlpine Cr with Indicated Trend ... 111-7 Annual Floods on L . Sugar Cr with Indicated Trend ... 111-8 Daily precipitation in May 1975 at Airport ...... 111-9 Hourly precipitation on 3 May 1975 at Airport ...... 111-10 Hourly Precipitation on 30 May 1975 at Airport ...... IV- 1 Differential Comparison of 20+2 and 100-Year profile IV-2 Distribution of ~ifferencesin W.S. El. 20-Yr Flood . IV-3 Distribution of Differences in W.S. El. 100-Yr Flood IV-4 100-Year Surface Profiles - Sudbury Mainstem ...... IV- 5 Hydrographs for Site by Current and Proposed Proc ... IV-6 Comparison of Pattern with Historical Storm ...... IV-7 Comparison of Pattern with Historical Storm ...... IV-8 Depth-Duration-Frequency Curves for Charlotte ...... IV-9 Ctr-Weighted 10-Yr. 24-Hr ~esignStorm for Charlotte IV-10 Comparison of SCS Type I1 and 10-Yr Storm ...... IV-11 Ctr-Weighted 10-Yr. 6-Hr Design Storm for Charlotte. IV-12 Comparison of 67% of 24-hr Storm with 6-hr Storm .... IV-13 Comparison of unit-~ydrograph & Proposed Procedure .. VI-1 100-Yr Flood profile with and without weirs ...... SUMMARY

The City of Charlotte, similarly to other North Carolina cities, experiences considerable damage and inconvenience associated with with the misbehavior of urban col lector streams, the term used here to designate streams having watersheds of sizes ranging from 20 acres to about one square mile. The misbehavior consists mainly of flooding and streambank degradation. The long range goal of the City, to which this project is directed, is to establish a stormwater management program for streams whose watersheds range from 20 acres to one square mile. The research is intended to provide information for City officials to use in accomplishing the goal. The investigators evaluated specific instruments that the City of Charlotte uses in stormwater management. Chief among these are the f lood-plain restriction of the subdivision ordinance, the detention provision of the zoning ordinance, the stormwater design manual and the storm-drainage repair pol icy. The flood-plain restriction of the subdivision ordinance of the City of Charlotte prohibits construction below the elevation of the 20-year flood, plus two feet. This provision predates the flood-plain regulatory standards applying to streams having watersheds greater than one square mile, which are based on the elevation of the 100-year flood. The elevation of the 20-year flood, plus two feet, is comfortably close to and slightly above the elevation of the 100-year flood in all cases analyzed. The detention requirement provides for the installation of stormwater detention ponds on site at developments of dense composition exceeding a certain area. The design objective is that the peak outflow from the site after development be no greater than the peak outflow from the site prior to development, the design storm being the ten-year storm. The project team reviewed and analyzed a sample of plans submitted, and inspected field installations. A majority of detention facilities are designed and installed according to the ordinance. The ordinance requires some detention basins to be constructed where no significant benefit would accrue from detention. A common example is where the developed site is quite close to a major stream. The ordinance does not provide an incentive to supply extra detention on sites where such is possible. There have been cases where lakes or other facilities were installed for recreational and aesthetic purposes without detention pools when such could have been provided to achieve significant benefit. Fol lowup inspections of constructed f acilities show that in some cases installation did not correspond to the approved plan. All such differences do not necessarily mean that the impoundments would malfunction, but they raise questions. It is possible under the ordinance to design and instal1 permissible detention facilities that are ineffective. The common cases observed involve dividing the detention-storage requ irement among several basins within the site, The interaction of the network of basins does not necessarily produce results commensurate with the objective of the ordinance.

It is recommended that the stormwater detention provision be revised substantially. In particular, the current requirement for detention to be included at each site should be replaced by one in which a fee is charged toward which credit can be earned for effective installed detention. provide for developers to install effective detention facilities toward a refund of the fee. Adequacy of proposed facilities should be judged in an engineering plan review. Refund of the fee should be contingent upon certification by a professional engineer that constructed facilities are in accord with the approved plan. Provide for payment to be made by the City to developments in which effective detention can be supplied in excess of that nominally required by the design objective of the ordinance. Remove the exclusion of small sites from coverage by the ordinance. Extend coverage to single family developments.

The Stormwater Impoundment Design Manual is in need of updating with respect to hydrograph formu lation and hydrau 1ic analytical procedures. If this is done, designers will unquestionably find that small sites will need larger impoundments than formerly expected. The hydrograph formulation method incorporating the Modified Rational Method should be deleted. Designers should be a1lowed the a1ternatives of £01 lowing the procedure of the Tabular Method of Technical Release No. 55, Soil Conservation Service, or of following the closely related small watershed method given in this report. Every stormwater impoundment submitted for approva 1 shou ld be accompanied by a flood routing computation verifying adequate performance in the design storm and reflecting the influence of basin geometry and outlet hydraulics.

A graduated set of measures of levels of service was prepared for suggestion as a basis for considerations and decision making in connection with the management of urban collector streams. Descriptors suited for communicating performance objectives between designers and laypersons were formulated for the following levels:

A, STREAM IS IN A NATURAL CONDITION. B. STREAM IS IN A DESIRABLE CONDITION. Ce STREAM IS IN A TOLERABLE CONDITION. De STREAM IS IN AN UNACCEPTABLE CONDITION. Em STREAM IS IN A DAMAGING CONDITION. Recommendations for improvements in productivity were made to the Engineering Department. A microcomputer-based project estimating procedure was developed based upon the experience to date with designs in response to citizen complaints and requests for improvements under the storm-drainage repair pol icy. A computer program for routine design of channels and swales was also provided. The comprehensive analysis of complaints and responses has led to a more systematic understanding of drainage issues and alternatives.

It is recommended that the large number of trivial complaints be dealt with differently. It is evident that many problems brought to the engineering group have to do with minor erosion, or shallow standing water, in residential yards. In such a case, educational help toward a simpler improvement might have been useful. The persuasion to the dominant pipe solution seems to be based on the desire of a11 concerned for a permanent, maintenance-free solution. But the reforming and seeding of the surface by the resident may be more economical and more reasonable for a11 concerned. Four approaches to stormwater system improvement were drawn. A cost estimate for implementing the approaches was derived for a sample watershed. The sample results were extrapolated to yield city-wide estimates. Because they are so approximate, the estimates are useful only to the extent of ranking the a1ternatives and aiding j udgments in relative comparisons with City investment in other areas, such as traffic management.

The approach now used may be described as a responsive approach to management of urban collector streams. A problem is identified, a solution is sought, a plan is put in place. problems likely to recur are addressed through such instruments as the f lood-plain ordinance and the detention requirement. The estimated annual expenditure in management under the current approach is approximately $260,000.

One a1ternative is to accept maintenance responsibil ity for streams to the extent of providing the engineering and field resources necessary to inspect and improve streams to level of service C (LOS C) over a f ive-year period. Based on conditions prevail ing in the sample area, the city-wide cost of improvement to LOS C is estimated at $1,800,000 annually for five years. Another alternative is to accept maintenance responsibility for streams to the extent of providing the engineering and field resources necessary to inspect and improve streams to level of service B over a five-year period. Based on conditions prevail ing in the sample area, the city-wide cost of improvement to LOS B is estimated at $3,600,000 annually for five years. Some concern has been expressed about the level of design

xii being the lo-year storm for design of urban collector streams against out-of-bank flooding. The point was made that such a low level generates many opportunities for complaints, and that a higher level of protection against out-of-bank flooding ought to be considered. The estimated cost of increasing the in-bank capacity of urban collector streams to the capacity of the 25- year flood is at least $7,400,000 annually for five years.

To achieve high performance goals for urban collector streams requires comprehensive treatment. Consideration of administrative structure for efficient comprehensive treatment leads directly to the concept of the stormwater utility.

The principal benefits to be accrued from formation of the utility are the establishment of a base for comprehensive treatment, long-range planning and secure financial support, to the end of achieving stormwater management goals with economic efficiency.

xiii CONCLUSIONS

Stormwater Management in urban collector streams is a two-part problem. Two aspects of the issue must be treated either separately or comprehensively. These are the upkeep of the existing stream system and the response to new developments. Most of the urban collector streams are now in watersheds that are essentially developed.

The elevation of the 20-year flood, plus two feet, is comfortably close to and slightly above the elevation of tne 100-year flood in all cases analyzed. The incentive structure of the Charlotte detention requirement and its implementing procedures encourages trivial add-on facilities resulting in nominal compliance and little or no effectiveness in some cases.

A majority of detention facilities are designed and installed according to the ordinance.

The ordinance requires some detention basins to be constructed where no significant benefit would accrue from detention, The ordinance does not provide an incentive to supply extra detention on sites where such is possible. opportunities to have effective regional detention facilities were missed. Followup inspections of constructed facilities show that in some cases installation did not correspond to the approved plan. All such differences do not necessarily mean that the impoundments would malfunction, but they raise questions.

It is possible under the ordinance to design and install permissible detention facilities that are ineffective. The 10-year storm remains as the magnitude providing the widest spectrum of reduction of out-of-bank floods. Some specific cases may arise in which it is found to be best to design for a larger storm, perhaps in the case of a regional flood-control facility. Flood magnitudes are increasing in watersheds being developed in single-family residential units. There is need for flexibility in the application of the detention requirement. Three distinct situations arise frequently enough to support the view: a. There are cases where detention is required, but it will likely be ineffective. b. There are cases where it is unusually expensive to include detention on the site being developed. c. here are cases where there are opportunities to supply significantly more effective detention than is required under the ordinance. The Charlotte Stormwater Impoundment ~esignManual is in need of updating with respect to hydrograph formulation and hydraulic analytical procedures. If this is done, designers will unquestionably find that small sites will need larger impoundments than formerly expected. Response time to a typical drainage complaint had become long due to the growth of demand for service and competing demands for engineering services, such as in subdivision plan review.

Files indicate that erosion is the most frequent complaint. The total cost of responding to flooding complaints exceeds the cost of responding to any other single category of complaint. Engineering responses to system complaints seem excessively expensive in some cases. Typical response to a minor erosion complaint is a piped system.

A sizable number of complaints have been for trivial backyard drainage problems to be expected under normal cond it ions. The concept of levels of service provides a common set of criteria by which to judge the performance of urban stream systems and a1locate resources for improvements. RECOMMENDATIONS There is no compelling reason to make any change in the f lood-plain restriction. The control 1ing criterion that construction is prohibited below the level of the 20-year flood, plus two feet, compares favorably with comparable controls based on the 100-year flood in streams whose watersheds exceed one square mile in area, The stormwater detention provision should be revised substantially. In particular, the current requirement for detention to be included at each site should be replaced by one in which a fee is charged toward which credit can be earned for effective installed detention.

Provide for site developers to pay a fee to account for stormwater system improvements needed to offset effects of development, The funds collected should be reserved for drainage improvements. Improvements should not be limited to stormwater detention facilities, nor should funds necessarily be used immediately below the site from which they were col lected. Provide for developers to instal1 effective detention facilities toward a refund of the fee. Adequacy of proposed facilities should be judged in an engineering plan review. Refund of the fee should be contingent upon certification by a professional engineer that constructed facilities are in accord with the approved plan.

Provide for payment to be made by the City to developments in which effective detention can be supplied in excess of that nominally required by the design objective of the ordinance. Payment would be based on downstream benefit from effective detention proposed by the developer as determined by the designated City administrator.

Remove the exclusion of small sites from coverage by the ordinance . Extend coverage of the ordinance to single family developments. Base the fee upon the land area to be developed and the percentage imperviousness of the new development.

The Charlotte Stormwater Impoundment ~esignManual be revised in two aspects: a. The hydrograph formulation method incorporating the Modified Rational Method should be deleted, and designs based on that method should be accepted no longer. In its place, designers should be allowed the alternatives of following the procedure of the Tabular Method, Chapter 5 of "Urban Hydrology for Small Watersheds," ~echnical Release No. 55, Soil Conservation Service, U.S. Dept. of Agriculture, or of following the closely related small watershed method given herein. b Every stormwater impoundment submitted for approval should be accompanied by a flood routing computation verifying adequate performance in the design storm and reflecting the influence of basin geometry and outlet hydraulics.

The large number of trivial complaints should be dealt with differently. It is evident that many problems brought to the engineering group have to do with minor erosion, or shallow standing water, in residential yards. The dominating pipe solution may be prudently avoided in some cases by adopting a less severe design storm that will permit uses of grass- 1ined swales.

A brochure should be prepared for routine dissemination to citizens who have minor drainage problems. Such a brochure could list the options for dealing with yard drainage, including instructions for citizen construction of small swales.

The City should adopt level-of-service criteria for allocating scarce resources for improvements to urban streams.

xvii 13. If the current management structure is maintained, at least four alternatives for improvements exist. The approaches and a rough cost estimate for implementation follow. The estimates are useful only to the extent of ranking the alternatives and aiding judgments in relative comparisons with City investment in other areas, such as traffic management. a. The approach now used may be described as a responsive approach to management of urban collector streams. A problem is identified, a solution is sought, a plan is put in place. problems likely to recur are addressed through such instruments as the flood-plain ordinance and the detention requirement. The estimated annual expenditure in management under the current approach is approximately $260,000.

One alternative is to accept maintenance responsibility for streams to the extent of providing the engineering and field resources necessary to inspect and improve streams to level of service C over a five-year period. LOS C is the tolerable condition. Based on conditions prevailing in the sample area, the city-wide cost of improvement to LOS C is estimated at $1,800,000 annually for five years.

c. Another alternative is to accept maintenance responsibility for streams to the extent of prov id ing the engineering and field resources necessary to inspect and improve streams to level of service 13 over a five-year period. LOS B is the condition most observers would describe as desirable. Based on conditions prevail ing in the sample area, the city-wide cost of improvement to LOS B is estimated at $3,600,000 annually for five years.

Some concern has been expressed about the level of design being the lo-year storm for design of urban col lector streams against out-of-bank flooding. The point was made that such a low level generates many opportunities for complaints, and that a higher level of protection against out-of-bank flooding ought to be considered. In the sample area an estimate was made of the expense of increasing the capacity of streams to accommodate the 25-year flood within banks over a five-year period. The estimate is tenuous, and likely low,

xviii because of the difficulty of anticipating the need for extensive structural modification at culverts and tight places in the sample watershed, and more so city wide. The estimated cost of increasing the in-bank capacity of urban collector streams to the capacity of the 25-year flood is at least $7,400,000 annually for five years. The principal investigator recommends that the in-bank design storm for urban collector streams be maintained at the lo-year level.

xix

I. INTRODUCTION

Background The City of Charlotte, similarly to*other North Carol ina cities, experiences considerable damage and inconvenience associated with with the misbehavior of urban collector streams, the term used here to designate streams having watersheds of sizes ranging from 20 acres to about one square mile. The misbehavior consists mainly of flooding and streambank degradation. Watersheds smal ler than 20 acres generally are drained by systems of gutters, inlets, pipes and small swales provided at the time of development. Streams in Mecklenburg County are maintained by the County if their watersheds exceed one square mile. The City Engineering Department seeks to integrate the management of urban collector streams with the comprehensive stormwater management programs in the City. Objectives --of the Research The long range goal of the City, to which this project is directed, is to establish a stormwater management program for streams whose watersheds range from 20 acres to one square mile. The research is intended to provide information for City officials to use in accomplishing the goal. The specific research objectives are:

1. To characterize the behavior of the current stream system as to location, severity and classification of problems identified through records of citizen complaints and excessive maintenance expenditures.

2. To define levels of service appropriate to urban collector streams.

3. TO define an assortment of feasible physical treatments that may be appl ied to various streams.

4. To estimate the magnitude of the investment required to bring the streams to appropriate levels of service. 5. To consider alternate means of financing the management program.

6. To propose elements of a management plan such as modifications of current ordinances, physical management strategies for the stream system, prioritized schedule of stream-channel improvements with preliminary cost estimates, and system maintenance policies. Description -of Effort The collector-stream system was characterized to determine the location, nature, extent and probable cause of misbehavior which generates citizen complaints. Systematic recurrences were analyzed against storm data as a basis for determining current levels of service. Current stormwater management ordinances and policies of the City were analyzed for effectiveness in control1ing adverse behavior of the stream system. As a product of the research, recommendations were offered for specific changes. The idea of defining levels of service for urban collector streams was tested as a basis for allocating resources to maintenance and reconstruction of channels and for gaging impacts of future development on existing stream networks. ~ppropriatephysical treatments for improving levels of service of streams were analyzed for effectiveness in improving observed adverse behavior of Charlotte streams. The treatments were collected in a set of procedures, computerized in part, that may be used efficiently to estimate needs and costs to respond to a given complaint or to assess developmental impact. With assistance and guidance of the City Engineering Department, appropriate levels of service were defined for a representative sample of collector streams, and the cost for bringing the citywide stream system acceptable performance was be estimated. Alternative arrangements were offered for financing recommended improvements over a realistic period of time. These included provisions for bringing misbehaving streams up to par, for responding to the impact of new development and for continuing maintenance of the system. I1 . LAYPERSON 'S DISCUSSION

To deal with urban drainage problems effectively, it is important to understand how the stream systems came to be in the shape in which they are found. The streams that drain the land in the city of Charlotte have evolved for many years from branches and creeks of the woods and farms that preceded urban development to the ditches, pipes, branches and creeks that exist today. This section is a brief discussion of the historical development of urban streams, current Charlotte practices, and an ordered set of alternatives for improvements in stormwater management. Nore detail follows in the remainder of the report.

PROBLEM STATEMENT Urban Collector Streams ~efined In the process of urbanization, stormwater management practice has been to depend upon the old natural stream system to convey water from commercial areas and residential neighborhoods to the significant urban streams such as Little Sugar Creek, Briar Creek, McMullen Creek and others. The members of the old natural stream system are termed in this project urban collector streams. Urban col lector streams are those having open channels draining watersheds of less than one square mile. heir behavior and management are the concerns of the project. Urban collector streams are small. Even one so old and ungainly as the principal investigator can leap over most of them in a single bound. They wind through side yards and backyards and along property lines. Most are in private ownership. To walk along such a stream is to see a variety of attitudes and treatments. Here the stream is a part of a garden, there it appears as a stony brook. Elsewhere it is screened from view by a hedge or fence. Most places it is simply neglected. It grows up in weeds, becomes eroded, and becomes clogged with debris. Where erosion threatens, it is treated expediently with materials at hand: waste concrete, stone, concrete blocks, fencing and the like. It is frequently the sink for urban refuse: tires, shopping carts, 1itter, leaves, garbage bags, discarded appliances. Historical perspective -of Watershed Development

It is instructive to follow a watershed as it has developed through time. Consider a typical stream draining five or six square miles in the east of Charlotte. At about the turn of the century, the chances are good that the land was in agricultural production, with row crops on the flatter slopes and woodland on the steeper slopes. There were a few farm dwellings and outbuildings in the watershed. The dirt roads followed the ridges to avoid as much as possible the necessity of bridges.

In a nearly natural watershed, it can be shown that stream channels evolve such that their capacities at bank full are approximately equal to the two-year flood. The two-year flood is that discharge, or rate of flow, that is equaled or exceeded, on the average, once in a two-year period. About every other year, a natural stream is flooded beyond its banks.

As Charlotte grew, more roads, some of which were paved, were cut through the area in a spider-web pattern radiating outward from central Charlotte. More buildings appeared in the watershed. Timber bridges were built at stream crossings. ~ypically,a country-road bridge was built by placing a fill on both sides of the bridge to minimize the span of the bridge. The small opening left for streamflow was a constriction that would hold back water upstream in large floods, but it was likely that there was nothing upstream to be adversely affected. Taken together, the system of short-span timber bridges and small-pipe culverts seem to have acted inadvertently as a flood control system, holding back floodwaters temporarily in the uplands and causing water to flow less deep in the lowlands.

At some point, perhaps in the late fifties or early sixties in the hypothetical watershed, it became the norm to develop land in moderately dense housing developments. Rooftops, driveways, residential streets and lawns a1 lowed less in£il tration of rain water; so, storm by storm, more streamf low was sent down to the stream system more rapidly in pipes installed where small swales used to be. It was the practice, and it remains so, that the system of inlets and pipes installed in a development conveyed stormwater to the nearest existing branch, or stream. The stream was viewed as the natural receptor for storm drainage, and likely it was assumed that nothing further was needed in the way of stormwater construction. As traffic volumes grew, narrow old timber bridges were replaced by wide mu1 ti-barrel box culverts which were not only more efficient for traffic, but were more efficient hydraulically as well. Floodwater that formerly was held back, perhaps in a basement that was built too close to the creek, was released. From the downstream point of view, the floodwater detention providec? by the constrictive timber bridge was lost. The effect of more stormwater arriving at the streams more rapidly was to increase the peak discharge from a given rain storm. The two-year flood became larger. Overbank flooding became more frequent, and streambank erosion became more severe as the streams responded to the change in streamflow pattern by enlarging the channels. ~oticein Figures 11-1, 2 and 3, the historical trends of floods in Charlotte. The data on which the trend computations are based are taken from stream gages of U.S. Geological Survey, and the stream gaging program is supported in part by the City of Charlotte.

The watershed of Little Sugar Creek has Tryon Street as its western ridge and Tyvola Rd. (later Archdale Dr.) as its outlet. During the gaging record, begun in 1924, much of the 43-square- mile watershed was urbanized, particularly the eastern half. The trend (Figure 11-1) shows that in the early part of the record floods became less severe, but since the early 1950s they have been increasing. The floods at the gage on Irwin Creek (Figure 11-2), in the western part of the City, show a mild increase in the twenty years studied.

The trend of floods on McMullen Creek at Sharon View Road (Figure 11-3) appears to be more definite in its rise. It is important to note that urbanization that has taken place in this watershed over the last twenty years has been predominantly of single-family residential composition.

These trend computations should be used as indicators, but not quantifiers, of the effects of urbanization. The year-to- year variation in storm intensity and the shortness of the record limit the precision of the estimate. Time-Honored Strateg ies One can observe four distinct strategies that people have used to address the problems associated with streams. They stay well away from the creek. Or they make the channel bigger. Or they make the flood smaller. Or they tolerate periodic flooding. The first three, and perhaps a11 four, are evident in the existing ordinances and policies of the City of Charlotte. The f lood-plain ordinance prohibits development within the flood- prone area along the stream. The City works in a variety of ways to increase the capacity of some streams and watercourses: in channel ization projects, in the storm-drainage repair pol icy, and in culvert enlargement projects. The detention requirement is intended to offset the increases in flood peaks associated with site development.

Management provisions intended as responses to new development will not necessarily promote satisfactory drainage of existing neighborhoods, and vice versa. TREND OF ANNUAL FLOODS L Sugar Cr @ Tyvola/Archdale 9.0

Water Yeor TREND o DATA

Figure 11-1 Annual Floods on Little Sugar Creek at Tyvola/Archdale with Indicated Trend. TREND OF ANNUAL FLOODS Irwin Creek near Charlotte

Water Year TREND o DATA

~igure11-2 Annual Floods on Irwin Creek at Wastewater Treatment Plant, west of Charlotte, with Indicated Trend. TREND OF ANNUAL FLOODS McMullen Creek @ Sharon Road

Water Year TREND o DATA

~igure11-3 ~nnualFloods on McMullen Creek at Sharon View Road with Indicated Trend. The Two-Part Nature of the Problem To improve the management of urban collector streams in Charlotte, it is important to observe that two aspects of the issue must be treated either separately or comprehensively. These are the upkeep of the existing stream system and the response to new developments. Most of the urban collector streams are now in watersheds that are essentially developed. Complaints associated with these streams cite erosion, out-of- bank flooding and the need for maintenance most frequently. Substantial new developments can be shown to increase the volume and rate of runoff downstream. The drainage facilities in and downstream of new developments should accommodate the post- development drainage demand.

EXISTING ORDINANCES, POLICIES AND PROCEDURES

The investigators evaluated specific instruments that the City of Charlotte uses in stormwater management. Chief among these are the f lood-plain restriction of the subdivision ordinance, the detention provision of the zoning ordinance, the stormwater design manual and the storm-drainage repair policy. The following describes the findings of the evaluation. Flood-Plain ~estriction--of the Subdivision Ordinance The flood-plain restriction of the subdivision ordinance of the City of Charlotte prohibits construction below the elevation of the 20-year flood, plus two feet. This provision predates the f lood-plain regulatory standards applying to streams having watersheds greater than one square mile, which are based on the elevation of the 100-year flood. The question arises as to the correspondence of the elevation of the 100-year flood and the elevation of the 20-year flood plus two feet. Is there need for the City to modify the flood-plain ordinance to correspond more closely to the regulatory effect in the larger watersheds?

The finding is that the elevation of the 20-year flood, plus two feet, is comfortably close to and slightly above the elevation of the 100-year flood in all cases analyzed. There appears to be no functional need to modify the flood-plain restriction. It neither carries a greater risk nor imposes a greater safety factor than it would if it were based upon the elevation of the 100-year flood. The analyses were conducted by computing water-surface profiles for both the 20-year and 100-year floods in a sample of typical stream reaches in watersheds of less than one square mile. On average, the computed elevation of the 20-year flood was 1.5 feet below that of the 100-year flood. There was no case found where the elevation of the 20-year flood was more than two feet below that of the 100-year flood. Detention Provision --of the zoning Ordinance The detention requirement provides for the instal lation of stormwater detention ponds on site at developments of dense composition exceeding a certain area. The design objective is that the peak outflow from the site after development be no greater than the peak out£ low from the site prior to development, the design storm being the ten-year storm. The detention provision was added to the ordinance in 1978. There are several aspects of the requirement that have been questioned. Is detention effective? DO facilities in the field perform as intended? Is the design-storm magnitude appropriate? Is the coverage sufficiently complete? Can appl ication of detention be made more flexible? Effectiveness Detention basins do reduce peak discharges of floods passing through the basin. The detention requirement of the City of Charlotte had that intent. Retrospective analysis of detention basins installed under the Charlotte ordinance shows that most are effective in reducing peaks to some degree.

The role of the detention requirement in management of urban collector streams needs clarification. ~etentionis effective in reducing flood peaks immediately downstream of the site at which the basin is installed. The smaller is the basin, the shorter is the stream segment along which detention may be expected to be significant. Therefore, detention is most appl icable where a radical densif ication of land use threatens to increase flooding below the site itself.

In a requirement like that of the City of Charlotte, detention does not reduce the ----volume of floodwater, it merely spreads out the flow in time. The increases in volume associated with urbanization of a particular site combine in unpredictable ways to increase volumes and peaks in the larger drainage region, with or without detention. Therefore, it is unrealistic to expect detention at small scale to control increases in flooding city wide. Field Performance

The incentive structure of the Charlotte ordinance and its implementing procedures encourages trivial add-on facilities resulting in nominal compliance and little or no effectiveness in some cases. Entrepreneurs engaging in conventional cost- minimizing behavior submit plans showing facilities devised under the ordinance. Once approval is obtained, it becomes the burden of the inspection team to insure that facilities are constructed in accordance with the approved plan. his regulatory approach and response is typical of other local and state programs reviewed by the principal investigator. The project team reviewed and analyzed a sample of plans submitted, and inspected field installations. The following conclusions were drawn:

A majority of detention facilities are designed and installed according to the ordinance. The ordinance requires some detention basins to be constructed where no significant benefit would accrue from detention. A common example is where the developed site is quite close to a major stream. If the site is small relative to the size of the watershed of the stream, there is little likelihood that peak floods in the stream would be appreciably increased by undetained discharges from the site. The ordinance does not provide an incentive to supply extra detention on sites where such is possible. There have been cases where lakes or other facilities were installed for recreational and aesthetic purposes that overrode the detention requirements at the sites in determining the sizes of the facilities. The quantities of detention storage supplied were vanishingly small compared to the quantities economically achievable. Opportunities to have effective regional detention facilities were missed. Followup inspections of constructed facilities show that in some cases installation did not correspond to the approved plan. For example, when compared to the plans, outlet pipes were in different locations or pavement slopes were too steep for storage. All such differences do not necessarily mean that the impoundments would malfunction, but they raise questions.

It is possible under the ordinance to design and install permissible detention f aci 1ities that are ineffective. The common cases observed involve dividing the detention-storage requ irernent among several basins within the site. The interaction of the network of basins does not necessarily produce results commensurate with the objective of the ordinance. Design-Storm Magnitude The appropriateness of the lo-year design storm has been questioned. Some suggest that a larger design storm be used. The investigators are f amil iar with detention requirements that have been based on the 2-, 50, 100, 25- ,SO- and 100-year storms; and in one case, all of these. The lo-year storm predominates, presumably because that is the prevailing return period for other designs associated with stormwater-collection systems. An effective detention basin designed for the lo-year storm will perform satisfactorily in the large storms of 90 percent of years. A detention basin designed solely for the 100-year storm will not perform as well at the lo-year level as one designed for the lo-year storm. So, as one increases the design storm magnitude the smal ler storms tend to sl ip through less we11 attenuated. The smaller storms passing a 10-year basin would be more likely to be within streambank capacity. The 10-year storm remains as the magnitude providing the widest spectrum of reduction of out-of-bank floods. Some specific cases may arise in which it is found to be best to design for a larger storm, perhaps in the case of a regional f lood-control facility, but these should be considered on individual merit as being beyond the scope of the detention requirement.

The intent of the detention requirement is to prevent increases in peak discharge downstream of radical changes in land use. When the requirement was implemented certain land uses were exempted. Among these were single family developments and any development of less than 20,000 square feet of impervious area. Analysis of the trend in annual peak discharge at the McMul len Creek and McAlpine Creek gaging stations indicates that flood magnitudes are increasing in watersheds being developed in single-family residential units. Adjacent, separately developed co~ercialparcels in strip developments generate runoff to the same degree as they would if they were developed at once under single ownership. In the latter case detention would be required, but in the former it would not. Some uses that contribute to increasing peaks fall outside the detention requirement. Flexibility

The development community, through its representatives on the project advisory committee, cite the need for more flexibility in the application of the detention requirement. The concern is valid. Three distinct situations arise frequently enough to support their view:

(1) There are cases where detention is required, but it will likely be ineffective.

(2) There are cases where it is unusually expensive to include detention on the site being developed.

(3) There are cases where there are opportunities to supply significantly more effective detention than is required under the ordinance. The needed flexibility is difficult to include under the current ordinance and its supporting procedural arrangements, because of the heavy investment in staff effort that would be required to treat all cases thought to be exceptions. Indeed, it is hard to conceive of any arrangement that would provide completely for individual interactions between engineering staff and designers.

Stormwater Impoundment Design Manual Detention facilities installed under the Charlotte requirements were analyzed by the project team. Some detention impoundments, a1though designed adequately by the Modified Rational Method given in the manual, were found to be ineffective when analyzed by independent procedures. The method given in the manual for use on small sites leads one to devise an impoundment having too small a volume of storage. In the design storm, one would expect water to overtop the emergency spillway and discharge flow at a rate greater than that specified for control.

The Charlotte Manual was prepared in 1978, At that time the methods given were widely used, and they still are. The ~odifiedRational Method remains as an alternative in the Urban Storm Drainage Criteria Manual of Denver, CO, although designers are encouraged to use more detailed hydrologic and hydraul ic procedures as described in an update of 1982. Modified Rational Method was also published as one method of choice by American Pub1 ic Works Association in its urban stormwater management report of 1981.

The finding is that the Stormwater Impoundment Design Manual is in need of updating with respect to hydrograph fonnulation and hydraulic analytical procedures. If this is done, designers will unquestionably find that small sites will need larger impoundments than formerly expected.

Storm Drainage Repair Policy The storm drainage repair policy of the City provides for the city to share the costs of certain repairs and improvements with the abutting property owners. Questions have arisen as to the effectiveness of the policy in responding to complaints by citizens of system problems. The Engineering Department keeps detailed files on complaints and responses to them. The project team selected a representative sample of 138 of more than 900 complaints on file since implementation of the storm drainage repair policy. As experience with the policy has grown, the procedures of City staff to respond have evolved, and demand has grown for service. Analysis of the complaints and responses led to the following findings:

1. Response time to a typical drainage complaint had become long due to the growth of demand for service and competing demands for engineering services, such as in subdivision plan review.

2. Files indicate that erosion is the most frequent complaint. The total cost of responding to flooding complaints exceeds the cost of responding to any other single category of complaint.

3. Engineering responses to system complaints seem excessively expensive in some cases. ~ypicalresponse to a minor erosion complaint is a piped system. In part thisrnay resultfromthe use of the ten-year storm for design. There is also considerable incentive both for residents and designers to favor the permanent solution of a piped system over the economically expedient solution of a grass- 1ined open channel that will likely need continual maintenance and may stimulate repeated complaints. It is also likely, and there is evidence in the complaint files, that the permanent solution is seen by the citizens affected as being too expensive even with cost sharing.

4. A sizable number of complaints have been for trivial backyard drainage problems to be expected under normal conditions. LEVELS OF SERVICE

There is need for a way for people to communicate with each other their satisfaction (or lack of it) as to the condition and performance of urban col lector streams. Similar needs have been addressed in the problem area of transportation by means of the concept of "levels of service." A driver on an artery operating at level of-service A (LOS A) can move unimpeded at any desired speed, aware of little or no competing traffic, but a driver on an artery operating at LOS E would be moving at a snail's pace. The key to successful application of the concept is to devise descriptors of quality of service that system managers and laypersons can use to agree upon the current and desired situation. Then the effort and cost of getting from here to there may be decided upon with reasonable efficiency. An objective of this project was to define levels of service appropriate to urban streams. A graduated set of measures of levels of service was prepared for suggestion as a basis for considerations and decision making in connection with the management of urban collector streams. The investigators are not aware of closely similar appl ications elsewhere.

The complaints filed with the Engineering Department show the nature and degree of citizen concern about conditions in urban streams. Virtually all stream complaints fall into the categories of erosion and flooding. Within those categories, there are the expected degrees of severity. The measures of level of service described here are sufficiently descriptive for laypersons to judge the quality of a stream segment to a resolution of five degrees, levels A through E. They are sufficiently quantitative for designers to analyze current capability and to determine what improvements are needed to bring a stream to a given level of service. The levels of service developed in the project are given in Table 11-1. In the development of the criteria, considerable field study was invested in test-rating Charlotte streams. City engineering staff reviewed earlier versions critical ly, and the Table 11-1 Recommended Criteria for Judging Levels of Service in Urban Collector Streams.

A. STREAM IS IN A NATURAL CONDITION. Undeveloped flood plain. NO adverse effects in 100-yr flood. No accelerated erosion. Some erosion on outside of bends.

Be STREAM IS IN A DESIRABLE CONDITION.

No property damage in 100-yr flood. Yard flooding may be experienced. Erosion is slight. Streams may be armored. Rare inconvenience to abutting owners.

STREAM IS IN A TOLERABLE CONDITION. Some inconvenience due to yard flooding. Property values not adversely affected. Erosion is slightly accelerated. Streams may be armored. Slight stream channel enlargement.

D. STREAM IS IN AN UNACCEPTABLE CONDITION. Property values diminished. Frequent nu isance flood ing . Erosion causes undermining or silting.

Ee STREAM IS IN A DAMAGING CONDITION. Significant reduction in property value. Damage to permanent structures recurs. Damage from erosion is occurring or imminent. Threats to safety may be present. project advisory committee reacted to the concept. The adv isory committee and some City staff noted that separate criteria could be developed for erosion and flooding ratings, and that additional designations can be added to indicate such facts as the level of treatment cost and the number of properties directly affected by the problem. Doubtlessly, these can be added; however, the view of the principal investigator is that such a rating system benefits from simplicity. Successful implementation certainly should begin with the simplest formulation and be modified if and only if it is necessary. ~evelsof service provide a common set of criteria by which to judge the performance of stream systems. They are useful for sharpening perceptions of a11 concerned in evaluations, surveys and planning efforts.

NEXT LOGICAL STEPS

The culminating objective in the project was to make specific recommendations for improvements to the instruments available to the City for stormwater management in urban col lector streams. Growing out of the findings described above in this section were the following recommendations.

Flood-Plain Restriction of the Subdivision Ordinance

There is no compelling reason to make any change in the f lood-plain restriction. The control 1ing criterion that construction is prohibited below the level of the 20-year flood, plus two feet, compares favorably with comparable controls based on the 100-year flood in streams whose watersheds exceed one square mile in area.

Detention Provision -of Zoning Ordinance

1. Provide for site developers to pay a fee to account for stormwater system improvements needed to offset effects of development. The funds collected should be reserved for drainage improvements. Improvements shou ld not be limited to stormwater detention facilities, nor should funds necessarily be used immediately below the site from which they were col lected.

provide for developers to instal1 effective detention facilities toward a refund of the fee. Adequacy of proposed facilities should be judged in an engineering plan review. Refund of the fee should be contingent upon certification by a professional engineer that constructed facilities are in accord with the approved plan. Provide for payment to be made by the City to developments in which effective detention can be supplied in excess of that nominal ly required by the design objective of the ordinance. Payment would be based on downstream benefit from effective detention proposed by the developer as determined by the designated City administrator.

Remove the exclusion of small sites from coverage by the ordinance . Extend coverage of the ordinance to single family developments. Base the fee upon the land area to be developed and t.he percentage imperviousness of the new development.

As discussed above in the findings, the current ordinance suffers from lack of functional flexibility and completeness of coverage. There is also a problem of quality control associated with the nature of enforcement. The recommendations are intended to provide the functional f lexibil ity sought by development interests and by City staff, and they are intended to change the nature of the incentive structure to assure that effective facilities are the end result. The principal investigator notes that the advisory committee could not reach a consensus on the matter of the fee substitution for the detention requirement. There is considerable concern by the development interests that the fee structure may be set arbitrarily high to be used as an impediment to development and a source of funding for solution of problems not brought about by the developments. ~evelopmentinterests should be heard on the issue of fairness of the fee. The overriding motivations for the fee-based ordinance derive from issues of flexibility and incentive. The best solutions to problems of watershed management frequently do not include the appl ication of detention. Channel-bank treatments, culvert enlargements and piping of channels also play a part. If detention facilities are extraordinarily expensive to provide on site, then off-site alternatives should be permitted. If a site is situated such that extra effective detention can be supplied economically, then the of f-site worth of the extra detention should be purchasable. For such transactions to take place, a medium of exchange is needed. The conventional medium of exchange is money, and the price-setting mechanism is the market. The setting of the fee is important. If the fee is too high, the value of detention basins will appear to be high. Too many basins will be built and the funds available for alternative uses will be diminished. If the fee is too low, it will be paid in virtually all cases, but too little money will be made available for a1ternative uses.

The fee must be fair. It must reflect the relative downstream impact of one development versus another. The evidence is that downstream impact of development, for both flooding and erosion, is most closely associated with increases in volume of runoff. In turn, volume of runoff is closely associated with the land area of the development and the extent to which the land surface is rendered impervious to rainfall. A fair and administratively simple measure of impact can be based on the two development attributes of developed area and percent imperviousness. The recommendation is that the fee should be computed by multiplying a basic acreage fee by the impervious ratio of the fully developed tract.

The fee should apply to all new developments. In principle, all impervious areas contribute to the problem, and all should contribute proportionately to the solution.

S tormwater Impoundment Design Manual It is recommended that the Stormwater Impoundment Design Manual be revised in two aspects:

1. The hydrograph formulation method incorporating the Modified Rational Method should be deleted, and designs based on that method should be accepted no longer. In its place, designers should be allowed the a1ternatives of following the procedure of the Tabular Method, Chapter 5 of "Urban Hydrology for Small Watersheds," Technical Release No. 55, Soil Conservation Service, U.S. Dept. of Agriculture, or of following the closely related small watershed method given in Section IV. Other methods, such as those comprising computer models, may be accepted by prior arrangement with the Engineering Department. 2. Every stormwater impoundment submitted for approval must be accompanied by a flood routing computation verifying adequate performance in the design storm and reflecting the in£luence of basin geometry and outlet hydraulics. The need for the updated hydrograph formulation was described above in the findings. The flood routing computation is intended to insure at plan submission time that the basin will perform adequately if built according to the design.

At the time that the manual was published, the methods and submittal requirements were consistent with current practice. Now, practice has progressed to the point that the revisions are both needed and reasonable to execute, both manually and by widely available computer-based methods.

It is repeated that design under the revised hydrograph formulation procedures will result in larger detention facilities than those done under the current method.

Storm Drainage Repair policy

The repair policy is accepted as a statement of policy of the City Council, and as such it is not criticizable from an engineering point of view. The performance of the system under the policy was described above in the findings. The largest number of responses has to do with erosion. The largest expenditures have been made to address flooding problems. The principal needs are to improve staff productivity and to sharpen the perceptions of all concerned as to the nature of a valid request or complaint under the policy. ~ecommendations for improvements in productivity were made to the Engineering Department earlier in the project, and these were implemented immed iately. A microcompu ter-based project estimating procedure was developed based upon the experience to date with designs in response to citizen complaints and requests for improvements under the storm-drainage repair pol icy. A computer program for routine design of channels and swales was also provided. The comprehensive analysis of complaints and responses has led to a more systematic understanding of drainage issues and alternatives. It should be stated that these improvelnents are not sweeping in their effect on productivity. The responses to the complaints were defensible, and most physical alternatives had been identified by the section that had responsibility. It is apparent that the efficiency gained by the computer application is significant, and that further improvements will be forthcoming as the system evolves. It is recommended that the large number of trivial complaints be dealt with differently. It is evident, as stated in the findings, that many problems brought to the engineering group have to do with minor erosion, or shallow standing water, in residential yards. Certainly, the citizens view these as significant problems; certainly, council members and engineering staff view the complaints as valid. Indeed, in counting and generating a caring response, they are treated much as a major flooding or erosion problem. Typically, the suggested design for a backyard problem involves the installation of a substantial length of pipe at a cost of several thousand dollars. It is reasonable and practical in many such cases to repair the offending surface by shovel work to improve the drainageway, installation of temporary protection against erosion and establishment and maintenance of a grass lining. In some cases examined, the pipe solution was not implemented because of the expense. In such a case, educational help toward the simpler improvement might have been useful. The persuasion to the pipe solution seems to be based on the desire of a1 1 concerned for a permanent, maintenanceof ree solution. But the reforming and seeding of the surface by the resident may be more economical and more reasonable for a11 concerned.

It is recommended that a brochure be prepared for routine dissemination to citizens who have minor drainage problems. Such a brochure could list the options for dealing with yard drainage, including instructions for citizen construction of small swales.

LONG-RANGE ALTERNATIVES

The system of urban collector streams is quite complex. It is managed in an ad-hoc manner. The system is sensitive to virtually any change that takes place in the watershed: new development, changes in roadway crossings, sudden clogging, weed growth, channel construction, channel evolution, dam building, and others. The system is mostly in private ownership, and each owner treats his/her part differently. There are many differing expectations of the behavior of urban streams. There is no system design. It has evolved from the old natural stream network that existed prior to urbanization. To repeat, there are only four responses to flooding: make the flood smaller, make the channel bigger, maintain a respectable distance from the stream, or tolerate periodic flooding. There are three actors in the process: the abutting owners, the City and the developers.

Depending on the resources that the City decides to commit to management of urban collector streams, there is a set of graded responses that can be made within the framework of the existing ad-hoc strategies. Costs of the responses can be shared among abutting owners, the general public and new developments in much the same way that they are now, perhaps with the addition of the proposed fee in 1ieu of detention. To go beyond ad-hoc management strategies to comprehensive watershed management is to proceed toward the concept of a stormwater utility, in which the City accepts complete responsibil ity for the design, construction and operation of the system of urban collector streams much as it does for water and wastewater systems and for streets and thoroughfares.

Graded Responses under -the Existing Ad-hoc Approach

Four approaches were drawn from perceptions of City staff, City council, advisory committee, the project team and experiences of other cities. A crude cost estimate for implementing the approaches was derived for a sample watershed. Assuming the sample to be representative, the sample results were extrapolated to yield city-wide estimates. Because they are so approximate, the estimates are useful only to the extent of ranking the alternatives and aiding judgments in relative comparisons with City investment in other areas, such as traffic management . A 900-acre portion of the upper part of the watershed of McMullen Creek was taken as the sample watershed. About 24,000 feet of channel was surveyed. Of that length, 25% was classified as level of service B (desirable condition), 41% as LOS C (tolerable condition), 29% as LOS D (unacceptable condition), and 6% as LOS E (damaging condition). The sample area elicited 28 complaints in the file, of which 22 were studied in detail. Of The 22, 20 cited erosion, 6 cited flooding, and others cited heal th or safety concerns (some cited mu1 tiple concerns). Continue Current Practice The approach now used may be described as a responsive approach to management of urban collector streams. A problem is identified, a solution is sought, a plan is put in place. Problems likely to recur are addressed through such instruments as the f lood-plain ordinance and the detention requirement. ~nevitably,a responsive approach leaves the feeling of being somewhat behind, of being open to criticism, of living in the present (or even the recent past). But you don't try to fix things that aren't broke. And you tacitly recognize that the stream system is in private ownership. ownership of streams, and their problems, is a nebulous issue everywhere. The current approach has the virtue of economy. In expressing the implicit value placed on the stream system by the affected population, the Charlotte practice is quite consistent with virtually every U.S. city known to the principal investigator. The estimated annual expenditure in management under the current approach is approximately $260,000. Improve -to Level -of Service -C One alternative is to accept maintenance responsibility for streams to the extent of providing the engineering and field resources necessary to inspect and improve streams to level of service C over a five-year period. LOS C is the tolerable condition (See Table 11-1). Based on conditions prevailing in the sample area, the city- wide cost of improvement to LOS C is estimated at $1,800,000 annually for five years. Improve -to Level -of Service -B Another alternative is to accept maintenance responsibility for streams to the extent of providing the engineering and field resources necessary to inspect and improve streams to level of service B over a five-year period. LOS B is the condition most observers would describe as desirable (See Table 11-1). Based on conditions prevailing in the sample area, the city- wide cost of improvement to LOS B is estimated at $3,600,000 annually for five years.

Some concern has been expressed about the level of design being the 10-year storm for design of urban collector streams against out-of-bank flooding. The point was made that such a low level generates many opportunities for complaints, and that a higher level of protection against out-of-bank flooding ought to be considered. In the sample area an estimate was made of the expense of increasing the capacity of streams to accommodate the 25-year flood within banks over a five-year period. The estimate is tenuous, and likely low, because of the difficulty of anticipating the need for extensive structural mod if ication at culverts and tight places in the sample watershed, and more so city wide. The estimated cost of increasing the in-bank capacity of urban collector streams to the capacity of the 25-year flood is at least $7,400,000 annually for five years. The principal investigator recommends that the in-bank design storm for urban collector streams be maintained at the 10- year level.

-A Stormwater Utility As one proceeds through the alternative approaches described above, the expectations of system performance are greater, and the level of acceptance by the City of responsibility for achieving adequate performance is 1ikewise greater. As one reviews the current management of the system, it is seen to have evolved from a natural stream system in which bankfull capacity is about a two-year flood, but the performance expectation is the ten-year flood or greater. Projects to transform the stream from the formerly rural expectation to the current urban expectation are fragmentary, with 1ittle comprehensive treatment. Indeed, given the private ownership of urban collector streams, comprehensive treatment is extraordinarily difficult. To achieve high performance goals for urban collector streams requires comprehensive treatment. Consideration of administrative structure for efficient comprehensive treatment leads directly to the concept of the stormwater utility.

The wastewater collection system is a close parallel. ~t the turn of the century, or thereabout, Charlotte had no sewer system. All properties had outhouses and garbage pits or similar on-site disposal facilities. Now, there is a complete waste management system operated by the City. The parts of the wastewater col lection system within developments are suppl ied by the developer and turned over to the City for operation and upkeep. The parts of the system that collect wastewater from neighborhoods, convey it, and treat it, are centrally planned, financed, designed, constructed, operated and maintained. The transition from a privately owned, fragmented system to the current wastewater management utility has been quite complete, but it took a number of years. Similar para1 lels can be drawn in the systems for water supply and distribution and for streets and thoroughfares.

It is feasible to form a stormwater utility. Some cities have: Bellevue, WA, Tulsa, OK, Cincinnati, OH, Tampa, FL, and Roseville, MN, to name some that come to mind. The principal benefits to be accrued from formation of the utility are the establishment of a base for comprehensive treatment, long-range planning and secure financial support, to the end of achieving stormwater management goals with economic efficiency. There are some barriers. Chief among them is the need to gain access to and control of the stream system. The hard question persists of how to move into the back yards of people to acquire rights of way, easements or other forms of permission to do work on a stream. Citizens are asked to give up freedom of decision in favor of a collective investment. Citizens do that in a large number of other ways.

The sharing of costs among the participants, general pub1 ic, developers, abutting owners, must be done equitably. A stormwater management fee, not unlike a sewer service charge, is common among places that have formed utilities. If a stormwater utility is established, results will not be immediate. There are a great many widely dispersed problems. If the sample watershed described above is representative, 35% of urban collector streams exist at an inferior level of service. As all inferior streets would not be widened at once, all inferior streams will not be fixed at once. A long-range plan to bring the system up to an acceptable level of service must be formulated and followed with real istic expectations.

A PERCEPTION An urban stream cannot appear or behave as a rural stream. But it need not be an open sewer. There must be something in between that makes sense. There is a suitable identity to be sought for the urban stream. It may not be the same for all segments of all streams, or for all segments of a given stream.

There are important questions to be asked and answered about urban streams. Who should own the stream? Who should respond to problems brought about by occupying land in flood prone areas? These are questions that must be answered in the political process.

111. CHARACTERIZATION OF STREAM SYSTEM IN CHARLOTTE

In order to determine the nature of the behavior of the stream system in the city of Charlotte, analyses were conducted of two available data sets. The first of these was the set of streamflow data recorded at nine gages in the City by U.S. Geological Survey. The second was the set of files on drainage system complaints kept by the staff of the City Engineer. The streamf low records were studied for evidence of trends in flooding. The system complaints were studied to identify the nature and extent of system misbehavior as perceived by citizens.

STREAMFLOW ANALYSIS

Watersheds Studied ~inegaging stations were selected for the study as listed in Table 111-1. The data were drawn from the Hydrologic Storage and Retrieval System (HISARS), a data bank resident at ~riangle universities Computation Center, Research Triangle Park, NC (Wiser, 1983).

Table 111-1 Gaging Stations Selected for Streamf low Study. Analysis ---of the data

The data sets selected for analysis are listed in Appendix A. The rightmost column is the approximate return period computed for each flood by a log-normal analysis of the station sample from which it was drawn. The station records were truncated at 1963 if they extended farther into the past. The records of Little Sugar Creek at Tyvola Road and at Archdale Drive were combined for this analysis.

It may be seen from the record that two floods having return periods of greater than 100 years were experienced in the storms of 30 May 75 on Irwin Creek and in the storm of 10 Jun 82 on McMul len Creek. Some twelve storms having return periods greater than ten years were included. Expected versus Observed Frequency -of Flooding Figure 111-1 shows the flooding occurrences at the nine gages as compared to the statistical expectation. There is no reason to believe that the City has been hit by an unusually severe set of floods, but the analysis behind the bar chart would not diagnose any trends that may be present in the series of floods. Figure 111-2 shows the return periods of floods in given years plotted against years of occurrence. There is some evidence that the larger floods experienced at the stations have come in the latter half of the record. Trends -at Selected Stations Figures 111-3 and 4 illustrate the year-to-year trends that may be observed in the records of Long Creek and Irwin Creek, whose watersheds are in the northwest part of the City. ~igures 111-5 and 6 similarly illustrate the trends at McMullen Creek and Mc~lpineCreek in the Southeast part. The trend lines were computed by a Fourier-smoothing routine which mathematically removes high-frequency variations that obscure longer range trends (Aubanel and Oldham, 1985). Similar trends may be brought out using mov ing-average techniques . The watersheds of McMullen and McAlpine Creeks seem more definitely to exhibit upward trends in recent years as compared to those of Long and Irwin Creeks. Records of flooding have been kept for the longest period at Little Sugar Creek. Combining the records for the Tyvola and Archdale gages (41.0 and 42.6 aq mi, respectively), the trend was computed and shown in Figure 111-7. Little Sugar Creek seems to have experienced a reduction of flooding from the early 1920s to about 1950. Since 1950, however, floods have been increasing. FLOODS EXPERIENCED IN CHARLOTTE

Return Period (yr) ry Observed vA Expected

Figure 111-1. Expected return periods of annual peaks versus occurrence in the sample of nine gaging stations.

29 FLOODS EXPERIENCED IN CHARLOTTE Return periods at 9 gages since 1963 0- 110 - t

100 -

90 -

70 i

50 6o 1 40 I i

:: 1 t t I 10 t t + ++ ii+*$4++dtd+Jtt$$+ t 0, I 1 I I

Water Year

Figure 111-2. Observed return periods of annual peaks versus year of occurrence. TREND OF ANNUAL FLOODS Long Creek near Paw Creek

Water Year - TREND o DATA

Figure 111-3. Annual Floods on Long Creek near Paw Creek, with ind icated trend. TREND OF ANNUAL FLOODS Irwin Creek near Charlotte

Water Year - TREND o DATA

Figure 111-4. Annual floods on Irwin Creek at wastewater treatment plant, west of Charlotte, with indicated trend. TREND OF ANNUAL FLOODS McMullen Creek @r Sharon Road

63 65 67 69 71 73 75 77 79 8 1

Water Year - TREND o DATA

Figure 111-5. Annual floods on McMullen Creek at Sharon View Road, with indicated trend. TREND OF ANNUAL FLOODS McAlpine Creek at Sardis Rd 7.0

Water Year - TREND o DATA

Figure 111-6. Annual floods on McAlpine Creek at Sardis Road, with indicated trend. TREND OF ANNUAL FLOODS L Sugar Cr @ Tyvola/Archdale 9.0

Water Year - TREND o DATA

Figure 111-7. Annual floods on Little Sugar Creek at ~yvola/Archdale, with indicated trend. The Fourier trend shown here agrees in substance with the moving- average trend described by Malcom in a previous analysis executed upon data collected to 1978 (Malcom, 1980). In that work, the declining and rising trends were found to be statistically significant. The recent data show the rising trend to be persisting. Significance -of Trend The record of annual peaks at Mcmullen Creek at Sharon view Road was studied for the significance of the trend. The 20-year record of annual peaks was divided into two samples, as shown in Table 111-2. Sample A consists of the natural logarithms of the annual peak discharge for the years 1963-1972. Sample B is similarly constituted for the years 1973-1982. The logarithmic transformation was performed to normal ize the samples. A t-test of the significance of the difference of two means was conducted (Ostle, 1963).

Table 111-2

Sample Summary for Annual Peaks of McMullen Creek at Sharon View Road.

An F-test indicated that the difference of sample standard deviations was insignificant at the 5% level.

The computed value of t for the observed difference in the sample means was 3.05. The probability of drawing two saaples as different as these from a single population is less than 0.5%. ~t was concluded that the trend is significant.

The values given in Table 111-2 for the two-year flood, 827 cfs and 1392 cis, respectively, appear to represent a difference in stage of approximately two feet at about bank full, based on the values of stage recorded for the floods in HISARS. Storms --of M~J ----1975 and June 1982 These storms produced significant floods in Charlotte. They were examined to find the characteristic storm duration that produces flooding in watersheds of the sizes to be studied (5 to 45 square miles). The dail precipitation data for May 1975 at Douglas Airport, as taken 2rom HISARS, are presented in Figure 111-8. The storms of 3 May and 30 May, Figures 111-9 and 10, are seen to be the largest. In each case, a spike of one or two hours seems to be the flood producer. Antecedent conditions in both cases were fairly wet. ~ouglas~irport precipitation for the month of June 1982 shows that the storm of 10 June occurred without significant antecedent rainfall. At the airport the maximum hourly increment was 0.85 inches, occurring at 1600 hours (4:OO PM). This is a small hourly value for such a rare flood as was recorded in the southeast part of Charlotte at the McMullen Creek gage. The problem of spatial differences of rainfall in given storms is quite evident, and it reduces the usefulness of the rich rainfall data set at the airport when analyzing rainfall/runoff events in the southeast of the City. ~dditionaldata have been gathered by USGS personnel at the Cameron-Brown Bldg. More than three inches of rain were recorded at that gage in a single hour in the storm of 10 Jun 82, a value more nearly in correspondence with the rare flooding event (Eddins, 1985).

Findings

1. Floods have been experienced in Charlotte at expected frequencies from 1963 to 1982. There has not been an unusually high number of 100-year floods when compared to the occurrences of smaller floods.

2. Based on the records of McMullen Creek, McAlpine Creek and Little Sugar Creek, rising trends are evident in watersheds in the southeast part of the city. The

trend in the watershed of , McMullen Creek is statistically significant.

3. The smaller gaged watersheds (5 to 15 sq mi) are sensitive to storms of one to two hours duration. RAINFALL IN MAY 75 Caused general flooding in Charlotte 4.00

Day of Month FA Location: Airport

Figure 111-8. Daily precipitation in May 1975, measured at

Doualas- Airport. STORM OF 3 MAY 75 Caused 5-yr Flood at McAlpine(2) Gage 1.20

Hour of the Day Location: Airport

Figure 111-9. Hourly precipitation on 3 May 1975, measured at Douglas Airport. STORM OF 30 MAY 75 Caused 100-yr Flood at Irwin Gage

Hour of the Day Location: Airport

Figure 111-10. Hourly precipitation on 30 May 1975, measured at Douglas Airport. SYSTEM COMPLAINTS The Engineering Department of the City of Charlotte has maintained a file of drainage system complaints serviced under the storm drainage repair policy. Of some 400 such complaints, a sample of 138 was selected by the project team for analysis. The sample is treated here and in Section V, on levels of service. The sample is listed in Appendix B.

Categories -of Complaints Of the 138 files in the sample, 39% included a complaint of erosion, 21% included a complaint of flooding and 46% included a complaint of a "blowout," a term applied to a failure of a pipe system that presents itself as a hole in the ground through which water emerges. (Some cited mu1 tiple categories.) The stormwater repair policy provides for cost sharing of up to 80% of corrective costs being borne by the City. Of the 138 complaints in the sample, corrective measures were implemented in 99 cases, or 72%. Implemented Solutions Of the 138 implemented projects, 31% cited erosion, 15% cited flooding and 54% cited blowouts. The total funds spent on the projects was approximately $775,000, unadjusted for inflation. Of this amount, 71% was borne by the City.

Of the funds spent, 16% went to projects citing erosion, 52% went to flooding, 8% went to blowouts and 18% went to projects citing aesthetic issues. Some complaints were judged to be for trivial yard drainage problems for which proposed solutions seem excessively expensive. These were discussed in Section 11. See also the discussion of complaints in the Section V, on levels of service. Find inas

1. The most frequent system complaint is for a breakdown of a pipe system. Corrective measures for pipe systems, however, comprise only 8% of expended funds.

2. With respect to urban collector streams, erosion is the most frequent complaint. The aggregate cost of flooding responses is higher than any other category.

3. The responses of the Engineering Department to minor yard drainage complaints sometimes appears excessively expensive. Typical response to erosion is a piped system. In part, this may be due to the adherence to a ten-year design storm for such projects. It clearly reflects the desire on the part of all concerned -- residents, designers and administrators, for a permanent, maintenance4ree solution, whereas an expedient repair of the surface channel would seem more economical. IV. ANALYSIS OF ORDINANCES AND POLICIES

Several ordinances, policies and procedures of the City of Charlotte were analyzed for effectiveness in stormwater management of urban collector streams. These were the flood- plain restriction of the subdivision ordinance, the stormwater detention provision of the zoning ordinance, the stormwater impoundment design manual and the storm drainage repair pol icy. This section is devoted to the analyses and findings.

Flood-plain Restriction The objective of this study is to compare the behavior of the 100 year flood water surface profile and the 20 year flood water surface profile plus two feet (20+2). Analysis of the water surface profile changes due to urbanization will be examined for a sample of the streams in the data set.

This subsection is a condensation of a larger project executed as an academic independent study (Bullard, 1986). Copies of the full report are filed in the Engineering Department of the City of Charlotte and in the Water Resources Research Institute of The University of North Carolina.

Floodway Regulations Creeks, streams and ad joining lowlands in Charlotte are subject to periodic inundation, and as such are designated flood hazard areas. Flood losses are caused by the cumulative effect of obstructions and occupancy of flood hazard areas by uses vulnerable to flood damage. In order to minimize the extent of flood damages the City of Charlotte and Mecklenburg County have adopted floodplain and f loodway ordinances. The City of Charlotte and Mecklenburg County restrict development in the floodplains, those areas subject to damage during the "regulatory flood". Floodway maps and flood profiles, developed to administer the ord inance and for flood insurance programs, are available for streams draining areas larger than one square mile. The regulatory flood elevation for streams draining greater than one square mile is the elevation of the 100 year flood crest. For streams draining less than one square mile the regulatory flood elevation is the crest of the 20 year flood plus two feet. Studies to determine the elevation of the 20 year flood crest have not been made for the region. Currently, this information is developed on a case-by-case basis by the City of Charlotte Public Service Engineer using accepted hydraulic analysis. á his investigation is concerned with the streams draining watersheds of less than one square mile within the boundaries of the City of Charlotte. These streams are subject to the floodplain and f loodway ordinances out1ined in the Subdivision Ordinance of City -of --- Charlotte. A portion of the ordinance regarding floodway protection is included below (Section 18- 5.1(0)): ~estrictionson the Subdivision . . . . Lots shall be construed to be subject to flooding when a flood crest recurring with a probable frequency of one time in twenty years would inundate any part of the proposed lot. . . . If any part . . . is subject to flooding, the prospective subdivider may make a determination of the crest elevation of a flood of twenty year probable frequency in accordance with generally accepted engineering practice. This determination must reflect the actual conditions imposed by the completed subdivision, and must give due consideration to the effects of urbanization and obstructions. . . . on the lot plan a line representing an actual contour at an elevation two feet above the twenty year flood. Such line shall be known and identified on the lot plan as the "building restriction flood line". ~ll buildings or structures . . . shall be located on such a lot so that the lowest usable and functional part of the structure shall not be below the elevation of the building restriction flood 1 ine. "Usable and functional part of the structurew shall be defined as .

A principal reason for conducting this study on small watershed water surface profiles is to determine the relationship between the 100 year flood elevation used on larger streams and the 20 year flood elevation plus two feet used on the smaller streams. Investigation -of Previous Work Stamper examined the mapping of floodways and floodplains on streams of watershed area greater than one square mile. The techniques and methods used in his analysis are similar to the methods used by this investigator on streams of watershed area less than one square mile. The principal purpose of the report is to state a method for establishing flood district and flood fringe maps using backwater analysis (Stamper, 1975).

Eddins and Jackson described a method for estimating flood heights on unregulated streams draining less than one square mile for the 10, 20 and 100 year return periods. Flood height is defined as the difference in elevation between the flood water surf ace and the streambed. The method described uses equations developed by regression analysis to describe the flood height. In order to develop the analysis of flood heights using regression analysis several assumptions were made by the investigators. It was assumed that the flood height is the same for streams draining areas of the same size. This assumption, of course, can only be crudely approximate, because the flood peak and channel geometry are not the same on all streams of equal drainage area. Nonetheless, the analysis was completed under the assumption that the watershed and channel geometry effects are negated by the tendency of channels and floodplains naturally to adapt to the magnitudes of floods experienced in the stream. According to the authors this tendency indicates that "the height of floods with equal frequency might fall within a limited range, and the average of many flood heights might be sufficiently close to the true value . . . ." The regression analysis for points on 62 streams were determined using step-backwater techniques. The majority of points used in the regression analysis were for streams with drainage areas of greater than one square mile. The following equations are produced by the lines of best fit:

Where H10,H2 and HIOO is the height of the 10, 20 and 100 year flood in fee?: and A is the drainage area in square miles. The standard error of estimate ranges from -19 to +26 percent, and at 0.5 square miles from -1.3 feet to +1.6 feet (Eddinsand Jackson, 1980 ) . Hydraulic -and Hydrologic Analysis The principal model used for hydraulic analysis is the ~ydrologicEngineering Center's water surf ace profile program HEC-2. The computer program uses a standard step analysis to perform the nonuniform flow calculations. A .version of the HEC-2 model designed to run on an IBM PC-XT with 512K of memory was use to complete the backwater analysis. Formatted data input files were produced using the spreadsheet so£tware LOTUS 1-2-3. Cross section data were obtained for the analysis from the files of the public Service ~ngineerfor the stream reaches of Fairfield Park, ~ollingHills, Village Lake Park, Lake Dell and Winding Brook ~ubdivisions. Cross sectional data for the Briar Creek tributary at Sudbury Road were taken from 1:200 scale city topographic maps with a two foot contour interval. Manning's "n" values and other data necessary to perform the HEC-2 analysis were based on field inspection of the stream reaches. Further information on the use and background of the HEC-2 model can be found in the users manual (U.S. Army Corps of Engineers, 1982).

Two hydrologic models were used in the investigation to estimate peak flows. Peak flow calculations on watersheds of less than 200 acres were based on the Rational Method as outlined in the --Charlotte S tormwater Impoundment ~esign---Manual. his method was used to calculate the 20 and 100 year peak flows for the stream reaches in Fairf ield Park, Rolling Hills, village Lake Park, Lake Dell and Winding Brook Subdivisions. Peak flow calculations for the Briar Creek tributary at Sudbury Road (Briar Creek trib No. 6) are developed using the Putnam Method (Putnam, 1972). This is the method accepted by City of Charlotte for streams with drainage areas greater than 200 acres. This method is based on lag time and imperviousness of the watershed. To analyze the effects of urbanization impervious area is used a measurement of development in the watershed. Hydrau 1ic analysis is completed for this stream and its tributaries at 16 and 35 percent imperv ious area.

Discussion The hydraulic analysis depends on the validity of ~ational Method and the Putnam Method to estimate the flood peaks for the 20 and 100 year flood events, and the accuracy of stream cross sectional data. The investigator is concerned that the hydrologic analysis may not adequately reflect the "true" 100 year flood event. The greatest uncertainty exists in the use of Putnam on the stream segments with less than 200 acres drainage area within the Briar Creek tributary at Sudbury ~oad. The analysis, however, was completed using the Putnam Method to avoid introducing error associated with mixing methods. Stream cross sectional data were taken from 1:2001 (2 foot contour) scale maps or from field cross sections provided by the Public Service ~ngineer'sfiles on previous 20+2 analyses. The cross sectional data taken from the 1:200 scale maps may not have the resolution necessary to provide a picture of the actual water surface profile behavior. For this investigation actual location of the building restriction flood line is not necessary and valuable information can be gained from comparison of the water surface profiles derived from the map cross section data.

The backwater effect, caused by the interference of culverts with stream flow, dominated the cases examined in this investigation. The relative frequency at which culverts appear in the urban watershed is high, causing a pool-drop-pool effect on the flood surface profiles. The effects of culverts on water surface profiles are not analyzed in great detail within the scope of this investigation. For purposes of simplicity it is assumed in this study that all the culverts act under inlet control. Further study of culvert behavior should account for outlet control, detailed analysis of overtopping, and hydraulic routing of the associated flood peak. Current City of Charlotte design standards allow the design of culverts on residential street subject to flooding at the 10 year flood level. Current design standards also limit the headwater to depth (HW/D) ratio to 1.2. These standards, 10 year flood and low HW/D ratio, produce culverts which regularly overtop in both the 20 and 100 year floods. The weir type behavior of road grade crossings allows the stream to carry greater discharges with only small changes in driving head. In effect, the roadway becomes an "emergency spillway" for the culvert. If the current design standards are maintained culvert overtopping will occur frequently during the 20 year and greater floods. The results of the water surface profile analysis using HEC- 2 indicate that the elevation of the 20 year flood plus two feet is approximately 1.4 feet higher than the elevation associated with the 100 year flood. Findings

1. The analysis of 16553 feet of collector streams reveals that the 20+2 water surface profile is conservative when compared to the 100 year flood water surface profile. The differences in the two water surface profiles range from 0.3 feet the 1.9 feet (See Figure IV-1). A central tendency is observed to occur at approximately 1.4 feet. This tendency is attributed to the effects of culvert controls on downstream backwater elevations. The effect of urbanization on raising the water surface profile is determined to be negligible. Analysis of 6900 feet of stream on the Briar Creek tributary at Sudbury Road at the 20 year flood discharge for 16 and 35 percent impervious area yields a mean difference in water surface elevation of 0.3 feet (See Figure IV-2). Correspondingly the 100 year water surf ace elevation yields a mean difference in water surface elevation of 0.2 feet (See Figure IV-3). Culverts are the controlling factors in the majority of analysis cases. Water surface elevations are determined largely by the backwater effects of the culverts. Culvert overtopping and broad overland flow WATER SURFACE PROFILES COMPARISONS DIFFERENCE OF 20+2 AND 100 YR WSP

0.00 0.40 0.80 1.20 1.60 2.00

DIFFERENCE IN FEET

~igureIV-1 ~ifferentialComparison of the 20+2 and 100-Year Flood Water Surface Profiles. DISTRIBUTION OF 20 YR DIFFERENCE (1 6% AND 35% IMPERVIOUS AREA) 20 ,

0.00 0.20 0.40 0.60 0.80 1 .OO

DIFFERENCE 35%- 1 6% (FEET)

Figure IV-2 Distribution of ~ifferences in Water Surface leva ti on in the 20-Year Flood (16% and 35% Impervious Area). DISTRIBUTION OF 100 YR DIFFERENCE (1 6Z AND 3% IMPERVIOUS AREA) 1

0.00 0.20 0.40 0.60 0.80 1 .OO

DIFFERENCE 35%- 1 6% (FEET)

Figure IV-3 ~istribution of Differences in Water Surface levat ti on in the 100-Year Flood (16% and 35% Impervious Area).

50 cause the doivnstrearr\ con tral elevations for the 100 year and 20 year floods to differ by approximately 0.5 feet, The overtoppiny of culverts in the 20 and 100 year floods is attributed to the design policy of the City of Charlotte regarding culvert design on residential streets. The policy allows for a minimum 10 year design flood to be used for culvert design on residential access street crossings which are subject to complete blockage by floodwaters. Secondary and arterial streets subject to blockage use the 20 year return period for culvert design. These backwater effects are apparent from the plotted water surface profiles (See Figure IV-4). WATER SURFACE PROFILE COMPARISON SUDBURY MAINSTEM (1 6% IMPERVIOUS) 770 ,

(Thousands) DISTANCE (FT) + 20YR o 20+2 - BOTTOM

~igureIV-4 100-Year Water Surf ace Profiles - Sudbury Mainstem (16% and 35% Impervious Area). Detention Provision --of the Zoning Ordinance

The detention requirement provides for the instal lation of stormwater detention ponds on site at developments of dense composition exceeding a certain area. The design objective is that the peak out£low from the site after development be no greater than the peak out£ low from the site prior to development, the design storm being the ten-year storm.

There are several aspects of the requirement that have been questioned. Is detention effective? Do facilities in the field perform as intended? Is the design-storm magnitude appropriate? Is the coverage sufficiently complete? Can application of detention be made more flexible?

Effectiveness

Detention basins do reduce peak discharges of floods passing through the basin. The detention requirement of the City of Charlotte had that intent. Retrospective analysis of detention basins installed under the Charlotte ordinance shows that most are effective in reducing peaks to some degree, but there are questions as to the adequacy of the degree of reduction.

Table IV-1 shows a summary of independent analyses of a set of basins selected at random. The analyses were conducted using the hydrograph formulation technique described below in the discussion of the stormwater impoundment design manual.

Table IV-1 Summary of Analyses of Detention Ponds on File...... SITE BASIN PEAK PEAK ROUTED ID AFTER BEFORE PEAK ------...... Icfsl Ecfsl kfsl Eastway Shop Ctr A 87.0 31.0 19.0 Summit Ridge Apt 1 5.7 3.6 5 .7 Atlantic Envelopetway Shop Ctr A 87.0 31.0 Summit Ridge Apt 1 5 .7 3.6 5.7 Atlantic Envelope Co 15.2 10.0 15.2 Cotswold Ofc Dvlpmt 23.0 10.2 23.0 St John Neumann Ch 10.5 2.7 3.6 Calvary Baptist Ch 30.9 8.5 Storage exceeded Lake point ~vlpmt 373.0 311.0 308 .8 Colonial Self Strg A 11.5 9.9 Storage exceeded Colonial Self Strg B 14.9 14.4 Storage exceeded Independence Datsun 96.0 26.0 Storage exceeded Char Pk Exec Ctr A+B+C Compos i te 4 .4 5 .0 Typically, in the analyses, there was lack of sufficient storage to reduce the peak to predevelopnent conditions in the ten-year storm. The main source of inadequacy was traced to the method of hydrograph formulation in the current design standards. The projects listed in the table generally were found to be adequately sized according current standards.

The role of the detention requirement in management of urban collector streams needs clarification. Detention is effective in reducing flood peaks immediately downstream of the site at which the basin is installed. The smaller is the basin, the shorter is the stream segment along which detention may be expected to be significant. Therefore, detention is most applicable where a radical densif ication of land use threatens to increase flooding below the site itself.

In a requirement like that of the City of Charlotte, detention does not reduce the ---volume of f loodbater, it merely spreads out the flow in time. The increases in volume associated with urbanization of a particular site combine in unpredictable ways to increase volumes and peaks in the larger drainage region, with or without detention. Therefore, it is unrealistic to expect detention at small scale to control increases in flooding city wide. Field Performance

The incentive structure of the Charlotte ordinance and its implementing procedures encourage trivial add-on f acilities resulting in nominal compliance and little or no effectiveness in some cases. Entrepreneurs engaging in conventional cost- minimizing behavior submit plans showing f acilities devised under the ordinance. Once approval is obtained, it becomes the burden of the inspection team to insure that facilities are constructed in accordance with the approved plan. This regulatory approach and response is typical of other local and state programs reviewed by the principal investigator.

The project team reviewed and analyzed a sample of plans submitted, and inspected field installations. The following conclusions were drawn:

(1) A majority of detention facilities are designed and instal led accord ing to the ordinance.

(2) The ordinance requires some detention basins to be constructed where no significant benefit would accrue from detention. A common example is where the developed site is quite close to a major stream. If the site is small relative to the size of the watershed of the stream, there is little likelihood that peak floods in the stream would be appreciably increased by undetained discharges from the site.

(3) The ordinance does not provide an incentive to supply extra detention on sites where such is possible. There have been cases where lakes or other facilities were installed for recreational an3 aesthetic purposes that overrode the detention requirements at the sites in determining the sizes of the facilities. The quantities of detention storage supplied were vanishingly small compared to the quantities economically achievable. Opportunities to have effective regional detention facilities were missed.

(4) Followup inspections of constructed facilities show that in some cases installation did not correspond to the approved plan. For example, when compared to the plans, outlet pipes were in different locations or pavement slopes were too steep for storage. All such differences do not necessarily mean that the impoundments would malfunction, but they raise questions.

(5) It is possible under the ordinance to design and install permissible detention facilities that are ineffective. The common cases observed involve dividing the detent ion-storage requirement among several basins within the site. The interaction of the network of basins does not necessarily produce results commensurate with the objective of the ordinance. Design-Storm Magnitude The appropriateness of the lo-year design storm has been questioned. Some suggest that a larger design storm be used. The investigators are fami 1 iar with detention requirements that have been based on the 20, 50, 100, 25- ,500 and 100-year storms; and in one case, a11 of these. The lo-year storm predominates, presumably because that is the prevailing return period for other designs associated with stormwater-co1 lection systems. An effective detention basin designed for the lo-year storm will perform satisfactorily in the large storms of 90 percent of years. A detention basin designed solely for the 100-year storm will not perform as well at the lo-year level as one designed for the lbyear storm. So, as one increases the design storm magnitude the smaller storms tend to slip through less well attenuated. The smaller storms passing a 10-year basin would be more likely to be within streambank capacity.

The lo-year storm remains as the magnitude providing the widest spectrum of reduction of out-of-bank floods. Some specific cases may arise in which it is found to be best to design for a larger storm, perhaps in the case of a regional flood-control facility, but these should be considered on individual merit as being beyond the scope of the detention requ irement . Coverage

The intent of the detention requirement is to prevent increases in peak discharge downstream of radical changes in land use. When the requ irement was implemented certain land uses were exempted. Among these were single family developments and any development of less than 20,000 square feet of impervious area. Analysis of the trend in annual peak discharge at the McMul len Creek and McAlpine Creek gaging stations indicates that flood magnitudes are increasing in watersheds being developed in single-family residential units. Adjacent, separately developed commercial parcels in strip developments generate runoff to the same degree as they would if they were developed at once under single ownership. In the latter case detention would be required, but in the former it would not. Some uses that contribute to increasing peaks fall outside the detention requ irement . Flexibility

(1) There are cases where detention is required, but it will likely be ineffective.

(2) There are cases where it is unusually expensive to include detention on the site being developed.

(3) There are cases where there are opportunities to supply significantly more effective detention than is required under the ordinance.

The needed flexibility is difficult to include under the current ordinance and its supporting procedural arrangements, because of the heavy investment in staff effort that would be required to treat all cases thought to be exceptions. Indeed, it is hard to conceive of any arrangement that would provide completely for individual interactions between engineering staff and designers. A- Fee-Based Ord inance The overriding motivations for the fee-based ordinance derive from issues of flexibility and incentive. The best solutions to problems of watershed management frequently do not include the appl ication of detention. Channel-bank treatments, culvert enlargements and piping of channels also play a part. If detention facilities are extraordinarily expensive to provide on site, then off-site alternatives should be permitted. If a site is situated such that extra effective detention can be suppl ied economical ly, then the of f-site worth of the extra detention should be purchasable. For such transactions to take place, a medium of exchange is needed. The conventional medium of exchange is money, and the price-setting mechanism is the market . The setting of the fee is important. If the fee is too high, the value of detention basins will appear to be high. Too many basins will be built and the funds available for alternative uses will be diminished. If the fee is too low, it will be paid in virtually all cases, but too little money will be made available for a1ternative uses. Table 111-2 illustrates the relationship between revenues and estimated increases in runoff for a fee base of $10,000 per acre.

Table IV-2 Sample of Fees versus Increased Runoff.

Based upon a 10-yr, 6-hr storm of 3.64 in and soils of ~ydrologicSoil Group B with Fee Base of 10000 $/ace

F ind inqs

1. Provide for site developers to pay a fee to account for stormwater system improvements needed to offset effects of development. The funds collected should be reserved for drainage improvements. Improvements should not be 1imited to stormwater detention facilities, nor should funds necessarily be used immediately below the site from which they were col lected.

2. provide for developers to install effective detention facilities toward a refund of the fee. Adequacy of proposed facilities should be judged in an engineering plan review. Refund of the fee should be contingent upon certification by a professional engineer that constructed facilities are in accord with the approved plan.

3. Provide for payment to be made by the City to developments in which effective detention can be supplied in excess of that nominally required by the design objective of the ordinance. Payment would be based on downstream benefit from effective detention proposed by the developer as determined by the designated City administrator.

4. Remove the exclusion of stnali sites from coverage by the ord inance . 5. Extend coverage of the ordinance to single family STORMWATER IMPOUNDMENT DESIGN MANUAL

The Stormwater Impoundment Design Vanual is in need of updating with respect to hydrograph formulation and hydraul ic analytical procedures. If this is done, designers will uncjuestionably find that small sites will need larger impoundments than formerly expected. Detention facilities installed under the Charlotte requirements were analyzed by the project team. Some detention impoundments, although designed adequately by the Modified Rational Method given in the manual, were found to be ineffective when analyzed by independent procedures. The method given in the manual for use on small sites leads one to devise an impoundment having too small a volume of storage. In the design storm, one would expect water to overtop the emergency spillway and discharge flow at a rate greater than that specified for control.

The Charlotte Manual was prepared in 1978. At that time the methods given were widely used, and they still are. The Nodified Rational Method remains as an alternative in the Urban Storm raina age Criteria Manual of Denver, CO, although designers are encouraged to use more detailed hydrologic and hydraulic proc2dures as described in an update of 1982 (Denver Regional COG, 1984). Xodified Rational Method was also published as one method of choice by American Public Works Association in its 13rban s tormwater management report of 1981 (APWA, 1981).

Zu rrent ----Procedure The procedure most often used in the designs on file is found in Section Three of the Charlotte Manual (City of Charlotte, 1978). A permissible out£ low rate is computed as the peak discharge in the ten-year storm from the site prior to development. The Modified ~ationalMethod is applied by formulating a set of trapezoidal hydrographs. Each has a flat maxiinurn discharge computed by tile Rational Formula using a rainfall intensity associated with the time of duration of interest. Each hydrograph rises from the origin to the maximum discharge at the time of concentra tion, and each begins to descend at the time of duration. For each storm hydrograph thus formed, a storage volume is estimated by deducting the estimated Release Volume (release rate multiplied by time of duration) from the Storm Runoff Volume (maximum runoff rate multiplied by time of duration). The required storage volume is the largest of those computed for the set of hydrographs formulated. A basin is devised to contain the required storage, and an outlet device is selected to discharge at the permissible release rate.

The Modified Rational Method has at least the following shortcomings:

1. The hydrograph shape does not correspond to that observed at gaged urban watersheds.

2. The hydrograph peak is systematically lower than is pred icted by other methods . 3. The volume of runoff (area under the hydrograph) is significantly smaller than would be estimated by other more defensible methods. In this method, the volume of runoff is not directly associated with observable properties of soil type and cover conditions.

Proposed Procedure

Background: The f 01lowing small watershed hydrograph procedure is intended for use in watersheds of less than one square mile. It may be used as a reasonable alternative to the ~abularMethod of SCS TR-55. Hydrographs formulated for the same site by both methods will be of similar size and shape in the important period of time around the peak. The design of stormwater detention impoundments under the Charlotte ordinance may be based on either. The small watershed method is based on three decisions regard ing important aspects of the design hydrograph :

1, Estimate the peak discharge by efficient and reliable means, The recommended means in Charlotte are the Rational or Putnam Methods. The design storm is the ten-year storm. 2. Estimate the -__-volume -of runoff, the area under the hydrograph. The specified volume in Charlotte is the rho££ from the six-hour, ten-year storm.

3. Adopt a pattern hydrograph of satisfactory shape. The specified pattern for Charlotte is a step-function approximation of the SCS dimensionless unit hydrograph. Procedure: The following is a procedure incorporating the decisions given above, based on the Rational Method: 1. Collect the following data for the location of the dam of the impoundment of interest: A = Area (acres) of the watershed of the impoundment.

H = Height (ft) of the most remote point in the watershed above the outlet,

L = Hydraulic length (ft), the length of the mainstem from the outlet of the watershed to the most remote point on the ridge.

Ca = Composite Rational runoff coefficient of the watershed af ter development.

Cb = Composite Rational runoff coefficient of the watershed before development.

CN = Composite SCS Curve Number of the waterhed after development. 2. Compute the time of concentration of the watershed by the Kirpich Equation or Chart:

in which Tc = Time of concentration (min),

3. Compute the ten-year rainfall intensity by the equation specific to Charlotte, or read the intensity from the chart for time equal to Tc:

in which I = 10-yr Rainfall intensity (in/hr). If Tc < 5 min, use Tc = 5 min. 4. Compute the peak discharge of the lo-year design hydrograph by Rational Method for the af ter-development condition :

in which Qp = Peak discharge (cfs). 5. Compute the runoff from the lo-year, 6-hr storm in Charlotte by the SCS Curve Number Method. The rainfall depth (PI in the 10-yr, 6-hr storm for Charlotte is 3.64 in:

in which P = Precip (3.64 in) S = (~OOO/CN)-10 Ro = Runoff depth ( in). 6. Compute the time to peak of the design hydrograph. Time to peak is measured from the time of significant rise of the rising limb of the hydrograph.

in which Tp = Time to peak (min).

7. Compute points on the design hydrograph by selecting various times and using the step-function of the pattern hydrograph:

For 0 < T < 1.25*~p:

For T > 1.25*Tp:

Notes on computation: a. The symbol indicates exponentiation. For example, N 2 is equivalent to NXN. "*" indicates multiplication. b. The function "cos" is the cosine, and its argument is in radians. Put the calculator in "radians mode" before computing the cosine. c. The function "exp" is the exponential function. It is the value "e" (the base of the natural logarithm) raised to the power of the value within parentheses. Other Values of Interest: In addition to the formulation of the design hydrograph, the designer may find the following helpful: 1. Compute the permissible peak out£low in the lo-year storm, Qo, by the expression

in which Qo = permissible peak outflow (cfs) and I and A are the same as step 4, above.

2. A rough estimate of detention storage required may be obtained by

in which Sreq = Estimated storage (cu ft).

3. For sizing the emergency spillway, or weir, the 50-year peak a£ter development may be computed in two steps: a. Compute the 50-yr rainfall intensity, Iw:

in which Iw = 50-yr intensity (in/hr)

b. Compute the 50-yr peak discharge, Qw:

in which Qw is the 50-yr peak discharge (cfs).

Computational Support: The procedures above can be efficiently executed manual ly, wlth a hand calculator, using the expressions given or the charts that are based on them. They are also expressly developed for devices such as hand-held programmable calculators and microcornpu ters. For microcomputers, the procedures are especial ly we11 suited for the popular spreadsheet applications. The following is an example taken from such a spreadsheet : INPUT VARIABLES:

A = 10 Watershed area [ac] 20 Height of most remote point above outlet [ft] 950 Hydraulic length [ft] Ca = 0.81 Composite runoff coefficient after development Cb = 0.32 Composite runoff coefficient before development CN = 85 Composite SCS Curve Number after development

COMPUTED RESULTS :

Tc = 6.8 Time of concentration, Kirpich method, [min] I = 6.7 10-yr Intensity at Tc [in/hr]' QP = 54 peak discharge, 10-yr storm [cfs] Tp = 17.2 Time to peak [min] Qo = 21 ~llowablepeak outflow [cfs] Sreq = 33793 ~pproximatedetention storage required [cu ft] Q50 = 68 50-yr peak discharge for emergency spillway [cfs]

10-YR DESIGN HYDROGRAPH By pattern hydrograph ------Tine increment = 2 min Time Discharge [min] kfsl ------I------0 3 2 2 4 7 6 15 8 24 10 34 12 43 14 50 16 54 18 54 20 51 22 44 24 38 26 33 28 28 30 24 32 21 34 18 36 15 38 13 40 11 42 10 44 8 46 7 Source Information: The following apply to the steps of the proposed procedure:

1. Additional discussions of elements of the procedure can be found in the References: Tabular Method of SCS TR- 55 (Soil Conservation Service, 1975); Putnam Method (Putnam, 1972); Six-hour, ten-year storm (U.S. Weather Bureau, l96l), SCS Dimensionless Unit Hydrograph (Soil Conservation Service, 1972); Kirpich qua ti on for time of concentration (U.S. Bureau of Reclamation, 1974). 2. The equations for 10- and 50-year intensity as a function of duration were derived by linear regression using National Weather Service data for Charlotte (Frederick,et-- a1,1977, U.S. Weather Bureau, 1961) with durations from 5 minutes to 120 minutes. The equations are in substantial agreement with Table IV of the Charlotte impoundment manual, the differences being attributable to the older data of Table IV. Proposed vs Current Procedure

To illustrate the difference in results to be expected of the current and proposed procedures, the proposed procedure was applied to Example 1, page C.2 of the Charlotte impoundment manual.

The site is a four-acre site to be developed to full imperviousness. From the spreadsheet applicat ion of the proposed procedure, the following is obtained :

INPUT VARIARLES:

A = 4 Watershed area [ac] H = 17 Height of most remote point above outlet [ft] L = 550 ~ydrauliclength [ft] Ca = 0.95 Composite runoff coefficient after developmerlt Cb = 0.3 Composite runoff coefficient before development CN = 95 Composite SCS Curve Number after development

COMPUTED RESULTS :

Tc = 3.8 Time of concentration, ~irpichmethod, [min] I = 7.1 10-yr Intensity at Tc [in/hr] QP = 27 Peak discharge, 10-yr storm [cfs] Tp = 19.7 Time to peak [min] Qo = 9 Allowable peak outflow [cfs] Sreq = 21992 Approximate detention storage required [cu ft] Q50 = 36 50-yr peak discharge for emergency spillway [cfs] ÿ able IV-3 contains the inflow hydrographs formulated by the two procedures and the computed out£ low hydrograph of the example. Figure IV-5 displays the same information.

The difference in hydrograph size is striking. Clearly, the detention pond designed in the example would perform inadequately when loaded with the storm hydrograph formulated by the proposed procedure.

able IV-3 Hydrographs Formulated for an Example Site by Current and proposed Procedures. HYDROGRAPH FORMULATION COMPARISON Using Manual Example No. 1

Time (min) 6 PROPOSED A EX. INFLOW x EX. OUTFLOW

Figure IV-5 Hydrographs Formulated for an Example Site by Current and Proposed Procedures. ~ustification-of Proposed Procedure The three important aspects of validity of the hydrograph f orrnulated for detention-basin design are the shape of the hydrograph, the volume of direct runoff and the magnitude of the peak. In the proposed procedure, the shape is set by the step- function, the volume is set by including the estimated runoff from the six-hour storm of the return period of interest, and the peak is estimated independently by Rational or Putnam methods. Shape --of the Hydrograph: The shape produced by the pattern function was compared to that of gaged watersheds in Charlotte. The gage data were obtained by U.S. Geological Survey at tributaries of Briar Creek at Shamrock rive (335 acres) and at Sudbury Road (360 acres), in eastern Charlotte. The recorded hydrographs of eight storms were evaluated, selection being made on the basis of size and compactness of the storm. The values of peak discharge and hydrograph volume of the step function (see step 7 of the proposed procedure) were set equal to those of each recorded storm, and the pattern hydrograph and recorded storms were plotted on common axes. The results for the storm of 10 August 1967 on the two watersheds are shown in Figures IV-6 and 7. These are typical of the eight storms evaluated. The goodness of fit deteriorated where the storm was spread out in time with multiple bursts of rainfall, as one would expect . The data support the selection of the step function as being representativeof the shape of the hydrographof a stormof known peak discharge and volume of runoff. Volume_- of- Direct Runoff: Conventionally, a center-weighted design storm is the source data set for hydrograph formulation procedures based on synthesis. The proposed procedure produces a hydrograph that matches satisfactorily the volume of runoff that would be included under the hydrograph formulated by the longer met hod s . The rainfall source data for the conventional design storm is taken from two National Weather Service documents (Frederick,et- __al, 1977; U.S. Weather Bureau, 1961). The full data for Charlotte are given in Appendix C and shown in the rain£a1 1 depth-duration-frequency curves of Figure IV-8. The center-weighted 24-hour design storm is devised for a given return period by obtaining incremental values of rainfall depth and rearranging them around the centered maximum value. The ten- year, 24-hour storm for Charlotte is shown in ~igureIV-9, divided into f ive-minute increments. The Soil Conservation Service has shown that for much of the nation the 24-hour design storm can be fitted to a common distribution of ratios of SHAMROCK DR. TRIBUTARY 10 August 1967 Storm 200

Time (min) - Goged + Function

Figure IV-6 Comparison of Pattern Hydrograph and Historical Storm - Shamrock Drive .

69 SUDBURY RD TRIBUTARY 10 August 1967 Storm 210 , I

Time (rnin) - Goged + Function

Figure IV-7 Comparison of Pattern Hydrograph and Historical Storm - Sudbury Road. RAINFALL DEPTH-DURATION-FREQUENCY CHARLOTTE, NC

Duration (hr) U 2-yr t 5 o 10 A 25 x 50 v 100

Figure IV-8 Depth-Duration-Frequency Curves for Charlotte.

71 CHARLOTTE: 10-YR DESIGN STORM 24-HR STORM @ 5 MIN INCR

Time (hr)

~igureIV-9 Center Weighted Ten-Year, 24-Hour Design Storm in 5- Minute Increments for Charlotte. precipitation at a given duration to precipitation at 24 hours (McCuen, 1982). That Charlotte storm patterns fit this distribution is supported by computing the ratios for the ten- year storm described above and plotting them with the SCS distribution (Figure IV-lo). The SCS Type I1 Storm is the rainfall basis for hydrograph formulation by the methods of SCS TR-55 and several of the computer models.

It can be noted that 71% of the rainfall in the scs Type I1 distribution is contained within the central six hours. Small urban watersheds are clearly most sensitive to the intense central portion of the storm. Analyses by SCS TR-55 show the substantial part of the hydrograph flow to occur after the 11th of 24 hours. It is reasonable to isolate the center six hours for watersheds of areas of less than two square miles. The hyetograph for the Charlotte six-hour, ten-year design storm with 5-minute increments is shown in Figure IV-11. Studies by the project team of design hydrographs formulated by SCS TR-55 indicate that 67% of the total runoff is contained in the high-discharge part of the hydrograph. Most of the remainder is in the protracted trivial discharge of the falling limb. In Figure IV-12, total runoff computed from 67% of the 24- hour rainfall is compared to that computed for the central six hours. The difference varies with SCS Curve Number, but in the range of CurveNumbers 80 to 95 the difference is quite small. The use of the six-hour rainfall to estimate the runoff, or volume of water in the design hydrograph is justified for urban watersheds of less than two square miles. -Magnitude --of the Peak: Given that the shape and area of the hydrograph are satisfactorily estimated, the magnitude of the peak is of interest. SCS TYPE II VS CHARLOTTE STORM 10-yr Return Period

Time (hr) + SCS TYPE I! o CHARLOTT'E NWS DATA

Figure IV-10 Comparison of SCS Type I1 Storm Distribution and the Ten-Year, 24-Hour Storm Distribution Formulated for Charlotte. CHARLOTTE: 10-YR DESIGN STORM 6-HR STORM @ 5 MIN INCR

9 10 11 12 13 14 15

Time (hr)

Figure IV-11 Center Weighted Ten-Year, 6-Hour Design Storm in 5- ~inuteIncrements for Charlotte. RUNOFF COMPARISONS 6-hr and 67% of 24-hr values

SCS CN 6hr A dif

~igureIV-12 Comparison of Runoff from 67% of the 24-Hour Storm with Runoff from the 6-Hour Storm for a Range of Curve Numbers. For the watershed of the tributary of Briar Creek at Sudbury Road, at the location of the USGS gage, a synthesis of the center-weighted ten-year storm was made using a unit hydrograph of time-to-peak equal to time-of concentration. The synthesis was done in 5-minute increments. Runoff was computed by SCS Curve Number method. The proposed procedure was used to estimate the hydrograph based on the same data, with the runoff coefficient being added. The common data were:

Watershed area = 360 ac eight of most remote point above outlet = 79 ft Hydraulic length = 5400 ft SCS Curve Number = 80 10-yr, 6-hr rainfall = 3.64 in The proposed procedure also requires estimating the runoff coefficient. The value used was 0.5 for single family areas, taken from Table I11 of the Charlotte Stormwater Impoundment ~esignManual. The two hydrographs appear in Figure IV-13. They have equal volumes of runoff and essentially the same shape. The peak is in question. The proposed procedure yields a peak of about 680 cfs. The synthesis yields a peak of about 500 cfs. It is the experience of the principal investigator that unit-hydrograph procedures lead to lower and flatter hydrographs for small watersheds than those of other methods. The statistical estimate of the 10-yr flood at the gage site as estimated by uSGS is 645 cfs (Putnam, 1972). The proposed procedure can be used with any reasonable estimate of the peak of the design storm. Recommended for Charlotte are the Rational and Putnam Methods.

Storm rain age ~epairpolicy

The storm drainage repair policy of the City provides for the city to share the costs of certain repairs and improvements with the abutting property owners. Questions have arisen as to the effectiveness of the policy in responding to complaints by citizens of system problems. The Engineering Department keeps detailed files on complaints and responses to them. The project team selected a representative sample of 138 of more than 900 complaints on file since implementation of the storm drainage repair policy. AS experience with the policy has grown, the procedures of City staff to respond have evolved, and demand has grown for service. PROPOSED VS SYNTHETIC PROCEDURES 10-Yr Storm on Sudbury Tributary

200 300 Time (min) t Synthesis Proposed

Figure IV-13 Comparison of Hydrograph Formulations by Unit- Hydrograph Synthesis and proposed Procedure for the Sudbury Watershed. The complaints studied are tabulated in Appendix B.

Analysis of the complaints and responses led to the following findings :

1. Response time to a typical drainage complaint had become long due to the growth of demand for service and competing demands for engineering services, such as in subdivision plan review.

2. Files indicate that erosion is the most frequent complaint. The total cost of responding to flooding complaints exceeds the cost of responding to any other single category of complaint.

3. Engineering responses to system complaints seem excessively expensive in some cases. Typical response to a minor erosion complaint is a piped system. In part this may result from the use of the ten-year storm for design. There is also considerable incentive both for residents and designers to favor the permanent solution of a piped system over the economically expedient solution of a grass- 1ined open channel that will likely need continual maintenance and may stimulate repeated complaints. It is also likely, and there is evidence in the complaint files, that the permanent solution is seen by the citizens affected as being too expensive even with cost sharing.

4. A sizable number of complaints have been for trivial backyard drainage problems to be expected under normal cond itions.

It is recommended that the large number of trivial complaints be dealt with differently. It is evident, as stated in the findings, that many problems brought to the engineering group have to do with minor erosion, or shallow standing water, in residential yards. Certainly, the citizens view these as significant problems; certainly, council members and engineering staff view the complaints as valid. Indeed, in counting and generating a caring response, they are treated much as a major flooding or erosion problem. Typically, the suggested design for a backyard problem involves the installation of a substantial length of pipe at a cost of several thousand dollars. It is reasonable and practical in many such cases to repair the offending surface by shovel work to improve the drainageway, installation of temporary protection against erosion and establishment and maintenance of a grass lining. In some cases examined, the pipe solution was not implemented because of the expense. In such a case, educational help toward the simpler improvement might have been useful. The persuasion to the pipe solution seems to be based on the desire of a11 concerned for a permanent, maintenance-free solution. But the reforming and seeding of the surface by the resident may be more economical and more reasonable for a11 concerned.

The level of service concept, broadly defined, is an attempt to measure the ualit of service by using quantitative factors. The term "leveP--_y of service" implies that some service is being provided to a given party, usually the general public. The term is used quite commonly in the area of transportation to describe the qua1ity of the transportation facilities being provided, usually by various government entities, to the users of those facilities, usually the taxpaying public. In regard to streams, the term will be used to describe the quality of the urban streams. This section is a condensation of a more extensive work done by one of the team in support of the project (Lancaster, 1985). The full report is filed with City of Charlotte and The Water Resources Research Institute of the University of North Carolina.

TRANSPORTATION LEVEL OF SERVICE The level of service concept has been applied throughout the transportation field. It is used in two distinctly different ways. Most commonly, level of service relates to the capacity of a facility -- with a very crowded condition being termed as a low level of service. In recent years, quality measures have been developed for highway pavement conditions and, a1though these measures are actual ly ref erred to as "pavement condition" measures and not "level of service" measures, they will be used to represent another level of service approach. The two approaches will be referred to as "capacity level of service" and "condition level of service".

Capacity Level -of Service The broadest use of the term "level of service" is in relation to highway capacity. Capacity is defined as the maximum number of vehicles a segment of highway can be reasonably expected to carry in a stated amount of time (usually measured in vehicles per hour). In the 1965 Highway Capacity Manual, estimates of the capacities of different types of highways were given. A standard method for determining the level of service on a segment of highway and at intersections was also introduced. The method was developed by a committee of the Highway Research Board whose stated intent was to provide guidelines to enable users of the manual to select a volume corresponding to the level of service best suited to a specific need. Level of service is- defined as "a qualitative measure of the effect of a number of factors, including speed-and travel time, traffic interruptions, freedom to maneuver, safety, driving comfort and convenience, and operating costs." A1though it was considered desirable that a11 of these factors be incorporated in a level of service evaluation, at the writing of the manual there were insufficient data to determine either the values or the relative weights of the six factors 1isted." (Highway Research Board, 1965 .) Qualitative Definitions

The levels of service range from A to F with A being ideal and F being intolerable. The qualitative definitions of the various levels of service are quoted as listed in the recently published 1985 Highway Capacity Manual:

Level-of-service A represents free flow. Individual users are virtually unaf feFted by the presence of others in the traffic stream. Freedom to select desired speeds and to maneuver within the traffic stream is extremely high. The general level of comfort and convenience provided to the motorist, passenger, or pedestrian is excellent. ----~evel-of-service -- -B is in the range of stable flow, but the presence of other users in the traffic stream begins to be noticeable. Freedom to select desired speed is relatively unaffected, but there is a slight decline in the freedom to maneuver within the traffic stream from level of service A. The level of comfort and convenience provided is somewhat less than at level of service A, because the presence of others in the traffic stream begins to affect individual behavior.

Level-of-Service C is in the range of stable flow, but marks the beginning of the range of flow in which the operation- of individual users becomes significantly af fected by interactions with others in the traffic stream. The selection of speed is now affected by the presence of others, and maneuvering with the traffic stream requires substantial vigilance on the part of the user. The general level of comfort and convenience declines noticeably at this level. ------Level-of-Serv ice -D represents high-density, but stable, flow. Speed and freedom to maneuver are severely restricted, and the driver or pedestrian experiences a generally poor level of comfort and convenience. Small increases in traffic flow will general ly cause operational problems at this level. Level-of-Service E represents operating conditions at or near the capacity level. Kll speeds are reduced to a low, but relatively uniform value. Freedom to maneuver within the traffic stream is extremely difficult, and it is generally accomplished by forcing a vehicle or pedestrian to "give way" to accommodate such maneuvers. Comfort and convenience levels are extremely poor, and driver or pedestrian frustration is generally high. Operations at this level are usually unstable, because small increases in flow or minor perturbations within the traffic stream will cause breakdowns. ----Level-of-Service -- -F is used to define forced or breakdown flow. This condition exists wherever the amount of traffic approaching a point exceeds the amount which can traverse the point. Queues form behind such locations. Operations within the queue are characterized by stop-and-go waves, and they are extremely unstable. vehicles may progress at reasonable speeds for several hundred feet or more, then be required to stop in a cyclic fashion. Level-of-service F is used to describe the operation conditions within the queue, as well as the point of the breakdown. It should be noted, however, that in many cases operating condition of vehicles or pedestrians discharged from the queue may be quite good. Nevertheless, it is the point at which arrival flow exceeds discharge flow which causes the queue to form, and level-of-service F is an appropriate designation for such points r ran sport at ion Research Board, 1985). Quantitative ~efinitions The quantitative measures selected to define levels of service for highways are travel speed and "v/C ratio" (the ratio of the traffic volume to the capacity of the highway). Theoretically, as the volume of vehicles increases, the travel speeds become lower until finally, in the level of service F range flow of traffic would stagnate with speeds and volumes approaching zero. The Capacity Manual specifies 1imiting values for each level of service for various types of highways. For example, Level of Service A for a two-lane highway is defined as a speed greater than 60 mph and a V/C ratio less than 0.35. Failure to meet either of these conditions means a level of service lower than A is being provided (Highway Research Board, 1965 ) . Applications

~lthougha facility with level of service A or B is desirable, of ten it is not economically feasible, particularly in an urban setting, to provide that degree of service. In general, level of service C is quite acceptable for an urban highway or intersection. Level of service D is often tolerated, due to frequent lack of any economically practical potential improvement. Level of service E, however, is generally a critical situation in definite need of improvement. Condition Level -of Service In recent years, there has been increasing concern at local, state and national levels of government over the deterioration of the nation's in£rastructure. Particular attention has been directed to "the pothole problem" of streets and roads. Many states and cities have revamped their highway and/or street maintenance programs by implementing what has become known as a I'PMS~ or pavement management system.

A pavement management system is essentially a data base of information intended to improve -the efficiency -and effectiveness of the decisions involved in maintaining pavements. The cornerstone of the data base is a periodic sur;ey of the condition of the roads in the network. In addition, a maintenance history of each pavement section is recorded along with the associated costs. Often, traffic data is also included in the data base since traffic volumes, particularly truck volumes, greatly aff ect the deterioration of pavements (Johnson, 1983; Shahin, 1980).

Pavement Condition Ratings The measure of pavement condition is called a "PCI" (Pavement Condition Index) or a pavement condition rating. The rating could also be called a condition level of service measure since it is an attempt to define the quality of transportation service as related to the condition of the facility. Numerous methods are available for measuring pavement conditions. Most are subjective. Personnel from a highway or street department will judge the condition based on different types of cracking and surface roughness. A1though there are many similarities between rating schemes, each one uses slightly different indicators and weights them a little differently. A typical scheme rates a pavement in "perfect" condition as 100 and deductions are made according to the type and extent of each evidence of deterioration. Some methods of measuring condition are objective, meaning that a machine is used to measure certain indicators, with those indicators being the only measure of pavement condition (Johnson, 1983; Shahin, 1980). The starting point of almost all pavement management systems is a rating of each pavement in the network. Sometimes sample areas of highway are assumed to be representative of large segments. Often, however, the entire system is surveyed. A low rating indicates needed repairs. The rate of drop of the rating over several years is also used as an indicator of problems. Development -of -a ~evel-of Service Scheme The usefulness of the capacity levels of service as defined in the Highway Capacity Manual has encouraged the development of level of service measures for other areas of transportation such as transit systems and airports. Several references have provided some use£u 1 insights regarding the process of defining level of service measures. A particularly helpful reference is one concerned with general highway maintenance. It describes in detail a systematic -procedure for developing maintenance levels of service (Malcom, 1980). Qua1itv Parameters

Before a level of service scheme can be developed, it is necessary to determine the qualities of service expected of the facility involved. In developing capacity levels of service, the Highway Research Board committee cited six qualitative factors related to highway capacity: "speed and travel time, traffic interruptions, freedom to maneuver, safety, driving comfort and convenience, and operating costs." The goal of the level of service scheme is to maximize the quality of service as perceived by the user of the facility (usually the peak hour driver). Travel speeds and traffic volumes were chosen as representative of the six qualitative factors (Highway Research Board, 1965).

Although condition levels of service generally include factors affecting driver comfort and safety, they actually measure the quality of service to the taxpaying instea8 of to an individual driver at a specific point in time. The quality goal of a PMS is cost-effectiveness -- preservation of the investment in highways for a minimum expenditure of funds. The factors used in defining a condition level of service or a pavement rating are the various types of pavement deterioration that require maintenance expenditures.

APPLICABILITY OF LEVELS OF SERVICE FOR URBAN STREAMS In the past 15-20 years much attention has been directed toward improving the management of urban stormwater. Rapid urbanization in many areas has resulted in increased volumes and velocities of storm runoff largely due to the increase of impervious areas such as streets, parking lots and rooftops. Management tools have included ordinances requiring developers to size pipes to accommodate a designed return period of flood, to build only above a certain minimum elevation (usually the 100 year flood elevation), and also, in some cases, to control the postdevelopment peak flood in such a manner that it does not exceed the pre-development peak. In spite of these tools, there are still a number of problems associated with the streams in urban areas, which in most cases have not been enlarged or improved to accommodate the increased urban floods. There is typically no comprehensive management policy directed to the maintenance of the urban streams (Poertner, 1980 ).

-The Level -of Service Concept --as a Management Tool In the transportation field, capacity levels of service aid the engineer in analysis of existing facilities and in design of future facilities. Condition levels of service are more appl icable in the area of maintenance of existing f acil ities. The network of streams present in urban areas existed before the area was developed. In one sense, all projects relating to urban streams could be considered maintenance projects since they are improvements of an existing facility. However, the Charlotte Engineering Department is not presently involved in routine maintenance of the streams. Instead, their work involves improvement projects that greatly alter the existing stream. The proposed use of the level of service concept for streams is intended to be applicable for analysis of existing conditions and design of improvement projects. It does not attempt to address the problem of routine stream maintenance, such as clearing debris from a stream reach.

Current Management -of Streams -in Charlotte Since 1978, the Charlotte Engineering Department has provided assistance to citizens with storm drainage problems through the Storm Drainage Repair Policy. Under this policy, the City provides a study of the drainage problem, at no cost, to any citizen making a written request, provided that he is a private residential property owner whose property receives drainage from a public street. When the study has been completed the citizen is given a written estimate of the cost of the repair including a breakdown of the property owner costs and the city costs. The city pays all costs involving areas within street right-of-way, as well as from 25% to 80% of the costs for improvements on private property. Currently city funds for storm drainage repair are appropriated on a first-come, first-served basis to those citizens who are willing and can afford to pay their share of the cost of improvements and/or repairs. The number of requests for drainage studies under the Storm rain age ~epairPolicy has increased over the past 2-3 years to the point that the City Engineering Department can no longer keep up with the requests and has been forced to hire consultants to handle some of the larger studies. Thus, it is apparent that the complaints regarding streams are continuing to increase and the citizens expect at least a certain amount of assistance from the City in the area of storm drainage. The City's current stream management policy is responsive in nature and any response is contingent on the participation of the property owner in sharing the costs. However, in seeking to improve its stormwater management policies, it is likely that at some point in the future the City of Charlotte will choose to assume greater responsibility for comprehensive planning and funding of improvements in the drainage system. A handful of cities in the United States have already adopted "stormwater utilities" where the property owners pay a user charge for maintenance of the stormwater facilities in much the same manner as they pay water and sewer charges. The local government in turn assumes responsibility for managing and maintaining the drainage system in a manner similar to the way streets, water systems and sanitary sewers are currently managed in most cities.

Role of Level of Service Measures in Future Management -7 - - It is in this context that a level of service scheme is deemed most desirable. Should the City assume full responsibility for stream maintenance and improvements, it would need some means of planning and prioritizing projects. Some performance standards for the urban streams would then be required. The City would need to have some way of allocating funds in an equitable, cost effective manner. This study of levels of service is being done as preparation for increased City responsibility in regard to urban streams.

A level-of-service scheme for Charlotte is intended to serve as the cornerstone of a management program for small streams. Such a program would be similar in nature to a PMS, with the general goal of the program being wise use of public funds to provide an adequate quality of service in the urban streams. The goals of the level of service measures would then include tne following:

1. To identify maintenance needs by setting a minimum level of service.

2. To prioritize projects according to level of service ratings.

3. TO standardize response based on degree and type of problem.

4. To be used as a basis for budget projections. Comparison -of Streams -and Streets One issue that is important to consider in the development of level of service measures for streams is that, unlike most transportation facilities, the majority of small streams are in private, not public, ownership. The private ownership of urban streams presents a major deterrent to the implementation of a stream management program since a property owner generally has the legal right to control access to his property. However, the quality of service provided by an urban stream on private property is both a public and a-private concern. part- of the rationale for pub1 ic spending on private property through Charlotte's Storm Drainage Repair Pol icy is that the eligible properties receive drainage from a public street and hence, the resulting problems are a -public responsibility to some extent. It is obvious, though, from surveying the complaint files and in talking to several riparian property owners, that the owners have widely varied expectations of how an urban stream should behave. In defining measures for level of service, some attempt should be made to ensure that the measures are equitable for varying densities of housing and different socioeconomic levels.

Capacity Considerations The most basic similarity between streams and streets is that they both carry flows. Any reach of stream or street has a certain capacity to accommodate flows. When the capacity of a street is exceeded, traffic stagnates. When the capacity of a stream is exceeded, the surrounding property floods. The goal in design of streets is to keep traffic moving under stable conditions. The goal in design of stream improvements should be to avoid negative flooding effects on the surrounding property.

It is true in transportation that the manner in which private property is developed may affect the capacity level of service on a nearby facility. The placement of driveway entrances and the additional traffic generated may lower the level of service provided to the daily commuter using the facility. It is also true in streams that the development of private property sometimes affects the capacity level of service in a stream by adding additional runoff. The analysis of level of service for a street is based on the existing traffic demands and the condition of the street. The analysis of level of service for a stream is based on the currently expected flood levels for storms of designated return periods and also the degree to which the surrounding properties will be affected by the flood levels. Transportation levels of service are sensitive to the development abutting a street only to the extent that traffic flow is affected by the development. Stream levels of service should be sensitive to the development abutting a stream to the extent that the property is affected by the stream flows. Condition Comparison The measures of the condition of a street are generally the type and extent of cracking that is visible. The measure of the condition of a stream is generally the extent of erosion that is visible. In pavement management systems, it has been found that it is cheaper to keep pavements in good condition than to let them deteriorate to a poor condition and then repair them (Johnson, 1983). It is likely that the same may be true in the case of streams. It certainly would seem cheaper to control erosion before the channel has greatly enlarged than to wait until the entire channel needs to be reshaped in order to repair the damage that erosion has done.

It has been suggested that the optimum time for a major investment in the maintenance of a pavement is just as the rate of deterioration of the pavement begins to increase. It may also be true that there is an optimum time to invest in improving a reach of stream. The optimum time may be just as a stream is beginning to experience an accelerated rate of erosion. The use of pavement condition ratings is beginning to provide managers with information that enables them to better understand the life cycle of a pavement. Level of service measures for streams may provide the same type of useful information regarding erosion processes in urban streams.

DEVELOPMENT OF LEVELS OF SERVICE FOR STREAMS The development process for various transportation level of service schemes has been reviewed. Similar processes have been applied to the development of levels of service for urban streams. Problems associated with the urban streams in Charlotte are discussed as a backdrop for development of the level of service measures. From this discussion, qualitative and quantitative definitions are proposed.

Assessment -of Urban Stream Problems As a part of the project the Storm Drainage Repair Policy files (referred to as complaint files) in the City of Charlotte's Engineering Department were reviewed. These provide use£ul information on the condition of urban streams in Charlotte and the costs involved in implementing improvements. Also, as part of the research project, much of the literature concerning urban stormwater management has been reviewed. These two sources form the foundation of this assessment of urban stream problems. Complaint File Findings

A study was made of the complaint files in order to find out what factors concerning the streams are regarded as problems by the riparian property owners. In the study, the McMullen Creek watershed was chosen as a sample watershed for the city. All of the requests for a drainage study in this watershed were investigated. (Complaints about failures in the pipe systems have been eliminated since they do not pertain to a study of open channel problems.) It is evident that flooding and erosion are the dominant causes of complaints in the urban streams. ~lthough the buildup of sediment of the streams is also a problem, it is often seen by the citizen only as a flooding problem (i.e. the sediment causes loss of cross-sectional area in the stream, reducing its capacity to carry floods). Citizens sometimes view the stream as a safety or health hazard. The safety complaint usually involves steep dropoffs caused by erosion. The health hazard is due to polluted water or the breeding of mosquitoes. The proposed level of service scheme is not intended to address water quality problems. The flooding complaints in the City of Charlotte involve minor property damage and inconvenience. The floods experienced along the small collector streams are not life-threatening and usually do not cause major property damage. The erosion complaints often involve damage to fences, storage houses or other backyard amenities. Also, an erosion complaint of ten reflects the citizen's concern that the stream in his yard not detract from the aesthetic value of his property. lood ding problems -in Urban Streams It has been well-documented in the literature that the urbanization of a watershed tends to increase the magnitude of flooding. In Hollist synthesis of studies on urban flooding effects, he concludes that "(1) small floods may be increased by a factor of 10 or more depending upon the degree of urbanization, (2) floods with a return period of 100 years may be doubled in size by the complete urbanization of a catchment if that urbanization results in at least a 30% paving of the basin, and (3) the effect of urbanization declines in relative terms as flood recurrence intervals increase." (Hall is, 1975.) In developing his conclusions, he used data from 15 different studies done in various U.S. locations, as well as in Japan and Britain. Martens, in his study of urban flooding effects in Charlotte, notes that basin lag time for a fully developed basin is about one- fourth the pre-deve lopment lag time. The increase in impervious area combined with the decrease in lag time associated with urbanization roughly doubled the discharge of the 20-year flood in the basins he studied. The increased discharges cause increases in flood stages. The computed flood elevations for the 20-year flood along 60 miles of stream channels in Martens' study show post-development flood stages to be as much as 6 feet higher than the pre-development stages (Martens, 1964). An additional effect of urbanization is of ten flood plain encroachment. Flood storage areas become occupied by structures that invite damage from flood waters.

Flooding Problems -in Charlotte The City of Charlotte has two provisions in the ordinances for controlling the tendency for urban flooding and flooding damages to increase as urbanization progresses. A floodplain ordinance has been in effect since the 1970% which prohibits the building of structures below the elevation of the 20-year flood plus 2 feet. Additionally, the City has imposed a detention ordinance on all development since 1978. ~etentionbasins as designed to 1 imit the post-development lo-year flood peak to the pre-development level.

Of the 33 complaints in the McMullen Creek watershed that are examined in this study, 15 of the complaints (44%) cite flooding as a problem. One- third of these flooding complaints are caused by an undersized pipe system. These are classified as a pipe problem instead of a stream problem and are not addressed in this study; thus, the remaining complaints total 28. Another one-third of the flooding complaints have to do with reaches of stream where the channel is undefined. In most of these cases the area is very flat. Only three of the 33 proposed solutions were actually implemented and two of these involved flooding complaints. In both cases, residences were situated very close to the open channel; thus, flooding threatened these structures. Several of the residents complain that their basement or crawl space is flooded during "heavy rains". In many cases, though, the property owner complains only of frequent and/or extensive yard flooding. It is obvious from looking at a number of these backyards that a considerable portion of the backyard (about 50% in some cases) would be flooded in a heavy rain. Apparently, this is in many cases not acceptable to the property owner. Thus, it is becomes apparent that flooding is perceived as a major problem along these small urban streams. According to city personnel, however, not a11 flooding problems generate a complaint to the City. In general, it is quite expensive to solve a flooding problem. Apipehas tobe replacedoranopen channel has to be piped in or enlarged. Any reshaping of a channel generally is accompanied by lining the banks with riprap. The expense appears to be general ly prohibitive to the property owners. Thus, it could be safely said that a majority of the flooding problems occurring along the collector streams are not being eliminated by the present policies.

Erosion Problems -in Urban Streams Erosion of the stream bed and stream bank is another often- mentioned effect of urbanization on the storm drainage system. The net result of urbanization, according to numerous investigations, is stream channel enlargement with width increasing more than depth (Nunnally --et --a1,1979). The enlargement is associated with the increase in flood levels, since the stream is said to seek to maintain its size such that bankfull flow is exceeded every one or two years. However, it is possible that urban streams continue to be more prone to lateral movement and scouring than rural streams, even after the channel enlargement process is complete (Hammer, 1973). Studies also indicate the possibil ity that the beginning of the channel enlargement process in an urban stream may be delayed until some type of threshold value is reached (Schumm, 1975). The channel may enlarge to as much as two to three times its original cross- section (Becker, --et al, 1973). Hirsch has found that downstream flooding is significantly increased when a channel is inadvertently enlarged by erosion processes. In his study of Holmes Run in Fairf ax County, Virginia, discharges downstream of an enlarged channel increased by 17% to 29% according to his model (Hirsch, 1977). Most of the studies involve drainage areas larger than one square mile. However, Hammer notes, "Very small streams are affected by urbanization in just the same manner as larger streams and in fact, the absolute amount of channel enlargement per square foot of impervious development is greater, the smaller the drainage area considered." (Hammer, 1973.) Geotemoeller notes that the most serious damage caused by accelerated discharge of stormwater from developed areas occurs in the smaller streams, since a 50 acre shopping center has much more affect on the stream at the point of a 200 acre watershed than at the point where the stream's watershed is several square miles (Goettemoeller, 1980).

At present the only ordinance in Charlotte aimed at the control of erosion downstream of new development is the Soil Erosion and Sedimentation Control Ordinance. It is chiefly designed to control on-site soil erosion, but it also states that "streambanks and channels downstream from any land disturbing activity shall be protected from increased degradation by accelerated erosion caused by increased velocity of runoff from the land disturbing activity" (City of Charlotte, 1975). The regulation is that the velocity in the channel during the lO-year storm must be controlled to the pre-development level or that the streambanks must be armored such that the lo-year flood could pass without causing erosion damage.

It should be noted that the present detention ordinance is not aimed at controlling erosion in the streams. In fact, studies indicate that detention may actually increase the erosion problem since the periods of bankfull flow are generally longer when part of a flood is detained (~ulkarni,-et --al, 1980). The more frequent floods (such as the two-year flood) are generally thought to be the more erosive floods. Whipple and Randall have shown that when detention is designed to control the lo-year flood, the 2-year flood peak may not be attenuated at all (Whipple, --et al, 1983).

~rosionproblems -in Charlotte Streams Roughly 80% of the citizen complaints generated in the McMullen Creek watershed list erosion as a problem. Erosion complaints span the entire spectrum of stream sizes for which the city is concerned (0-1 square mile of drainage area). In all cases the stream is in someone's yard area and is not buffered from sight by a wooded area. If a fence has been placed between the stream and the backyard area, the complaint is often that the fence is endangered.

Qualitative Level -of Service Measures The definition of level of service measures for streams is being approached in a manner similar to the measures defined for highways in the Capacity Manual. The quality factors intended to be incorporated in level of service measures for streams include safety, property damage, convenience, property value, aesthetics, and limit of sediment loads in streams. These will each be discussed in some detail. Parameters

In almost every case where a property owner has mentioned a concern for safety in his complaint to the City, the concern has been for the neighborhood children. Frequently, the problem area is at a discharge from a pipe where severe erosion has occurred and there is now a steep cliff-like dropof f. In some places, the entire stream has been experiencing bed erosion such that it is relatively deep and narrow with steep sides. Any potential safety hazard indicates a low level of service to the surrounding neighborhood. Property damage involves a fairly wide range of degree and type of damage. Certainly, there are homes along the small streams where there would easily be a foot or so of water (at least) during a 100-year flood. However, it is unlikely that any buildings would actually be destroyed, even by a very major flood. Alesserdegree of damage occurs whena basement or crawl space receives flooding. A basement is defined as a level of a residence that is at or below ground level at any point. While a basement may be considered living space by some property owners, it should be not accorded the same degree of consideration in a city policy as the first floor of a home. b his is done partly for simplicity and partly for equitability of the level of service measures. Another type of property damage is caused by erosion. While it is conceivable that erosion could endanger a permanent structure, it is much more frequently the case that it is a fence or a small shed that is being undermined. The proposed level of service definitions chiefly address damage only to permanent structures in order not to bias the measures in favor of owners who place backyard amenities too close to the stream.

Convenience is mentioned as a consideration in order to include backyard flooding. Although there is frequently no measurable property damage, the flooding may present an inconvenience to the property owner. For instance, the rear edge of a lot is often a favorite site for a garden. When it floods, the owner may have to spend some time cleaning up the debris left behind by the flood. This may be true for a very conscientious property owner even if he does not have a garden. Thus, a high level of service is being provided when inconvenience to the property owner is minimized. Certainly, the value of property is affected by the condition of the stream running through it. In a study of the effects of flood protection benefits on property value changes, Soule and Vaughan (1973) note that, for a major flood control reservoir in Kentucky, the value of the protected downstream property increased by 121%. This exceeded the increase in value of the shoreline property along the new lake, which increased by 94% due to the recreational benefits (Soule and Vaughan, 1973). While a major flood control project is very different in scale from flood control on an urban collector stream, it can at least be extrapolated that a home experiencing flooding may sell for less than the same home on higher ground. It is also likely that erosion effects property values if the stream is not buffered by a wooded area and particularly if it is in a front or a side yard. The less the stream detracts from the value of the surrounding property, the higher the level of service being provided.

Aesthetics will be considered in this level of service scheme only to the extent that it is likely to affect property values. This is based on the premise that neighborhood beautification is not the responsibil ity of an engineering department. There has been a large amount of government regulatory effort aimed at preventing soil erosion during construction. Silt fencing and sediment basins have become common at any construction site. The goal has been to keep the sediment out of the streams. However, there is a large amount of sediment in the streams that comes from the streambanks themselves. Little has been done on a broad scale to control streambank degradation. Studies indicate, however, that the amount of sediment in the streams originating from the streambanks may be on the same order of magnitude as sediment from construction sites (Hirsch, 1977). A higher level of service is provided if the streambank degradation is control led. Definitions Six levels of service are defined qua1itatively as £01 lows with examples of flooding or erosion conditions that are associated with the particular level of service: ----Level --of ------service A represents a stream in its "natural"------condition. The floodplain is undeveloped. No structures or yards are affected by the 100-year flood. No accelerated erosion is occurring. Erosion is occurring only on the outside of bends. The stream does not appear to have widened or to be affected by development in any way.

Level of service B represents a stream in a desirable condition. No property damage- to permanent structures is experienced in the 100-year flood, a1though yard flooding may be experienced. Erosion is slight or stream banks are armored against erosion. The stream in no way detracts from property values. The riparian property owner rarely experiences any inconvenience caused by the stream.

Level of service C represents a stream in an tolerable condition. Some inconvenience due to yard flooding may be experienced by riparian property owners, but not enough that the stream would detract from property values. ~rosionmay be occurring, but the stream cross-section has altered only slightly. ---Level -of ---service -D represents a stream is in an unacceptable condition. The value of surrounding property is slightiy affected due to erosion or flooding. The riparian property owner is frequently inconvenienced by problems with the stream. ---Level -of ---service -E represents a stream in a damaging condition. It is causing a significant reduction in the value of surrounding property. Damage to permanent structures is occurring. ~ar~e amounts of sediment may be moving from the bed or banks. A safety problem may exist along the reach.

Quantitative Level -of Service Measures The quantitative measures considered were not developed completely enough or sufficiently precisely for immediate use. The reader is referred to the original work (Lancaster, 1985) for the details.

A CASE STUDY The case study for application of level of service measures involves two separate samples of stream reaches. The first sample consists of 28 reaches of stream in the northwest tributary area of McMul len Creek. The McMul len Creek watershed is located in the southeast area of Charlotte. The second sample is the set of 33 complaints in the McMullen Creek watershed, 28 of which involve reaches of stream (5 are flooding problems caused by an undersized pipe system). There is some overlap between the two sets since 7 of the complaints are about stream reaches in the sample tributary area.

Sample Tributary Area The sample tributary area includes 900 acres in the headwaters of the McMullen Creek watershed. For this study, the streams were generally divided into reaches by road culverts or a fork in the network, but in a few cases they were divided by a bend in the stream or by a change in land use. Reaches range from 400 feet in length to about 1500 feet in length. All of the reaches of natural streams in the area are included in the case study. A few swales are included that may have been man-made or at least altered by developers, but in general the man-made swales, such as roadside ditches, are not included in this case study. Procedure After the reaches were marked out and numbered on a topographical map with drainage overlays, a site visit was made to each of the reaches. However, this did not include walking the entire length of each reach. Many sections of streams are bounded by fences or hedges or are buffered by thickly wooded areas with a lot of brush and vines so that it is difficult to get close enough to judge the condition of the stream. In general, a stop was made at each reach at the upstream and/or downstream end where it could easily be seen from the road. Further investigation was done only if it seemed desirable or necessary. Cross-sections were measured in some cases, but were judged by eye in others. Often the cross-section of the stream was highly varied. The general condition of the stream was noted as to erosion or sedimentation. Several conversations with property owners proved very he lpfu1 and informative. Obviously, flooding problems could not be judged by a mere site visit. However, property owners provided a history of flooding for three of the reaches. Houses that seemed to be at a low elevation relative to the top of the stream bank were noted during the site visits. Later, estimates for 2, 10 and 100 year peak flows were made for each reach of stream using TR-55 graphical solutions (Soil Conservation Service, 1975). The peak flow estimates apply to the downstream -end of each reach and were made by estimating the time of concentration at that point and using TR-55 as a lumped parameter model. Manning's equation was then applied to each reach of stream for the 2 and the 10-year peak flows. The depth and the average velocity was calculated in each case based on the observed width and side slope. his was done is order to estimate whether or not the stream cross-section is adequate to handle the 2 and/or the lo-year flow. Obviously, the calculated depth of flow would not approximate the actual depth once bankfull flow is exceeded. Even so, the depth calculations are indicative of the extent of flooding that could be expected. It should be noted that flooding classifications are based on the worst performance level within a reach. For example, flooding of only one basement during a 2-year storm qualifies a reach as level of service Do On reaches where the flooding parameter indicates level of service D or lower based on this rough analysis, a more detailed study, including backwater computations, would be recommended to - provide an adequate assessment of the problem. Results

Of the 28 reaches of stream in the sample tributary area, 6 are tentatively classified as level of service B. Three of these have been lined with riprap, apparently the work of county forces. The B classification is made assuming the streams carry the 2-year flood within their banks and that the lo-year flood inundates less than one-fourth of each abutting lot. One reach was riprapped by the developer of the property and was also greatly enlarged. A lo-year flood could easily be carried within the streambanks. One reach is a grass-lined swale with an eroded low-flow channel that looks to be adequate to handle a 10- year flood without excessive yard flooding. The last reach classified as level of service B is a small rather undefined channel running through a thickly wooded area. ~ough calculations indicate that only the wooded area is flooded during a lo-year storm. Thirteen reaches are classified as level of service C. These generally include reaches of stream where the banks are undercut or are otherwise eroded, but slumping is either not present at all or is negligible. At least half of these reaches are largely in wooded areas either on undeveloped property or in wide buffer strips between subdivisions. One reach is a grass- lined swale where part of the surrounding property is low relative to the stream and is subject to some yard flooding. One reach is presently performing at level of service C, but the abutting property on one side of the stream is currently being developed in a manner that is likely to degrade the level of service in the reach. Several feet of fill have been placed along the entire stream bank; and, thus far, no attempts have been made to stabilize the banks. Other reaches are experiencing moderate erosion or some yard flooding.

The level of service D category includes 7 reaches of stream. Of the seven, five are designated level of service D due to potential flooding problems, one due to erosion, and one is categorized as D because of a combination of erosion and flooding. Three of the flooding problems are along reaches with a very flat cross-section where extensive yard flooding occurs. On reach number 3, the house on Barwick Circle at the pipe entrance is sometimes flooded due to the inadequacy of the pipe or the poor entrance conditions, according to one of the neighbors. On one reach, one apartment building is below the level of a nearby manhole and crawl space flooding is likely during a 10-year storm. The erosion on one reach is characterized by undercutting and exposed roots along both sides of the bank. About half of the reach was surveyed and the whole area was in equally poor condition. No vegetation was growing along the stream (with the exception of large trees). A complaint has been made by the property owner at 4307 Emory Lane stating that their basement sometimes floods. Based on that complaint, the reach was tentatively designated as level of service D. An erosion problem is also evident in the stream with tree roots exposed all along the banks and the banks washed out behind 4307 Emory Lane in the area where there are no trees along the bank. Any reaches that are tentatively designated level of service D may actually be performing at level of service E. A more complete analysis is required to differentiate between levels of service D and E due to flooding. Two reaches are classified as level of service E. Flooding of living space could be expected at one house along a reach where the stream is about 8 feet (measured horizontally) from a front corner of the house. Calculations indicate that the 2-year peak flow is about bankfull flow. However, the culvert under the driveway would restrict the passage of the flood and could cause water to flood under the house. The other reach in this category is a reach in a wooded area behind Emory Lane north of Sharon ~mity. Uprooted trees and large amounts of coarse sand sediments can be seen in the stream bed during low flows. Fresh slumping can be observed on the banks.

The total length of channel surveyed in the sample tributary area was 23,900 feet. Of the total length, 25% was classified as level of service B, 41% as level of service C, 29% as level of service D and 6% as level of service E. Level of service C is proposed as a minimum quality standard for urban collector streams. Thus, 35% of the streams in this 900 acre area were substandard.

Sample -of McMu llen- Creek Complaints

As mentioned earlier, 28 complaints are included in the set used for this study. However, only 22 were actually included in a visual survey of the complaint sites. Two of the property owners who received assistance through the Storm Drainage Repair Policy installed pipe, and thus there is no longer an open channel on their property. One installed pipe at his own expense. Two owners complained of a destroyed retaining wall and neither of these complaints is included in the sites visited. One, however, was on a reach in the sample tributary area and had chosen to repair his wall at his own expense. One site was inadvertently omitted. The 22 complaints included in the visual survey are listed on the following page. Procedure The other 22 sites were inspected as to their current condition relative to erosion. The city's study of the problem was reviewed. ~stimatesof the degree of flooding are based on comments in the file from property owners and city engineers. No independent analyses of peak flows and flooding were performed for these sites. Table V-l Summary of Complaint Files ...... ADDRESS DATE OF TYPE OF DRAINAGE RESPONSE COMPLAINT PROBLEM ...... 3811 ABINGDON RD 5/5/83 EROSION 5420 ADDISON DR 2/22/8 2 EROSION 86.4 AC PIPE 1301 ARDBERRY PL 3/3/82 EROSION 10.8 AC PIPE/ CHAN IMP* 1822 CAVENDISH 7/15/82 EROSION/ 151.5 AC CHAN IMP FLOODING 2422 CLOISTER DR 9/29/80 EROSION REPAIR/ CHAN IMP 1741 DALLAS AVE 3/7/84 EROSION 79.9 AC CHAN IMP 5941 DEVERON DR 3/26/84 EROSION/ 7.7 AC PIPE FLOODING 4307 EMORY LANE 3/7/83 FLOODING/ 433 AC CHAN IMP EROSION 801 GLENDORA DR 7/13/82 EROSION 23.3 AC CHAN IMP 1033 GOSHEN PL 3/20/80 EROSION 11.5 AC CHAN IMP 1040 GOSHEN PL 2/25/82 EROSION 11.5 AC CHAN IMP 5900 GROSSNER PL 4/28/82 EROSION (10 AC PIPE 2129 KNICKERBOCKER 3/20/79 EROSION 102 AC PIPE 2143 KNICKERBOCKER 3/30/84 EROSION 100 AC PIPE OR CHAN IMP 2321 KNICKERBOCKER 4/28/81 FLOODING 57.9 AC PIPE KNICKERBOCKER/ 9/15/78 EROSION 149 AC PIPE MCNAIR 4/10/79 1138 LYNNBROOK 4/2/79 EROSION/ 25-40 AC PIPE/CHAN HEALTH PROB CHAN IMP 715 RAMA RD 6/30/80 EROSION/ 5.5 AC PIPE SAFETY 5320 RANDOLPH RD 6/18/79 EROSION 51.06 AC PIPE 5328 RANDOLPH RD 3/30/79 EROSION/ 51.06 AC PIPE BLOWOUT 5348 SUNBURY LN 3/29/84 FLOODING/ 15.6 AC PIPE EROSION ...... 4420 WHITBY LANE 2/14/83 FLOODING 45 AC CHAN IMP

* CHAN IMP is an abbreviation for "channel improvement". Results

Five complaint sites have been riprapped to hinder erosion. Three were lined entirely with riprap by the property owners themselves and two were shaped and lined with riprap at a discharge point by city forces. Three can be assumed to be performing at level of service Be The site at 1822 Cavendish, however, is more likely to be level of service C or D based on flooding parameters, since the owner had complained of flooding under his house, and was warned in the city's letter of recommendation that the widening of the stream would not solve his flooding problem entirely. The complaint at 1033 Goshen Place concerned erosion at a pipe discharge. The file was unavailable for review, but riprap has recently been installed at one point along the stream. Although the riprap should prevent further erosion in that area, the rest of the stream has erosion that would categorize it as level of service C.

All of the other complaint sites are classified as level of service D or E for various reasons. Three of these are flooding problems in .the sample tributary area and have already been discussed. Of the remaining 14 sites, all but one are classified as chiefly erosion or sedimentation problems. The one site considered to be chiefly a flooding problem is at 2321 Knickerbocker Drive. Two minor tributaries join in the backyard, only about 30-40 feet from the house. The owner claimed, in his letter of April 1981, that overbank flow had occurred at least once per year during the time he had lived in the house. The reach is classified as level of service D since the crawl space of the house at 2321 would probably flood in a lo-year storm. Erosion is also a problem along this reach. Further downstream, below Stockwood Drive, the erosion problem becomes worse. Three of the complaints concern areas along this reach, upstream and downstream of a 66" CMP. The City Engineering Department performed a study of the problem in 1979, the ~nickerbocker/Mc~airstudy, giving the surrounding property owners an estimate of the cost of extending the pipe. A complaint from six upstream property owners in 1984 prompted the city to make an estimate of the cost of a channel improvement alternative. The present cross-section is deep and relatively narrow. It is up to twelve feet deep in the downstream section and in some places the sides are almost vertical. The reach is classified as level of service E because of the danger posed to neighborhood children and the degree of widening that is occurring. Erosion is occurring along most of the reach with slumping evident in a few spots. Although bed erosion is assumed to have occurred based on the present cross-section of the stream, the elevation of the pipe culvert relative to the bed elevation could not be noted during the site visit since it had been raining and water was flowing in the channel at a depth of 1-2 feet. Several hundred feet downstream of the pipe there is a go0 bend in the channel and the property on the outside of the bend is flooded during heavy rains. The extent of the flooding is not noted in the file. Two other complaint sites are classified as level of service Eo One is at 2422 Cloister Drive where erosion has broken off the last three sections of 54" pipe. According to one of the Engineering Department's field service engineers, the dropof f there is 15 feet. No trespassing signs are posted at the site. This is classified as level of service E because of the safety hazard. The other level of service E area is at 5420 Addison Drive. The banks are not undercut; instead slumping appears to have occurred over time. A 10' x 10' area close to one property owner's driveway is beginning to slump. At the back of the property, the stream appears to be migrating. It is scoured up to the top of the bank on one side and a sandbar is building up on the opposite side. Of the remaining complaints, the ones on Randolph, Sunbury, Deveron, Rama and Grossner involve very small channels about 1-2 feet wide and 1 foot deep. There is a channel in the front yard at 5328 Randolph Road. Although the complaint was made in 1979, slumping is continuing to occur. The slumping classifies the problem as level of service Do The erosion on the reach running from 715 Rama Road and behind the house at 5941 Deveron Drive is similar, but a little worse. Additionally, there is one area where there is no defined cross-section for the channel and yard flooding is occurring. Both of these reaches are in grassed areas of the yard, with no buffer of hedges or trees. Part of the channel along Sunbury Lane is also in a grassed area. In this area of the channel, sediment has been deposited and the cross-section of the channel is not evident at all. It is therefore considered to be level of service Do Further downstream, erosion is occurring. One property owner is concerned that his rare sequoia trees are endangered. It is true that erosion has exposed the roots, but the trees are not in danger of falling because the stream is only about a foot deep. In general, the erosion would classify the reach as level of service C. The stream behind the house at 5900 Grossner Place has a similarly small cross-section. The concrete splash pad at the discharge of the pipe has been washed out. Bed erosion (headcutting) seems to be occurring and 100% of the banks are undercut. The reach is given a level of service D rating. The two remaining complaints involve streams with larger cross-sections. At 3811 Abingdon Road, there is severe scour at the discharge of a 36" CMP. The pipe discharges at a 90° turn in the stream and erosion is occurring on the outside of the bend and around and above the pipe itself. It looks like the property owners have attempted to hinder the erosion by placing rocks, bricks and debris in the scour-prone areas. Level of service D describes conditions in the discharge area. Fifty feet downstream there is no scour and erosion is at level of service C. The last complaint is at 1301 Ardberry Place. A small tributary discharges from a pipe along the side of the backyard and runs toward McMullen Creek behind the back of the lot. Downstream of the pipe, erosion is severe, with scoured banks and a cross-section that is 4-5' deep and 3' wide. There is also a small swale on the other side of the backyard. The rear of the lot is extremely flat and there is no well-defined cross-section for either flow once it reaches the back of the yard area. his area is described as level of service D mainly because of yard flooding .

The stream problems observed at the complaint sites should be divided into several separate categories. Small swales are distinctly different from a major channel. Erosion in a small swale is not as serious a problem as the same measure of erosion in a major channel. For this reason it is recommended that level of service D be the lowest classification assigned to a small swale experiencing erosion and that small swales and large channels be considered in separate categories. Another separate category should be made for reaches where erosion is occurring only at the discharge point of a pipe. It is not reasonable to rate an entire reach of stream at a low level of service only because of an isolated erosion problem.

FINDINGS

1. The proposed level of service measures represent a reasonable approach to assessing the performance quality of an urban stream. The measures are a useful tool to aid in the management of an urban stream network.

2. A major benefit of using the measures would be the opportunity to view the stream system as a whole instead of as a series of citizen complaints. Reaches of stream in need of improvements are readily identified by the use of the level of service measures. The downstream effects of various a1ternatives can be more easily assessed when the downstream conditions are already known. Better planning is an inevitable result of the management's awareness of the levels of service being provided in the various reaches of the stream network. 3. The prioritization of projects based on level of service measures would allow public funds to be spent more equitably. Under the present Storm Drainage Repair Policy, the allocation of public funds is based on a property owner's willingness to pay their share of the costs. In the process, some of the worst stream problems are never addressed while public funds are being spend on some of the more minor problems.

4. The use of level of service measures for urban streams should provide useful information about urban stream processes and the impact of various treatments. For example, understanding of erosion processes in urban streams could be greatly advanced by a periodic check of the condition of all the streams in an area. The cost effectiveness of various treatments could be assessed. This type of information would help to standardize treatments based on degree and type of problem and would eventually make it possible to base budget projections on the level of service measures. VI. PHYSICAL TREATMENTS FOR URBAN STREAMS

It is proposed that level of service measures be used in design of improvements. The designation of a stream reach as level of service D or E makes it a candidate for improvements. However, it does not dictate the treatment required to correct the problem. This section contains suggestions for various methods of improving a stream's level of service and policy changes that could be applying in designing these improvements.

CONVENTIONAL TREATMENT ALTERNATIVES

The standard treatments currently implemented in Charlotte are pipe installation and channel improvements that usual ly include widening the stream and stabilizing the banks. Variations of current channel treatments are proposed. Additionally, stormwater detention and velocity reduction structures are proposed as alternatives.

Pipe Installation

Pipe installation is typically the treatment that the property owner prefers. Of ten, however, it is cost prohibitive for the owner. One advantage to installing pipe is that, in general, little maintenance is required to maintain the capacity of the pipe. The disadvantages of extending the pipe system are the expense of installing pipe and the fact that flow velocity in that reach may be greatly increased, possibly increasing downstream erosion and flooding. If the stream network is to be managed as a whole, it is generally recommended that treatments which hasten the floodwaters downstream be avoided (Urban Land Inst., 1975; ~ossmiller,1983).

Channe 1 Improvements

Most channel improvement projects done in the City of Charlotte include bank stabilization. Lining with riprap is the method most commonly used in Charlotte. Similarly to pipe, most bank stabilization measures also increase flow velocities since the Manning n-value along the sides of the channel is usually reduced. However, there are cases where bank stabilization is necessary in order to control an erosion problem. A Corps of Engineers pub1 ication on "Streambank protection Measures" provides detailed descriptions of different types of bank stabilization methods (US. Army Corps of Engineers, Oct 1983). The booklet recommends that "before" and "after" pictures be taken for documentation and that improved sites be inspected periodically for maintenance needs. This should be particularly helpful for improving future designs and procedures. In conjunction with bank stabilization measures, a stream is usually widened and its banks laid back on a 2:l slope. In many cases, the stream is also straightened. This usually involves clearing the trees from at least one side of the banks. However, Nunnally's approach to bank stabilization, ref erred to as stream restoration, presents an alternative to traditional methods of stream channelization (Nunnal ly and Keller,lWg). He advocates leaving the stream's natural meanders in place and using riprap only in scour-prone areas such as the outside of bends. Stream restoration is less environmentally damaging than traditional approaches to stream channelization. The trees and meanders provide a better habitat for fish and make the stream more pleasing to the eye.

Another modification of traditional stream treatments is the use of riprap only along the toe of the banks, instead of up to the top of the bank. ~ractiveforce analysis shows that the area of the banks subject to the most stress is along the toe (Malcom, 1980). Wilson has observed that riprap of the lower one-third of the banks hinders slope instability in some cases. He recommends this procedure for streams with depths of 12-15 feet (Wilson, 1983). Several reaches of stream in Charlotte have been improved since 1977 using some of Nunnally's ideas and variations of traditional stream treatments. Two of these sites were inspected during the summer of 1985 and appeared to be in good condition. On Briar Creek below Central Avenue, most of the riprap was still present along the toe of the banks and erosion does not seem to be a problem in this reach. The only area not in good condition was Briar Creek just upstream of Eastway. County forces were working on that area during the summer of 1985, placing riprap up to the top of the banks for about 100 feet upstream of the culvert. Further upstream the banks are in good condition with riprap visible along the toe of the banks. It is possible that riprap was placed further up the banks and is not visible because of the heavy vegetation.

Stormwater Detention

A third classification of treatments is stormwater detention. It has already been mentioned that Charlotte requires that lo-year peak discharges from all new development be detained such that the pre-development level is not exceeded. When flooding is a problem for property owners adjacent to a stream, the only two plausible solutions are to hasten the floodwaters through that reach or to reduce the flows coming into the reach. In general, the hydraulic capacity of the reach in question is improved so that water moves through the reach more quickly and flooding of property is reduced. However, it may be possible in some instances to find ways of detaining water upstream of the f lood-prone area to the extent that the downstream flooding is effectively reduced. Strategic stormwater detention is not considered to be a generally applicable solution, but it is one of the tools that should be considered when investigating possible ways of improving the level of service on a reach that is prone to flooding. The design should be based on the flood reduction desired rather than a particular design storm.

Velocitv Reduction Structures

When erosion is a problem on a particular reach, consideration should be given to instal1 ing velocity reduction structures. There are a variety of types and sizes of structures but the common purpose of all of them is to reduce the velocity in the stream in order to lessen the tendency to erode the bed and banks. When a dam is placed across a stream, the water stored behind the dam forms a "linear lake". Whipple warns that the dams, or other types of drop structures, are only effective if designed in conjunction with an energy dissipating device that is effective during high flow periods (Whipple -et --al, 1981). These velocity reducing structures do, however, increase flood stages. Running a backwater program with and without the dams or drop structures will provide estimates of the degree to which flood stages are likely to increase.

Suggested Pol icy Changes Regarding Stream Treatments

In addition to consideration of alternative treatments, there are several policy changes proposed in conjunction with implementation of the level of service measures. The intent of the policy recommendations is to standardize treatments according to the type of problem.

Lower Limit -on City Projects The City should place a lower limit on the size of channel problems that it is willing to address. Several of the complaint sites studied had problems small enough for a the citizen to take care of himself. The standard recommendation that the citizen receives from the city in such cases is that a pipe be installed. ~ypically,the citizen does not elect to pay his share of the cost and proceed with implementation of the City's recommendation. In most of these cases, it would probably be more helpful to the citizen if the City would provide advice, and possibly materials, to help the property owner maintain and/or improve the condition of his channel. A general rule should be that if a vegetated swale can adequately handle 2-year flows, the city should avoid involvement with any channel improvements. Limited --Use of Pipe It is recommended that the installation of pipe be avoided unless it is the least expensive alternative. Exceptions to this general policy would be the improvement of short reaches of channel between two pipes. Especially when flooding problems are caused by water building up at the downstream inlet, the installation of pipe may be the most reasonable alternative. Variable Design Storm Currently, it is the policy of the City to design all storm drainage improvements to accommodate the lo-year peak flow. However, streams in their natural state accommodate only a 2-year flow. As long as the overbank flows do not cause property damage or frequent inconvenience to a property owner, the 2-year flow should be considered a standard design for reaches of stream. Where flooding of abutting property is unacceptable, the stream should be widened such that flooding problems are minimized, but not necessarily such that the lo-year flood is carried with no overbank flooding.

Undeveloped Reaches

If erosion problems exist in a reach of stream where the abutting property is undeveloped, the problem should be addressed as the property is being developed. The developer could be required to improve the level of service in the stream to an acceptable level or the City could require the developer's cooperation in a City-sponsored improvement project. These sites should be considered prime candidates for a linear lake. THE LINEAR LAKE ALTERNATIVE Objective

The primary objective of this section is to explore the application of a linear lake on an urban stream segment in Charlotte, North Carol ina. The appropriateness of this treatment will be discussed in terms of erosion control, aesthetics, economics and environmental impact. This section is a condensation of a more extensive work done by one of the team in support of the project (Avera, 1985). The full report is filed with the City of Charlotte and The Water Resources Research ~nstituteof the University of North Carolina.

Results The linear lake concept is a feasible treatment for appropriate urban sites as supported by the case study:

1. Since the toe of the streambank would be protected by riprap, slumping and undercutting would decrease. 2. Sedimentation would not pose a serious problem because of sufficient bottom scour in large storms.

3. The quality of the stream is usually high enough for secondary recreation and fishing.

4. Algae blooms might occur periodically in times of low flow, but would be flushed from the system in rain.

5. The weir system can be fitted into the stream without degrading the aquatic and vegetative ecosystems.

6. The gabion weir design and grassed banks fit in naturally with the surrounding area.

7. The 100-year flood level is not significantly raised with the weirs.

8. The linear lake system is approximately one third the cost of conventional r iprap treatment.

9. The linear lake system would improve the aesthetics, utility and preservation of the stream system.

10. The linear lake system is especially appropriate at Arborway Road, the location of the case study, since the greenway is already establ ished, resulting in minimum disagreement about property rights. Recommend ations

prior to construction,

1. the stream segment should be monitored for the severity and frequency of algal blooms in naturally ponded water,

2. the potential problems of mosquitos and water weeds should be further explored, and

3. a study of the relationship between water depth and oxygen depletion should be performed on the system in order to assess the possibility of installing weirs higher than 2.5 feet.

Introduction

Urban communities throughout the country are suffering from a lack of open space for recreation and the passive enjoyment of nature. In response to this shortage of open space, cities are increasingly turning toward the use of flood plains as recreation sites. Commonly called "greenways" these areas are unsuitable for development because of property damage, as a result of flooding and erosion. A greenway system usually includes various facilities that promote recreational activities suited to a long corridor space. These activities include hiking, picnicking and horseback riding. The City of Charlotte is itself very interested in establishing a greenway system (U.S. Army Corps of Engineers, August 1983). However treatment of an area generally stops at the streambank, In this section, a treatment coined "linear lake" is evaluated. The treatment consists of a series of low weirs across the stream. These weirs pond the water upstream and make a system of slow moving elongated lakes. The benefits are many, for the system of pools and cascades is aesthetically pleasing, promotes water-based recreation and encourages people to interact with instead of avoid the urban stream.

Case Study

The idea of the linear lake was applied in concept to a reach of stream in Charlotte. The stream segment of interest is located within the McMul len Creek watershed and begins approximately 270 feet above Arborway Road. The watershed area at the first weir is 5.36 square miles, and has a land use composition of mostly residential areas. Even though individual property owners own the land adjacent to the stream, they have come together as a group and established a greenway along the east bank of the stream. This strip is approximately 100 feet wide, grassed, maintained regularly and has a paved foot trail. Mecklenburg County is responsible for maintaining the stream segment since the watershed area is greater than one square mile. The streambanks are very eroded and undercut and many trees have fallen into the stream or are in danger of doing so.

Engineering Design Considerations

In the process of designing a system such as a linear lake, one must evaluate several design considerations. These include erosion, sedimentation, water quality, effects on the existing ecosystem, and streambank and stream bottom treatment.

Erosion Control To avoid slumping and streambank failure, the most important part of the bank to protect is the toe. As far back as 1937, engineers recognized the benefit of dams, which raised the water level, in stabilizing the toe of the bank (Schoklitsch, 1937). It has been shown that the tractive forces exert a greater cumulative influence just above the toe of the bank (Malcom, 1980). Thus installing riprap on the bottom third of the bank, as in this linear lake design, would protect the region which receives the maximum tractive force from erosion. Another source of streambank failure is the rapid drawdown of the water surface. When the water surface elevation is raised, a pressure balance exists between the water in the channel and that in the saturated bank. d his balance keeps the bank in place. If the water is suddenly drawn down, the pressure of the stream is removed and a pressure imbalance develops. This is especially true of soils, such as in Charlotte, that are tightly bound and drain slowly. If the bank does not have sufficient strength, the bank could slough off (U.S. Army Corps of Engineers, October 1983). A linear lake would prevent a rapid drawdown of water in the area of the toe, and thus decrease bank failure. Streambank Treatment In order. to further protect the bank from eroding and to stabilize the already undercut banks, sloping the banks back above the normal water level is suggested. The slope could be on the order of 2:l and planted with a locally adapted grass such as fescue or bermuda (U.S. Army Corps of Engineers, August, 1983). Care should be taken to protect the grass mulch and seeds until the grass has taken root. Jute mesh, staking or an asphalt spray are possible protective devices. The bank shaping could be performed by a few strong backs and mattocks, instead of heavy machinery (Barick, 1985). This technique would decrease the number of trees that would have to be cleared in order to get the machinery to the stream. The removed dirt should not be thrown into the stream or up on the bank. It should be hauled off and disposed of properly. Trees in danger of falling into the stream should be removed and stumps left intact if at all possible. Stumps that would significantly protrude into the stream should be removed, for they would be a source of eddy currents which would eat away at the streambank (U.S. Army Corps of ~ngineers, October 1983). Overall bank treatment should be kept to a minimum in order to maintain the "naturalness" and wild1if e habitats of the area (Harp, 1981). In addition natural meanders and the stream bottom should be preserved (Wildlife -In North Carolina, 1976). Sedimentation Sedimentation is usually a problem in reservoir-type designs. When sediment laden water reaches a reservoir and slows down, it deposits its sediment and a delta is formed. For example the stilling basins located in a linear lake system in Arlington, Virginia, filled up within 6 months (Matthews, 1985). In contrast the sediments at Lassiter Mill Dam on Crabtree Creek in Raleigh, NC, jump the dam in times of high flow. In analyzing the segment of interest at Arborway Road, it was concluded that sedimentation will pose no serious problem for the following reasons. The greatest amount of erosion takes place in the 2-year storm (Dunne and Leopold, 1978). Settling will be hindered because the channel velocity in the 2-year storm is at least three times greater than the settling velocity of quartz, a common material found in sand. In addition significant scour would occur in the 2-year and greater storms, for the channel velocity is greater than the scour velocity. Other ideas that support the above conclusions follow:

1. A slower flow would deposit sediments, but these flows are usually not carrying a large sediment load. 2. The fact that the linear lake design would protect the toe means less sediment in the first place. 3. Natural streams in urban areas are often clogged with bank growth, brush and fallen trees. These barriers decrease the channel velocity and promote sedimentation. In contrast a stream segment which is maintained in a clog-free manner will not decrease velocities and thus not promote sedimentation because of clogged conditions. Clog-f ree maintenance involves clearing banks of trees in danger of falling and periodically cleaning the trash screen located upstream of the weirs. In summary, the combined effects of the above suggest that large amounts of sediment will not build up behind the weirs.

Water Quality The water quality in a stream is a factor of the groundwater system, the overland water flow, the precipitation directly into the stream and the effluent discharged into the stream by industries and wastewater treatment plants. McMullen Creek had no permitted dischargers as of July 1, 1982 (U.S. Geological Survey, 1984). Thus most pollution in the stream would be from the groundwater system, precipitation or overland water flow. The United States Geological Survey conducted a reconnaissance study of the water quality in the streams in Charlotte from 1978- 1981 (US. Geological Survey, 1984). Samples were taken at both low and high flow periods. Theoretically, low flow samples would represent ground water qua1ity and any point-source effluents, while the high flow would represent overland runoff and point sources. Caution should be taken in interpreting high flow values, for no attempt was made to distinguish between samples in the rising or falling limb of the hydrograph. Concentrations of pollutants can vary widely during these periods. When interpreting the USGS water quality data, one should keep in mind that McMullen Creek is classified as Class C (NC Environmental Management Comm., 1974). This classification imp1 ies fishing and secondary recreational uses. The substances which violated standards or criteria follow.

1. Phosphorus- All low flows and one of the high flow measurements had phosphorus levels higher than EPA suggested levels. USGS contributed these levels during high flow to runoff from lawns and construction sites.

2. Copper- All low flows may have had, and all high flows deflnltely had copper levels higher than EPA criterion for freshwater aquatic species. No possible sources were cited. However since the definite violations were during high flows, adverse impacts might be limited. --Iron and Mercury- Both substances violated NeCo surface water standards and EPA criterion for freshwater aquatic species during high flows. Care should be taken in interpreting this result because of the method of measurement. The technique measures total iron associated with suspended sediment. Iron and mercury tightly bound to sediment may be released during treatment. Thus all of the iron and mercury measured is not available to the aquatic species. In addition high levels of mercury cou ld be attributed to background sources. 4. -Zinc- Levels of zinc during high flows may have exceeded EPA levels for freshwater aquatic species. No possible sources of zinc were cited. ~ikecopper adverse impacts may be 1 imited because of possible violations only during high flow events.

5. -BOD- High flow values of BOD exceed levels suggested by Nemerow (1974). USGS accounts for these levels by overland runoff. However dissolved oxygen levels were within the acceptable range for aquatic species.

From the USGS report it appears that McMullen Creek's water qua1ity is not seriously degraded. Phosphorus is the substance with the largest potential for degradation.

It was reasoned that phosphorus would not pose a serious water quality problem in the linear lake system. The stream would be shaded, and thus the algae could not photosynthesize to maximum levels; and the detention time during one half average yield flow was about eight hours. However on a visit to the site in September, a reddish-brown scum was noticed on naturally ponded water in the shaded areas, and definite green-colored algae mats in the sunny stream segments. Considering that the algae occurred in September and it was reddish-brown in color, it was likely a blue-green algae bloom. This occurrence of a bloom was a definite adverse effect on the aesthetics of the system. A redeeming factor is that the algae was flushed from the system when it rained. ~husthis negative aspect may be a periodic instead of constant problem. Other biological water quality factors to consider are water weeds and mosquitos. Water weeds generally occur in depths less than four feet. They could prove to be a nuisance. In addition mosquitos could be bothersome in the summer months. More study should be done in these areas. Floating debris and trash can also deteriorate the water quality as well as the aesthetics of the lakes. A trash screen, located upstream of the uppermost weir would be effective. Periodic cleaning of debris from the screen and general maintenance of the weirs and streambank would be required.

It is interesting to note the results of a survey conducted in the early 70's on people's feelings toward the River Walk in San Antonio, Texas. The survey resulted in a very strong positive response even though half of those interviewed said the water was dirty (Gunn, 1979). This indicates that when water contact sports are not allowed, aesthetics are more important than water quality (Gunn 1974). Effect --on the Ecosystem Fish: In 1978 Cloutman and Olmstead issued a report that identified fish species at different sampling points in the Charlotte stream network. At the Sedley Road station, very close to Arborway Road, they found five species of fish. They included two shiners (Notro~is------Altipinnis and ------Notropis ----Procne), a mosq uitof ish (Gambusia Af f inis) and two sun£ish (~epomisGulosus and Lepomis -Macrochirus) (U.S. Army Corps of Engineers, August 1983 ) Frank Barick (1985), former Chief of the North Carolina Wildlife Resources Commission, was contacted about the effects of a linear lake system on the present fish population. He shared the following concerns and suggestions:

Do not pond water much greater than two feet, for one runs the risk of stagnation and anaerobic conditions, especially in low flow periods.

If one can create a splash over the weir, reaeration will occur. The existing fish species should grow to a larger size in the ponded areas.

Direct the flow over the weir to the center of the channel in order to decrease scour around the weir during periods of low flow.

Protect the area around the weir with riprap or trees to further reduce scour during high flow. Leave the bottom of the stream intact in order to maintain a high quality substrate for the benthic organisms. 7. The ponded areas will provide needed water for the fish in low flow periods.

Trees -and Vegetation: ond ding water in the stream to a depth of 2.5 feet will raise the water table depth in the area. In the long run stresses will be felt by the surrounding trees and vegetation (Gregory, 1985). Over a long period of time trees that can not adapt to the raised water table will die and may be replaced by others more tolerant of the higher water table. Once a tree dies provisions should be made to replant the spot with a water tolerant tree. This action will ensure the preservation of habitat and shade which are needed by the fish and wildlife of the area.

Weir ~esign

Material Choice Concrete and large and small gabions are the three materials considered for the weirs. Regardless of the material chosen, the design should be as cost-effective and easily maintained as possible. In addition the system should be simple, to avoid the problems inherent in complex systems, such as the weir with rotating gates at Freedom Park in Charlotte. Concrete and gabions have both advantages and disadvantages in their appl ication. Concrete is general ly more expensive per cubic yard, for the stream must be diverted during construction and ski1led labor is required for instal lation. However the concrete weir yields more intricate designs than the gabion weir. For example small notches could be included in order to concentrate the flow at low periods. This concentration of flow would increase the turbulence and thus the reaeration of the water in times of potential stagnation. In addition to flexibility in design, the concrete weir would be more durable, but more prone to damage by uplift.

Gabion weirs are less expensive to build, for the stream does not have to be diverted and unskilled labor may by used. Even though the gabion weir is not as flexible in design as the concrete weir, it is more flexible in terms of streambank settlement. The gabion weir is less prone to uplift than the concrete weir and can be provided with a six-inch concrete cap to protect against damage from debris (~accaferriGabions, 1970). Neoprene fabric can be put on the upstream side of the weir to hasten the process of sediment accumulation and waterproofing. In terms of appearance the gabion weir is less imposing and artificial in appearance, for it is constructed from stone, which blend in better with the natural surroundings. Design Specifications Gabions were selected for the case study because of their lower cost than concrete, ease of installment and appearance. In response to Frank Barick's suggestions for a design which is compatible with the natural ecosystem, the weir height was set at 2.5 feet, and spaced such that the water at normal depth does not fall below 1.5 feet. The foundation should be placed three feet below the channel bottom to prevent undermining. In addition an upstream apron of 3-6 feet is provided for additional uplift protection and a downstream apron of 9-12 feet to protect against scour (Maccaferri Gabions, 1970). Likewise the weir should be keyed three feet into the bank to protect against scour around the sides (Wilson, 1983) and a protective mat of riprap, spanning 15 feet, should be placed on each streambank of the weir. Each base gabion should be anchored into the channel bottom by a one-inch diameter steel rod. Since the gabion is porous, the upstream side of the weir should be covered with a material such as neoprene fabric. The fabric is flexible and will not crack, thus catching sediment and making the weir more or less impervious.

Aesthetics

The linear lake system must be analyzed not only with engineering tools, but with aesthetic criteria as well. The design can elevate the appearance of the stream from an eroded and stressed stream to a stable lake- like system. A part of the urban stream system which is ugly and eroded may be transformed into a segment which is pleasing to the eye and therapeutic to the stream. The book, Water -and Landscape, (Litton, et. al., 1979), used the three terms, unity, variety and vividness, to describe the aspects of the aesthetic experience of water streams and bodies. With respect to the linear lake design at Arborway Road, the three terms are supported as follows: UNITY

Water Element

1. littleseasonalwater level fluctuation,

2. ability to see a considerable distance along reaches of the stream, 3. homogeneous water color,

4. mirror-like quality of the water surface (if no algae is present) Shore Element 1. balance between shore and water, 2. uniform treatment of the streambank, 3. features are repetitious,

4. riparian vegetation is natural and continual.

VARIETY Water Element

1. cascades alternate with pools of water,

2. natural drift material is present, no trash, 3. meanders left intact, 4. nonuniform width of the water

Shore Element

1. bank treatment alternates between grass and r iprap

VIVIDNESS Water Element 1. turbulence at the weirs,

2. uncommon expression of water in the region, 3. species of fish swimming about

Shore Element 1. shore edge stands out in contrast to water. The linear lake design succeeds in providing unity and a bit of variety. Vividness is not strongly expressed because the natural system is itself not vivid. The design is constructed from natural materials and makes no drastic changes in the adjacent land. Thus the overall goal is to fit into the environment in form and function, reflective instead of competitive .

HEC-2 Water Surface profile Model ~pplication

The HEC-2 Water Surface profile Model was used to determine how much the system of weirs in the linear lake design raised flood levels. The 20, 10- and 100-year floods were run through the stream segment, with and without weirs. The 2-year flow was calculated by the Putnam method, the 10-year from the U.S. Army Corps of Engineers data and the 100-year from the Official USGS Flood Map. As one can see from the example of Figure VI-1 the flood level did not rise to a large degree. As expected the 2- year level was raised the most, while the 100-year level was raised the least. This occurs because the 100-year level encompasses a larger area over which to distribute the raised flood level, resulting in a smaller increase in stage.

The 100-year flood level determines the official f loodway and floodway fringe areas. If the linear lake system had raised this elevation appreciably, the extra land and houses inundated would have had to be assessed for flood damages. However the 100-year flood level with the linear lake weirs inundated a negligible amount of new land. There were no new houses flooded, just a bit more of the ones already in the flood area. Thus this potential cost will be neglected in the benefit/cost analysis. McMULLEN CREEK AT ARBORWAY ROAD

SECTION NUMBER W/O WEIRS + W/WEIRS - BOTTOM

~igureVI-1 100-Year Flood Profile of McMullen Creek at Arborway Road, with and without weirs. Economics

Theoretically a design should generate social benefits greater than social costs. However social benefits are often difficult to evaluate, for they usually include intangible impacts and values which are important to present and future generations (Elfers and Hufschmidt, 1975). These intangible benefits might include recreational, aesthetic as well as social impacts (Dean and Shih, 1975). Attempts have been made to quantify these intangible benefits for a water park, including a travel cost model and market value of surrounding property model.

Debo (1977) measured the attitudes of residents which lived close to four residential lakes in Atlanta, Georgia. In terms of benefits he found that the residents felt that the lake had a positive impact on the value of their home, was a positive factor in buying their house, provided recreational benefits and promoted a neighborhood atmosphere.

On the negative side residents were concerned about the continual maintenance required, water quality problems of sedimentation and trash, and safety enforcement. In addition they would agree to pay for maintenance only of they were assured access to the lake and its facilities.

The observations from Debo's study can easily be applied to the linear lake system. Other benefits accruing specif ical ly to the linear lake system include improved aesthetics, increased utility of an urban stream segment and the preservation of open space for future generations. Overall the linear lake design had an initial capital cost of $58,000 (1985$). This amount is in direct contrast to the conventional treatment used by Mecklenburg County (Nunnally and ~eller,l979), which has jurisdiction over this stream segment. The total cost for laying the banks back and riprapping the banks and channel bottom is $188,400 (1985$). The comparison between these figures is astounding. Even when one adds a sum for routine maintenance of the grassed banks and trash screen, the linear lake design is far less expensive than the conventional treatment. Examples -of Other Linear Lakes

-San Antonio, Texas In the early 1940's a horseshoe bend section of the San ~ntonioRiver, which goes through the city, was dammed up (Gunn, 1979). This action resulted in the main river diverting around this horseshoe bend section. The water level in the bend was kept stable by an elaborate hydraulic system. Sidewalks, trees, plants, shops, restaurants and hotels soon lined the canal. The result, The River Walk, became a very positive combination of a park in the central business district (Gunn, 1974, 1979). To this day the River Walk remains a popular attraction for the residents of and visitors to San Antonio. In the early 1979% a survey was conducted to assess people's feelings about the River Walk (Gunn, 1979). The majority of people had very positive feelings about the River Walk and considered it an asset to the City.

Many of the streams in England are controlled by weirs and take on the appearance of a linear lake. They are very picturesque and provide a variety of recreational opportunities.

Hopkinsville, Kentucky The City of Hopkinsville, Kentucky, built a mid-city river park similar to the San Antonio River Walk (Baird, 1979). The park is one-half mile long and located within the central business district. The project was a recreation project from the start and sought to revitalize the failing downtown section of the city. Weirs are located in the river and pool water three feet deep, a depth thought to be safe for children fishing along the banks. The weirs are notched to accentuate the cascade and to decrease scour (Baird, 1979).

Arlington, Virginia The design of a linear lake system was recently awarded the 1985 Outstanding Civil Engineering Achievement Award from the ~ationalCapital Section of the American Society of Civil Engineers (Matthews, 1985). During the past twelve years, Four Mile Run, a stream draining two thirds of Arlington (25 sq mi), had eroded down six to eight feet below the original stream bottom. In the process a sewer line had been exposed, and left vulnerable to damage from floating debris. The goal of the project was to protect the sewer line and save a park-like setting for the area. Mr. Matthews designed a system of eight check dams with stilling basins that decreased the water velocity and encouraged sedimentation around the sewer line. The stream bed filled in over the sewer line in a six-month period (Matthews, 1985). Mc~lpineCreek -in Charlotte, North Carolina Located in the upper portion of the McAlpine Creek watershed, the City of Charlotte has established a greenway system. The greenway includes playing fields, a lake and walking path. A series of weirs are installed to convey the walking path across the stream. The weirs are broadcrested, and have three corrugated metal pipes to carry the water at low flow. During a visit to the site children were fishing and swimming in the pools with great enjoyment. Lee McLaren (1985), a landscape architect in the City, described two problems the County had encountered with the design. The first was the problem of scour which was undermining the downstream side of the weir. The County installed gabions in the area to protect the weir. In addition a concern about safety had arisen. Even though signs were posted to stay off the weirs in high flow, children will be children. During some such high flow, a young girl fell into the upstream pool, was sucked through a pipe and bobbed up at some distance downstream. Fortunately she was not seriously hurt. This accident prompted the County to install reinforcing bars across the upstream side of the pipes to avoid the same sort of accident. Freedom --Park in Charlotte, North Carolina Located on Little Sugar Creek the weir is broadcrested and equipped with radial gates. During low flow the gates are to be closed, so that water will back up in the stream and create a linear lake. Theoretically during high flow periods the gates would be lifted to allow water and sediment to be discharged over and under the weir (Wilson, 1983). b his rotation of the gates would decrease flooding and sedimentation. However the gates broke and still remain open, resulting in a dry stream bed. Remedial work is needed to bring about the intended effect. ~assiter---Mill Dam in Raleigh, North Carolina The dam at the old Lassiter Mill creates a linear lake on Crabtree Creek. In contrast to the other examples, the banks are wooded rather than grassed. The section is within the Raleigh Greenway system and appears to generate a significant amount of use. The greenway has biking and hiking trails and the lake is suitable for fishing and boating activities. An interesting and encouraging observation is that the sediments jump the dam during high flows. This is shown by the accumulation of sediment along the bank below the dam. From the above examples one can learn of both good and bad design features for a linear lake. The advantage of a water park system is shown. The linear lake concept can be applied to produce an aesthetically pleasing as we11 as economical design. In addition the system can be placed successfully in a central business district, grassed park or wooded area. Areas of potential problems to be noted from these examples are scour below and around the weir, safety, liability and the pitfalls of a complex design. VII. FINANCIAL MANAGEMENT OF STORMWATER SYSTEMS

It is now widely accepted that urbanization increases the quantity of stormwater runoff. In turn this creates erosion and flooding problems for residents downstream of urban areas. In response to growing numbers of drainage problems, cities have begun to design and implement more comprehensive stormwater management programs. A necessary component of a management program is its financial support. This section describes traditional and innovative financial management practices for stormwater systems.

SUMMARY OF ALTERNATIVES

Traditional financing schemes can be divided into those for existing and new developments (Whipple et al, 1983). Sources for existing developments include generaltax funds, special assessments, bonds, federal and state funds and private funds. rain age facilities for new developments can be funded by developer fees, developer-provided facilities, dedications and flood plain regulation. Each option has advantages and disadvantages.

Existing Developments General -Tax Revenues There is considerable conventional dependence upon general tax revenues for provision and maintenance of stormwater systems. Some argue that competition for scarce resources makes the general fund unreliable for capital expenditures for major system improvements. General funds might best be used for mapping, planning and other activities for which benefits are widely spread among public works interests. Special Assessments Special assessments are charges levied on selected properties for improvements which benefit the property owner. State government, through a local or special purpose district, has authority to implement such assessments. Special assessments can be initiated three ways (~ebo,1980):

1. With consent of the property owners, who must sign a petition.

2. Without consent of property owners, but the initiation can be stopped by an opposing petition or protest. 3. Without consent of property owners, and the initiation can not be stopped by petition. The assessment formula must ensure that the amount of each assessment is in proportion to benefits received. The total assessment must not exceed the total cost or benefit of the project. The difficulties of fulfilling these formulation requirements and passing through the initiation process often make the special assessment option inadvisable. public opposition can be fierce. Toledo, ~hio,encountered substantial opposition because benefits were not easily defined and were not confined to the drainage area (Colony, 1982).

In addressing the question of defining benefits, the Colorado Supreme Court ruled that an assessment could be charged to any property where improvements provide :

1. An increase in market value of property.

2. Capacities for discharging increased surface runoff from higher to lower land.

3. Property adapted to a superior or more profitable use. 4. Reduced health or sanitation hazards.

5. A reduction in maintenance costs of property.

6. An increase in convenience or a decrease in inconvenience to property owners, including access to and travel over streets. 7. Recreational improvements.

The Court ruled that special assessments must be founded upon "special benefits." "Special benefits," which accrue directly and solely to the property owner in question, differ from "general benefits," which accrue to an entire community. Taken as a body, these findings provide for assessment of people living on ridges and hills as well as those living in the flood plain (Shoemaker, 1974).

In theory, the special assessment matches those who pay to those who benefit. However, its cumbersome legal hurdles in the definition and valuation of benefits reduce its potential effectiveness. Bonds

Bonds are useful in that they provide a vehicle for borrowing against future revenues to obtain capital for public works projects. They are not a source of additional revenue, since the liability must be met with future revenue. There are two types of bonds, general obligation bonds and revenue bonds. General obligation bonds are guaranteed by the credit of the issuing entity. They may be repaid with revenue from a fee, backed by general tax revenues if the fee is insufficient. The interest rpte for general obligation bonds is lower and more erratic than revenue bonds, and a two-thirds majority is required for approval. Obtaining approval for general obligation bonds is made less likely by the perception that clear benefits will not accrue to the voter. For example, DeKalb County, Georgia, failed to pass two bond issues for drainage improvements. Defeat was attributed to opposition to increased taxes, belief that the bond issue had not been adequately planned, belief that property owners who built in flood plains deserved to be flooded and apathy due to lack of personal involvement (Brown, 1980). In general, to increase the probability of passage of a general obligation bond issue, it has been suggested that items other than drainage be included in the plan. The added benefits, such as parkland and nature trails, make the investment more attractive to residents nor directly affected by poor drainage performance (Debo and Williams, 1979).

In contrast, the revenue bond is not guaranteed by the issuing entity, and a popular vote is not always needed for approval. It is repaid entirely from project-generated revenues. Stormwater facilities rarely generate revenues, so revenue bonds have not been suitable for financing stormwater system improvements.

Other Sources State and federal funds have been a prime source of support for drainage improvements. Currently, with budget-reduction programs being the norm, this support is diminishing. private sources are not commonly used because drainage improvements are usual ly too expensive for a given property owner, and because flooding typically affects more than one resident. The difficulty of organizing groups of owners to share costs discourages common projects. The storm drainage repair policy of the City of Charlotte provides financial and planning assistance to citizens who request it and elect to participate. -New Developments Developers have historically provided such drainage facilities as curb and gutter, inlets and storm sewers. Recently, detention has been added to the requirements in many municipalities. Most communities prohibit or carefully regulate construction in flood plains. With respect to financing new facilities serving more than one development, developer fees have been used to spread over basin-wide developments the cost of providing common drainage facilities. Such a fee was an alternative recommended in this report to the City of Charlotte as a means to provide greater flexibility in applying the stormwater detention requirement.

Debo (1980) summarized four arrangements to allocate costs for drainage system improvements:

1. Developer of the proposed project pays the total cost of facilities.

2. Developer of the proposed project shares the cost with one or more additional developers whose projects benefit from the drainage facilities.

3. Developer shares the cost with the City.

4. Developer shares the cost with the City and one or more benefited developers.

The cost is allocated in proportion to impervious area, not including existing developments with adequate drainage.

Stormwater Utility Innovative techniques for f inanc ing s tormwa ter management include the concepts of user charges and the stormwater utility. Several western cities have implemented these techniques with positive results. They seem to be gaining popularity as federal and state support of drainage facilities diminishes.

A user-charge system relates the fee to the level of service provided. Area of impervious cover is one measure used as a basis for the fee. A distinct advantage is that collected fees can be used to finance revenue bonds. Some issues that must be satisfactorily dealt with include (Whipple, --et al, 1983): 1. The immense quantity of information necessary, such as property size, location, impervious area, ownership, etc . 2. A billing system for each property.

3. The resistance of upland residents who feel that they need not pay for drainage services since they experience no flooding . Denver, Colorado, and portland, Oregon, administer user charges based on the amount of impervious area. These charges provide funds for capital improvements, operation and maintenance. Denver tried in 1974 to initiate an earlier version of its charge system, but it was canceled in response to a citizen class-action suit. Lack of public knowledge of the need for the charges was cited as the main cause of the suit (APWA, 1981) . Billings, Montana, enacted a similar set of charges in 1978. These charges, in conj unction with revenue bonds, provide money for construction, operation, maintenance, depreciation and replacement of storm sewers. Rather than being based on impervious area, these charges are allocated with respect to total lot area and zoning classification (APWA, 1981).

The stormwater utility concept extends the user-charge idea. In general, a municipality has duties to protect the health, safety and general weltare of its inhabitants. ~istorically, these duties have included sewage disposal, provision of potable water and collection of solid waste. More recently, in some places, the duties have been extended to include prevention of erosion, landslides, stream pollution and flooding (Appel, 1980). Just as a municipality has the right to collect sewage so that it will not flow from one property to another, the municipality may do the same with stormwater runoff. A service rendered justifies an equitable charge. In this respect a stormwater utility provides for equitable distribution of costs to those who use the system. Some disadvantages cited are high front-end costs of engineering, legal and financial studies; necessary billing and collection systems; new administrative staff and data base maintenance (Gallagher and Laredo, 1983).

Even though the cost of overcoming these obstacles is significant, several cities have established stormwater utilities with apparent success. Boulder, Colorado, initiated one in 1974 to serve capital needs. They consider it to be equitable and excel lent. Annual revenues are pred ictable and assured, which leads to more re1 iable master planning and implementation. Sent with monthly water bills, fees are based on the lot area, computed runoff coefficient, site location and nearness to the flood plain. If detention is provided on a commercial or industrial site, the rate is reduced. Rates are increased 1.5 times for f lood-plain properties. Operation and maintenance is paid from the city share of the state sales tax and from the general fund (APWA, 1981). Bellevue, Washington, established a stormwater utility in 1977. Fees are collected bimonthly with the water bill. Revenue is provided for capital improvements, operation and maintenance. Revenue bonds, used to support major improvements, are repaid with service revenues and land development fees. Rates are based on lot area and a measure of intensity of development which in turn is based on imperviousness (APWA, 1981) . Aurora, Colorado, has two charges for drainage services. The first is a one-time charge on undeveloped property for which a building permit has been requested. The second is a user fee for storm drainage and flood control which is placed on the water bill and is proportional to meter size (APWA, 1981).

Other cities that have stormwater utilities are Tacoma, Washington; Corval lis, Oregon (APWA, 1981); and Rosevil le, Minnesota (Honche11,1986). Local governments that are in early stages of formulating or implementing utilities are Cincinnati, Ohio; Clark County, Nevada; Tampa, Florida; and Tulsa, Oklahoma (Cyre, 1985). While the above cities report excellent results with the stormwater-util ity, Lakewood, Colorado, is a negative example. The City attempted to form a utility in 1983. Anticipating resistance, the City embarked on a publ ic education program which included mailed notices to about 100 civic organizations, presentations and two information pamphlets sent to all registered voters. Public response was lukewarm, and the measure failed in the election. Property owners felt that the fees were excessive and that administrative costs of the utility were high relative to the simple expedient of raising the property tax or sales tax (Plastino, 1984). Clearly, the experiences of Denver and Lakewood suggest that publ ic education and involvement is essential to successful initiation of a stormwater utility. Poertner (1980) cites the need for for full participation of local elected officials. According to Cyre (1983), successful financing options associated with stormwater utilities must possess the £01 lowing attributes:

1. Perceived equity and public acceptance.

2. Flexibility in response to program growth and the physical complexities of the drainage basin. Capacity to meet present and future revenue needs. Accepted levels of cost of implementation, Compatibility with existing policies, practices and systems, Reasonable upkeep requirements for fee systems.

A balanced financing strategy.

~ppropriatetiming for implementation, Incorporation of unique geographical and jurisdictional considerations,

It is recommended that the City of Charlotte consider the concept of the stormwater utility for orderly development of the stormwater management system,

In Sections I1 and V, the influence of stream ownership on the formation of a stormwater utility is discussed. private ownership of streams that transport commonly generated stormwater is a problem with which to deal. [Blank page here] VIII. REFERENCES

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Avera, ~arianE., 'A Linear Lake Application in Charlotte, North Carolina," ~ndependentStudy Project for Degree of Master of Civil Engineering, Civil Engineering Department, North Carolina State University, December, 1985.

~aird,~obert. We, "soil Conservation and Urban Design- A New Downtown Concept," Water --And The Landscape, edited- by Grady Clay, McGraw-Hill Book Company, 1979. Barick, Frank, Personal communication, Former Chief- Wildlife Resources Commission, Raleigh, N.C., September 1985.

Becker, B.C., Clar, M. L., and Kautzman, Re Be, "Approaches to Stormwater Management", Hittman ~ssociates, Inc., Columbia, MD, November, 1973.

Brown, L. A., "~oliticalAspects of Urban Stormwater Management ," --J, of Water Resources Planning -and Management, ASCE, v. 106, n. WR1, March, 1980.

Bullard, Charles M., "Analysis of Water Surface Profiles on Small Urban Collector Streams in Charlotte, NC," Independent Study project for Degree of Master of Civil Engineering, Civil Engineering Department, North Carolina State University, March 1986. City of Charlotte, "City of Charlotte Soil Erosion and Sed imentation Control Ordinance", Ordinance No. 529, North Carolina, Jan., 1975, City of Charlotte, ---Charlotte Stormwater Impoundment Design --Manual, North Carolina, Department of Pub1 ic Works, Engrneering ~ivision,Charlotte, NC, September 1978. colony, D. C., nothe her Look at Financing Urban Drainage projects," urban Stormwater Quality Management and Planning, Be C. Yen, ed., 1982. - Cyre, He A., "~ewOptions for Stormwater Financing," APWA Reporter, April 1983. ---

Cyre, Ho Ao, Personal communication, President, Water Resource Associates, Inc., Kirkland, WA, 1985.

Dean, Joe Ho and C.S. Shih, "~ecisionAnalysis for the ~iverWalk Expansion in San Antonio, Texas," Water Resources -~ullet in, Val. 11, NO. 2, April 1975.

~ebo,To No, "Man-Made Lakes: ~ttitudeSurveys of Their Value in Residential Areas," Water Resources ---BU-1 letin, Vol. 13, No. 4, August, 1977.

Debo, To No, "Elements of Management Programs for Detention Storage, Finance, Maintenance and Ordinances," presented for Water Management Science, Inc., Washington, DC, February,

1980 0

Debo, To No, and Jo T. Williams, "voter Reaction to Multiple-use raina age projects," Jo of Water Resource Planning -and Management, ASCE, v. 5,n. WR2, September, 1979. Denver Regional Council of Governments, "Urban Storm Drainage Criteria Manual," Vol 1, Denver, CO, 1969, As amended through 1985. Dunne, Thomas and Luna B. Leopold, Water -in -Environmental - planning, W.H. Freeman and Company, San Francisco, 1978.

~ddins,Harold A. and Jackson, No Macon,Jr., A Technique -for Estimating Flood Heights -on __Small streams-in the City of ---Charlotte -and --Mecklenburg County, North ---~af;i;;lXa, Unrta States Geological Survey, Water Resources Investigations 80- 106, USGPO, Washington, DC, December 1980.

~ddins,~arold Ao, Personal Communication, U. S. Geological Survey, Charlotte, NC, 1985.

Elfers, Karl and Maynard Mo Hufschmidt, Open Space -and Urban ----Water ----Manapement, ---- Phase -I: ---Goals --And Criteria, Water Resources Research ---Institute of the University------of North Carolina, Report 104, Raleigh, NoCo, January 1975.

~rederick,R.H.! V.A. Myers, EoP. Anciello, "Five to 60 Minute ~recipitatlonFrequency for the Eastern and Central United States," NOAA Technical Memorandum NWS HYDRO-35, National Weather Service, NOAA, U.S. Dept. of Commerce, Silver Spring, MD, June 1977. Gregory, J.D., Personal communication, NCSU Department of Forestry, ~aleigh,N.C., October 1985.

Goettemoel ler, Re L., "Erosion Control and S tormwater Management for Urban Soil Sediment ~ollutionControl: A Workable Ohio Standard1', ------Proceedings --of a ------National ------Conference on Urban -----Erosion ---and ------Sediment ------Control: ------~nstitutions-and~echnology, ------Downing, W. I,., ed ., U. S. Environmental protection Agency, Jan., 1980.

Gunn, Clare A*, John We Hanna, Arthur J. ~arenzinand Fred M. Blumberg, Development of Criteria -for Evaluating Urban River Settings -for ouri ism-~Fcreation Use, Texas Water Resources Institute, Texas A&M, Technical Report No. 56, June 1974.

Gunn, Clare A*, "~iverWalk Generates IS trongt Positive Response ,I' Water --And The Landscape, edited by Grady Clay, McGraw-Hill Book Company, 1979. Hammer, T.R., "Effects of Urbanization on Stream Channels and Stream Flow", Regional Science Research Institute, Philadelphia, PA, Nov., 1973. Harp, J.F., utilization, Planning -and Problem Evaluation of Urban Arterial Waterways -and Corridors --as an urban ~esource, Off ice cf Water Research and Technology, November 1981.

Hirsch, Re M.,"The Interaction of Channel Size and Flood Discharges for Basins Undergoing Urbanization", Pub1 ication -NO. 123, ~nternationalAssociation of Scientific Hydrology, 1977.

Hollis, G. E., "The Effect of Urbanization on Floods of ~ifferentRecurrence Intervals", Water Resources Research, Vol. 11, No. 3, June, 1975.

Lancaster, C. C., "Application of the Level of Service Concept to Urban Streams," Master's Thesis, Civil Engineering Department, North Carolina State University, December 1985.

~onchell, C. V., "creating a Stormwater Utility," APWA Reporter, January, 1986.

itt ton, Re Burton Jr., Robert Jo Tetlow, Jens Sorenson and Russell A. Beatty, Water -and ~andscape,Water Information Center, Inc., Port Washington, New York, 1974.

Highway Research Board, Highway Capacity Manual, Special Report -87, Washington, DC, 1965. Johnson, C., "pavement (Maintenance) Management Systems", APWA Reporter, Vol. 50, No. 11, NOV., 1983.

Kulkarni, Ro Bo, Golabi, Ko, Finn, F. N., Johnson, Rot "A Systematic Procedure for the Development of Maintenance Levels of Service", Transportation Research Record - 781, Transportation Research Board, Washington,DC, 1980.

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Malcom, Ho Rooney, A Study of Detention in Urban Stormwater -Management, ~at%r~esources ~esearcr ~nstitute of the university of North Carolina, Report NO. 156, Raleigh, NoCo, July 1980.

Martens, Lo A., "Flood Inundation and Effects of Urbanization in Metropolitan Charlotte, North Carolina", ------Water-Sue~ly - - Paper ------1591-C, U. S. Geological Survey, Washington, DC, 1964.

Matthews, Danny, Personal communication, Arlington County Department of Public Works, Arlington, Virginia, September 1985.

McLaren, Lee, Personal communication, DPR ~ssociates,Charlotte, NoCo, September 1985. McCuen, Richard He, A Guide to Hydrologic- --Analysis Using -SCS Methods, renti ice-~all, 1982.

Nemerow, NoLo, Scientific Stream -~ollution ~nalysis, McGraw-Hill, New York, 1974. North Carol ina Environmental Management Committee, --New Classifications Adapted -and Assigned to various Streams -and Segments of Streams in the Catawba ~GerBasin, ~esolution 74-49, ~eg: No. ~~~-2~85>ugust22, 1974.

Nunnally, NOR. and Eo Keller, Use of Fluvial Processes to -Minimize Adverse Effects of -re= mgnnerization, water ~esources-~esearch ~nstitrteof the university of North Carolina, Report NO. 144, Raleigh, NoCo, July 1979. Ostle, Bernard, Statistics -in Research, 2d Ed., Iowa State Univ. Press, 1963.

~lastino,Ro J., "~eathKnell Sounds for a Storm rain age Fee," public Works, v. 115, n. 4, April 1984. Poertner, H. G., "Stormwater . Management in the United States: A Study of Institutional Problems, Solutions and Impacts", Office of Water Research and Technology, U. S. Department of Interior, Bolingbrook, Ill., Sept., 1980. Putnam, Arthur L., Effect -of Urban ----Development -----on Floods in the ------piedmont Province------of ----North ------Carolina, United States ~eologicalSurvey, Open File Report, Raleigh, NC, 1972.

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Soule, D. M. and Vaughan, C. M., "Flood Protection Benefits as Reflected in Property Value Changes", Water Resources -Bulletin, ~ol.9, NO. 5, ~ct.,1973. Stamper, ~illiamG., Flood Mapping in Charlotte -and Mecklenburg County, North ---Carolina, ~niter~tatesGeological Survey, Open ~ileReport, Raleigh, NC, 1975.

rans sport at ion Research Board, Highway Capacity Manual, Special Report -209, Washington, DC, 1985. Urban Land Institute, "Residential Storm Water Management: Objectives, Principles and Design Considerations", with ASCE and ~ational~ssociation of Home Builders, Washington, DC, 1975. U. So Army Corps of Engineers, HEC-2 Water Surface Profiles:-- Users Manual, The Hydrologic Engineering Center, Davis CA, September 1982.

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Wiser, Eo He, "HISARS -- Hydrologic Information Storage and Retrieval System -- User's Guide," Report NO. 201, Water Resources Research Institute of The University of North Carolina, Raleigh, NC, September 1983 (Updated 1984). IX. APPENDICES

SELECTED CHARLOTTE STREAMFLOW DATA

...... GAGE YEAR PEAK M D Y STAGE WYR R ...... kfsl [ ftl [yrl BRIAR 1963 1450 9 -29 -63 9.67 63 3 BRIAR 1964 1070 1 -25 -64 8.36 64 1 BRIAR 1965 1260 10 -16 -64 9.05 65 2 BRIAR 1966 1480 3 -4 -66 9.82 66 3 BRIAR 1967 1520 8 -23 -67 9.9 67 3 BRIAR 1968 1360 6 09-68 9.36 68 2 BRIAR 1969 807 10 -13 -68 7.26 69 1 BRIAR 1970 1100 12 -10 -69 8.46 70 1 BRIAR 1971 2220 5 -13 -71 11.79 71 35 BRIAR 1972 1510 10 -16 -71 9.87 72 3 IRWIN 1963 2720 3 06-63 11.64 63 1 IRWIN 1964 1780 4 -7 -64 9.46 64 1 IRWIN 1965 2500 7 -28 -65 11.49 65 1 IRWIN 1966 2850 3 -4 -66 12.2 66 1 IRWIN 1967 3320 8 -23 -67 13.12 67 2 IRWIN 1968 2720 6 09-68 11.93 68 1 IRWIN 1969 1840 8 -2 -69 9.65 69 1 IRWIN 1970 1750 12 -10 -69 9.41 70 1 IRWIN 1971 3450 6 -21 -71 13.33 71 2 IRWIN 1972 2900 10 -16 -71 12.29 72 2 IRWIN 1973 3750 2 -2 -73 13.78 73 3 IRWIN 1974 3760 9 -6 -74 13.8 74 3 IRWIN 1975 8880 5 -30 -75 18.04 75 111 IRWIN 1976 2960 10 -8 -75 12.07 76 2 IRWIN 1977 6740 10 -9 -76 16.38 77 25 IRWIN 1978 3180 1 -25 -78 12.39 78 2 IRWIN 1979 4090 9 -30 -79 13.77 79 3 IRWIN 1980 2920 5 -20 -80 9.77 80 2 IRWIN 1981 2740 9 -7 -81 9.45 81 1 IRWIN 1982 6400 6 -10 -82 14.98 82 19 LHOPE 1967 1110 8 -23 -67 8.12 67 4 LHOPE 1968 1020 6 -9 -68 7.91 68 4 LHOPE 1969 487 7 -24 -69 6.39 69 1 LHOPE 1970 320 12 -10 -69 5.7 70 1 LHOPE 1971 788 5 -13 -71 7 . 32 71 2 LHOPE 1972 1240 7 -26 -72 8.39 72 6 LONG 1966 1260 3 -4 -66 10.15 66 2 LONG 1967 1350 8 -23 -67 10.38 67 2 LONG 1968 830 3 -12 -68 8.45 68 1 LONG 1969 874 3 -19 -69 8.67 69 1 LONG 1970 543 7 -6 -70 7.29 70 1 A. SELECTED CHARLOTTE STREAMFLOW DATA (Continued) ------...... GAGE YEAR PEAK M D Y STAGE WYR R [cfsl [ ftl [yrl ------,----,------LONG 1971 972 8 -2 -71 10.71 71 1 LONG 1972 774 1 -13 -72 9.51 72 1 LONG 1973 2250 2 -2 -73 11.2 73 5 LONG 1974 1180 9 -6 -74 9.86 74 2 LONG 1975 3720 5 -30 -75 11.46 75 22 LONG 1976 1180 10 -8 -75 8.49 76 2 LONG 1977 3480 10 -9 -76 11.3 77 18 LONG 1978 1550 1 -26 -78 9.25 78 3 LONG 1979 1360 2 -24 -79 9.73 79 2 LONG 1980 814 3 -28 -80 7.83 80 1 LONG 1981 530 9 -7 -81 6.19 81 1 LONG 1982 4300 6 -18 -82 11.7 82 36 LSUGARl 1963 4040 9 -29 -63 12.73 63 1 LSUGAR1 1964 2770 4 -6 -64 10.17 64 1 LSUGARl 1965 4660 8 -21 -65 13.62 65 2 LSUGAR1 1966 3920 3 -4 -66 12.25 66 1 LSUGARl 1967 4010 8 -23 -67 12.42 67 1 LSUGAR1 1968 3890 6 -9 -68 12.18 68 1 LSUGARl 1969 2140 7 -3 -69 9 69 1 LSUGAR1 1970 3060 12 -10 -69 10.65 70 1 LSUGARl 1971 5190 5 -13 -71 14.53 71 3 LSUGAR1 1972 4480 10 -16 -71 13.25 72 2 LSUGARl 1973 8440 6 -15 -73 18.2 73 21 LSUGAR1 1974 3630 9 -28 -74 11.75 74 1 LSUGARl 1975 7800 9 -23 -75 17.3 75 14 LSUGA.1 1976 3610 10 -8 -75 11.33 76 1 LSUGARl 1977 6960 10 -9 -76 16.32 77 8 LSUGAR2 1978 5000 1 -25 -78 11 78 2 LSUGAR2 1979 6900 3 -23 -79 12.44 79 8 LSUGAR2 1980 3900 5 -20 -80 10 80 1 LSUGAR2 1981 6540 9 -7 -81 12.22 81 6 LSUGAR2 1982 7220 6 -10 -82 12.25 82 9 MCALPINEl 1963 2250 3 -6 -63 11.2 63 1 MCALPINEl 1964 3400 4 -8 -64 13.06 64 2 MCALPINEl 1965 3280 10 -16 -64 12.9 65 2 MCALPINEl 1966 3170 3 -4 -66 13.15 66 1 MCALP INEl 1967 2730 8 -23 -67 12.46 67 1 MCALP INEl 1968 2190 3 -12 -68 11.38 68 1 MCALPINEl 1969 2000 7 -3 -69 11 69 1 MCALPINEl 1970 1160 2 -16 -70 8.54 70 1

MCALPINEl 1971 2970 3 -3 -71 12.86 71 A1 MCALPINEl 1972 3120 10 -16 -71 13.12 72 1 MCALPINEl 1973 3820 4 01-73 13.84 73 2 MCALPINEl 1974 1570 1 -1 -74 9.89 74 1 A. SELECTED CHARLOTTE STREAMFLOW DATA (Continued)

...... GAGE YEAR PEAK M D Y STAGE WYR R ...... kfsl ftl [yrl ~ALPINE~ 1975 4190 5 -18 -75 14,17 7 5 3 MCALPINEl 1976 1340 6 -20 -76 9.12 76 1 MCALPINEl 1977 5510 10 -9 -76 15.34 77 5 MCALPINEl 1978 3000 1 -26 -78 12.73 78 1 MCALPINEl 1979 6690 3 -24 -79 16.7 79 10 MCALPINEl 1980 2440 4 -8 -80 11.04 80 1 MCALPINEl 1981 2640 6 -12 -81 11.41 81 1 MCALPINEl 1982 5940 6 -10 -82 16.18 82 7 MCALPINE2 1975 5960 5 -4 -75 10e68 75 4 MCALPINE 2 1976 1690 1 -1 -76 5.98 76 1 MCALPINE 2 1977 6480 3 -30 -77 12.16 77 5 MCALPINE2 1978 5520 1 -26 -78 11.23 78 3 MCALPINE 2 1979 6010 3 -24 -79 13-5 79 4 MCALPINE2 1980 4230 3 -21 -80 10.38 80 2 MCALPINE2 1981 3520 2 -11 -81 9.72 81 1 MCALPINE2 1982 5870 6 -11 -82 11.67 82 3 MCMULLEN 1963 939 9 -29 -63 6.66 63 1 MCMULLEN 1964 1080 7 -30 -64 8.4 64 2 MCMULLEN 1965 794 10 -16 -64 6.65 65 1 MCMULLEN 1966 876 3 -4 -66 7.16 66 1 MCMULLEN 1967 916 8 -23 -67 7.41 67 1 MCMULLEN 1968 789 6 -9 -68 6.83 68 1 MCMULLEN 1969 534 2 -8 -69 5.34 69 1 MCMULLEN 1970 476 12 -10 -69 4,97 70 1 MCMULLEN 1971 1020 5 -13 -71 9.21 71 2 MCMULLEN 1972 1130 7 -26 -72 7.88 72 2 MCMULLEN 1973 1210 6 -15 -73 8.31 73 2 MCMULLEN 1974 1240 7 -7 -74 7.89 74 3 MCMULLEN 1975 1630 9 -23 -75 9.19 75 6 MCMULLEN 1976 602 11 -12 -75 4.9 76 1 MCMULLEN 1977 1440 10 -9 -76 8.55 77 4 MCMULLEN 1978 925 10 -26 -77 6-62 78 1 MCMULLEN 1979 2390 3 -23 -79 9.91 79 26 MCMULLEN 1980 1380 4 -8 -80 8.43 80 3 MCMULLEN 1981 1340 9 -7 -81 8.31 81 3 PICMULLEN...... 1982 3150 6 -10 -82 10.89 82 101 Be LISTING OF COMPLAINT FILES STUDIED

CITY ADD LOCATION MO YR PROBLEM COST COST PAYMENT ...... ------.------3700 MEDALLION DR 9 82 ? 946 703 ONSITE 6127 AMBERLY LANE ? 2,458 1,230 PETITION FOXCROFT RD MEDIAN 1 84 AESTHET. 166,583 97,206.PETITION 1844 PINEWOOD CIRCLE 5 82 BL/ER 1,715 1,019 ONSITE 619 MANNING DR 4 83 BL/ER 3,166 1,969 ONSITE 1109 BELGRAVE PL* lo 83 BL/FL 4,019 2,171 ONSITE 329 RENSSELAER AVE 6 83 BLOW/SILT 2,451 1,345 ONSITE 9009 WINDSONG DR 9 82 BLOW/SILT 516 428 ONSITE 2120 KENMORE AVE 3 82 BLOWOUT 81 72 ONSITE 3700 MEDALLION DR 1 81 BLOWOUT 970 815 ONSITE 2622 BUCKNELL AVE 5 82 BLOWOUT 723 397 ONSITE 1534 COVENTRY RD 8 80 BLOWOUT 31 0 ONSITE 1518 N 1-85 SERVICE RD 4 82 BLOWOUT 428 313 ONSITE 4262 DENBIGH DR 8 81 BLOWOUT 881 300 ONSITE 3021 NORTHAMPTON DR 12 82 BLOWOUT 4,399 2,648 ONSITE 3301 EASTBURN DR* 6 81 BLOWOUT 3,716 2,114 ONSITE 4015 BROOKVIEW DR* 11 79 BLOWOUT 1,772 1,164 ONSITE 2934 EASTBURN RD 8 81 BLOWOUT 1,597 1,167 ONSITE 5912 POWDER HORN RD 9 80 BLOWOUT 295 207 ONSITE 5909 FARM POND LANE 3 80 BLOWOUT 472 389 ONSITE 5636 PRESTON LANE 5 84 BLOWOUT 202 169 ONSITE 3126 FERNCLIFF RD 11 79 BLOWOUT 109 119 ONSITE 2101 QUEENS RD EAST 12 80 BLOWOUT 164 129 ONSITE 7012 HEATHERFORD DR* 11 79 BLOWOUT 991 290 ONSITE 2221 QUEENS RD EAST 5 82 BLOWOUT 1,533 1,085 ONSITE 2807 INVERNESS RD* 9 82 BLOWOUT 350 329 ONSITE 1524 RAMA RD 8 81 BLOWOUT 244 126 ONSITE 1722 CANDLEWOOD RD* 12 79 BLOWOUT 669 328 ONSITE 1209 BRAEBURN ROAD* 5 80 BLOWOUT 318 270 ONSITE 1815 LANSDALE DR 12 80 BLOWOUT 131 110 ONSITE 3917 RHODES AVE 4 83 BLOWOUT 675 510 ONSITE 3415 CHAMPAIGN ST 9 82 BLOWOUT 191 338 ONSITE 3507 ROUND OAK RD* 9 79 BLOWOUT 826 531 ONSITE 5401 DONCASTER DR* 5 81 BLOWOUT 587 539 ONSITE 2916 RUSTIC LANE 3 82 BLOWOUT 545 336 ONSITE 2927 EASTBURN ROAD 5 80 BLOWOUT 1,117 708 ONSITE 2216 SHARON FOREST DR 2 81 BLOWOUT 334 287 ONSITE 1318 CAVENDISH CT* 8 81 BLOWOUT 466 399 ONSITE 4615 SOMERDALE LANE 9 83 BLOWOUT 543 231 ONSITE 4101 IRVINGTON DR 1 81 BLOWOUT 310 270 ONSITE 1728 STARBROOK DR 5 81 BLOWOUT 1,988 1,267 ONSITE 2630 BUCKNELL AVE 7 82 BLOWOUT 1,517 1,142 ONSITE 1835 THOMAS AVE 10 81 BLOWOUT 2,912 1,118 ONSITE 2927 EASTBURN RD 2 83 BLOWOUT 2,005 1,385 ONSITE B. LISTING OF COMPLAINT FILES STUDIED (Continued)

THURMOND PLACE 8 80 BLOWOUT 927 438 ONSITE HIDDEN FOREST DR 4 80 BLOWOUT 357 261 ONSITE WHEELOCK RD 10 82 BLOWOUT 475 343 ONSITE DAWNSHIRE 6 83 BLOWOUT 606 442 ONSITE KNIGHTSWOOD DR 12 82 BLOWOUT 908 592 ONSITE FARM POND LANE 6 83 BLOWOUT 638 540 ONSITE ASPEN CT 5 82 BLOWOUT 307 204 ONSITE WRISTON PL 1 81 BLOWOUT 368 295 ONSITE OLD POST ROAD 11 79 BLOWOUT 477 400 ONSITE RAMBLING ROSE DR 9 80 BLOWOUT 265 175 ONSITE DUDLEY DR 8 83 BLOWOUT 15,645 8,275 ONSITE COVENTRY ROAD 8 80 BLOWOUT 162 442 ONSITE SCOFFIELD RD 6 84 CAVED MH 3,041 2,041 ONSITE JOHNNY CAKE LANE 8 83 DAMAGE 311 125 ONSITE TABLE ROCK RD 5 81 ER/BL 2,418 1,644 ONSITE DRAKESTONE CT 9 83 ER/BL? 4,087 2,037 ONSITE STRANGEFORD AVE 5 82 ER/FL 9,735 6,491 PETITION BLENHEIM ROAD ER/ FL 9,172 6,115 PETITION FARMHURST DR 7 82 ER/HEALTH 5,506 3,185 ONSITE IDEAL WAY 5 82 EROSION 3,710 2,749 ONSITE CHAMPAIGN EROSION 5,934 3,956 PETITION SNOW WHITE LANE 9 82 EROSION 2,497 1,810 ONSITE MCALWAY RD 8 80 EROSION 2,918 969 ONSITE DAWNWOOD DR 10 82 EROSION 3,695 2,187 ONSITE EASTWAY DR 6 83 EROSION 5,531 4,254 ONSITE WINFIELD DR 9 80 EROSION 2,687 1,959 ONSITE COLUMBINE CIRCLE 11 83 EROSION 8,842 3,578 ONSITE BUCKNELL AVE 7 81 EROSION 1,538 589 ONSITE GLENDORA DR 3 83 EROSION 763 382 ONSITE TIMBER LANE 2 82 EROSION 2,834 1,395 ONSITE HEATHER 7 82 EROSION 3,370 2,321 ONSITE HARRIS ROAD 4 80 EROSION 16,922 13,016 PETITION HEATHER LANE 10 81 EROSION 3,517 2,416 ONSITE SARATOGA DR 6 82 EROSION 1,756 795 ONSITE TUCKASEEGEE RD 8 83 EROSION 12,314 9,164 ONSITE WENSLEY DR 12 80 EROSION 2,797 1,599 ONSITE SOMERDALE LANE 9 83 EROSION 1,883 1,033 ONSITE BRAXTON DR 10 83 EROSION 7,195 2,763 ONSITE SUFFOLK PL 12 82 EROSION 1,430 559 ONSITE BARNCLIFF RD 10 81 EROSION? 3,406 2,032 ONSITE POINDEXTER DR 1 83 EROSION? 7,207 3,376 ONSITE DREXEL PL 9 83 EROSION? 3,546 1,833 ONSITE N. SHARON AMITY 10 82 FL/ER 1,193 880 ONSITE PINCKNEY/EVERETT 8 80 FLOODING 129,000 103,807 PETITION B. LISTING OF COMPLAINT FILES STUDIED (Continued)

...... ----- CITY ADD LOCATION MO YR PROBLEM COST COST PAYMENT ...... ------.---,------6201 DELIAH LANE 7 84 FLOODING 1,506 1,127 ONSITE PEPPERCORN LANE 6 82 FLOODING 8,585 5,723 PETITION 6317 BIRMINGHAM DR* 2 84 FLOODING 23,058 8,073 ONSITE 3138 AIRLIE STREET* 6 79 FLOODING 2,278 1,820 ONSITE 4528 STRANGEFORD AVE 9 83 FLOODING 8,125 8,125 ONSITE WALKER RD 5 82 FLOODING 8,765 3,572 ONSITE BRANDON ROAD 1 82 FLOODING 181,267 168,001 PETITION 1234 ROMANY RD 9 83 FLOODING 5,032 3,250 ONSITE 601 EDGEGREEN DR 3 83 FLOODING 3,911 2,818 ONSITE 624 FORTUNE ST 7 82 FLOODING 23,420 19,707 ONSITE 3727 STOKES AVE 6 82 STOP-UP 565 409 ONSITE 1272 CHANDLER 3 83 BL/ER 2,915 ONSITE R 4224 COLUMBINE CIR 4 80 BL/FL 1,345 ONSITE R 2101 CLOISTER DR 11 83 BLOWOUT 2,574 ONSITE R 3324 FIELDING AVE 5 83 BLOWOUT 2,827 ONSITE R 1101 REDCOAT DR 8 81 BLOWOUT 852 ONSITE R 6024 BISMARK PL 3 79 BLOWOUT 946 ONSITE R 1107 HALESWORTH 3 79 BLOWOUT 3,224 ONSITE R 5114 CHAPLIN LANE 10 83 BLOWOUT 1008 GOSHEN PLACE BLOWOUT IN PROGRESS 2422 CLOISTER DR 2 80 REPAIR 11,524 ONSITE R 5328 RANDOLPH RD 3 79 ER/BL 8,522 ONSITE R 801 GLENDORA DR 7 82 ER/FL 763 ONSITE CH 5348 SUNBURY LN 3 84 ER/FL 95,100 PETITION 1822 CAVENDISH 7 82 ER/FL 10,401 ONSITE CH 2110 CORTELYOU 7 79 ER/FL 3,042 ONSITE R 5941 DEVERON DR 3 84 ER/FL 45,000 PETITION 4307 EMORY LANE 3 83 ER/FL 75,000 PETITION 1138 LYNNBROOK 4 79 ER/HEALTH 16,190 ONSITE R 715 RAMA RD 6 80 ER/SAFE 3,878 ONSITE R 2143 KNICKERBOCKER 3 84 EROSION 48,000 PETITION 5801 DONCASTER 7 82 EROSION 2,374 ONSITE CH 1033 GOSHEN PLACE 3 80 EROSION 4,632 ONSITE 426 CHILLINGWORTH 10 79 EROSION 13,375 ONSITE R KNICKERBOCKER 9 78 EROSION 84,000 PETITION 2420 ADDISON DR 2 82 EROSION 66,450 ONSITE R 134 MCALWAY WAY 11 79 EROSION 3 ,111 ONSITE R 2129 KNICKERBOCKER 3 79 EROSION 13,040 ONSITE R 5320 RANDOLPH RD 6 79 EROSION 7,243 ONSITE R 1301 ARDBERRY PL 2 82 EROSION 5,750 ONSITE H 5900 GROSSNER PL 4 82 EROSION 3,956 ONSITE R 1741 DALLAS 3 84 EROSION 1040 GOSHEN PLACE 2 82 EROSION 5,453 ONS ITE 111 CIRCLEWOOD DR lo 84 FL/BL ONSITE R B. LISTING OF COMPLAINT FILES STUDIED (Continued) Cm RAINFALL TABLES COMPUTED FOR CHARLOTTE, NC

5 min 0.47 0.82 NOAA HYDRO-35 15 min 0.97 1.74 NOAA HYDRO-35 60 min 1.67 3.52 NOAA HYDRO-35 24-hr 3.50 7.00 TP-40 ...... USWB l[<---- DATA ---- > 'I