Urban Stormwater Management Needs in North Carolina (Report No. 199) August 1983

[This is not the full report.]

The management of urban watersheds has become increasingly complex in recent years. Rapid urbanization in the Southeast has been accompanied by dramatic hydrologic changes. With the development of urban areas have come land-disturbing activities and an increase in the degree to which watersheds are composed of impervious surfaces such as streets, parking lots, and roof-tops. Natural drainage patterns have been altered, and changes have occurred in the quantity and velocity of stormwater runoff.

A variety of problems accompanies these changes. Increased stormwater velocity and peak flow can cause urban streambanks to become eroded and undermined. The streams become wider and shallower. The frequency and magnitude of flooding increase. Streams are polluted by sediments, nutrients, bacteria, heavy metals, and other contaminants.

Management of these problems has been fragmented and localized. Technical considerations are predominantly site specific. Local public works managers who must confront these and other stormwater management problems find that the problems are related. It is difficult to address one without touching on another. Yet they are dealt with separately. Ongoing problems are divided among federal, state, and local agencies, with each addressing a narrowly defined subset of urban watershed management issues.

The purpose of this project was to examine current practices in stormwater management and provide recommendations for improvement, particularly in the areas of institutional arrangements and technical support. The general objective of this research has been to establish and initiate use of state and local guidelines for management of urban drainage and flooding, sedimentation, stormwater-related land use, and the quality of urban runoff in the United States. These guidelines should provide local governments with the tools to interrelate and take full advantage of the many ongoing programs in the area of urban watershed management.

Specific project objectives were as follows:

1. identify current watershed management programs (state, local, and federal) for the southeastern states of Virginia, North Carolina, South Carolina, Georgia, and Alabama;

2. formulate a conceptual model of the interactions of these programs within each state;

3. identify the needs for coordination among these programs and the issues that presently overlap, conflict, or are overlooked within each state;

4. inventory and evaluate the current resource materials available which might provide guidance;

5. identify and develop the additional guidelines required; and

6. develop a program of information dissemination and transfer of research results.

The objectives were pursued through literature searches; interviews with public works officials, consulting professionals, builders, and others involved in making storm-drainage decisions; seminars and short courses on stormwater management for municipal officials and design professionals; and the design of a microcomputer program to provide a basis for interested groups to study specific cases in their own locations.

The following conclusions have been reached:

1.Institutions arise in response to different problems. It is unreasonable to expect a single set of controls to be applicable over wide areas of the nation. It is likewise unreasonable to expect local solutions in one area to be transportable to other areas without localizing changes.

2. Technical guidelines evolve similarly. Direct transfer of technical control measures will not likely result in an optimum control strategy because of differences in weather patterns, soil types, topography, development densities, and institutional arrangements. Even technical issues, such as hydrograph formulation, are subject to settlement by local consensus, and different communities will decide differently on such matters.

3. Given interactions among diverse governmental interests, conflicts in purpose, design, and operation of facilities are virtually certain to arise. To document systematic conflicts, however, is quite difficult. Rarely are institutional arrangements designed to resolve drainage conflicts openly (except in the courts).

4. Problems associated with urban runoff (flooding, sedimentation, and inferior water quality) are concentrated in the set of open channels that constitute the bulk of the drainage network in the urban area. Design, construction, and maintenance of this system are quite site specific. Comprehensive urban watershed planning is primitive and fragmented. Any program of improvement in any of these three areas of concern must deal with the size and complexity of this physical system, and it must likewise deal with the awkward institutional arrangements and systematic neglect that have accompanied the development of the city around its streams.

5. Stormwater detention, used for a variety of purposes in stormwater management, can aggravate streambank degradation. By causing stormwater to flow in the channel at a shallow depth for a protracted time, detention leads to accelerated undermining of channel banks.

The following recommendations can be made:

1. The urban storm drainage system should be managed as a public utility, in parallel to the way the street, water, and sewer systems are managed. The emphasis should be in achieving comprehensive planning, design, and maintenance of the storm drainage system and in extending and improving the care of the network of open channels and formerly natural streams.

2. The analytical resource of computer simulation of urban watersheds should be developed and applied systematically in making day-to-day design and planning decisions. The commitment to modeling, if made, should be sufficient to justify development of adequate rainfall and streamflow gauging stations to assure reliable calibration and validation of the model.

3. The microcomputer should be considered for adoption as the tool of choice in support of comprehensive watershed analyses and designs. It should be used for data storage and as a terminal to larger remote computers on which watershed models currently reside, and for ancillary computations.

4. The decision to seek significant improvements in the behavior of urban stream systems with regard to flooding, sedimentation, and inferior water quality carries with it the assumption that the public will support substantial increases in resources allocated to management. This assumption that the public considers the urban stream system worth development to some higher level should be carefully validated before the management commitment is made.

5. Detention remains a valuable tool for watershed management, even allowing for the conclusion of this project that questions its use for control of streambank degradation. Rather than use detention for this purpose, it is recommended that designers conduct cross-section design for new channels and bank protection for new and old channels, taking into account the magnitude of shear stress and the time over which the stress is applied. Impulse analysis constitutes one possible approach.