Characteristics of the watershed area directly affect the hydrologic analysis. Basic fea­tures of the watershed basin include size, shape, slope, land use, soil type, storage, and orientation.

The size of the watershed basin is the most important characteristic affecting the determination of the total runoff. It is generally measured in acres, square miles, or square kilometers and is defined by the limits of the topographic divide. A topographic divide is a line that separates water flow between basins, thus causing the rainfall that falls on one or the other side to flow into a particular watershed. The location of this divide, and thus the perimeter of the basin, may be determined from aerial photographs, topographic maps available from the U. S. Geological Survey (USGS), and field surveys.

The shape of the watershed primarily affects the rate of water flow to the main channel. Because the rainfall in narrow watersheds reaches the main stream relatively quickly, a narrow basin generally has a low peak discharge compared with a fan – or pear-shaped basin of otherwise similar characteristics.

The main effect the slope has on water flow is on the time of concentration, or the time it takes the rainfall to flow from the farthest point in the watershed to the point under consideration. Everything else being equal, steeper slopes cause a shorter time of concentration, and thus a higher peak discharge, than do flatter slopes.

The use of the land and the type of surface the precipitation falls upon have an obvious impact on the flow of water. Developed areas covered by asphalt or concrete will allow a much greater percentage of the rainfall to flow to the point under deliber­ation than will an undeveloped vegetated area.

Peak flows may be reduced by the effective storage of drainage water. Of the three main types of storage—interception, depression, and detention—detention storage has the major impact in determining runoff. Interception refers to storage on aboveground fixtures such as plants, and depression refers to storage in depressions in the ground surface. Interception storage will eventually evaporate, and depression storage will either evaporate or infiltrate into the ground. Detention storage is runoff that is either in transit to the main channel or in storage in a pond, swamp, basin, or constructed detention chamber prior to transmission.

The final characteristic of the watershed basin is orientation. Taking into account the slope of the basin, if it is north – or south-facing, the runoff may be affected. If the basin accumulates snow and faces north, the snow may not melt until the late spring. If the snow melt is caused by a spring rain, the total runoff will be increased. On the other hand, if the basin faces south, the snow melt may come much earlier in the year, and with evaporation and infiltration, it may not contribute as greatly to the runoff. Basin orientation for small, steep basins also affects the peak rates of runoff. Where these basin types are in line with prevailing storm movements, the watershed responds with higher, shorter peak discharges.

The science of hydrology is concerned with the estimation of the intensity of rainfall, the
distribution of the flow of the rainwater over the land, and the determination of the flow quan-
tity (peak and total) that eventually reaches some specified point, the “point of solution.”

Of primary concern to the highway engineer is the frequency of occurrence of the peak discharge. Although many methods for determining runoff have been proposed over the years, making an accurate prediction is difficult, because of the many and varying para­meters that contribute to the complexity of the problem. These parameters include the affected drainage area, the rainfall intensity, the time of concentration of the rainfall, and the percent of the rainfall that will actually reach the point of solution. In addition to the difficulty in forecasting flows due to the inaccuracies in measuring and predicting the above parameters, different techniques that are commonly used to predict flows may pro­duce significantly different results for a specific site and situation.

The objective of hydrologic analysis is to estimate the quantity of runoff for which a specific hydraulic structure must be designed. The magnitude of the study must be propor­tional to the risks involved. Those risks include the potential for damage to the roadway and adjacent property and the importance of the roadway in the transportation system.

Principal Hydraulic Engineer
E. L. Robinson Engineering
Columbus, Ohio

A properly designed highway requires a well-designed drainage system. This requires a determination of the quantity of runoff reaching the drainage structures and an accurate analysis of water flow through the structures in order to properly size them. Also, a working knowledge of structural characteristics of buried pipe systems and effects of environmental factors is necessary to provide for long-term performance. Timely inspection and maintenance of drainage facilities will ensure satisfactory service life. If all of these issues are properly addressed, an efficient drainage system can be devel­oped. Because a large percentage of highway funds is spent on culverts, storm drains, and other drainage facilities, it is incumbent upon the engineer to use funds wisely and create an efficient drainage system. Thus, the roadway and adjacent property will be protected without wasting taxpayers’ money.

This chapter includes a review of fundamental hydrology considerations and runoff estimation, fundamentals of the hydraulics of open-channel flow, and design considera­tions and methods for the various components of highway drainage. The design, construc­tion, and service life of both flexible and rigid pipe are addressed, as well as rehabilitation and maintenance. The range of products is broad, extending from small-diameter drainage pipe to long-span structures that may be used for the replacement of short-span bridges. Article 5.6 may be referred to for a general description of the major products available.

In recognition of the serious potential destructive effects of earthquakes, AASHTO specifications contain comprehensive provisions for seismic design. Although earlier speci­fications contained some provisions, the more comprehensive provisions were not adopted until the 1980s. They were based on a detailed study by consultants who were specialists in that field, with review and participation by bridge engineers and design firms. The standards developed apply to conventional steel and concrete girder and box girder construction with spans up to 500 ft, but do not cover suspension, cable – stayed, arch-type, and movable bridges.

Bridges and components designed to the AASHTO seismic provisions may suffer damage under severe seismic events, but should have a low probability of collapse due to ground shaking. The general philosophy adopted in the development was

• Small to moderate events should be resisted elastically without significant damage.

• Realistic seismic ground motion intensities should be used in design.

• Large events should not cause bridge collapse, and damage that occurs should be

readily detectable and repairable.

Seismic performance categories are assigned on the basis of a ground acceleration coefficient for the site determined from a contour map of the United States, and an importance classification of “critical,” “essential,” or “other.” Different degrees of design complexity are specified, depending on the seismic performance category. Each bridge is assigned to one of four seismic zones, and one of four different site coefficients is applied to approximate the effects of the site conditions (soil profile) on the response. Lateral forces and displacements may be determined from a single-mode spectral analysis, a multimode spectral analysis, or more rigorous procedures. Elastic response is assumed in the analysis, but forces are adjusted with response modification factors. The lateral forces are applied in orthogonal directions in combination to account for the directional uncertainty of earthquake motions. An important require­ment specifies the minimum length of the bearing seat supporting the expansion ends of girders, determined as a function of the span length and the height of the supporting columns. Foundation design is also treated.

Seismic retrofit is a major consideration for older structures, particularly in the western United States. Serious distress and collapse of some bridges in California during the Loma Prieta (1989) and Northridge (1994) earthquakes received wide publicity. However, the problem structures were generally those designed and constructed to earlier standards. Where bridges were built according to modern methods, problems were minimal. Problems included failure of reinforced-concrete rigid-frame supports, failure of reinforced-concrete columns, columns punching through decks, and collapse of a structural steel span where the longitudinal displacement was excessive.

Active programs are in place to retrofit older structures to current criteria, but it is a massive undertaking that requires several years to accomplish. Some of the tech­niques being applied include (1) increasing the length of the seats for the bearings to provide a greater tolerance for longitudinal displacements, (2) adding cable restraints and hold-down devices at supports and hinges to restrict excessive movement and keep members in place, (3) adding spiral reinforcing steel and steel jackets or composite overwraps to strengthen concrete column piers, (4) replacing obsolete bearings with energy-dissipating types having lead cores or shock absorbers, and (5) adding founda­tion tie-down rods inserted into holes drilled into the soil. In the Northridge earthquake, several structures that had recently been retrofitted survived intact, giving confidence to the retrofit program. Retrofitting is not limited to the west but is under way in other parts of the United States as well.

In the design of a new bridge, provision must be made for maintenance inspection. For example, plate girders can be provided with safety handrails, safety railings can be specified on top of wide piers for inspectors to check bearings, and safety ladders can be installed to provide access to elements of the bridge otherwise difficult to reach. For deck-type bridges of moderate span and width, it will be possible to access the superstructure from special bridge inspection vehicles operating on the deck. For longer spans where the depth of girder exceeds the vertical capacity of a boom, and for wider bridges where the horizontal reach of the boom is not adequate, it may be necessary to provide catwalks or permanent movable inspection platforms. These devices are becoming increasingly popular as inspection and maintenance require­ments are given the attention they deserve in the design process.

4.16 SCOUR

Stream scour can undermine bridge piers or abutments, resulting in collapse of spans and loss of life. Several such incidents, including collapses in Alabama, New York (Schoharie Creek bridge on the New York Thruway, 1987), and Tennessee, caused FHWA to mandate the evaluation of all highway bridges for scour vulnerability by 1997.

An insidious aspect of scour is that soil around a foundation can be removed and rede­posited during a flood without leaving clear evidence that this has occurred, so that material may be present but may not provide the required support. Beyond surveying the stream bot­tom for local lowering of the flow line and inspecting around the pier by visual, manual, and remote means, current techniques for determining whether a loss of support has occurred are limited. They include physical probing and use of ground-penetrating radar.

Bridge scour evaluation requires input from hydraulic engineers as well as from structural and geotechnical engineers. Following the determination of a total scour prism, all three disciplines should be involved in providing structural stability.

Scour Study. A scour study at an existing bridge will include some or all of the following:

• A channel bottom physical inspection

• A channel bottom topographic inspection

• A localized scour evaluation conducted around each substructure element

• Photographic or video recording of observations

• Hydraulic analysis

• Soils investigation including laboratory testing

Hydraulic Analysis. In the hydraulic analysis, depth of scour is calculated for 100-year and 500-year floods. Inclusion of the 500-year flood calculation reflects a change of thinking in regard to bridge hydraulics that has taken place in the last 20 or 30 years. Previously, it was thought acceptable to have a very small percentage of bridges wash out in a severe flood, and if this did not occur the hydraulic design requirements were considered excessive. The current thinking is that a complete washout should be avoid­ed, even in very extreme floods because total cost of a bridge failure would be more than design for scour. A difficulty in implementing this policy, as in earthquake engi­neering, is that the hydrologic database has been developed over a relatively short period of time in the United States.

AASHTO LRFD specifications require the bridge foundations to be investigated for the following two conditions:

• 100-year design flood for scour or an overtopping flood of lesser recurrence interval

at strength and service limit states

• 500-year check flood for scour or an overtopping flood of lesser recurrence interval

at extreme event limit state

AASHTO Model Drainage Manual contains guidance on design procedures and computer software for hydrologic and hydraulic design.

A series of hydraulic analysis computer programs are available to assist in scour analysis. They include HEC-18, Evaluating Scour at Bridges; HEC-20, Stream Stability at Highway Structures; HEC-23, Bridge Scour and Stream Instability Countermeasures; HEC-RAS, River Analysis System; HY-8, Culvert Hydraulics Analysis; and FHWA’s water surface modeling program, WSPRO. These programs are primarily for inland streams. In coastal areas, tidal velocities and hurricane surge velocities may also cause scour. To perform the hydraulic analyses for these condi­tions, it may be necessary to obtain data from the Federal Emergency Management Agency (FEMA) and the National Oceanic and Atmospheric Administration (NOAA) and to use other analysis techniques.

Soils Investigation. The objective of the soils investigation is to determine whether and to what degree the soils are subject to being eroded. Grain size is of particular interest.

Countermeasures. Where a potential for undermining is found, countermeasures will be required to ensure the stability of the bridge. Countermeasures include riprap, poured-concrete protective aprons with keyed edges, cabled-concrete sections, pre­cast-concrete units, rock-filled basket mattresses, and protective piles or sheet-piling. In the case of new bridges, where more opportunities for preventing scour exist, some of the available options are a larger waterway opening that reduces stream velocity, location of piers out of the scour-vulnerable zone, use of deeper piles, and selection of a different pier shape.

Design Information on Plans for New Bridges. Information from the scour analysis for a new bridge should be placed on the construction drawings so that a permanent record of scour estimates, and their effect on design, is readily available for future inspections and for improvement of this design process.