TABLE OF CONTENTS
ABSTRACT
INTRODUCTION
THE
URBAN HYDROLOGY APPROACH
THE STATE OF PRACTICE IN BRITISH COLUMBIA
The Rational
Method
Burkli-Zigler
and Talbot Equations
the
Burkli-Ziegler equation
the Talbot
method
Soil
Conservation Service (SCS) Method
California
Method
SUMMARY
OF EMPIRICAL METHODS
THE WAY
FORWARD
Design Criteria
Standardization of Flood Estimation Methods
Watershed
Changes
SUMMARY
AND CONCLUSIONS
LITERATURE
CITED
Reprinted with permission from the Journal of the American Water Resources Association as appears in Journal of the American Water Resources Association, Vol, 34, No. 4, pp. 787-794.
ESTIMATING THE Q100 IN BRITISH COLUMBIA: A PRACTICAL PROBLEM IN FOREST HYDROLOGY
By Lynne Tolland , Jaime G. Cathcart , and S.O. Denis Russell
Estimates of peak flows, with specified return periods, are needed in practice for the design of works that affect streams in forested areas. In the province of British Columbia (BC), Canada, the new Forest Practices Code specifies the 100 year instantaneous peak flow (Q100) for the design of bridges and culverts for stream crossings under forest roads; and many practitioners are engaged in making such estimates. The state of the art is still quite primitive, very similar to the state of urban hydrology 30 years ago, when popular estimating techniques were used with little consideration given to their applicability. Urban hydrology then evolved on a much more scientific basis, such that within about a 10 year period, standard approaches to design were developed. Forest hydrology should follow the same pattern, at least as far as estimating design flows is concerned. Popular present day design procedures include the rational method and other empirical approaches based on rainfall data, as use of the standard flood frequency approach is limited by the paucity of relevant flow data. Estimating procedures based on peak streamflow measurements and statistics are likely to evolve, and these will include distinctions for rain, snowmelt, and rain on snow floods. Guidelines will also be developed for selecting and applying appropriate procedures for particular areas.
(KEY WORDS: Forest Hydrology; Urban Hydrology; Peak Flow Estimation; Design; Forest Roads; Stream Crossings)
At the Forest Hydrology Symposium held in Vancouver, BC, in March of 1997, several speakers drew attention to the distinction between scientific hydrology and practical or engineering hydrology, as applied to forested landscapes. Both approaches use similar methods - observations, measurements, field experiments and computer modeling; but the emphasis is different. In scientific hydrology the aim is to further the understanding of hydrologic processes in the forested environment, in both qualitative and quantitative terms. In contrast, the aim of engineering hydrology is to make estimates of flows for design purposes. This involves developing standard procedures and collecting the necessary data to compute design values of parameters of interest. The best example in BC is the estimation of the Q100 for the design of forest road stream crossings. The Q100 is the instantaneous flow from a particular watershed that on average would only be equaled or exceeded once in 100 years. As several of the more scientific speakers at the Symposium pointed out, the concept of the "100 year flood" is an artificial one, and some contend that it tells relatively little about how the stream behaves. Nevertheless, it is important in practice as it is a major determinant in the design of stream crossings and other in-stream works.
At present, the hydrologic methods utilized by the forest industry in BC for estimating flows for design purposes are relatively primitive, and in some ways comparable to the state of urban hydrology in the United States (US) in the 1960's and early 1970's. This paper reviews the present state of practical forest hydrology in BC, as illustrated by the procedures used for estimating the Q100, and compares it to the state of urban hydrology 25 to 30 years ago. Further, the evolution of urban hydrology is explored as a model for the future development of forest hydrology.
In the 1950's and 60's, as rapid urbanization became a fact of life in many parts of the US and Canada, it became apparent that urban drainage problems were increasing. At the time there were a number of relatively simple design methods and formulae in use by various cities and municipalities, with different designers using a variety of methods and criteria. As the number of flooding and overflow incidents increased, the need for a more rational and coherent approach was recognized. It was soon established that peak flows did indeed increase dramatically with urbanization. This caused particularly severe problems in drainage basins where urbanization proceeded in an upstream direction, as the downstream drainage works, which were sized for pre-development conditions, became more and more inadequate as development continued. Eventually, a national study was completed under the auspices of the concerned professional organizations which produced a short, but very influential report entitled "Stormwater Management" (Urban Land Institute, the American Society of Civil Engineers and the National Association of Home Builders, 1975). Another somewhat earlier, but also very influential report was "Urban Stormwater Drainage", a "how to do it" design manual for the Greater Denver area (Denver Regional Council of Governments, 1969). This manual gave detailed instructions and criteria for the design of stormwater drainage systems for the Denver area. To the great surprise of the writers of the manual, Denver was inundated by requests for copies of this manual from all over the US, Canada, and overseas. The reason being, that this was the first published manual on the practical aspects of urban storm drainage and as such, it provided a "how to do it" example for cities and municipalities throughout North America and other parts of the world.
The main points presented by the "Stormwater Management" manual in what was then known as the "modern" approach to urban stormwater management, were: (1) use a "dual design" approach; (2) use detention storage to offset the effect of urbanization on peak flows and hold post development peak flows closer to pre-development values; (3) keep drainage channels as natural as possible (e.g. keep rivers natural instead of encasing them in underground conduits); and (4) encourage local areas to prepare drainage manuals with criteria that could be used by different professionals, yet result in a consistent approach to the design of stormwater facilities in the area. This general approach quickly gained acceptance and it has been widely applied throughout North America, including much of BC. The main points are discussed in more detail below.
The "dual design" approach involves designing stormwater facilities for no damage during relatively frequent storms of the order of 1 in 5 to 25 years and no unacceptable damage during more extreme floods such as the 1 in 100 or 1 in 200 year floods. Previously, storm drainage systems were just designed for floods in the order of 1 in 20 years, and no thought was given to what might happen during much larger floods. This new approach also considered what might happen during more extreme floods and allowed for some level of acceptable damage in such cases. The "dual design" approach leads to designs that roughly approximate the theoretically correct design that minimizes total expected cost, which includes the capital cost and the expected monetary and environmental cost of dealing with flood damage.
The second major development in modern urban drainage was the use of detention storage to reduce peak floods to pre-development levels. In some cases, such as where short channels or pipes discharge directly into major rivers or water bodies, this is not necessary. However, in most cases, reducing peak flows is both economically and environmentally advantageous. The reduction of peak flows is generally achieved by providing detention storage, which fills during a flood, thereby temporarily delaying the passage of some of the water until the peak flood period has passed. The stored water is subsequently released in a controlled fashion, freeing up storage which can be utilized during the next flood event. This peak flow attenuation system plays an important role in flood management as it permits the use of smaller pipes and channels and minimizes erosion problems associated with high flow velocities.
The third development of modern urban drainage was the use of natural channels wherever possible. Generally, this has been widely applied and has resulted in much more attractive and environmentally sensitive facilities, which in many cases are also less expensive than the more traditional buried pipes and conduits.
The fourth component of modern urban drainage is the development and use of local master drainage plans. These plans are drawn up for local areas to ensure that the best practice available is used and that the design methods and criteria employed are suitable to the area and incorporate local data. Another general aim of a master plan is to make readily available standardized methods and criteria that would ensure consistency between different designers. This is an important consideration for administrators as it helps mitigate conflicts between professionals representing the differing interests of developers, municipalities, etc. This approach has resulted in practical concepts such as basing designs on specific return interval flood peaks. While such concepts may be artificial and somewhat simplistic, they do ensure consistent standards and guard against any designs being "too far out".
THE STATE OF PRACTICE IN BRITISH COLUMBIA
In BC, there is a great need for a better understanding of how to determine appropriate culvert and bridge sizes for forest road stream crossings. In the past, culverts and bridges were usually designed to pass floods with return periods ranging from 10 to 50 years. Since the occurrence of washouts was relatively infrequent, and the cost of repair and replacement were modest, the failure of culverts and bridges was considered acceptable by some and tolerated by most. However, now that the environmental costs of culvert or bridge failure have also been taken into account in forming the Forest Practices Code, all stream crossings, except temporary bridges, have to be designed to pass the Q100. In addition, culverts must pass this flow with the head pond level no higher than the top of the culvert (British Columbia Government, 1995). This design criterion is likely warranted for streams where the damage from failure could be very large, but it may be excessively safe and unwarranted in many instances. Regardless of this standard design level, however, numerous different peak flow estimating approaches are employed by different designers, resulting in a very large range of possible design flows for a particular stream crossing.
Generally, the most reliable approach for estimating flood flows is the statistical analysis of historical flow records, commonly known as flood frequency analysis. Peak flow values are used to estimate parameters of standardized distributions, with which return period flood estimates are generated. The applicability of this approach, however, is severely limited by the availability of data. The historical flows represent a sample from a population, and consequently, the validity of the approach is very dependent on sample size. In addition, most design situations related to forest works involve drainage basins which are isolated and relatively small (often < 25 km2), and have storms of relatively short duration which often do not contain any peak instantaneous flow values. Consequently, most practitioners in BC favour the use of the most simple empirical approaches.
A large number of empirical and semi-empirical approaches for estimating design flows have been developed throughout the world and can be found in popular texts, such as those by Ponce (1989), Maidment (1993), McCuen (1989), and Chow et al. (1988). As a result of the large number of equations available in the literature, the following discussion will be limited to those which are commonly used in BC. These are the rational method, the Burkli-Ziegler equation, the Talbot formula, the Soil Conservation Service method, and the California method.
The rational method is one of the earliest and best known techniques for estimating peak flows. Its origin has been traced back to the work of Mulvaney in 1851 (Ward, 1975), and despite its age, and considerable criticism about its adequacy, it still finds extensive use today throughout the world, particularly for estimating urban drainage flows for storm sewer design. As a result, it has received considerable research attention (Singh, 1988; Hromadka and Whitley, 1994; Hromadka and Whitley, 1996) and despite the many modification efforts that have been made, it has remained essentially unchanged.
The rational method is used in BC to estimate peak flows for all types of small basins. However, it generally only produces reliable results in urban watersheds where the input parameters have been properly quantified and it usually tends to overestimate design floods for non-urban basins. One study by Hiemstra and Reich (1967) found that the rational formula overestimated flows in over two-thirds of the rural basins considered, and that the average of the estimated peaks was more than twice the magnitude of the observed average. The following provides a brief description of the rational formula and its various components, including discussions of efforts made to improve the method's applicability to BC's forested watersheds.
The rational method is based on the idea that the maximum rate of runoff from a drainage area will occur when the entire drainage is contributing to the runoff. The most common form of the rational equation is:
Q = CIA,
where, Q = runoff (cfs); C = runoff coefficient; I = rainfall intensity (in/hr); and A = basin area (acres). This equation derives its name "rational" from the fact that in the given form, it is logical for the quasi-steady state assumed, and the units of the quantities involved are numerically consistent.
The basic assumptions associated with the rational formula are: (1) the peak rate of runoff at the outlet of the basin is a function of the average rainfall rate for a duration equal to the time of concentration; (2) the rainfall intensity is constant for the duration of the storm; (3) the rainfall is uniformly distributed over the entire basin; (4) the frequency of rainfall is equated to the frequency of runoff; (5) the runoff is primarily by overland flow; (6) the storm duration equals or exceeds the time of concentration; and (7) the watershed system is linear.
The runoff coefficient is the proportion of the rainfall that runs off directly, thus representing a fixed ratio of peak runoff rate to rainfall rate. The proportion of rainfall which contributes to runoff depends on the characteristics of the drainage basin, such as slope, ground cover, soil type and degree of saturation. Proper selection of the runoff coefficient requires judgment, experience, local knowledge, and is key to obtaining realistic flow results. Runoff coefficients are theoretically restricted to a range of 0 to 1.0, but may exceed 1.0 in special cases to take snowmelt into account.
The rainfall intensity is the average rainfall rate for a storm with a return period equal to the design return period, and a duration equal to the time of concentration for the basin in question. For example, with a time of concentration of 1 hour, one would use the 100 year, 1 hour duration rainfall in estimating the Q100.
The time of concentration is usually defined as the time it takes for a drop of water falling on the most remote point of a basin to reach the outlet of the basin. This value can be computed if both rainfall and flow records are available for the basin in question; or estimated by using formulas. In practice, designers always have to use a formula and it is through this estimation that significant error typically occurs in applying the rational formula to forested watersheds in BC. Bondelid et al. (1982) found that upwards of 75% of errors in peak flow estimates can be attributed to errors in the time of concentration.
Numerous time of concentration equations have been developed, and examples can be found in most hydrology texts, such as those by Chow et al. (1988), Maidment (1993) and McCuen (1989). Unfortunately, most equations are unreliable when applied to areas different from those used for their development (Loukas and Quick, 1995). The BC Ministry of Environment, Lands and Parks (MOELP) (British Columbia Government, 1991) recommends the use of three different time of concentration estimating procedures, depending on the basin type. The well known Kirpich equation is suggested for urban watersheds, the Hathaway equation for agricultural watersheds or small interior basins with light forest, and an empirical chart for all other basins, including forested areas. However, despite the effort which is made to assign different estimating equations to appropriate basin types, many practitioners still rely on the Kirpich equation.
Loukas and Quick (1995) made a comparative study of many popular time of concentration formulas and found all of them to significantly underestimate measured values for two forested mountainous watersheds in BC. This poor performance was largely attributed to the fact that the dominating runoff mechanism in forested watersheds is shallow subsurface flow, while the majority of formulas are based on overland flow. Clearly, there is a need for a simple BC specific time of concentration formula.
The greatest strength of the rational formula is its simplicity and ease of application. This method is the most widely used empirical approach and it is recommended by the MOELP (British Columbia Government, 1991) for estimating peak flows for ungauged basins smaller than 25 km2. The conditions of constant temporal and spatial rainfall, storm duration exceeding time of concentration, and overland flow as the dominant runoff mechanism, dictate that the rational method be restricted to small basins.
BURKLI-ZIEGLER AND TALBOT EQUATIONS
In addition to the rational method, the Burkli-Ziegler and Talbot equations have also found considerable use in BC. These two equations are essentially modifications of the rational formula. The Burkli-Ziegler equation provides a similar general peak flow estimating approach, while the Talbot Formula was developed specifically for sizing small flow passage structures such as culverts. These two equations are recommended for use by the "Forestry Handbook for British Columbia" (Watts, 1983), and have been used extensively in BC, particularly by the forest industry. This recommendation is based on the results of a UBC Forestry undergraduate thesis by R.G. Bellamy, in 1975, entitled "Calculation of Peak Stream Flows for the Purpose of Determining Culvert Sizes for Logging Roads". This made a very simple study of the applicability of various peak flow formulas for estimating flows on mountain streams surrounding Vancouver. The study compared the results of the various formulas with flow values estimated with the rational formula, and found the Burkli-Ziegler and Talbot equations to give good results. This is not surprising, as both equations have the same basic form as the rational formula. Unfortunately, the study did not address the fundamental question of whether the rational formula provides realistic estimates of peak flows.
The Burkli-Ziegler equation is a modification of the rational formula which explicitly incorporates basin slope into the estimating procedure. The rational formula implicitly accounts for basin slope through the time of concentration/rainfall intensity determination, but the Burkli-Ziegler equation further emphasizes the effect that basin slope has on peak flows. The suggested form of the equation (Watts, 1983), for estimating peak flows for ungauged basins is:
Q = CIA(S/A)0.25
where, Q = runoff (cfs), C = runoff coefficient, I = rainfall intensity (in/hr), A = basin area (acres), and S = basin slope.
The runoff coefficients for the Burkli-Ziegler formula are similar to those commonly used for the rational method, although they are sometimes more specific for different slopes and types of forested watersheds (Rothwell, 1978). The Burkli-Ziegler equation was developed in Europe using 1 hour intensities and is not known to have been widely used in North America outside of BC.
The Talbot method, as outlined by Watts (1983), provides an example of a simple empirical peak flow estimating technique which was developed specifically for sizing waterways and small flow passage structures such as culverts. It has been used frequently by logging companies in BC, particularly for determining flow passage areas for bridge crossings. The Talbot equation is based on the Burkli-Ziegler formula (McCuen, 1989) and was developed on the basis of an assumed waterway flow velocity of 10 ft/s and a rainfall intensity of 4 in/hr. This velocity is considered to be the usual velocity through a typical culvert under flood conditions (CSPI, 1972), while the intensity represents an upper bound value above which culverts are not normally designed. Unfortunately, it is not known what return period is associated with this upper bound, but it is likely in the order of 25 or 50 years, as these were popular design standards in the past.
The suggested form of the equation (Watts, 1983) is:
W = A0.75CR
where, W = waterway cross-sectional area (ft2), A = basin area (acres), C = runoff coefficient, and R = reduction factor of i/4 for rainfall intensities of less than 4 in/hr. W values can be equated to peak flows through use of the relationship Q = VW, where V is the assumed flow velocity of 10 ft/s.
The runoff coefficients for the Talbot formula are similar to those for the rational and Burkli-Ziegler methods, but are generally not as well defined. The Talbot formula provides a simple means of sizing culverts, but like the other approaches discussed, it has some deficiencies, the most obvious being the assumption of a constant flow velocity. However, this is more of a hydraulic concern, rather than a hydrologic one. In terms of hydrology, this equation does not provide any advantages over the other methods as it is subject to similar runoff coefficient and basin response time inaccuracies.
SOIL CONSERVATION SERVICE (SCS) METHOD
The SCS method was developed in 1954 by the US Soil Conservation Service (US Soil Conservation Society, 1985) and is widely used in North America for estimating peak flows on small to medium sized ungauged basins. It came into common use in the mid-1950's and has been adopted as the required procedure by many municipal and regional authorities, replacing the rational formula in many instances. It was developed on the basis of data obtained from infiltrometer tests and measured rainfall and runoff on small basins in the mid-western US, but has been extended by way of practice to the entire US and other countries, including Canada.
The methodology is too extensive to describe in this discussion, but in very general terms, a series of equations and relationships are used to determine and distribute a storm runoff volume. The method essentially relies on only one parameter to translate rainfall into runoff, and that is the runoff curve number (CN), which varies according to soil type and condition, land use, surface condition, and antecedent moisture condition.
Once the storm runoff volume is computed, the corresponding peak runoff value can be determined from a triangular approximation of the storm hydrograph. The shape of the triangle is related to the response time of the basin estimated from time of concentration equations (US Soil Conservation Service, 1975).
The SCS method has many of the same drawbacks as the rational formula, as it also involves the estimation of the time of concentration and a runoff coefficient. In addition, it requires a complete design storm instead of just an average intensity. It does appear to provide an advantage by permitting greater "fine-tuning" of the model for specific site conditions, as the CN value more specifically accounts for the physical processes in the basin that affect runoff. However, the method is very sensitive to curve number and antecedent moisture conditions, and published CN values have largely been developed for urban and agricultural/rural basins with little guidance provided for estimating values for mountainous forested watersheds.
The California method is another approach which has been designed specifically for sizing culverts, but rather than involving the estimation of basin parameters and rainfall intensities, it simply determines culvert size on the basis of a relationship between culvert size and stream bank-full cross-sectional area. This relationship is very simple and it equates the cross-sectional area of a culvert required to pass the 100 year flood to 3 times the bank-full area of the stream. That is:
W = 3Ab
where, W = culvert cross-sectional area; and Ab = stream bank-full area.
This equation is based on the various assumptions which are: (1) the average velocity in the stream is the same as the average velocity in the culvert during flooding; (2) the average velocity in the stream is the same during the 2 year and 100 year events; (3) the bank-full flow represents the 2 year flood level; and (4) the 100 year flood flow is three times as great as the 2 year flood flow. A more sophisticated version of the method is to estimate the bank-full flow using the Manning equation and then compute the Q100 as three times the bank-full flow.
The main difficulty associated with this approach is determining the high water, or bank-full flood level, particularly for small, steep streams. Also, if the Manning equation is used in steep streams, it over-estimates the flow. In general, these errors lead to the substantial oversizing of culverts, however, the approach does directly use information from the stream in question, which is not employed in the rainfall based approaches.
All of the methods presently in use for estimating the Q100, apart from the relatively crude California method, rely on rainfall data. The main problem being that apart from Vancouver Island and some coastal areas (accounting for less than 10% of the province), most of the floods in the remainder of BC result entirely or largely from snowmelt. Hence, the rainfall based methods described are simply not appropriate. In areas such as Vancouver Island where the largest floods do result from rainfall, and where the topography is so mountainous, the few recording rainfall gauges are located in valley bottoms, making it very difficult to get reliable estimates of rainfall intensity.
As outlined in the previous section, the state of forest hydrology in BC, as applied to the derivation of flows for design purposes, is relatively primitive, and unstandardized, with each designer simply using their preferred design method. It is helpful to look at what has to be done in light of the precedent of urban storm water drainage, and this is discussed under the headings of design criteria, standardization of flood estimation methods, and watershed changes.
The relatively new Forest Practice code specifies that all new forest road stream crossings be designed to pass the Q100, with the water level no higher than the top of the culvert. This is a very stringent requirement, more so than that for main highways in many locations. For example, in California, highway culverts have to pass the Q10 with the water level no higher than the top of the culvert, and the Q100 without the water level overtopping the road embankment (American Iron and Steel Institute, 1984). The authors were not able to identify the source of the BC requirement, but believe that at the expense of better engineering, this approach was adopted in an effort to keep the code requirements simple. A more rational criterion could be to follow the urban drainage precedent and design for no damage from the 10 year peak flow and no unacceptable damage from the 100 year flood. The concept of risk should also be considered, as under the current system, the likelihood of failure is much greater for a culvert operating for 25 years than one required for only one year of service.
The question of continuing to use the Q100 for designing all forest road stream crossings should probably be studied by a well qualified, impartial group, such as a university team, which could support a technically competent committee, widely representative of all interests, including industry, government and environmental groups. Then, if there was wide agreement, a recommendation could be made to the government to change the code requirement to a more appropriate one.
STANDARDIZATION OF FLOOD ESTIMATION METHODS
For urban stormwater design, peak flows are usually estimated from rainfall data measured in the area of application. Rainfall data is easy to obtain and associated flows are relatively easy to estimate due to the predominance of overland flow in urban areas. However, with forested basins in BC, rainfall is difficult to measure, and the rainfall-runoff process is difficult to model. In different parts of the province, floods result from rainfall, snowmelt, or a combination of rain and snowmelt. Obviously, for a given area, flood producing mechanisms have to be at least roughly understood; there has to be an accepted methodology for estimating flood flows; and there has to be a local data base with which to calibrate the methodology. It is anticipated that a combination of modeling, peak flow measurements, precipitation measurements and statistical regionalization techniques will end up being the norm for design flood estimation. Since most of BC is quite mountainous, and since rainfall varies considerably with elevation, it may be more practical in most instances to measure peak flows than to measure rainfall. It should be relatively easy to measure peak flows at existing culverts under forest roads, since most culverts are steep enough to have "inlet control", which means that the peak flow can be computed if the peak water level is known. Peak water levels can be measured with simple crest gauges, and research is underway to develop simple reliable peak water level gauges and simple flow estimation methods. This standardization should also incorporate guidance for the consideration of the potential watershed changes, and not simply design for present state conditions.
Forest harvesting and the construction of forest roads clearly change a forested watershed from its natural state, but the effects on peak flows are not clear. For example, it is well known that snow melts more quickly when the forest has been clear cut; but while it is likely to result in an earlier flood peak, the peak may be no higher than it would have been before the harvest, when the later melt would likely have been the result of higher temperatures. There has been considerable research over the years on the effects of logging on flood peaks; but the results have been highly variable and inconclusive (Jones and Grant, 1996; US Department of Agriculture, 1989; Harr, 1986; Ziemer, 1981; and Hibbert, 1967). The effects are usually smaller than the natural variability, which makes it difficult to detect whether or not apparent changes are real and significant. Perhaps the practical approach would be to add a safety factor to take account of possible effects and then gradually reduce the factor as knowledge accumulates. This will clearly require further research and could incorporate committee deliberations to determine suitable factors to use.
Clearly there are many tools available to practitioners in BC for estimating peak flows. This review of the current state of practice leads to the conclusion that there is still much room for improvement in the techniques available for estimating peak flows in BC. The biggest weakness in the current system is a lack of knowledge about the use of the available systems on the part of practitioners. With the advent of micro-computers, many techniques have been made user friendly and are more accessible to the average user, and it is cautioned that there are problems inherent in making a system too user friendly (Klemes, 1986). Users sometimes become complacent and develop the misguided belief that the use of a computer somehow adds legitimacy to a methodology and input data, and that computer generated values are somehow more correct than those generated by hand. Developers of operational hydrologic techniques should provide explicit guidance for the use of techniques, with explicit warnings about the consequences of misuse. Instead of incorrectly using the simple rational formula, which is a common occurrence at present, users may be inclined to incorrectly use other techniques which may not be as inherently conservative as the rational formula.
At present, there is no generally agreed methodology in the approach for estimating design floods in BC, and not enough data to allow reliable use of any of them. The situation is similar to that in which urban hydrology found itself about 30 years ago. Forest hydrology, where it is involved in providing advice and estimates for design purposes, seems likely to follow the pattern along which urban hydrology evolved. Looking at this pattern provides useful insights into the probable directions along which forest hydrology is to develop in BC. This will involve research on estimation methods and on data collection techniques; discussions between researchers and practitioners; the development of a consensus on the most appropriate approaches for the different hydrologic zones in the province; data collection programs; and the development of the necessary design criteria.
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