1. Introduction
Flooding problems frequently occur because drainage infrastructures have insufficient capacity [
1]. These structures capture runoff from urban basins whose hydrological analyses are generally carried out with heterogeneous criteria. This is because they depend on the experience of those in charge of each urban storm drainage project [
2].
The design discharge is usually the main variable for designing storm drainage systems. However, there is a lack of a single criterion to calculate it [
3]. Such criteria may include factors such as the definition of runoff coefficients, times of concentration, rainfall intensities, and basin areas, as well as the calculation method itself. In many cases, hydrological studies are conducted without considering the basin as a basic unit [
4]. Instead, the calculations may have a geographic scope that corresponds to the layout of real estate developments, even though their runoff drains into the same storm drain because they are in the same urban basin [
5]. In addition, basic information is often scarce in cities located in developing and even developed countries [
6]. On the other hand, in general, there is no measurement of stormwater discharges into the surface or subsurface storm drainage network in cities [
7]. Therefore, such discharges must be estimated indirectly from rainfall data registered in one or several stations.
For example, for the case study presented here, there are only two climatological stations in the city, one in CONAGUA (DGE) and the other in the School of Biology at the Autonomous University of Sinaloa. This makes it difficult to sectorize precipitation along the urban territory to simulate the different precipitations that have been observed during the same day in the city in recent years [
8]. In addition, there are areas of cities where information on the layout of the surface and subsurface drainage network is lacking [
9].
The case of the City of Culiacan, Mexico, may be representative of several cities subject to torrential rainfall regimes [
10]. In 2013, Culiacan was affected by tropical storm Manuel, which caused major damage due to its high winds and flooding. The latter was due to the intense rainfall that occurred over approximately three days, which included the historical maximum of 250.3 mm of accumulated rainfall in the city in 24 h [
11]. Following this event, the Culiacan City Council focused its attention on the hydrological studies that were previously established as a requirement in Art. 274 Bis of the Building Regulations for the Municipality of Culiacan, Sinaloa [
12].
According to these regulations, to approve each real estate development, a hydrological study is required. The purpose of this is to ensure that the rainwater generated by the development planned for construction, as well as the rainwater generated by other developments that pass through the site, can be quickly and safely drained.
The hydrological basin is a convenient geographic unit for analyzing the surface water flow pattern in urban areas. This is because the directions of the water currents of the main urban storm drainage network converge and thus facilitate the understanding of the flow pattern. However, the division into urban basins may not be officially established, so it should be generated for each project. Generally, this is achieved based on digital terrain models obtained from freely available databases that have lower resolutions than those required for an urban basin [
13].
The heterogeneity of criteria used to carry out hydrological studies in different areas located in an urban basin makes it difficult to analyze the flow pattern over territories that drain into the same storm drain. This is because each discharge may have been calculated using different methods and assumptions, which are sometimes not specified in the calculation report. In addition, it makes it difficult and laborious for researchers and authorities to understand and review the different studies.
In summary, the main problem that the proposal of this article seeks to solve is the heterogeneity of criteria to determine the input data for calculating rainfall discharge in urban areas. Complementarily, the proposal supports the solution to the problem of inefficiency in the review of hydrological studies of areas located in the same basin and the scarcity or lack of cartographic information to delineate urban basins, especially in developing countries.
4. Discussion
There are several other methods commonly used for storm flow design. The selection of the method depends on several factors, including the project requirements, available data, and local regulations. Engineering references, guidelines, or local authorities are usually consulted to determine the most appropriate method to calculate the storm flow design in a specific location. According to this, no standardized methodology has been proposed in the literature. In addition, references generally do not include standardized procedures for delineating urban basins, and consequently, the dimensions of their areas and even their shapes can influence the magnitude of rainfall–runoff.
Some of the commonly used methods are the unit hydrograph method, soil conservation service (SCS) method, time of concentration method, rational method, and hydrologic models. The unit hydrograph method involves developing a hydrograph, which represents the flow rate of stormwater over time, based on the rainfall characteristics and the catchment’s response [
48]. The unit hydrograph is derived from the observed hydrographs to estimate the peak flow rate and hydrograph shape for a given storm event. The SCS method is commonly used in the United States and uses a hydrologic soil group classification system to estimate the runoff volume and peak flow rate [
49]. This classification system is based on the soil properties, land use, and rainfall characteristics to calculate storm runoff. The time of concentration method is based on the time it takes for the entire catchment area to contribute runoff to a specific point [
50]. However, several specific factors, such as flow path length, slope, surface roughness, and some hydraulic properties, must be provided to determine this time. Once the time of concentration is known, this time can be used to calculate the peak flow rate.
The rational method provides an estimate of the peak flow rate and is based on several assumptions. It is commonly used for the preliminary design and sizing of stormwater management systems. It is used to estimate the peak flow rate of stormwater runoff from a given catchment considering the land use and surface characteristics, and the rainfall intensity obtained from local weather records and using rainfall intensity–duration–frequency (IDF) curves specific to the study area [
51]. In addition, there are various hydrologic models available that simulate the rainfall–runoff process. These models utilize complex algorithms and data inputs to estimate the peak flow rate and hydrograph. Examples of commonly used hydrologic models include HEC-HMS, SWMM (stormwater management model), and TR-20 [
52,
53,
54]. These models consider the presence of green infrastructure, detention/retention facilities, or other stormwater management practices that affect the runoff characteristics.
Since the values of rainfall intensities are defined by zone or calculated with information that is generally not available in developing countries, the use of simple methods with many assumptions is common. As an example, the delineation of urban basin boundaries is traditionally carried out with automatic digital tools. These tools are based on the use of DEMs with poor resolution and do not properly consider the effect of urbanization [
55], which can distort the understanding of the surface rainfall flow pattern in the study area. In addition, the presence of the subsurface storm drainage system can greatly modify this flow pattern.
The proposed methodology is simple and has advantages over other more sophisticated methodologies that require detailed basic information [
6,
20,
21,
56]. This information on various occasions is limited or nonexistent, especially in terms of data corresponding to the topography of the surface of urban areas where the natural terrain conditions have been modified [
9]. Obtaining accurate data, such as rainfall data, soil properties, land use, and topographic information, can be challenging and time-consuming in developed countries [
25]. In our methodology, this problem is solved by generating a high-resolution digital surface model using photogrammetry, which is a cost-effective and less time-consuming option than traditional methodologies that require ground-level topographic surveys. This allows for accurate and rapid basin mapping, which facilitates the study of surface water flow on a basin basis rather than by urban development zone. Then, this methodology does include a procedure to delineate the basins under study. In this same sense, our methodology involves using a procedure to estimate rainfall intensity whose parameters are adjusted based on local information, which has yielded appropriate results in Mexico.
The rational method is proposed because it continues to be an international standard due to its simplicity and wide use. It requires the determination of the runoff coefficient, which is based on land use and type. Land use can be determined based on the photographs obtained as a partial product of the UAV flights of this methodology. However, the determination of soil type will depend on previous studies in the study area.
We recognize that the methodology proposed here has some disadvantages. One, perhaps the main one, which is also common to other methodologies, is the lack of model calibration. This is because it does not include field corroboration by means of a measurement network of the design flow rates.
In addition, the design flow rate corresponds to the flow rate of the basin as a whole; however, our study shows that it is not only discharged through a single point of interest (in this case, the location of the sewer) but also through other points of water parting. These correspond to streets that mainly constitute the surface drainage network of the area under study. Therefore, the flow rate flowing through the sewer is lower than that calculated with the rational method.
Our case study does not include the transfer of flows from the basin under study to other basins through the secondary or subway storm drainage network, except for the flow through the pipe connected to a sewer. This did not allow the influence of such transfers to be included in the methodology.
5. Conclusions
In this study, we proposed a standardized methodology to design a storm drainage system for a basin in Culiacan, México, using UAV, geographic information systems, and hydrological methods. The urban basin was delineated using a high-resolution DEM obtained from a photogrammetric process by using a UAV for image acquisition. The basin had an area of 0.42 km2, a perimeter of 2.8 km, a length of 1.1 km, a slope of 0.02, and a time of concentration of 11.56 min.
The probability distribution that best fitted the rainfall data was Gumbel of two populations, and this was used to estimate the maximum daily precipitation for different return periods. The high-resolution orthophoto was used to identify different land uses in the study area, which were then used to calculate the runoff coefficients for each return period. The intensity–duration–frequency curves were obtained using the isohyet curves and Chen’s formula, and a design storm was defined with a return period of 10 years and a duration of 11.56 min. The resulting rainfall intensity was 166.9 mm/h.
With the proposed methodology, it is possible to perform standardized hydrologic studies to calculate design surface water flow rates in urban basins with little basic information. Such standardization would facilitate the review of hydrologic studies by researchers or authorities to analyze surface water flow or to authorize or deny construction permits. The most appropriate methods existing in the literature were selected for urban areas with scarce basic information, such as the case study area. This methodology includes the delineation of urban basins with precision, considering the layout of roads and urban and real estate storm drainage works. In addition, it emphasizes the study of urban surface flow patterns with a basin vision instead of an urban development zone. This would allow for the integration of results from sub-basins with the results for larger basins.