Development Of Simplified Hydrological Method for Dimensioning of Storm Water Detention Basins/Ponds

Adel R. Awad⁽¹⁾and Azm S. Al-Homoud^(2)
(1)Dept. of Environmental Eng. Faculty of Civil Eng., Tishreen University, Lattakia-Syria
⁽²⁾School of Engineering American University of Sharjah Sharjah, United Arab Emirates

Abstract

The paper handles the application of storm water detention basins or ponds in urban catchments for the protection of the cities or parts from floods risks, and for the purpose of protecting the water resources from the pollutants running off with storm water flow. The simplified mathematical hydrologic relationships were derived in the research work aiming to determine volumes of rainfall detention basins/ponds based on patterns of rainfall charts (rainfall intensity versus rainfall duration and return period), applying this method for rainfall floods of return period (n) between 0.5 (probability of one flood per 2 years) and 0.2 (probability of one flood per 5 years) on case study of Lattakia city as an example for a Coastal Arab Region, and on case study of Damascus as an example for an Arid or Semi-Arid Region. The results came accurate besides being attained through simple application of this developed method, with the possibility to generalize this method to other climatic regions where rainfall charts or rainfall stations are available.

Introduction

Deriving Mathematical - Hydrologic Relationships:

The curve that relates among rainfall height, rainfall duration and return period is represented in the following exponential function in general. (Figure 1):

Volume determination of storm water detention basins/ponds

substituting Eq. (10) into Equations (6) and (7) gives the required design volume (without considering runoff duration volume).

Flow determination (Q_effl.)

In detention basins / ponds where outflow is not regulated and fixed, but depends on water level at outlet, this outflow increases with the rise of water level. Then it would be important to find a relation between design (Q_effl.) and maximum flow (Q_{effl. max}). In detention basins / ponds where outflow is not regulated and fixed, but depends on water level at outlet, this outflow increases with the rise of water level. Then it would be important to find a relation between design (Q_effl.) and maximum flow (Q_{effl. max}).

In basins / ponds with vertical walls (rectangular basins) and when the outflow (Qeffl.) is little in comparison with the inflow (Q_infl.), that leads to filling the detention basin with constant speed approximately, then the actual flow will be calculated with the following formula that represents storm overflow:

Applied Examples:

With the assumption of the following data:
studied drainage area A = 50 hectare
impermeable area A_red. = 25 hectare
maximum flow Q_effl.max = 750 l/s
minimum flow Q_effl.min= 0 l/s
Time of concentration t_f = 25 min
Design outflow:

It is noticeable here that with the increase of safety condition against flood risks from return period n = 2 years to n = 5 years, volume of required detention basin / pond increases accordingly at a rate of 40%. Arid or Semi - Arid region (Damascus as an example)
Rainfall data: return period n = 2 years (Tab. 2)

Rainfall data: return period n = 5 years (Tab. 2)

It has been found out that Damascus region does not need detention basins / ponds with volumes more than 7% to 8% from those needed for Lattakia region, within the design return period (n) of 2 to 5 years. This big difference in the detention basin volumes between the two regions (arid and coastal) is due to the different meteorological data such as rainfall intensity, frequency, duration, season and run off pattern.

Conclusions:

The developed hydrological method is characterized with its simplicity and accuracy, which enables the environmental engineer or hydrologist to design basins / ponds for storm water detention directly based on the patterns of rainfall charts known or pre-determined for any studied site aiming to protect from floods risks, which enables generalizing and apply this method in any climatic region wherever it be provided to have the rainfall charts or rainfall stations. It was clearly seen that this method is distinquished from the tradititional design method employed in some countries, where the differences were plain in the detention volumes at the same selected rainfall return period (n). In addition to the large variation in the volumes of detention basins / ponds between the arid and coastal regions due the wide variation in the nature of climatic / rainfall data regarding rainfall intensities, durations, return periods and runoff patterns.

References:

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Table 1. Rainfall height hN(mm) versus rainfall duration T(min) and return period n (years) for Lattakia city [9, 10]

Table2. Rainfall height hN(mm) versus rainfall duration T(min) and return period n (years) for Damascus city [9, 10]