Category Archives: Hydrology

Estimating the design flood based on design storm rainfall intensity/depth

rainfall-runoff transformation (image from alanasmith.com)

For Malaysian, we have a procedure (manual) to design flood hydrograph based on the design storm rainfall intensity, such as DID ((1976; 2010). Those manual are explained very well on the corrected procedures and methods to estimate the design flood hydrograph on the gauged and ungauged catchments.

The following procedure is recommended by DID (1976) for estimating the design storm:

  1. the design recurrence interval is selected on the basis of the guidelines suggested by Heiler and Tan (1974).
  2. the design storm depth for the required recurrence interval is calculated from the procedure by DID (1982). The rain depth or intensity is obtained from Intensity-Duration-Frequency curve.
  3. The areal reduction factors recommended by DID (1982) are used to convert point rainfall to catchment mean rainfall.
  4. The temporal pattern of the design storm is not considered.
  5. the design storm duration is taken as that duration which gives the highest peak discharge.

Note: the design storm duration is usually adopted as that duration gives the maximum discharge. this critical duration is found by trial and error by calculating the design flood for a range of storm duration. From the experience, the duration within the catchment lag (time concentration) gives the maximum discharge.

References:

DID (1982), Estimation of the Design Rainstorm in Peninsular Malaysia. Hydrological Procedure No. 1.
DID ( (1976). Design Flood Hydrograph Estimation for Rural Catchments in Peninsular Malaysia. Hydrological Procedure No. 11.
DID (2010). Estimation of design flood hydrograph using clark method for rural catchments in Peninsular Malaysia. Hydrological Procedure No. 27
Heiler T.D and Tan H.T (1974). Hydrological Design Return Periods, Provisional Hydrological Procedure.

Calculate Flood/Rainfall Frequency from cumulative distribution function (CDF) of the distribution

Specifically, we will use the cumulative distribution function (CDF) of the fitted distribution to calculate the annual exceedance probability (AEP), or the probability that the event is equaled or exceeded in any single year. For example, considering a 200,000 cfs level, the exceedance probability is calculated in the following way:
Exceedance_P = P{X≥200} = 1 – P{X<200} = 1 – F(200) = 1 – 0.975 = 0.025 Note: F(200) is the CDF at 200.

To obtain the return period (also known as the recurrence interval) of the event, we should calculate the reciprocal of the exceedance probability:

Return_Period = 1 / Exceedance_P = 1 / 0.025 = 40 years.

F(x) is CDF

F(x) is CDF

The interpretation is that in a very long series, the 40-year flood value would be exceeded every 40 years on the average. For example, about twenty-five 40-year floods can be expected during a 1000 year period (on the average).

Source: http://www.mathwave.com/applications/flood_frequency.html

Exceedance Probability

Introduction

Sometimes a hydrologist may need to know what the chances are over a given time period that a flood will reach or exceed a specific magnitude. This is called the probability of occurrence or the exceedance probability.

Let’s say the value “p” is the exceedance probability, in any given year. The exceedance probability may be formulated simply as the inverse of the return period. For example, for a two-year return period the exceedance probability in any given year is one divided by two = 0.5, or 50 percent. Continue reading →

Design, safety and management of the reservoir

I found very good online article on the dam management on the view of hydrology and hydraulic. The article was achieved from Dr. Sobri’s academic website (Link: http://civil.utm.my/sobriharun/drama/). Very interesting. The article as follows:

A Dam Management team requires knowledge of civil engineering (structural, hydraulics, hydrology, geology and geotechnical), electrical and mechanical engineering. The main objective of dam safety management is to ensure that the dams able to sustain on two aspects; strength and operation. The standard operation procedures (SOP) determine the successful of dam safety management.The strength normally related to structural, geology and geotechnical assessments. The knowledge in hydraulics, hydrology, electrical and mechanical engineering are required in the reservoir operation.Construction of a large dam ( > 15 meter high) or a major large dam ( > 30 meter) would impose the risk to nearby inhabitants. Dam risk management program would help the operator and manager on minimising the risk and maximizing the safety.

To ensure the safety of the inhabitants along downstream of dam during the heavy rainfall, the flow modelling through spillway adopting minimum of 100 years design (ARI or return period) rainfall shall be performed by the dam managers. Several critical storm patterns could be inspected during the simulation of outflow hydrograph and potential flood inundation map along the downstream river.

Fig. 1: Reservoir system

Sediments flow into reservoir system due to erosion from upstream catchment could reduce the reservoir storage capacity in long-term. The erosion control at upstream catchment is less costly compared to removal of sediments in the reservoir. The nature of inflow pattern into the reservoir system also changes with time and this variation would affect the spillway capacity of reservoir system. The computed probable maximum flood (PMF) value shall be below the designed value to indicate that the dam is safely design. It is the PMF condition with adequate capacity of the spillway to discharge the PMF to the downstream in the event of extreme rainfall. PMF provides an upper limit of the interval within the engineer must operate and design.

The spillway capacity of a dam is regularly inspected for at least 10,000 years return period of PMF. The PMF is derived from probable maximum precipitation (PMP) using rainfall-runoff model. For several stages of spillway operation (gradual releases) of a major large dam, the capacity could be inspected from 10-yr, 100-yr, 1000-yr, 10,000-yr and 100,000-yr. If the spillway capacity below the design level, the possibility of upgrading the system should be adopted by dam manager. Gradual water release has advantages that would alter the shape of rising limb for the outflow hydrograph through spillway due to extremely huge flood magnitude.

Dam failure is possible due to lack of management on two attributes; the structural strength and optimal operation. Most of the dams in the world failed due to the overtopping and piping. In general, the overtopping failure occurs after unexpected extremely heavy rainfall and the piping failure occurs due to unsatisfactory maintenance of seepage in the earth dam.

The dam break modelling also necessary to reveal illustration of potential flood inundation due to the greatest possible heavy rainfall (probable maximum precipitation, PMP). Normally, the hydrologists adopt within 10,000 years to 100,000 years design rainfall that equivalent to probability of occurrence by 0.01 percent and 0.001 percent respectively. The results of dam break modelling are the timing, depth and extent of flood the inundation along downstream that would likely possibly happen due to dam failure.

The international organizations related to dam building and safety management are the International Commission on Large Dams (ICOLD) and the World Commission on Dams (WCD).