Sanitary Engineering Lecture 4

Storm water estimation many factors affect the nature of storm water runoff: there factors must be consider for any hydrological estimating, some of these factors include: Rainfall amount and storm distribution. Drainage area shape, size, topography and orientation. Land use and soil type. Slopes of terrain and stream channels. Storage potential (ponds, wetlands, reservoirs, channels, etc) Characteristics of the local drainage system.

Hydrological Methods: For any storm water system the most important data is the runoff , The term "runoff' is used for water that is "on the run" or in a thawing state, in contrast to water held in storage or evaporated into the atmosphere. Since such flow conditions take place in various stages of a hydrological cycle, there are two main various types of runoff: Surface runoff or overland runoff is that part of the runoff that travels over the surface of the ground to reach a stream channel and through the channel to the basin outlet. (To be precise, the surface runoff includes the precipitation directly falling over the channel reach, and the overland runoff excludes the channel precipitation.) Surface runoff appears relatively quickly as stream flow. Subsurface runoff is that part of the runoff that travels under the ground to reach a stream channel and to the basin outlet.

Factors that control runoff : Rainfall characteristics (duration, amount, intensity, frequency and distribution). Climatic factors; temperature, humidity, wind velocity, pressure variation, etc. Watershed physical conditions (size, shape, slope, topography, geology, soil type, land use and cover, orientation, depressions, streams, etc).

Method to estimation of Runoff 1. Rational Method:   The Rational method is a formula for determining the peak runoff rate, where it's characterized by consideration of the entire drainage area as a single unit, estimation of flow at the most downstream point only and the assumption that rainfall is uniformly distributed over the drainage area and it's constant over time. The rational method is based on the premise that the rate of runoff for any storm depends on: the average intensity of the storm. the size of drainage area. the type of drainage area surface.

Rational Method Equations:   The peak rate of runoff calculated by the rational formula at any location in a watershed is as a function of the runoff coefficient, mean rainfall intensity for a duration equal at lest to the time of concentration (Tc), and the drainage area. The formula of the Rational Method is expressed as follows: Where: Q = maximum rate of runoff (m³/hr) C = runoff coefficient I = intensity of rainfall for a selected frequency and duration equal at Least the time of concentration (mm /hr) calculated from tables or from the intensity duration curve. A = drainage area (m²).

Relationship between impervious surfaces and surface runoff

2. The Soil Conservation Service (SCS) Method This method develops by the US Soil Conservation Service (SCS), this method can be used to determine the peak rate of discharge. The fundamental premise used in developing this method is that the depth of runoff (h) depends on the rainfall (P). SCS Method Equation: The following equation –calculates the peak flow of an area through steps explained below: Where: A = drainage area (mi²) q ¯p= unit peak discharge [ft³/sec per inch of runoff per mi² of catchments area] for 24-hr storm.

h = runoff (in) can be determined by the formula: P = rainfall S = potential maximum retention after runoff begins, can be determined by the formula: CN = the runoff curve number that varies from 0 (previous) to 100 (impervious).

The Water Budget Calculation Method Water budget comprises the components of the hydrologic cycle where it's an accounting of the inflow, outflow and storage of water in designated hydrologic system. For specific time period, we can apply the continuity equation by balancing the gains and losses of water in a region with the quantities of water stored in the region. Water Budget Equation:   Water balance is defined as the change in volume of the permanent pool resulting from the total inflow minus the total outflow. Where: I = total input. O = total output Δ S = change in water storage.

Water Budget Equation:   Water balance is defined as the change in volume of the permanent pool resulting from the total inflow minus the total outflow. Where: I = total input. O = total output Δ S = change in water storage. Where: P = precipitation (mm). R = R2 – R1 {average runoff (m³/s)}. ET = evaporation transpiration. G = G2 – G1 (lateral outflow -- lateral inflow).

Water Budget Equation:   In large watershed and over long period of time, the entire surface and subsurface system under steady state conditions and that there is no occurrence of groundwater lateral inflow or outflow the water balance equation become:

Design of W.W. Collection System Design criteria: Waste water flow: Flow varies according to: The season (monthly variations) Weather conditions Week of the month , day of the week, time of the day. Estimation of the design flow Qdes: Data needed: Average daily water consumption per capita for domestic areas (L/c/d), (Qavg). Average daily water consumption per capita for institution ( school, offices, ….etc. ), (Qavg). Average daily water consumption for commercial and industrial areas. Infiltration, inflow: Qinfil is taken as [24-95 m3/day/km] or [0.5 m3/day/diameter (cm)], take the bigger value of the two. Qinflo is taken as 0.2-30 [m3/ha/day]. ( hectare = 10,000 m2 ) Qdes = Qmax + QI/I ( if found) QI/I = Qinfil + Qinflo Qmax = [0.80* Qavg] * Pƒ ( 0.8 > 80% return from water supply). This equation is for domestic users only. Qmax for institutions, commercial activities, and industries are calculated according to the type of industry, and cannot be calculated from this equation. Each industry has its specific average wastewater production and peaking factor that can be taken from published references or from the records of these industries or institutions.

Sewage flow diagram for a small town 149 86 43 0 2 4 6 8 10 12 14 16 18 20 22 24 hour 1.8 1.5 1.0 0.5 0.0 Flow coefficient Flow (L/s) Peak coefficient Average day flow Average 24 hr flow Average night flow Sewage flow diagram for a small town

Note: [Qavg]w = 0.8 Qavg , which is the average domestic wastewater production , while Qavg is the average water consumption.

Example Solution