Storm water runoff is the excess water from a rainfall event that does not infiltrate into the soil and is discharged from a land surface. This flow is part of the total flow, which corresponds to the runoff and groundwater (base flow). This research will focus on the runoff and the base flow, estimated as part of the status quo of imperviousness level in the catchment and storms. We assume a base flow from which changes in imperviousness will change only the runoff from the properties.
Figure 2-4 Relationship between (A) rainfall events, (B) IC allowances, (C) flows at control points, and (D) flood damage distributions in the catchment. CP1 and CP2 are
control points, sstorms probabilities, and i, ii and iii are final IC allowances (ICL).
The runoff flow may be connected to channel systems, storm sewers, pipes, and streams; thus, these flows convey and reach different points in the catchment such as a reservoir, lakes, rivers, estuaries, and the sea. These flows may exceed threshold capacities in channels and streams, producing flooding problems.
Rainfall event 1 s 2 s s1 2 s
Impervious level (ICL) i with Forest (30%)
Impervious level (ICL) ii with Crops (70%) CP1 CP2 Flows at outlet Probability Flood damage ICL ii ICL i
Probability ICL iii
Impervious level (ICL) iii with Concrete (100%) ICL
River/stream (B)
(A) (C)
The overland runoff flows can be quantified through monitoring or modelling; however, monitoring is expensive due to the logistics needed in the field to estimate flows. Alternatively, modelling is widely used to estimate runoff flows, and the rational method is probably the simplest runoff model available to determine peak flow discharges from a drainage area based on runoff coefficients, rainfall intensity and surface area. This method, however, does not allow estimating the flow movement in routed channels and streams, nor allows estimating runoff according to different impervious levels (assumes 100% of
imperviousness). Actually, the method3 does include a level of imperviousness through the
loss coefficient and the calculation is only for estimating peak flows. Complex models estimate runoff by using components of a hydrological cycle. This is called flood routing. These models are also able to approximate flows in channels and streams. In addition, hydrological models can be linked to geographic information systems (GIS) such as HEC- HMS (HEC 2008a) and SWAT (USDA-SWAT 2008) in order to improve the spatial data management and visualisation (Pasche 2007). A certain method may be more appropriate than another under specific scenarios of location, vegetation, topography, catchment size, weather, and land uses; and without such information it is difficult to determine the best method (Haan et al. 1994; Pasche 2007).
A runoff hydrograph is a record of runoff flows by time. Flows depend on upstream land characteristics such as land cover (IC), soil moisture, soil type, management practices, and the rainfall distribution, which affect the infiltration patterns and groundwater (Pasche 2007). Figure 2-5 illustrates runoff hydrographs from two types of hypothetical rainfall distributions categorized as type I and II (whose peaks occur at 1/3 and ½ duration of the storm, respectively) as well as storm events with different probabilities. Changes in rainfall distribution and probability affect the concentration time, peak time, and total flows discharged.
Figure 2-5 illustrates runoff hydrographs resulting from different IC allowances A, B and C. Increasing IC decreases infiltration, increasing the total volume of flows (area under curve) and possibly reduces the time of concentration4. For instance, increasing IC, by
3 The rational method calculates peak flows (Q) as follows: Q = C x i x A, where C is loss coefficient, i is
rainfall intensity, and A is area.
4 The time of concentration (tc) is closely related to discharge and infiltration. This time corresponds to the
changing the land use from forestry to horticulture land would reduce infiltration and accelerate concentration time. Consequently, peak flows at channels and flooding control points could occur earlier.
Figure 2-5 Runoff hydrograph curve of two rainfall distributions. RA is a type I storm and
RB is a type II. RA1 rainfall event 1 and RA2 rainfall event 2, RA1 > RA2. A, B and C are runoff hydrographs with different land uses or IC allowances (A<B<C) under similar rainfall events. qp is the peak flow, tp is the time peak, and tb is the total time duration of runoff.
BMPs and technologies will modify the runoff hydrographs. For instance, retention ponds slow the release of water. In this case, A, B and C represent different runoff control practices (for a specific land use). Changes in land use, implementation of runoff control technology, and other BMPs may be approximated to an equivalent IC allowance in the catchment (Haan et al. 1994). Such practices may change the peak time as well as the total runoff flows (Figure 2-5) from the property, and consequently the routed flows and peak
quantifies the time from the overland flow and the conveyance time in the channels, streams, junctions, rivers, street gutters and storm sewers, etc. Similar to runoff estimations and routing, different methods can be used to quantify. The tc is mainly affected by overland and channel factors such as surface roughness, slopes, sectional shape, length, and storms (NRCS 1975; SCS 1985).
Time RB RA B A Runoff qp tp tb RA1 RA2 C
flows at flooding areas. These changes in IC have a spatial and temporal flow effect at channel and flooding areas.
Hydrological models such as HEC-HMS (HEC 2008a) and WIN-TR55 (NRCS 1975) simulate runoff processes and flow hydrographs. Hydraulic models such as HEC-RAS (HEC 2008b), SHE model, MIKE SHE and MIKE11 (Pasche 2007) are used to simulate changes in water levels in rivers and streams. Flows obtained from hydrological models can be used in hydraulic models to simulate flooding. This thesis will use HEC-HMS and HEC-RAS to illustrate a market application in Chapter 7.